Wing tip stalls

Common knowlege about Glider wing designs says that a high taper ratio will
lead to Wing tip stalls.

However in our flights and research we have found that a straight or forward
swept flying wing will not stall a wing tip despite the very narrow chord
( renolds numbers of 325,000) and lack of washout at the tip. I can attempt
a stall in the Pioneer IId by bringing the stick slowly all the way back
giving a CL of about 1.5 at 32 mph. We do have some separation at the root
but there is no sudden wing tip stall despite no washout at the tips. The
glider itself will not stall.

In the gliders I also fly with tails, Grob 103 and 102 there is also a
reluctance to not stall a wing tip but that is perhaps due to the washout at
the tips.

Does the wing tip stall suddenly in some of the high performance ships that
may not have the amount of washout that the lower performing ships? What
gliders if any do stall a wing tip easily?

-mat

mat Redsell wrote:

Common knowlege about Glider wing designs says that a high taper ratio will
lead to Wing tip stalls.

However in our flights and research we have found that a straight or forward
swept flying wing will not stall a wing tip despite the very narrow chord
( renolds numbers of 325,000) and lack of washout at the tip. I can attempt
a stall in the Pioneer IId by bringing the stick slowly all the way back
giving a CL of about 1.5 at 32 mph. We do have some separation at the root
but there is no sudden wing tip stall despite no washout at the tips. The
glider itself will not stall.

The "modern" gliders don't have problems with tip stalls under the same
straight ahead stalls, either. They might have some washout, but may
instead use different airfoils at the root and tip. Winglets can also be
used to prevent tip stalling, and many gliders now have swept back
leading edges that probably reduce the tip stall likelihood. Taper ratio
isn't a very useful predictor for this problem, and it is barely
mentioned in Fundamentals of Sailplane Design.

Turning stalls are the ones you need to study, because almost every
glider can have a tip stall during a turn.


In the gliders I also fly with tails, Grob 103 and 102 there is also a
reluctance to not stall a wing tip but that is perhaps due to the washout at
the tips.

Does the wing tip stall suddenly in some of the high performance ships that
may not have the amount of washout that the lower performing ships? What
gliders if any do stall a wing tip easily?

I don't think this is a problem in the "modern" high performance glider.
The ones I've flown are generally quite benign in stalls, both straight
ahead and turning.

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Eric Greenwell
Washington State
USA

On Wed, 10 Mar 2004 14:13:54 GMT, "mat Redsell"
wrote:

Common knowlege about Glider wing designs says that a high taper ratio will
lead to Wing tip stalls.

Does it?
According to my data stall depends on AoA, nothing else (at least as
long as the wing is not swept). The advantage of a forward-swept weing
is that its unstalled tip is not affected by the turbulence from a
staled inner wing.


Does the wing tip stall suddenly in some of the high performance ships that
may not have the amount of washout that the lower performing ships? What
gliders if any do stall a wing tip easily?

Never heard of this. The consequence of a tip stall would be a really
rapid snap roll, independent of CG - and I'm sure that any glider that
shows such a behaviour would become (in)famous immediately.


Bye
Andreas

Does it?
According to my data stall depends on AoA, nothing else (at least as
long as the wing is not swept). The advantage of a forward-swept weing
is that its unstalled tip is not affected by the turbulence from a
staled inner wing.


See "Fundamentals of Sailplane Design," page 42 in the English
translation by Milgram.

"Aspect ratio and wing taper have a pronounced effect on circulation
and lift distribution.... Fig. 58 shows the influence of taper on
lift distribution. The sharp increase in lift coefficient observed
near the tip of a highly tapered wing indicates that the stall will
develop first in this region. This is why highly tapered wings are
often associated with poor stall characteristics. Rectangular and
moderately tapered wings present less of a problem in this respect."

As far as I can see, there's no explanation there for why this
happens. Stinton, in "The Design of the Aeroplane," presents a
graphical method for estimating lift coefficient on a
non-constant-chord wing, but doesn't explain why taper affects lift
coefficient distribution either.

Sweep isn't discussed by Thomas, but Stinton says (p. 146)

"Swept back wings have two disadvantages at increased angles of attack
and reduced speeds:
- Wing boundary layers tend to drift outboard, assisted by the
spanwise component of flow in fig. 4.13a, which causes them to thicken
and separate prematurely at the tips.
- Sweep staggers the vortices shed across the span, so that those
shed inboard are ahead of those shed further out. This causes
increased upwash ahead of the tip, with a corresponding increase in
tip angle of attack, accompanied by premature peaking of the lift
coefficient.

The overall result is that swept back wing tips stall before the
root... Forward sweep avoids premature tip stall, because the root
stalls first. However, forward sweep has an adverse effect upon
directional stability, and larger fin area is needed than with
sweepback."

If you look at a beginner's hang glider on the launch ramp, sighting
along the leading edge while the wing is developing lift in the wind,
you'll see enormous amounts of wing twist, with the tip clearly at a
negative angle of incidence to the flow but still developing lift!
This is the clearest visualization I've come across of the way that
the "tip" vortices shed inboard result in a higher local AOA on the
outboard portions of a swept wing: the tip is at a negative incidence,
but a positive AOA. With forward sweep, the reverse could happen: the
center section might have a negative angle of incidence but a positive
AOA.

Mat seems to be seeing the two effects offsetting each other: the
forward sweep of his wing (looks like a straight leading edge, forward
swept trailing edge, which is a forward swept wing) tends to make the
root stall first, while the taper ratio would tend to make the tip
stall first. It looks like the sweep is winning.

However, I agree that taper ratio and sweep alone don't seem to be
enough to predict tip-stall behavior, which will also depend on wing
twist and the airfoil used at the root vs tip.

I clearly need to fly more and spend less time with these books!

thanks for your fine reply on wing tip stalls,

I tend to concentrate on the flying and building... and leave the book work
for late at night.

I have produced a number of DVD's on the Pioneer which to me is a most
interesting glider that needs a lot more study.

As to the root stalling first -yes that is as we found it but it is very
gentle. Dave Wells tufted the wing root and yes it progressed from the root
to the tip. In my movie it readily shows that when the one attempts a stall
the Pioneer stays at a certain angle of attack but it can slowly go off to
one side but it can be controlled by the rudder.

If you would like to view this for yourself I will send you a DVD of a
cross country flight with a save from 700 ft agl and include a number of
attempt stalls and a high speed run to 95 mph. I would like your input.

In a turn the Pioneer will not drop a wing tip... I think this needs further
investigation and the new Pioneer III with a modern arifoil, that should fly
this year, will be another glider to study.

-mat

Consider also that it is partially irrelevant whether the taper results in a
forward or backward sweep.
As the wing chord reduces so the Reynolds number changes (for any given speed),
and the behavior with it for any given airfoil. The lower Reynolds number at the
narrow chord will result in more tetchy behavior for the laminar profiles
preferred for the overall wing. I assume that this is one of the reasons the
aerodynamics types make quite substantial airfoil changes on the tapered
sections of the wings. This is probably a larger factor in design than the
minimal degree of sweep forward or back possible in a high aspect ratio
sailplane wing.

Depending on what you want from your airplane it may be a desirable compromise
to live with the more interesting high AOA behavior of a compound taper wing.
For the performance improvements possible it is presumably worth the effort for
high performance XC ships.

Presumably this is one reason why the Piper Cherokee has it's "hershey bar"
wing. Only washout effects, so easier to predict behavior. The other reason to
go with simple wing planforms is the cost involved...

Bruce

mat Redsell wrote:

thanks for your fine reply on wing tip stalls,

I tend to concentrate on the flying and building... and leave the book work
for late at night.

I have produced a number of DVD's on the Pioneer which to me is a most
interesting glider that needs a lot more study.

As to the root stalling first -yes that is as we found it but it is very
gentle. Dave Wells tufted the wing root and yes it progressed from the root
to the tip. In my movie it readily shows that when the one attempts a stall
the Pioneer stays at a certain angle of attack but it can slowly go off to
one side but it can be controlled by the rudder.

If you would like to view this for yourself I will send you a DVD of a
cross country flight with a save from 700 ft agl and include a number of
attempt stalls and a high speed run to 95 mph. I would like your input.

In a turn the Pioneer will not drop a wing tip... I think this needs further
investigation and the new Pioneer III with a modern arifoil, that should fly
this year, will be another glider to study.

-mat