ORIGINAL: tripower222
Why don't airplanes with high lift wings (like flat bottom trainers) have lifting/airfoil horizontal stabs. I would think if the lift and tail moment where calculated correctly once the elevator is trimmed, the plane would fly level at any airspeed, above stall of course. I would think it would make a more stable plane which is idea behind a trianer.
High lift wings produce a nose down pitch.
That is why a wing that gets deattached from a model in flight rotates rapidly while it free falls.
For that reason, the horizontal stabs must push down instead of lift, resisting the nose down tendency.
In my opinion, airfoiled stabs are stronger and less draggy that flat stabs; however, they are harder to build.
As foamies demostrate, a flat wing can fly very well; however, they are weaker and more draggy (good enough for 3D flying, but not for racers).
For a plane to fly level at any airspeed, the decalage must be zero; hence, the model would be less stable.
A model that is stable in pitch, needs to have certain amount of decalage.
A more maneuverable model must be less stable, and viceversa.
The attached pictures show us that decalage is there, even if the geometrical incidence angle is similar for wing and stab.
The downwash that the wing produces, makes the stab work as it would have negative incidence.
The AOA for the stab is negative in most of the cases, creating negative lift.
Symmetrical airfoils produce no nose down pitch moment; hence AOA for the tail should be zero for level flight.
By deflecting the elevator up or down, we are making the stab cambered in either direction, modifying the amount and direction of the lift that the stab generates.
Copied from
http://www.americanflyers.net/aviati.../chapter_3.htm
"Most airplanes are designed so that the wing’s center of lift (CL) is to the rear of the center of gravity. This makes the airplane “nose heavy” and requires that there be a slight downward force on the horizontal stabilizer in order to balance the airplane and keep the nose from continually pitching downward. Compensation for this nose heaviness is provided by setting the horizontal stabilizer at a slight neg-ative angle of attack. The downward force thus produced, holds the tail down, counterbalancing the “heavy” nose. It is as if the line CG-CL-T was a lever with an upward force at CL and two down-ward forces balancing each other, one a strong force at the CG point and the other, a much lesser force, at point T (downward air pressure on the stabilizer). Applying simple physics principles, it can be seen that if an iron bar were suspended at point CL with a heavy weight hanging on it at the CG, it would take some downward pressure at point T to keep the “lever” in balance."