Sep 29, 2012, 06:21 AM
Registered User
Denmark
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Discussion
boundary layer thickness

I have been looking to find the actual thickness of the boundary layer thickness of models planes. I am in the process of designing a fast +200 MPH edf model and need to fined some sort of value. I am aware that is has to do with speed shape and smoothness of the model. To try to explain the problem I have add a pic. It is at the point where the back of the fuselage that goes in to the nacelle I am worried about.... Any thoughts on that.

# Images

 Sep 29, 2012, 01:14 PM The Great Filter Joined Jan 2007 3,537 Posts First, why are you worried about the boundary layer thickness? EDFs routinely ingest the boundary layer from long inlet tubes, so I don't think that is a big issue. Total separation at high AoA is quite possible in the area you show. That might cause some vibration? The boundary layer thickness depends on how far back from the front of the body you are (l), how fast it is flying, a small dependency on altitude, and whether the flow is laminar or turbulent. On a flat plate in laminar flow: d (thickness) = 5.20* l/(Rl)^1/2 Rl = Re at that length For turbulent boundary layer on a flat plate: D= 0.37*l/(Rl)^0.2 Here l = length of plate since the beginning of transition. With those chines, the safest approach wold be to assume turbulent flow from the nose, and just use the turbulent thickness calculation. I can calculate that thickness for you if you give me a distance and a speed estimate. Kevin
Sep 29, 2012, 04:14 PM
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Denmark
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863 Posts
Hi kevin, thanks for your replay....My worries is that the air flow from the fuselage will distort the flow from the rest of the nacelle, causing turbulence in front of the fan.

I am not sure I fully understand the math here, so I have added a new pic with some mesourments....reg. air speed I hope for +200 MPH.

# Images

 Sep 29, 2012, 04:46 PM Grad student in aeronautics United States, GA, Atlanta Joined Oct 2010 435 Posts How much thrust will your motor give at 200mph?
 Sep 29, 2012, 04:56 PM Sink stinks United States, GA, Atlanta Joined Apr 2005 4,515 Posts It is actually possible to delay transition to turbulence in the boundary laryer by changing the shape of your fuselage, but that won't be an easy task and would probably only work for a specific flight condition. That said, I agree with Kevin that boundary layer turbulence is probably not a big deal for you. I would be more concerned about separation on that sloped back part of the fuselage, because having the inlet subjected to separated flow would probably hurt performance and cause vibration. If you need the fuselage to be that thick in order to store the battery and radio equipment, you might consider changing the shape a little so that the bulge is biased more towards the bottom. This would keep the slope smaller in front of the ducted fan and would make separation less of a concern. Could you include a side view so that it's easier to see the fuselage profile?
 Sep 29, 2012, 05:50 PM The Great Filter Joined Jan 2007 3,537 Posts The worst case boundary layer thickness (assuming turbulent from the front, altitude 100m) might be about 12mm at 200mph, and about 18mm at 25mph. I would think you would get some laminar flow at low AoA, so the boundary layer should be thinner than that at speed. I doubt the boundary layer will be much of an issue in any case. Kevin
Sep 30, 2012, 04:47 AM
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Denmark
Joined Sep 2005
863 Posts
Thanks again...I realized that I mix op the terms boundary layer and separation, sorry. But I am surprised at the boundary layer was that high, I was thinking more like a couple of mm.

The actual fuselage has it's basic from a wing foil that I made, with some modifications is made of course. But my main goal is to integrate the nacelle in the fuselage and at the same time making it as smooth as possible.

I have no idea how much thrust I have at 200mph.... The fan that I am using is a cut down CS90, it has been cut down to 70mm, this allows my to use a Turnigy XK4074-B-1400KV Brushless Inrunner. I have made some static thrust test and it is giving me 3.6 Kg at 3.5 KW, this is a very power full fan.

We did a flight test with Claus's super sling jet, but it was ripped a part when Jonas was trying to make a high speed pass.

Please not that the drawing it a sketch...

# Images

 Sep 30, 2012, 09:21 AM Grad student in aeronautics United States, GA, Atlanta Joined Oct 2010 435 Posts Well here are a few quick calculations to help out (if you want) v = 293.33 ft/s rho = 0.0023 slugs/ft^3 (1000 ft elevation) Then depending on your size (S, ft^2) and your drag coefficient (CD), this is how much drag you will have: CD__S__D__ 0.01 4.0 3.972 0.02 4.0 7.944 0.03 4.0 11.916 0.01 7.0 6.951 0.02 7.0 13.902 0.03 7.0 20.8531 0.01 10 9.93 0.02 10 19.8601 0.03 10 29.7901
 Sep 30, 2012, 02:00 PM Registered User Denmark Joined Sep 2005 863 Posts Thanks for your cal. DPATE. I made some cal. to find Cd at 200mph, with a average core length of just about 1 feet I found that re would be 1.800.000 at this point the airfoil has a Cd at 0.0038 (according to Profili). The area is roughly 2.7 Sq feet on one side so I guess it has to be multiple by two...right? If my cal. are correct then D (drag) would ending op being 2.14 for the model. But what dose this number tell me? Last edited by frank40; Oct 01, 2012 at 11:45 AM.
Oct 01, 2012, 01:37 PM
The Great Filter
Joined Jan 2007
3,537 Posts
Boundary layer ingestion can be a good thing for propulsion efficiency, because you get to accelerate the slower moving air more:

"They also moved the engines from the usual wing-mounted locations to the rear of the fuselage. Unlike the engines on most transport aircraft that take in the high-speed, undisturbed air flow, the D-series engines take in slower moving air that is present in the wake of the fuselage. Known as the Boundary Layer Ingestion (BLI), this technique allows the engines to use less fuel for the same amount of thrust, although the design has several practical drawbacks, such as creating more engine stress."

http://web.mit.edu/newsoffice/2010/nplus3-0517.html

Dr. Drela headed the design study.

I'm not clear enough on exactly what makes an EDF most efficient to say whether the boundary layer would hurt of help in this case.

Kevin

# Images

 Oct 01, 2012, 01:52 PM B for Bruce The 'Wack, BC, Canada Joined Oct 2002 11,254 Posts Good point Kevin. We've got so many more tools available to us now that can be used to examine and optimize shapes that it makes such stuff possible. Where before much of this sort of thing would have taken many hours or even days of computations we can make a change in shape and map the pressure and flow charactaristics in seconds. Such ability can make the application of such properties more practical than previously. As an example this Boeing blended wing/body design is the inspiration for my reply and is far more typical of what we've seen on most designs; All in all though for Frank's project I'm pretty confident in saying that he stands to get more MPH per Kw of battery and very likely better slow speed acceleration from a belly mounted fan unit that has as little structure in front of it as possible.
Oct 01, 2012, 02:33 PM
The Great Filter
Joined Jan 2007
3,537 Posts
An updated version of the blended wing below, with what look a a lot like ducted fans at the trailing edge...

At 3.5kW, I don't think acceleration will be an issue.

Kevin