Devices for avoiding VNE?

Concorde, when it was acelaring through transonic speeds had to do a
large fuel xfer to the aft tanks to conpensate for the strong nose
down trim shift.

It was rumoured to be certified

I've already written about this : in a supercritical airfoil (read
supersonic design) the Aerodynamic center does move back as the speed goes
ABOVE MACH 1.
Also, delta-wings tend to be challenging to manage in terms of CofG, so,
they usually require some form of fuel xfr to keep'em in balance.
Classic example is the british Vulcan bomber, that required substantial
amounts of weight (30 tons come to mind) just to be in balance...

So these will become issues when we start flying supersonic gliders with
delta-wings...
Maybe the space-shuttle pilots out there can share their views.

"Arnold Pieper" wrote in message
. com...

With the flaps in full negative, the big old glider easily accelerated
to
Vne. As it accelerated, the elevator forces diminished as expected. At
Vne, the stick force per G was essentially zero signifying neutral or
slightly negative static stability. While controllable, and trim-able
it
would diverge nose up or down with the slightest nudge.


"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.

I bet they did, you just mis-identified it. If you think the elevator gets
stiffer as the airspeed increases and more stick force per G is required,
I'd have to conclude you haven't flown many gliders fast. In fact, at Vne,
very tiny stick forces will produce large G forces.

The certification requirements (both JAR and FAR), spell out that stick
forces have to increase with increasing G-loads, all the way to VNE.

I didn't say that the stick forces wouldn't have increased with increasing G
loads. I didn't test this. The flight was kept very close to 1.0G. BTW, I
didn't say the Nimbus 2C is standard category either, it's experimental, at
least in the USA.

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.

If the airspeed returns to the trimmed airspeed but doesn't stabilize there
does it pass the test? Apparently, yes. 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.

The problem with gliders is that, with highly cambered airfoils, the center
of lift moves aft with increasing airspeed (decreasing AOA) or, put another
way, the airfoil generates a nose down torque about the lateral axis with
increasing airspeed. This nose down torque opposes the nose up trimming
forces required for static stability. This diminishes the static stability
margin as airspeed increases.

If the trimming system is weak, as with a bungee spring, the nose down
pitching moment will overcome the spring at some high airspeed and the nose
will want to continue down unless the pilot intercedes. (Divergence)

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.


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.

The key here is "short period oscillations", i.e. 1 - 2Hz, not 15 - 20
second phugoid oscillations. NO high performance glider has a damped
phugoid - period. The only way a phugoid is damped is with drag and, by
definition, a high performance glider has little drag.

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.

Bill Daniels

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?
JJ Sinclair

The Nimbus 2C was flying with the CG at 80% aft and at a wing loading of
6.1
PSF confirmed by a recent weighing. This particular Nimbus has a separate
stabilizer and elevator and not the all-moving stab. The structure and
rigging has been checked by a well respected shop within the last year.

With the flaps in full negative, the big old glider easily accelerated to
Vne. As it accelerated, the elevator forces diminished as expected. At
Vne, the stick force per G was essentially zero signifying neutral or
slightly negative static stability. While controllable, and trim-able it
would diverge nose up or down with the slightest nudge.

This behavior was almost certainly unrelated to any wing twisting since
the
carbon wings are extremely stiff. I suspect the airfoil is the root cause
since it has a particularly negative pitching moment. With the flaps in
full negative, the wings pitching moment is probably moved slightly toward
neutral stability where an unflapped glider would most likely exhibit more
negative pitching moment.

One easily forms the impression that flying this glider above Vne would be
most unwise. It's also very easy to see how a nervous pilot could get
into
trouble at Vne since it requires a cool hand to fly it there. I am
certain
that one unintended tug on the stick would send the G loading way above
the
ultimate load factor in the blink of an eye. It makes me think that some
of
the in-flight break-ups were overcontrol followed by G-LOC and then
airframe
breakup

Seeing a pair of gliders circling about two miles ahead, I started the
nose
up with a tiny bit of backpressure. The glider responded instantly and,
to
prevent unintended G buildup, I needed to push slightly to control the
pitch-up until the IAS dropped below 110 knots where the control forces
became more normal.

I don't think this is particularly unusual behavior since it confirms what
I
have seen on other high performance gliders. If you are going to fly near
Vne, do so with a cool hand and steady eye. It can get pretty unforgiving
up there.

Bill Daniels

Bill
I fly my glider the same way, with The C of G way back
What you are describing shows your glider is tuned just right if your
horizontal
stab / elevator if it has no concave.
The FX 67 shows a centre of pressure of about 38%MAC at Cl of .2 with
a minus 8 deg flap. With an extra -2 deg deflection the CofP will move
further forward ahead of the CofG. At that speed and - 10 deg. flap
the nose should come up gently.

As for the concaved elevator,
if one runs out of nose down trim and one has to hold the stick still hard
forward to maintain high speed, as on a ridge or in very strong
conditions, the concave on the stab acts like a servo tab and could in fact
contribute to a sudden pitch up at high speed if one is inattentive for a
second.

As for VNE and recovery, I had one unintended spin entries in my glider,
caused by an avoidance manoeuvre. I was banked about 35 deg when an other
glider was heading toward me. If it would have been one second later the
glider would have been in a blind spot. I cranked the glider over still
further ( instead of pushing down hard) and the outside wing stalled due to
more
aileron input and tightening up the circle. I found myself vertical in a
fraction of a second. I neutralized the stick and started recovery just a
bit to soon
I had a small secondary stall, after that the nose came up gently the
recovery was
completed in about 500ft that included the pull up to bleed off the extra
speed. The recovery may in fact have taken 600 ft. The maximum speed was
about 115 kt. No spoilers where used. (I have no spoilers). I like to think
because I was
exposed to unusual and sudden attitudes early on in my flying, things went
smoothly.
The altitude was 3000ft.

Regards
Udo

Bill Daniels wrote:

The problem with gliders is that, with highly cambered airfoils, the center
of lift moves aft with increasing airspeed (decreasing AOA) or, put another
way, the airfoil generates a nose down torque about the lateral axis with
increasing airspeed.

From my reading of "Fundamentals of Sailplane Design", the center of
lift remains constant (by definition), as does the pitching moment
coefficient (by measurement on a typical airfoil), with AOA. This is for
a _fixed_ airfoil. The pitching _moment_ will increase with speed (even
though the coefficient doesn't), of course.

This nose down torque opposes the nose up trimming
forces required for static stability. This diminishes the static stability
margin as airspeed increases.

This is were I get puzzled: you were flying the glider with negative
flap, which changes the airfoil to one with a positive pitching moment.
Shouldn't this increase, rather than diminish, the static stability?

If the trimming system is weak, as with a bungee spring, the nose down
pitching moment will overcome the spring at some high airspeed and the nose
will want to continue down unless the pilot intercedes. (Divergence)

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.

This might be true with a positive cambered airfoil, but during the
flight test you did with your Nimbus, you used a negatively cambered
airfoil.


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

Eric Greenwell
Washington State
USA

"Eric Greenwell" wrote in message
...
Bill Daniels wrote:

The problem with gliders is that, with highly cambered airfoils, the
center
of lift moves aft with increasing airspeed (decreasing AOA) or, put
another
way, the airfoil generates a nose down torque about the lateral axis
with
increasing airspeed.

From my reading of "Fundamentals of Sailplane Design", the center of
lift remains constant (by definition), as does the pitching moment
coefficient (by measurement on a typical airfoil), with AOA. This is for
a _fixed_ airfoil. The pitching _moment_ will increase with speed (even
though the coefficient doesn't), of course.

This nose down torque opposes the nose up trimming
forces required for static stability. This diminishes the static
stability
margin as airspeed increases.

This is were I get puzzled: you were flying the glider with negative
flap, which changes the airfoil to one with a positive pitching moment.
Shouldn't this increase, rather than diminish, the static stability?

If the trimming system is weak, as with a bungee spring, the nose down
pitching moment will overcome the spring at some high airspeed and the
nose
will want to continue down unless the pilot intercedes. (Divergence)

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.

This might be true with a positive cambered airfoil, but during the
flight test you did with your Nimbus, you used a negatively cambered
airfoil.

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

Eric Greenwell
Washington State
USA


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.

Bill Daniels

"JJ Sinclair" wrote in message
...
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.

This is a guess, but I think their concern might have been about the wing
having a tendency to twist leading edge up in a high G pull-up. If I
recall, this is called a divergent bending moment. It's more commonly found
on swept forward wings. The lead in the LE would tend to counteract that.

As I say, only a guess.

Bill Daniels

"JJ Sinclair" wrote in message
...
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?
JJ Sinclair

To prevent flutter

Bill Daniels wrote:

"Eric Greenwell" wrote in message
...

Bill Daniels wrote:


The problem with gliders is that, with highly cambered airfoils, the

center

of lift moves aft with increasing airspeed (decreasing AOA) or, put

another

way, the airfoil generates a nose down torque about the lateral axis

with

increasing airspeed.

From my reading of "Fundamentals of Sailplane Design", the center of
lift remains constant (by definition), as does the pitching moment
coefficient (by measurement on a typical airfoil), with AOA. This is for
a _fixed_ airfoil. The pitching _moment_ will increase with speed (even
though the coefficient doesn't), of course.


This nose down torque opposes the nose up trimming
forces required for static stability. This diminishes the static

stability

margin as airspeed increases.

This is were I get puzzled: you were flying the glider with negative
flap, which changes the airfoil to one with a positive pitching moment.
Shouldn't this increase, rather than diminish, the static stability?

If the trimming system is weak, as with a bungee spring, the nose down
pitching moment will overcome the spring at some high airspeed and the

nose

will want to continue down unless the pilot intercedes. (Divergence)

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.

This might be true with a positive cambered airfoil, but during the
flight test you did with your Nimbus, you used a negatively cambered
airfoil.


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

Eric Greenwell
Washington State
USA



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

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