High Lift airfoils for UL's

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BBerson

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The original Lazair was a fairly clean ultralight with two small (9hp, I think) direct drive engines.
Two of those DA-150's might do it. And with small props, the noise might be bearable.
BB
 

caribeanbound

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I live on a 44 foot sailboat near West Palm Beach
Wonderous Mountain: Sounds like you are using the same airfoil and cord of my Janowski J1-B. Clark Y 13% with a one meter cord. I am using the standard span of 25 feet. The performance numbers can be found on the Janowski web site for three different HP engines. It is hard to beat the old clark Y for performance in the Ultralight to LSA range. I should cruise at around 105 with a top end of around 120 on 50 - 60 hp. Stall will still be around 40. I wont care much as long as the cruise is near 100 and the stall remains low. I have always liked the underhung aileron system as it lets you get a really simple smooth wing. I wonder how much you gain in efficiency with this arrangement. Does the aileron gain since it has a higher aspect ratio? Does the airflow over the wing improve much? I'm looking at these things for my next plane.
 

WonderousMountain

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The Clark airfoils are outdated by anyones standard. Their stall performance is pretty good, and the aileron responsiveness is predictable. I like the looks of your airplane. Underhung airfoils are probably the simplest, most trustworthy way of creating high lift. The simplicity and ease of manufacture has probably given them a good reputation. Simplicity often results in actually meeting goals.

As for the high AR aileron, biplane interaction will likely ruin any benefit you may see. You won't gain any efficiency with this arrangement, simply speaking you'll have to account for the drag of both airfoils all the time instead of the one. Taken on a total chord basis you'll have a shorter wing depth for the main airfoil and moment will vary significantly. However, you could go with a slightly thicker 18-21 percent? Wing airfoil and maybe the reduced wing area and increased structural depth will more than offset the penalties.

As for my airplane, riblet's GA airfoils offer high lift, low drag, and good stall characteristics. Read Mountain Flyer if you're curios.

Wonderous Mountain
 

mstull

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I have used underhung, normally called "Junkers style" ailerons on a few of my designs. They have a couple advantages and a couple disadvantages.

Advantages... They are fully effective even when the wing is stalled. They also generally have less adverse yaw than conventional ailerons. And you can use them to gain lift... particularly near stall speed... but if you give them some incidence to produce lift in cruise, they create more adverse yaw.

Disadvantages... They add drag compared to conventional ailerons. They can be difficult to support on a light weight wing. They work best the farther aft you put them. And they are more efficient the farther below the wing you put them. But how do you support them stiffly and strongly enough, back and down there, when the only thing nearby is the relatively flimsy trailing edge of the wing?

I found that to be quite problematic on my U/L wings. Violent, destructive flutter was the result. So if you do try them, mount a long, strong, stiff bracket to both spars. It helped a little to counter-balance them, but they really limited my Vne. Also, if they're fairly long, make their spars large enough diameter so they have good torsional stiffness. Flutter is a fickle (and dangerous) beast.

Here's a picture of one of my designs that used them:
 

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Norman

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biplane interaction will likely ruin any benefit you may see. You won't gain any efficiency with this arrangement, simply speaking you'll have to account for the drag of both airfoils all the time instead of the one.
Actually Junkers flaps are one of the few examples of constructive interference. The pressure field of the external airfoil flap reaches forward and onto the wing and decreases its drag. This drag decrease is almost equal to the drag that the small airfoil would have in isolation so the total drag of this double wing, or "doppelflugel" as Dr Junkers called it, really isn't much more than a wing with a slotted flap.
If you want to learn about constructive interference read "The aerodynamics of sail interaction"

And specificaly about Junkers flaps
 

caribeanbound

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I Know what flutter can do to an aircraft. I saw the wrecked Motor Glider version of a Taylorcraft after the Test pilots and their parachutes were removed. The aileron castings failed during flutter. The ailerons were not balanced and when the top speed was increased with all those nice fairings it simply came apart in the air. I am not sure how they lengthened the wings with the associated hardware. That could have more to do with it. The castings were all broken though. The sad thing is that both test pilots had heard the flutter (buzzing sound) starting on the previous flight and flew again without changes. That was a real pretty plane and I had watched it fly over my home on every flight including the last.
My design does not call for balanced ailerons but I am going to do it anyway. I will also be very mindful of cable and linkage adjustments.
 

WonderousMountain

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Sometimes I feel my aeronautical diet consists mainly of incomplete, faulty, inaccurate, outdated, impertinent, unreadable and otherwise useless data.

That's all I wanted to say,
thanx Norman

Wonderous Mountain
 

Norman

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Sometimes I feel my aeronautical diet consists mainly of incomplete, faulty, inaccurate, outdated, impertinent, unreadable and otherwise useless data.
Don't we all

Books written by people with credentials are usually pretty good but even a guy with a PhD will sometimes have a blind spot with regards his own pet theory. Many crackpots have advanced degrees and even the good guys can be a bit myopic in some areas. What it really comes down to is learning who you can count on to be unbiased ie not a crackpot and try to get multiple sources even when you think you're dealing with a good source. Orion and Billski are two of the good guys
 

clanon

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What about Irving Culver's airfoil used on the British
Shadow ultralights ?
I know is a rotor blade airfoil the first 33%(plywood laminar i think) and then
only straight lines to trailing edge(dacron).
"This was a wing without twist being built
straight, but the D-box structure leading edge
had washout progressing from the root
(centre), which was symmetrical section to
minus 2.3° washout at the tip. The tip looking
very similar to a Clark Y section."
From 20 to 100mph range.
UN-STALLABLE
 

Dana

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Nothing is "UN-STALLABLE".

-Dana

I've noticed that the press tends to be quite accurate, except when they're writing on a subject I know something about.
 

PaulS

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Using a rather unconventional airfoil like the Lissaman 7769 which was used on the Gossamer Albatros and Condor would give very high lift at very low reynolds numbers. The high pentroof design will cause a lot of turbulence and drag at speeds higher than the design speed.
Paul
 

clanon

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Well it seems there is an issue there with your LE radius...:ponder:
IMHO
 

bmcj

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Check this out and workout for center of lift for this wing profile,
I've seen camber lines on paper with more thickness.

As already pointed out, sharp leading edges can have some bad stall traits, and thin airfoils mean narrower spars. The reasons for choosing thin wings with a pointy nose vs undercamber seem to contradict one another.
 

danmoser

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Check this out and workout for center of lift for this wing profile,
this airfoil will have a very narrow range of acceptable angle of attack.
Terrible separation will occur at AoA either above or below that narrow range.
At least round the nose, or better yet, use an airfoil designed for ultralights.
If you want high CL max, with lots of under-camber, something like this might do..
http://m-selig.ae.illinois.edu/ads/afplots/fx60126.gif
 
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