To calculate Horizontal Stabilizer Area one needs to know the Tail Moment Arm. Can you please explain what where the TMA is measured from i.e. from the wing TE to the Hori.stab LE or TE or what

(please mention your procedure for both rectangular as well as tapered and elliptical planforms).

From : Don Stackhouse

The tail moment arm is always measured in the direction parallel to the aircraft centerline, from the aerodynamic center ("AC") of the wing to the aerodynamic center of the tail. Of course for things like the horizontal tail volume coefficient you use the aerodynamic center of the horizontal tail, and for the vertical tail volume coefficient you use the aerodynamic center of the vertical tail assembly (i.e.: fin + rudder) For the vertical tail in particular it can be a little tricky, because you often need to include some part of the aft fuselage as part of the tail area. The Gee Bee racers would be one example of this. Exactly how much to include can be a bit of a "judgement call".

The aerodynamic center of a flying surface is located at 25% back from the leading edge of its Mean Aerodynamic Chord ("MAC"). For most normal planforms, the MAC is the chord that divides the surface so that half of its area is inboard of there and the other half is outboard. For a trapezoidal planform, it's typically a little bit inboard of the location of the average chord. There are other articles in the "Ask Joe and Don section of our website that discuss how to find the MAC of a trapezoidal (i.e.: single-taper) surface. To find the MAC of a more complex planform, you can break it up into several trapezoids that together adequately approximate its shape, find the MAC and AC of each trapezoid, then use the ratios of their individual areas to find the MAC and AC of the total surface.

There are also some programs available that will calculate the MAC and AC of a complex flying surface. Chuck Anderson has one, and Eric Sander's "CompuFoil" airfoil, rib and planform plotting program also includes this function.

Don Stackhouse
DJ Aerotech