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Author Topic: Boundary Layer depth  (Read 2277 times)
Steve Fitton
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« on: May 07, 2012, 12:02:45 PM »

How thick is the boundary layer on a stunt wing? Is it laminar or turbulent?
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Tim Wescott
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« Reply #1 on: May 07, 2012, 12:11:32 PM »

I suspect that the best answers that you're going to get are "it varies" and "it depends" for the first question, and "yes, it is" for the second.

Someone with a lot more aerodynamics gravitas will correct my details, but here's what I know:

The boundary layer at the leading edge is going to be very thin and laminar.  At some point past that it will separate -- this point of separation will be earlier in the airflow on the lifting side, and later on the non-lifting side.

Exactly where it separates, and how thick it is is going to depend a lot on the details of the airfoil, the angle of attack, the flap deflection, and the surface finish -- a nice smooth surface, and a nice smooth airfoil are going to make it separate later.  A rough surface at the right place (See PJ Rowland's turbulators) will make it separate sooner but will keep it thinner back to the back of the wing.  In level flight it will show a lot less turbulence, much farther back on the airfoil, than in a square corner.  Taping the hinge lines will (I think) keep it thin at the flap leading edges, and will (I know) make it more consistent from wing to wing, flight to flight, day to day, etc.

Someone really needs to build a wind tunnel that operates at 50 MPH and will hold a 12-inch chord wing, and then take some pictures of smoke trails at various angles of attack and flap deflections.
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« Reply #2 on: May 07, 2012, 12:50:21 PM »

Tim, all true,, but first it needs to be determined what the actual angle of attack achieved is on said stunt wing, I have seen a lot of discussion on this topic as well and there seems to be some discord on what the actual real world number is for the angle of attack achieved,,
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Tim Wescott
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« Reply #3 on: May 07, 2012, 01:03:57 PM »

So -- another "It depends"?

AoA should be something that one could tell by careful video-watching.  I'm sure you'll find that it varies with how the plane is set up.  I know that the flapless stunters that I currently fly go at a much higher AoA in the squares than my flapped Waiex stunter did -- the flapless ones kind of "push with the tail" while the Waiex looked like it just pivoted around the center of curvature with a string to the center of gravity (much prettier -- when I stop regularly crashing I'm definitely going back to flapped stunters).  And it just makes sense that for any given plane, as you increase the elevator throw with respect to the flaps you'll get more and more of that "push with the tail" effect (and hence higher AoA).

So you can't even ask the question "OK, what angle of attack is achieved" without getting wishy-washy answers!
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Howard Rush
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« Reply #4 on: May 08, 2012, 02:07:34 PM »

See http://stunthanger.com/smf/index.php?topic=25877.0, post 29.  That's for an Impact with flaps at 30 degrees and an angle of attack of 10 degrees. 

Here is Impact boundary layer transition for a couple of flap positions.  Y axis is fraction of chord. 

* Impact BL Transition.pdf (53.07 KB - downloaded 92 times.)
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« Reply #5 on: May 09, 2012, 05:50:45 AM »

I not sure of what value knowing the thickness of the boundary layer would be.

Tim The device know as a "Turbulator" is somewhat different to what Im using - it simply distrupts the airflow WhereasThe Vortex Generators are creating a high energy directed spinning current of air that transitions all the way down the wing.

Most of the tubulators I've seen have been on Free flight gliders in the for of Strip tape.
Generators are seen on tons of things from F1 Race cars, boats to athletic racing helmets and of course - Some Stunt ships.


Its fairly small however the Maths to work it out is considerable Larger  Roll and beyond me.
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« Reply #6 on: May 09, 2012, 07:34:41 AM »

See http://stunthanger.com/smf/index.php?topic=25877.0, post 29.  That's for an Impact with flaps at 30 degrees and an angle of attack of 10 degrees. 

Here is Impact boundary layer transition for a couple of flap positions.  Y axis is fraction of chord. 

Thanks Howard, my apologies that I forgot you had already documented it in that thread from last month.
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« Reply #7 on: May 09, 2012, 07:17:34 PM »

I not sure of what value knowing the thickness of the boundary layer would be.

Most modelers wouldn't need to know that.  It would only be useful if you're making vortex generators or something.


Tim The device know as a "Turbulator" is somewhat different to what Im using - it simply distrupts the airflow

Distrupts?  People use grit, tape, or such doodads to force transition from laminar to turbulent flow farther forward than would happen naturally.  From the plots above, it looks like natural transition happens pretty far forward on an Impact at any significant lift.
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« Reply #8 on: May 09, 2012, 07:23:11 PM »

Howard ; That was my point, during my VG development I read countless papers on Boundary layer effect in an attempt to understand it better despite my lack of degree's to support such a topic. It seemed quite small and trying to calculate it for our specific useage was difficult.

Interesting you mention seperation happens quite early - Im guess probably around 10 % .. What I found is the Vortex Generators were not as effective at that point - but they clearly worked better further back. Ive had them back as far as 35 % with good effect - I just tend to lean on "At the highpoint" it seems to work quite well at that point.

Obviously the further down you move, the less effect you acheive - with the least effective spot being infront of the flaps and I can quite easily explain why.


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« Reply #9 on: May 10, 2012, 01:59:23 AM »

It seemed quite small and trying to calculate it for our specific useage was difficult.

It's easy.  Just use Javafoil or Profili, free and cheap respectively. I've given references to them and to a site that explains the boundary layer stuff, which I don't understand either.


Interesting you mention seperation happens quite early - Im guess probably around 10 % .

I mentioned the transition from laminar to turbulent flow, which, according to the plot above, happens at about 10% for an Impact at 15 degrees angle of attack.  Separation is something else.  It's what vortex generators fix.

Obviously the further down you move, the less effect you acheive - with the least effective spot being infront of the flaps and I can quite easily explain why.

No doubt you will, but can you?  It would be helpful if you can figure how long it takes for one of those vortices to do half a turn.  I've been curious about that, but too lazy to do it.
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« Reply #10 on: May 10, 2012, 05:57:49 AM »

The last time I tried to bring stuff like this up to the NACA brain trust at Langley, there was some spirited argument that what we were flying in some ways should not work at all.  Cut and try seemed to have trumped theory.  Then again, all the Langley people built models first to answer the most basic question: will it fly at all? 
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« Reply #11 on: May 10, 2012, 04:30:47 PM »

Cut and try is how to do this toy airplane stuff, especially in this case, where the theory is especially deep.  PJ has done a lot of cutting and trying.

This boundary layer talk reminds me of story, as so many things do.  Folks have done different things for boundary layer control.  Early F-4s blew engine bleed air through slots in the wing leading edge and at the front of the flap to energize the boundary layer.  Various people have looked into applying suction through little holes in the wing surface to remove the slow boundary layer air.  Onward to the story: I ran into a former boss at work one day.  I asked him what he was up to.  "Natural boundary layer control," he said.  "You mean blowing and sucking?" I asked.  "No," he replied, "that's unnatural."
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« Reply #12 on: May 10, 2012, 08:50:47 PM »

Not only is it "unnatural"  it's illegal in Arkansas. Layingdown Layingdown Layingdown Layingdown Layingdown Layingdown
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« Reply #13 on: May 10, 2012, 08:54:25 PM »

Too much cutting and trialling - but its been fun and Im fairly confident I have the correct configuration. Most people who have run the system agree of the benifits..

There is only 1 person who Im especially interested in hearing his test report..
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« Reply #14 on: May 20, 2012, 05:22:53 PM »

..and here I thought those "vortex generators" on the aft bottom of the winning Corvettes were part of some sort of heat radiator (oil cooler, maybe).

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* Corvette-Racing-side-by-side.jpg (190.39 KB, 672x448 - viewed 117 times.)
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« Reply #15 on: May 20, 2012, 07:48:29 PM »

Thats simply a rear diffuser.

To help channel the high speed air flow coming from underneath the car, diffusers redirect the high speed air flow to transition smoothlyinto the low pressure vacuum in the rear. This encourages a smoother flowing air flow underneath and allows for better performance from other aerodynamic components such as aerodynamic wings. The diffuser alone also offers increased downforce and drag benefits.
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If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them. - Bruce Lee.

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Tim Wescott
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« Reply #16 on: May 20, 2012, 08:11:31 PM »

Are you sure those aren't just heat sinks for the ESCs?
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