2 Primal thermalling questions

[Jurgen]

Hi all,
At the moment I fly a fully moulded RG15 hotliner (CHK SPIRO) converted as glider, CG aft as possible (decalage 0) and launch on a winch. It flies super this way but could use some poly for the turns. A better plane for thermalling is on the way.

1) The aft CG shows lift and sink very good. More than once when i'm in a good strong lift zone, suddenly the plane rolls a full 90° into knife edge (no snap roll !) due to thermal activity, followed by a short drop ofcourse and need for recovery . Does this indicate the core-center or outer line limit of the thermal ?

2) I readed a thermal flying book (old buzzards...) and it states several times "thermal wave, and wave activity". What is a thermal wave activity?

Maybe its novice questions but i don't care :cool:
Many thanks in advance,
Jurgen.

[DFW_HLG_TG]

Your wingtip drop may be a sign that you need washout in your tips. You may have actually stalled and not realized it. I have found that planes with a tip-stall tendency are very hard to fly. Also, thermals can be very violent. Building and maintaining a little energy can help. The core will try to push you away but before long you will learn to deal with it.

[Jurgen]

* It might been tip stall indeed. Its a fully moulded wing so perhaps a little turbulator at the tips can do something good. I can remove it later when not good anyway.

* And yes i noticed some behavior in lift-zones that the plane becomes pushed aside as it seemes "forbidden" to fly there. I'm always doubting on moments like that,
1) should i return straight into the "forbidden zone" with an extra stick input to overcome the side-push,
2) or should I answer with the planes hint not to fly there again (as planes tend to know better were the thermals are than we do).

According to you i better not let me pushed away doing the first alternative.
Thx

[carrinsr]

Please allow me to share a few tips I've learned over the years that seem to always help me when searching for (and flying in) thermals here in Northeast Florida.

It has been my experience that the turning radius of any RC sailplane I've ever flown (and I've flown 78-3/4" wingspan up to 17'-2" wingspan) is large enough that there isn't a problem encountering the eye of a thermal, as long as two factors remain constant:

(1) Sufficient air speed is maintained
(2) Stay anywhere inside the thermal

First, every sailplane I've seen entering a thermal head on, the tail will always lift first.

If you'e anywhere in the northern hemisphere, after flying into the thermal for a few moments, circle RIGHT first. I suggest turning right first because thermals in the northern hemisphere always circle counter clock wise, and you will be heading into the wind of the thermal (and therefore need less ground speed to gain altitude).

If when you circle right and you remain in the thermal, you're there...happy flying!

But if at any time during your first 360 degree circle to the right you leave the thermal, continue circling to the right until you enter the thermal again. Then when you enter again -- immediately fly straight for a few moments...then circle LEFT.

You may experience faster ground speed circling left, but as long as you're gaining altitude...happy flying!

While searching for a thermal you see the nose of your sailplane lift first, then you've probably just encountered a breeze and not lift. In this case the closest thermal may be in the direction of the breeze. Soooo...

If you're far enough up wind then I recommend turning with (not into) the breeze in hopes you're heading toward a thermal. Be sure to maintain good airspeed so you can cover a lot of ground in a hurry. But...

If you're not upwind when the nose goes up, then turn into the breeze and hunt for another thermal, maintaining a good airspeed.

If a thermal is encountered by a wing tip first, then obviously that wing tip will lift first -- in which case you simply turn the sailplane in the direction of the wing tip that lifted. If the tail then lifts, fly straight for a few moments and then begin circling to the right (see above as to why to circle right first).

Tornados and hurricanes have an eye at their center where there is calm air. Since thermals are tonados in miniature they also have an eye. However, in all the years I've been flying I've never encountered a thermal eye that was large enough to effect the upward movement of an RC sailplane. This is because the turning radius of even two meter sailplanes is usually larger than the eye -- hence if you fly into the eye while in a circle pattern in a thermal you will also fly back out of it before there is any appreciable loss of altitude. Keep circling...happy flying!

In my humble opinion if you ever encounter the eye of a thermal with an RC sailplane, good for you! You are DEFINATELY going to go UP more than DOWN...unless you're flying a brick! :0)

David

[NWade]

Excuse me, But I'm in the middle of training for an Aviation degree; and I just got done with a Meteorology class.... I need to clear up a myth:

Thermals do NOT "spin" either direction based on the hemisphere you are in. The Coriolis Force (which is partly responsible for the spinning of large air masses like Hurricanes and Typhoons) only applies as an object moves over a large distance across the globe - as the globe spins beneath it. Similarly, your sink, toilet, or bathtub does not swirl in a particular pattern because of this effect.

Thermals are relatively small, rising blobs of warm air - and any "spin" or rotation in them is a function of the winds in your area and the terrain over which they pass. As they rise, they slowly weaken, but they tend to spread out horizontally over a larger area (which is why they're easier to catch 200 or 300 feet up, as opposed to close to the ground). However, thermals do not grow large enough or stay intact long enough to become affected by Coriolis Forces.

David does correctly point out that the plane will tend to roll AWAY from any thermal it encounters - especially if it only brushes the edge of a thermal. Additionally, thermals drift with the wind - so you want to get into a thermal and slowly drift downwind as you circle.

One thing that took me awhile to learn (and I'm still learning) is to try to keep your airplane's wings relatively flat while circling. Proper use of rudder (not just ailerons) helps a lot here. If the thermal is small, you may have to bank tighter to stay in it; so there's no simple solution to fit all circumstances. Reading the air based on how your plane reacts (and learning what to do as a result) is *the* major skill in thermal soaring, IMHO.

Be aware of the "sink" that surrounds any medium-to-large thermal: as the warm air rises, the cooler air it is rising through will pass down and around the outside edges of the thermal. This means that when you fly into or out of a thermal, you'll see the plane sink some when its near the edge (but outside of) the thermal. You want to hustle your way through this zone, to minimize your altitude loss. The faster a thermal is rising (i.e. the stronger it is), the more cool air it is displacing - so the stronger the "sink" usually is.

I'm not sure what they are referrring to with "wave activity" - although they could be referring to a local bit of terrain that is steadily producing warm air, so you get a succession of thermals pass over your site one after the other. In this case, you drift with a thermal downwind for awhile, then gain some speed and work your way back upwind to hunt for the next one that comes through from the same direction/location. OR, it could be referring to the fact that you can use a combination of thermals and slopes to soar. When a thermal begins to form, it tends to drift along close to the ground - almost as if it is stuck to the ground (imagine a piece of taffy being stretched, but with one end stuck to the ground). Certain terrain types (slopes being one of them) are known for "popping loose" these thermals from the ground, so that they rise and begin to spread out more.

Whew, lots of info there and this is getting long... Plus I'm still learning a lot myself - so I'll cut off here. Hope this info helps!

Take care,

--Noel

P.S. To answer one of your questions succinctly: If you are sure you were already in the lifting part of a thermal when the plane rolls, I would guess that you're encountering the edge of the thermal. If you're not yet in good lift, then the rising wingtip is hitting the "good" part of the thermal. In either case, turn back TOWARDS the lifted wing to find the core of the thermal.

[DFW_HLG_TG]

Noel, Here in Texas we have Dust Devels. They spin like crazy and will lift a brick. Is that not a thermal on steroids? I really never paid much attention to which direction the rotation is though I have found that most of them do rotate.


I always turn into what turns me away and never really cared about whether it was left or right. Once you are in lift, ratating or not, the plane does not care which direction it is moving.


Jurgen, If you have ailerons that span from the center of each wing to the tip, you might try to raise both of them slightly with your servo centering. You could also do this at the clevis then go fly and see if this tames the beast.

[DFW_HLG_TG]

David, our weather systems rotate due to our position noth of the equator but I do not think this carries enough into individual thermals. I do not believe they are affected by thier hemisphericle position.

[Dolphinman]

Excuse me, But I'm in the middle of training for an Aviation degree; and I just got done with a Meteorology class.... I need to clear up a myth:

Thermals do NOT "spin" either direction based on the hemisphere you are in. The Coriolis Force (which is partly responsible for the spinning of large air masses like Hurricanes and Typhoons) only applies as an object moves over a large distance across the globe - as the globe spins beneath it. Similarly, your sink, toilet, or bathtub does not swirl in a particular pattern because of this effect.


Noel

You may be in the middle of training for an aviation degree, but you are incorrect about Coriolis acceleration. The Coriolis effect is indeed responsible for the counter-clockwise swirl of water going down the drain, as it is for the counter-clockwise swirl of low pressure systems in the Northern Hemisphere. You will notice that no matter what direction hurricanes or other low pressure systems move, or even stagnate, the same counter-clockwise rotation occurs. I recall an excercise in my engineering dynamics class where we had to prove this using vectors for both of the earth's hemispheres. As such, I would think it possible that a thermal might show some clockwise rotation (not counter-clockwise as others have theorized on this board) as a consequence of the Coriolis effect and the movement of the air *away* from the center of the earth. If you would consider a "dust devil" to be a thermal "on steroids", perhaps the theory of thermal rotation would carry more credence. FWIW.

Scott

[David Taylor]

Thermals have been visually verified to spin both directions in both hemispheres, they may be affected by coriolis forces but not enough to make them all spin in one direction. As for thermalling, your glider WILL climb better if you fly against the rotation, which ever direction it is going, against the rotation the glider can fly slower at lower bank angles whereas trying to thermal with the rotation you have to fly faster circles (tailwind ) and keep higher bank angles because of that speed.

If you don't believe it all you have to do is pay attention and look at the weeds or grass and you will be able to tell which direction it's rotating; If you keep track over a few months your tally of clockwise VS counter will be nearly equal, including dust devils.
There have been many times that I have been climbing out fairly weakly and when I had enough altitude I gained a little extra speed and quickly changed my rotation in the thermal and was able to climb out faster, although sometimes it can make things worse if you were doing it the right direction in the first place. This is most obvious with a handlaunch glider because it thermals so slowly and can be close enough to be readily seen.

Dave Taylor

[Jurgen]

This is all very good information THX again. This forum realy works :)
Nature's trick box might be big enuf to create all kinds of thermals (even in the shape of nice women, darn we dont see them) and certain air/terrains conditions might advocate some kind in favourite (e.g. counter clockwise rotating long haired blondes)
I never readed about flying against rotation before, next time i'll sure check it out upthere. Very good hint !
As for the sudden roll-effect, the plane never ever tipstalls when flairing out in landing even with full up at the end, only in thermals itself it becomes more bold as like that thermal woman shape kicking my plane as it tends to enter her ...... :censored:

As for the mistical "wave" i quote the book: (old buzzards soaring book)
"Four open-class ships at 500 ft, riding a wave off the left .......etc etc .....praying for the clock to run out before the wave does."
Maybe i'm searching to far , "wave" might not be a special aerodynamic creature but simply a word for a series of thermals one following the other.

[Tuomo]

One thing that took me awhile to learn (and I'm still learning) is to try to keep your airplane's wings relatively flat while circling. Proper use of rudder (not just ailerons) helps a lot here. If the thermal is small, you may have to bank tighter to stay in it; so there's no simple solution to fit all circumstances. Reading the air based on how your plane reacts (and learning what to do as a result) is *the* major skill in thermal soaring, IMHO.


Why? That would mean side-slipping the airplane?

To me this ofter referred idea of turning wings flat does not make sense. Thermal turns are just like any other turns. To fly them efficiently you need to bank the plane to preven side-slipping. Sometimes when working a smalla pubble low down, the bank angle needs to quite high. higher up thermals are usually larger and larger ("more flat)" turns can be made.

You are right about use of all controls though. Aileron sailplanes do not generally turn perfectly if you trust on aileron->rudder mix without using rudder independently.

[DFW_HLG_TG]

Thermals have been visually verified to spin both directions in both hemispheres, they may be affected by coriolis forces but not enough to make them all spin in one direction. As for thermalling, your glider WILL climb better if you fly against the rotation, which ever direction it is going, against the rotation the glider can fly slower at lower bank angles whereas trying to thermal with the rotation you have to fly faster circles (tailwind ) and keep higher bank angles because of that speed.

If you don't believe it all you have to do is pay attention and look at the weeds or grass and you will be able to tell which direction it's rotating; If you keep track over a few months your tally of clockwise VS counter will be nearly equal, including dust devils.
There have been many times that I have been climbing out fairly weakly and when I had enough altitude I gained a little extra speed and quickly changed my rotation in the thermal and was able to climb out faster, although sometimes it can make things worse if you were doing it the right direction in the first place. This is most obvious with a handlaunch glider because it thermals so slowly and can be close enough to be readily seen.

Dave Taylor




Sorry Dave but your wrong. The glider does not care which direction the rotation is going. The plane flies a constant speed regardless. The ground speed may change but not the air speed.

[Vinkie]

Hi Jurgen,

The description of how your plane sometimes drops out of a thermal made me think about my recent experiences. I recently started flying two new electric sailplanes and I also like a rearwards CG. Both my new planes reacted similar to what you describe when I was still testing for the best CG and put it maybe a bit too far aft. With both planes shifting the CG a hair forward (almost unmeasurable, maybe a .7mm forward or so) and decreasing elevator travel (or adding some expo to it) solved the problem completely.

Coring thermals is a skill that needs some practice to get it right. I mainly use rudder to control the radius of the turns and to counteract the push away from the core. Once I'm circling I only use ailerons to keep the banking constant.

In my experience there's no real point in trying to keep your wing as flat as possible. Try to fly coordinated turns instead, ie the tighter the circle the more banking you need.

Hope this helps a bit, good luck,
Martijn

[rvsixer]

Sorry Dave but your wrong. The glider does not care which direction the rotation is going. The plane flies a constant speed regardless. The ground speed may change but not the air speed.Whew I'm glad someone else said it :D .

[JimNM]

Dust Devils are the "footprints" of thermals - really it is the surrounding air rushing in to fill the void left by the mass of hot air (THERMAL) leaving contact with the surface. The "real" lift is already gone before the Devil appears. The winds inside a whirlwind/dust devil can be quite strong - and are most certainly very chaotic. IF you can get in one with a ship that'll take the stress, you can have a very fast and exciting ride in one. Ask my Gentle Lady back in '87 - that was a ride she nor I will ever forget.

The image of "mini-Tornados" is quite wrong, as thermal formation takes place in the boundary layer of the atmosphere (very low altitude) while Tornadic activity is spawned by much larger and much higher altitude factors, namely hot air mass and cold air mass colliding and producing violent wind shears.

I have watched through polarized lenses several times as thermals boil up out of the desert - working outside has its advantages, sometimes. Thermals are really ugly at birth - just a big blob of roiling air at ground level. The top layer boils like a cauldron, throwing up columns and little bulges, until the hot air manages to break through the heavier cold air layer above it. Once there is a sizable breach, the hot air looks like steam escaping from an open pot. Once the thermal has reached a certain escape velocity, it elongates into the more familar column shape. Strong surface winds blow into the area vacated by the heated air, but not always froming a classic dust devil.

YMMV
JimNM

[BMatthews]

...Thermals do NOT "spin" either direction based on the hemisphere you are in......

As mentioned, tell that to the dust devils.

But you're right in that they don't start off spinning. Blowing cattail fluffies up into rising puffs as part of my free flight flying has shown me that but once the thermal is established and has a larger vertical size component they do seem to seem to develop some spin as seen by the fluffies.

Jurgen, when the model tries to kick up onto a wingtip turn the ailerons hard toward the upper wing. The model is rolling due to you flying through a shear area and turning into the high wing will either turn you into the lift or away from the sink. Either way it's best to turn toward the higher wing and get the turn going before the model ends up on a wingtip. Assuming you're not flying in stormy conditions the turbelence induced roll should be slow enough that you can catch it before it rolls to vertical or even much past 30 degrees of bank.

[Daemon]

Sorry Dave but your wrong. The glider does not care which direction the rotation is going. The plane flies a constant speed regardless. The ground speed may change but not the air speed.

Nope, you're wrong here.
Imagine a rotating thermal or dust devil, where a dust mote
in the air say 20 feet from the central core moves at 15mph in a circular
path around the core. A glider flying the opposite direction to this rotation
with an airspeed of 15mph, also 20 feet from the core would hold absolutely
still relative to both the ground to the core (the axis), and thus require no
bank angle to maintain its position. The glider is *not* flying in a circle.
The air is coming to it in a circular path. The same glider flying the opposite direction with 15mph of airspeed would circle the center of the
thermal at 30mph (same as groundspeed) thus requiring a steep bank angle
to maintain the turn radius.

This is *not* the same as the eternal argument about whether
your airspeed changes with upwind/downwind turns (it doesn't)
because unlikely a uniform airmass moving in a straight line,
a rotating air mass is not an inertial frame of reference.
Objects trapped in a rotating airmass experience centripetal
acceleration.

It's like walking on the edge of a merry-go-round (we actually had one that
you could do that with when I was a kid). If you walk or run at exactly
the same speed it's rotating, near the edge in the opposite direction then
you hold your position with no lean required. Stand still on the edge, or go
the opposite direction and you must lean further and further in.

What this means to thermal pilots is circle in the opposite direction
to the rotation of the thermal, if that can be determined. If there
are birds in the thermal, they're probably going the optimal direction.

ian

[DFW_HLG_TG]

Your logic is wrong. The plane does not know or care what the direction of the wind is. The only change is the relative speed to the ground. You cannot change physics.

[rvsixer]

Your logic is wrong. The plane does not know or care what the direction of the wind is. The only change is the relative speed to the ground. You cannot change physics.Whew I'm glad someone else said it (and again correctly) :D .

[Daemon]

Your logic is wrong. The plane does not know or care what the direction of the wind is. The only change is the relative speed to the ground. You cannot change physics.

I'm not wrong. And it's not about the glider's relative speed to the ground.
It's about its relative speed to the central axis of the rotating thermal.
In this case, that is zero. Bank angle is determined by the tangential
velocity and radius from that axis. If the tangential velocity is zero, then
the bank angle is also zero. In the scenario I laid out above
the thermal *happened* to not be drifting, only rotating, so the
glider's speed relative to the thermal's axis of rotation and to the
ground were the same.

The rotating air mass is *not* an inertial frame.

If the thermal were drifting in a straight line over the ground
then ground speed would be non-zero and the glider would know nothing
of it, but the glider's speed relative to the axis of rotation would *still*
be zero.

ian

[Daemon]

Inertial frame of reference
http://id.mind.net/~zona/mstm/physics/mechanics/framesOfReference/inertialFrame.html

Non-inertial frame of reference
http://id.mind.net/~zona/mstm/physics/mechanics/framesOfReference/nonInertialFrame.html
Take special note of the third example of a non-inertial frame. "The frame
could be traveling along a curved path at a steady speed."
Therefore a rotating air mass is not an inertial frame of reference. In
fact even the earth's surface itself is not a true inertial frame of
reference, which is why the Coriolis Force (http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml)exists.
At the human scale however, the coriolis force is
undetectable (which includes sink drains and toilets which rotate
about 10 thousand faster than the 1 revolution per day of the earth).

VRML example of rotating non-inertial-frame
http://id.mind.net/~zona/mstm/physics/mechanics/framesOfReference/nonInertialFrame.html

A uniform airmass moving at a constant velocity (constant speed and
direction) is an inertial frame of reference, and a glider can know nothing
about the velocity of that frame. That's where the the upwind/downwind
turn myth comes into play, but it does not apply here.

A non-inertial frame gives itself away however, because you must accellerate
to stay within it. Travelling in a circle requires accelleration and for
a glider requires some bank angle. The bank angle is proportional
to the angular velocity about the axis of rotation and inversely proportional
to the radius of the circle. If the glider is holding a fixed position relative
the axis of rotation of the rotating frame of reference, then the glider is
effectively not in that frame of reference. It can be influenced by
the air that that frame drags around with it, but can know nothing of
the angular velocity and distance from the axis of rotation therefore
it has no bank angle.

ian

[DFW_HLG_TG]

OK, It does sound like you are very educated in what you are talking about but your demo themal does not exist except in your theory. I have been in RC and a pilot for 32 years and have never worked a thermal without banking the wings. I also never really cared which way the rataion was cause it really does not matter.

At this point I will let someone else that can discuss this beter than I take over.

Dwain

[NWade]

1) I wasn't saying that thermals can't or don't spin - I was saying that the Coriolis Force has nothing to do with any spin, and will not cause every moving air mass in the hemisphere to rotate the same direction.

And no, Coriolis Forces do not apply to your sink, toilet, or tub!! The distance covered is just too small... This is a very often-repeated old myth, but it has no basis in fact. Ask any college professor or professional meteorologist.

http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html

Tornadoes in the northern hemisphere TEND to spin in the same direction as Hurricanes and other large air masses that spin as a result of Coriolis forces - but that is because the tornadoes are born out of these larger sorts of low-pressure systems in the first place (which *are* large enough to be subjected to the Coriolis effect/force). When the existing rotational tendancies and pressure gradients are increased and concentrated with some severe wind shear or other phenomena, the tornado forms.

2) I said to keep your wings relatively flat in thermal turns. I was not advocating severe skids/slips in turns... But keeping your wings flatter causes more of the lift component generated by the wings to be directed UP (against gravity). At a higher angle of bank, more of the wing's force is directed sideways, "pulling" the plane into the turn. Also, a higher angle of bank requires more elevator force to be applied to keep the plane from losing altitude. At bank angles below about 30-degrees, the additional force required isn't much - but once you get to 60-degrees of bank, it takes roughly 2G's of force to keep a plane from sinking (in dead calm air), and above 60-degrees of bank the force required to maintain altitude quickly moves towards infinity. Applying the elevator to generate these forces means that you're increasing the aerodynamic forces on your tail-surfaces, thus increasing drag.

So in summary: A steeper-than-absolutely-necessary turn reduces the "upward vector component" of lift from the wings, AND it increases drag and slows the plane (from the tail-surface movement).


As for the "headwind" vs. "tailwind" argument:

It is perfectly true to state that a plane travelling with 40 knots of groundspeed into a headwind of 20 knots has the exact same characteristics as a plane flying at 80 knots over the ground with a tailwind of 20 knots.... Both planes' wings "feel" 60 knots of air moving over them (and that is what the airspeed indicator onboard both airplanes would read). However, these are steady-state examples.

In our flying, we are often turning into (or away from) the wind. At these moments, our planes are in transition. Their changes are not instantaneous, and it takes time for the pressure of the wind against the surface of the plane to change it's speed. This is especially true since our planes are designed to slip through the air with minimal disturbance/drag. So in real life, things are a lot more "messy" than in theory (aren't they always?).

For an example of this "transition in action", take a really REALLY lightweight foamie plane, fly it over the ground in a 5 - 10mph wind. Fly it upwind and slow it to almost a stall. Now gently turn downwind - the plane will want to stall as you first turn downwind, but after a couple of seconds (during which it will probably lose some altitude) it will be carried downind and pick up some speed. Your higher ground speed will make it appear to take "longer" to turn around - but relative to the surrounding air, the plane is behaving the same as on the upwind leg... It just looks different to us because we're standing on the ground, not moving with the wind.

Take care,

--Noel

[Daemon]

OK, It does sound like you are very educated in what you are talking about but your demo themal does not exist except in your theory.

Generally true, but a rotating thermals *do* exist, and if you fly it one
direction you'll have a lower angular velocity and require lower bank angle
than if you fly it in the other direction, and the glider will generate
less drag holding a lower bank angle. As I said, the birds generally circle
the direction they do for a reason.

Another thing to consider here is that while most people thermal primarily
from flat ground, I thermal primarily from the lee side of a 1000 foot
high slope so I get to see and fly very well developed thermals of all
sorts. Things that people argue about here in theory I can observe
literally at eye level.

ian

[Jurgen]

So, on one hand we have scientific reason to fly against rotation with exerience that confirm this, on the other hand we have experience that it doesn't come into account for better results.
Why say wrong on either way? Personal flystyle, planetype, mind-setting/conviction, terrain, airtype, so may reasons and infuences determines the outcome followed by interpretation.
I stay open for both opinions and incorporate them gradualy into my own experience which might change if i fly other plane or place, lets keep our knowledge alive and open for suggestion at any time.
Science is true purity but sometimes narrow whereas convictions are headstrong, lets stay flexible and admit were all limited.

What about this, climb out counter spin for minimal banking, when high enuf change rotation to gain inertia momentum and shoot youself outa thermal-sink zone using that momentum. Some say do that trough the core for a burst.

JimMN cold blooded killed my dreams of thermals being shaped like female beauties but still they behave like them anyway. Al kind of forms and sizes, then there is none, then they are in batches, very unpredictable despite scientificaly determinable and they teasingly push you away when they actualy wait for a ride into heavens :rolleyes:

[flystoolow]

As for the "headwind" vs. "tailwind" argument:


In our flying, we are often turning into (or away from) the wind. At these moments, our planes are in transition. Their changes are not instantaneous, and it takes time for the pressure of the wind against the surface of the plane to change it's speed.

I certainly hope they're not teaching this in a class you paid for. "It's speed" (airspeed) is different than observed groundspeed, and should remain constant in a well co-ordinated turn regardless of "Wind pressure".


To answer an original question of this thread:

WHAT IS WAVE LIFT:

Wave lift occurs when a moving air mass is forced to flow over a stationary object. It's effect can be noticable downwind of the object. In the case of a long mountain range, for example, the 'wave' created by the mountain range can continue to oscillate for many miles downwind of the mountains. In this example, flying a full scale glider, you may find yourself in an excellent band of lift over a flat plain, miles from any mountain or hill, that may take you to well over 20 000 feet.

Flying rc gliders, wave lift can be found downwind of long objects.
Some examples: low hills, a long row of close houses, even a long high fence (if your glider is light enough to show the lift).

[Jurgen]

So in flat fields (where i live) we might expect less usefull wave activity. I saw them as nasty turbulence more than usefull.

Then ofcourse there comes the obvious question:

How do I know i ride a thermal or a wave? So waves should be rock-steady predictable according to wind-speed (+ direction), either in lift and sink. Waves should be more reliable right? Kind of phantom sloping :D

Does waves have particular cloud-forming effects revieling them? Are this the cloud streets I sometimes see? But then those streets aren't always perpendicular to the wind direction as I would expect, yes they are sometimes in the wind direction itself, thats confusing abit.

The practical question:
is it in any way important for us RC-thermallers to know wheather we ride wave or thermal, we are rising after all, case closed?

Is there general knowledge about our lower heights activities (we aren't full scale after all) how much value schould we give on clouds for finding lift-zones?

I know its a bunch of questions :o

I want to thank all contributions, they are worth gold! And if opinions don't stroke it becomes very interesting instead of incompatible.

[NWade]

I certainly hope they're not teaching this in a class you paid for. "It's speed" (airspeed) is different than observed groundspeed, and should remain constant in a well co-ordinated turn regardless of "Wind pressure".


This argument of yours ignores drag (especially form-drag and induced drag). The wind will provide different amounts of drag and other effects on an airplane based on the angle at which it strikes the aircraft. Any movement through the air, combined with the movement of the air itself, provides a "relative wind" that the plane feels. This relative wind will of course change as the plane changes heading. Speed changes are not instantaneous, and no system is 100% efficient at converting energy into motion or vice versa. I had a huge long explanation and thought-experiment I was going to post, but I realize that it would probably not be read, because of its length.

Fundamentally yes, a plane travelling at 100mph in a 20mph tailwind performs the exact same as a plane travelling at 80mph in calm air (all of these being groundspeed measurements. In both cases, the plane's airspeed indicator would simply read "80mph"). I am NOT disputing this fact. But these are steady-state examples. Once you add in the element of transition from upwind-to-downind (or calm-air to moving-air), you have to deal with the fact that changes in direction and speed are not instantaneous - and while the system is in transition, there will be relative differences which exert forces on the aircraft. Finally, as you maneuver through this transition state, your control-surface deflections will add their own set of forces and drag onto the plane.

Bottom line: Real life is far, far messier than the chalkboard.

Take care,

--Noel

[Daemon]

Noel, you're wrong on this one.
The only steady state that matters here is that of the airmass in
which the plane flies, not the plane itself. The plane can do whatever it wants.
If it's a relatively uniform mass of air moving at a constant velocity (meaning
both constant speed and direction), then it represents a true inertial frame of
reference, and anything moving through it, whether it's maneuvering or not
can't "feel" the velocity of the airmass itself, regardless of its inertia,
drag, or accelleration.. You claim there's a difference depending on which
direction the air strikes the the airframe. That's true, but an aircraft moving
through a uniform inertial airmass always experiences a straight on headwind
regardless of which direction it's travelling. While manuevering the angle
of attack changes but only relative to the airmass, and the change
in angle of attack, lift and resulting drag is identical whether it's turning
upwind, downwind, or crosswind. From the glider's perspective there is only
a headwind. Unless it can *see* the ground, it can't know how fast or
what direction that air mass is moving relative to the ground. Or in our
case, you the pilot can see it.

The only time the plane can detect what the airmass is doing directly
is if the airmass is not in an inertial frame of reference. E.g. Crossing from
one airmass to another, or the airmass is being accellerated by an
external influence (rapid pressure change, slope compression, and
drag or turbulence from other ground features.. etc).
Rotation is also a form of acelleration, which is why a rotating air mass
is not an inertial frame of reference, while one moving straight
at a constant velocity is.

I agree that in the real world it's messy, but not in the way you're
claiming because your explanation defies the laws of physics (some
of the most basic). Most of the effects you're attributing to
the glider knowing if it's turning upwind or downwind are more properly
explained by the effect that the ground has on the wind itself,
basically creating a non-inertial frame for the glider to fly through.
For instance, as you approach the ground you're flying into a wind
shear gradient such that wind speed is lower near the ground than up
high. If you decend into this slower moving air while flying upwind
then your airspeed effective decreases. If you decend into this slower
moving air while flying downwind then your airspeed effectively increases.
Your example of the glider stalling when it made a turn from upwind hover
to downwind run is explained by the same.
But all this is possible because its a non-uniform airmass and thus not
an inertial frame of reference. When a glider turns in a uniform
inertial airmass it can't know whether it's turning upwind/downwind, or whether there's wind at all.

Please check out the references for Inertial and Non-Inertial frames earlier
in this thread.

ian

[Whorges]

Usually in the 4000-7000 foot altitude above sea level in the North East there is a temperature inversion. In otherwords as you gain altitude above this hieght the temperature starts to increase. Now imagine that your a large mass of hot air (not hard for some people) and the wind blows you up the side of a mountain and the sinking cold air places an upward boyancy force on you. You accelerate up the side of the mountain as a run of the mill thermal. As you leave the mountain you continue upward because of the boyancy force. As you gain altitude the local pressure drops and you expand, and as you expand your temperature drops. Then you hit the inversion and all of a sunden you're colder than the ambient air and your boyancy force is gone. But you still have mass and upward velocity so your momentum caries you higher into this cold air and you continue to expand and get colder. Now your momentum has run out and you start to sink again, you gain momentum and sink below the inversion and as you sink you compress and gain temerature. Your boyancy is back, and back up you eventually go... hence "wave". Up and down you go as you drift down wind way past the original mountain. On the up stroke planes will find lift, and on the up stroke as the air temperature drops below dew point clouds will form. On the down stroke no clouds will form. So if clouds form from wave lift it is typically a series of clouds perpendicular to the wind direction and parallel to the ridge, and they get thinner further down wind b/c the air has less moisture. This type of lift is unlikely to be found near the ground in my opinion b/c the scale is wrong in terms of momentum, and no inversion likely. Now I am by no means an expert, b/c the thing I don't understand is that wave lift will oftem carry you to altitudes much higher than any thermal will, which means the wave increases in amplitude over the original thermal. Also there is something called a Lennie, which is a type of wave lift? This is great for hang gliders and sail planes, but good luck seeing your RC plane at 4000 feet!

[Daemon]

So if clouds form from wave lift it is typically a series of clouds perpendicular to the wind direction and parallel to the ridge, and they get thinner further down wind b/c the air has less moisture.
Lots of nice photos of wave clouds here (http://images.google.com/images?q=wave%20cloud&sourceid=mozilla-search&ie=utf-8&oe=utf-8&client=firefox-a&rls=org.mozilla:en-US:official&sa=N&tab=wi)
Since the prevailing winds are from the West here, we see wave clouds
underlit by the setting sun quite often above the front range.

In slope flying I've often found another "wave" of sorts above
the normal slope compression lift. This is what I observe. I'm cruising around
about 20 feet above the hill in moderately light air and the glider just
won't climb any further no matter how close or far from the hill I am.
I can pump it up a little bit but basically am stuck below a certain
altitude. Above that altitude the lift is either too weak or the
horizontal component is too large and the glider will blow back.
Then I see a hawk cruising easily about 50-100 feet higher than my
glider, sometimes parked sometimes working along the ridge. Now
you'd say "oh, he's better at it than you and can fly in less lift". But no.
I can cheat. I drop over the backside, make 2 or 3 decent DS turns
and then punch back out to the front and climb to the same height
as the hawk, and what do I find there.. lift. Plenty of lift to keep
the glider up. The way it *feels* as I fly in is as though there's a
second layer of compression, a wave if you will. Also even if the wind
speed is up, penetration in the wave is usually not a problem. And
having found and flown in it, it I now notice that most soaring birds
around here hang in that wave lift when they're moving cross country
above any slope, rather than in the primary slope lift itself. Also,
because it's so high up, it's a great place to catch a passing thermal,
which they then often do.

ian

[Batmanwpg]

Boy, it looks like the Enginerd's and Technogeeks (fun term of endearment in our group, so don't get offended!) ;) have high jacked this thread from what started as a simple question! 3 pages of cr :censored: p, OK, that maybe a little harsh. I get a little excited when I read some things! Jurgen, first off your converted Spiro is not a great plane to be thermal flying in the first place. Did any of you guys find a picture of this converted hotliner and see its lack of dihedral? Some thing that is really important for successful thermal flying! Sorry a converted hotliner could be at home on the slope but is not best for pure thermal work.
Second, planes do not care which way they turn in a thermal. If it where really important to turn "A" specific direction for best thermal climb do you not think that a world class competitive Free Flighter would have figured this out by now? I guess some are just dumb as some FF planes are trimmed to turn left and others right. Both turn directions have won WC's. Flyers from the northern hemisphere have won WC's in the southern hemisphere and do you think they re-trimmed their plane to turn the other way to win? Hardly!
The glide turn direction for say an F1B is determined by what ever technological devices or specific trim set up that gives best climb for that model. These models that must extract every bit of possible altitude from a tick fart for a 5:00 am flyoff flight could be trimmed for turn in either direction.
What is really important for thermal planes is proper overall design such as sufficient EDA for roll response, proper washout, proper CG, is it built straight, etc. 25 years of competitive FF and 5 years RC soaring and I don't care which direction I core my thermals in. That is dictated by which direction I feel the core is and how fast I can get my glider into it. Do thermals rotate? Some do and others not. I have thermaled air with fluffies clearly rotating and others that did not. Expand your horizons people, and try other facets of this great hobby and you'll be surprised at what you may learn!

Bear :cool:

[Jurgen]

Hi Bear Batmanwpg,

Spiro is 8,5% thick RG15, span= 1.8m(5.9ft), weight RTF= 1.17kg(41.2oz), wingloading= 35gr/dm²(11.4oz/ft²), dehedral= 3° each wing.
Removed the powerplant, installed towhook, setup CG + decalage for “neutral”. So You’re right, its no match for a good thermaller, in an upward fart it keeps sinking but it can thermal, and for now it is the only winchable plane I have (love the rocket zooms)
Advantages are:
* in a moderate thermal it stays up (hardly rising) but one have to work for it, errors get magnified = good for learning the hard way.
* give it an good thermal and you rock around as no “real” thermaller can, while a floater would struggle to come down this plane becomes adrenaline.

For floating I still have an Ellipsoid (elektro) and as we speak the post is handling my ordered Carbon Corado2000 (Zenith), the very first “serious” glider in my very life :cool: . Next year I hope to buy a F3B plane as an adrenaline-replacement for the little Spiro.

If you say 3 pages of cr :censored: p, that’s for your account to generalise, feel free to do so. But be aware of the conviction “there is nothing more to find out” , its not encouraging for the hobby . I do appreciate your input/experience that thermal rotation has no exploitable factor nor benefit for a better result.

[Batmanwpg]

Jurgen
At the bottom of your first post you stated "is this a newbie question" There fore the assumption was that you are. So my statement that a converted hotliner is not a great thermal airplane. Recommended dihedral for a thermal aileron ship would be 5° etc,etc. I know hotliners can thermal .

“* give it an good thermal and you rock around as no “real” thermaller can, while a floater would struggle to come down this plane becomes adrenaline.”

You can bring floaters down fast (not as fast as your Spiro) if flipped inverted or pull spoilers and diving with figure 8 circles. You don’t reach terminal velocity.

“, errors get magnified = good for learning the hard way.”

Good for you if you can learn the hard way. I am that way too but it is not always good for every “newbie” and the degree that this thread took on technical wise is not always good for newbie’s either. Some will get scared off or get left scratching their heads even more.

“If you say 3 pages of cr :censored: p, that’s for your account to generalise, feel free to do so. But be aware of the conviction “there is nothing more to find out” , its not encouraging for the hobby .”

Thirty years of modeling and I can still learn new things, yes. I just get tired of misconceptions being fed to “newbie’s”. In my country our birds thermal in both directions just like my planes!

Regards
Bear

[DFW_HLG_TG]

Gota admire and FF pilot. I'm a former CL jock. You know, the rock and the string thing.

[Jurgen]

I’m 23 years into rc flying, and did some elektrogliders along, always thinking my plane scared thermals away because they wouldn’t rise motor off. For serious gliding I feel newbie. The real thing is called hunting and exploiting thermals, this is very new to me, that’s why I posted and eager for info.
(BTW actualy the years don’t matter, I know guys fly 50 years and never flew properly, they fly fysical threats instead of planes :D )

[John Gallagher]

[QUOTE=Batmanwpg
In my country our birds thermal in both directions just like my planes!

Regards
Bear[/QUOTE]

The other day my Phrophet was in a thermal and three buzzards come over and joined the party. Two circled in the opposite direction of my turns and the other circled in the same direction. Sometimes I've noticed that when one buzzard finds a thermal the first buzzard that joins him flies in the opposite direction. This might have to do with a social rule rather than an aerodynamic need.

[tw126a]

John, does this mean us full scale pilots have been getting it wrong all this time? :)

Tom

[Jurgen]

When the birds fly opposite they encouter headon eye to eye each circle telling jokes about us flying, if not they look constantly into eachothers a.. :)

[T Tail]

Jurgen, you have a great sence of humor!
Bear, your right about the discushion, too thecky.
But it's also nice to know, that we have a couple of highly educated
flyers aboard. Some days you sky out, others your down in three minuets
when the task is ten!
I have ridden a wave, much better than a thermal!

[carrinsr]

"I'm right, you're wrong"..."I'm smarter, you're less educated"..."I have more experience, you're talking about something different"...I didn't mean to start a war here, folks.

Having thus said, come to northeast Florida and let's fly. Maybe it is just different here. Maybe real grits (not instant) really will cook in five minutes here (seen the movie, "My Cousin Vinney"?)

No matter, we won't need to test what everyone's said to see who is right and who is wrong. After all, nothwithstanding the importance of the truth or the error of anything, isn't peace important too?

So when you arrive here let's just launch and fly and enjoy life! Who cares if I stay up for an hour (and you stay up longer), and then I deliberately land because I get concerned about the batteries? (No kidding, that'a just something I usually do.)

But then again, a few club members have accused me of being able to see therrmals...and I don't even "polarize"! :o

Regardless, to anyone who reads this, you have certain gifts I DON'T have, so especially as we enter this holiday (holy day) season, Happy Thanksgiving, Merry Christmas, Happy New Year, and Peace to one and all!

David