Re: Mechanics of Elevator Trim. In Detail.

Tina wrote:
> One point about the lift fairy sitting on the tail I'd like to
> understand is this  -- actually a serious question. As I understand
> it, nearly aways the tail is exerting a downward force, since the
> center of lift is aft of the center of gravity on general aviation
> airplanes (that is true, isn't it -- that the cg is forward of the
> center of lift?). If so the tail really is imposing an increased load
> on the airplane, adding to its effective weight.  The question I have
> is, how many pounds of weight is imposed aerodynamically for an
> airplane that might be loaded with its CG at the forward limit? I
> don't know where the center of lift is on ga airplanes -- a third of
> the way aft of the leading edge of the wing is an ok approximation,
> but a few inches error on an airplane weighing what ours does at max
> could make a huge change in the required force to overcome the nose
> heavy moment.

A rule of thumb is that the force on the horizontal tail
is 5 to 10 per cent of the wing lift. This translates
to a loss of 10 to 20 per cent of the raw gross lift
availbale from the horizontal airfoils.

> I'm obviously thinking about increased efficiency -- extra weight
> added because of either fat people, full fuel, or aerodynamically
> imposed, all cost horsepower (OK, watts for you purists) to move
> around.

This is the reason why modern military aircraft are designed
aerodynamically unstable, and the electronic gnomes of the
flight control system have to work all they can do.

The loss of gross lift is the proce to pay for simple and
safe longitudinal stability.

-- 

Tauno Voipio
tauno voipio (at) iki fi