Solved Sizing Control Surfaces on Models

[Inq]

I am using scale drawings of various airplanes... i.e.

All RC model plans always have enlarge ailerons, rudders and elevators. Is there some rule of thumb to enlarge these surfaces? Are those oversized controls to do acrobatics? If I want more scale realistic flying qualities would I use the scale sized controls or do I need the extra sized controls?

 

[Tench745]

Air doesn't scale, so you generally need more surface area to achieve the same aerodynamic effects. Even more importantly, wind and runway surfaces don't scale, so you need enough authority to overcome incredibly rough terrain and scale gusts in excess of 60 MPH! (10 mph gust at 1/6 scale).

Here's a diagram I like to share that has some "rule of thumb" proportions for RC planes.
The drawing is from "Basics of R/C Model Aircraft Design" by Andy Lennon. It's a good book that takes a math-based approach to designing model planes; from airfoil selection to control surfaces, to float design. It's focused around mid-sized nitro and gas power planes, but the principles are similar.

 

[Inq]

Here's a diagram I like to share that has some "rule of thumb" proportions for RC planes.

Thank you... just what I needed.

 

[Air-headed Aviator]

I've had the same question before, and I learned that its a a fairly complicated question! The math to actually calculate the size of the control surfaces for a desired effect involves calculus, derivatives, and matrices Maybe one day I'll learn the math proper...but that may be only after I practice aerospace engineering for real! For the time being, I try to take inspiration from existing model aircraft that move in a way I like, and copy their proportions. I've at least learned that the amount of control surface needed is overestimated in most applications.

 

[Tench745]

I've had the same question before, and I learned that its a a fairly complicated question! The math to actually calculate the size of the control surfaces for a desired effect involves calculus, derivatives, and matrices Maybe one day I'll learn the math proper...but that may be only after I practice aerospace engineering for real! For the time being, I try to take inspiration from existing model aircraft that move in a way I like, and copy their proportions. I've at least learned that the amount of control surface needed is overestimated in most applications.

The book I mentioned earlier has that math, or at least as much as any modeler will ever need.

 

[telnar1236]

A bit late to this thread, but Tench745's post is good as a rule of thumb for conventional designs, and he is also right about how complex the math can get. Static stability derivatives are pretty easy to calculate, but dynamics get messy fast. While you can do them by hand, I wouldn't recommend doing them without software. Even in full-scale aircraft, control surfaces are initially sized based on rules of thumb (typically including a bit, but not much, more math) and then refined.

If you really want to get into it, AVL (Athena Vortex Lattice) is a free software that lets you analyze stability and controls using linear inviscid aerodynamics (so most of what we would be concerned with). There is an extremely steep learning curve since it is mostly run from the command line, but it is also extremely powerful. XFLR5 is another free software that can do stability pretty well, and it has a nice GUI, but it doesn't do controls particularly well and it has problems with fuselages. Ultimately with either of these approaches, though, you get back into a combination of statics and dynamics or need to use rules of thumb for the values the software outputs. If you enjoy needlessly complicating things as I do, then it's pretty fun to play around and try to get as nice of characteristics as you can, but it isn't really necessary to make something that flies well and the level of software a hobbyist can play with means any analysis is a bit suspect as anything but preliminary.

AVL (mit.edu)
XFLR5