I noticed for the first time when flying at speeds above 200 km/h, my wingtips start to bend down a bit, has anyone ever noticed this? Glider is a DG-100.

Yes.

I flew a DG-100 at the F1GP. Approaching redline the downward bend was impressive.

On Wednesday, September 11, 2019 at 10:00:12 AM UTC-4, Senna Van den Bosch wrote:
> I noticed for the first time when flying at speeds above 200 km/h, my wingtips start to bend down a bit, has anyone ever noticed this? Glider is a DG-100.

Yep. The twist that helps provide the very docile handling hurts at higher speeds.
The airfoil is not very efficient above 150 or so. Not much reason to fly very fast.
FWIW
UH

On 9/11/2019 8:11 AM, wrote:
> On Wednesday, September 11, 2019 at 10:00:12 AM UTC-4, Senna Van den Bosch
> wrote:
>> I noticed for the first time when flying at speeds above 200 km/h, my
>> wingtips start to bend down a bit, has anyone ever noticed this? Glider
>> is a DG-100.
>
> Yep. The twist that helps provide the very docile handling hurts at higher
> speeds. The airfoil is not very efficient above 150 or so. Not much reason
> to fly very fast. FWIW UH

"What UH said."

Me knowing zero/zip/nada about the DG-100's airfoil choice(s?), reason for the
downward-bend-phenomenon at higher speeds lies in some flavor of
geometric/aerodynamic twist from root to tip, where - in essence - the tip
airfoil is at a lower/reduced angle of incidence (and hence angle of attack)
to the root airfoil...done for the reason UH mentions, i.e. to ensure the root
stalls prior to the tip, helping ensure low-speed behavior is benign. None of
this "sudden tip stalling" excitement to worry about. Twist is a
less-graceless manner of avoiding (say) stall strips often seen inboard on
power planes (e.g. Bonanzas).

At higher speeds (lowering AoA), reversed aerodynamic effects become apparent,
as you've now seen. The tip will reach negative AoA prior to the root. Cool!

If the reason is geometric twist from root-to-tip (i.e. the same airfoil),
sometimes you can actually *see* that by gazing from tip to root along the
apar. Kinda cool, for the wannabe aerodynamicist/designer in you!

Bob W.

---
This email has been checked for viruses by AVG.
https://www.avg.com

On Wednesday, September 11, 2019 at 10:11:06 AM UTC-4, wrote:
> On Wednesday, September 11, 2019 at 10:00:12 AM UTC-4, Senna Van den Bosch wrote:
> > I noticed for the first time when flying at speeds above 200 km/h, my wingtips start to bend down a bit, has anyone ever noticed this? Glider is a DG-100.
>
> Yep. The twist that helps provide the very docile handling hurts at higher speeds.
> The airfoil is not very efficient above 150 or so. Not much reason to fly very fast.
> FWIW
> UH

Yup. The wing twists because of inadequate torsional stiffness.
IIRC, as you go faster, the center of lift moves aft, so twists the wing.
And as it twists, it aggravates the situation by further reducing tip AOA.
Later designs use a variety of trickery to minimize this effect.
Carbon wing skins (stiffer) help, hence big reduction in twist from
LS6a to LS6b with corresponding high-speed performance improvement.

Recent designs (eg from Boermans, Antares and later) use tailored airfoils
rather than twist to obtain gentle stall and minimize this.
On open and higher-aspect-ration ships this is quite challenging!
Concordia design worked hard to minimize twist (Dillinger's Phd thesis).
Don't know about Nexis.

Interesting stuff...
See ya, Dave

Op woensdag 11 september 2019 16:54:07 UTC+2 schreef BobW:
> On 9/11/2019 8:11 AM, wrote:
> > On Wednesday, September 11, 2019 at 10:00:12 AM UTC-4, Senna Van den Bosch
> > wrote:
> >> I noticed for the first time when flying at speeds above 200 km/h, my
> >> wingtips start to bend down a bit, has anyone ever noticed this? Glider
> >> is a DG-100.
> >
> > Yep. The twist that helps provide the very docile handling hurts at higher
> > speeds. The airfoil is not very efficient above 150 or so. Not much reason
> > to fly very fast. FWIW UH
>
> "What UH said."
>
> Me knowing zero/zip/nada about the DG-100's airfoil choice(s?), reason for the
> downward-bend-phenomenon at higher speeds lies in some flavor of
> geometric/aerodynamic twist from root to tip, where - in essence - the tip
> airfoil is at a lower/reduced angle of incidence (and hence angle of attack)
> to the root airfoil...done for the reason UH mentions, i.e. to ensure the root
> stalls prior to the tip, helping ensure low-speed behavior is benign. None of
> this "sudden tip stalling" excitement to worry about. Twist is a
> less-graceless manner of avoiding (say) stall strips often seen inboard on
> power planes (e.g. Bonanzas).
>
> At higher speeds (lowering AoA), reversed aerodynamic effects become apparent,
> as you've now seen. The tip will reach negative AoA prior to the root. Cool!
>
> If the reason is geometric twist from root-to-tip (i.e. the same airfoil),
> sometimes you can actually *see* that by gazing from tip to root along the
> apar. Kinda cool, for the wannabe aerodynamicist/designer in you!
>
> Bob W.
>
> ---
> This email has been checked for viruses by AVG.
> https://www.avg.com

Wow, never knew about this until now, I could actually see a nice curve going from the root of the wing to the tip, pretty cool to see, but had me a little worried (220 kph and I see that for the first time). I usually don't fly above 150, however that was fun to pass alongside the airfield at low altitude at the end of the day :)

On Wednesday, September 11, 2019 at 7:11:06 AM UTC-7, wrote:
> On Wednesday, September 11, 2019 at 10:00:12 AM UTC-4, Senna Van den Bosch wrote:
> > I noticed for the first time when flying at speeds above 200 km/h, my wingtips start to bend down a bit, has anyone ever noticed this? Glider is a DG-100.
>
> Yep. The twist that helps provide the very docile handling hurts at higher speeds.
> The airfoil is not very efficient above 150 or so. Not much reason to fly very fast.
> FWIW
> UH

What always impressed me was the up down flex of my Nimbus 4 wings. Especially in Sierra turbulence those wing tips must have been bouncing 12 feet.

On Wednesday, September 11, 2019 at 9:54:32 AM UTC-7, Senna Van den Bosch wrote:
>
> Wow, never knew about this until now, I could actually see a nice curve going from the root of the wing to the tip, pretty cool to see, but had me a little worried (220 kph and I see that for the first time). I usually don't fly above 150, however that was fun to pass alongside the airfield at low altitude at the end of the day :)

The LS4 was the same. The washout that makes it so nice to fly at normal speeds does that.
The best fix is... Don't look at the tips near red line.
Jim

Op woensdag 11 september 2019 19:54:37 UTC+2 schreef JS:
> On Wednesday, September 11, 2019 at 9:54:32 AM UTC-7, Senna Van den Bosch wrote:
> >
> > Wow, never knew about this until now, I could actually see a nice curve going from the root of the wing to the tip, pretty cool to see, but had me a little worried (220 kph and I see that for the first time). I usually don't fly above 150, however that was fun to pass alongside the airfield at low altitude at the end of the day :)
>
> The LS4 was the same. The washout that makes it so nice to fly at normal speeds does that.
> The best fix is... Don't look at the tips near red line.
> Jim

I still have the standard tips, which have a downward skid, which are the same as you can see on this picture: http://aviation.derosaweb.net/dg101/images/N101RP_deploy.JPG
Would having the optional TN 301/20 wingtips/winglets fitted make a difference to this downward flexing?

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 picture: http://aviation.derosaweb.net/dg101/images/N101RP_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

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 picture: http://aviation.derosaweb.net/dg101/images/N101RP_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 picture: http://aviation.derosaweb.net/dg101/images/N101RP_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 . .