Devices for avoiding VNE?

It seems like there is a fixation on the "negative pitching moment",
forgetting the global forces acting on the glider.
The CG is ahead of the Aerodynamic Center of the wing, and thus generates a
nose down moment that is counteracted by the horizontal stabilizer.
For practical purposes, the Aerodynamic Center does not change with AOA (or
speed), and it is what is utilized to analyze stability.
I'll say that again : For stability analysis, the AC does not change (check
FOSD or similar publication).

The balance of forces between the CG, wing (AC) and the horizontal
stabilizer is such that the glider tends to stay at the trimmed airspeed,
from stall speed all the way to Vne. It MUST be so, and it has to be
demonstrated to be so under all conditions of CG position (always ahead of
the AC) and all airspeeds, from stall to redline.
"The speed must settle within 15% of trimmed airspeed"... etc.

As far as the stick forces increasing with airspeed, it must be this way or
we simply wouldn't need a pitch trim control.
The JARs and FARs don't really require a pitch trim control, if the stick
forces are light enough to hand fly stable in all possible speeds in the
envelope, and the "trimmed speed" is 1.4 or 1.5 the speed of stall (can't
remember exactly), which means, if the stick is released, the speed has to
settle on 1.4 or 1.5 the stall speed.
I've never flown a glider or airplane that doesn't have a pitcth trim
control, so, that speaks for itself.
But always, the stick forces must be progressive and perceptibly so.

If the negative pitching moment was this big monster you seem to think it
is, VNE wouldn't be determined by VD or structural design speed.
It would be determined by when we run out of "nose up" force on the elevator
and go into the infinite inverted outside loop mode, which has never been
heard of. Frankly, that must have come from someone who hadn't finished
reading the whole aerodynamic book yet, and started jumping to conclusions.

We usually run out of "Nose down" trim, at very high speeds, and have to use
some "push forward" force on the stick to maintain the high speed and
prevent the nose from coming up too quickly, such as when I want to recover
from a high speed dive or a low pass, high speed finish.

A divergent mode is unstable and therefore unacceptable for the europeans
and americans.



"Bill Daniels" wrote in message
news:dKLbc.174977$1p.2106507@attbi_s54...
You're right, the negative flaps would tend to reduce the airfoils
nose
down
pitching moment and increase the static stability. My feeling is that
the
effect of just 7 degrees of negative flap just isn't enough to negate
the
whole wings' pitching moment.

Your feeling is probably right. I just found the pitching moment diagram
for the FX 67-K-150 airfoil (FOSD, page 93), which is used on the outer
part of the wing of the Nimbus II. At -8 deg deflection, it is very
close to zero, but still negative. I'm assuming the FX 67-170 airfoil
for the inner part of the wing is very similar.

--
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change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA


Of course, we're dealing with the whole glider, not just the wing, and
that
means down wash effects on the stab, stab/elevator section, trim bungee
spring rates, 3D flow around the fuselage, etc.. all summed together in
the
static stability equation.

You know, proof reading the preceding paragraph makes me think about that
screen door spring thingy connected to the green knob that's pretending to
be a trim bungee. I suppose those things get old and weak. I wonder what
effect that would have....

Bill Daniels