ASW 20 SPIN CHARACTERISTICS

Martin Gregorie wrote:

This is the usual result of a stall, and is what occurs in the typical
training situation, but it isn't the definition of a stall. Generally, a
stall begins when the airflow starts to separate from the wing at
increasing AOA. It is this separation that keeps the lift from
increasing and sets the maximum lift coefficient.


Usually major airflow separation coincides with a stall and the drag
increase ensures that a stall will happen because of the associated
loss of airspeed. However, flow separation is not the same as a stall.

Perhaps we are not discussing the same thing. It sounds like you are
talking about "a stall", meaning the aircraft's behavior from the pilots
viewpoint (buffeting, loss of lift, poor control, etc), and I am talking
about the aerodynamic situation during "a stall" (high AOA leading to
flow separation and constant or diminishing lift coefficient).

Many aircraft have quite a high degree of flow separation during low
speed flight. In the model world we assume separation always occurs at
about 60% chord at min.sink and this would appear to be close to the
mark for sailplanes judging by Will Schumann's experiments.

I think our modern airfoils have very little separation at minimum sink,
and certainly far aft of the 60% point. Instead of "separation", perhaps
you mean the transition from laminar flow to turbulent flow? That does
occur somewhere around the 60% point (maybe 70% or so) on modern airfoils.


A wing can be stalled and still produce plenty of lift; for example, in
a high speed pull up done with too much elevator can stall the wing, but
the stalled wing will still have more lift than the weight of the
aircraft because of the high speed.


I would normally call that a high drag flight regime rather than a
stall.

I agree it is not "a stall", but I think is sometimes referred to as
"stalled flight", and the wing is considered "stalled". For some
aircraft, like fighters with their powerful engines, it is a useful
situation. For gliders, I think any time the AOA is high enough to stall
the wing, the glider will suffer "a stall", regardless of the load on it!


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Eric Greenwell
Washington State
USA

On Sun, 18 Jul 2004 08:27:23 -0700, Eric Greenwell
wrote:

Perhaps we are not discussing the same thing. It sounds like you are
talking about "a stall", meaning the aircraft's behavior from the pilots
viewpoint (buffeting, loss of lift, poor control, etc), and I am talking
about the aerodynamic situation during "a stall" (high AOA leading to
flow separation and constant or diminishing lift coefficient).

I think that's partly true. I meant 'A stall' as in what happens as
the wing becomes no longer able to support the aircraft, not what
happens if you keep the stick back and the situation stabilises with a
high but constant descent rate.

I think our main disagreement is whether the aircraft really reaches
the constant Cl, increasing Cd region, let alone the diminishing Cl
region. It may do that, but the AoA would need to be very large indeed
- over 20 degrees at a guess.

I've not played with calibrated AoA indicators. If you have, what AoA
was reached at the stall? I'm curious.

Many aircraft have quite a high degree of flow separation during low
speed flight. In the model world we assume separation always occurs at
about 60% chord at min.sink and this would appear to be close to the
mark for sailplanes judging by Will Schumann's experiments.

I should read back more carefully before hitting SEND. I meant 80%.
Sorry 'bout that.

I think our modern airfoils have very little separation at minimum sink,
and certainly far aft of the 60% point. Instead of "separation", perhaps
you mean the transition from laminar flow to turbulent flow? That does
occur somewhere around the 60% point (maybe 70% or so) on modern airfoils.

Depends on the surface texture and Re number: the turbulent transition
is just behind the hi-point with a paper covered surface and Re =
50,000. I'd guess the separation point was about at the aileron hinge
line on a Discus 1 - otherwise why put the turbulator there? Its job
is to increase the boundary layer energy by forcing a transition from
laminar to turbulent and hence causing separation to be delayed.
Without measuring the wing, that must be in the 80% ballpark.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Martin Gregorie wrote:


I've not played with calibrated AoA indicators. If you have, what AoA
was reached at the stall? I'm curious.

I haven't used calibrated ones either, so I don't know.


I think our modern airfoils have very little separation at minimum sink,
and certainly far aft of the 60% point. Instead of "separation", perhaps
you mean the transition from laminar flow to turbulent flow? That does
occur somewhere around the 60% point (maybe 70% or so) on modern airfoils.


Depends on the surface texture and Re number: the turbulent transition
is just behind the hi-point with a paper covered surface and Re =
50,000. I'd guess the separation point was about at the aileron hinge
line on a Discus 1 - otherwise why put the turbulator there? Its job
is to increase the boundary layer energy by forcing a transition from
laminar to turbulent and hence causing separation to be delayed.
Without measuring the wing, that must be in the 80% ballpark.

I was talking about the separation on the top surface at high AOA
during a "stall situation". I now realize you were talking about laminar
flow separation on the bottom surface, which isn't related to the stall
situation.

For the modern laminar airfoils, the transition (from laminar flow to
turbulent flow on the bottom of the airfoil) is at least 80% or more. On
my ASH 26 E, the turbulators are on the flaps and ailerons at about 95%.

The transition from laminar flow to turbulent flow on the top of the
airfoil is sooner, perhaps in the 60%-80% range. There is rarely a
laminar flow separation, though the Speed Astir is a well-known example.

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Eric Greenwell
Washington State
USA

On Sun, 18 Jul 2004 12:45:02 -0700, Eric Greenwell
wrote:

I was talking about the separation on the top surface at high AOA
during a "stall situation". I now realize you were talking about laminar
flow separation on the bottom surface, which isn't related to the stall
situation.

Actually, I was talking about upper surface separation at low speed -
thermalling regime. Our oldest club Discus has zigzag turbs just ahead
of the aileron hinges and those were the turbs I was thinking about.
I'm not sure how common there are, come to think of it, because the
other club ship (Czech-bult with tiplets) doesn't have them.

I forgot about the lower surface turbs, but I think their placement is
due to airfoil shape rather than anything else. I've only seen them in
front of the narrow undercambered area under the TE and assumed they
were to stop separation in the undercamber dish at the top end of the
speed range.

I hope I didn't cause too much confusion there.

For the modern laminar airfoils, the transition (from laminar flow to
turbulent flow on the bottom of the airfoil) is at least 80% or more. On
my ASH 26 E, the turbulators are on the flaps and ailerons at about 95%.

Interesting - I've never seen a 26E close enough to know what its
airfoil looks like. Does it also have a somewhat hooked trailing edge?

The transition from laminar flow to turbulent flow on the top of the
airfoil is sooner, perhaps in the 60%-80% range. There is rarely a
laminar flow separation, though the Speed Astir is a well-known example.

Could it have been more of a problem on the early glass? I've read
Will Schueman's article about the development of his triple break
leading edge a couple of times. The separation bubble on his ASW-12
seems to have been huge and thick. His analysis of the problem and the
way he went about developing the fix is a classic.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Martin Gregorie wrote:

On Sun, 18 Jul 2004 12:45:02 -0700, Eric Greenwell
wrote:


I was talking about the separation on the top surface at high AOA
during a "stall situation". I now realize you were talking about laminar
flow separation on the bottom surface, which isn't related to the stall
situation.


Actually, I was talking about upper surface separation at low speed -
thermalling regime. Our oldest club Discus has zigzag turbs just ahead
of the aileron hinges and those were the turbs I was thinking about.
I'm not sure how common there are, come to think of it, because the
other club ship (Czech-bult with tiplets) doesn't have them.

Turbulators on the top of the wing are uncommon. Except for a Speed
Astir, I haven't seen any, not even on Discus(es?). Generally, I believe
the separation that occurs while thermalling is not laminar flow
separation (which would start around 60% or so on the airfoil), but
turbulent flow separation starting at/near the trailing edge at the
onset of stall (flying too slowly).

I forgot about the lower surface turbs, but I think their placement is
due to airfoil shape rather than anything else. I've only seen them in
front of the narrow undercambered area under the TE and assumed they
were to stop separation in the undercamber dish at the top end of the
speed range.

I hope I didn't cause too much confusion there.


For the modern laminar airfoils, the transition (from laminar flow to
turbulent flow on the bottom of the airfoil) is at least 80% or more. On
my ASH 26 E, the turbulators are on the flaps and ailerons at about 95%.


Interesting - I've never seen a 26E close enough to know what its
airfoil looks like. Does it also have a somewhat hooked trailing edge?

Well, it is a flapped ship, so the trailing edge can deflected down 10
degrees or so. The flap and aileron seem to have a slight concavity on
the top side. The ASW 27 is essentially identical, and they both use
blow turbulators, like the ASW 20 models.


The transition from laminar flow to turbulent flow on the top of the
airfoil is sooner, perhaps in the 60%-80% range. There is rarely a
laminar flow separation, though the Speed Astir is a well-known example.


Could it have been more of a problem on the early glass?

I haven't heard that it was. I think it was eventually found on the
Astir because it performed so far below expectations, that much effort
went into discovering the cause.

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Eric Greenwell
Washington State
USA