Laminar flow over canopy seam?

Theory predicts that laminar flow is easily triggered to turbulent. Once we go to 3D bodies with a proverse pressure gradient, the flow is much less sensitive and we can get extensive laminar flow.

Looking at wind-tunnel and tow tests from Carmichael, Raspet etc, on an axisymmetrical body the flow is still easily triggered to turbulent, save on the extreme nose. Even a small fly would cause a turbulent wedge.

Now, most wind tunnels have higher levels of turbulence as the atmosphere, so it could very well be that it isn't that bad in reality. Unfortunately, I haven't been able to find any actual testing in flight to determine whether the flow over the canopy and the canopy seam was completely laminar, completely turbulent or something in between.

Obviously, cockpit pressure is a big factor; overpressure most certainly will result in turbulent wedges. Still, does a Mandl (sic?) extractor and a cockpit with mild underpressure result in completely laminar flow over the front fuselage and the canopy?

Talk with Boermanns as he and his pals are the gurus. What you're most likely to get is the hope for re-laminatization after a trip to turbulent. Rough equation is Re_x 100 for even the hope to get this action in a proverse gradient. X is a step or gap or roughness element or even waviness perhaps..

Some nice color flowvis you've probably seen out of the Akafliegs as well, but even there with no seams and proverse gradient, the flow trips to turbulent about 30-40% of the canopy highlight and moves aft from there as you get to the seams. Will also be function of the pressure field around the wing root since that is now a boundary condition for the canopy.

Yaw string definitely trips to turbulent, but we need those.

Under pressure or suction at the seam due to an extractor? If you could get suction on the whole canopy surface then yes, but if it's only at the canopy gap then no because youre still bounded and hoping for re-laminarization, or at best the calibrated amount of suction at the seam, the flow would still see a perfect wall and thus you're back into the proverse gradient question of the canopy/fuse as a whole.

Then you'll get the argument for total drag vs speed and how much comes from what part of the glider. Wing, fuse, tail. Canopy drag is combined in total fuse drag of course.


Good question to ask and will make people think.
Britton

I too had wondered about this question; my PIK
had ( until I fixed it) a 1/2 mm step at the frame,.
Are there degrees of turbulent as one goes downstream,
or just a thickening BL?
Have to do something about the yaw string; how about tiny ports each side
of the nose feeding a vario differentially
and meter on the cowl Audio optional.
John F

At 14:12 05 July 2014, wrote:
Talk with Boermanns as he and his pals are the gurus. What you're most
like=
ly to get is the hope for re-laminatization after a trip to turbulent.
Roug=
h equation is Re_x 100 for even the hope to get this action in a
proverse=
gradient. X is a step or gap or roughness element or even waviness
perhaps=
..=20

Some nice color flowvis you've probably seen out of the Akafliegs as
well,
=
but even there with no seams and proverse gradient, the flow trips to
turbu=
lent about 30-40% of the canopy highlight and moves aft from there as you
g=
et to the seams. Will also be function of the pressure field around the
win=
g root since that is now a boundary condition for the canopy.=20

Yaw string definitely trips to turbulent, but we need those.=20

Under pressure or suction at the seam due to an extractor? If you could
get=
suction on the whole canopy surface then yes, but if it's only at the
cano=
py gap then no because youre still bounded and hoping for
re-laminarization=
, or at best the calibrated amount of suction at the seam, the flow would
s=
till see a perfect wall and thus you're back into the proverse gradient
que=
stion of the canopy/fuse as a whole.=20

Then you'll get the argument for total drag vs speed and how much comes
fro=
m what part of the glider. Wing, fuse, tail. Canopy drag is combined in
tot=
al fuse drag of course.=20


Good question to ask and will make people think.=20
Britton

I asked around a bit in Delft. Seems no documented in-flight tests have been done, at least I couldn't get hold of them. Surprising given how simple that'd really be, though a respirator might be advisable...

Problem of course is that in wind tunnels we don't have seems, nor are levels of ambient turbulence anywhere as low as in the atmosphere.

I guess Britton is referring to wind tunnels tests like these performed in Delft?
http://www.dropbox.com/s/354b4wpodglrw7j/Untitled.jpg

There it's just the effect of the wing/fuselage intersection that pushes the transition point forward.

What I'm wondering about is whether flow over the canopy looks anything like the above test, or we basically have a mostly turbulent canopy, with turbulent wedges originating from all over the canopy seam.

Because if that's the case, a fully flush canopy (that by definition would have to slide fwd for ingress) would cause a major drag reduction. Something like the MOBA:
http://www.dropbox.com/s/vozyc6qby8x...bafullview.jpg

Yes, those are from the family of images I was referring to. Thanks for linking them for everyone's benefit.

Too bad they didn't score the model around the canopy to test this. I know from experience it's hard to take a knife to your perfect model considering what they probably paid for it. Although they still could've added a trip that's appropriately scaled for physical size (if they were matching Reynolds number that is), or scaled appropriately for boundary layer thickness (if not matching) to see the effect.

No doubt that you've spawned some discussion for flight tests in the Delft crowd. Good job.

Britton

J. Nieuwenhuize wrote, On 7/7/2014 12:13 PM:
I asked around a bit in Delft. Seems no documented in-flight tests
have been done, at least I couldn't get hold of them. Surprising
given how simple that'd really be, though a respirator might be
advisable...

Problem of course is that in wind tunnels we don't have seems, nor
are levels of ambient turbulence anywhere as low as in the
atmosphere.

I guess Britton is referring to wind tunnels tests like these
performed in Delft?
http://www.dropbox.com/s/354b4wpodglrw7j/Untitled.jpg

There it's just the effect of the wing/fuselage intersection that
pushes the transition point forward.

What I'm wondering about is whether flow over the canopy looks
anything like the above test, or we basically have a mostly turbulent
canopy, with turbulent wedges originating from all over the canopy
seam.

Because if that's the case, a fully flush canopy (that by definition
would have to slide fwd for ingress) would cause a major drag
reduction. Something like the MOBA:
http://www.dropbox.com/s/vozyc6qby8x...bafullview.jpg

Based on a conversation with Gerhard Waibel 10-15 years ago, the modern
glider canopy is almost entirely laminar. The yaw string will cause a
wedge of turbulent flow behind it; consequently, he recommended
attaching the yaw string as far aft as possible.

I've observed evidence of his statement when in flying cool, humid
conditions. A wedge of condensation will form inside the canopy, behind
the yaw sting, but nowhere else (except possibly some of the rearmost
portions of the canopy). I attribute this to the turbulent air cooling
the canopy under it more than the laminar air elsewhere.

It's my understanding a well-done canopy seam at the front does not trip
the laminar flow into turbulent flow, because the expanding
cross-section of the fuselage in that area produces pressure
distributions that promote laminar flow.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl