Wing flex at higher speed DG-100

Op woensdag 11 september 2019 20:33:06 UTC+2 schreef Steve Leonard:
On Wednesday, September 11, 2019 at 1:16:02 PM UTC-5, Senna Van den Bosch wrote:

I still have the standard tips, which have a downward skid, which are the same as you can see on this pictu http://aviation.derosaweb.net/dg101/...1RP_deploy.JPG
Would having the optional TN 301/20 wingtips/winglets fitted make a difference to this downward flexing?

No. Any small change in the last 10-12 inches will not change the twist that has been imparted of the last 10 feet of the wing.

Steve Leonard

Still very interesting to see that happen. Are there any other gliders that have this effect at higher speeds?

On Wednesday, September 11, 2019 at 4:02:17 PM UTC-4, Senna Van den Bosch wrote:
Op woensdag 11 september 2019 20:33:06 UTC+2 schreef Steve Leonard:
On Wednesday, September 11, 2019 at 1:16:02 PM UTC-5, Senna Van den Bosch wrote:

I still have the standard tips, which have a downward skid, which are the same as you can see on this pictu http://aviation.derosaweb.net/dg101/...1RP_deploy.JPG
Would having the optional TN 301/20 wingtips/winglets fitted make a difference to this downward flexing?

No. Any small change in the last 10-12 inches will not change the twist that has been imparted of the last 10 feet of the wing.

Steve Leonard

Still very interesting to see that happen. Are there any other gliders that have this effect at higher speeds?

Almost all do, but to varying degrees. Modern ships generally have accomplished the aerodynamic objective by using airfoil changes toward the tip to get benign stall, and by having structures that a more stiff in torsion.
UH

On Thursday, 12 September 2019 06:02:17 UTC+10, Senna Van den Bosch wrote:

Still very interesting to see that happen. Are there any other gliders that have this effect at higher speeds?

Yes. Particularly prevalent in the open class ships of the 80s & 90s vintage. Watch an ASH 25 come screaming across the line at close to VNe. Looks like a giant McDonalds sign!

A twist of this nature is called washout generally. In one famous example,the Slingsby KingbKite was accidentally built with the opposite, wash in. It’s spin behaviour was character building to say the least.

On Wed, 11 Sep 2019 15:56:27 -0700, Justin Couch wrote:

On Thursday, 12 September 2019 06:02:17 UTC+10, Senna Van den Bosch
wrote:

Still very interesting to see that happen. Are there any other gliders
that have this effect at higher speeds?

Yes. Particularly prevalent in the open class ships of the 80s & 90s
vintage. Watch an ASH 25 come screaming across the line at close to VNe.
Looks like a giant McDonalds sign!

Not quite the same, but its also fun to watch an Ash 25 on finals. See
how it changes from an elegantly curved wing to anhedral near the tips
when the brakes are opened.

More to the point: at thermalling speeds the centre of lift is around 33%
of the wing chord - pretty close to the spar position, but flying faster
moves the centre of lift rearward away from the spar, so this can twist
flexible glass wings toward a lower AOA. This will cause an increase in
washout at the wingtip.

Do 201 Standard Libelles do this? I can't recall ever looking at the
wings when flying fast in mine.


--
Martin | martin at
Gregorie | gregorie dot org

Libelles definitely do it, although not as pronounced as the DG100. The Standard Cirrus would probably do it if it’s VNE was high enough.

Cheers,
Nick

That's not do much connected to the airbrakes, but rather to the outboard flaps moving to a negative position when setting flps to the negative position. Same principle as with ASW20's.

On Thu, 12 Sep 2019 04:46:06 -0700, Tango Whisky wrote:

That's not do much connected to the airbrakes, but rather to the
outboard flaps moving to a negative position when setting flps to the
negative position. Same principle as with ASW20's.

Agreed - predictably I remembered that after I'd hit 'send'.

--
Martin | martin at
Gregorie | gregorie dot org

On Wed, 11 Sep 2019 15:56:27 -0700 (PDT), Justin Couch
wrote:

Watch an ASH 25 come screaming across the line at close to VNe. Looks like a giant McDonalds sign!


Err - no.

In the ASH-25 wing flex is used to check the correct flap settings:
The wing must have a constant bending over the wing span in the
complete speed range.

If the bending changes with varying speeds and flap settings,
something is worn. The flaps need to be re-adjusted. Not a trivial
task.

Ask me how I know.

Interesting is the landing setting where the ASH-25 wing is designed
that the complete outer trapeze creates negative lift at speeds over
90 kp/h, therefore creating the impressive anhedral during landing.

BTW: Nothing beats the ASW-20L with the 16.60m wing tips in flaps 5
and red line (120 kp/h).

Cheers
Andreas

The 25 has an "interesting" "gearbox" for the control surfaces on the
wings from negative flap to flap 4 the trailing edge moves in a fairly
uniform way (all 3 trailing edge controls per wing)
so coming in, in full negative to do a "beat up" the wing bends up quite
impressively.
As you slow down and find yourself underbraking due to the pitiful
brakes and go to flap L
All bets are off on the trailing edges, the inboard "flaps" come down
almost vertically to act as brakes, the mid wing control surface comes
down a little to act as flap and the outboard ailerons get kicked up to act

like spoilerons (still work as ailerons but almost zero positive effect)

The RC guys use the same effect and call it "Crow" braking (as it mimics
a crow coming in to land hot)

An underbraked approach into a short field has a 25 going from wings
point up to wings point down, quite entertaining.

There is a dodge used by 25 pilots to stop a bounced landing of having
P2 ready on the flaps and as the main gear touches, go from flap 4
(recomended landing flap) to full negative flap.
It does stop a bounce, but can end up belting the tail off the deck quite
positively.
Alternate bounce prevention is to "fly it on" and have the wheelbrake
applied a bit, that pitches the nose forward as the wheel bites thus
"sticking" the glider (which has an effective enough pitch stability to
avoid touching the nose (usually, YMMV)
Alternatively, you can land it properly with a fully held off landing . .
rarely managed any of those . .

Complete sidetrack, aplogies, Miss the old 25 sometimes.
Not times that involve rigging, ground handling, flying fast or thermalling

. . but sometimes . .