Answer” Increase of speed
generates pull forces.” Is incorrect. If the CG is
ahead of the neutral point we will have a stable configuration, so an increase
in speed would require a push force.
Answer “Aeroplane nose up
trim decreases the stick force stability.” Is incorrect.
If you trim nose up, you would have to keep a forward pressure on the stick to
maintain straight and level. If you increase speed, the forward pressure would
increase as the elevator become more effective, and that is in the same sense
you would expect with a speed increase, so stick force stability increases.
Answer “Stick force
stability is not affected by trim. “ Is incorrect. If
you trimmed nose down you would require a pull force to keep the nose up, and
fly straight and level. As you increased speed the pull force would increase,
which is not what you would expect, so that is an unstable stick force. So trim
does affect stick force stability.
You can calculate the difference of a 10 kt change at low speed and high speed
to prove the answer. It goes about the percentage change in lift.
If you assume a CL of 0.3 in straight and level and substitute changes of speed
maintaining the same angle of attack, then you will see that the percentage
change in lift, and therefore stick forces, will be greater at lower speeds. L
= CL x V2 (no change in the ˝ ρ or S so we can leave them out, in this
calculation) (I also left it in Kts, as it will not
affect this specific calculation, but we should use M/sec)
Change in speed 60 70 120 130
Squared 3600 4900 14400 16900
X 0.3 1080 1470 4320 5070
Change 390 750
Percentage 36% 17%