RC Airplane Design

How to design a blended wing body (BWB) RC Airplane

 

The BWB can offer very good advantages over a tube and wing, but requires about 5 times the aero analysis to get it. This is because the wing’s airfoil stack varies radically from root to tip. If you have a copy of Mark Drela’s AVL code I would recommend using this to determine the best airfoils and twist needed to get good performance. Here are the Guiding principles to design blended wing body BWB RC Airplanes,

1. Near elliptic spanload at cruise (Inviscid e =~1 without winglets and ~1.1 to 1.2 with winglets)
2. Use normal airfoils outboard (normal thickness 12-16%).
3. Use reflexed airfoils inboard (increased thickness 18-22% is AOK).
4. Blend very smoothly across the kink in the planform.
5. Use ~25-35 degrees of wing sweep to get pitch and yaw stability (AVL stability estimates are good).
6. Aim for a Static Margin of ~25%mac
7. In AVL adjust chord and twist to locate the stall-critical station inboard of ~70% semi-span for good stall handling.
This is a very iterative process, but if done correctly you will get higher CLmax and L/D that a tube and wing design.
Also, I use a trapezoidal wing definition for Sw, ARw, and mac. So the extra chord in the Centerbody is not included in the reference wing area. Using this definition, a typical CG fro good stability is -10%mac (yes that’s 10%mac ahead of the mac’s leading edge), way forward of a typical airplane. Also this trapezoidal wing should have a taper ratio of 0.8 to 1.0 to get stability with good stall characteristics.

Special Thanks to Mark Page.