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Author Topic: P-51 type Laminar flow wings  (Read 8039 times)
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Tron
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« Reply #50 on: September 15, 2016, 09:54:29 PM »

Very interesting discussion.

For those interested in airfoil plotting, this website gives airfoil plots for root and tips by entering dimensions, and even prints wing sheeting if required.

http://airfoiltools.com

The price suits my budget too, free.

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lincoln
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« Reply #51 on: November 04, 2016, 02:47:57 PM »

I was just re-reading this thread. I wonder if, for a park flyer, you could make the wing hollow with suction at the control surface gaps? Then maybe the airfoil could be more scale-like without as many problems? Maybe push the air out the cooling exit? Or maybe make the trailing edges slightly thicker, but hollow, and blow the air out there. The fan for this would probably use up a significant amount of power, but maybe, for a park flyer, that doesn't matter very much. Maybe, to save power, this could be used only when flying slowly.

I wonder if this could enable a scale airfoil for a B-29? http://m-selig.ae.illinois.edu/ads/afplots/b29root.gif

Obviously this won't work for a rubber scale or other low-powered model.
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packardpursuit
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« Reply #52 on: November 10, 2016, 08:45:45 AM »


lincoln-
I suspect that many modelers see/worry about "airfoil problems" where none actually exists. Shocked

Of course, you might  have a real problem if that B-29 ROOT specific airfoil is also used  at semi-span and wing tip. But then,... it wouldn't be a scale anything, would it? Grin

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MarkSSC
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« Reply #53 on: November 12, 2016, 04:49:55 PM »

Ricardo,
You are correct about the thin section of the Spitfire wing. The challenges were to maintain the required stiffness as power and speed increased. The structure of the later wings was very different. There is a very good book called "Secrets of the Spitfire" by Lance Cole that is the story of Beverley Shenstone, a Canadian who was the areodynamicist behind the Spitfire. The remarkable thing was there was alsmost no wind tunnel time as the design was essentially done by mathmatics. Shenstone spent several years in Germany at Junkers, Heinkel and worked with Lippich and the Horten Bros.in the early 1930's.
 Shenstone was also part of the British Purchasing Commission with some involvement in the P51. One irony was that he almost went to work with Hawkers but was messed around by Sidney Camm and then went to Supermarine and clearly fitted in with team. Mitchell had a knack of surrounding himself with very talented people.
Rickys

This book is a great read and quite technical in places. It also draws attention to how they cleverly decreased drag with the wing fairing. Such details are often overlooked. A scale plane can be cleaned up aerodymically to reduce drag and this must be a plus for the flight characteristics. It also looks nicer. Some of the cheaper ARF planes lack this detailing which is a good reason to scratch build.
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packardpursuit
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« Reply #54 on: November 14, 2016, 02:47:44 PM »

Huh?
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basiliscus
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« Reply #55 on: December 12, 2019, 05:20:18 PM »

Hi Guys ,
I have been flying for about 10 years , mostly ARF'S But have just recently started building from scratch and also trying some designing .Has anyone tried duplicating the P-51 Laminar flow wing for use on model aircraft ? Is there formulas or discussions available in the forums for designing such a style wing for model use ? I am not real swift on the engineering aspect of airfoil designs so I may be way out of line in even considering this .Any comments will really be appreciated ---------------Dave Huh Huh

Airfoil design is all about the care and feeding of the boundary layer.  Given the size of the wing and the speed at which the Mustang flew, the problem they were trying to solve was to delay the transition from laminar flow to turbulent flow in the the boundary layer.  To do this, they designed a section shape that at the design lift had a constant pressure for about 60% of the chord and then increased linearly to the trailing edge.  This kind of section is not really suitable for use in a model for several reasons.

Given the size of the typical model and the speed at which it operates, the problem isn't maintaining laminar flow in the boundary layer, the problem is getting rid of it gracefully!  The issue is flow separation.  To the boundary layer, negotiating the pressure changes from the leading edge to the trailing edge is a lot like a bicyclist crossing Grand Canyon.  They both start at rest - the boundary layer at the stagnation point where the flow divides between passing above vs below the wing, and the cyclist at the edge of the rim.  There is then a rapid acceleration to the maximum speed as the boundary layer whips around the leading edge and the cyclist drops into the canyon.  At the stagnation point, the pressure is at its highest and at the point of maximum speed the pressure is at its lowest, so the boundary layer is being sucked along and life is easy.  But then comes the hard part.

For the airfoil, the pressure at the trailing edge is near the ambient pressure, so the boundary layer experiences increasing pressure as it moves from the point of minimum pressure to the trailing edge.  This is what the aerodynamicists call an adverse pressure gradient.  It's like the cyclist struggling uphill.  The boundary layer loses energy and gets "tired".  If the pressure increases too fast, the boundary layer will be brought to a stop and pushed backwards, just like a cyclist that can't surmount too steep a grade and rolls backwards.  Imagine the plight of a poor blob of air in this situation.  It can't go forward because the increasing pressure is pushing it backwards.  It can't go back, because of all fast moving air coming from the max speed point.  It can't go down, because of the solid wall of the wing.  It can only go one direction - outwards into the outer flow.  That's what causes flow to depart from following the wing contour, or flow separation.

A laminar boundary layer is more prone to flow separation than a turbulent boundary layer.  It's like driving on ice.  Smooth and fast, but when you try to slow down it's easy to break free unless you do it very gently.  That takes more room, and if you have to slow down by a certain point, then you're in trouble.  A turbulent boundary layer is much more robust with regard to separation.  It's like driving on packed snow.  There's more friction, but the traction is better so you can slow down more quickly.

For the laminar boundary layer, there is a maximum adverse pressure gradient that is the most deceleration that the boundary layer can take without separating.  At model sizes and speeds, that pressure gradient is less than it is at full scale sizes and speeds.  Remember that the pressure has to get back to ambient at the trailing edge.  If the boundary layer can't decelerate as rapidly, it is going to need a longer distance to get slowed down.  So the position of the peak velocity has to be further forward on a model airfoil in order to give more distance for the deceleration to occur.

The Mustang's problem was disturbances in the boundary layer being amplified until they became turbulent flow.  Their solution was to move the point of peak velocity way back on the airfoil so as to stretch out the acceleration portion and use the favorable pressure gradient to keep the disturbances damped down.  They could get away with this, because they could do a rapid deceleration to the trailing edge without separating the flow.  A model can't get away with the same deceleration. 

So using something like a NACA 66,2-(1.8)15.5 A=.6 (the P-51H root section) is going to be very disappointing.  It will be prone to separation on the aft third of the airfoil because the laminar boundary layer will separate at the beginning of the pressure increase rather than transitioning to turbulent flow.  And the NACA 6-series sections formed a sharp pressure spike at the leading edge when they were operated at angles of attack above the design lift, which in models results in a really abrupt and nasty leading edge stall when laminar separation there makes the whole wing let go at once.

With regard to formulas, you can use a tool like Xfoil (https://web.mit.edu/drela/Public/web/xfoil/) to design a section for your requirements.  With Xfoil, you can specify the design pressure distribution and calculate the shape that will give you those pressures.  Then you can see how the boundary layer behaves and what the section's lift and drag are.  Some really good examples of airfoils that have been designed this way would be Mark Drela's glider sections.
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lincoln
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« Reply #56 on: December 13, 2019, 04:03:46 PM »


lincoln-
I suspect that many modelers see/worry about "airfoil problems" where none actually exists. Shocked

Of course, you might  have a real problem if that B-29 ROOT specific airfoil is also used  at semi-span and wing tip. But then,... it wouldn't be a scale anything, would it? Grin


Suction for the park flyer was in the realm of "because I can" rather than solving a problem. Way too much work compared to a thinner airfoil. Maybe for a gold plated 1/4 scale model!

I may have missed it, but I don't seem to recall any mention of how big or fast the OP's plane was going to be. Nor whether it's supposed to have a wide speed range. The best choice for a pistachio scale model's airfoil is going to be quite different from an RC, 1/4 scale fighter. I suspect that, for the pistachio, a very thin, even "single surface" cambered airfoil is best. A look at some papers on UAV's led me to fooling around on Profili (which incorporates Xfoil). One airfoil that had good results for flying slowly was almost a flat plate, with 15 percent leading and trailing edge flaps, deployed at 14 degrees. (I suspect a somewhat different amount of flap would still work ok.) Without flaps, one could round the corners a bit, similar to the airfoil in a paper I remember. I have to admit I don't remember how much better than an "arc" airfoil this would be. The rounded version ought to be good for rubber scale, though in the FAC, you're not supposed to use an airfoil that's concave on the bottom unless the real aircraft had it. In this case, a Neelmeyer doesn't look too bad. Maybe thinned a little? For larger planes, maybe including RC, one can look at Drela airfoils. ( http://www.charlesriverrc.org/articles/drela-airfoilshop/markdrela-ag-ht-airfoils.htm ) The page includes some advice about which airfoil to use, but it's  aimed primarily at RC sailplanes. If you look through old Soartechs, you can find some Selig and other airfoils that are actually aimed at larger RC models, including ones that are intended for aerobatics. The Soartech's include wind tunnel test results at appropriate Reynolds numbers, and even measurements showing how far off the test airfoil was from the nominal one, and where those errors are.
https://m-selig.ae.illinois.edu/soartech/
https://m-selig.ae.illinois.edu/uiuc_lsat.html  (note that volume 4 includes some results for a few AG airfoils)

Note also the page listing airfoil usage on real airplanes:
https://m-selig.ae.illinois.edu/ads/aircraft.html

More fun:
https://m-selig.ae.illinois.edu/ads.html

They even have data on props at low Reynolds numbers.
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