boundary layer thickness - RC Groups
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Sep 29, 2012, 07:21 AM
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frank40's Avatar

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.
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Sep 29, 2012, 02:14 PM
Cognitive dissonance
kcaldwel's Avatar
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.

Sep 29, 2012, 05:14 PM
Registered User
frank40's Avatar
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.
Sep 29, 2012, 05:46 PM
Grad student in aeronautics
How much thrust will your motor give at 200mph?
Sep 29, 2012, 05:56 PM
Sink stinks
Montag DP's Avatar
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, 06:50 PM
Cognitive dissonance
kcaldwel's Avatar
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.

Sep 30, 2012, 05:47 AM
Registered User
frank40's Avatar
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...
Sep 30, 2012, 10:21 AM
Grad student in aeronautics
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:

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, 03:00 PM
Registered User
frank40's Avatar
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 12:45 PM.
Oct 01, 2012, 02:23 PM
B for Bruce
BMatthews's Avatar
Frank, you're picking the worst possible place to put your fan unit and in the worst possible mount by inluding it on the rear of the wing and inset down flush like this with the surface of the wing. Then you add some sexy looking spines that lead up to the fan to further potentially trip the local airflow around the mouth of the inlet.

As you suspect you want to worry about turbulence either from boundry layer effects or trailing turbulence off the structure. The equipment hump just ahead of the inlet looks sexy but it could well become a source for turbelence off the rear face due to poor pressure gradients at high speeds leading to separation bubbles. And even if that doesn't occur there's going to be some amount of turbulent and accelerated wake behind the bump and due to the wing itself....

All in all this is why despite seeing lots of artist's conceptions for upper surface mounted engines and jets this idea is seldom seen in final designs. And even then on most designs the jet inlets are typically spaced up off the wing or fuselage surfaces to attempt to minimize or avoid ingesting that turbulence off the wing and any fuselage structure and any skin bound boundry layer.

Even on fighter jets with forward inlets you typically see "splitter plates" to avoid ingesting the tubulence and spiralling flow that is found attached to the skin and immediate zone above the skin. So inletting the EDF unit down into the surface as you have done is most certainly going against this design factor.

So how thick is the boundry layer? As I understand it you may as well ask "how long is a piece of string?". It depends on the speed and airflow and shape of the wing or structure. It's thin in some spots and thickens in areas of lower local pressure as it flows across the surface.

And lets not forget what slow speed is going to be like on your delta wing. As the angle of attack rises the flow and lift from a delta wing shifts to a strong rolling vortex coming up and over the leading edge and rolling inwards. The upper surface of your wing will transition from fairly smooth during max speed to almost washing machine like during low high AoA flying. And your EDF is trying to suck up and spit out such air.

All in all this is why pretty much any delta wing aircraft has the inlets either under the wing or positioned well forward and ducts the air to the engine. If you're looking to get the most out of your 3.5Kw of power you might want to consider doing the same. Instead of on top tunnel out the lower side similarly to the top. The top side could then be a little arched in the rear with the pointed "duck tail" swooping up and over the half tunnel exhaust area.

Again, asking how much the penalty for mounting it on top is like asking about the string. Without CFD or wind tunnel testing of the design it's impossible to guess at the effect of any boundry layer or trailing turbulence off the forward hump. But it is almost certain that there would be SOME effect. Mouting the EDF up on some mini pylons that lifts it about a half inch clear of the wing surface would aid by likely letting any turbulent boundry layer slip by at high speed. But there may be a bad suction bubble behind the forward hump that feeds the inlet. Again, without testing it's nearly impossible to say for sure.

And then there's the low speed behaviour. Trying to use the very turbulent air that WILL be found on the top side of a delta for low speed thrust is almost certainly going to ensure that accelerating out of the high alpha vortex flow and getting the wing back up "on to the step" where the air flows more or less straight over the wing is likely going to be delayed and require more power than if the EDF was mounted on the underside where it would be seeing cleaner and more direct inlet air.
Oct 01, 2012, 02:37 PM
Cognitive dissonance
kcaldwel's Avatar
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."

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.

Oct 01, 2012, 02:52 PM
B for Bruce
BMatthews's Avatar
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, 03:33 PM
Cognitive dissonance
kcaldwel's Avatar
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.

Oct 01, 2012, 04:28 PM
Registered User
frank40's Avatar
Hi Bruce and Kevin

Thanks a lot for your inputs, I relay appreciate it. I can see that my model dose have some similarity with the Boeing blended wing/body, so it might work after all. But like you say Bruce I do have a tendency to sex it up a bit, with some lines that isn't necessary always aerodynamic correct.

One of my thoughts with the fan position was to help the separation bobble issue, the fan should help sucking down the bobble due to the massive airflow going into the fan. But it's just an idea.
Oct 01, 2012, 05:39 PM
Registered User
richard hanson's Avatar
Ducted fans do best with least flow impedance at inlet - fastest exit aft the prop
They just don't act like turbojets
They are also not positive displacement devices
restrict either inlet or outlet and the other side has reduced flow
Unless there has been some changes to ducted fans that I have not seen, you cant beat a short,l seperate unit
The great setups we see on current electric ducted fans are compromises - just like the rest of an airframe.

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