Understanding RC Planes: Scaling Down & Performance

[Batmoosemike]

This question is slightly related to my other plane, but I didn't want to post in the same thread as I didn't want the other one to get derailed.

If I have blueprints for an RC plane, and I scale down everything by a ratio (let's say 1/2), would it still work just as well? Or would it follow the square cube law and perform differently?

 

[mrspeedybob]

Depends what you mean by "work just as well"...

It's a lot more complicated then the cube/square relationship.

If you scale a plane down by %50, you have 1/4 the wing area and 1/8 the volume. If we assume volume is going to be proportional to mass, then 1/8 the mass. 1/8 the mass on 1/4 the wing are will mean the stall speed will be lower and the plane will be more maneuverable. 1/8 the mass probably also means 1/8 of the power and 1/8 of the energy reserves (fuel), so your top speed and range will be lower.

The reality is that you can't really just scale an aircraft up or down and expect it to work well. You have to design a different aircraft. The size of the aircraft will be determined by the job it is supposed to do.

 

[David Lewis]

The scaled down version will generally not perform as well. It will require more skill to fly, it will not be as steady in turbulence, it will have a higher drag coefficient, and it won't be able to carry as high of a wing loading. To compare wing loading between different size airplanes on an apples-to-apples basis, you go by cube loading (= weight / wing area ^1.5).

 

[Batmoosemike]

Depends what you mean by "work just as well"...

It's a lot more complicated then the cube/square relationship.

If you scale a plane down by %50, you have 1/4 the wing area and 1/8 the volume. If we assume volume is going to be proportional to mass, then 1/8 the mass. 1/8 the mass on 1/4 the wing are will mean the stall speed will be lower and the plane will be more maneuverable. 1/8 the mass probably also means 1/8 of the power and 1/8 of the energy reserves (fuel), so your top speed and range will be lower.

The reality is that you can't really just scale an aircraft up or down and expect it to work well. You have to design a different aircraft. The size of the aircraft will be determined by the job it is supposed to do.

I think mass would barely be affected as a large majority of the weight would come from the motor (which wouldn't change). A lower stall speed is a positive, although I wouldn't want the plane to be too maneuverable and hard to fly.

The scaled down version will generally not perform as well. It will require more skill to fly, it will not be as steady in turbulence, it will have a higher drag coefficient, and it won't be able to carry as high of a wing loading. To compare wing loading between different size airplanes on an apples-to-apples basis, you go by cube loading (= weight / wing area ^1.5).

If I make the wing bigger compared to the chassis, would the plane be easier to fly?

 

[David Lewis]

Yes, I believe so. An easy to fly airplane has, among other things, a low wing loading and a high maximum lift coefficient. Also in my experience bigger models tend to be easier to fly. Everything happens more slowly, which gives you time to react, and they are not as sensitive to wind or turbulence. According to a law of dynamic similitude, time speeds up as the square root of the scale factor.

 

[Nugatory]

think mass would barely be affected as a large majority of the weight would come from the motor (which wouldn't change).

If that's your plan, then you aren't scaling down everything by one-half, you're scaling down some things but not others. That's actually a much more sensible approach to designing larger or smaller plane... But it also reinforces what everyone else in this thread is telling you, namely that there's more to the problem than just a simple scaling.