huge control surfaces?

[Danny]

On aircraft for 3D flying, why do they have such huge control surfaces?

It contradicts a modeling aerodynamics book that i've been reading. It says that increasing the control surfaces (ailerons for example) from 20% to 30% of the chord at 10 degrees deflection only increases the CL slightly. From approximately .59 to about .60. The book also says that this increase in control surface size also puts a considerable increase of load on the servo and linkage.

So from reading about this, I'm a little confused. Any help on explaning this contradiction would be of great help.

[Ollie]

You might be suprised at how much design is based on "logic" and legend rather than solid facts. Logic like,"If 20% is good, then 40% must be twice as good."

I've been following model design and doing some my self for over thirty years. I have only found one modeler, Dr. Mark Drela, who does a really thorough job of engineering design that rivals the best full scale engineering efforts.

Many economy ARFs have emphasized the production efficiency of the design to the point of seriously compromising both strength and performance.

[Sparky Paul]

Those huge surfaces are for instant reponse at low to zero airspeed, using propellor blast instead of slipstream.
Aerodynamic finesse is not part of the solution.

[flyfalcons]

SP has it right. For 3D flight you need outlandish pitch control, and to a lesser extent rudder and aileron. To get that thrust vectoring ability it is nice to have the movable surfaces to be at least 50% of the vertical and horizontal tail area. When setting up a 3D plane you set a low rate to fly normally and a 3D rate for 3D maneuvers for all the control movement you can get. There is nothing really aerodynamic about flying at extremely high Alpha; it's all about vectoring thrust and making non-standard flight attitude transitions (as in a blender).

3D flight really is based on logic and legend because that is what works. It's not just the economy ARF's that use this design; look at all the high end 3D planes of modern design and you will see huge control surfaces.

[Ollie]

Sparky Paul and flyfalcons have a point. The smooth, wind tunnel flow condition that the published data were taken under were quite different than the turbulent, swirling flow in a prop blast over the rudder and elevator.

[steve lewin]

For a fair amount of their lives 3D planes are behaving as much like helicopters than conventional planes. It's not so much conventional aerodynamics as the triumph of power over gravity.

If you think about it when a plane is hovering stationary in a vertical plane the Cl of the wings has almost nothing to do with anything ;).

Steve

[Danny]

Hey,

Thanks a lot for your replies. They've been a great help in understanding all of this.

Dan