Andy Durbin wrote:
"Bert Willing" wrote
in message ...
You're right - the elevator produces lift (same direction as the
wings) at low speeds, not at high speeds. Got mixed up.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
om...
"Bert Willing"
wrote in message ...
That doesn't make sense to me. At high speeds, the elevator
produces
lift
so
in case of structural failure, the bits would go upwards.

--
Bert Willing

ASW20 "TW"

Bert,

The elevator does produce lift, but in the opposite direction as
the wings (most of the time anyway).



Ok, so let me see if I've got this straight now. I cruising along
at
60 kts in trim and elevator close to neutral. I want to go 140kts
so
I push the stick forward, the elevator goes down, which pushes the
tail up, which pushes the nose goes down, I go faster. And all this
is
because the elevator is producing more lift in the downward
direction?

For a fixed stab with moving elevator don't we have to consider the
forces on both components separately to predict the failure mode?

I think we are confusing transient and steady forces here. The
overspeed problem occurs with the controls almost in neutral, but the
plane in a dive, where the speed builds up steadily. In this case the
tailplane will be generating an increasing downward force in relation
to the longitudinal axis of the aircraft. This downward force is to
counteract the forward rotation force generated by the wing. At a high
enough speed these forces will increase beyond the capacity of the
structure to support them.

The transient case is when a large control excursion is input at high
speed, and in this case the force on the tailplane could be in either
direction, depending on the direction of control input. However
downward total force is likely to be more severe in a pull-up than an
upward force in a push-over, since the contribution of the elevator
adds to the existing downward force in the first case and subtracts
from it in the push-over case.

That's my 2c worth...
Cheers, John G.