ASW 20 SPIN CHARACTERISTICS

Jack wrote in message

They unflex because the load is removed. In order to stall they would
have to unflex faster than the load is removed. If the pilot is pushing,
the stall speed can go _way_ down. It's when he starts to load the wings
again that his technique, or failure to track his loss of airspeed in
the pull/push, can bring about the stall.


Jack

Isn't it also true that lift will be reduced if a highly flexed wing
increases its angle attack beyond the critical angle of attack. If
the wing is producing less lift it cannot maintain the flex that
existed before the critical angle of attack was exceeded. As the wing
unflexes the tip angle of attack increases.

Which of those assumptions is invalid?

Andy

Chris OCallaghan wrote:
....
When thermalling, use flap position 4, or drill a hole between
positions 3 and 4 if you want less drag.

Thank you, Chris. My 20B had a 3.5 hole drilled when we bought it but I
had no advice on its purpose or use. I was puzzled when I couldn't
relate the Flight Manual descriptions to what my glider had. Eventually
I worked it out.

I would be interested in any advice anyone can give on the use of 3.5.
I tend to use it for nearly all thermalling and only use 4 for landing.
Am I right? Is a 3.5 hole common? Did Schleicher's get it wrong?

GC

On Sat, 17 Jul 2004 00:36:28 +1000, Graeme Cant
wrote:


Thank you, Chris. My 20B had a 3.5 hole drilled when we bought it but I
had no advice on its purpose or use. I was puzzled when I couldn't
relate the Flight Manual descriptions to what my glider had. Eventually
I worked it out.

I would be interested in any advice anyone can give on the use of 3.5.
I tend to use it for nearly all thermalling and only use 4 for landing.
Am I right? Is a 3.5 hole common? Did Schleicher's get it wrong?

It depends what you use 4 for: For thermalling at normal bank angles
(20-30 degrees) 3 is the better setting (the 20 converst excessive
speed into height a lot better in 3 than in 4), but very tight turns
combined with high wing loading (or forward CG) need 4.

Many 20 owners drilled the 3.5 hole, but I have to admit that I tested
this setting and I never felt 3.5 to be an advantage over 3 (I hold
the flap handle at 3.5 before I decided not to drill a hole there). At
3 the nose is significantly higher than in 3.5, but I think the 20's
airfoil loves high AoA's. If the AoA of the 20 is too low (in other
words: Flap setting too positive for current airspeed/g-load
combination), the drag rise is drastic - very easy to feel the
deceleration.

One more thing why I love the 20: The flap handle tells you which
position it wants to be in - it moves itself into the optimum position
(if you help it overcome the friction with your hand).

Bye
Andreas

On 16 Jul 2004 07:03:55 -0700, (Andy Durbin)
wrote:


Isn't it also true that lift will be reduced if a highly flexed wing
increases its angle attack beyond the critical angle of attack. If
the wing is producing less lift it cannot maintain the flex that
existed before the critical angle of attack was exceeded. As the wing
unflexes the tip angle of attack increases.

Which of those assumptions is invalid?

All of threse assumptions are valid.
But if the tip is already stalled it doesn't matter a lot if it has
exceeded its maximum AoA by 2 degrees or 6 - the resulting stall is
the same.

But to stall a 20 with sufficient g-load to get significant wing flex,
you need a lot of speed and a nearly fully pulled-back stick.

I havent's flown a 20 for a couple of years now - maybe one of the
current 20 drivers could check this out, but I doubt that at 4g the
wing flex is more than three ft compared to level flight. The
resulting AoA change as the wing reflexes will be less than 1.5
degrees then.... neglectable.

Bye
Andreas

Andy Durbin wrote:

Jack wrote in message

They unflex because the load is removed.

Isn't it also true that lift will be reduced if a highly flexed wing
increases its angle attack beyond the critical angle of attack. If
the wing is producing less lift it cannot maintain the flex that
existed before the critical angle of attack was exceeded. As the wing
unflexes the tip angle of attack increases.

I see nothing in your comments which is invalid. If the wing unflexes
due to stall and resultant loss of lift, the further increase in the
angle of attack during the unflex may be of little consequence.


--
Jack

In article ,
Andreas Maurer wrote:

But if the tip is already stalled it doesn't matter a lot if it has
exceeded its maximum AoA by 2 degrees or 6 - the resulting stall is
the same.

No it's not. There isn't a "maximum angle of attack". There is only an
"angle of attack for maximum lift". As you approach that angle of
attack the rate of lift increase gets smaller and smaller, then you get
the same amount of lift at slightly increasing angles of attack, and
then with still more angle of attack you get less lift. The more you
take the angle of attack past the point of maximum lift the less lift
you get.

So, yes, it does matter whether you are 2 degrees or 6 degrees past the
angle of attack for maximum lift.

-- Bruce

On Fri, 16 Jul 2004 09:22:52 +0200, "Bert Willing"
wrote:

Stalling of a wing is connected to AoA in the first place, nothing else.

I must respectfully disagree - the load being carried by the wing is
at least as important as the AoA.

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

In article ,
Martin Gregorie wrote:

On Fri, 16 Jul 2004 09:22:52 +0200, "Bert Willing"
wrote:

Stalling of a wing is connected to AoA in the first place, nothing else.

I must respectfully disagree - the load being carried by the wing is
at least as important as the AoA.

I'm afraid that turns out not to be the case.

Stalling depends on the AoA, and only the AoA (Reynolds number effects
aside).

The amount of lift generated depends only on the AoA and the airspeed.

The amount of lift necessary to support the aircraft against an
acceleration of 1 gravity depends on the load being carried. For each
load there is a minimum airspeed below which the amount of lift
necessary to support that load against gravity can not be generated.
But if you don't insist on trying to support the load against gravity
(that is, trying to increase the AoA until sufficient lift is generated,
thus stalling the wing) then you can be in perfect control and not
stalled at as low an airspeed as you like.

Which brings us back to: stalling of a wing is connected to the AoA,
nothing else.

-- Bruce

Bruce Hoult wrote:

In article ,
Martin Gregorie wrote:

...the load being carried by the wing is
at least as important as the AoA.

[snippage]

...if you don't insist on trying to support the load...then you can
be in perfect control and not stalled at as low an airspeed as you like.

Bruce, it would appear that you and Martin are in agreement.


Jack

Jack wrote:
Bruce Hoult wrote:

In article ,
Martin Gregorie wrote:


...the load being carried by the wing is

at least as important as the AoA.


[snippage]

...if you don't insist on trying to support the load...then you can

be in perfect control and not stalled at as low an airspeed as you like.

Bruce, it would appear that you and Martin are in agreement.

Appearances can be deceiving...

If you look at the Coefficient of lift diagrams for airfoils, you see
that it is dependent only on AOA, not load. In other words, a wing will
stall at the same AOA at .5 G, 1 G, 2 G, etc. I think this is what Bruce
is saying. Martin is wrong to say the load is as important as AOA, and
that is why some ras posters think we should have AOA indicators in our
gliders.

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

Eric Greenwell
Washington State
USA