Downwind Turn

[pimmer]

I keep reading posts with the term "Downwind Turn" and "it's having a struggle when flying into the wind".

Let's get one thing straight. There is NO SUCH THING as a "downwind turn" or a "dowwind turn stall". This is basic physics 101.

Let the arguments begin....

[davidfee]

All we can do is hope... but, unfortunately, "conventional wisdom" is more prevalent in this world than an education in basic physics. And yes, it's amazing that people who have been flying models for years will still insist that the conventional wisdom is correct.

Oh well, let them crash their models and blame it on "interference," "that darned carbon fiber stuff," "flux discontinuities" and, of course, the dreaded "downwind turn."

-David

[Andy W]

or "tip stalls"..

[JRuggiero]

There are no such things as "tip stalls?"

Jim R

[Andy W]

Find me an aviation reference that describes them and I'll amend my postion.
..a

[fhhuber506771]

or "tip stalls"..

"Tip Stalls" are a proveable thing... it is when the wingtip stalls before the center of the wing or (more common) one wing stalls before the other. It CAN and does happen. If it didn't happen, and wasn't predictable, you wouldn't be able to do a snap-roll using just rudder and elevator. (and they've been doing that for more than 60 years on purpose... longer by accident.)

The entry to a spin involves having one wing stall before the other.. (and a snap roll is sort of a spin... both are stall based maneuvers causing fast rotation of the aircraft.)

*******

The "downwind turn" problems are an effect of perspective. If you maintain a constant pabnk angle and rate of turn.. then the airplane doesn't care about the wind... but the "center" of the turn will drift downwind. By trying to maintain the turn in a constant radius about a fixed point on the ground, you have to vary the bank angle and turn rate. When you increase the bank and pull to increase rate of turn going from downwind to upwind, you increase the G load on the aircraft and add drag. This can be asking for a stall if you were already flying slow.

********

Full scale pilot's handbooks (Stick and Rudder... and the AOPA's "The Proficient Pilot") discuss the phenomenon we CALL "Tip-Stall" and the effects of wind relatied to making a turn about a FIXED point on the ground.

We might be using different words... but the physics of them are the same for the models and the airplanes people ride in.

*********

Maintain thy AIRSPEED (not ground speed..) lest the ground rise up and smite thee.

You are standing on the GROUND and observing GROUND speed. And that often has very little to do with the model's airspeed.

[Ollie]

http://www.wfu.edu/albatross/atwork/dynamic_soaring.htm
For days to weeks the albatross don't stall with repeated, again and again, downwind turn!!!
DS R/C flyers do the same:
http://members.tripod.com/douglasturner/id27.htm

With stalling margin of airspeed (not ground speed) there in not a downwind turn problem!!!

When gusty wind, increase stalling margin of airspeed.

A bad flyer confuses ground speed with airspeed!!!

[leccyflyer]

Find me an aviation reference that describes them and I'll amend my postion.
..a

Will this do a. ?

http://aerodyn.org/Wings/larw.html

[Tom Harper]

Downwind Turn, Weather Vaning, Ballooning etc.

After reading one of these 'downwind turn' discussions I went to our field and watched the wind sock for a while. It was the usual New Mexico spring breeze. I would have described it as constant SW at 5-10 mph. However, after watching the wind sock for a few minutes I noticed that air was moving across the field in packets or bubbles. The wind was always slowly but constantly accelerating (signed velocity).

So, a model is not simply in a moving mass of air. It is constantly adjusting to changes in air velocity. Due to inertia, there is a lag in the model's response. This inertial lag results in a net change in the flow of air over the model's surfaces. A model with dihedral that turns right angle to an accelerating air mass will experience a roll force. A model running down wind in an accelerating air mass will experience a loss of lift, and control. A model that turns up wind to an accelerating air mass will balloon and even stall.

However, if the air mass were laminar and moving at a constant rate the model would not respond to it. Such air movement is rare.

[Tom Harper]

Ollie,

This mode of flight demonstrates that there is a downwind turn 'problem'. The Albatross is constantly changing it's rigging in order to gain energy by diving in and out of the boundary layer. If the Albatross were simply static in moving air mass it's flight would end after a few seconds.

[Sparky Paul]

When sloping we see gust effects on the planes all the time.. particularly when going across the slope. One wing can totally stall, and the plane roll 90 degrees instantly.
The air does come in packets... watching the grass down the slope move as Tom mentions with the wind sock shows the effect of the packets relative to the larger moving mass.
But the proverbial "downwind turn" is purely a perception problem due to the moving plane and the fixed pilot.
The albatross uses the essentially fixed sea wave for its reference point to get the dynamic soaring effect it thrives on.

[Ollie]

The good flying albatross has a small head. The small head can't stuff the "downwind turn problem" in the small head. A good flyer allows the stalling margin airspeed above the stalling speed.

In a RC flyer in a floating a hot air balloon. When wind is high, even with the balloon drifts along. The RC flier flies perfect circles at constant speed and constant angle of bank. The same RC flyer on the ground and at the same wind has a problem in his head because of the reference to the ground speed.

The ground speed is referenced to the maneuver for power training. Sailplane flier is trained to the air speed and other gusts and air changes.

"TK was dragged out to a power field on a Sunday morning to see an old
friends immaculate 1/3 scale Piper Cub that he had just finished over the
winter fly for the first time. Well after they got the motor and
everything straightened out on the ground, he asks TK to take the first
flight, "just in case". He wasn't expecting to do the test flying as it
was a big power club and there were lots of "experts" in attendance, but he
didn't want to see his friend break his model either so he agreed to take
the first flight. So here is a senior guy, a brand new 1/3 scale Piper
Cub, a glider guy to do the test flying, and the club "experts" have now
broken away from running gas thru their engines on the ground to checking
this out like a bunch of vultures.

TK taxis the Cub out, takes off, trims it out, starts backing off the
throttle a bit and the engine sputters and quits. Next thing he knows guys
are screaming up and down the flight line, "DEAD STICK - - DEAD
STICK" He's like thinking to himself, no big deal, got flying speed, got
altitude and over the field, why are these guys screaming... so he goes to
set up an approach and the wing gives a bobble. He sets the Cub in a
thermal turn and spends the next 10 minutes specking it out. Brings it
back down, sets up a nice approach on their paved strip, and rolls it out
to his feet... and yells DEAD STICK."

Thoughts in some heads and not in others.

[Sparky Paul]

Ollie.... :D
All power fliers need glider experience.. "Dead stick" from takeoff to landing..

[leccyflyer]

Great story Ollie!

[Ollie]

It is a matter of attitude. Some use the air wind gust energy flying with finesse. Others can carry energy in the planes to stomp the air into submission.

The small downwind turn problem is part of a flier's attitude.

[leccyflyer]

Standing by my son last weekend with a stiff breeze almost in our face he started by flying the most commonly prevailing left hand circuit direction. That meant that his third turn, into the downwind leg, presented him with a model that had the appearance of rapidly accelerating towards him - compounded by him putting the nose down as he turned downwind. The subsequent turn onto crosswind was rushed, raggedly and over rapid to avoid the no-fly zone on that side of the site. I told him to switch to a right hand circuit direction, more appropriate for the conditions, meaning that groundspeed increase occurred a hundred yards away, and not directed at him and the no fly zone and from then on things proceeded much more smoothly. Same turn, same wind, same leg, same model, same pilot - the only difference was the relationship of the model to what was on the ground.

It catches even experienced pilots out and is, as has been said, a matter of perception and the position of the observer more than anything actually happening to the aircraft.

[pimmer]

I think the important point I wanted to get across was that, in a constant wind, the airplane should be flown the same reguardless of the direction in which the turns are made. When traveling downwind, resist the temptaton to throttle back to slow the plane down. Conversely, resist the temptation to throttle up when going into the wind. I have seen pilots throttle back to almost idle after turning downwind, and then wonder why the plane is so sluggish. The assumption is that after turning the plane downwind that is has lost airspeed from now going with the wind. The reality is that they have throttled back without thinking. Sometimes without even being aware of it.

A good thing to practice in a stiff breeze is to take the plane up to about 100 feet, set the throttle to 1/2 to 3/4 and take your thumb off of the throttle/rudder stick and fly circles. Set your thumb on the transmitter completely off of the throttle. You should be able to fly around level, even with abrupt turns upwind or downwind with no imaginary "zooming" or "dropouts".

[fhhuber506771]

pimmer.... but that means askign the guys to learn to fly the airplane... not the engine :p ;)

[Ollie]

"It catches even experienced pilots out and is, as has been said, a matter of perception and the position of the observer more than anything actually happening to the aircraft."

Or:
"Flying For Proficiency" by Mike Lee
"The ultimate idea of flying models is for the enjoyment and recreational value of modeling. To this end, many of us accomplish this every time we put a bird into the air. Those of us who are the sporting type of pilot often watch the better pilots and wish that we also had the "touch" to flying like the other fella. You, too, can have the "touch" if you simply put your mind to it by flying for proficiency.

Flying for proficiency is not stressful and certainly does not detract from the enjoyment of flying. It is simply telling yourself that you have a goal, and today we are going to strive to achieve that goal. The only person you need to help you with this process is yourself.

You can begin this process by asking yourself what you wish to accomplish. Don't task yourself with being the World Champion right off the bat. Instead, task yourself with small goals, such as landing within a 30-foot spot. Believe me, most power plane pilots couldn't hit a 30-foot spot if their lives depended on it. In this task, get up and fly around and enjoy the flight. When you decide to land the ship, simply concentrate the landing portion on the spot landing. That way, you enjoy the wonder of flight while worrying only about the minor portion of landing. Once you find that this task is well within your grasp, decide on another small task, such as getting only a 5-minute flight. Next can be 7-minute flights, and the list goes on.

The whole idea here is to fly with a purpose. As you grow into this mode of self-instruction, you will find that you will pull the smaller tasks together, eventually making the entire flight a singular task. You will find that by this time you are more at ease in accomplishing your goals, you fly better, and you are now one of the guys who get watched by the other pilots.

Lastly, take the following advice: never push yourself too hard. Doing so will result in banged-up aircraft or worse, destroyed aircraft. Don't worry about not achieving your goal every time you try. This is a learning process we are going through, and you will fail once in a while. Stay with the same aircraft. By learning the way the one aircraft handles, you will learn how to get the most out of it. Always keep yourself in a friendly state of mind. You cannot fly effectively if you don't like what you're doing, let alone enjoy the marvel of flight. A relaxed pilot flies confidently and effectively. Fly for proficiency and you will find the real pilot within you."

And:
'Learning To "See"' by Mike Lee
"While there is no doubt that a large number of people must wear glasses for perfect vision, there is a little-known fact that people only see what they want to see. Most of the time, we see only those things which are in close proximity to us and ignore the rest. But when we fly aircraft we must be able to see or we are lost. There is a way to help yourself.

Good eyesight doesn't require you to borrow the eyes of an eagle to be great. Even if you wear glasses, the trick to seeing well is to train yourself to do so. This is really easy to do, and you can do this anytime that you are awake and looking around.

The first thing to do is try to notice everything around you. Look especially for details of items like the leaves in the trees, not just the trees. Look for the name emblem on a car as well as the entire car. Watch for the things within your peripheral vision without moving your eyeball from the spot it is focused on. After awhile you will find that you are seeing things you didn't see before.

Next, move on to more distant objects. Don't just look at a mountain in the distance, look at the detail of the mountain in the distance. When you are out driving, look ahead, way ahead, for the street signs and read them. You will soon find that you are teaching yourself to see better. Your eyesight was always there for you -- it just wasn't fine tuned to see everything.

If you practice seeing with your eyes, you will soon find that your ability to identify the altitude of your aircraft at distance is easier than before. You won't worry about whether that last control input should have been left, or was it right? Consequently, you will recognize lift faster and run from sink sooner. It is a very conscious effort to train your eyes to see, but the results will last you the rest of your life."

[Tom Harper]

Those who think it is only perception have never had to recover a free flight model after a long downwind run.

[davidfee]

Those who think it is only perception have never had to recover a free flight model after a long downwind run.

Huh? I would argue that those who fly free flight are best able to understand that the "downwind turn" problem is 99% perception and 1% misunderstanding. Free flight models are trimmed for one glide speed...so they are very sensitive to changes in velocity. If they speed up, they rise... if they slow down, they sink. And yet, a trimmed free flight model just goes about its business, circle after circle, as it drifts downwind. The 1% I included is because gusts do exist and they can cause a problem if the glide speed is too slow.

Chasing a FF model on foot, you definitely get the PERCEPTION that the model slows down when it heads into the wind and speeds up when it heads "downwind," but this is a perfect example of why it's a perception problem. You are standing very close to the model and your feet are fixed to the ground, while the model is flying in a moving mass of air... you are in different inertial frames!

And believe me, I've recovered my fair share of FF models (F1B mostly).

[Tom Harper]

Then I assume you have experienced a free flight turning downwind and running straight as an arrow.

[Ollie]

First fix attitude. Then fix perception.

Who is stubborn? You? or Me? ;) ;)

Friends?

[Tom Harper]

Thee of course,

Hey, is that hamburger joint and bar still open down on the water front in Punta Gorda or did it get blowed away last year?

[davidfee]

Then I assume you have experienced a free flight turning downwind and running straight as an arrow.

When my glide trim was not correct, yes... :D

Friends. Hmm... a burger sounds nice. :)

[Ollie]

I was riding it out two miles from last summer's hurricane eye. At the time I didn't think about downwind turn. Five of the restaurants at Fisherman's Pier are open.

[fhhuber506771]

I sometimes trim an RC model as if it were FF... then set the engine speed for a very slow climb and just watch... Its an interresting way to see what the AIRPLANE "thinks" is a constant radius circle in a steady breeze.

[Throne]

Free flite trim-
I fly slope, early in the day when the wind is lite I put up my little 50" balsa plane, I consider it a slow lite lift flyer. My slope is inland so we get boom'n thearmals. When setting trim I just send strieght out into the wind (breeze) and trim for "dead-stick".
it'l fly strieght and level until it bumps into a big thearmal. The para-gliders sitting around on the ground (not enough lift for them) get kinda worried when I set my tx down to lite a ciggeret (nasty habbit). Then we put empty coke bottles (with sand in the bottoms) on a boulder and see who can knock it over. When the wind pick up out go the rocket mouldies for some high speed bank and yank burn em up thrills.
Bottom line is fly the plane not the prop (if you use one), the wind is our friend and alot of fun when we figure out how to play it.

[pimmer]

Wow, I am very pleased to find that there are a lot of rational right mided people here. Usually, when I bring up this subject, I get whole elaborate diatribes about how when you turn downwind you suffer the loss of airspeed equivalent to the differential between airspeed and wind speed. And they don't mean turbulent air either. A lot them claim to be full size pilots, and that their flight instructor backs them up in their opinions. And these diatribes are delivered almost like religeous dogma, but all if them are utter hogwash. All in all, it has a great deal of entertainment value. :)

[kallend]

You can't see turbulence and you can't see windshear, both of which can screw you when turning. It's not ALL a matter of perception.

[vintage1]

Well I can see both when flying in a straight line, with the model jumping and twitching all over the place.

[Throne]

Its much better when we fly the wind rather than allowing the wind to fly us. like vintage says if we just watch the model "twitching all over the place". Then thats all its going to do. We need more energy when flying in stiff winds and we can get that energy from the wind itself. Use the upwind leg to gain altitude, downwind leg for speed, and by all means dont fight altitude loss during "downwind turn". That altitude loss should be used to create speed (energy)= lift.

[vintage1]

You need more energy in turbulence. More speed in stiff winds if you want the model to stay in the same place.

The trouble with wind is it isn't constant.

I've had the model lifted and dumped maybe 50 feet at a time sometimes, flipped sideways, tail wagging, bumping along as well..
Its all very challenging, and I can fly in wind, but its not real fun - I like precise flying and turbulence is just an exercise in model survival.

[fhhuber506771]

wind shear and turbulence bother you if you are turning or not. Thats what bounces the properly trimmed for straight and level airplane around when the controls are in neutral.

But the guys that consistantly turn too sharply on the "downwind turn" and stall the airplanes.. its perception, because the turbulance and wind shear are NOT in the same place every time. They won't normally be found in the exact same place in 2 consecutive laps around the airfield, unless the "turbulance" is from a "rotor" of wind interacting with a tree line or building and the wind is constant. (we have one of those come up at our field... 100 yds straight in line with the runway, and in the final approach path, when the wind is from the S.)

[vintage1]

agreed, no argument.

My free flight experience taught me what to expect tho.

[biber]

It is definately the perception! Even fullscale pilots have this problem near the ground and don't have it in high altitude. Because in high altitude they are less tempted to fly ground based circles. And yes, I am fullscale pilot too and thus know the problem by own experience. But it's easy to overcome it with the right picture in mind, the air mass shifting over the ground is a good picture for it despite the fact that there are things like thermals and turbulence too. They just add to that basic air mass movement.

biber

[Ollie]

"It's not ALL a matter of perception."

Perception is never perfect. There is a huge, huge difference perception between some fliers and other fliers. Some can tell "see" the stalling margin air speed by sluggness of controls. Some can read the bank angle and control response for very nice turning with out stalling. Etc. etc. etc.

I would go more than 50 miles to see a master flier perform. He sets up the plane on the edge of unstability so he can see the small gusts. Yet he flies like on rails. He can focus on flying better than me. He can see and correct for gusts sooner than I can see the gusts.

[pimmer]

All I am saying is that in a CONSTANT wind, there is NO DIFFERENCE in the airspeed the model experiences as you turn full circle. In order to maintain the same circle over the ground, however, the "downwind turn" will actually be shallower, and the upwind turn will be steeper.

Several of the responses here indicate to me that there is a belief that the downwind turn somehow results in coming closer to stalling the plane. This is where the fallacy lies. My goal in starting this thread was to dispell this misconception as to the physics of the situation. Pilots will claim that the model "zooms" up when turning into the wind, and that it "drops" when making a downwind turn. The airplane will do NO SUCH THING unless you command it to with your stick movements. This is not a matter of opinion, this is physics FACT.

[fhhuber506771]

part of it is the definition of "downwind turn" too...

you can define it 2 ways (and both got used here) You can define it as the turn made to change from going downwind to upwind... or the turn made to go from upwind to downwind. (both nvolve "downwind") And either can present a problem when trying to maintain constant radius related to the ground, because HALF of each has the modle pointed into the wind (relative to ground track) and half has the model pointed with the wind and to keep the same radius (relative to the ground) throughout... you need varying bank angles.

Highly recomended reading (if you can find it):: The Proficient Pilot by Barry Schiff. AOPA. Read the section on "Turn Dynamics" (Chapter 1 part 5... page 27 to 32 in my copy {special attention to latter part of page 30 to end of section}) This talks about "Ground track maneuvers" with a good explanation of the bank angle changes needed for constant radius (ground track) turn about a point.

[portablevcb]

Constant wind and turbulence. Vast differences. Full scale or model.

We fly mainly at two fields, one large and open (half mile square) with almost no trees or other obstacles. The other is a couple of soccer fields with houses, trees and an arroyo (drainage canal to the rest of the world :) ). A 10-20 mph wind at the large field is something to enjoy, allowing landings at a standstill (or even backwards!). At the small field if you can feel the breeze it is almost dangerous to fly! Large field had large 'air masses' with little tubulence. Small field has very small 'air masses' with LOTS of turbulence.

Watching a master pilot is a joy. I do, almost every time I fly cause one of the guys at the field is one. He flies planes in such a scale manner sometimes I confuse his planes with the real ones at the local airport, regardless of wind!

R/C = Guided FF! I flew one of my R/E/T models Sunday (10oz electric). Launched, climbed out, flew circuits. Then noticed the wind was at about 10mph. Got a little altitude and trimmed a right hand large circle. Watched it fly for a bit. Set down tranny, got out lawn chair and set it up. Got out stogie and lit it up, got soda from icebox. Sat down and picked up tranny, flew plane back to field! What a joy steady air is! Only issue was it flew through a thermal and had gained about 500ft alt :)

Downwind turn problems in a steady wind are definitely a pilot perception problem. I'm a relative newbie and I know enough to set the throttle for a comfortable stall margin and leave it there. Upwind the groundspeed is slower, downwind faster. I set approach speeds by throttle setting as well, chosing the stall margin according to the tubulence. Still have to watch the bank angle when turning final!

Full size flying has some applicable parts, but, this is one area where the R/C pilot can get into more trouble, faster. Full size you have an airspeed indicator that helps alleviate the issue (not totally, just most of the time).

charlie

[Soar_dude]

Okay I have these questions then. Why does it appear that a plane turns more quickly in one wind direction and slower in another? I agree that downwind turns are a pilot on the ground Perception issue. I have a scenario for you. Two planes one flown from the ground the flown with a AV system barring quibbling about weight they are idenitical will they react the same making turns in the wind?

Soar Dude

[fhhuber506771]

Okay I have these questions then. Why does it appear that a plane turns more quickly in one wind direction and slower in another? I agree that downwind turns are a pilot on the ground Perception issue. I have a scenario for you. Two planes one flown from the ground the flown with a AV system barring quibbling about weight they are idenitical will they react the same making turns in the wind?

Soar Dude

This is EXACTLY the stuff described in that section of "The Proficient Pilot" I referenced.

Even though that book is aimed at full scale light plane pilots (and is usefull to even 747 pilots..) it has a TON of information that is applicable to the models. and the explaination of the difference between "ground Track maneuvers" and flying at altitude is the answer to the flying in wind questions.

***********
Two IDENTICAL planes.. with duplicate GPS maneuvering autopilots set for a 200 ft radius turn about a fixed GPS location... one with no wind. the other with a constant 12 mph wind:

The one with no wind will hold a constant bank angle, and can follow the circular track precisely. (within the accuracy of the autopilot)

The one with wind will continuously vary the bank angle trying to stay on the circular track, but due to the constantly varying angle of the wind to the aircraft's direction of travel the circle might wander a few feet from the programmed constant radius. (the wandering will be more than the windless one)

the person on the ground observing the 2 aircraft will see the first doing perfect round circles at constant speed.

The person on the ground will see the second as doing the round circles.. but the percieved speed will vary with the aircraft's heading in relation to the wind direction. (and the bank angle changes will be visible, though the deviation from circular path might not be if its a GOOD GPS autopilot)

[portablevcb]

The only 'extra' piece you have to remember is that as the bank angle changes, the minimum airspeed to maintain flight changes. So, if you are banking more to stay in a particular ground path, you are closer to stall speed. This is also covered in flight training books. Especially since turns on approach are usually done at slower speeds and are referenced to ground points (usually).

Perceived 'speed' of a turn is applicable mainly to the R/C pilot. When making a turn from downwind to up wind it will appear 'slower' since the plane is drifting away from you. When making a turn from upwind to downwind the opposite is true. If you maintain the same center point on the ground the turn 'speed' will be the same, but, the difference in bank angle and the amount of elevator/rudder applied will make one 'feel' faster than the other. This is compounded by the ground speed of the plane coming out of the turn.

charlie

[HELModels]

I've got some very recent(This Evening) "downwind turn" experience. I dont have alot of stick time but am not totally new to R/C or flight either. Winds on the ground were non existent and leaves in the trees were not moving, so I went to the field with my recent creation, V-tail, pusher, CDROM powered, trainer. All went well as I quickly gained altitude and observed effects of various intermittent forces aloft, i.e., winds and/or thermals. This field is very large and open except for where I stand which is a corner with trees and houses marking the edges. I got way up and out and decided that was too high and too far out, so chopped power and started a gently turning descent by giving the occasional bump on the stick for direction. Then I experienced what this thread is all about. A little left stick and still drifting right over the trees and houses while losing altitude. Alot more left and still drifting and now the plane is banked 60 degrees and really losing altitude and gaining on the tree tops. An image flashed in my head of all the work stuck up in a tree, so I released the stick and hit full throttle and climbed. I sensed/saw a lull in the drifting and then turned left and out back over the field. That was the dreaded downwind turn as in I was downwind trying to get back over the field. As I saw it, it would not turn but was glad to bank and lose altitude. :eek:

[Sparky Paul]

Storch, it's comprehending the reason for the plane not responding when going downwind as expected that reveals the amount of flying you have to do.. can't just let the plane do it for you.
Not infrequently it's tightening the turn that does the final deed to the plane.

[Ledbetter]

"Not infrequently it's tightening the turn that does the final deed to the plane."

Amen. Ask my dead Spectra about it. That packet thing was good too.

Speaking of which: It seems to me that in moderately gusty wind, flying straight downwind and starting a turn, ALL of the packets are pushing against the top of the plane/wing. Also, at this instant, the largest possible profile of the plane (top down) is presented to the oncoming wind. The plane at this instant is also losing energy/velocity from altering its momentum.

As opposed to upwind or lateral flight relative to the wind, in which the packets are blowing, to some degree, across both wing surfaces, top and bottom, at the initiation of the turn.

Wouldn't this be the same as "loss of lift" and contribute to a stalling tendency at the margins of the flight envelope?

My thought is that what is occurring simultaneously in the downwind turn is "downforce" on the entire wing due to the wind coupled with normal loss of velocity in the turn. The RC pilot perceives no loss in groundspeed and confuses this with airspeed. This effect would be seen less on full size planes (perception aside) due to their greater momentum; same with higher wing-loaded RC planes.

The consequence is unexpected, even if the phenomenon doesn't exist. :eek:

[HELModels]

"Not infrequently it's tightening the turn that does the final deed to the plane."



My thought is that what is occurring simultaneously in the downwind turn is "downforce" on the entire wing due to the wind coupled with normal loss of velocity in the turn. The RC pilot perceives no loss in groundspeed and confuses this with airspeed. This effect would be seen less on full size planes (perception aside) due to their greater momentum; same with higher wing-loaded RC planes.

The consequence is unexpected, even if the phenomenon doesn't exist. :eek:

Yes, I Flew the plane out of a bad situation. My gut knew that perceived groundspeed is not necessarily airspeed and that 60-90 degrees of bank and then yank up on the elevator could be direct ticket to a stall/spin which is why I leveled the wings(by releasing rudder/elevator stick) and climbed out(using full throttle) and prayed that winds would pass and allow a shallower turn.

[pimmer]

Ledbetter, your belief is the entire reason for this thread. There is NO downforce such as you describe. The airplane is physically unaffected by ANY amount of constant wind. The wind field is simply a moving frame of reference. Picture yourself in a warehouse with the doors closed flying your airplane around. Now change that warehouse into the hold of a ship moving at 30mph. That is just like the wind field at any appreciable altitude about our heads. It is an entire chunk of air, like the hold of a ship, moving at 30mph. The plane has no idea that there is any wind.

[Sparky Paul]

Led, the wind isn't "pushing on the top of the wing".. The plane is immersed totally in the moving air packet, and would turn normally at the bank angle it's at, but the turn is elongated from circular in the down wind direction, so it is drifting downwind as it is turning.
If the bank angle is held constant, it will eventually come around, but a lot further away than usual.. the natural reaction is to tighten the turn to bring the nose back upwind. With added power, to add the upwind thrust needed, nothing odd will occur.
If there's no power, then diving to get the speed needed to come back upwind can work, with sufficient altitude.
I've had any number of planes which couldn't get back upwind -with- power in these conditions. :)
On their second flights (presuming they survived the landing off-field) , I don't let them get downwind at all!

[Ledbetter]

I like the packet theory better than the chunk theory.

I've seen RC planes hit with an oncoming gust of air and come to a virtual standstill. When the gust is gone, they fall or stall. They are not immersed totally in a chunk of air moving at the same speed. Gusty air is turbulent and not moving coherently.

"Pushing on top of the wing" may be the wrong words, but it still seems to me that the air pressure on top of the wing is greater than the pressure on the bottom. That is why the plane is "drifting downwind" in the turn, yes? This condition is still the opposite of "normal" conditions where the wing is generating lift from air pressure differential.

I appreciate this thread and all of your thoughts very much. I'm not arguing with you except to seek explanation for what (I think) I see. :)

Regards to all.

[portablevcb]

No that's not why the plane is drifting downwind in in the turn. It is drifting downwind cause the air mass is moving and the plane is flying relative to the wind. The wing is still generating lift the way it always does, by airspeed (not groundspeed!).

Turbulent conditions can be completely baffling because the air is not moving at a steady rate or direction. You get updrafts, downdrafts, side flows, etc and they are not predictable. If you see someone hit a gust of wind, the plane appears to slow down. The 'drop' you may see at the end of that gust is not the plane stopping flying, it is probably because the turbulent air is flowing down and the plane is going with it. That's why it is so difficult to fly in turbulent air, especially at low altitude.

This is not an easy concept for a lot of folks. Relative velocities is one of the more confusing topics to teach in physics because everyone is conditioned to what they oberve from their reference. This is especially true when viewing conditions caused by moving air simply because you can't see the air moving.

That's one reason why I 'fly the throttle' a lot. I know the settings for stall (or I figure it out during flight) then I use that knowledge for landings, not the observed speed of the plane, which is almost not useful for a landing.

charlie

[kristianb]

I must agree with Tom Harper, there is not only a question of perception, the inertia of the airplane also is a factor to consider.
My best example is from flying model helicopters which are able to make a very fast 180 degree turn.
A helicopter that hovers steadily over the ground against the wind have some extra lift from fresh air coming into the rotor (some of the work to pull air downwards comes from speed relative the air).
If the pilot turns the chopper 180 degrees and leave everything as before, the helicopter will start to move with the air. It will after a while reach the air speed and loose lift.
This is the danger of downwind turns with airplanes, after a sharp turn at low speed, especially with a heavy model, it will take some time before the plane picks up the previous air speed and the risk of stalling is higher!
KristianB

[vintage1]

That does not compute.

[portablevcb]

Kristian,

Your mention of helicopter performance is correct in one respect. If you have one setup for hovering in a wind, you are actually in forward flight! If you then turn downwind you will go through a speed where you are at zero airspeed, putting you back into an actual hover and you will lose lift for a bit. Then as you gain forward airspeed to the point where you were trimmed to start with, you will be at the same flight conditions, but, at a forward ground speed twice what the wind speed is.

If you just rotate 180 deg holding the hover, then you are stationary, but, actually have a backwards airspeed.

If you turn a 'heavy' plane at low speed (actually any plane) there is no major change in airpseed (unless you really yank it around). There is a loss of vertical lift due to the banking of the plane, ie, some of the 'lift' generated by the wing is being used to turn the plane, not keeping it in the air. Stall speed is then a bit higher than normal. The perceived 'loss' of airspeed is actually a loss of groundspeed because you are seeing double the effect of the wind (downwind to upwind).

If you bank a plane a lot (over 30deg) and pull it around a turn, the increased drag due to the turn rate will slow the airplane somewhat, aggravating the problem. At steep bank angles you are losing half or more of the lift of the wing. If you don't increase speed, or start with a comfortable speed margin over stall, you will stall in the turn, regardless of upwind or downwind turning. The plane will regain the original airspeed once out of the turn just as fast as it would in an upwind to downwind turn. It just seems different because of the distance travelled during the acceleration.

This is only really an issue because folks fly low and slow on downwind turn to final, usually after having slowed the plane when flying downwind due to a perception that their speed is too high. Then the combination of airspeed near stall and pulling a hard turn combine to cause a stall during the turn.

The combination of airspeed near stall and a hard turn will cause a stall whether you are turning upwind or downwind.

charlie

[kristianb]

I fully agree with your high quality comment! What I wanted to show with the helicopter example is that it is not fully true that an aircraft always keep the airspeed in a turn, the inertia change required take some time. The helicopter is a good example since it have quite some weight but are still able to turn 180 degrees quickly. For an airplane the effect is marginal since it have quite some time to adapt the airspeed after the wind and as You said, other factors, like increased drag from the turn is of much higher importance. I have unfortunately seen some heavy scale models meet it´s destiny during windy competition, in downwind turns where I´m sure the problem to increase the momentum is one factor with some influence!
It should be fairly easy to calculate, if air is blowing with 5 meters/second, model flies at 15 m/second, weights 5 kg and a 180 degrees turn takes 5 sec, how long time will it take for the model to regain airspeed?
Maybee something for Vintage to compute?
Best regards, KristianB

[pimmer]

I am afraid that you do not understand at all what is happening. THERE IS NO DIFFERENCE IN A TURN FROM UPWIND TO DOWNWIND. Even if the wind is at 1000 mph. or 10,000 mph. or 5mph. There is no "regaining airspeed", there is no "drop out" there is no "zooming upward" in a turn from downwind to upwind.

[Ledbetter]

I think that a gust of wind at the right moment during a "downwind turn" can retard the forward/upward momentum of the airframe, relative to the direction of wind, to the point of stalling, falling and subsequent bawling.

[biber]

I think that a sufficiently strong gust of wind at the right moment during any turn can bring a model to the point of stalling, falling and subsequent bawling. :eek: :D :D

biber

[portablevcb]

A sufficient gust of wind while flying straight and level can cause enough loss of airspeed to cause a stall!

Kristian, Contrary to your statement, any changes in airspeed in the turn are not inertia related, they are drag related, AND, are not any different in any type of turn, regardless of wind direction, and, are not different because of model size or weight. Full size planes can have the same thing happen.

Crashes on turn to final are almost always because the pilot lets the plane get too slow in airspeed, even though the plane looks like it is going fast enough. The rest are simply because the pilot trys to turn too tight at such a slow airspeed when correcting a bad approach.

The reason you MIGHT see more larger models fail because of this is because of how close to stall the pilot is flying the plane due to high wing loadings. It is still pilot error, not a condition of windspeed.

charlie

[kristianb]

sorry folks,
I dont follow You any more. If You believe that a gust when flying straight can cause a stall, why cant a pilot-induced gust created when making a 180 degree turn cause a stall?
If model flies into a 5 m/sec head wind and then makes a 180 in 5 seconds and then have a 5m/sec tail wind, there is no difference from flying straight and wind changes direction completely in 5 sec from head to tail wind.
I´m convinced that this can cause a stall! But I also agree that other factors are more common reason for the majority of the stalls we see, but this effect is one part of the problem.
vbr KristianB

[portablevcb]

A wind change in 5 sec is not enough to cause a stall. The airplane will 'fly with' that slow a change in wind velocity, maintaining its airspeed. A wind gust strong enough to cause a stall such as described above has to take place in much less than a second with a very large change in wind velocity. FWIW, I have never seen such a gust. What you describe as a "pilot induced gust" is exactly what we are talking about, a hard turn that causes a stall, but, it has nothing to do with wind direction.

You can have a stall when flying in a 5m/s headwind and turning 180 deg in 5 sec and then have a 5m/s tailwind! People don't normally do this when flying low and slow so you don't hear about many incidents from this condition.

You can see this if you set up a good amount of elevator throw. Put your plane into a slow hard turn, then pull back hard and fast on the elevator. The plane will immediately stall, sometimes entering a spin. This happens regardless of wind direction. This is essentially what happens on low and slow approaches when the pilot misjudges the angle of his turn or his airspeed.

The effect of wind gusts can contribute to crashes. It happens every week at our soccer field because of the turbulence from local trees. That is not really what we are talking about here though. This is limited to steady winds and the confusion of groundspeed and airspeed during a turn.

Too many folks blame an errant gust of wind or the "downwind to upwind turn effect" instead of admitting their poor piloting. I am not immune to the poor piloting part. I too have pulled a turn too hard on final and stalled into a crash. It was with a strong wind, but, it was not the wind's fault, it was my 1) bad approach and 2) the error in trying to fix it incorrectly.

The message is to learn to fly a good approach, and stop blaming the wind when it's your thumbs that cause the crash.

charlie

[biber]

Kristianb, try to imagine yourself standing in the cabin of a freefloating balloon steering your model. The ground is moving now from your point of view and the air does not. Maybe its foggy down there and you can't even see the ground. Will you notice any wind? Will your model notice any wind? Will it stall suddenly in a turn or circle because the ground is moving in some direction? What will tell the model in wich direction the ground actually moves to decide where it has to drop out of the air?

biber

[Ledbetter]

Can we agree that this is more likely to happen during a downwind turn because of increased chance of pilot confusion of groundspeed with airspeed, among other factors? :o

[rorywquin]

Question.....

If I am flying downwind, relatively slowly for the airframe, and need to start turning to cross wind, do I then add power before I start the turn to keep the airspeed up?

This is the first crash, that I have had like this and it really caught me by surprise and I believe it was to do with this downwind problem or have I completely misunderstood this discussion?

It was quite a windy day and I was flying down wind with the u/c down and possibly some flap - not too sure anymore, and was slowing the plane up to do a slow flypast on the up wind leg (that was my first mistake - I did it because somebody on the flight line suggested it). So I banked to the right (I think the wind speed had picked up some) probably pulled some up elevator to tighten the turn (the control tower had earlier told me not to stray too close to the full-size runway - so that was probably in the back of my head) and as it was coming round to cross wind it rolled / flipped over to the left and inverted, I managed to correct the worst of it but was too low to avoid a meeting with mother earth. :(

[Sparky Paul]

In a steep turn, any wind speed, any direction, you must add power to keep the altitude you're at. Otherwise the plane descends.

[HarryC]

Rory, it sounds like a simple case of going too slowly. Because we are fixed to the ground we tend to judge the model speed relative to the ground, so when going downwind there is a tendency to go much too slowly through the air. By the time you come to turn across wind you can be very close to stalling despite having quite a speed over the ground. Any turn increases the stalling speed, at 60 degrees of bank the stall speed goes up to 1.4 times what it is in level flight. Therefore you can get the model inot a dangerous situation - low airspeed at the end of the downwind leg, stalling speed rising rapidly due to banking, speed falling rapidly due to drag as you yank on the elevator. The illusion of speed when going downwind has caught a lot of model fliers.

H

[rorywquin]

Rory, it sounds like a simple case of going too slowly. Because we are fixed to SNIP The illusion of speed when going downwind has caught a lot of model fliers.SNIP

H

Thanks - however one Boomerang damaged and one lesson well learned !!

[portablevcb]

I like to 'test' an airplane before landing. Fly it at altitude in the approach pattern and let it slow by gradually decreasing the throttle. Note the throttle setting when it stalls (and the mushy control response). Then add at least two notches of throttle (depending on the plane) and use that as the approach speed all the way to the runway. Then, again at altitude, fly a full approach at that setting to verify that it will work for the kind of turns you make.

That way I don't fool myself into going too slow. Not foolproof as I can still turn too sharp and stall, but, at least I have some reference other than my poor vision.

charlie

[Gary Warner]

Want to have some fun?

First, see the attached image.

Here's the exercise question:

From the steepest bank angle to the least bank angle, can you correctly identify the correct numbered order in the diagram for a turn-about-a-point ground reference maneuver?

Example post: (not necessarily correct - just an example)

(steepest bank )
3
2
1
4
(shallowest bank)


Only post the sequence of numbers you come up with. After a day or so lets discuss it, but for now, just contemplate the answer without comment.

Understanding the correct answer will go a long way in understanding the down-wind probelm.

This might be an eye opening experience or a no-brainer. Lets see.

Gary
--

[zoomzoooie]

It's all relative. The aircraft to the ground, the air and to you while it is in the air. :cool:

Airspeed: speed at which the aircraft flies in relation to the air around it.
Groundspeed: speed at which the aircraft flies in relation to the ground.
Windspeed: speed at which the wind moves in relation to the ground.

An aircraft looks like it is going slow into the wind and looks like it's going faster downwind. This is the effect of ground speed.

Airspeed has no relation to groundspeed or windspeed. It is only the speed at which the aircraft is moving through the air, whether the air is moving or not. In no wind conditions the airspeed and ground speed will be the same. An aircraft at the same attitude and power is actually trying to fly at a constant speed through the air regardless if it's into or with the wind.

The ground speed is the speed at which the aircraft is flying through the air plus the speed at which the air is moving in relation to the ground. When the air moves it also moves the aircraft with it at the same speed it moves, however the airspeed of the aircraft does not change. You don't notice the airspeed changes of the aircraft in wind but you do notice the ground speed changes of the aircraft into and with the wind.

You try to keep the aircraft at the same ground speed by adding or decreasing power or attitude. This will change the airspeed regardless if it is in wind or not. When you decrease power going down wind you are decreasing airspeed. When you enter into the downwind turn you may be close to the stall speed. You try to keep the plane from drifting with the wind so you turn tighter increasing G loading. As the G loading increases so does the stall speed, then aircraft stalls. You need to keep the same power setting going downwind and add some power going into a turn to keep airspeed up. Fly as you would in no wind.

Ground speed becomes critical when the aircraft comes close to an object(you or the ground). This is why we take off and land into the wind. We want to have a slow ground speed to clear objects around it or at least have a slower impact speed. :eek:

When an airmass moves no matter the direction, it takes the aircraft with it. An airmass also has movment within it'self. You see this when the aircraft is "bumped" around or drops a wing. Air does not move at the same speed across the entire aircraft. Any air movement acting on an aircraft effects the area it is in contact with. The resultant forces of those air movements in contact with the aircraft effect it as a total. The aircraft attitude is change by pressure differences in the air. As a wing passes through air moving in a different direction or pressure, it changes the attitude of the aircraft. When a wing passes through virtical moving air greater pressure effectively pushes on the wing moving it up or down. When a part of a wing moving horzontally passes through different pressures it acts aerodynamically on the wing as if the wing increased or decrease airspeed causing more or less lift resulting in what you see as a wing drop or tip stall. Any part of the wing including the tip can stall if the airspeed decreases to the stall speed. :eek:

It's all relative. The way you see it is perspective.

ZZ

[biber]

(steepest bank )
2
3,1
4
(shallowest bank)

biber

[pimmer]

2
3
1
4

[zoomzoooie]

(steepest bank )
3
2
4
1
(shallowest bank)

ZZ

[kristianb]

(steepest bank )
2
3
1
4
(shallowest bank)

KristianB

[rorywquin]

Rory, it sounds like a simple case of going too slowly.
H

I have been going over and over why I crashed and why this had not happened to me before.

What I now realise is that usually, I am losing height while slowing the aircraft up for a landing so the turn from down wind to cross wind has always been made whle losing height = an increase in speed. While a flat turn (maintaining same altitide) without added throttle, is an accident waiting to happen.

This has been a useful thread for me - thanks guys!

[HarryC]

Sorry to hear of the accident Rory, was it badly damaged? Perhaps just as well you started with a relatively straightforward jet rather than something scale like the Sabre! If you're ever touring out in Gloucestershire direction give me a shout and we will go up in the full size and you can practice flight at slow speed then you can see very clearly from the instruments what happens when you turn.

As someone who has spent hours and hours flying slow speed cirlces in winds in full-size gliders, I can honestly say I have never once encountered this mythical downwind turn loss of height and into wind turn gain of height that so many model fliers like to believe in. It just doesn't exist. You only know there is a wind because you can see your position over the ground is moving, there is absolutely no indication of a wind from the way the plane behaves, from its change of height, from its airspeed, or from the feeling of constant G load in the turn. If you are circling in or above cloud and can't see the ground you have no indication if there is a wind across the ground at all.

Any change of height is caused by the model pilot altering the elevator position to try and compensate for the odd shape circle the model is flying over the ground.

[biber]

That's EXACTLY right, HarryC :cool:!

Do you still go fullsize?

Seems to me that for killing the myth of downwind turn many modellers should take a ride with a fullsize glider to get a feel for what is happening and what is not when turning. Should be fun anyway. There can be learned a lot about flying by experiencing it on your own body sitting in the plane yourself and not only watching it from the ground.

biber (who still goes fullsize too besides modelling)

[HarryC]

That's EXACTLY right, HarryC :cool:!

Do you still go fullsize?

Seems to me that for killing the myth of downwind turn many modellers should take a ride with a fullsize glider to get a feel for what is happening and what is not when turning.

I only fly full-size power these days, means I can guarantee enough enough height to do aerobatics on every flight! I agree entirely, if every model flier took a ride in a glider they would learn so much that would dispel these ground based myths for good. It is so easy to understand when you are with the plane and can see the instruments, and so hard to understand when you have only ever stood on the ground.

H.

[biber]

Well, I only do gliding. But for glider aerobatics the needed height is easy to achieve too if using aerotows ;) . We have a H 101 Salto and a DG 1000 to do that funstuff with. Aerobatics are a very good opportunity to improve skills and have lots of fun at the same time. Further going upside down adds a new fresh look on mother earth :D :D .

biber

[zoomzoooie]

There is NO lose or gain in alt in a turn into or with the wind.
Myth BUSTED! :D

HOWEVER:

R/C pilots use perspective from the ground to try to judge what thier plane is doing. They try to compensate for what they perseive is happening and it is all relative to them.

Powered R/C pilots tend to reduce power too much, downwind to compensate for fast ground speed. Then into the wind they over power to compensate for very slow ground speed. The aircraft is trimmed for one power setting or another. If the trim is not adjusted the plane will either loose or gain alt with a change to the power setting. This is the gain or lost of alt they are talking about.

R/C glider pilots use attitude to adjust speed so in doing so climb or dive to reduce or gain speed to compensate for what is perceived. Again speed and trim dictate what the aircraft will do.

As speed is reduced, so is lift. As speed is increased, so is lift.
The aircraft will loose speed in a bank. The varing bank angle and diameter of the turn varies the speed. Stall speeds increase with G loading.

R/C aircraft fly fast and there is not always time to adjust trim.

If an R/C pilot does not try to compensate for ground speed (No attitude or power adustment) there will be little no change in alt.

R/C pilots that have full scale experience understand the flight the dynamics involved. Everyone should go get some. :)

ZZ

[Gary Warner]

2
3
4
1

[fhhuber506771]

Want to have some fun?

First, see the attached image.

Here's the exercise question:

From the steepest bank angle to the least bank angle, can you correctly identify the correct numbered order in the diagram for a turn-about-a-point ground reference maneuver?

Example post: (not necessarily correct - just an example)

(steepest bank )
3
2
1
4
(shallowest bank)


Only post the sequence of numbers you come up with. After a day or so lets discuss it, but for now, just contemplate the answer without comment.

Understanding the correct answer will go a long way in understanding the down-wind probelm.

This might be an eye opening experience or a no-brainer. Lets see.

Gary
--

3 is steepest,
1 is shallowest
2 and 4 should be EQUAL

[Gary Warner]

Two of the responders were right. The correct order is:

2
3
1
4

Congrats Pimmer and Kristianb :) .

(I knew the correct order, but I had a finger-fart when I posted :rolleyes: - so I got it wrong :p )

Here's some supporting text from this site (http://whitts.alioth.net/Pagea6Ground%20Reference.htm#TAaP_):

With a wind, the first turn will require more than a 90-degree angle of turn. The angle beyond 90-degrees is used to set up the crab required by the crosswind. The upwind turn will be gradually decreased so that when directly upwind the wings will be most nearly level. This is where the ground speed is slowest. The bank is gradually increased but crab must be held into the crosswind to keep the circle from flattening on the top. Once across the top of the circle, the bank must be gradually increased to make the circle conform to the added ground speed caused by the tail wind. The steepest bank is held when we are directly downwind. All banks are gradually increased and gradually decreased.


I'm still trying to re-find a site that diagramed the same kind of circle I posted and explained each cardinal point bank angle, relative to the other cardinal points.

Short on time today - got to get back to work.

Gary
--

[Sparky Paul]

The Tactical Mode Flight Director in the P3C ASW aircraft tells the pilot how to fly a perfect circle around a broadcasting sonobuoy, by moving the command bars on the ADI to indicate how much the bank angle must change to hold the constant radius desired for best signal reception..
"With a wind, the first turn will require more than a 90-degree angle of turn. The angle beyond 90-degrees is used to set up the crab required by the crosswind. The upwind turn will be gradually decreased so that when directly upwind the wings will be most nearly level. This is where the ground speed is slowest. The bank is gradually increased but crab must be held into the crosswind to keep the circle from flattening on the top. Once across the top of the circle, the bank must be gradually increased to make the circle conform to the added ground speed caused by the tail wind. The steepest bank is held when we are directly downwind. All banks are gradually increased and gradually decreased.
We worked this system out in the middle '60s for the Navy, which did the manuver at very low altitudes and slow airspeeds.

[HELModels]

Great! Forget the VFR flight training manual with all the cartoons for simple comprehension. The R/C pilot yanks and banks as needed to get the job done. Somewhere in the back of his mind is respect for the limitations that can bring him down. Those limitations are the boundaries of that planes flight envelope. Recognizing where the plane is within the envelope and knowing what movements to make or not make keep it flying. Some are smoother at it than others. If you measured the amount of glucose used by the brain of a skilled R/C pilot vs. the less skilled, the more skilled would actually be using significantly less glucose. To successfully recall all that is written in these forums about how to fly in a moment of imminent trouble(read stall) would require a very large amount of glucose.

I flew yesterday and used less glucose than the flight before, but with more complicated maneuvering. :p
The furthest thing from my mind was this forum, as it aint part of my planes flight envelope :D

[biber]

Now then tell me why 3 should be steeper than 1.
If the text you quoted is right (I think he actually is) that would mean the points 3 and 1 should be equal banked. Don't have the time yet to make a drawing to visualise the geometric relations but that will follow soon.
Yes, finally you got me to the point where I will sit down and take a pencil to figure it out. :confused:

biber

[Sparky Paul]

3. the plane is being moved out of the circular track. More bank needed.
1. the plane is being moved into the circular track. Less bank needed.. relative to still air.

[Gary Warner]

Now then tell me why 3 should be steeper than 1.
If the text you quoted is right (I think he actually is) that would mean the points 3 and 1 should be equal banked. Don't have the time yet to make a drawing to visualise the geometric relations but that will follow soon.
Yes, finally you got me to the point where I will sit down and take a pencil to figure it out. :confused:

biber

Hehe... Don't use 'too much glucose'.

I'd like to see that pencil work too. I've been told by a number of instructors that 3 and 1 are close to the same, but not equal bank angles. It has to do with setting up the anticipated amount of crab angle and flying through that point in the turn are varying crab angles. For the purpose of this discussion, I think we could accept that 3 and 1 are the same. It really gets down to splitting hairs.

The way I see it, when getting close to point 3, the heading change is over 90 degrees from point 2 (establishes the crab angle). And when getting close to point 1 the heading change is less than 90 degrees from point 4, also because of the crab angle. At two points in the turn there will be two identical bank angles. But I don’t think that they occur exactly on the cardinal points.

Don't feel bad. I always seemed to have trouble with turns about a point when in primary flight training. Somewhere along the line, I adopted the idea that point 3 (base leg) was the steepest turning point of the circle. I felt so strongly about it, that when the FAA examiner asked me which point was the steepest, I argued with him. I told him he was wrong. Then after about a minute into debating this with him, it hit me.

BTW, I WOULD NOT recommend arguing with an FAA examiner. It all worked out just fine. He didn't fail me. He told me afterwards that I was so 100% right about everything else that he was just glad to see that I was NOT perfect. He did make me demonstrate the turns about a point correctly. Great... :rolleyes: it was like flying a totally new maneuver for the first time with the FAA on board (and my ticket in his hands :o ).

Gary
--

[Gary Warner]

3. the plane is being moved out of the circular track. More bank needed.
1. the plane is being moved into the circular track. Less bank needed.. relative to still air.

I think I can visualize biber's logic. Step through this with me:

If points 1 and 3 were extended into strait flight (perpendicular to the wind) each would have the same amount of crab angle correction and have the same ground speed.

The correct (and same) crab angle offsets at points 1 and 3 makes the ground speeds equal at both points.

"Bank angle" and "rate of heading change" are interchangeable in level coordinated flight.

"Rate of heading change" would be identical at points 1 and 3 since ground speeds are the same.

Since points 1 and 3 would have the same ground speed, wouldn't the bank angles be the same (same rate of heading change)?

--

The problem with this logic is that "rate of heading change" differs a bit because entering point 3 requires more than 90 degree "heading change" from point 2 to establish the crab. Entering point 1 requires less than 90 degrees "heading change" to establish the crab from point 4. This makes points 2 to 3 a faster "rate of heading change" and points 4 and 1 a slower "rate of heading change" (remember I claim "bank angle" and "rate of heading change" are interchangeable in level coordinated flight).

It's the need to establish the crab that makes the bank angles different, not just the blown in or out of the circle problem.

Gary
--

[pimmer]

This is all wrong. The point was to hold a perfect circle. There is no "crab angle" involved. That would only be true if it was a racetrack pattern, with some part of the circle that had straight legs. There is not. 3 and 1 are NOT close to the same. 2 is the steepest because that is the part of the circle that has the greatest velocity downrange and would thus require the smallest radius of turn relative to the air. 3 follows because it has the next highest velocity downrange. And so on and so forth.

[Gary Warner]

...There is no "crab angle" involved...

In Turns About A Point the 'point' is a ground reference (say the center of your flying field) and the circle is perfectly round (theoretically) and centered over the 'point'. It's a maneuver with constant air speed and constant altitude (non-commercial version). Crab is a must except exactly at points 2 and 4. Flight Maneuvers 101.

Gary
--

[kristianb]

When I started rc-flying in the early seventies I soon learned that the model flies relative the wind, it doesnt know there is a wind blowing. After started with model helicopters I learned that this is not the complete truth, but I draw the wrong conclusion as described in my previous mail.
Thanks to everyone involved, this is a good discussion to increase our understanding!
To understand better I plotted circles as seen from ground respectively from the wind, for example from a balloon, see enclosed picture.

What I found out, not surprising, is that a circle that is round relative wind looks awful from the ground, but at least the model is comfortable!
If You passes over the model airfield in a balloon and someone is flying perfect circles relative the ground, the model have to perform a path relative the wind as on the lower picture (if I got it right). Now it gets very interesting, just from geometry and wind speed the model have to make a very tight turn with the wind, example (speeds etc) comes from my previous mail. Required radius is roughly half of circle radius. When flying slow, most of the drag is induced drag, and making a tight turn adds a lot of drag. This slows models requiring more elevator increasing induced drag even more (increase is exponential). Summary, to maintain speed needed power increase can be extremely substantial, and is definitely avoided by pilot since it makes it by feeling even harder to make a nice circle relative ground. So when making a nice downwind turn relative ground (for example on a scale contest) I would not be surprised if certain combinations of model, engine and ambition to fly in a scale like manner can not be managed even by the best pilot!
And that long I was correct in my first mail, when You loose speed it takes a long time to regain it!
Downwind turn is a pilot problem in the aspect that the understanding of all consequences are not widely spread, so in opposition to what my mother says, flying low and slow are not safer!
Best regards Kristian B

[biber]

Now it's time for a drawing. It's not perfect qaulity but I hope it's enough to show what I had in mind.

I assume the ground based pattern to be perfectly circle and constant groundspeed.

North should be top of the page, wind comes from east. The plane does its 360 starting at the very west point of the circle pattern, moving north and heading north with a slight east component to compensate the east wind (that's the crap angle into the wind).

I divided the circle pattern in 12 parts, each of them is same increment of angle and thus same increment of time since groundspeed should be constant.

To get the described ground based pattern the plane has to fly a pattern relative to the air as shown below. That's simply the circle with an overlayed linear movement against the wind to get it exactly compensated.

The time needed from one of the 13 (0-12) points to the next is as mentioned ever the same, the distance relative to air is not, neither is the airspeed. The longer the distance from one point to the next the higher is the speed.

The needed bank angle depends on the current airspeed and radius flown relative to the air.

The points that are called 3 and 1 in Gary Warners example are equivalent to the points 12 and 6 in my drawing (while 0 and 12 are the same points on ground).

Do you see the symmetry between point 6 and 12?

biber

[biber]

Oh, kris you where faster and produce the more beautiful pictures :eek: !

Very well done!

biber

[kristianb]

thanks for the compliment Biber!
quality of drawing is not important, seeing your picture gives me some comfort that maybee my thinking was not too bad when we managed to produce so similar drawings...
vbr Kristian

[Gary Warner]

I brought this Turns About A Point into the conversation to demonstrate the effects of down-wind turns when we try to fly with reference to the ground (like trying to avoid that tree at the approach end of the runway). The turn about a point maneuver is used to teach pilots how to understand the attitudes and headings in a traffic pattern. Each of the four quadrants of the turn can be directly applied to the four turns used in the pattern.

(lefthanded pattern)

The cross wind turn (4 to 1)
The downwind turn (1 to 2)
The base turn (2 to 3)
The final turn (3 to 4)

In training, the most importance is placed on the base and final turns. As it turns out, in real planes this is statistically the most dangerous part of the traffic pattern for inadvertent unusual attitudes (stall, spin, etc.). One might think that the base turn would be the most dangerous turn, but it's the final turn that gets more pilots in trouble.

There are several reasons for this. Even though the base turn is at a steeper bank angle, it's generally performed at a higher speed and altitude. This affords the pilot with many more options in the event an unusual attitude is entered. In real planes, much like in our models, the planes speed is ideally slowed down gradually for a landing. Each leg has a decreasing target airspeed. The final turn is where the plane is slower, lower and subject to in increased tendency to slip or skid.

These in themselves are good enough reasons to consider the base and final turns as more dangerous. But in model flying, we are much more likely to error in not following the Turns About A Point theory as well as other principals of flight.

I see it nearly everyday at the field. Everything is just fine on the downwind leg. The base turn starts out correctly, but in an attempt to 'fly' this rectangle pattern, many pilots roll out of the turn too soon. The plane begins to drift downwind. Now a correction is offered by the pilot. Usually it's to simply roll back towards the pattern (left for a left hand pattern). Now the plane is flying kind of at a 45 degree angle to the correct pattern 'line'. As this happens, ground speed is reduced. The plane is now farther away than anticipated and there for 'looks' lower on the horizon, suggesting an excessive loss of altitude. Too many pilots apply 'up' elevator to re-gain the appearance of additional altitude.

Now they have got the plane flying slower, kind of on the base leg and it's time to turn final. They begin the turn, apply the 'normal' amount of 'up' elevator used to turn and the speed falls off even further. Add some abrupt aileron input to induce a slip or skid and they are on the edge of a stall/spin.

Most of the time they make it back to the runway ok. But add increasing levels of error and model imperfections (like wing twist, CG too far back, etc.) and the results are sudden and bad.

To reduce the chances of getting 'out side the box', modelers could learn a few real-flight principals:

Airspeed is controlled with elevator.

Altitude is controlled with power.

Stalls without slip or skid are strait-ahead stalls.

Stalls with slip or skid are spins.

Ground reference maneuvers nearly always need to take crab into account if there is any wind.

Learn and understand the Turns About A Point principals.

Learn that the Angle Of Attack (AOA) decides when a plane's wing will stall - not the airspeed, weight, bank angle or whatever. Though they do effect the point at which a stall is entered, ultimately it's an excessive AOA that stalls a wing.

Good flying all...

Gary
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[Gary Warner]

Biber,

I assume the ground based pattern to be perfectly circle and constant groundspeed.

That should be constant airspeed, not ground speed. Does this change your drawing?

Gary
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[biber]

Gary, I'm very well along with your last posting. Especially the emphasis on the importance of the AoA. But one should keep in mind that the AoA directly depends on weight, airspeed and bank.

Kris, how did you generate that nice awful patterns drawing?

biber

[biber]

Biber,



That should be constant airspeed, not ground speed. Does this change your drawing?

Gary
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Yes, it does, but does not destroy the symmetry I mentioned to show that the points "Gary3" and "Gary1" are equal banked.

biber

[Gary Warner]

Yes, it does, but does not destroy the symmetry I mentioned to show that the points "Gary3" and "Gary1" are equal banked.

biber

That's fair enough. We can call points 1 and 3 as equlal in bank angle. Good work with the drawing.

Gary
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