physics of the downwind turn
 

Okay, although this is mainly a physics question, it has an aerodynamic aspect to it so I will post it here.
I believe the argument that the "downwind turn" phenomona is bogus but am always troubled by the inertia issue that is raised in the hypothetical case where a model is going the same airspeed as a headwind, thus has no groundspeed. After a 180 turn it will be going the same airspeed as before but will now have a groundspeed twice the airspeed (or it will have fallen out of the sky...)
Its inertia or momentum relative to the earth's frame of reference has changed without question. The forces involved have to be aerodynamically generated during the turn but it is not obvious to me how this happens.
Allan

RE: physics of the downwind turn
 

Oh boy, this has potential:eek:. I'll try to put the fire out before it starts.

It's an AIR plane. The frame of reference that matters is the AIR. That is the frame of reference that is moving with the air. Unless it hits you or something else on the ground, the ground frame of reference doesn't affect the plane while it's flying.

The inertia that you talk about is not an absolute quantity. If you are on a bike going 10 mph west and a park flyer doing 15 mph west overtakes you and hits you in the back, you do not feel the effects of 15 mph worth of inertia. You feel the effects of the difference. 5 mph. Make sense?

RE: physics of the downwind turn
 

I am going to stay out of this thread because I know that the ideas about downwind turns will just blow me away.:D

RE: physics of the downwind turn
 

I know what you're describing, but don't know how to respond in kind. Empirically, you should fly my Cub in your described scenario to see what happens. As you turn from upwind , through the one-eighty your vector relative to wind direction is constantly changing, as is the attitude (and angle of attack and the wind direction over the wing). This requires an application of power to maintain altitude through the turn, otherwise there is an energy loss from the change in lift during the turn, and the plane loses altitude. Either way, through the acceleration due to gravity or due to the power increase, lift is maintained about the same even though the groundspeed is changing significantly; the overall airspeed over the wing and the lift component is remaining pretty much constant. I can't explain it in the terms I'd like. What I see flying the Cub is a tendency for the bank to increase as I turn cross wind, with the attendent drop in altitude unless I increase power. If I keep power the same, as I complete the turn to downwind, I am in a slight dive, accelerating to maintain lift that I lost turning out of the wind. If I have applied power in the turn, I maintain the altitude and accelerate as I roll out downwind from the increase in power. Either way, once I match the airspeed I had upwind, I have to either level out from the dive or reduce power, or I am going faster, in terms of both airspeed and groundspeed, than I was during upwind flight.

I have actually managed to fly my Cub at almost zero groundspeed (I had about 15 kts headwind). I's kind of scary, you want to think it's going to stall and crash, but as long as you stay directly into the wind, it was okay. Once I got the least bit crosswind, though, it started getting interesting. The big wing on the Cub is really sensitive to airflow, and banking it want to flip it over when the wind gets under the upwind wing.

RE: physics of the downwind turn
 

Well, here we go. round -n- round. My answer is simple, to the point, and correct in all cases. All the other stuff is about pilots perception and what they think is happening to the plane because of the wind, is actually the different way they fly the plane in reaction to the wind. End of story.

And now...

:eek::eek::eek:

...I'm unsubscribing. I'll check back in a couple weeks once the hot-air front has passed. :D

RE: physics of the downwind turn
 

Generally speaking, there should be no difference in altitude loss between a downwind turn and an upwind turn, but the wind velocity versus altitude gradient at only a few feet altitude can cause some interesting effects. For example, as soon as you bank into a turn while flying upwind, the low wing will be flying in lower velocity air, causing it to lose lift and, if it is anywhere near stall, also develop more drag, since its true angle of attack will also be higher than that of the high wing. This will tend cause the low wing to drop further, steepening the bank angle, resulting in the nose dropping. This is the only reason I can see for the widely held belief that downwind turns result in dramatically more altitude loss than do upwind turns.

Years ago, when I flew freeflight quite a bit, I noticed that a model that was trimmed for slow circling would tend to fly straight downwind after it reached a sufficiently low altitude at close to stall airspeed. I think that the reason for this might be if the model tries to turn a bit, the low wing will be at a slightly higher airspeed, since the wind velocity will become lower as the ground approaches. Let's say that the model is flying at an airspeed of 15 MPH with a 15 MPH tailwind, with a 30 MPH groundspeed. If a wing dips a sufficiently to get into 14 MPH wind, that wing will have an airspeed of 16 MPH, tending to increase its lift and causing it to raise slightly. The high wing may, similarly, be in 14 MPH wind, tending it to lose lift, and lower slightly. The reality is probably far more complicated, what with the effect of ground-induced turbulence burbles and all.

RE: physics of the downwind turn
 

There's 37 threads in Aerodynamics alone which mention "downwind".
I recommend ALL of them be read before this old chestnut expands any more.... :(

RE: physics of the downwind turn
 

My free flight models do an upwind and downwind turn about once every 10 seconds in a continous circle. They just fly nice and even the whole way around other than LOOKING like they slow to a stop while nose first into the stronger winds.

My RC gliders do the same thing as long as I can avoid trying to "help" them. If I trim for a nice turn and just hold it the glider turns 'round and 'round all pretty and on it's own with nary a nose up or down cyclic response in the lot.

But I too wondered about the mass inertia thing and concluded that it isn't a factor because of the above.

RE: physics of the downwind turn
 

once plane is in air -it moves with air
Just like a dog running up and down the aisle of a train as it travels 60 mph

.
To someone watching from the ground the dog appears to running at 60 mph and having the ability to instantly reverse directions!
To someone driving a car at 60 -alongside the train -- the dog is simply doing about 5 mph
It's all in the relative vantage point .

Our need to orient with OUR fixed spot on the earth causes ALL of the conjecture

RE: physics of the downwind turn
 

Dick is absolutely right on this one. If you have ever been on one of the moving sidewalks common at many airports, it is obvious that turning around and walking the other direction causes no acceleration force on your body, but the visual sensation is impressive. The airplane is moving relative to the air mass through which it is traveling. All accelerations are related to that mass, and the appearance of motion relative to another reference (such as the earth) is irrelevant.

RE: physics of the downwind turn
 

I am certainly glad to see this nipped in the bud before it detracts from more important topics, like AMA, which airplane should I buy, etc. Obviously, the way to prevent the effects of the dreaded downwind turn is...(wait for it)...don't turn! Problem solved. A friend of mine once took off in his full-size Cub in about a thirty-plus knot ground level breeze, climbed to several hundred feet, throttled back, and backed up the length of the runway, chopped the throttle, did a vertical approach and landed, completing a flight without making a turn. Also, once in a spirited discussion about same topic, I half-kiddingly posed the hypothetical case thusly: if you were in a motorboat going up a river and made a turn and started going downstream, would the boat sink? The sputtering lasted for several minutes! Jim

RE: physics of the downwind turn
 

how wide was the river ?

RE: physics of the downwind turn
 

...trouble maker.... :D

RE: physics of the downwind turn
 

Let me restate my question with a little more specificity.
First, with regard to the PERCEPTION issues, they are fascinating and powerful. Indeed they are so compelling that it is hard to accept the idea that they are only an illusion and many flyers will argue forever that there is a real effect. However, I have convinced my head (not yet my gut) that it is only an illusion - and I am asking about the APPARENT physics/momentum "problem" raised by the example I gave.
I have no problem talking about the perception issues if people want to, but I still would like to see my question answered.
-------
Yes, I did search this site (and others) before raising this issue but none of the posts comes close to satisfying my specific query. And, I feel no need to apologize for raising this issue, in spite of all the eye-rolling, heavy exhaling that it was certain to create .
-------
Of course there are different frames of reference one can select, the airplane-air reference and the airplane-earth reference, to name two. My question is asked from the airplane-earth reference frame, which is perfectly valid. After all, gravity is a major part of this situation and it is relative to the earth, not the air mass.
Newton's laws should be totally resolvable from ANY of the reference frames anyway. Furthermore, since we are not dealing with relativistic masses and velocities, we don't need Einstein to resolve this matter. Newton will do just fine. So... inertia IS an issue, pretty much an ABSOLUTE issue in fact.
I like the dog-on-the-train analogy. Even with it, one can resolve the inertia/momentum issue easily. Any change of speed by the dog has to be translated to the train and thus to the earth, so there is no momentum "problem". For example, the dog (running forward) decides to stop. His momentum is transferred to the train and it speeds up a tiny bit. He starts running back and the train slows down. The momentum of the train relative to the earth is affected and, if the engineer hits the brakes, this force is transmitted to the track and then to the earth. Ultimately the earth "feels" the dog's inertial effect.
Once again, Newton's laws should be totally resolvable from ANY of the reference frames anyway. Even the airplane-airmass-earth-traintrack-train-dog reference frame (the "airplane-dog" frame for short) should be completely resolvable as to ANY momentum changes.
I guess the air mass is the train and the dog the airplane. However the plane is not coming to a stop but going through a curved path while maintaining airspeed. This introduces angular momentum into the picture and it gets even more intellectually muddy for me, which is why I am looking for help.

Regards,
Allan

RE: physics of the downwind turn
 

I've had similar thoughts. Don't have much time these days to devote to thinking it through, however.

Getting the same clear answer in both frames of reference is essential for me to be happy with my own understanding. Something is out of whack in the all the explanations until the answers are correct in both frames of reference. Can't say I'm there yet....

Impatience with the question isn't an answer.

RE: physics of the downwind turn
 

The momentum picture is exactly the same with wind and no wind.

If Momentum is p and Velocity is v, then:

No wind case you start with v make a 180 degree turn and you're going -v. Momentum has gone from p to -p and Deltap=-2p.

For the wind case looking from the ground you go from 2p to zero so Deltap=-2p. In the wind frame of reference you go from p to -p and DeltaP=-2p.

I think its impressive how fast you can turn a balsa wing plane without breaking it. There is a large force (F=DeltaP/DeltaT) that must be mostly elastic to turn a plane that is going 100 mph and have it come out 180 degrees with almost the same energy, WOW.

Carl

RE: physics of the downwind turn
 

Bravo! :D:D

RE: physics of the downwind turn
 

Ok, I didn't read all 37 threads, but I do have a question. I fully understand the the airplane moves with the air mass, but it seems to me that if you are flying crosswind you are actually moving slightly slower than the airmass by some (probably negligible in the bigger picture) amount due to drag, etc? I'm not arguing upwind/downwind, just tryign to verify my assumptions.

Duke

RE: physics of the downwind turn
 

The speed when moving crosswind is the "dead air" speed.
The plane OTOH is drifting with the wind.
As mentioned in one of the threads, the Tactical Mode Flight Director developed for the P3 Orion to get the plane to fly a true circle around a sonobuoy computed bank angles to alter the flight path to allow for the wind's different effects on the plane.
The cross-wind angle turns out to be less than the dead air angle would be for the same radius of turn, due to the wind carryng the plane.
The upwind angle is the lowest, while the downwind is steepest, when computing the angle needed to maintain a constant radius from the signal source.
It was up to the pilot to keep the airspeed constant thru the turn.

RE: physics of the downwind turn
 

All of the flying questions would be instantly answered for anyone if they did this:
stand on the ground and fly a controline model in the wind- you can easily detect speed changes upwind downwind crosswind.
Now
step onto a balloon and fly the model while the balloon drifts
you could flyupwind /downwind cross wind - and never detect which was which.


PS ---don't -step- off'n- the -balloon

RE: physics of the downwind turn
 

Allan:

Do you know anything about resultant vectors? If you do it'll help you wrap your brain around the issue.

Let's take a scenario: You are shrunken to model size and you are riding in your plane. Your airspeed is 40 mph. You turn in a constant bank at a constant speed. What's your airspeed? It's 40 mph of course. Now assume it's a breezy day and you execute the same manuveur. What's your airspeed? It's 40 mph. There's no change from the airplane's perspective.

Now stand on the ground and do the same thing. What is your airspeed? It's still 40 mph, but the model will be drifting away from you at the speed and direction of the wind.

The reason for the downwind turn phenomina is perception. You as a ground based pilot can not tell what the aircraft's frame of reference is. You fly it from your perspective. You make corrections to keep the plane in your "box" and not drift downwind. Hence the plane appears to be performing very differently. In reality, though, it is all in response to your inputs.

The inertial picture isn't different because of the wind. It is different because of the pilot. I can express for you the Newtonian equations from either reference if you wish, and show that they resolve the same. I'm lazy and would rather not. Hopefully, this explanation will suffice.

RE: physics of the downwind turn
 

Actually, Siefring had given some math in his post which demonstrates that the change in momentum (delta P) is the same regardless of the wind. Thanks, by the way.
When I initially searched this subject, I found the most compelling arguments on glider sites and free flight sites, which makes sense because with free flight one cannot control the plane with a radio and the glider guys like to find a thermal and trim their plane to circle in it. In both cases, the altitude remains constant regardless of the wind. What I find interesting on the PERCEPTION issue is that these guys are flying HIGH and there are not the usual visual references that I might be dealing with on my little 1/8 scale Fokker E.V running a few yards up. I think this lack of reference makes the drifting with the wind less noticable and therefore the pilot would feel less need to "correct". We have all seen this effect with the full moon which looks enormous when it is low but tiny when it is high in the sky.
Where the vector analysis has helped me is in resolving the forces on the plane in a 180 turn, where the plane ends up going the opposite direction at the same speed (minus just a bit due to losses). This is a different situation from the dog in the train which starts and stops. It is more like a bobsled going around a turn on a very wide train. The fore-aft forces resolve the same as the dog but the plane, like the bobsled, has sideways vectors to deal with (fortunately they zero out after 180 degrees).
Thanks everybody, for bearing with this.
Allan

RE: physics of the downwind turn
 

Let me throw this in...

I see many pilots confusing the effects of constant wind and gusts. Constant "clean" wind is just that - it caries your plane with it and the plane doesn't know the difference. It flies absolutely normal in it and if something goes wrong, it's definitelly your fault and not wind's.

Now add +/- 10mph gusts to let's say 30mph constant wind. Let's say most of the gusts will happen in the same direction as your constant wind. If your plane is pointed into the direction of the wind, any gusts will get to both wings at the same time and it kicks you around up/down forward/back since your average 60mph airspeed fluctuates between 50 and 70 mph. No big deal overall.

You now start turning and point the plane 90 degrees to the average wind direction. Some gusts will now affect one wing before the other and really give you hard time. Same goes for any other direction gusts. Most people will blame this on the "downwind turn", "the wind took it" etc.

From my personal experience, I love flying in 30mph CLEAN wind. I hate flying in gusty 10 mph wind.


P.

RE: physics of the downwind turn
 

Paf, you'd be quite unhappy on the slope!
Particularly when paralleling the slope, one wing can be in the slope lift, with a gust coming along and stalling the other wing completely!
90 degree banks in an instant..
Keeps one's attention.. :)
Sloping should be a requirement to get a pilot's license.
You'll learn more about flying there than anywhere else, and your flight times are limited only your endurance, or the plane's batteries.
Highly recommended to everyone.

RE: physics of the downwind turn
 

Gusts have nothing to do with the idea of "downwind turn", and everything to do with the relative wind across the wing. No matter if the wind is 30 gusting to 40 or 0 gusting to 10. Effects are the same.

Add turbulent air due to the fact that models fly fairly low to the ground, and flying fields have trees and buildings near them, and you can throw a lot of discussions out the window. Trying to navigate across very choppy seas gives you all kinds of problems, and when you add a strong current to boot, it's enough that you have to constantly add correction after correction. Now add the fact that the modeler is outside of the airplane, on a fixed platform trying to control a model in a constantly-varying inertial field that's in no way connected to the modeler's fixed platform, and you now have the basis for a lot of incorrect conclusions based on too little actual knowledge of the systems involved.

The real answer? Just fly the plane!