Mad (Flt) Scientist
Quote:
Originally Posted by Dani
Are there some reasonable ways to guess from the trim setting if you are in the positive or negative range? Any types with a marker on the trim wheel or so?
You can make a guess based on estimated the angle of attack at the tailplane.
alpha(tail)=alpha(wing) + tail angle - downwash
Any time you have any significant amount of TE flaps deployed, the downwash will be large, AND you'll have a nose down moment which will usually require a significant negative tail angle. So alpha tail will basically "always" be negative for a flaps config, and so there will be a download.
For the cruise/zero flaps case, you could take a guess at the downwash being about half the wing aoa - and that is a guess. Therefore if the tail angle is greater than 0.5* the wing aoa, you might have a positive alpha-tail, and so upload on the tail.
If you actually KNOW the downwash number, you can use that and not just guess.
If you want a really wild guess, positive tail angle = positive tail load, negative tail angle = negative tail load. That's pretty much assuming that the AoA and downwash are both small in cruise - which isn't quite true, but not a million miles away either.
Don't forget that for anything but a symmetrical Airfoil the zero lift AOA will be negative so even with positive incidence a non symmetrical stab flying in down wash will be creating down force.