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Effect of wind on maneuvers
Matt Piatkowski:
Hello,
A single propeller stunt plane has a tendency to speed up in maneuvers when it is windy.
The plane with the counter-rotating propellers does not show this behavior or shows very little of it.
Why?
Thank you,
M
Dave_Trible:
I doubt it really makes any difference . The speed build up comes from the wind pushing the airplane from behind and added side forces from wind on the fuselage sort of like wind in the sails of a boat. Most of any help comes from the prop if the power is suppressed somehow like a piped IC engine set up where the pipe acts like a governor to prevent the engine gaining rpm from being forced forward and unloading with the tailwind. The direction of rotation can have other affects but not much in regards to winding up in the wind. If you fly a twin that seems to act that way I'd suggest perhaps the second prop disc is offering a little more forward air braking affect when pushed, like pushing a ping pong paddle into the wind. If the engines aren't winding up very much they can seem to have some braking ability in strong winds once the prop disc isn't showing it's broadside into the wind.
Dave
Brett Buck:
--- Quote from: Matt Piatkowski on November 01, 2019, 07:28:07 AM ---Hello,
A single propeller stunt plane has a tendency to speed up in maneuvers when it is windy.
The plane with the counter-rotating propellers does not show this behavior or shows very little of it.
Why?
--- End quote ---
Probably because of something else being different, unrelated to counter-rotating props. The same effect can almost certainly be replicated or exceeded with a single prop. There's some other factor or factors (or actually, a lot of them) that are undoubtedly different, too, causing the difference you see. Because you think you have found a new "killer feature", you are attributing any differences you find to that, and that alone. Basically, it's not a controlled experiment.
Brett
Matt Piatkowski:
Hi Brett,
In 2019 season, I have seen at least fifty F2B practice and competition flights in the wind with contras and the speed of these planes in maneuvers was almost constant.
Hi Dave,
The wind indeed adds the side force on the fuselage - we all can feel it as increased lines tension. The effect I have written about is undoubtedly caused by the wind velocity (Vw) added as vector to the plane velocity (Vp) but I cannot understand looking at the vectors diagram what part of Vw causes speeding. Lets say you fly a single prop model and perform the inside loop. The plane of this loop is tilted 22.5 deg. w/r to the wind direction. Vw = 5 m/sec (16.5 ft./sec). Should this value be multiplied by sin 22.5 deg. and arithmetically added to the model's speed of 25 m./sec? This will give the plane's speed of 26.9 m.sec. Of course the velocity of the model is not constant while it flies the loop but the example, if correctly described, shows the increase of Vp.
I am, however, completely at loss trying to quantify the behavior of the models with contra in the wind. What causes the contra powered F2B planes to fly the maneuvers with almost constant speed in the wind?
The answer for this question will shed a new light on the flight dynamics of the C/L models.
Best Regards,
M
Brett Buck:
--- Quote from: Matt Piatkowski on November 02, 2019, 06:23:46 PM ---Hi Brett,
In 2019 season, I have seen at least fifty F2B practice and competition flights in the wind with contras and the speed of these planes in maneuvers was almost constant.
Hi Dave,
The wind indeed adds the side force on the fuselage - we all can feel it as increased lines tension. The effect I have written about is undoubtedly caused by the wind velocity (Vw) added as vector to the plane velocity (Vp) but I cannot understand looking at the vectors diagram what part of Vw causes speeding. Lets say you fly a single prop model and perform the inside loop. The plane of this loop is tilted 22.5 deg. w/r to the wind direction. Vw = 5 m/sec (16.5 ft./sec). Should this value be multiplied by sin 22.5 deg. and arithmetically added to the model's speed of 25 m./sec? This will give the plane's speed of 26.9 m.sec. Of course the velocity of the model is not constant while it flies the loop but the example, if correctly described, shows the increase of Vp.
I am, however, completely at loss trying to quantify the behavior of the models with contra in the wind. What causes the contra powered F2B planes to fly the maneuvers with almost constant speed in the wind?
The answer for this question will shed a new light on the flight dynamics of the C/L models.
Best Regards,
M
--- End quote ---
Again, you are jumping to conclusions and the experiment is uncontrolled. The obvious answer is that the contra-rotating systems have a different response to load and velocity than single-prop systems you have seen/used. But it's very easy to come up with single-prop systems, even with IC, that range from virtually no speed stability, to far too much speed stability. I guess that for reasons unrelated to the fact that the props contra-rotate, you have seen systems with better speed stability.
I don't know how the various systems you are comparing respond; your IC systems never worked even to first approximation, not even close, so I am not surprised that if you got or saw better systems that were working correctly, and they happened to have contra-rotating propellors, that they worked better.
BTW, the dynamics of the "whip-up" are a lot more complex than you describe. For example, the induced drag is much lower at the top of the loop than it is at the bottom, because it requires far less deflection at the top of the loop than the bottom, since at the top, the wind want to decrease the radius, whereas at the bottom it has no effect. Another effect is that if you had perfect *airspeed* stability, it would go much faster "down" than "up" relative to the ground, because of the vector sum of the airspeed and wind speed along the X axis - not Y or Z. The side force you describe is nearly trivial with regard to speed stability. That's also why you don't actually want perfect airspeed stability in many cases of wind.
The value of contra-rotating propellors is undeniable in the area of reducing axial coupling; that *is* a good reason to do it, if there was no other factor. I expect you are attributing a better system to the fact that it is contra-rotating, rather than the many other aspects that are infinitely more likely to cause the effect you notice. It's hardly a new phenomenon - most new tuned pipe users attribute every effect they see to the "pipe length", and Rabe rudder users tend to attribute every trim effect to "rudder adjustments". In any of these cases, there are a myriad of other effects that may also be relevant, but the new enthusiast to a particular gadget focuses in on that to the exclusion of everything else.
Brett
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