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  #21  
Old 08-29-2020, 03:20 PM
scsmith scsmith is offline
 
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Quote:
Originally Posted by PhatRV View Post
Thanks for the correction. I re read the article and it says the turbulent flow has less drag because it has lower skin friction drag but higher form drag
Other way around. Turbulent boundary layers have higher skin friction. This is because the small eddies embedded in the boundary layer mix higher velocity flow down closer to the surface, so there is more surface shear stress.

But turbulent boundary layers are more resistant to separation, so less chance of causing a bunch of pressure drag (form drag) from a separated flow region. They are more resistant to separation for the same reason as the higher surface shear stress -- the eddies mixing in the boundary layer bring fresh, higher energy flow down into the boundary layer.

The laminar bubble that forms as part of the transition process generally is very small and creates very little drag. Laminar bubbles can get big enough to create some drag, and the alternative is to force transition to a turbulent boundary layer ahead of where the laminar bubble would form. This is done with zig-zag tape or dimpled trip tape, or with pneumatic blow holes. This technique of tripping the boundary layer to avoid laminar bubbles can be quite successful, as it was on the Std Cirrus.
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  #22  
Old 08-30-2020, 02:53 PM
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snopercod snopercod is offline
 
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When mounting a blade-type transponder antenna underneath my Lancair, I first tried using tufts to determine the direction of the airflow. That was an abject failure because the propwash during takeoff just slammed the tufts all over the belly.



So I drilled a small hole in the belly where the coax would go. Then, on the inside of the baggage compartment, I stuck a 3' length of 1/4" Tygon over the hole using modeling clay. On the other end of the tubing I inserted a small syringe filled with Champion spark plug thread lube. I could reach the syringe while flying. Then I flew the plane. When I got up to cruise configuration, I discharged the spark plug fluid out into the airstream. It made a nice, black streak back from the hole at a 7 degree angle to the centerline of the aircraft. That's how I mounted my antenna.



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  #23  
Old 08-30-2020, 03:06 PM
scsmith scsmith is offline
 
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Very clever!
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  #24  
Old 08-30-2020, 06:07 PM
BillL BillL is offline
 
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I like that idea, I have 4-5-6 small tubes from the cabin to the engine compartment, I can use this to check the flow exiting the chute!

Steve, update on the UV of the aeroshell 15w50 - I went in the dark and could not see the oil. Is it just the W100 oils that lights up with UV?

I have some oil UV dye and thinking about using that. I'll test a paint sample first to see if it can be cleaned off.
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  #25  
Old 08-30-2020, 06:20 PM
Terry Lutz Terry Lutz is offline
 
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Default Flow Visualization

Many years ago we did a study to determine where the flow transition from laminar to turbulent was on a Blanik L-13 sailplane. We tried two techniques, the oil method and sublimating chemicals. For our testing, the oil method worked quite well. But as Steve mentioned, it may not be useful for 3D flows such as the ones you want to investigate.

What worked the best for us was Mobil 1 motor oil with lamp black mixed in. We simply painted it on the wing, approximately back to the point where we thought the transition would occur, and were towed aloft. The point of flow transition showed as a darker color, and there were obvious v-shapes indicating where bugs and other flaws tripped the flow.

We used a chase airplane and photographed the glider at different angles of attack. Today it is much simpler and better to use properly placed GoPro cameras to record the flow, as the oil does shear off after a period of time and the visual aspect is lost. Takes a bit of clean-up after the test!
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  #26  
Old 08-30-2020, 07:59 PM
scsmith scsmith is offline
 
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The presence of turbulent wedges from small roughness elements can be key in correct interpretation of transition. Sometimes we put 'bugs' on the leading edge on purpose to achieve that. Thanks for the good story. Too bad L-13s are now pretty much recycling bait. Really nice-flying glider.

Quote:
Originally Posted by Terry Lutz View Post
Many years ago we did a study to determine where the flow transition from laminar to turbulent was on a Blanik L-13 sailplane. We tried two techniques, the oil method and sublimating chemicals. For our testing, the oil method worked quite well. But as Steve mentioned, it may not be useful for 3D flows such as the ones you want to investigate.

What worked the best for us was Mobil 1 motor oil with lamp black mixed in. We simply painted it on the wing, approximately back to the point where we thought the transition would occur, and were towed aloft. The point of flow transition showed as a darker color, and there were obvious v-shapes indicating where bugs and other flaws tripped the flow.

We used a chase airplane and photographed the glider at different angles of attack. Today it is much simpler and better to use properly placed GoPro cameras to record the flow, as the oil does shear off after a period of time and the visual aspect is lost. Takes a bit of clean-up after the test!
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RV-8 N825RV
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  #27  
Old 08-30-2020, 09:40 PM
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RV8JD RV8JD is offline
 
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Yup, the Blanik L-13s are rather versatile sailplanes, and great in their intended training role. Spent lots of time in the rear seat.

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Last edited by RV8JD : 08-30-2020 at 09:47 PM.
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  #28  
Old 10-22-2020, 09:14 PM
BillL BillL is offline
 
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OK, I could not find tempura powder locally, but tried blue snap line chalk. I mixed it with oil and smeared it on a white paint sample for a test. It mixes well, and did not stain the white paint sample.

Tempura is apparently an egg based paint and is not good for any painted surface, so I have some available for face painting.

Next: it gets flight tested on some white parts when weather permits.
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Lord Kelvin:
I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.
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