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        Question Holes in wings for boundary layer blowing?

#1 JaRaMW Jan 15, 2011 08:14 AM

Holes in wings for boundary layer blowing?
 
Did anyone try this yet to avoid separation?
I want to increase lift on a foamy wing for slow flight. Drag does not play a big role. I thought of a spanwise line of small holes at maybe 1/3 chord. The channels would be slightly tilted backwards (bottom to top). No idea how big the holes would need to be and if the loss of lift due to equalization of pressure would outweigh a possible boundary layer stabilisation.

#2 Yak 52 Jan 15, 2011 08:43 AM

What sort of Re numbers are we talking about?
ie...Whats your chord and speed?

Jon

#3 JaRaMW Jan 15, 2011 09:00 AM

Chord would be 15-20 cm, airspeed around 10 km/h or less if possible. So Re < 30k.

#4 BMatthews Jan 15, 2011 01:31 PM

A better way is to just use turbulator strips ahead of the region where you think the air will separate. The tubulence along the skin behind the strip acts like a glue to hold the upper laminar flow onto the surface for longer.

Consider too that 10 km is a jogging/slow running pace. The time tested method for flying this slowly and even slower is a low wing loading. Free flight models have been flying at this sort of speed range for literally 100 years now. You could do a lot worse than to adopt that pattern of design and building so that you achieve the same results.

Since the key to this at such reynolds numbers is light weight I'd suggest that the stuff needed to blow or suck air through the wing will increase the wing loading more than it may help. If you want to experiment with it then great. But if your goal is to just fly super slowly then make your models like the free flight guys make their models.

In full size circles blowing is generally used for encouraging extra lift from flaps during takeoff and landing. So the vents are in the wing where they can blow higher velocity air over the upper leading edge of the flaps.

For separation control I think you'll find that the more normal method involves sucking away the boundry layer. And where you suck it away matters. At low reynolds numbers and high Cl's separation can occur much earlier than the high point of the wing. It can even be just barely behind the leading edge. Free flight modelers found this out and it was common to glue on turbulator trip cords from coarse thread to the upper side of the leading edge strip on many competition towline gliders back in the 50's through to the late 70's at least. They may well still be doing this in some events.

So the odds that your blowing or sucking system would be located at the right chordwise position for best effect would be highly risky unless you can do some wind tunnel or wind tunnel like testing to determine where the separation occurs at the expected Cl and Reynolds number and then experiment with position, hole size and spacing and how much to blow or suck to control any separation bubbles.

In the end I guess it would come down to how much extra lift can you get out of the wing from such a system. Would it even provide enough of a difference to make up for the extra weight of the pumping system components? Factor this against a proven methodology demonstrated by free flight model design coupled with lighter weight.

#5 JaRaMW Jan 17, 2011 02:52 AM

Thanks for the explanation! I would expect turbulator strips to be quite effective if properly sized and positioned, but my feeling is that sucking or blowing the boundary layer would generate a stronger effect.

I did not think of a separate suction/blowing system but simply holes allowing high pressure air from below the wing to the upper side. This would create a thinner BL at the bottom (thin BL is good I think - but not really necessary at the bottom because separation is usually not such a big issue there) and increase the upper BL's kinetic energy. If the holes are tilted backwards, this may work. Otherwise, when blowing the air out perpendicular to the surface, this would probably rather stall the BL.
This would all be rubbish if the BL stabilisation effect - if there would be any after all in such a naive attempt - would be over-compensated by the loss of lift due to the equalisation of pressure through the holes.

#6 BMatthews Jan 17, 2011 11:22 AM

By all means play with such a setup if you wish but my feelings from reading about the blown flaps systems suggest that it won't be enough at all. The big guys that did this used a bypass off the high pressure area of the engine compressor area. That's both serious volumes and serious pressure. The idea is that it's not just a bleed of air but a high velocity sheet that blasts over the upper side of the flaps. Just leaking air from below will not have anywhere near that sort of energy. Especially with a light model where the pressure difference is only fractions of an oz per sq inch. Think about it... at a wing loading of around 4 to 6 oz per sq foot, which is needed to fly at around 10 kph or a medium running speed. The pressure difference per sq inch is 6/144 or just .042 oz per sq inch. Or putting this into pounds per sq inch or PSI it's .042/16=.0026 psi That isn't going to move air with any sort of energy at all. So to do this in a way that won't reduce your lift by merely leaking air from below you'll need a pump system that can provide the sort of energy needed to actually pull the air down.

#7 JaRaMW Jan 17, 2011 12:07 PM

Oh yes, now that makes sense! I'll rather go with turbulator strips then.

#8 morgans Jan 21, 2011 01:12 AM

In the mid 70's there was a british single skin hang glider called the Vortex which had a line of holes along the 1/4 chord line towards the tips supposedly to prevent tipstall. Dont know if it worked.

I suppose a slat or slot could be considered as a continuous hole , these work for full size aircraft to delay separation, does anyone know if they are effective at model Re numbers?

#9 morgans Jan 21, 2011 01:18 AM

There was a single skin british hang glider called the Vortex in the late 70s which used a row of holes towards the tips supposedly to prevent tipstall, dont know if iot worked.

I suppose a slat or slot can be considered as a "closely spaced" row of holes, these delay separation on full size aircraft, does anyone know if they are effective at all at model Renolds numbers ?

#10 JaRaMW Jan 21, 2011 02:19 AM

As I see it, a slat is arranged in such a way that it blows high energy air to the upper surface's boundary layer. However, I remember a paper from a sailor (?) who stated that's not entirely true, the slat being a small wing in front of the main wing that alters the total circulation in a more favourable way. Supposedly foresails on sailboats are similar to slats.

The gap between slat and wing needs to be of considerable size so that enough air can flow through, otherwise the kinetic energy would be too low to have any useful effect. A line of holes would not allow a flow anywhere as strong unless the holes were quite large. Furthermore the exit direction of the air on the upper side would not be parallel to the surface unlike the flow from a slat.

My guess is that the air that passes through such a line of holes rather works as a turbulator, simply tripping the BL into a turbulent state rather than directly increasing its kinetic energy.

#11 Jurgen Heilig Jan 21, 2011 06:52 AM

While there have been full size A/C experiments in the past, I don't think tiny holes would work very well in model aircraft, due to the much lower Reynold's numbers.

That being said, I found a fairly recent patent about holes used for boundary layer control:

http://www.freepatentsonline.com/20100270434.pdf

:) Jürgen

#12 nmasters Jan 21, 2011 11:15 AM

Quote:

Originally Posted by JaRaMW (Post 17168974)
As I see it, a slat is arranged in such a way that it blows high energy air to the upper surface's boundary layer. However, I remember a paper from a sailor (?) who stated that's not entirely true, the slat being a small wing in front of the main wing that alters the total circulation in a more favourable way. Supposedly foresails on sailboats are similar to slats.

Sounds like you've read one of Arvel Gentry's articles. He's not a sailor he's a retired aeronautical engineer and a yachtsman (sailor is a job yachtsman is a hobby).

Quote:

The gap between slat and wing needs to be of considerable size so that enough air can flow through, otherwise the kinetic energy would be too low to have any useful effect.
The slot must be wider than the thickness of the boundary layers of both surfaces. That's not necessarily a lot

Quote:

A line of holes would not allow a flow anywhere as strong unless the holes were quite large. Furthermore the exit direction of the air on the upper side would not be parallel to the surface unlike the flow from a slat.
There has been considerable confusion between two entirely different use's of blowing in this thread. My reading of post #1 lead me to believe that he was asking about blowing little jets vertically from the upper surface to act as turbulators. These little jets are perpendicular to the airstream and only have to blow through the boundary layer plus a millimeter or two so the pressure inside the wing doesn't have to be very high. This system requires either a pump or an air scoop below the wing to pressurize a plenum inside the wing. It is not in any way similar to a blown flap which requires a significant volume of high pressure air blowing tangentially to the surface.

Quote:

My guess is that the air that passes through such a line of holes rather works as a turbulator, simply tripping the BL into a turbulent state rather than directly increasing its kinetic energy.
Bingo:)

--Norm

BTW holes are listed as item "F" in this group of aerodynamic bandades

#13 AirAusquin Feb 17, 2011 10:27 AM

Quote:

Originally Posted by JaRaMW (Post 17110509)
Did anyone try this yet to avoid separation?
I want to increase lift on a foamy wing for slow flight. Drag does not play a big role. I thought of a spanwise line of small holes at maybe 1/3 chord. The channels would be slightly tilted backwards (bottom to top). No idea how big the holes would need to be and if the loss of lift due to equalization of pressure would outweigh a possible boundary layer stabilisation.

I was very fortunate that a future colleague sent me this thread.

The first thing that came to my mind after reading the initial question was the following video (please set aside all the religious/UFOs stuff and concentrate in the aeronautical portion): http://www.youtube.com/watch?v=7-C7XO-QH-s
Check out those leading edges...

If JaRaMW intention was to blow the BL, I will suggest to execute the holes from below the Leading Edge line defined by the chord line of the wing profile, very tilted back or even curved in order to have the channel exit nearly parallel to the wing's upper surface.

Thanks Bruce, Jürgen and Norm for the links, I am making good use of the info found there!

Alejandro
Aeronautical Engineer
Venezuela

#14 riker84 Feb 17, 2011 07:12 PM

Hello,

I am new here, and I am currently creating a wing as part of a final year project in university. My project relates to boundary layer control, and one part of my project is to evenly distribute air through these holes to distrupt the boundary layer.

My project is loosely associated with Mc Larens F-duct, which uses blown air to alter the B.L. over the wing.

I am using an NACA profile (23012) built in parts by a rapid prototype stacked paper machine. At about 3/4 lengthways along the chord, a cylindrical cut will be made, along which, on the underside of the wing, will be a series of holes.

My problem is, how am I going to evenly distribute this air through these holes. Also, how many would be needed to distrupt it successfully?

Thanks for any help guys, it is much appreciated.
Patrick


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