Circling in Lift question

[Rayven]

When you get into lift and start to circle, I understand that you try to maintain a constant bank angle with the elevator, and relatively shallow at that. Question--how do you make the adjustment to the bank angle: with the rudder or with the ailerons or with both mixed together? Seems to me you would not mix so you could move the rudder toward the outside of the curve to flatten, move the ailerons to tilt inward in order to minimize any altitude loss. Does this make sense?

[Ollie]

Great Article by Mark Drela "Using the Rudder in Thermal Flying"

http://www.polecataero.com/
Select menu from articles.

[Robglover]

It's always different. Use your eyes to tell you if the plane is coordinated, use BOTH of your sticks to make it coordinated.

Most pilots forget they have a left stick. Sad. It's helpful, and OK, to couple rudder to ailerons when you are first learning how to fly. But coupling alone will never be right in all conditions. As you get comfortable (and safe) you should turn the rudder coupling off.

[histarter]

I have always flown mixed, Rudder stick still works when needed! When mixing, one merely creates a center balance that can be modulated.

[Landi]

Hi

I use the ruder for the turn and the airlerons to control the bank angle.

Sometimes when the thermo is very weak I even circle almost flat to reach that you turn the ruder in one direction and the airlerons into the other to flatten your turn.

Elevator is used to compensate for the up wind and down wind side so the plane is always going up and does not det to fast in the downwind side.

This is the way it works for me wiht great results.

Saludos,

Landi

[machild]

Looks like most everyone is on the same page here. Here are some rules I try to observe:

When the airplane "signals" lift, make one smooth 360-degree turn. If the plane climbs or maintains altitude, stick around. Conversely, if the plane sinks, continue hunting. Don't "sinkle."

If you enter lift close to the ground, the base of the thermal will usually be tighter. Tighter base=steeper bank angle to keep the plane in lift.

As the plane climbs towards the top of the thermal, the area of lift (or core) will broaden, and the diameter of your circling turns should increase. Try to show as much of the wing's area to the lift. Decrease bank angle (with aileron stick) and hold nose into turn (with rudder stick). If you are circling to the right, you will hold left aileron (to flatten bank angle a bit) and right rudder to keep the nose heading in the right direction.

Most modern TD ships fly best when thay are "on step" so use the rudder -- coordinated with the elevator -- to keep the fuse parallel to the ground (or better yet, nose-down just a touch to keep the tail outside the radius of the turn) and opposite aileron to vary the bank angle. Narrow core of lift -- steeper bank angle. As core widens, decrease the bank angle with opposite aileron.

It takes a little practice, but once you discover your aileron stick and learn to use it independently of the rudder, you should notice an improvement in both your technique and your plane's performance. Drela's article addresses sideslip and the drag and lost L/D it produces when the pilot's left and right thumbs go dumb…

[flystoolow]

Have you not been reading the latest in thermalling science? The thing to do, if you want to gain altitude, is, and I quote,

"Dive plane OUT of thermal at steep (not too steep) angle and when comes toward you must start turn, no not turn - actually gentle curve very important. Now when curving and desending very rapidly look very carefully, ABSOLUTELY CRITICAL DON'T MISS BOTTOM BOOST! Pull up gentle yet steady NOT TOO HARD 5 POUND BALLAST LIKE BOMB UNDER WING. Wow! Look as she climbs! WATCH OUT CESSNA!!! :eek:

[Rayven]

Thanks everyone for the response....but there seems to be some contradictions that continue to baffle me: if you are cross controlling, rudder one way, ailerons the other to keep wings flat, then--as Drela points out--the plane is flying very inefficiently. It is losing altitude as a result. And why do we want to keep the wings flat/parallel to the ground when in lift anyway? The plane doesn't "know" its in lift, its still falling through the air--it just so happens that the air is going up (a corrolary of Einsteins theory of Special Relativity!). It also doesn't know if its upwind or downwind--that is relative to us on the ground, not to the plane in the moving air. So I'm still not sure what is best way to maintain the bank to minimize the sink rate.

[Rayven]

Have you not been reading the latest in thermalling science? The thing to do, if you want to gain altitude, is, and I quote,

"Dive plane OUT of thermal at steep (not too steep) angle and when comes toward you must start turn, no not turn - actually gentle curve very important. Now when curving and desending very rapidly look very carefully, ABSOLUTELY CRITICAL DON'T MISS BOTTOM BOOST! Pull up gentle yet steady NOT TOO HARD 5 POUND BALLAST LIKE BOMB UNDER WING. Wow! Look as she climbs! WATCH OUT CESSNA!!! :eek:

Are you talking here about using a Dynamic Soaring technique to utilize the wind gradients around the thermal??

[dephela]

He's[flystooslow] pulling your leg!

Let me try to clear one thing up, the "cross-controlling". Over the years. I've been told that our planes have too much vertical fin and are "spirally divergent". This means that a plane put into a banked turn, if left alone, will probably want to drop the inboard wing, drop the nose and enter a "death spiral"! As we see the plane doing this and sometimes we do it while adding more rudder to position the plane in the thermal, we know that we don't want the steeper bank angle that the plane is driven to by its design fault. We put some opposite aileron in as a correction for the bad behavior. Why the too large a vertical fin? I dunno but my guess is that we launch better with it.

Dennis

[histarter]

I am afraid of falling out of a chair laughing here. :D

1. A plane turns because it banks to generate a composite lift vector. Advanced cross sticking to attempt to flatten the bank angle cancels the turn with advanced drag - causing sink greater then the needed bank angle would generate - and at a larger spiral diameter than desired!

2. Only airspeed controls bank angle for a given radius! Slow allows smaller spiral or lower bank angle! There is no way around this!

3. Un happy with with a design that won't spiral tight?

A. turbulate,
B. add flaps,
C. buiild a new wing with a thicker profile.

or A+B, A+C, B+C, A+B+C!!

And that is it. [It is very difficult to make an existing machine lighter!!] :eek:

[Ollie]

Not thickness but mean camber for tighter turn (same function as flap droop)!

[histarter]

More MCL is more for performance retention. Thicker is lighter, stiffer, less inertia transient, and also has more MCL. All depends upon what one wants to capitolize upon. :confused:

CLA and Large Rudder:
All Sailplanes do a small forward slip in a spiral - models and full size! A fact of life. This small slip can be harnessed for thermal sniffing! When wanting to spiral tight in lift, use flap to get airspeed down, which also washes out the inboard tip for advantage. If lift is strong, it will try to accellerate an aircraft with large vert stabilizer area into a spiral dive, that should be held off with cross stick! And this is healthy - especially with differential ailerons! With practice a pilot can feel how much cross is needed, and determine thermal strength - to help adjust flap to slow machine further to maximize climb rate. I had used probably 12 degrees of flap (throwing away potential L/D) on my Spirit 100 for tight (because of slow airspeed) to get 200 + FPM climbs.

This is a better situation than a B 25 with one motor gone - so the guns ammo and crew gets tossed out! :)

[Miami Mike]

Elevator is used to compensate for the up wind (upwind) and down wind (downwind) side so the plane is always going up and does not det (get) to (too) fast in the downwind side.Ouch! Why haven't you guys said anything about this yet? Do you agree with it?

[schrederman]

Fly the glider in the airmass, not with respect to the ground track. While it may look like it is speeding up going downwind, it isn't speeding up in the airmass. If you slow it on the downwind side, and speed up on the upwind side, you will find yourself flying out of it unless it's huge in diameter. There are some inertial things to consider, but they are far less worrisome than you think, so I basically ignore them.

Forget trying to turn flat. The wing not only lifts the model, but it's the wing that turns it, too. Coordinated flight in tight turns may require as much as 60 degrees of bank, with the stick held on the backstop. In small, tight thermals, it may be the only way to stay in the thermal. The sink rate goes up... YES! But the core of the thermal is usually so much stronger than the fringes that it way more than makes up the difference. While many think this is not good advise, I've proven it to myself over and over. The slower and tighter you can thermal, in the core of course, the faster your model will climb.

If there's a wind component, it may appear to rocket downwind and almost stop coming upwind. LET IT! It's flying at it's airspeed within the airmass, and the thermal is moving as an airmass. You're in the airmass, not on the ground. Fly airspeed, not groundspeed. If it isn't stalling when it appears to almost stop, don't make any correction. The stronger the wind, the more pronounced this will be.

When circling, use the controls as designed. The ailerons are for roll control, but so is the rudder coupled with whatever dihedral your model has. Without dihedral, I'd hate to try to fly a Windfree or any straight winged model that doesn't have ailerons. I still use the ailerons primarily for roll and the rudder to counter adverse yaw. Coordinating your turns is one of the first things a student glider pilot learns. Usually lead very slightly with the rudder, with the corresponding aileron input to achieve the desired roll rate. Once the bank angle is established, keep the bank angle constant with the ailerons, and the turn coordinated with the rudder. Keep the airspeed where you want it with the elevator. While in the thermal, it may be necessary to keep the bank angle constant with the ailerons and a touch of rudder to the outside of the turn. This can compensate for the fact that the outside wing is flying a touch faster than the inside wing. This results in slightly uncoordinated flight, but may also help you to slow the model a bit more, resulting in a tighter turn. Again, I've proven these techniques to myself. They take practice, practice, and more practice.

Before you flame me too badly on this, come fly with me... or ask those that do on a regular basis.

Jack Womack

[DFW_HLG_TG]

When you get into lift and start to circle, I understand that you try to maintain a constant bank angle with the elevator, and relatively shallow at that. Question--how do you make the adjustment to the bank angle: with the rudder or with the ailerons or with both mixed together? Seems to me you would not mix so you could move the rudder toward the outside of the curve to flatten, move the ailerons to tilt inward in order to minimize any altitude loss. Does this make sense?



If you read too much, you will never be able to fly. Heck, I'm getting a headache reading all of this. Forget the crossing it up. Have some rudder, not much, mixed with the ailerons. Find the speed and the bank angle that your plane likes to fly and work on duplicating it every time. A perfect circle would require no aileron or rudder input but we also have the tubulent air of the thermal to deal with. Try to maintain your turn and if at any part of the circle you see that one wing wants to go up, go further into the side with the high wing. Keep adjusting the position of your circle untill your plane requires the least amount of input to maintain that circle.

I would not worry about using the rudder stick untill you have the circle and thermaling down.

[Miami Mike]

There are some inertial things to consider...Good post, but what are these inertial things?

[Rayven]

Jack..

Thank you for your input...this sounds right to me and I'm going to practice what your preaching next time I'm out to the field. I've finally come to understand myself in this last year that there really is no such thing as upwind and downwind turns or lift acting on wings--except in our minds!! Like you say, you've got to fly the airmass, not the ground!!

[schrederman]

The inertial portion of the downwind/upwind turn has been debated among R/C soaring pilots constantly since the late '60s... possibly before that. That's the first time I can remember the discussion.

While the ship does fly in relation to the airmass, the actual movement of it's mass is not constant, except in still air. It's movement, and therefore it's inertia, are greater during it's downwind side of the circle, and it's harder to overcome that inertia with the normal forces of the turn. As the upwind turn nears it's completion, the actual movement may be almost nothing, and it's consequential lack of inertia can cause problems. I generally find that if you are circling as tightly as I like to, this doesn't manifest itself nearly as badly as it does for someone that circles in large lazy 500' circles.

Hope that explains it well enough.

JW

[Sparky Paul]

If the thermal isn't 500 feet in diameter, then a circle that large -will- get into air moving in a different direction than the air in the thermal, and it will bump the plane around..
When the thermal is larger, then circling in both directions can be done.. find the direction your plane really likes..

[histarter]

JW
Your system is excellant, providing design has some residual dihedral to it to make the increased vectoring of the forward slip effective.
Zero dihedral would make the offset rudder increase slip drag with more rudder drag, making Carter's comments much better - since his technique just doesn't care!

X machine with mixed rudder and aileron that has the ratio finessed by experience, will be effective by holding off excessive bank with just the prime stick - no complications needed. Slow thinking pilots (like myself) prefer this state of affairs, and it still is dog-gone efficient! Simplicity makes for superior thermal pilots (less juggling when working lift). Obviously, I try to make my aileron machines emulate my 2 channel machines - using mix ratio for an adjustable dihedral effect. KISS :D

[Definitition: Excessive bank is when spiral dive is struggling to take over. It is quite obvious, and experience will quickly educate.]

[Miami Mike]

The inertial portion of the downwind/upwind turn has been debated among R/C soaring pilots constantly since the late '60s... possibly before that. That's the first time I can remember the discussion.I don't see how anyone who understands physics would debate such a fundamental and obvious principle. An airplane flying within a mass of air isn't affected by the way the ground is moving down below it.

[Landi]

@ shrederman

Your theory might be right and the forum is here to discuss all points of view so we all can broaden our horizons and become a better flyer.

I do not agree with your theory regarding the up wind and down wind.

I noticed during my flights that there is a loss in climbing performance if you do not compensate for the up and downwind.

Why I know this well I have a Vario, Picolario Talk< with a enrgy compensation probe in my Schueler DG 505 (http://www.xl-sailplanes.com/index_files/page0025.htm)

This setup lets me monitore the altitude in real time and the energy compensation wont let me have a fake thermo so I always know when I'm in raise or in sink.

In the beginning I use to do it just as you described or something similar but with time and experimenting several setups I learned how to make the most of it.

I agree that in a narrow thermo you have to circle in a high bank to keep the plane in it but still you compenstate for the up and dowm wind side. In a angle of 60 degree or more sometimes I even use the ruder as elevator and the elevator as ruder just like Knife edge and guess what it does work.

Circling flat or with not to much bank is only usefull in a very weak thermo BUT the thermo has to be wide if not you will fall out of it all the time.

So no one theory works 100% you just have to make the best out of it.

What ever works for you/me is the best... :D :D :D

Saludos,
Landi

[flystoolow]

In a angle of 60 degree or more sometimes I even use the ruder as elevator and the elevator as ruder just like Knife edge and guess what it does work.

Great, I will launch without the wings next time I fly in small, strong thermals.

PS. Any hints on getting down safely?

[DFW_HLG_TG]

Lets see, as a pilot I should know this. a 45 deg. bank angle equates to 2gs. What would 60 deg. equate to. I know it's non linear so I guess around 6gs. With a model @ stall of say 10 mph x 6 = Oh well, I'm going flyin.

[flystoolow]

Lets see, as a pilot I should know this. a 45 deg. bank angle equates to 2gs. What would 60 deg. equate to. I know it's non linear so I guess around 6gs. With a model @ stall of say 10 mph x 6 = Oh well, I'm going flyin.

LOVE IT! :D

[gustabmo]

I've finally come to understand myself in this last year that there really is no such thing as upwind and downwind turns or lift acting on wings--except in our minds!! Like you say, you've got to fly the airmass, not the ground!!
One thing I do is to trust the stick: I try to ignore the speed of the plane in relation to the me (=ground) and just trust that I'll be able to reproduce that stick movement that worked on the upwind turn... it ought to work on the downwind one!

(thought the trajectory of the plane in relation to the ground will be different)

Gustavo

[gustabmo]

Thanks everyone for the response....but there seems to be some contradictions that continue to baffle me: if you are cross controlling, rudder one way, ailerons the other to keep wings flat, then--as Drela points out--the plane is flying very inefficiently. It is losing altitude as a result. And why do we want to keep the wings flat/parallel to the ground when in lift anyway? The plane doesn't "know" its in lift, its still falling through the air--it just so happens that the air is going up (a corrolary of Einsteins theory of Special Relativity!). It also doesn't know if its upwind or downwind--that is relative to us on the ground, not to the plane in the moving air. So I'm still not sure what is best way to maintain the bank to minimize the sink rate.
I agree entirely with your position.

I'd go as far as saying that the plane doesn't even know it's turning. When flying full scale sailplanes I did a test and closed my eyes while piloting: you are able to maintain coordinated flight but after a few seconds it's impossible to maintain your route, you just can't determine wether you are turning left, right or going straight, the plane would go all the way into a spin without the pilot beeing able to know which way to pull the rudder to correct!

This is a well documented symptom of flying in clouds without an artificial horizon.

[Sparky Paul]

Lets see, as a pilot I should know this. a 45 deg. bank angle equates to 2gs. What would 60 deg. equate to. I know it's non linear so I guess around 6gs. With a model @ stall of say 10 mph x 6 = Oh well, I'm going flyin.
.
http://www.fly-imaa.org/imaa/hfarticles/flying/v2-3-52.html

[rcbrust]

The inertial portion of the downwind/upwind turn has been debated among R/C soaring pilots constantly since the late '60s... possibly before that. That's the first time I can remember the discussion.

While the ship does fly in relation to the airmass, the actual movement of it's mass is not constant, except in still air. It's movement, and therefore it's inertia, are greater during it's downwind side of the circle, and it's harder to overcome that inertia with the normal forces of the turn. As the upwind turn nears it's completion, the actual movement may be almost nothing, and it's consequential lack of inertia can cause problems.

Hmm, how does the fact that the glider, airmass, and surface of the earth are all moving about 1000mph due to the earth's rotation come into play when you talk inertia?

Randy

[flystoolow]

Hmm, how does the fact that the glider, airmass, and surface of the earth are all moving about 1000mph due to the earth's rotation come into play when you talk inertia?

Randy

I thought PROInstructor already answered this, when he let us know that
5 POUND gliders can thermal. :eek:

[Miami Mike]

It looks like this discussion of inertia is picking up momentum.

[rcbrust]

...or the fact that the earth is traveling around the sun at 66,000 mph and our solar system is traveling through our galaxy at... :D

[Hostage-46]

Forget trying to turn flat. The wing not only lifts the model, but it's the wing that turns it, too. Coordinated flight in tight turns may require as much as 60 degrees of bank, with the stick held on the backstop. In small, tight thermals, it may be the only way to stay in the thermal. The sink rate goes up... YES! But the core of the thermal is usually so much stronger than the fringes that it way more than makes up the difference.

Before you flame me too badly on this, come fly with me... or ask those that do on a regular basis.

Jack Womack

Yeah the voice of reason finally shows up..... didn't even need any higher math to dazzle the masses!

[schrederman]

A couple of points...

These inertial forces are relative. Nobody quoted how fast the Milky Way Galaxy is hurtling through space... I'm disappointed...

If you constantly correct for wind as our friend Landi suggests, each correction moves your circle slightly upwind. This is the way we move our circle when we need to center the core of the thermal; model or full-scale makes no difference. This knowledge comes from experience in thermalling a real sailplane in tight lift, close to the ground... 600 to 800 feet... where you're not supposed to be thermalling, but landing, instead... scratching we calls it... In that scenario, the wind in West Texas can make a full scale ship seem to just about stop on the upwind side, too. Start correcting that and you WILL be landing. Applied to hand-launch type lift with an open class model, it's all scaled down, including the narrow little thermal 40' above the ground. Even if you don't lose your thermal altogether, you will not effectively core it.

These are my opinions, not to be taken as fact. Experiment for yourself, and do what works for you... I'll just outclimb you and go on about my business... hehe!

JW

[schrederman]

By the way, the Region 10 Soaring Contest begins at Texas Soaring Association on July 3. I'm going to be up there as a spectator visiting old soaring friends for a couple of days. If not for this torn tendon, I'd be competing... There's always next year...

Jack Womack

[mhmitchell]

As an ex full sized glider guider you initiate turns with the ailerons and coordinate with the rudder. (see yarn on windshield very sensitive insturment) After you establish your bank you actually reverse the aileron slightly to counteract adverse yaw (you'de have to be there) In a tutn the out wing is traveling faster than the inner creating more lift so to keep the bank you use crossed controls . The bank angles should be fairly small near the ground but as the thermal gets highr its diameter decrease and you have to increas the bank with the ailerons. I have seen full sized sailplans racked over at almost 90 degrees and still climbing. Full sized guiders have an advantage as they can watch the variometer to establish what degree of bank gives the best climb.

I have often suggested the rc glider pilots take a couple of fli=ghtd in a sailplane , insure they will let you fly it in a thermal, the dynamics will become so clear that its almost second nature to you.

[DFW_HLG_TG]

As an ex full sized glider guider you initiate turns with the ailerons and coordinate with the rudder. (see yarn on windshield very sensitive insturment) After you establish your bank you actually reverse the aileron slightly to counteract adverse yaw (you'de have to be there) In a tutn the out wing is traveling faster than the inner creating more lift so to keep the bank you use crossed controls . The bank angles should be fairly small near the ground but as the thermal gets highr its diameter decrease and you have to increas the bank with the ailerons. I have seen full sized sailplans racked over at almost 90 degrees and still climbing. Full sized guiders have an advantage as they can watch the variometer to establish what degree of bank gives the best climb.

I have often suggested the rc glider pilots take a couple of fli=ghtd in a sailplane , insure they will let you fly it in a thermal, the dynamics will become so clear that its almost second nature to you.



Sorry to burst your bubble but I can tell you are not a pilot. In many, most I have been in you start the turn with the rudder and coordinate with the ailerons. The tail moment is much shorter than the models we fly. Not untill you get past about a 20 deg. bank angle is any opposite aileron needed to stay in the turn. You will never see one go past a 45 deg. bank angle. Anything above 20 when thermalling is wasted. At 60 deg. The g force to maintain altittude would cause one to pass out fairly quickly.

[Hostage-46]

At 60 deg. The g force to maintain altittude would cause one to pass out fairly quickly.

If you're a big old sissy :)

[mhmitchell]

Well my bubble is not busted . Will be happy to take you up if you wish to pay for the tow, or will mail you copies of my log book. A 90 degree bank if it is tight enough it will cause high G forces, but one can fly 90 degrees of bank on a straight course with no turn at all, the airplane does not fall out of the sky. In reality the G forces rise as you try to keep the nose up by use of the elevator. Ask a sailplane pilot what it means to "put your foot in the hole" and he will tell you that refers to the yaw string and you press the rudder pedal on the side with the most space "the hole" Rudder is almost never used in power aircraft except to steer in on the ground! If you are flying straight and level and press full rudder unless your plane has some dihederal it wont turn but rather will point its nose in the direction you pushed and fly in the yawed position. Rudder is used to coordinate the turn and help prevent a slip. Rather than get into a heated argument about your calling me a liar I will not comment farther. Go fly and see for yourself!!

[DFW_HLG_TG]

I think your about to get smoked.

[Hostage-46]

... A 90 degree bank if it is tight enough it will cause high G forces, but one can fly 90 degrees of bank on a straight course with no turn at all, the airplane does not fall out of the sky. In reality the G forces rise as you try to keep the nose up by use of the elevator.

Well ....

It is true that you need to use your feet in a full scale glider. When I transitioned from high performance military and light civil power planes, it took a bit of getting used to. In fact, in the Grob, a glass two seater, I recall having to lead with rudder, then aileron and recorrect with rudder, all tied to the yaw string. Which, by the way, is an excellent insturment.

Get in a 2-33 and skid around a turn and watch the vario, you'll never do it with with your model again. It's all about keeping the string in the middle.

With regard to 90 degrees bank, no you cannot maintain level flight in a coordinated turn at that bank angle as there is no vertical lift element. Before you hit reply and bring up a knife edge, refer back to the previous sentence.

I personally have flown a full scale glider at 60 AOB in 6 KTS of lift, it's between 2 and 3 G's no biggie really. Of course one does not generally fly at that bank angle but sometimes a tight turn, at high AOB is better as it keeps you in the core where as a lasy floating turn will drive you out of the lift.

DJA

[histarter]

The ONLY 90 degree spiral I have witnessed in my life time was executed by a 25 lb 1/4 scale Stearman.
The pilot was asked to demonstrate different forms of spins. He did the classics, the flat spin, the inverted flat spin and I thought that was all there was - only to have him climb to altitude and initiate a spin that made me rub my eyes afterward! I witnessed what I thought was the impossible! The Stearman banked to 90 degrees (like in edge flight) rotating axially about the wing centerline, while falling out of the sky at a tremendous rate. The pirouette was then converted to a flat spin - and recovered accordingly! Could this be the 90 bank refered to? :eek:

It probably would work well if one was coring a Oklahoma tornado! :p

[mhmitchell]

iF you will go back and re read what I said. I have witnessed high performance sailplanes racked over nearly 90 degrees. Efficent or not .If you have a really boomer thermal you will maintain and gain altitude but I do not think they stay that way for the entire circle. Yes you need to pull the nose up to maintain altitude. When you are in a steep bank the elavators actually do more ruddering and the rudder does mor elevataing (sic). I was taught to initiate the turn with the aileron maintain coordination with the rudder. After the turn was intitiated one may have to apply opposite aileron to counteract adverse yaw which of course means that you would be using the rudder to maintain the turn. The amount of bank is really determined by the vario. I always tried to keep the max climb using the vario. Turning tighter and increasing bank until the rate of climb dropped off. For the 1-26 and the thermals in my area this was abot 30 degrees and if the thermals were smaller in diamete up to 45 drgrees. I have quite a bit of time in Cessna aircraft and again the only time I had to use the rudder is on the ground and crabbing into the wind during crosswind landings (the other method would be to drop the upwind wing but that always felt awkward to me.)

Finally the point of the conversation I was trying to make was go fly in a full sized sailplane and you wont have any more questions about flying in thermals with a model other than how to find them in the first place.

I guess in reality that once you learn to fy a sailplane the movements of rudder and elevator are so intertwined that you really dont distinguish which comes first or second. The turns indeed can be intiated with the rudder. Just as I learned to fly I was taught that it was better to use the ailerons first. The next time I go flying I will try rudder first to see if there is any difference. Not worth arguing over in any case, just interesting conversations.

[histarter]

1. Full size sailplanes have tremendous inertia compared to models. If wing gets to 90 degrees one is executing a stall turn - and not a spiral!
2. Training was to start the bank, then add the rudder for cleanup while adjusting elevator (and throttle if bank is steep) to maintain airspeed and altitude. Rudder only yaws and will not turn - unless enough dihedral present to convert yaw to roll. Obviously sailplanes have some weak dihedral and can inefficiently skid a turn!
3 All sailplanes spiral with a slight INSIDE forward slip (circular airflow). Skidding works with large regions of lift where throwing away L/D for improved lift vectoring is advantageous. (My full size friends employed this technique after it was explained).
4. In my lifetime I don't think I banked a turn more than 45 degrees under any circumstanses. Stall in turn airspeed goes up drastically after 30 degrees.
5. I have made interactive programable charts describing sink, airspeed compensation, elevator compesation correction, and delta diameter loss - and the numbers get rediculous after 45 degress. :p

[Sparky Paul]

The yaw string..

[tw126a]

I believe it, histarter. I don't think your gaining anything by going over 45 degrees. The other thing is that most pilots overestimate their actual bank angle, ie. 30 degrees may seem or feel like 45 degrees. Consequently, most newer pilots don't bank steep enough, to core a small, narrow thermal requires 45 degrees.

Tom

[mhmitchell]

Gee I wonder if this pilot has passed out from the extrem G force of any bank over 60 degrees...I rest my case this is one of several dozens I found of sailplanes at high angles of bank

[schrederman]

Some of you guys need to get your facts straight. A 60 degree banked turn IS a 2g turn, basically. In other words - If you enter a turn at 45 knots and hold 45 knots during the 60 degree banked turn, you WILL be pulling 2g. It's in the most basic of flight manuals, and can easily be proven with a g-meter. Beyond 45 degrees is of NO value(?)... maybe where you fly, the thermals are huge areas of lift, and in that area, you are correct. A 20 degree bank would be ok if the conditions warrant. Fly a 20 degree bank around me and in the lift I fly and I'll thermal up through you and probably have a word with you when we get back on the ground, as I did just recently with a PW-5 pilot. He was at 4000 and seemed wandering aimlessly around in about an 800' circle to the left. (There's your 20 degree bank.) I entered the thermal about 500' below him and 2 circles to the left IN the thermal has me almost head to head with this guy. When we landed he was pissed at me for not "clearing" his thermal. We spoke about this on 2 different occasions, the last with the board of the club. The fact is I was in the thermal, and he wasn't. Protocol isn't the question, on this thread, though. I was circling at 45 knots at... you guessed it... 60 degrees of bank. He stated the thermal was 1 to 2 knots. Actually I was going up at 6 to 8 knots, and continued happily to 6900 in short order.

OBTW, as a current glider pilot, I can tell you that a properly coordinated turn requires a lot of rudder applied ALMOST simultaneously with a lot of aileron. It takes a lot of practice to keep that 10" piece of yarn centered on the canopy. When you do accomplish it, you will notice that you are leading with the rudder but only by a couple of hundredths of a second... almost imperceptible.

As far as passing out goes, if you are in reasonable health, it takes about 6g to make the average guy black out. Fighter pilots in the prime of life/health can do 9 or so wearing g-suits.

90 degrees of bank should never be used if you are trying to gain altitude. It should be used only as a defensive move or when flying aerobatics. I can't think of any sailplane that has enough fuselage side area to maintain altitude with ALL the wing's lift vectored to 90 degrees. The wing lifts, propels, and turns the sailplane. With the nose slightly down, as in gliding flight it must be, the lift is vectored forward slightly and drag and the forward-vectored lift reach equilibrium at a certain airspeed. Drop the nose and the sailplane acelerates forward because the forward-vectored lift is then greater than the drag until again, equilibrium is reached at a higher airspeed.

The turn works the same way. Wings-level and the remaining lift all points straight up. Drop a wing a bit and the vector points into the turn, ie. the vectored component of the lift pulls it that direction. The airspeed increases a little to make up for the lift component that no longer just holds the glider up. To maintain airspeed, you pull back a little, increasing the angle of attack, keeping the airspeed the same but compensating for the vectored force that's being used to turn the airplane. That's why the stick goes to the backstop to maintain 45 knots in a 60 degree bank. It's a VERY tight turn. Go on believing the myth that beyond 45 degrees is of no benefit, and please come fly in out contests...

Yes, PLEASE... I love dispelling myths.

Jack Womack

OBTW, I do know what a 60 degree bank looks like, both from the cockpit and from the ground, holding a transmitter.

[dephela]

Does a pic of a sailplane in a steep bank indicate a sustained thermal turn or just an impressive shot?

[schrederman]

I promise you that NO competitive sailplane pilot fools around with a 20 degree bank, unless the lift is very large and very weak and he's just trying to it make a landable field. Most pics I see of sailplanes in thermals are at 45 degrees or more and the elevator position is usually quite evident.

JW

[Sparky Paul]

Few airline passengers will ever see 30 degrees of bank, which is about 1.2 g.
4g is "interesting"!
One of our "guest" pilots getting the feel of a Tristar was instructed to bank to 45 degrees.. From his age, apparently the last plane he'd flown was a Bf-109... he rolled to the last mark on the ADI.. which is 60 degrees!
Those of us standing up in the plane were immediately sitting down.. near a seat or not! :)

[Miami Mike]

Yes, PLEASE... I love dispelling myths.Post #49 (http://www.rcgroups.com/forums/showthread.php?p=3947322&postcount=49) was very informative and impressive, and there's no doubt in my mind now that you know exactly what you're talking about. The only thing you left out were the "inertial things" that you alluded to in posts #15 and #19. How does the speed and direction that the ground is moving down below affect your flying?

[schrederman]

Miami Mike,

Reply # 19 tells it pretty well as to what's happening. I usually don't correct for the inertial portion at all as I usually find them to be very small, particularly if turning tightly. Thermals are NEVER perfect columns of air. The core can dance all around and corrections are constantly being made to remain in the core, as much as possible. The inertial effects are but a very small part of this puzzle and are moot when trying to stay with a small, sometimes violent thermal. In the large-diameter, smooth area type lift, the circle isn't usually moved much anyway, and the interial effect isn't large enough to be detected. I usually find this type of lift is not accompanied by much wind, anyway, so the downwind drift isn't much of a factor, if any.

Hope that helps.

Jack Womack
Ol' Lefty... or ROBOARM... until July 21. I may actually return to full-scale flight as early as October...

[Miami Mike]

I still wish you'd answer the question. What are these inertial effects? Where do they come from?

[DFW_HLG_TG]

[QUOTE=schrederman]Some of you guys need to get your facts straight. A 60 degree banked turn IS a 2g turn.



My bad, your right, it is 2g @ 60 deg. However, your stall speed is increased by 41%. I am curious as to the stall speed of your plane if you maintained 45kts @ 60 deg.

[schrederman]

Stalls at 36 kts with +8 degrees flap.

JW

[DFW_HLG_TG]

Cool, I want one. I was working on my sailplane endorsement a couple of years ago. I lack 7 hrs. I bought a 150 and that took up most of my mad money. The last time I went soaring was about two years ago, what a rush, there is nothing like it. I sold my 150. I only flew 99 hrs the year I had it and it would be cheaper to rent. I may go back and work on the endo. again.

[histarter]

Old saying;
There are bold pilots and there are old pilots; however, there are no old bold pilots!

I am the old pilot from my group of private pilots! [All my friends that played with home builts, ultra lights, concentrated upon IFR as prime, and performed occasional simple acrobatics in violation to FAA and VFR - are now deceased]. :eek:

The closest I got was trading time of Tcraft for Rotec time. One flight in a "legal" American Ultra light convinced me they were not strong enough. The UL crashed a month later killing its pilot. As time passed (5 years), so did the other 4 corporate pilots in various UL crashes - completely wiping out the corporation.

As an old man I think it stupid that it is acceptable for me to drive, and unaccetpable for me to fly a Cessna 140. Sticking to VFR and private strips is a hell of a lot safer for all partys concerned! Instead the world of the private pilot has been destroyed. Insufficient economic growth!

[histarter]

Vmtrs/sec BA Degrees L/D Sink fps Spiral Dia ft.

20.000 0.000 21.000 3.124 Infinite

20.350 15 20.283 3.291 1033.607

21.494 30.000 18.182 3.877 535.136

23.779 45.000 14.855 5.250 378.170

28.279 60.000 10.504 8.831 308.811


FS with 8 degree flap

15.000 0.000 17.000 2.894 Infinite

15.263 15 16.420 3.049 581.404

16.120 30.000 14.719 3.592 301.014

17.835 45.000 12.026 4.864 212.721

21.210 60.000 8.503 8.182 173.706


Table is concervative from "Golden Age Of Model Aviation". L/D is currently higher. However physics formulahs still apply and everything is in proportion. It demonstrates the high loss in going from 45 to 60 degree spiral - regardless. Flaps make the 45 degree bank usable. :)

[mhmitchell]

Our club and contest flyers (full sized) always led with the ailerons though it is pretty acedemic and not really that important . I agree that pure 90 degrees is wasted maybe 80 is inefficent but aint pretty. As for myself I wasnt and will never be a contest pilot. I fly to relax not to watch the clock and worry all the time. Yes I have hot dogged a little but more closer to 60 degrees rather than 80 or 90. 3 Gs isnt even a grunt for most people . Again the poibt was to not skid aroubd but coordinate whether tou "steer" with the russer or bank first is of no consequence to me since I wont change what is natural and comfortable with me. An intesting "fact" if you can belkieve facts on the net is the term "stick and rudder" (see book of same name) was origionally started as a mnenomic to remind pilots to use the stick (ailerons) before the rudder.

Now tell us how to stay in lift once you find it.

[schrederman]

Sorry, but I've been gone a couple of days... and my work network filters me enough as to not be able to get to this site.

Intertia is the force that keeps mass moving in a straight line at the same velocity, unless acted upon by another force. When making the upwind turn, inertia is diminished due to the lower true speed, so the turn is sharper because the side-vectored lift component that's turning the glider remains constant. On the downwind side, the true speed is increased, as is the inertia, and therefore, the turn is wider for the same bank angle, as again, the the vectored lift component remains the same.

Is it noticeable? Yes, when the wind is high. When the wind is light to moderate, the effect is so minimal that the fight to keep up with the core of the thermal will negate any notice of it.

Your formulae are correct, but are truly only HALF of the problem. While you have your physics book out, look up the velocities of flow within a tube. The fact is that these higher sink rates at higher bank angles are the price paid for being in the heart of that MUCH faster flow in the center of the tube. Along the outside wall of the tube... or of the thermal... the flow is turbulent and interrupted by friction with the outside air that is sinking around it. The CORE OF THE THERMAL is where I'm trying to get you to fly. That's the part that is going to reward your efforts and your 2g turn for several minutes at times. Above 60 degrees, I agree, the sink rate is so bad that you need to be in a tornado to go up. Start flying cross country in a 2-place sailplane with an experienced pilot and the first corrections he will make for you are in your thermalling technique. Even if you are not racing, but just out to get your badges, you will need to improve your cross-country speed, which will first and foremost depend on your ability to gain altitude fast. If you're afraid of a steep bank, you won't fly 500km in thermals.

How does this equate to models? Easy... Thermals have durations. The average cumulus cloud lasts 20 minutes. The thermals that produced them often last much less than that. Many of them are tall skinny bubbles, especially down close to the ground where we fly models, limited by sight, of course. While they often get stronger and bigger as they go up, that's not always the case.

Again, I'm not trying to dismiss anyone's opinions, as they have merit and technical basis. I just don't think you are seeing the whole picture on the bank angle thing. In the end, you have to fly your model or real sailplane, and I have to fly mine. My advise came to me via names like Dick Johnson, Karl Striedek, Dick Schreder, etc. Their facts are proven over many years, and I have proven them to myself... with real sailplanes and models.

Have fun...

Jack Womack

OBTW... My sailplane is a licensed aircraft with a redline airspeed of 143 knots and stressed to +7g and -5g... NO ultralights here, thanks...

[Phil Barnes]

This is a thread that I've been following primarily for entertainment value. I have no meaningful input to the initial question about what to do with the ailerons and rudder to maintain a constant bank angle in a thermal turn so I have not posted until now. A link was given in post#2 to a Mark Drela article on the topic. That article gives the correct answer in great detail.

Mark's article gives the precise, theoretical answer to the question. Few, if any, of us are capable of coming close to the theoreticaly perfect control inputs as depicted in his diagrams and our models are usually too far away to directly see what input is required to keep turns coordinated. As a result most people develop simplified ways of thinking about the control inputs which will hopefully produce something close to a coordinated turn most of the time. I think it is also true that most RC pilots say one thing about how they coordinate their aileron and rudder inputs but do something entirely different when they actually fly a model. You can check this for yourself when you time for someone. Just have a casual conversation about this topic of rudder usage prior to the flight then discretely look over their shoulder when they fly to see what they actually do. You will probably see little if any resemblance between what they say they do and what they actually do. You can also try asking a number of different really good thermal pilots about their methods of rudder coordination. You will very likely get answers that are not only very different but conflicting. So I would say that the mental picture used by most RC pilots for turn coordination and rudder usage is something that is only useful to that particular pilot since this mental picture typically bears little resemblance to reality. I think this is a case where the conscious mind does not really know what the subconcious mind is doing. The subconscious mind is doing the job of turn coordination but when you ask the RC pilot what he is doing, it is the conscious mind that answers. The above is my best explanation for why you are not likely to get meaningful answers to the question of real world turn coordination on RC models. It may also explain some of the conflicting answers you have already gotten.

[Miami Mike]

Intertia is the force that keeps mass moving in a straight line at the same velocity, unless acted upon by another force.So far, so good...When making the upwind turn, inertia is diminished due to the lower true speed, so the turn is sharper because the side-vectored lift component that's turning the glider remains constant. On the downwind side, the true speed is increased, as is the inertia, and therefore, the turn is wider for the same bank angle, as again, the the vectored lift component remains the same.Sorry, but that is pure rubbish. You've already read and acknowledged the comments from rcbrust, which were meant to illustrate that there's no such thing as "true speed", and you seem to have a decent grasp of physics, at least superficially, so it's a real puzzle that you would make such an absurd, unscientific statement as that one. It just doesn't make sense.

Speed is relative, and there is therefore no such thing as "true speed". That's Physics 101. My avatar illustrates the concept: No matter how fast and no matter in which direction the ground moves below the glider, the glider itself still flies in circles relative to the mass of air around it. The ground below has no effect on the glider's flying characteristics. As one of our local club members puts it, "Wind is the navigator's problem, not the pilot's."


.

[Phil Barnes]

This thread is a wonderfully amusing mix of mythology and accurate information. It is unfortunate that many people have no way to determine which posts contain mythology and which contain accurate information. I won't try to correct all of the mythology but will say to Rayven that you have a better handle on the topic than some of the people who responded to your question as you demonstrated in your post#8.

I don't need to correct most of the mythology because Jack Womack has corrected most of it already. If you could just delete all of this stuff from his posts about "inertial effects" then all would be well. here are some quotes from Jack's posts:


Fly the glider in the airmass, not with respect to the ground track. While it may look like it is speeding up going downwind, it isn't speeding up in the airmass.

If there's a wind component, it may appear to rocket downwind and almost stop coming upwind. LET IT! It's flying at it's airspeed within the airmass, and the thermal is moving as an airmass. You're in the airmass, not on the ground. Fly airspeed, not groundspeed.

These inertial forces are relative. Nobody quoted how fast the Milky Way Galaxy is hurtling through space... I'm disappointed...

You would think after reading all of that that Jack has a good handle on things since those points are all correct. I guess the only thing to disagree with or the thing that Jack does not seem to understand or agree with is that the correct frame of reference is the moving air mass that the glider is flying in. You only see these "inertial effects" if you attempt to analyze the glider's motion from the incorrect, ground based frame of reference. If you look at the glider's motion as an observer within the same moving air mass that the glider is in then you will not see any "inertial effects". The glider is simply doing steady circles within an air mass that may or may may not also be moving relative to the ground. As others have pointed out; the model does not know or care whether the ground is moving below the air mass that the model is flying in. If you use the correct frame of refence when analyzing the glider's motion then that is the situation you are faced with. You and the air mass are stationary, the glider is doing perfect circular turns in the stationary air mass. In this frame of reference there is no such thing as wind. What a ground based observer sees as wind you would see as the ground moving underneath your feet. The movement of the ground has no effect on the model's behaviour.

A mind game

Imagine a model flying perfect circles inside a railroad car. You are the RC pilot flying this model and you are sitting within the railroad car. Are there any "inertial effects" to consider when piloting this model?

Now imagine that the railroad car is moving along the tracks at a certain constant speed. You are still piloting the model from within the railroad car. Are there any inertial effects now?

Now imagine that the railroad car is moving at the same speed and the same direction as the wind is blowing at that moment. The air outside the railroad car is not moving relative to the railroad car. You are still piloting the same RC model from within the railroad car. Are there any inertial effects to consider yet?

Inside the railroad car is another passenger who is a really good magician. We ask the magician to say abra-cadabra and make the sides and top of the railroad car go POOF! and disappear. Now since the railroad car and the outside air mass were moving at the same speed and direction, there will be no effect on the model. The model will continue circling above the now flat railroadcar. You, the magician and the air mass will continue moving along at a constant speed and you will see the model continue circling and moving along with the airmass. Are there any "inertial effects" to consider when piloting the model yet?

At this point someone slams on the brakes of the railroad car and you, the magician and the railroad car come to a stop on the tracks. The model continues circling in the moving air mass and is now "drifting" relative to you, the RC pilot. Are there any "inertial effects" to consider now?

So I ask; At what point in that scenario do you begin to see "inertial effects" that need to be considered while piloting that model? If the answer is never then perhaps we can put that bit of mythology to rest.

[Phil Barnes]

Miami Mike managed to get a post in while I was typing my second post. He's on my side:-)

[Miami Mike]

He's on my side:-)You bet! Good post.

An alternative to the railroad car is to imagine flying a model plane inside a hot air balloon that's being carried along by the wind. Now imagine that the balloon disappears and leaves you flying in a thermal.

[schrederman]

OK Mike, so you want to split hairs. All you need to do is substitute my absurdity... "true speed" with "relative speed", and the rubbish as you call it, goes away. Yes, I have a small grasp of physics. May I ask if you do? If so, answer your own question about the inertial effect in upwind/downwind turns. We've yet to see your wisdom on the subject. College physics was a long time ago, and I haven't been dragging the book out - nor will I. OBTW, it was me that said to fly in the airmass, and stop flying in relation to the ground, and I haven't mentioned the ground since.

I also said the effect was small enough as to not be noticeable most of the time... but that wasn't good enough for you, so I attempted in my layman's way to explain it. The guy that said the wind is the navigator's problem was probably flying a bomber, not a sailplane.

Phil Barnes hit the original question on the head. Dr. Drela's explanations are as you would expect... excellent. However, If you want to actually see how coordinated your turns are, do some low launches with a 3' orange string taped to the fuselage, or better yet, taped to the flap skeg. At low altitudes where you can see it, it will tell you how coordinated your turns are at the entry, during, and at the exit of your turns. Any deviation from right down the fuselage means the coordination is off, i.e. you're either slipping or skidding. It's not quite as good as one taped to the canopy with you on the other side, but it will be an eye opener. Sorry... just more rubbish coming your way because I have only my left hand usable, and can't fly and I'm bored to tears.

Jack Womack

[Miami Mike]

If you want to actually see how coordinated your turns are, do some low launches with a 3' orange string taped to the fuselage, or better yet, taped to the flap skeg...That's a great idea! I can't wait to try it.

[schrederman]

BE CAREFUL!!! Make sure the yarn can't foul the tail or the towhook...

JW

[Rayven]

So here's where I've gotten to after reading all the posts to my original question: I'm now going to go out and practice flying my XP-4 DLG in circles. Forget about the whole "flying in lift" premise that I started this thread with. It seems to me now, thanks to so many good post by Miami Mike, Schrederman and Phil Barnes (congrats on your first place showing at Poway International and Polecat Challenge by the way!!) and others, that the goal is to be able to fly in a circle with as little loss of altitude as possible. If the airmass the plane in happens to be going up, then you can afford to be less efficient in the turn and increase the bank angle (and/or slow down with up elevator and more camber and maybe even turbulators?) to tighten the circle and stay in the core of the uprising air.

Now that that's settled, maybe we can talk about why its worth ballasting up a glider so it will fall through the air faster than the same glider without ballast when everybody knows that gravitational acceleration is independent of mass!

[rcbrust]

Phil and Miami,

I agree with you guys completely regarding the inertia question. Inertia plays no role in the circling of a glider in a moving airmass. When you calculate inertia, you have to have an inertial frame of reference, and that is the moving airmass in the case of the glider.

A very simple way to demonstrate this is to sit in a car and spin a weight tied to the end of a string. You will feel a constant pull on the string due to the centripetal force needed to make the weight move in a circular path. This will not change if the car is moving.

In fact, if the weight is spinning at 20mph and the car is moving 20mph, at one point in every revolution the velocity of the weight with respect to the ground will be zero. The weight will not fall out of the air at this point, and to you, the observer inside the car, you will feel no difference in the amount of pull on the string.

Randy

[histarter]

Now that that's settled, maybe we can talk about why its worth ballasting up a glider so it will fall through the air faster than the same glider without ballast when everybody knows that gravitational acceleration is independent of mass!

Ecellant! Accelloration in 9.81 mtrs/sec/sec regardless of mass. :)

Two Identical sailplanes other than profile:
Profile 1. S-3021 loaded to 18 oz/ft
Profile 2. Dr. Drela's hottest profile loaded to 6 oz/ft.

From 600 ft AGL with both flying at the same airspeed to handle same conditions: Which would be the first sailplane down (Without thermal or slope assistance)? :confused:

If you use available profile tables correctly, the answer just may supprise you. :D

[gustabmo]

Two Identical sailplanes other than profile:
Profile 1. S-3021 loaded to 18 oz/ft
Profile 2. Dr. Drela's hottest profile loaded to 6 oz/ft.

From 600 ft AGL with both flying at the same airspeed to handle same conditions: Which would be the first sailplane down (Without thermal or slope assistance)? :confused:
If you impose a sailplane with 1/3 of the wing loading to fly at the same airspeed as the higher loaded one, you are throwing away most of the good qualities it may have.

[gustabmo]

You only see these "inertial effects" if you attempt to analyze the glider's motion from the incorrect, ground based frame of reference. If you look at the glider's motion as an observer within the same moving air mass that the glider is in then you will not see any "inertial effects".
Actually there won´t be any inertial effects as long as your measurements are based on an inertial referential, the ground being one.

I´d like to drop in something that I´m sure everbody had in mind all the way but was not mentioned specifically: all this assumes an air mass moving at a constant speed... no wind gusts. (beeing picky: constant speed in relation to an inertial referential, e.g. the ground :rolleyes: )

[Phil Barnes]

Actually there won´t be any inertial effects as long as your measurements are based on an inertial referential, the ground being one.It sounds like Gustabmo is more technically literate than I am. Part of the reason why the "inertial effects" myth was allowed to go unchallenged or only partially challenged for so long in this thread is that many people know the correct answer but don't feel qualified to talk about it due to being unfamiliar with the exact technical terminology required to get it right.

Gustabmo has pointed out that any inertial reference frame could be used to analyse the glider's motion and no inertial effects would be seen. Certainly the easiest inertial reference frame to use for the analysis would be the one that the glider is in. To use a different inertial reference frame (such as ground based) would yield the same answers but would be more complicated and would involve a lot of new terms that would all drop out of the equations in the end.

[Phil Barnes]

It seems to me now...... that the goal is to be able to fly in a circle with as little loss of altitude as possible. If the airmass the plane is in happens to be going up, then you can afford to be less efficient in the turn and increase the bank angle .....to tighten the circle and stay in the core of the uprising air.This may be the most important point that has been made so far. In all of my DLG flying to this date this is the only area where I've even attempted to work on my turn coordination. Up until maybe two years ago I just used aileron to rudder coupling and I never touched the left stick. It has only been in the last couple years that I have attempted to do some independent, uncoupled rudder inputs. It's not that I don't understand or agree with the benefits of coordinated turns, it's just that I've never felt like I have the ability to see the effects of my rudder inputs and I have maybe 30 years of two channel RC flying habits to overcome.

Despite whatever success I have had with DLG flying, there are some weak areas in my contest flying skills. One of the biggest is my ability/inability to work weak lift. To work weak, small thermals requires excellent turn coordination. The smaller the thermal is, the tighter the model needs to turn to stay in the lift. If the lift is also weak then the model will sink out of the lift if the turns are not well coordinated. There are a few people that I know of who do very well flying in this sort of small, weak lift. Those are the people who will consistently beat me in contests where the weather conditions offer that kind of lift. Oleg and Tom Kiesling are two of those people. What makes Oleg so hard to beat is that he combines a very high launch with that ability to work the weak, small thermals. This means that he starts out high and doesn't waste any of the available air that he encounters. I, on the other hand, may not recognize the weak, small thermal in the first place or if I try to work it I will quickly fall out of it. Tom survives as one of the top DLG piolts in the country even with a relatively low launch due to his incredible abilities as an air reader and his unsurpassed thermal flying abilities which allows him to work the smallest and weakest of thermals. One of the reasons that I have done well at the IHLGF in Poway is that the conditions there rarely involve working weak, small thermals. The lift there is much more likely to be large and strong so it is just a matter of knowing where the lift is and getting to it. There isn't a whole lot of finesse required to fly the lift once you get there.

I don't have much actual advice on rudder usage for turn coordination since I'm not very good at it but here is the little that I can offer for whatever it's worth; The proper, independent use of the rudder becomes more important the tighter the turn radius. A simple aileron to rudder mix without any attempt at independent rudder usage is likely to yield better results than a clumsy use of the rudder for more shallow turns or even moderate sized thermal turns. Tom uses an elevator to flap mix which has the effect of increasing camber on the wing whenever up elevator is applied. I do not yet use this mix since I have a bad habit of pulling too much up elevator in parts of my thermal turns and the addition of camber at the same time I'm pulling too much up elevator would be especially bad. Tom's advice to me has been to use more and more inside rudder as the thermal turn gets tighter and tighter. I hope Tom reads this and makes a post of his own.

Guys who learned to fly with full house models at an early stage such as pattern fliers or helicopter fliers or sailplane pilots who never flew two channel models and/or never used aileron to rudder mix on full house gliders are much more likely to do well without aileron to rudder mix. Those of us who flew two channel models for 30 years or who just don't have the perceptual speed to directly see turn coordination even at close distances may be better off just using an aileron to rudder mix.


maybe we can talk about why its worth ballasting up a glider so it will fall through the air faster than the same glider without ballast when everybody knows that gravitational acceleration is independent of mass!Of course gravitational acceleration is independent of mass and two objects of dissimilar mass will fall through a vacuum at the same rate, but an airplane flying through an air mass has additional forces acting on it. A heavier model requires more lift to sustain it against gravity. The generation of this extra lift also generates extra drag. The extra drag is overcome by the use of a faster sink rate.

Ballast is useful when conditions are such that the thermals are strong but they may be far away or it may be difficult to get back on field after thermalling. In these conditions a higher sink rate while thermalling is not a problem since the lift is strong enough that the model will still climb. The higher wing loading due to ballast may allow the model to get to the lift and get back on field after thermalling.

[mhmitchell]

Ah finally sink rate reference. Is not a low sink rate more important than a good L/D? If not why are we building so light? Weight, unless taken to a silly level has no effect on L/D But it does for pentration and sink rate. EG. A ballasted full sized sailplane L/D will not change when the pilot dumps the ballast.

That would lead to a different question. Would it not be better to fly faster , eg. have a heavier sailplane to minimize the time spent between thermals of course that assumes that you are better than I and can find another thermal. Slow down in the thermal shouldnt matter really what the sink rate is since even small thermals can cause a 800 pound sailplane to climb.

So if you had an fish tank with 20 gallons of water (aprox 160 pounds) and you put in a 10 pound fish how much would the combination weigh?

[gustabmo]

Would it not be better to fly faster , eg. have a heavier sailplane to minimize the time spent between thermals of course that assumes that you are better than I and can find another thermal. Slow down in the thermal shouldnt matter really what the sink rate is since even small thermals can cause a 800 pound sailplane to climb.
You are confusing weight and sink rate. If you have a sailplane as light as 4oz (example) with a 10ft/sec sink rate (example), it will only rise on the stongest boomer thermals.

Also, a slower plane can circle on a tighter radius, so it can better core thermals at low altitudes (the diameter of the thermal grows with altitude).

[Rayven]

[So if you had an fish tank with 20 gallons of water (aprox 160 pounds) and you put in a 10 pound fish how much would the combination weigh?]


There is definately something fishy about this question! The sum total depends if the 10 pounds refers to the weight of the fish out of water or in the water. If the fish is neutrally bouyant it doesn't weigh anything relative to the water around it.

[Daemon]

1. Confusing *added* weight and increased sink rate is ok, just not useful.
The same plane with more ballast will have a higher sink rate.
What's important though is whether it also has a higher airspeed.
Higher airspeed is what you need to minimize your time in sink or to
fight back upwind to find the next thermal. In general the
airspeed increases no more than the square root of the increase in
wing loading, but that can still be a useful amount as long as that's
what you're really getting. If you double the wing loading and it only goes
10% faster (because it's super draggy), then that's not useful. You'll just
fall out of the sky faster.

2. Ideally what you want instead of a heavy fast plane, is a light
fast plane that can be slowed way down. It's just that light planes
with high lift airfoils generally aren't always fast enough for all conditions
so it's appropriate to ballast up for higher winds or when the thermals
are further apart.

3. A fish has the same mass whether it's in the water or out
of the water. "weight" is an arbitrary measure of the force that
1 G of gravity exerts on a given mass. The total mass
is the same whether it's neutrally buoyant (or floating on top of the water).
It all exerts the same "weight" pressing down on the surface of
the earth. The earth's crust floats on the earth's mantle. That doesn't
mean that it weighs nothing.

4. Someone's probably discussed this earlier in the thread but I think
it bears repeating. The real danger
of using the radio to mix aileron -> rudder is that it produces the
opposite of the desired effect when flying a tight thermal turn. Yes it'll
reduce adverse yaw when you first initiate a roll, but if you're trying to
work a little thermal flying a 10 foot diameter circle with aileron
and elevator only, you'll find that you have to hold some opposite
stick on the aileron to keep it from spiralling further into the turn.
If you've got aileron->rudder mix on, and you're flying a tight left
hand turn, while holding a little bit of right aileron, then you're
also producing some right rudder. Left turn with right rudder
produces side slip and thus more drag.

5. The best argument I can make for understanding whether and how
turns should be coordinated, is to go take a ride in a full scale glider
with a turn-slip indicator or just a simple tell-tail ribbon stuck to the front
of the cockpit window.
Initiate a left turn with ailerons only in neutral air, (hold rudder straight)
and then use only aileron and elevator to maintain a constant radius turn.
Firstly you'll notice airspeed will increase, sink rate will increase
and the tell tail on the windscreen will show that you're slipping
sideways a few degrees. It's impossible for this to be the most
efficient configuration to fly a turn, because the side slip is
casting a wind shadow on the outboard wing and overall the fuse is
presenting a larger cross section to the wind. Now bring the rudder
into play. Apply some left rudder until the tell tail straightens out,
and you'll find that you can decrease the airspeed while maintaining
the same radius turn, which also means a lower bank angle is required.
If you hold the turn radius with the rudder, and use the telltail (or watch
the slip and turn indicator) to tell you what to keep the bank
angle at, you'll see the airspeed go down, and the sink rate
decrease as well. Execute all of this in a thermal and climb rate
will increase.

Everything that's true for the full scale glider also applies to R/C
gliders although some of these effects can be masked with lots of
di/polyhedral or by flying a fuseless wing/plank. Virtually every
convential tailed glider I've seen when working a tight turn in a
themal with ailerons and elevator only is side slipping badly and
it takes a while after I point it out before people recognize how bad it is.
It's simple to avoid. Initiate the turn with rudder and aileron sticks
pushed the same direction, and then using the rudder and
elevator to hold a constant turn radius, while ailerons are used
to set the appropriate bank angle so that the radius of the turn
neither decreases nor increases (assuming a fixed rudder/elevator input).
If the radius of the turn remains constant and the fuse is flying a
path that is a tangent to the circle then you've got a coordinated
turn.

I was just thinking about tell tails.. You could stick a little
peg on the bottom sticking down a couple inches and tape a long piece
of cassette tape to it, the same length as your fuse. It'll produce a
little more drag but if your fuse and tape ribbon are oriented in different
directions while turning then you're flying an uncoordinated turn and you
know it's side slipping. See if you can coordinate it with aileron
and elevator only.

ian

[mhmitchell]

You are confusing weight and sink rate. If you have a sailplane as light as 4oz (example) with a 10ft/sec sink rate (example), it will only rise on the stongest boomer thermals.

Also, a slower plane can circle on a tighter radius, so it can better core thermals at low altitudes (the diameter of the thermal grows with altitude).
Yes a slower plane can circle tighter. I do not believe I am confused about weight and sink rate( but check with our sailplane pilot/physics major he knows his stuff). A ballested sailplane has a higher sink rate but it flys faster due to its inertia. How small are our thermals? If they are 400 feet across I surmise it matters not how tightly we turn after all we are so small. Conversation of energy would lead us to believe that if thermals get larger the higher we go (I would suggest that at model altitudes they are not much smaller) then the rate the column of air is rising would decrease.

But lets talk more about inertia.

Take a baseball and a hollow plastic baseball that comes with the plastic bat that young kids have. Throw them each in turn as hard as you can horizontally (dont we throw our HLG as hard as we can?) Which ball will travel the farthest before hitting the ground? Ans= baseball. Why (inertia. Since the
only difference in the two balls is it weight we have to assume that weight must be the most important factor here since the initial velocity of each is the same (as hard as we can) . Whiich one hits the ground first assuming each was launched at the same angle and speed. Why, WOW! they both hit at the same time.(high school physics) So the factor involved here is gravity , and it pulls equally on both balls. What has this to do anything? Well I am just doing mind experiments here as I know little about physics and I really would like to know. But when we launch are not we trying to get they sail plane as high as possible? A heavier sailplane will go higher due to inertia.My theory, and its only that, is if we build a sailplane a bit heavier it will launch higher (be a bit easier our our arm muscles (dont know about you but I can throw a baseball for an hour as hard as I can and suffer no ill effects but twice with a wiffle ball and I hurt). So if there is less rising air than still or sinking air I feel that we need to cover as much space as we can looking for the rising air, higher speed will allow us to do that( the baseball goes farther than the other one in the same time period.) Perhaps we should research which airfoils will allow us to fly faster while maintaining minimum sink (which usually is not the best glide speed (L/D).Comment from sailplane pilots besides myself would be helpful here.) Anyway I have a relative that wins more than his share of model sailplane contests by flying fast and to do that he ballasts almost always. Yes I know at best glide you will travel farther but that is aerodynamics you speed will be slower and you will be in sinking air longer so you actually will have to land sooner. The common rule is fly fast in sink slow in thermals.
The fish question was for entertainment I really know nothing about fish.

[schrederman]

I'm not convinced... Unfortunately, all we can come up with on the "myth" argument is just that... that the inertial forces in a circle are a myth. The explanations of the rail car and the baloon don't work for me. Why? Because you are now talking about a static air mass enclosed in something. The spinning object is yet another problem, i.e. the gyroscope has it's own rules, and the sailplane isn't tethered to a fixed axis spinning many rpms. It has no outside froces moving it along. I've said that these are almost totally unnoticable. However, you can take notice of them as the birds do... Let's face it, your sailplane gains more altitude on the upwind half of the circle because it turned into the wind and it's airspeed went up. The buzzards and hawks are masterful at taking advantage of this. Where does that extra bit of airspeed come from? Mythical.. I think not. If it is, please explain this phenomenon... in physical terms... no baloons or rail cars...

Jack Womack

By the way, the fish question is easy... 160 + 10 = 170... if you are weighing everything. If you're just trying to weigh the fish in the water, you'd have better luck trying to heard cats.

[gustabmo]

I do not believe I am confused about weight and sink rate( but check with our sailplane pilot/physics major he knows his stuff).
By what you wrote on your last message I truly believe you know the difference between these, but the phrase you wrote and I quoted was misleading.

How small are our thermals? If they are 400 feet across
At DLG save altitudes they are more like 20 feet in diameter, sometimes less than that.

But when we launch are not we trying to get they sail plane as high as possible? A heavier sailplane will go higher due to inertia.
Physical arm strain, as you noted, is an important factor here.

The common rule is fly fast in sink slow in thermals.
For this, see the theory behind MacCready´s ring http://www.skynomad.com/articles/axc9.html (I didn´t read this page thoroughly... it showed up on a Google search and seemed worth posting the link)

[gustabmo]

The explanations of the rail car and the baloon don't work for me.
What about this one: someone in a hot air ballon at the same altitude and very close to our sailplane circling in lift, looking at it. Don´t you agree this guy will see the sailplane draw perfect circles across the sky?

Let's face it, your sailplane gains more altitude on the upwind half of the circle because it turned into the wind and it's airspeed went up.
I fully disagree with the "it´s airspeed went up" part, and also with the part where you say the sailplane gains more altitude on the upwind half.

schrederman, there were so many posts here that I got confused... do you fly full scale sailplanes?

If yes: can you tell while circling a thermal which way the wind is blowing without looking at the ground/horizon?

If no: do it! At least one pleasure ride... it helped me a lot to understand how to coordinate turns, how to react to adverse yaw, how to hold the nose while turning, and it´s just plain very fun! ;)

[flystoolow]

[QUOTE=schredermanLet's face it, your sailplane gains more altitude on the upwind half of the circle because it turned into the wind and it's airspeed went up. The buzzards and hawks are masterful at taking advantage of this.

Jack Womack[/QUOTE]


Let's be fair to Jack here and admit what has frustrated us all. When thermalling in a small, strong thermal, especially one that is leaning dramatically and drifting rapidly, I usually wonder, "How is it possible that the glider seems to gain 40' during the turn 'into' the prevailing wind, and yet gains nothing durning the turn 'with' the prevailing wind." Try as you might to better center the thermal, the overall results lean toward this scenario when the wind is high.

I would like to bring up a final point for discussion. When several birds enter a small, challenging thermal, they usually all seem to choose the same direction of rotation...and we can, with fair certainty, expect that the direction they choose is somehow more effecient than the direction they did not choose.

[markdrela]

If you double the wing loading and it only goes
10% faster (because it's super draggy), then that's not useful. You'll just
fall out of the sky faster.
If you double the weight, the various speeds (min sink, best L/D, etc) all will increase about 41%. Regardless of whether the glider is "super draggy" or not.

4. The real danger of using the radio to mix aileron -> rudder is that it produces the opposite of the desired effect when flying a tight thermal turn. Yes, it will reduce adverse yaw when you first initiate a roll, but if you're trying to work a little thermal flying a 10 foot diameter circle with aileron and elevator only, you'll find that you have to hold some opposite stick on the aileron to keep it from spiralling further into the turn. I think one should be more specific here. A little bit of Ail->Rud mix can be quite beneficial. Consider two control setups:

1) A small amount of Ail->Rud mix is used, just enough to cancel adverse yaw at an intermediate glider speed, making the aileron stick mostly a pure roll control. This is what I use. It allows me to think of the rudder as a pure yaw control, and not burden it with the additional chore of cleaning up after the ailerons after each small roll correction. The simpler the control task the better. I still have to give an independent rudder kick when rolling into a turn to start the yawing motion, since the aileron stick won't do this (it's a pure roll control).

2) A large amount of Ail->Rud mix is used, so that rolling the glider into the turn with ailerons also starts the yawing motion as well. Great initially, but this causes real sideslip control problems once the steady turn is established and slight opposite aileron is applied. I don't use this kind of setup for this reason.

[gustabmo]

When thermalling in a small, strong thermal, especially one that is leaning dramatically and drifting rapidly, I usually wonder, "How is it possible that the glider seems to gain 40' during the turn 'into' the prevailing wind, and yet gains nothing durning the turn 'with' the prevailing wind." Try as you might to better center the thermal, the overall results lean toward this scenario when the wind is high.
I´d say this is an optical illusion due to our point of view, see pic.

[markdrela]

Let's be fair to Jack here and admit what has frustrated us all. When thermalling in a small, strong thermal, especially one that is leaning dramatically and drifting rapidly, I usually wonder, "How is it possible that the glider seems to gain 40' during the turn 'into' the prevailing wind, and yet gains nothing durning the turn 'with' the prevailing wind." Try as you might to better center the thermal, the overall results lean toward this scenario when the wind is high. If you're truly centered in the thermal, there can't be any increase in the climb rate when going into the wind. Basic physics.
I agree that it looks that way sometimes, but I think it's an illusion due to the pilot's viewpoint. I find no such apparent behavior when circling in a thermal which is upwind or "sideways" from the field rather than downwind.

PS. Gustabmo beat me to it with his diagram.

[Sparky Paul]

...

I would like to bring up a final point for discussion. When several birds enter a small, challenging thermal, they usually all seem to choose the same direction of rotation...and we can, with fair certainty, expect that the direction they choose is somehow more effecient than the direction they did not choose.
.
I've watched a ball of pelicans migrating thru here. and they circle both directions. Same with vultures.
I expect their onboard wetware checks 12.. :)
Ravens are too erratic, getting frisky with each other, so nothing concrete can be observed very often with them.

[erich]

I fly full size sailplanes (Grob, 1-26,1-34,sprite,2-33). Have never found more lift to be dependent on the direction am facing in relation to ground . But find more lift depending where I am in the thermal (centered in lift or not), and that can be encountered in any direction in relation to wind and or ground. Have really no sense of wind speed or direction, unless looking at the ground. If the wind is high you get a sense of (when looking at ground) almost stopping when heading into wind, and speeding up greatly when turning downwind. If however you stop looking down, you have no sense of speeding up or slowing down (airspeed remains constant, depending of course on how steady you are on the controls).

erich

[flystoolow]

Let's be fair to Jack here and admit what has frustrated us all. When thermalling in a small, strong thermal, especially one that is leaning dramatically and drifting rapidly, I usually wonder, "How is it possible that the glider seems to gain 40' during the turn 'into' the prevailing wind, and yet gains nothing durning the turn 'with' the prevailing wind." Try as you might to better center the thermal, the overall results lean toward this scenario when the wind is high.



I would like to back up my claim with psychics (that's sort of like physics).

PLEASE NOTE: I have used 'upwind' and 'downwind' in quotation marks only to clarify the portion of the thermal I am referring to; more clearly these would read, "toward the prevailing wind" and "away from the prevailing wind".

Seriously, we have not yet considered that a thermal, and especially a small strong thermal, spirals as it climbes. If there is no prevailing wind, then yes, it is understandable that if flown well centered within the thermal, a glider will rise at a constant rate. However, when tipped at an angle, the observed climb rate will vary within the thermal.

Here we consider the SMALL, STRONG THERMAL (40 feet across) in a strong wind:

Let us first imagine the spiralling, rising air of a thermal as a stretched out coil spring (like the front suspension spring on your car). When held vertically, the 'coils' rise at a constant rate. However, when held at a 45 degree angle (as a thermal might lean in a stiff wind), the coils rise at a very high rate during the 'upwind' portion of their travel. Conversly, the coils fall at an angle negative of horizontal during the 'downwind' portion of their travel. Now I know the coils will not be tipped at the same angle as the thermal as a whole (45 degrees), but it is hard to imagine that in a tiny thermal, the coils would not be tipped even slightly. Note here I am not talking about thermals 1/4 mile across that have their own planets, moons, and the like.

From this, can we say:

1: A glider in a leaning and drifting thermal actually gains more altitude (with ground level as reference) during its flight through the 'upwind' half of the thermal; and less altitude during its flight through the 'downwind' half of the thermal. If you hold a spring in front of yourself at an angle, this theory will become clear. I realise that the thermal as a whole is rising, but the air within still has a spiralling current. If this is to be contested, can anyone show me the data collected from a thermal of 40' diameter or less, that is very strong and affected by a 15mph wind.

There is yet another factor that can increase a glider's climb in the 'upwind' portion of the thermal. We know that thermals carried by the prevailing wind travel at a slower rate than the wind. Thus we can figure that the wind must penetrate them to some extent. It makes sense that the 'upwind' portion of the thermal will be infused with more of this prevailing wind than the 'downwind' portion, as the 'upwind' portion will take the brunt of the intrusion by the wind. Therefore, when flying into the 'upwind' portion of the thermal, your glider will experience a slight increase in airspeed which can be exchanged for extra altitude.

I have shown two reasons why a glider flying in a drifting, leaning, small thermal will gain proportionally more altitude, on each pass, as it flys through the side of the thermal that is toward the prevailing wind. I may be wrong, but if one considers that thermals ARE affected by the prevailing wind, and ARE NOT encased in an impenetrable shell, I may be right. :)

[gustabmo]

When held vertically, the 'coils' rise at a constant rate. However, when held at a 45 degree angle (as a thermal might lean in a stiff wind), the coils rise at a very high rate during the 'upwind' portion of their travel. Conversly, the coils fall at an angle negative of horizontal during the 'downwind' portion of their travel.
Your assumptions are not correct.

The wind doesn´t tilt the coil, it slants the coil. Vertical speed of the thermal is the same no matter if upwind or downwind.

Edit: Wind or no wind, the coil is still a coil... it only looks slanted to someone standing on the ground

[gustabmo]

We also know that thermals can be dissipated by the wind.
Wind doesn´t dissipate thermals... What wind does to thermals is:

1) Wind doesn´t let the air mass stay too long over the heat source (which is invariably in the ground)... air doesn´t get a chance to get heated all that much so thermals are weaker

2) Turbulence and wind gusts may break a thermal bubble into smaller bubbles, or fragmentate a thermal cone into bubbles... if a thermal bubble is small the relationship volume/area gets smaller and it cools up faster, if it´s too small it may cool up so much that it disappears. This is an effect caused by turbulence and wind gusts, but not at all caused by a clean and constant wind.

[Phil Barnes]

The explanations of the rail car and the baloon don't work for me. Why? Because you are now talking about a static air mass enclosed in something.The air mass inside the rail car or inside the balloon is not static. Those air masses are moving relative to the ground in exactly the same way as an air mass which is not enclosed in anything. Both the rail car and the balloon scenarios were chosen and set up such that the enclosure (balloon or rail car) were moving with the wind. In this situation the model's motion relative to the ground is exactly as it would be if the enclosures were not there. There would be no difference in how the model flies whether that enclosure exists or not. If it is your contention that the model will fly differently without the enclosure than it would with the enclosure then you will have to show some interaction between the enclosure and the model.
your sailplane gains more altitude on the upwind half of the circle because it turned into the wind and it's airspeed went up. ...... please explain this phenomenonThis is easy. Sailplanes do not gain more on the upwind side of a thermal than they do on the downwind side. The sailplane's airspeed is constant all the way around the circle. the phenomenon you cite is just another myth. You can force this behavior to happen by alternately adding up and down elevator while you circle but that behavior most certainly does not happen otherwise. Have you ever watched a free flight model? Do they exhibit such galloping behavior as they circle? Of course they don't because they don't have an RC pilot at the sticks fulfilling a preordained philosophy of flying behavior.

[billwolfe]

Even if we accept the leaning coil theory, your plane could be circling in the direction opposite that of the coil, in which case you would expect to observe opposite flight characteristics.

Does wind penetrate the thermal? I don't know. May be. Does the wind penetrate a hot air balloon's envelope?

In addition to the optical illusion scenario and the self-fulfilling prophecy scenario, I think another reason these myths are perpetuated is that the plane is NOT centered in the thermal. If you fly INTO the core upwind, the plane rises with the thermal and falls downwind outside the thermal. If you fly OUT of the thermal upwind, the plane could rise while bleeding off airspeed and then sink or rise modestly downwind, in the thermal, because of the airspeed lost while outside the thermal. The pilot thinks he is centered in the thermal and concocts theories that attribute flight characteristics outside the thermal to the thermal itself.

--Bill

[Daemon]

Just to throw a monkey wrench into the mix, I primarily fly
slermal conditions, and have found that many of the things
that are taken as gospel about thermals above flat ground are
quite different when a thermal backs itself up against a slope.
Those of interest are
1. A thermal doesn't have to move at the same speed the wind is
blowing, if at all.
2. Some thermals can lean upwind at higher altitudes
3. Some thermals are defined only by a region of lift while the wind
basically blows right through them.

Together this means that the glider *does* climb faster while
turning upwind, and sometimes it's best to just point straight
into the wind and go up and out. And no, I'm not talking
about slope lift here. Mt. Zion is a prime slermal spot for both
R/C gliders and paragliders. The upper level winds are usually
from the West so most of what we fly on the East side of the
mountain ranges from small to enormous thermals driven by
a rotor from the Rocky Mountains that drifts in all the way
from just past Denver. On a good slope day (cold front moving
in from the east) a paraglider can ride the slope lift maybe
500-700 feet over the top of the ridge, but on a good slermal
day, sky's the limit and it's not uncommon to see someone
hook a thermal and ride it up and out away from the hill 2000 feet
over (about 9000 ft ASL) and half a mile out before it stops feeding and
then they'll try to stick with it as it drifts back over the mountains.
On a great slermal day they'll take it to 12-14k ASL and then fly
off to Boulder 30 miles away.

My explanation for the 3 phenomenon above has to do with the way a
big thermal feeds. Picture it as a big 3d dimensional vortex .
Air rising rapidly up the center, and decendending all around. At
ground level warm air moves horizontally towards the base of the
thermal and the sinking air all around it tends to keep the feeders
from rising and forming new thermals. The feeders are forced to flow horizontally
and the path of least resistance is towards the base of the closest
organized thermal. Basically the thermal vacuums up all the warm
air around its base from all sides equally if given a chance. The thermal
will persist as a column as long as it draws in warm air at the base.
When it runs out of warm air and starts pulling in cool air, it shuts
off andthe warm air above continues rising, so now the thermal has a
"bottom", and the whole thing is now a bubble.

But when a drifting thermal column runs into a big hill it faces a problem
(yes I anthromorphosize it a little). The downwind side of the thermal
is uphill, and it's much harder to pull warm air down a slope (especially against
the increase airspeed created by normal slope compression) than up a slope,
so the thermal feeds very little from the downwind/uphill side, and most
of the new warm air flows into it from the upwind/downhill side.
This causes the effects observed above. The thermal *stalls*. The
background wind is still blowing. But the thermal is only feeding on
the upwind/downhill side so the region of active lift remains fixed or even
starts to extend forward upwind. The downwind side of the thermal
can't draw any warm air downhill so it sort of peters out and blows
away gradually. The net effect is that the thermal is strongest on
the upwind side, and the wind actually blows through the region of
maximum lift. If you attempt to circle in this type of thermal you'll
notice that
1. the glider rises while flying upwind and decends while flying downwind (and it's
neither a myth nor a self fullfilling prophesy, since the paragldiers and
hang gliders experience exactly the same thing while holding a constant
radius turn, not to mention that I'm not a moron and do know how to hold
the sticks still)
and
2. if you keep circling and let the glider drift with the wind, it'll
fall right off the back of the thermal and start sinking. Turn back
upwind and you can drive back into the thermal. Sometimes you can just
park pointed into the wind several hundred feet out and go straight up
to speck altitude. The paragliders do it all the time. I've done it with
more gliders than I can count including some heavy ass slopers.

As for leaning upwind. That's mainly because the air near a slope
is initially moving faster than the air above it so the base of a thermal can be
accelerated as it works up the slope leaving the top of the thermal
further upwind of the base. Strange thing though is combine this leaning
out shape with the above tendency to feed only on the upwind/downhill
side and I've seen thermals that will basically stay fixed in place wind
blowing through them leaning out and upwind. If I start behind
the thermal and fly the glider straight out it goes up, and then it falls
off the front edge, but fly circles extending each one a little further
upwind as the altitude increases and it'll keep going up and out.

Now how does any of this relate to you flatlanders? Even without
a slope, few thermals actually feed equally from all directions at the base
especially if there are any serious air heaters (bare earth, parking lots,
sun facing depressions in the ground) and/or distinct triggers (roads,
treelines, rocky outcroppings etc), or wind blocks (tree lines). If
you visualize the active flatland thermal as column with tendrils reaching
diagonally and horizontally to the sources of greatest warm air and
the "core" as being the region of greatest average lift, then you'll
see that a drifting flatland thermal will tend to be drawn toward
the heaters and triggers (not so much that the mass of air moves
sideways but that it grows on one side while dissipating on the other
so the region of greatest lift moves just as I see on the slope). And
if a thermal's greatest source of warm air is upwind, it may stall out
until its sucked dry (sucked cold really), and you may find that circling
in a big thermal that it rises on one side and falls on the other because
not all the air in the thermal is rising at the same rate.

Another interesting phenomenon that the paragliders report is
"slope lift" on the front side of the thermal. If the air aloft is blowing
harder than that below it (which is generally the case) then an rising
air mass is always moving slower than the air right above it, so by the
time it gets there the wind above has to push on it to get it moving.
That means on average the thermal is moving just a little slower
than all the surrounding air and the wind on the upwind side of it
will be deflected upwards as it pushes against the side of the
thermals airmass. The way the paraglider's experience this is
like this. When they're flying straight downwind and stumble upon a
thermal they'll often experience a region of increased lift but decreased
airspeed (like flying downwind into the lift band of a slope), and then
they drop into the thermal proper and experience an increase in airspeed
and general consistent thermal lift. If they fly into the same
thermal from the back they experience a dip instead of a bump.
There's a region of increased sink at the back edge of the thermal
and then they ramp up to the climb rate of the thermal proper.
Watching a half dozen paragliders converge on one gu who's cored
a boomer you can see this play out, and since I often fly my glider
out to join the fun, I've seen the glider do it as well.
Reason I mentioned this one is that it's almost impossilbe to stay
in that little region of lift on the front edge of the thermal, but if you
circle through it the glider pops up and decends every time.

Take all that however you want. I feel I have a different perspective
on thermal flying because I literally have a different perspective standing
on a slope 500-900 feet up. I watch their affects on gliders when
they've already had some altitude to develop, so I can watch a
glider fly into a well developed thermal right at eye level. No tricks of perspective.
I've also caught many a thermal only 100-200 feet AGL
(600-700 feet below me) and brought em back up so I can see how the glider
moves relative to the ground from above rather than below.

ian

[histarter]

If you impose a sailplane with 1/3 of the wing loading to fly at the same airspeed as the higher loaded one, you are throwing away most of the good qualities it may have.

Beautiful and succinct! It is difficult for the followers of the 'light', who are blind to this simplicity, and are amazed that heavy machines work as well as they do. [At CL 0.2 vs. CL 0.6, heavy is operating closer to Max L/D so sink is slower. :)

This is the condition that wind (which requires higher operational V) creates. Moving the CL by velocity to about 1.0 for the heavy, it is close to peak L/D; wheras, for thin to keep up, and is still undercoupled at CL 0.333. :rolleyes:

Decreasing V (for calm air) so thin is now running CL 0.8 (its peak L/D), thick cannot get to CL 2.4 (i.e. is clipped at 1.1) so it is just flying too fast! The only salvation is to take the ballast out (if possible) and fly at 9 oz/ft to make it competitive - and just slightly handicapped.

Hope this helps explain why heavy works (with its correponding profile) so well in the upper air! :)

[mhmitchell]

wow! back to the starting point!

Tell you what, I think I will just go out throw my airplane as hard as I can and hope for the best! If the left wing rises I will turn and or bank and or skid or slip my way to left and I will hold that turn until I think I am too low then I will turn up wind or down wind not depending on where I think the thermal is but rather to move the sailplane away from the trees and/or highway or pond. Then if the right wing lifts ......well thats why I dont fly contests. all things considered I prefer mythology to physics...I sorta like believing horses and dragons can fly.

[Sparky Paul]

Ian, I've seen the same.. a lift band way out upwing from the slope lift, which can take a plane to speck height, without it blowing back to the slope.
The ravens usually escape from my planes doing that. :)