Harder to stall in a steep turn?

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 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.

Anyone know any ref to this? I'm not certain this is correct,
but it seems to work for the Piper Tomahawk :-P

On Mon, 28 Jul 2003 10:29:48 -0600, "Bill Daniels"
wrote:


"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.

Bill Daniel

Your statement about the glider becoming nose-heavy from the load
factor in a turn gives me a very clear picture of what is going on.
Thank you.

"Eric Greenwell" wrote in message
.. .
In article ,

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)

Eric, that's an interesting observation. One should never forget the
effects of local flow around an aircraft. No doubt the effectiveness of the
elevator is effected by the downwash from the wing - even if it's a "T"
tail.

I would tend to think, however, that the wing producing 2x lift would have
an increased downwash which should increase the negative AOA of the
stabilizer thus opposing the nose-down effect of the CG being ahead of the
center of lift. This would tend to increase up-elevator authority. But
then these are difficult things to visualize.

What is clear is that the down (relative to the lateral axis) vector from
the CG is twice as large in a 60 degree bank as in level flight and that the
lift from the center of lift is also twice as large producing a large
nose-down moment that must be opposed by the stab/elevator. If the
up-elevator authority limit is reached before the wing stalls, the case
described by Derek is true.

Bill Daniels

"Bill Daniels" wrote in message
...


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.

Thank you for that. I suspect that there may be a little more to it
than that, but your explanation is clear, concise, and portable enough that
a few of us CFIGs will probably be stealing it.

Vaughn


Bill Daniels

Jim writes:

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."

Please be patient with a long-time power pilot attempting to make the
transition to gliders, but I am having considerable difficulty imagining
that any aircraft which can be brought to a stalling angle of attack with
the elevators at a given speed should have so much more difficulty doing so
in one attitude than another. Surely what we have here is a statement by
Piggot the truth of which rests upon some unspoken assumptions and a rather
more specific scenario than we are attributing to him.

I have not read "Gliding" by Piggot, but I am currently reading his
"Understanding Gliding". His explanations of maneuvering flight regimes seem
to suffer from an attempt to explain flight dynamics in layman's terms.
Piggot, for all his vast experience in gliding and teaching, is sometimes as
awkward to read as was Langewiesche with his references to "flippers"
instead of "ailerons".



Jack

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

Empirically - It is harder to stall in a steeply banked turn.

Just don't expect all aircraft to be stall proof at 60 degrees. My
Cirrus would bite anyone silly enough to try it...

It depends on the design, but most conventional (as opposed to all
moving) tail designs will reach a load and aerodynamic situation at some
bank angle where they will not stall.

My attempt to explain : -

Load on tail increases with speed. This is a function of the stability
requirements.
As mainplane AOA increases so the tailplane AOA decreases. Limiting case
here is that once the glider is fully stalled the the tailplane will now
produce an up load - pitching the nose down.
Maximum deflection of elevator gives some fixed AOA relative to the
aircraft centerline.
In any attitude where the wings are being subjected to 2G the aircraft
must be describing a "pull up" path (circular if the load is constant)

This implies the mainplane is at a relatively large AOA, say X degrees
larger than 1G. Trigonometry says the maximum AOA of the
tailplane+elevator is reduced by this amount. (They are both exposed to
roughly the same relative wind, although downwash from the mainplane can
change the angle at the tail slightly)

So - assume your wing needs to be at 4 degrees above the 1G point to
generate 2G, then your effective elevator range at 2G is the same as if
you limited your tailplane+elevator to 4 Degrees less.
Add to this that the load needed to overcome the nose down torque will
be higher than at 1G, because the CG is in front of the AC and AC moves
backwards as AOA increases (until the stall).
4 degrees is a significant fraction of the total AOA range of the
tailplane which will only operate effectively up to around 16 degrees.
You have reduced your control authority by around 25%.
Add to the fact that the airflow will probably be more turbulent and the
tailplane therefore less efficient and you have an explanation.

Sound right?

In article ,
Jack wrote:

Piggot, for all his vast experience in gliding and teaching, is sometimes as
awkward to read as was Langewiesche with his references to "flippers"
instead of "ailerons".

Perhaps because the term "flippers" was referring to the elevator?

-- Bruce

I fully agree, and I might add that entering a spin from a 60 deg bank -
when it happens - is a totally different story than spinning out of a
shallow bank. The difference in behaviour increases as wing span increases.
Spinning a 25m glider out of a 60 deg bank is something I experienced once
and I don't want to experience it another time.

Whether or not a specific glider will spin out of a steep bank is nothing to
learn from books. When you're low on a ridge, or centering a strong core at
600ft agl in flat country, there is more to flight dynamics that elevator
authority - a strongs gust or wind shear in such a situation makes any
sailplane stall.

--
Bert Willing

ASW20 "TW"
"Dave Martin" a écrit dans le message
de ...
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