May 03, 2014, 08:53 PM Registered User United States, OH, Bradford Joined Jun 2005 3,933 Posts The plane's drift due to the wind-induced component is CONSTANT, in both direction and magnitude, hence, exactly zero acceleration due to the wind. However, there is an acceleration going on. The sideways component of lift due to the plane's bank angle is continuously pulling the airplane to the side, which is what pulls the airplane around the turn, balancing centripetal force. This acceleration around the turn is exactly the same regardless of whether the plane is encountering wind or is in perfectly still air. The downwind turn "problem" is the result of a misleading visual cue, due to the pilot observing the plane's ground speed, while the plane flies in reference to its airspeed. A properly trimmed free-flight model has no problems with downwind turns. Last edited by Don Stackhouse; May 03, 2014 at 09:01 PM.
May 03, 2014, 09:44 PM
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Quote:
 Originally Posted by eflightray What, no body here flown control line, on calm days and windy days.
That's a different kind of circle (not round relative to airmass reference frame)
May 03, 2014, 10:08 PM
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Quote:
 Originally Posted by Don Stackhouse The downwind turn "problem" is the result of a misleading visual cue, due to the pilot observing the plane's ground speed, while the plane flies in reference to its airspeed. A properly trimmed free-flight model has no problems with downwind turns.
You are absolutely right. The problem for some people to understand is, as you stated, the "visual clue". This is similar to the "sound clue" with the doppler effect.

But, it seems more difficult, in this case, for someones to understand the value of reasoning (putting themselves) in a different referential (i.e. ground v.s. airframe).

Once again, just do the equilibrium of an aircraft in a steady constant radius turn (aircraft referential) and add a "referential drift" component. You either have your initial referential "drifting" with the aircraft still making its perfectly balanced turn; or, if you are in the "drifting component referential" you will have your aircraft motion and equilibrium in this "drift referential": in our case the ground.

NOTE: the "drifting referential" describes a perfect steady air mass moving parallel to the "ground referential". As soon as you introduce gusts, updraft/downdraft, turbulences it becomes much more complicated. But with the same "referential frame" approach may be explained/solved. In the simple case of a steady air mass motion (perfect drift) it is very easy to understand and quantify the equilibrium at the aircraft referential border (envelop) within the drift referential (ground): it is a steady motion (no acceleration) of constant value (constant speed) in a fixed direction (fixed angle in the ground plane reference). All the other potential variables are constant (temperature, pressure, other speed components, etc).
 May 04, 2014, 12:02 AM Registered User Toowoomba, QLD, AUSTRALIA Joined Jan 2008 613 Posts I wonder if the guy floating along in the balloon has the same problem with his RC car on the ground seemingly speeding up and slowing down when driving circuits, thus requiring braking on the downground turn and accelerating on the upground turn to maintain constant groundspeed?
May 04, 2014, 03:02 AM
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Quote:
 Originally Posted by bjr_93tz I wonder if the guy floating along in the balloon has the same problem with his RC car on the ground seemingly speeding up and slowing down when driving circuits, thus requiring braking on the downground turn and accelerating on the upground turn to maintain constant groundspeed?
You, as the observer, are going to have a different impression/feel depending on your altitude above the ground as you are introducing an angular factor.

Furthermore, you will have a visual range problem rapidly since you (the observer) are moving and the item you want to analyze is not (the car) in you reference frame. Once again amplified (or minimized) by your altitude introducing an angular factor.

But, yes: when the car moves in the same direction than your ballon you will "have the impression" that your car slows down and when the car moves in the opposite direction you will have the impression that it is speeding. But this "impression/feeling" is linked to your altitude above the circuit (ground) because of the introduction of the angular factor. This is the exact same thing than when doing the last down wind turn before landing all other things (2D motion v.s. 3D motion) aside.
 May 04, 2014, 07:19 AM Registered User Zurich Joined Apr 2006 3,537 Posts magic or illusion? Regardless of how one may interpret a circling Redtail's cyclic wingplanform, extension and sweepback changes, I still have video proof of my "strange" highly inertially sensitive rotorcraft [invention/discovery] displaying similar and easily seen physical changes while circling FF in a wind. And many tests showing the FF circle drift being approx. 1/3 the wind speed. [its RC flt seems like steering through an autopilot.] Of course one can use inertial guidance -- as I presume the Redtails do, "feeling" airspeed [onflow-vector] changes relative to their flight -- to get similar results. In either case some relatively "fixed" inertial frame is used. I again apologize if it seems I am being stupid, ignorant or making stuff up. But the video did show that, and inertial guidance or a good pilot can also produce similar results > many years ago I read of a test re: "seat-of-the-pants" flying in in which the pilot of a fullsize helicopter had his butt thoroughly Novocained .... and he had great difficulty flying! Lee
May 04, 2014, 10:32 AM
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Quote:
 Originally Posted by aeronaut999 (I'm talking about constant-radius turns around a balloon drifting freely with the airmass, not around a fixed point on the ground as per those silly, non-useful ground reference maneuvers required by the FAA for private and commercial pilot's license tests...)Steve
"those silly, non-useful ground reference maneuvers required by the FAA for private and commercial pilot's license tests" come in pretty handy when turning for base and final approach to the runway... At least in full scale flying.
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 May 04, 2014, 11:52 AM Registered User Joined Oct 2004 2,733 Posts Hmm, no, it would be better to turn for a fixed radius than over a fixed ground point from a safety standpoint. That's why there's bank angle markers on the attitude indicator after all. If you drift a little away from the runway is no big deal compared to starting the turn too slow.
May 04, 2014, 11:59 AM
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Quote:
 Originally Posted by bjr_93tz I wonder if the guy floating along in the balloon has the same problem with his RC car on the ground seemingly speeding up and slowing down when driving circuits, thus requiring braking on the downground turn and accelerating on the upground turn to maintain constant groundspeed?
Nicely put!
 May 04, 2014, 12:08 PM Registered User The Willamette Valley, Oregon Joined Dec 2008 1,101 Posts (duplicate)
May 04, 2014, 12:10 PM
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Perhaps-- or just start you turn a little early/ late and don't go for perfectly equally-radiused corners on each corner of the pattern-- personally, I feel like I'm doing something wrong if I'm finding myself banking up super-steep whenever the wind is at my tail. To each his own; there's nothing intrinsically wrong with banking steeply in a tailwind as long as you don't fall prey to the "high ground speed must equal high airspeed so I'll pull the stick back" illusion.

I think the usefulness and training efficacy of at least some of these maneuvers is open for debate. Anyway, the basic physics of flying in a steady wind shouldn't be a matter of debate...

Quote:
 Originally Posted by kenh3497 "those silly, non-useful ground reference maneuvers required by the FAA for private and commercial pilot's license tests" come in pretty handy when turning for base and final approach to the runway... At least in full scale flying.
May 04, 2014, 01:19 PM
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Quote:
 Originally Posted by aeronaut999 Perhaps-- or just start you turn a little early/ late and don't go for perfectly equally-radiused corners on each corner of the pattern-- personally, I feel like I'm doing something wrong if I'm finding myself banking up super-steep whenever the wind is at my tail. To each his own; there's nothing intrinsically wrong with banking steeply in a tailwind as long as you don't fall prey to the "high ground speed must equal high airspeed so I'll pull the stick back" illusion.
I just couldn't stand those long drawn out (Cessna type) pattern approaches in my short winged RV6. I flew downwind close to the runway, and did a quick desending, and somewhat "steep" arc. Much less time for some wayward plane to get into the pattern (as one looks for on base), and the fact that the RV, with a "braking" constant speed prop...........needed a quick descent to maintain airspeed. Never looked at the attitude indicator, just airspeed. Landings were either steep, or with some power. Steep approaches bug some passengers.
May 05, 2014, 07:32 AM
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The wind velocity gradient might somewhat explain the diverese viewpoints, concepts and experiences. and that effect could be multi-fold additive or even multiplicative, including:

1. banking the wings of a light polyhedralled-wing FF or 2ch model on the upwind/crosswind part of the circle [more force on the upper/outside wingtip], this excess bank resulting in altitude loss on the downwind side of the circle, reduced bank on the downwind/crosswind side leading to excessive climb on the upwind side, then repeating

2. disk-like chunk of air [incl. plane and its affected air-momentum system, incl. vortices, etc.] through/in which model circles being tipped, as is a section of a smoke or thermal plume

3. precession of the disk [or its elements]

These effects might never be noticed or significantly exist if one flies a large/r fast/er plane while "centering the ball" [or having it electronically or otherwise centered, etc.] at greater speeds, circle radii and heights, or by a manned pendulum-stabilized hangglider, for ex.

I might also argue that the velocity-gradient rotation [tilting] is an acceleration which thus makes the moving-air/wind a non-inertial frame-of-reference somewhat inappropriate in the above case. And maybe inconsequential otherwise ....?

http://en.wikipedia.org/wiki/Non-ine...eference_frame

But it still doesn't really explain to me why these few special [but differing from each other!] designs of mine like to point into the wind in any flight condition, find lift and avoid sink

Lee

P.S. the tripe bounced ping-pong ball model* [representing a theoretical statistical element of a "body at rest" in a gravitational field] is a
not-totally-unreasonable attempt to "explain" the weight-force .... which should come from an acceleration, as would occur also paddling the ball away in free-space [i.e., otherwise the body doesn't seem to be accelerated -- as I understand velocity-change -- as it rests on a scale on Earth].

*incl. likely "do-si-do" effect as the up/down elements get diverted by each other, tending to sideways flatten the body! [approx. like jumping up/down on a sand-filled waterbed]
Dunno, never been a ping-pong ball ...

# Images

 May 05, 2014, 09:47 AM Wreckreational User Joined Sep 2007 211 Posts I had a brief thought about thermal feed at low levels and the effects on airplanes with a low level of directional stability, but I can’t get past that you said they point into the wind. I pulled my questions so I wouldn’t waste your time. Last edited by Steve Anderson; May 05, 2014 at 09:06 PM.
May 05, 2014, 12:47 PM
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Quote:
 1. banking the wings of a light polyhedralled-wing FF or 2ch model on the upwind/crosswind part of the circle [more force on the upper/outside wingtip], this excess bank resulting in altitude loss on the downwind side of the circle, reduced bank on the downwind/crosswind side leading to excessive climb on the upwind side, then repeating
In my experience I've never seen this. But then I know better than to rely on the apparent speed as seen from the ground. Instead my glider/sailplane flying has taught me to trust the plane to return to the trimmed airspeed on it's own and I let the model fly that way regardless of how slow or fast it looks to me on the ground.

As I mentioned in my post in the Prandtl thread I've often flown lighter rubber powered free flight models in wind conditions that exceed the flying speed of the model. The wings and nose totally ignore the wind and the bank angle, such as it is, does not change. Neither is there an altitude loss and gain with each circle in any sort of cyclic manner.

What DOES occur is that I see this sort of stuff when the model interfaces with a thermal. In that case until it's centered well into the core I will see the nose rise and fall and the wings increase and decrease the bank angle in a cyclic manner as it flies through the transition zone between the outer air and the core. It'll keep this up until, hopefully, it centers itself due to the changes in the glide circle caused by the nose and banking changes or it gets kicked out of the thermal zone by the same reactions to the changing airspeed around the thermal.

You make mention of bird altering their wings and tail as they circle. I've seldom or never seen bird steadily circling other than when hunting or thermaling. And mostly it seems to be when they are thermaling. As such if they are working small and patchy lift then they too are likely seeing this same effect as their circle carries them through the somewhat more stable core and into the turbulent area between the core and surrounding air.

And I feel that this points out a very valid problem in measuring this sort of stuff. On windy days there's a lot of turbulence in the air. And at model altitudes we can't discount the wind shear and rolling turbulence. We also typically get vertical turbulence in the form of thermals that pass through the area.

Having missed my fair share of thermals over the years or purposely avoiding them for the sake of test trim flights in a breeze I have had a good chance to view my models in relatively calm/steady wind. Again I stress that if they fly in a way that suggests no thermal help or downdraft hindrance that they simply do not show any signs at all of this sort of upwind vs downwind altitude variation you are suggesting.

Quote:
 [/But it still doesn't really explain to me why these few special [but differing from each other!] designs of mine like to point into the wind in any flight condition, find lift and avoid sink [head-scratching image here]
Odd things can happen with some models when trimmed in a certain way. I won't even try to suggest any options for your situation since I've not seen your models fly. Instead I'll offer one story that occurred to me.

It was a free flight A-2 class towline glider. We were flying on a calm morning with almost no breeze. I put up my first flight without much hope but then back then I didn't really know how to find thermals anyway. So a nice calm "dead air" flight seemed like a nice compromise from later when I'd likely find all the sink.... The model came off the line and proceeded to wander all over the field turning both into and against the trimmed glide circle. At first I was all concerned that the model was broken or the dew had "softened" the doped tissue and the wing was warping. But then I realized that it WAS STAYING UP! ! ! !

Turns out that the rudder turn working with the cross warping of the wing was creating a self hunting flight that would lift and turn tighter in lift and dive and straighten in sink. The model wandered all over the field willy nilly but I got my first 5 maxes and a 5:13 on my 6 minute attempt.

The moral here is that model trim can react in odd ways with the turbulence in the air on windy days to do odd things. If you find your models behave oddly it's likely due to factors which you don't yet understand or know about. For me and my mystery maxes it was about 20 to 25 years later when I had learned enough to piece this apparent "miracle" out and be able to explain it.
Last edited by BMatthews; May 05, 2014 at 12:53 PM.