Delta stability ?

[dvint]

I am building a 29" span delta with reflexed airfoil -about 12%
thick. How stable are deltas on take off ?
thanks

[Ollie]

If there is insufficient air speed on takeoff so that an extremely high angle of attack is attempted, even a low aspect ratio delta could stall. Low aspect ratio deltas can achieve very high angles of attack without stalling. At the angles of attack between stalls, pitch stability is achieved by placing the CG sufficiently ahead of the aerodynamic center. The low aspect ratio of deltas and the large sweepback angle of their 25% chord lines make calculation of the true aerodynamic center dependent on vortex lattice methods rather than the easier geometric methods which work well for higher aspect ratios and less sweepback. The practical solution for modelers, who do not have access to vortex lattice methods, is to put the initial CG at about 15% of the mean aerodynamic chord and adjust it aft in small increments during flight testing, reducing the initially excessive stability until the desire stability is achieved. Forward CG locations require excessive reflex to achieve pitch trim. As the CG is moved aft, less reflex (up-trim) is needed and the airfoil becomes more efficient. During flight testing you can find a suitable compromise between stability and airfoil efficiency.

Yaw stability is achieved with a large vertical surface placed aft of the aerodynamic center.

Roll stability is achieved through the ample sweepback of the 25% chord line inherent in low aspect ratio deltas.