On Tue, 27 Sep 2011 22:28:05 -0700, Beryl wrote:

wrote:
...
The elevator - and the trim tab - do a LOT of work on a race plane at
500mph. And a lot at 200 too.

Explain! I haven't seen any explanations here, just claims. Richard
claims the nose must forcibly be held down at 500 mph.

The airplane is nose-heavy already, it must be held up. If it takes 200
lbs of downforce at the tail to hold the nose up at 200 mph, it also
takes 200 lbs of downforce to hold the nose up at 500 mph.

We want that 200 lbs of force to come with as little drag as possible at
500 mph. Can anyone explain how full nose-down trim tab deflection +
pushing the stick forward will produce the necessaty 200 lbs of
downforce at the tail?

It will do it because the stabilizer incidence is all wrong for 500 mph.
The stock stabilizer rigging is a compromise, to work acceptably through
the whole speed range. At 500 mph it's pushing the tail down too hard,
so Richard has to trim, and push the stick, to help hold the tail back
up. The tail surfaces are working against each other. Pitched up/down
crap hanging out in the wind.
The trim TAB is a servo mechanism that causes a small amount of
force to control a large amount of force. The little trim tab is what
makes the control input neutral. At 500mph, the force produced by a
couple degrees of "angle of incidence" on a tab 20 inches wide and 2
inches long - just as an example is VERY SIGNIFICANT - Stick your hand
out the window at 50mph and change the angle - feel the force. Now
remember aerodynamic drag increases at the cube of speed increse. The
lift and drag work directly in concert.Double the speed - 4X the
force. You are going to go 10 times as fast. What does that do to the
forces? And that tab is aerodynamically a lot cleaner than your hand.
It is also SIGNIFICANTLY more area - A few degrees of tab trim will
input a lot of force - particularly at the trailing edge - up to
several feet from the pivot. That trim makes the elevator (in this
case) follow along at the correct angle of incidence for straight and
level flight with no control input force (stick pressure).
Now, let that trim tab come loose at one end and start flapping in
the breeze, 2 feet farther back from the pivot than where it should be
- or simply 15 degrees or more off from where it should be - and all
of a sudden LARGE AMOUNTS of control input are required to hold the
elevator at the right position for level flight. Several hundred
pounds of force on the stick would be required INSTANTLY to correct
for the separation - and if that correction is not made INSTANTLY, the
quick movement of the elevator control surface through a significant
degree of movement causes a dangerously violent change in attitude -
forcing the tail surface down - and on a LONG lever - the down force a
LONG way back from the center of lift - which acts as the fulcrum. It
does not take a lot of force that far back to really toss the aircraft
out of straight and level flight. The up-pitch of the plane cuased by
the quick drop of the tail in this case caused well over 12 G's of
force on the plane- and the pilot - making it virtually impossible for
him to correct and control the plane - particularly when that close to
the ground.
The probability is VERY high that the 12 Gs of force caused the
(average)20 lb human head to weigh 240 lbs plus - instantly snapping
the pilot's neck in the process.

Anyone who doubts the effect of a trim tab at speed has never looked
seriously at aerodynamics or the flight characteristics of an
airplane. (and has likely never been at the controls of an airplane)