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.

--
-----
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

On Sun, 04 Apr 2004 00:06:23 GMT, "Bill Daniels"
wrote:

No glider will stabilize itself at
the trimmed airspeed because the phugoid is undamped. It will oscillate
around the trimmed airspeed with ever increasing amplitude.

I think things are getting esoteric now.
I've flown about 40 kinds of gliders so far, an ALL of the stabilized
at the trimmed airspeed within a couple of oscillations. None was
showing the behaviour that you describe.
What am I doing wrong?




I have no trouble believing the stories about uncontrollable vertical dives.
The nose down pitching moment created by the airfoil is very likely
powerful enough on some gliders to completely overcome the up elevator
authority at some speed above Vne.

Well... it's simple: If you are flying faster than Vne, you become a
test pilot. But staying below Vne is extremely simple in a glider...



Take any glider and trim it for best L/D, then push it up to 10 Knots above
best L/D and release the stick. The pitch oscillations will increase in
amplitude until you take control again. This is true whether the stick is
free or fixed. To demonstrate the drag effect, just open the spoilers and
watch the phugoid damp out.

Hmmm... is it possible that you are flying with an extremely aft CG?


Bye
Andreas

On 04 Apr 2004 00:27:33 GMT, (JJ Sinclair) wrote:

I got a question for you aerodynamasists (is that a word?)

When Schleicher added a bit to the wing span of the ASH-25 (25 m to 26.5m) they
required that 3 Kg of lead be mounted in the leading edge (inside the D tube)
it was to be spread down the D tube for about 10 feet and then glassed in
place.

Why? and why in the leading edge?

To change the flutter frequency of the wing.

It's problematic if the natural oscillation frequency of the controls
and the part the control is mounted on (elevator/ rudder and fuselage,
airleron/flaps and wing) is similar.

If natural oscillation frequencies are similar, an oscillation that is
started on a certain part might cause other parts to ascillate in
harmony - therefore changing the weight or CG of a part is used to
make sure that each moving part has a different oscillation frequency.



Bye
Andreas

"Andreas Maurer" wrote in message
news
On Sun, 04 Apr 2004 00:06:23 GMT, "Bill Daniels"
wrote:

No glider will stabilize itself at
the trimmed airspeed because the phugoid is undamped. It will oscillate
around the trimmed airspeed with ever increasing amplitude.

I think things are getting esoteric now.
I've flown about 40 kinds of gliders so far, an ALL of the stabilized
at the trimmed airspeed within a couple of oscillations. None was
showing the behaviour that you describe.
What am I doing wrong?

Flying very low preformance gliders or flying with the spoilers open?


Hmmm... is it possible that you are flying with an extremely aft CG?

If the CG were aft, then it wouldn't have a restoring force and so wouldn't
oscillate. It's when the CG is toward the forward limit that the
oscillation is worse.

Bill Daniels

My Mosquito has a long term phugoid. Try this. Get everything just
perfect. In trim, stable on airspeed, then let go of the stick for 10 minutes.
If it wanders off the trimmed speed, then corrects but overshoots, you
have a long term phugoid. If it eventually diminishes, it's damped. Most
pilots never notice a long term phugoid. The air is seldom smooth enough.

In article , Andreas Maurer
wrote:
On Sun, 04 Apr 2004 00:06:23 GMT, "Bill Daniels"
wrote:

No glider will stabilize itself at
the trimmed airspeed because the phugoid is undamped. It will oscillate
around the trimmed airspeed with ever increasing amplitude.

I think things are getting esoteric now.
I've flown about 40 kinds of gliders so far, an ALL of the stabilized
at the trimmed airspeed within a couple of oscillations. None was
showing the behaviour that you describe.
What am I doing wrong?




I have no trouble believing the stories about uncontrollable vertical dives.
The nose down pitching moment created by the airfoil is very likely
powerful enough on some gliders to completely overcome the up elevator
authority at some speed above Vne.

Well... it's simple: If you are flying faster than Vne, you become a
test pilot. But staying below Vne is extremely simple in a glider...



Take any glider and trim it for best L/D, then push it up to 10 Knots above
best L/D and release the stick. The pitch oscillations will increase in
amplitude until you take control again. This is true whether the stick is
free or fixed. To demonstrate the drag effect, just open the spoilers and
watch the phugoid damp out.

Hmmm... is it possible that you are flying with an extremely aft CG?


Bye
Andreas

On Sun, 04 Apr 2004 20:43:19 GMT, "Bill Daniels"
wrote:

What am I doing wrong?

Flying very low preformance gliders or flying with the spoilers open?
Neither, nor. Nearly all of these gliders have an L/D between 40 and
60.


Hmmm... is it possible that you are flying with an extremely aft CG?

If the CG were aft, then it wouldn't have a restoring force and so wouldn't
oscillate. It's when the CG is toward the forward limit that the
oscillation is worse.

Never notived that (my CG is very forward if I don't have water/lead
in the tail). The phugoid is always there, and is always (nearly)
dampened away after a number of oscillations.

Bye
Andreas

d b wrote:

You got it. Stability isn't really defined as the aerodynamics of the plane.
It is defined as what the pilot sees. You didn't mention that
dynamic instability is quite common and is usually not a serious issue.

???

I think you are mixing it all...

Stability *is* defined by aerodynamics and is quite difficult for a non
specialist pilot to determine...

*Dynamic* instability is very serious and may result in a loss of
control (pilot induced oscillations) and/or overloading

*Static* instability in not a serious issue as long the glider remains
dynamically stable, which is the case except with very static-unstable
designs - but it should not be "quite common" because the certification
standards do require a positive static stability (whether this
requirement is pertinent or not is another debate)


In article , "Arnold Pieper"
wrote:

But aircraft need to have a predictable behaviour and a predictable response
to control inputs.

Unstable aircraft have a very predictable unstable behaviour ;-)

And unstability (either static or dynamic) does not mean that control
inputs response (at least in its primary effect) is inverted...


--
Denis

R. Parce que ça rompt le cours normal de la conversation !!!
Q. Pourquoi ne faut-il pas répondre au-dessus de la question ?

I thought the BRS parachutes were a good idea, and
maybe they still are, but from a short visit to tehachapi
and seeing the HP-24 being made, and talking to the
fiberglass guy, he said that putting a big hole in
the fuselage requires a bit of work (and weight)
to restrengthen.

I still find it an elegant system, but I have no info
on how making that big hole allows twisting or causes
cracks later. I suppose the motorglider
guys would know more about it...

I am quite happy that the Russia and Sparrowhawk
offer BRS as an option. It will be interesting to
see any "saves" in these, and if this favorably
affects insurance enough that the insurers give a rate
discount for this device...
--

------------+
Mark Boyd
Avenal, California, USA

Bill Daniels wrote:

A glider with zero static stability will have zero stick force per G and no
tendency to hold any particular airspeed - elevator trim will be
unneccessary. It's easy to confuse this with stability since the glider
will not change its attitude very much in response to turbulence and no
phugiod will be evident. I actually prefer this since, to me at least, it
represents the lowest pilot workload.

I agree - except for "zero stick force per G"... the force per G is a
dynamic behaviour and not static. If you have zero static stability
(which is, as you said, the situation where the position of the stick
at 1 G is the same at different speeds) you still have a positive
dynamic stability (therefore a positive, though perhaps very light,
"force per G") because of the moment induced by pitching speed...


--
Denis

R. Parce que ça rompt le cours normal de la conversation !!!
Q. Pourquoi ne faut-il pas répondre au-dessus de la question ?

Arnold Pieper wrote:

"The elevator forces diminished as expected"...
I don't know why you expected this behaviour, since this goes against
certification requirements and against normal flight behaviour.
None of the gliders and aircraft that I've flown in the past 24 years
present this characteristic.

What's your weight ?

The certification requirements (both JAR and FAR), spell out that stick
forces have to increase with increasing G-loads, all the way to VNE.
Static stability requiremens for certification say that the airspeed has to
return to within 15% (10% in the case of FARs) of trimmed speed, for all
trimmable speeds between stall speed and VNE, and any significant change in
airspeed HAS TO cause a variation in stick force plainly percepbible to the
pilot.

The rules are there to be transgressed - either by older gliders
(grandfather right) or because some design may get derogations to these
rules in the certification process


JAR-22 says about Dynamic Stability that "any short period oscillations
between Stall Speed and Vdf must be heavily damped" with the primary
controls both free and fixed. Vdf is the demonstrated design speed, VNE is
90% of Vdf.

May you please recall us how long is a period of a "short-period
oscillation" on a glider ?


--
Denis

R. Parce que ça rompt le cours normal de la conversation !!!
Q. Pourquoi ne faut-il pas répondre au-dessus de la question ?