Airfoils

[jberg]

Normally you need *thin* airfoils at low speed with small planes (low Reynolds numbers). The air has not enough energy to follow a too much bent surface and forms a laminar separation bubble. To prevent this you need a turbulator, but thin airfoils are considered best at low speed. The MA409 shows excellent results in a test made by M. Selig, a real aeronautical guru. This airfoil is just 7% thick and well cambered. It is used in HLG planes.

[jimwalker]

Hey, thanks for the quick and informative input! Let me get this straight:


If I remember correctly from watching jet design shows on the wings channel, laminar flow is the thin sheet of air molecules actually touching the wing surface. I think they were doing experiments on an F-16 to reduce laminar flow seperation at high supersonic speeds to reduce drag. The wing had many tiny holes and a mechanism sucked ever so slightly across the entire wing surface to eliminate the seperation. It worked and drag was significantly reduced, however it wasn't practical the way they did it.

Back to slowflyers, I guess your saying that at very low speeds, with a highly curved and thickened airfoil, you get the same effect of tiny pockets across the wing creating drag and reducing lift. You said that turbulators would remedy this effect. Then you said a thin airfoil is better. I would assume then that even turbulators cannot overcome the seperation problem, or can they? From your paragraph I could say that with enough turbulators across the leading edge, I could overcome the laminar seperation and use the thick and cambered airfoil.

I'm not trying to argue, just trying to understand exactly the concepts you put forth. Please discuss this more if you don't mind. I really am fascinated by the art of air molecule manipulation....

[jberg]

Jason,

if you are interested in this stuff then you should read the source of my little knowledge yourself. Martin Hepperle has created a very good (english) page about model aerodynamics: see his page (http://members.tripod.de/MartinHepperle/Airfoils/theory.htm).

If reading that does not bother you, then this page (http://soaring.cnde.iastate.edu/calcs/frames.shtml) will show you the differences between airfoils at different Reynolds numbers.

Beware, all this is heavy theoretical stuff, although fascinating (at least to me). And all heavier-than-air airplanes follow the same physics!

[ 08-07-2001: Message edited by: jberg ]

[jimwalker]

Just a few thoughts to throw into the conversation pot so to speak.

I have a NSP Virus which I like very well. I think the reason I like it so well is due mostly to it's airfoil. It is a simple flat bottom airfoil with the only distinctive feature being how thick it's cross-section is at the spar (approx 1"). The plane is ultra stable and a real "floater" that's able to really slow down without a hint of stall.

I read this months RCM article on airfoils and ideas have formed. The results clearly show that undercamber provides tremendous lift at micro-flyer speeds. Turbulator spars helped a little more. What if a person made a wing with an exceptionally thick cross-section, undercamber, and a turbulator spar or two? Would you have the ultimate slow flyer? Able to float along indefinitely at 3 mph?

It sound like a fun experiment, any opinions?

[jimwalker]

Thanks again, I plan on reading and soaking up as much as possible. I'll let you know when I come up with the "miracle slowflyer foil".

What little I've read so far reminds me of a show I watched where some scientists were trying to explain why a dragon fly could do what it does. On the surface it seemed as if the insect shouldn't be able to fly at all, much less hover endlessly. After much study they discovered 2 microscopic turbulators at just the right spot on each of the 4 wings. Also in slow motion wind tunnel w/smoke films, they documented vortices created by the constant angle of attack changes that the wing motion created. These vortices created pressure changes equivalent to a tornado. Once again we're just discovering what nature has long ago designed and implemented....

[ 08-08-2001: Message edited by: Jim Walker ]

[gjohnson]

Here are some links courtesy of David Theunissen http://www.flyelectric.ukgateway.net

Profili http://www.angelfire.com/sports/profili/
is an exceptional airfoil plotting program. Not only is it free but it comes complete with 1800 airfoils. Easy to search, view, modify, print or export in DXF CAD format. Does any skin thickness and tapered wings. Only weakness is that it cannot draw spars.

Foils http://web.onyxnet.co.uk/Hunt.Family-onyxnet.co.uk/foils.htm
is another airfoil plotting program which is currently free (437kb). I like the washout option. Cannot draw spars. It only comes with a few airfoils, but there are over 1100 individual 'dat' files to choose from on the UIUC http://amber.aae.uiuc.edu/~m-selig/ads.html Airfoil Data Site.


Center of Lift http://home.no.net/~thomanie/ is a free beta version of airfoil plotting software which used to draw spars. The new 'version 4' does not seem to do this any more though. Here is my copy of the old version 2 (77kb) which is not complete but does work. The main advantage it offers over the previous two is leading/trailing edges and spars (if you don't mind them all drawn 5mm wide). At only 77kb there's no reason not to download it. Also does DXF exports. Use continuous stationery if your airfoil is larger than 1 sheet. Also needs the UIUC data files for airfoils.

The Incomplete Guide to Airfoil Usage http://amber.aae.uiuc.edu/~m-selig/ads/aircraft.html provides a list of airfoils used on over 3900 fixed wing aircraft (+ numerous helicopters). Root and tip airfoils provided. Here's a zipped Excel spreadsheet (98kb) copy of the list which makes for interesting reading when you sort by airfoil (Clark Y features rather well!). Ever heard of a "Wide #2 Pencil" airfoil??? The site also has Dave Register's program which is just 44kb in size (I've not really used this one yet).

[ 08-08-2001: Message edited by: gjohnson ]

[flyingbuddy]

Gordon, I've seen that Profili link before, but when I click on it, the host, Angelfire, says that the site was removed due to violation of their terms of service. I was wondering if maybe you or Allan could host the program on your site, or maybe you could email it to me UNzipped?

Examples of successful single-surface undercambered airfoils in full-scale aircraft and models include the Quicksilver line of ultralights, hang gliders, and competition free flight powered (timed) models. Flying Models magazine usually has an example "Darn Good Airfoil" in their free flight column.

Jim, as I recall, the adhesion layer is the molecules of air attached to the surface, the boundary layer is the transition from non-moving to moving air, and the laminar layers are above that, in smooth airflow.
Just to get remedial, chord is the distance from the forwardmost point of the leading edge to the aftmost point of the trailing edge. The chordline is the straight line between them. Camber is the maximum displacement of the surface from the chordline, expressed as a percentage of the chord. For example, the Litestik wing has a chord of 6.75" and is 0.7" high from the chordline, so 0.7/6.75= 10.4%camber.
In general, under 10% is common, with some high-lift sections being as high as 25%, but this has a corresponding increase in drag.
With the flaps fully deployed, the NASA QSRA has a camber of more than 40%!
See: http://www.hazegray.org/navhist/carriers/odd/odd04.jpg
In this photo, the flaps are still only partially deployed. The NASA Ames/Moffett Airshow is this weekendand maybe I'll get some pics.

[gjohnson]

John, I had the same problem. I hoped that it was temporary like Ezone not being available today. It's the only link I have, which came from the source listed at the top. Sorry.
--Gordon

[flyingbuddy]

I think it has been dead for a couple of months, at least that I know of.

[gjohnson]

Jochen,
Thanks for the links. I am planning to write Dave Roblen and ask for the links he mentions in his RC Microflight article. If he sends them to me I'll post them here. His article whet my appetite for more, but I haven't searched out all the web sites on airfoils and don't know where to start. You links are much appreciated.
--Gordon

[David T]

Hi guys. Profili has a habit of disappearing so I've loaded my copy on my web site again. http://www.flyelectric.ukgateway.net/links.htm

[Joe]

Jim - If you are still in the market for some custom cut foam wings you may want to contact Vic Plath in Albuquerque at 828-9499.

He's a pretty old dude and may not be able to do it anymore but I'm sure he knows who will. As long as you provide the foam and templates, I doubt that Vic would charge you anything to do a couple.

If you are not a member of the Albuquerque Soaring Association, you may want to look into that group. The current president, Charlie Zaffery is top-of-the-line in the human department and will give you all the help you need.

Joe :)

[jimwalker]

Thanks, if I'm ever up that way I'll have to stop in and visit him.