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Thread: High Lift airfoils for UL's

  1. #1
    Registered User PTAirco's Avatar
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    High Lift airfoils for UL's

    FAR 103 presents some interesting challenges to a designer; I am toying with an ultralight design and have always thought the low stall speed requirement woiuld be better met with a wing with a very high Cl operating at a relatively high (for an UL) wing loading rather than the usual (abominable excuse for an) airfoil used on most UL's and a large wing area. A smaller wing means bending moment, less structure; smaller chord and span means smaller tail volume and/or shorter fuselage all of which helps with weight.

    Since we have no need for high speed flight it would seem one could capitalise on the other end of the spectrum by using a very high lift airfoil, something that would look like an airliner's wing with everything hanging out (though everything would be fixed of course, not retractable). Or like some of the early German Goettingen glider airfoils with huge amounts of camber.

    Does anyone have any sources for data on very high lift airfoils?
    "Aeronautical engineering is highly educated guessing, worked out to five decimal places. Fred Lindsley, Airspeed."

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    Registered User xj35s's Avatar
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    Re: High Lift airfoils for UL's

    I agree. Here's some good info on airfoils.

    http://www.zenithair.com/kit-data/ht.html

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    Moderator Dana's Avatar
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    Re: High Lift airfoils for UL's

    There are some interesting high lift airfoils out there, but as always everything is a compromise. With high lift comes also high drag and pitching moment, so you don't really gain that much. There are certainly some good applications for exotic airfoils, but typically they're optimized for one particular flight regime, not for the overall good handling you want on a typical ultralight.

    I looked into a few of the thick, highly cambered high lift airfoils recently for a project, but settled on the good old Clark Y section.

    -Dana

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    Registered User WileEZ's Avatar
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    Re: High Lift airfoils for UL's

    My take on this:

    I also had the same idea, but after a while I realized that I would need a heavier and more powerful engine to overcome the drag generated by the high lift wings, even though I would not be cruising more than 55 MPH.

    There are trade-offs and compromises to be made.

    There's a reason why so many of these ultralights are made for midgets. (Thus rendering them useless for me.)

    While I can stand to lose a few more pounds, I've been told by several doctors that my ideal weight would be at or slightly above 200 lbs, that's with minimal body fat. Right now I'm at 240 lbs and working to drop a few more pounds. I'm just one of those stocky guys at 6'.

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    Registered User mstull's Avatar
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    Re: High Lift airfoils for UL's

    Here's a high lift airfoil that works great. I took a Got.387 upper surface, and under-cambered the heck out of it. It glides fantastic, stalls around 24 mph, cruises from 28 mph to 60 mph, and climbs great. Even when you throttle way back, it still wants to climb. You have to point the nose down several degrees to keep from climbing.

    I made this wing to climb with a tiny CorsAir M25Y, 24 Hp engine... which it did. Later I put my MZ201 direct drive on it. With the MZ201 with reduction drive, it would climb at ridiculously steep angles and rates. There's no drag penalty if you keep it down below 55 mph. I love the way you can throttle way back and float at very low speeds or ride thermals.

    Most thick camber airfoils are inefficient because of the large angle of the trailing edge. If you can get that angle near zero (with deep under-camber), you get huge amounts of lift from the under side of the airfoil. That makes the wing loading on the upper surface much less. So a high lift airfoil can be very efficient at legal U/L speeds.

    All designs are compromises. In this case there wasn't room for a rear spar. So I designed the wing with a single, tubular, 6" diameter, carbon spar. That spar made the wing come out a bit heavy. But it still makes the weight limit with the MZ201, even with the reduction drive and big prop.

    It doesn't have any problems with pitching moment. It would easily climb with a 250# pilot with about 40 Hp. I used Junkers style ailerons on this wing. If I was to make it again, I would probably use spoilerons for roll control.

    Here's a photo cruising at about 35 mph. You can see the negative angle of attack clearly, that I had to use to keep from climbing... even at that low speed.
    Attached Thumbnails Attached Thumbnails High Lift airfoils for UL's-eld4.jpg   High Lift airfoils for UL's-winters1.jpg  
    Mark E. Stull
    mstull@wtxs.net

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    Registered User steveair2's Avatar
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    Re: High Lift airfoils for UL's

    Hey Mark!
    I really like what your doing. I hope you plan on offering some drawings one day. Your bipe is cool! Keep up the good work.

    Steve

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    Re: High Lift airfoils for UL's

    Mark,

    I always learn something from your posts and it usually sparks a question at the same time. For the dummies (me), is a "pitching moment" a tendency for a wing to try to "roll forward" or nose a plane down in flight as speed increases? Sorry for my un-informed questions.

    Lynn

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    Registered User mstull's Avatar
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    Re: High Lift airfoils for UL's

    Right. The center of lift varies a little with angle of attack. Different airfoils vary different amounts. If your horizontal stabilizer is far enough aft, and your CG is in range, it's not a problem.
    Mark E. Stull
    mstull@wtxs.net

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    Re: High Lift airfoils for UL's

    Thanks Mark. Just to make sure I understand... the undercamber does make the Center of Lift "move", depending on angle of attack, more than a similar, yet flat-bottomed, airfoil profile? To counter-act that movement of the center of lift one would need a bit higher ratio of chord-to-horizontal-stabilizer distance to regain the pitch authority?

    FWIW, I'm currently reading the book "Stick and Rudder" so I'm reading and learning a LOT about angle of attack

    Thanks again.

    Lynn

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    Registered User mstull's Avatar
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    Re: High Lift airfoils for UL's

    Lynn,

    Yes, the center of lift of a deeply under-cambered airfoil probably does move more than other common airfoils. The center of gravity has to be forward of the entire center of lift range, whatever airfoil you choose.

    Yes, you need enough pitch "authority" so when the center of lift is at its farthest aft, and your CG is farthest forward, your horizontal stabilizer and elevator can generate enough down force to counter that. But for that to come into play, your CG would probably be too far forward, out of range.

    The main reason, for moving the horizontal stabilizer aft, is efficiency. If the the horizontal stabilizer is close to the wing (short coupled), the plane will be inefficient. The reason is the lack of leverage the horizontal stabilizer has from such a short arm.

    If the horizontal stabilizer is say half as far aft, it will need to generate twice as much down force to balance the plane, because it has half the leverage. That extra down force has the same effect as adding weight to your plane. So the wing will be less efficient lifting the extra weight, and the horizontal stabilizer will generate more induced drag too, at its high angle of attack.

    I wouldn't go so far as to say that you'd have to move the horizontal stabilizer farther aft because the center of lift moves an extra inch on an under-cambered airfoil. Another solution would be to limit the CG range.

    Yes, I like to measure how close coupled a plane is, by how many wing chords the center of lift of the horizontal stabilizer (with elevator) is from the center of lift of the wing. This is where you see compromises for various reasons (like looks, weight, and cost). Some planes use just 2 or even less... like a Quicksilver. Efficient gliders go way up closer to 10. 3 or more is good.
    Mark E. Stull
    mstull@wtxs.net

  11. #11
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    Re: High Lift airfoils for UL's

    Mark....Thanks again for the additional information and answers.

    Lynn

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    BDD
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    Re: High Lift airfoils for UL's

    Take a look at the NACA 4415 or maybe the 4418. Allows for very deep spars or a deep D-section for strength. Has excellent stall behavior for safety and has a very high coefficient of lift potential in exchange for a reasonably low drag coefficient. The CL can be up to 1.6 or so with flaps. That would produce a nice slow and relatively safer landing speed. Or you could land faster if you want without the flaps, depending on conditions. Wind gusts would be a factor here.

    For an ultralight where wind gusts and light wing loadings impose greater load factors a relatively deep airfoil would be a good idea, especially if wou want an efficient cantilever wing.

    Also for ultralights I think that safety and stall behavior are very important.

    These are also well known airfoils that are not critically affected by dirt, roughness, insects, etc. I.e. they will keep working and aren't as critical as laminar airfoils are. One of the airfoils used on a certain canard design seemed to be very adversely affected by even rain.
    Last edited by BDD; February 24th, 2008 at 07:54 PM.

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    Registered User WonderousMountain's Avatar
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    Re: High Lift airfoils for UL's

    While I'm not building an ultralight, the stall speed and wing chord is similar and finding information on airfoils in Ultralight Reynolds number is still difficult.

    What I'm trying to do is get a low wing area, Which will reduce bending moment and increase skin thickness. The loss in aspect ratio will be missed, but probably unnoticable.

    Cruise Cl will be about .3. Reynolds .6 to 1 million full flap landing, and 3 to 1.5 cruise. Ailerons will likely be set @ fast cruise Cl.

    Pivot mechanisms are fine, but rollers, cranks and other gizwhizzys are beyond my patience.

    Blessings,

    Wonderous Mountain

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    Registered User bmcj's Avatar
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    Re: High Lift airfoils for UL's

    Quote Originally Posted by WonderousMountain View Post
    The loss in aspect ratio will be missed, but probably unnoticable.
    Don't underestimate the effect or importance of aspect ratio.

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    Registered User WonderousMountain's Avatar
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    Re: High Lift airfoils for UL's

    Ummmm, sorry about that.

    My chord is "fixed" at 1 meter. Exactly why I'll accept a 1M root chord and no more, is a trade secret....

    In any case, the more lift I get, the less wing I need for my stall speed, which was chosen by another trade secret....

    So not only do I get to make a smaller wing with more lift, I also get to make a shorter one. Reducing surface area means higher wing densities for more strength, shorter wings mean less bending moment for less force which is almost as good as more strength. Weight will remain the same due to yet another trade secret...

    Hope this helps,

    Wonderous Mountain

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