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I'm currently designing an aircraft with the following specifications-

s=1.8m^2, AR=7-8, trapezoidal planform with a taper ratio of about 0.45, a load of 25KGs and a cruising speed of 13-15m/s.

I'm barely getting any improvement in drag reduction or tipstall delay after testing twists upto -4deg. A twist of -3 is delaying tipstall by about 0.7deg while reducing Cl by 0.08 in xflr.

Is a twist even necessary for my model?

Also I've read that a twist optimizes drag reduction and lift distribution for a single aoa in non-elliptical planforms. Should this angle be the aoa for the Cl I get for my cruise speed?

Also how would my planes angle of incidence depend on the twist that I select?

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Is a twist even necessary for my model?

Yes, with that amount of taper I would recommend twist. Those 4° look rather low on first sight. The tapered wing will produce induced loads on the outer wing due to the amount of suction of the deep inner wing. Below is a plot of where flow separation starts for different sweep angles and aspect ratios (source):

flow separation for different sweep angles and aspect ratios

I would classify your wing as a high taper wing (second from top, right column). Without twist it will start to stall from the tip. My recommendation is to use 0.7 taper even if that means higher root bending moments. With 0.7 taper and the same stall angle of the inner and outer wing airfoil, 2° of twist are usually sufficient.

Twist optimizes drag reduction and lift distribution for a single aoa in non-elliptical planforms. Should this angle be the aoa for the Cl I get for my cruise speed?

Yes.

Also how would my planes angle of incidence depend on the twist that I select?

Proportionally to the lift loss on the outer wing with increasing twist, the whole wing needs more angle of attack for the same lift. You will, however, need many degrees of twist before the wing's incidence will increase by more than one degree.

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