The dreaded down-wind turn, fact or fiction? - Page 17 - RC Groups
 Oct 12, 2012, 11:59 AM Tossing planes into the snow Here is an interesting experiment to try, which basically amounts to temporarily turning your plane into a free-flight model. First, it is assumed you have the plane trimmed and CG is set so it will glide straight, without porpoising, and hands-free on a calm day. Now it's up there surfing the wind, and it's taking very little input from you to keep it there. Put on a bunch of clicks of aileron or rudder trim to get it turning, and maybe some up-elevator trim to compensate. Now let it circle without any input from you. It should look just like a hawk circling, and it should not gain or lose altitude depending on the wind direction. It should be making perfect circles from the hawk's perspective, but they will look like ovals from the ground, as the center of the circle gradually moves downwind.
 Oct 14, 2012, 10:53 PM Foxman I suppose the ultimate test would be to fly a control line model with on board data recording equipment for the airspeed. This would determine whether the airspeed changes going from a downwind to an upwind situation. It would obviously be flying from a fixed point on the ground. I remember from my control line days the lines blowing back a bit more as it came into wind. Jim
 Oct 15, 2012, 12:33 PM Tossing planes into the snow I would be more tempted to put the air speed indicator on the free flight model. If it showed the airspeed to be steady, that would go a long ways towards proving that downwind stalls are caused by the pilot and not the wind. The control line example introduces some new forces and factors that do not occur in regular aviation. It's an interesting idea, but we could end up comparing apples and oranges.
 Oct 15, 2012, 09:31 PM You can't take the sky from me Or you could just go up on a really windy day in a full scale airplane and fly steep turns, one set starting into the wind and the other starting with the wind. You will find that the air speed is identical for both. I know it works because I used to do this for students to get them past this belief that if they turn into the wind they will get a boost in lift.
 Oct 16, 2012, 12:05 PM Tossing planes into the snow That confirms my theory, and I am assuming the pilot is performing reasonably good turns. What about the times when he does not do a proper turn, and the airspeed actually does drop, causing the plane to feel mushy or stall? How do you convince him that it was the pilot and not the wind that caused it?
Oct 16, 2012, 01:55 PM
An itch?. Scratch build.
Quote:
 Originally Posted by Jovanx That confirms my theory, and I am assuming the pilot is performing reasonably good turns. What about the times when he does not do a proper turn, and the airspeed actually does drop, causing the plane to feel mushy or stall? How do you convince him that it was the pilot and not the wind that caused it?
You don't.

An RC pilot will always try to convince themselves and others that it was,... the planes fault, the wind, the sun, too cloudy, wind sheer, a downdraft, etc etc.

We do not make mistakes, and we most definitely do not listen to others who say we made a mistake.
 Oct 16, 2012, 08:51 PM You can't take the sky from me My favorite is when they stall spin and claim interference, or brown out (lock out) now, because they had full opposite aileron and full up elevator and it just wouldn't respond.
 Oct 17, 2012, 10:47 AM Foxman The problem with comparing full size and models is that the modeller is fixed to the ground and has no instrumentation or seat of the pants feelings to help him/her make a nice coordinated turn. They only have their eyesight and experience to tell them what is happening. I have flown full size gliders, and many other powered aircraft and you don't notice the wind at all except that you may have got blown downwind a bit during turns. You also need to allow for it if you fly cross country. The control line test would actually work but I don't know what the results would be. Jim
 Oct 17, 2012, 11:36 AM Tossing planes into the snow In some ways, a control line model is more like a ball on a string than an airplane. It doesn't need rudder or ailerons to turn, and if it had enough power, it wouldn't even need wings. Centrifugal force would be enough to make it "fly". If we put an air speed indicator on it and determined that it was or wasn't constant, how would we interpret those results?
 Oct 17, 2012, 11:45 AM You can't take the sky from me Again the comparison to full scale is very relevant, as it shows it is totally a perception problem and not an aerodynamic problem. When you understand what is really going on it will make it easier to handle all situations.
Oct 18, 2012, 09:44 AM
Bending gear since 1984
Quote:
 Originally Posted by cfircav8r The whole inertia thing is rampant in full scale pilots as well.
Inertia can play a part if your aircraft is unstable (fighters and aerobatics planes are intentionally unstable) and travelling at speed. If the aircraft can be manouvered so rapidly that the inertia carries the plane on a vector which makes the lifting surface completely un-aerodynamic(presenting the top, bottom or tip of a wing to the airflow). You can see this regulary at airshows in the form of aerobatics. The flight surfaces are stalled for a large part of the display. Only inertia, prop and oversized motor are keeping the plane aloft during these manouvres. So long as the plane maintains sufficient kinetic or potential energy and can regain enough forward motion or return airflow to the flight surfaces, it can recover and return to normal flight.
Inertia is also important when timing a flare in something the weight of a moving van.

To identify inertia as a factor (or illiminate it ) do the manuoevre while trailing smoke. If the smoke surrounds the plane or overtakes it and travels in a different direction, inertia may be creating enough of a difference in the motion of the plane relative to the airflow that it stalls. If the trail neatly traces the aircrafts path then inertia is not having any effect.

In my short 2 crash RC experience, inertia may play a part in induced pilot error. My first foam cessna would turn on a dime and if i wanted to stall it, I'd go through 2 or 3 batteries before I managed. When i moved to my heavy low wing warbird, i would often overshoot the turn onto final as the plane would continue to travel base even through the nose was already pointing at me. The natural response to this visual input was "pull harder" resulting in more of the speed bleeding off... bam! tip stall, onto its back and into the dirt - airspeed, airspeed, airspeed. This could have been avoided by starting my turns sooner and keeping them smoother.

I've not yet stalled on a downwind turn.

I see few people reducing speed after doing a highspeed upwind show off pass before turning crosswind. I guess that may have a bit to do with the phenomenon too.
People race into the downwind turn requiring a steep turn which bleeds off speed and reduces lift, at the same time, the throttle is coming back in anticipation of the groundspeed increase due to the tailwind. Each item reduces the plane's ability to maintain sufficient lift. It's possibly one of those moments when we are feeling a buzz from the highspeed / show pass, we are not too close to the ground, we are not doing anything that stands out as risky flying and yet the plane is now inexplicably falling to earth nose first.

Must have been an unknown atmospheric condition from behind the plane, or aliens or my TX / RXor possibly a solar flare??
Last edited by Roux; Oct 18, 2012 at 10:44 AM.
Oct 18, 2012, 12:35 PM
Tossing planes into the snow
Quote:
 Originally Posted by Roux When i moved to my heavy low wing warbird, i would often overshoot the turn onto final as the plane would continue to travel base even through the nose was already pointing at me. The natural response to this visual input was "pull harder" resulting in more of the speed bleeding off... bam! tip stall, onto its back and into the dirt - airspeed, airspeed, airspeed. This could have been avoided by starting my turns sooner and keeping them smoother.
This is good advice, and I'm finding that making smoother turns works for me as well. That's what it would do if it was in free-flight mode. Another reason people rush turns is because so many planes are being flown in spaces that are a little on the small side. Instead of flying at airports, the modern electric RC pilot is often flying in small parks, back yards, front streets, and indoors. That means that trees and buildings often force us to turn quicker and tighter than we should.

On the subject of inertia, and unstable fighter jets and other high-powered 3D flying machines, I think they are stretching the definition of "airplane" and might be more comparable to helicopters and rockets. As they squirt about in the sky, they are less susceptible to wind and more to inertial forces. They are often deliberately flown in a stalled state.

In the case of a "normal" airplane that needs to keep up it's speed to avoid stalling, I do not believe inertia is a factor whether turning upwind or downwind.
 Oct 18, 2012, 06:33 PM You can't take the sky from me Yes inertia plays a big roll in flying but wind direction has nothing to do with it and you will get the same results no matter what your direction in relation to the wind. Inertia plays a role when you are talking about turbulence, ie. wind shear, gusting, up/down drafts, rotors etc... or high load maneuvers. Again these situations will occur at any point of the flight not just during a specific turn in relation to the wind.