Harder to stall in a steep turn?

On 28 Jul 2003 11:54:47 -0800, (Mark James Boyd)
wrote:

I also recall something about aspect ratios and AOA. I think
there was something about a higher aspect ratio wing stalling
at a lower AOA than a lower aspect ratio wing with
the same wing area and loading.

This is not the case. Stall AoA on relatively high-aspect ratio wings
such as on gliders is dependent on airfoil only.

Delta-wings with very low aspect-ratio indeed have a higher stall AoA.


Bye
Andreas

With regard to the original message,

I don't think Derek is arguing specifically that the effectiveness of the
elevator depends on the angle of bank, rather that pulling back on the stick
with the wings level or in a shallow bank leads to a nose high attitude from
which the glider will slow down and stall. In a steeply banked turn, pulling
back on the stick tightens the turn more than it raises the nose so you
won't slow the glider down very much, any pre-stall buffet is more likely to
be the result of stall speed increasing with higher g.

Its easier to reach that pre-stall buffet (inadvertently or otherwise) by
slowing down than by pulling g, thus it is easier to use the elevator to
stall a glider from straight flight or in a gentle turn than in a steep
bank. Either way, the recovery is the same, ease the stick forward.

If your instructor asks you why its harder to stall a glider in a steep bank
and you reply that the extra g makes the glider nose heavy, you are liable
to get a few demonstrations as to why that isn't true. The reason why its
hard to stall in a 60 degree bank is not because the glider is nose heavy
but because you are using virtually all the elevator authority to make it
nose heavy.

I'm sure its not what Bill meant to suggest, but its important that you
understand that just because you're pulling 2g, the glider is not too nose
heavy to stall.

Ed

That's why I gave a very specific example and also mentioned proper rigging,
which includes that the elevator actually moves within the design
deflections at annual time. There's also no accounting for pilots that
can't coordinate a 60deg bank or think a 45deg bank is 60degs. Of course
there are gliders that will flick into a spin with little or no warning from
this attitude, so try it at altitude, away from the crowd, and with your
instructor if necessary.

Frank

"Dave Martin" wrote in message
...
The danger here is that we are talking theory where
we may start to confuse pilots. It is harder to stall
with 60 degrees of bank. Gliders like the K13, by
design run out of elevator in straight and level flight.
They are difficult when flown with heavy pilots to
develop more than a mushing stall in sraight and level
flight.

Put light -- bottom weigh pilots in and they become
a different glider.

The Puchacz on the other hand has plenty of rear elevator
even when banked, quite steeply.

There can be some dangerous assumptions that gliders
will not spin.

The pilot must know the limitations and characteristics
of the glider he/she is flying. This can only be achieved
by carefully experimenting with different configurations
and different flight situations.

Gliders with reputations that they will not spin, can
catch pilots out who load them wrongly, fly them badly
or worse combine both.

Dave Martin

Get some empirical experience. Hop in a G-103, circle
at 60deg bank and
bring the stick back to the stop. If properly rigged,
it will not stall.
Do the same in straight and level flight. It will
stall, in a mushy sort of
way depending on loading. Also, in a G-103, you will
get more elevator
authority in tight turns by moving the trim forward.

This is not true of all gliders, but clearly in a 60deg
bank, the G-103 is
stall proof by design.

Frank Whiteley

Hi Eric

I am pretty well convinced that the CG causes the majority of the
effect of keeping airplanes form stalling in a steep turn.

The CG will determine the AOA of the stabilizer at 1G and the
stabilizer size will determine the ratio of how the AOA varies with G
loading. A small stabilizer will reach the critical angle of attack
quickly and large one will not. For Example if the CG is at the
Center of lift on the wing then a symetrical stablizer would be flying
at an AOA of zero in level steady flight. Obviously in the AOA of the
Stablizer has to increase (in the down direction) to increase the AOA
of the wing. The size of the stabilzer will determine how much the AOA
increases.

On a side note this is actually one method that is used by test
pilots determine the aft CG limit for aircraft. They incremently move
the CG aft and put the aircraft into a steep turn at low airspeed. As
long as the aircraft exibits positive control pressure through out the
turn the CG limit is adequate. As the control pressures go neutral the
aircraft is at it's aft limit. If the CG goes any further back the
pilot would have to put forward stick into the turn to keep the the
airplane from increasing the AOA to the point that the airplane
stalled. I know this because it is how we determined the AFTCG limit
for the Thunder Mustang.

My HP16T had the opposite problem when I bought it, the CG was so far
forward that I could not fly at less than 60kts in a 60deg bank. The
stabilizer would stall and the nose would pitch down with the stick
all the way back. I would have to roll out of the bank to bring the
nose back up.
After a Weight and Balance and 4lbs in the tail and I can turn at
45kts an a steep bank and the nose has no tendance to drop.

Brian Case
CFIIG/ASEL

Eric Greenwell wrote:

In article ,
says...

"Jim" wrote in message
...
In his book Gliding, p100, Derek Piggott writes:

"In most modern gliders, the elevator power is not adequate to pull
the wing beyond the stalling angle in a steep bank and it is only just
possible to reach the pre-stall buffet with the stick right back.
This is very different from straight flight and gentle turns where a
movement right back on the stick would definitely stall the aircraft,
requiring a significant loss of height to pick up speed before full
control is regained."

If this is the case, what are the aerodynamics that account for
this? Does it have something to do with the elevator's limited
power to deal with the load factor resulting from a steep, level turn?

I'll give this one a try.

As we all know, the pitch stability/control system is like a seesaw with the
download produced by the horizontal tail balanced by the downward force of
the weight of the glider acting at the center of gravity with the center of
lift acting as the fulcrum.

In level flight the downforce at the center of gravity equals the all-up
weight of the glider and there is sufficient reserve up elevator authority
to stall the wing.

In a 60 degree bank, for example, the downforce at the CG is twice the
weight of the glider due to the centrifugal force of the turn. However, the
elevator effectiveness is the same as in level flight so it cannot overcome
the increased downforce at the CG and bring the wing to a stalling AOA.

As Derek points out, with most modern gliders in a steep turn, the wing
cannot be brought to a stalling AOA. The glider is, in effect, becoming
nose-heavy due to centrifugal force.

Since the wing is able to develop twice the gliders weight, the
elevator should also be able to develop twice the force it normally
does, shouldn't it? And this explanation would suggest the elevator is
unable to generate more than 2 g's, even in level flight, but we know
it can do that.

I believe the reason the elevator becomes less effective in circling
flight is due to the change in relative airflow between the wing and
the tail. Because the glider is turning partly in the pitch plane
(mostly in the pitch plane at 60 degrees bank), the airflow at the
tail meets the tailplane at a higher angle of attack than it does at
the wing. This higher angle of attack means more "up elevator" is
required to produce the same download. At the point the elevator
reaches it's stop, it is then producing less download than it can in
level flight, and is unable to force the wing to the stall AOA.

A quick glance at "Fundamentals of Sailplane Design" didn't find an
reference to it, but Frank Zaic described the effect 50 years ago for
model airplanes, using the term "circular airflow".
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)


I completely buy your explanation (which I would have posted if you didn't
do it before) and think all other explanations based on increasing forces
with increasing G are wrong. If this would be true, this would also apply
in level flight, i.e. ballasting a glider while keeping its CG in the
same place should have the same effect, and it is not the case. There is
a simple experiment that everybody can easily do showing the effect of
the change in AOA on the tailplane due to the circular airflow. Fly a
stabilized turn at e.g. 45 degrees bank (load factor = sqrt(2) ~ 1.414)
and set the trim to have a neutral stick. The fly stright and level
at the same AOA, i.e. ~ 15% slower (sqrt(1/1.414) ~ .85), without changing
the trim. You need to push on the the stick to obtain that, altough both
wing and tailplane should be at the same AOA relative to the local airflow,
the change in forces is only due to the change in speed. This shows that
the direction of the airflow changed on the tailplane.

"Dave Martin" wrote in message
...
The danger here is that we are talking theory where
we may start to confuse pilots. It is harder to stall
with 60 degrees of bank. Gliders like the K13, by
design run out of elevator in straight and level flight.
They are difficult when flown with heavy pilots to
develop more than a mushing stall in sraight and level
flight.

Put light -- bottom weigh pilots in and they become
a different glider.

The Puchacz on the other hand has plenty of rear elevator
even when banked, quite steeply.

There can be some dangerous assumptions that gliders
will not spin.

The pilot must know the limitations and characteristics
of the glider he/she is flying. This can only be achieved
by carefully experimenting with different configurations
and different flight situations.

Gliders with reputations that they will not spin, can
catch pilots out who load them wrongly, fly them badly
or worse combine both.

Dave Martin

You make a good point. Some gliders are very resistant to stalls and others
will stall readily - especially with light pilots. It seems that trainers
made in Eastern Europe come equipped with large, effective elevators that
can stall the wing in any attitude. On the other hand, many German single
place glass gliders often have small elevators with limited up authority.

For example the Blanik L-23, IS28 b2 Lark and, as another poster pointed out
the Puchacz, can be stalled from a steep bank easily. For this reason, they
make good trainers since the student must learn to be constantly aware of
pre-stall buffet.

However, the point the Derek was making is that it is more difficult, but
not impossible, to stall in a steep turn. I've had this discussion with
pilots who feared steep banks. I suggest that thermalling is steep banks is
easier in that the glider is more difficult to stall and fewer corrections
are needed to stay in the thermal since the turn diameter is smaller.

Bill Daniels

On Tue, 29 Jul 2003 08:05:21 -0600, "Bill Daniels"
wrote:


"Dave Martin" wrote in message
...
The danger here is that we are talking theory where
we may start to confuse pilots. It is harder to stall
with 60 degrees of bank. Gliders like the K13, by
design run out of elevator in straight and level flight.
They are difficult when flown with heavy pilots to
develop more than a mushing stall in sraight and level
flight.

Put light -- bottom weigh pilots in and they become
a different glider.

The Puchacz on the other hand has plenty of rear elevator
even when banked, quite steeply.

There can be some dangerous assumptions that gliders
will not spin.

The pilot must know the limitations and characteristics
of the glider he/she is flying. This can only be achieved
by carefully experimenting with different configurations
and different flight situations.

Gliders with reputations that they will not spin, can
catch pilots out who load them wrongly, fly them badly
or worse combine both.

Dave Martin

You make a good point. Some gliders are very resistant to stalls and others
will stall readily - especially with light pilots. It seems that trainers
made in Eastern Europe come equipped with large, effective elevators that
can stall the wing in any attitude. On the other hand, many German single
place glass gliders often have small elevators with limited up authority.

For example the Blanik L-23, IS28 b2 Lark and, as another poster pointed out
the Puchacz, can be stalled from a steep bank easily. For this reason, they
make good trainers since the student must learn to be constantly aware of
pre-stall buffet.

However, the point the Derek was making is that it is more difficult, but
not impossible, to stall in a steep turn. I've had this discussion with
pilots who feared steep banks. I suggest that thermalling is steep banks is
easier in that the glider is more difficult to stall and fewer corrections
are needed to stay in the thermal since the turn diameter is smaller.

Bill Daniels

This is an extremely important caution. Gliders do not all have the
same behavior. Along this line, I have been cautioned that in a turn,
the inside wing, even in a coordinated turn, is flying at a higher
angle of attack than the outside wing. The degree of difference would
vary with the bank angle. Thus, some gliders may not only stall in a
steep turn, they can flick into a spin "out the bottom" in the blink
of an eye. This may not be the case with many (most?) "modern"
ships, but none-the-less is worth keeping in mind I guess.

I don't think the point was that a glider won't stall in a steep turn.
I think the point was that in some gliders with the CG far enough
forward and a small enough stabilizer, the stabilizer can not produce
enough down force when in high G turn to get the wing to the critical
angle of attack and thus stall. Limiting the stabilizer/elevator is a
well known trick to prevent (or least make it more difficult to stall)
stalls if power aircraft, both Ercoupe and Stinson used this trick.
Flying glider with an aft CG and/or large stabilizer will definently
stall in a steep turn. Other might be made to, but it are more
difficult to stall in a turn.

Brian
CFIG/ASEL



"Jose M. Alvarez" wrote in message ...
Don't agree to the idea that the glider won't stall in a steep turn.
First, the stalling speed at 60 degrees of bank is higer due to G forces,
and second, if you point your nose up (with left rudder in a right handed
turn, for example) the speed will drop and cause a stall, even a spin. This
could be very dangerous in a crowded thermal. You can also pull up your
nose, as when entering a thermal at hight speed and pull up, and then bank
hard to center the thermal. Nose up, speed drops, steep bank. Then stall,
maybe spin.
Stall is only a function of AoA, wich is in turn dependant of speed, wing
load and G-load (hope I don't miss anything). Nothing about bank angle
there, IMHO.

Good flights,
Jose M. Alvarez.

"Bill Daniels" escribió en el mensaje
...

"Dave Martin" wrote in message
...
The danger here is that we are talking theory where
we may start to confuse pilots. It is harder to stall
with 60 degrees of bank. Gliders like the K13, by
design run out of elevator in straight and level flight.
They are difficult when flown with heavy pilots to
develop more than a mushing stall in sraight and level
flight.

Brian Case wrote:

I don't think the point was that a glider won't stall in a steep turn.
I think the point was that in some gliders with the CG far enough
forward and a small enough stabilizer, the stabilizer can not produce
enough down force when in high G turn to get the wing to the critical
angle of attack and thus stall. Limiting the stabilizer/elevator is a
well known trick to prevent (or least make it more difficult to stall)
stalls if power aircraft, both Ercoupe and Stinson used this trick.
Flying glider with an aft CG and/or large stabilizer will definently
stall in a steep turn. Other might be made to, but it are more
difficult to stall in a turn.


I completely disagree with the idea that you need more down force
(at any given bank angle, including zero) in order to get a more
nose up attitude. This would mean that the wing airfoil is stable,
which is not the case except for flying wings. What you need is more
elevator deflection, but not because you need more force, rather because
the airflow at the tail plane has a different direction when the attitude
is more nose up, needing a higher deflection even to produce a lower
force. If this would not be the case, the whole aircraft (wing + taiplane)
would be unstable.

A finite wing is one that ends (in a wing tip).

Looking at the evolution from ASW17 to ASW22 to ASW22BL to Eta, the idea of
a finite wing seems to be repellant to many soaring pilots :-)

--
Bert Willing

ASW20 "TW"


"Mark James Boyd" a écrit dans le message de
...
http://www.geocities.com/CapeCanaver...pectratio.html

A picture is worth a thousand words. I have no idea what a
"finite wing" is, but apparently it's important.

I also recall something about aspect ratios and AOA. I think
there was something about a higher aspect ratio wing stalling
at a lower AOA than a lower aspect ratio wing with
the same wing area and loading.

This is not the case. Stall AoA on relatively high-aspect ratio wings
such as on gliders is dependent on airfoil only.

Delta-wings with very low aspect-ratio indeed have a higher stall AoA.



P.S. Also, it's apparently
not important for the FAA to put an index in the
back of the Pilot's Handbook of Aeronautical Knowledge (1997).

And no index for the FAR/AIM? No wonder people buy the
commercial products instead of the gov't ones.