Dave was explaining this to some of the pilots
 at Reno.

 Most tractor airplanes have down force on the tail
 to counter the lift on the wing. That is because
 the CG is always ahead of the center of lift on the
 wing. That makes for a longitudinally stable airplane.

 Cambered airfoils have a negative pitching moment (Cm) tending
 to pitch nose-down with an increase in speed.

 With no elevator input and an increase in power and speed
 the airplane will pitch down.

 Conversely as you slow the airplane you must pull aft
 on the stick raising the elevator and pitching up to prevent a
 decrease in altitude and maintain straight and
 level flight.

 The down force on the tail needs to be increased with
 up elevator to counter this. It can be done by pulling back
 on the stick. It can also be done by moving
 the trim tab down forcing the elevator up. The trim tab
 is an aerodynamic servo. This neutralizes the stick force
 for straight and level flight.

 If the trim tab departs the elevator the elevator will
 tend to stream line it self placing a forward force
 on the stick.

 If the pilot holds the stick fixed the aircraft
 will stay where it is. If he releases the stick
 the aircraft will pitch down.


 Scenario number 2.
 If the trim tab is up forcing the elevator down
 due to perhaps an aft CG problem. Supposedly there
 was a large tank in the fuselage that held ADI fluid.
 In this tank was a water to water heat exchanger.
 The idea here was the ADI fluid would boil off
 cooling the water to water heat exchanger for a total
 loss cooling system. No conventional cooling
 system with its inevitable drag.

 Then the aircraft would pitch up and the stick force
 would be aft.

 Perhaps I am wrong about all this.

 Paul Lamar


Yea, I think you have it wrong Paul.

The h-stab is set at a negative AOA to create a downforce (plane pitch up -
that way when the h-stab stalls, the nose pitches down and you can recover
from the stall).  But that aerodynamic down-force is dependent on airspeed,
while the force needed to balance the weigh of the engine is fixed.  So to
compensate for various speeds (and CG loading) the trim is used.  As the
plane goes faster, the aerodynamic down force of the empenage increases
(aircraft pitch up), so pitch-down trim must be added for level flight.

The P-51 going 470 mph will have a very large pitch-down setting on the trim
because that is well above the aircraft's max speed.

Suddenly remove that pitch down trim tab and the plane will suddenly pitch
up (the tab is up, pushing the elevator down, pushing the tail up, pitching
the plane down)

It sure seems like that is what happened to the Ghost.  The sudden pitch up
from loss of the tab caused GLOC.  The aircraft landed in a random location,
which was a bad spot, but it could have been worse.