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Low aspect ratio ultralight

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Starman

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After running my other design up in complexity and cost I turned around and ran it back down so now I'm thinking about making an ultralight ... or something pretty light that I might try to qualify as an ultralight for starters. The main reason is to get in the air ASAP.

I like the idea of low aspect ratio because it is possible to get a lot of wing area and still keep the weight down enough to be able to use a more powerful engine for what ends up being more climb rate than a high aspect ration UL. However I read about the Snedden M7 and about the full power required by the rules and wonder if he is getting away with it? I suppose someone can get away with it if they make just one but not if they are manufacturing. Not that I have my mind set on cheating, I'm just curious. I suppose I could start it out as an ultralight and then maybe add some power and make it experimental. I do like the low stall speeds and want to keep that whether it is a UL or experimental. I have lots of ideas so far but no specific direction chosen yet except I'm liking a fabric covered design - pretty much a requirement for a large wing area.
 
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Dana

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From what I've heard, I seriously doubt that Snedden's design is a legal ultralight.

If you build something for your own use that's not quite legal, you'll likely never have a problem if it looks like an ultralight and nothing's too blatant (like hanging a huge engine on it so it's obviously going to be too fast). For a manufacturer, it's a different story (or should be).

-Dana

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Starman

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One potential shape for an ultralight is a low AR delta like a facetmobile (except I would use curves). The facetmobile only weighs 350lb, and that's just 100lb over the limit. It seems a design with this shape should be about the easiest to get under the limit, but then it doesn't have wire bracing, it's cantilever ... but there are cantilever ultralights out there with more or less 'normal' AR.
 

BBerson

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You might search for a low aspect ratio ultralight called "Milt's Little Bird".
Probably in an old EAA publication.

I think the low efficiency of low aspect ratio can be offset somewhat with clean cantilever design. Low aspect ratio combined with low span loading should be good.
 

mstull

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Starman,

Be sure to read Part 103 and AC103-7 before you make any decisions. The problem is passing the stall speed limit. Low aspect ratios might work fine on an R/C plane, where the Reynolds number is low. But on an U/L, a lifting body design like the Facetmobile wouldn't come close to passing the stall speed limit. It takes an awful lot of wing area to pass the stall speed limit.

You're right, that lower aspect ratio wings are less aerodynamically efficient, and need more power/thrust.

Lower aspect ratio wings tend to be simpler, less expensive, and lighter, since the spars can be so much shorter and lighter. But they tend to need more power. The bigger engine will be heavier, more expensive, and burn more fuel, so can cost more in the long run.

I've designed and flown U/Ls with aspect ratios from as low as 3.3 to as high as 9.5. For a given wing area, higher aspect ratio wings will tend to climb better, glide better, stall slower, and burn less fuel. They are simply more efficient.

If you want to get into the air ASAP (and safely), buy a used U/L. You'll learn a lot that will help you design your own. Be sure to take your scale when you look at used U/Ls. If it's over weight, you'll have extra bargaining power.
 

Dana

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Also if you put on a big enough engine to overcome the inefficiency of a low aspect ratio, you're likely to run afoul of the Part 103 max speed limit, especially as you probably won't have all the drag producing wires and struts that keep a conventional ultralight's speed down.

Starman, the taglines are shamelessly stolen from wherever I find one I like, and added to and randomly selected from a list.

-Dana

In general, liberalism consists of A & B getting together to see what they can make C do for poor old D.
 

ultralajt

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Starman, dont forget that at loss of power, low AR ultralight will fly as a brick.
Noething wrong when plenty of landing area within a (bad) glide path...but...

Mitja
 

Starman

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Starman, don't forget that at loss of power, low AR ultralight will fly as a brick.
Nothing wrong when plenty of landing area within a (bad) glide path...but...
I think it depends on the details. For one, a lot of wing area (I'm thinking more wing area than required by the regulations) makes up for lack of span because the wing flies at a lower CL, and the effect of CL on induced drag is a squared function so it can dominate. This is the reason the low aspect ratio Avro Vulcan has less induced drag than a B-52.

Another thing is that even if there was higher induced drag you would only get the high sink rates if you tried to slow it down too much, which is the instinctive reaction. If you keep the speed up then low AR planes glide pretty well.

I found a detailed analysis and explanation of the aerodynamics of the facetmobile and anyone with concerns about low AR should really read that. It does just about everything better than a 152, and with a lower power to weight ratio, (I think) Don't want to sound to much like Pie in the Sky :D
 

ragflyer

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I often find using L/Dmax = 0.89*effectiveSpan/sqrt(flatPlateDragArea) rather than the traditional formula that uses aspect ratio and cl easier and more appropriate for design.

It brings out the important factors clearly- for high L/D you need as little drag area and as much span, the latter being more critical.

It avoids the pifalls in thinking in terms of AR in isolation and it is easier to see why the avro vulcan and B47 have an almost identical L/D ratio

Both formulas are offcourse exactly equivalent from a mathematical point of view.
 

Starman

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Also if you put on a big enough engine to overcome the inefficiency of a low aspect ratio, you're likely to run afoul of the Part 103 max speed limit, especially as you probably won't have all the drag producing wires and struts that keep a conventional ultralight's speed down.

Starman, the taglines are shamelessly stolen from wherever I find one I like, and added to and randomly selected from a list.
OK, well they are being stolen for a second time =)

The big problem is exceeding the speed limit. I see that Mark Stull put a throttle limiter on his plane but that wouldn't work for this because it would handicap takeoff and climb too much. I could just copy the snedden idea of a throttle governor, but that's cheating, and if everyone starts cheating the feds will dump on us.

Another idea is to have a drag producing governor, some kind of speed brake that would automatically deploy to prevent further increases in speed. Any of these things though, could be removed as soon as an inspector walks away, even Mark's throttle limiter.

Actually I'm thinking ultralight 'style' rather than ultralight legal. I still want to play the game to try to make it legal at first, but will switch it to experimental as soon as I get my license so I can use more power and fuel, with no speed limiting devices.
 

Topaz

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...Another thing is that even if there was higher induced drag you would only get the high sink rates if you tried to slow it down too much, which is the instinctive reaction. If you keep the speed up then low AR planes glide pretty well....
Speed for minimum sink (longest time to come down) and speed for best L/D (farthest distance from a given altitude) are set by the configuration and wing loading.

You can only get to those 'best' performance points by flying at those speeds. If you fly any other speeds than those, power off, you'll come down more quickly or with a shorter glide distance, respectively.

Generally the speed for minimum sink is lower than the speed for maximum L/D.
 

autoreply

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Generally the speed for minimum sink is lower than the speed for maximum L/D.
It always is :)

As for the relation between everything:
It's quite easy to plot everything in a spreadsheet and fiddle around a bit. Especially if you're concerned about a reasonable low amount of power, going below 4 or 5 won't give you a serious structural benefit, but it will give you a big weight penalty in power.
 

Dana

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Throttle stops, governors, automatic speed brakes... all of these are acceptable means of limiting maximum speed for FAR 103, provided they can't be defeated in flight. What you do on the ground is your decision... but as soon as it's removed or disconnected, you no longer have a legal ultralight, just like if it's overweight or 2 seats.. No problem until you get ramp checked by the FAA, or have an accident or complaint and they investigate (unlikely but not unheard of). If it "looks like" an ultralight, you almost certainly won't have any trouble even if it's not quite legal... but if it looks like it's going 200 mph standing still, it better be legal 'cuz it's gonna attract attention.

Autoreply, just as an example as I mentioned in the other thread, for the design I'm working on with a wing area of 140 ft², increasing the aspect ratio from 4.4 to 5.2 decreases induced drag by about 12# at a Cl of 0.8, or about 2HP at 40mph.

-Dana

In the 60's people took acid to make the world weird. Now the world is weird, people take prozac to make it normal.
 

Starman

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You might search for a low aspect ratio ultralight called "Milt's Little Bird".
Probably in an old EAA publication.

I think the low efficiency of low aspect ratio can be offset somewhat with clean cantilever design. Low aspect ratio combined with low span loading should be good.
Thanks, that's what I had in mind too. I looked up Milt's little bird and found a picture of one of his low AR planes. This is a type planform I'm contemplating.

I haven't decided on any planform yet, but I was thinking about the wing tip props, which I wrote about in a different thread. My idea is to use powerful electric motors and a generator to drive the wing tip props in addition to the main prop driven mechanically off the engine.

I have another hybrid design in mind, but this one probably shouldn't be low AR, but this idea is even further back on the back burner than the previous hybrid idea.

The idea is to use a small gas engine in addition to a small amount of batteries and pure electric drive. The idea is to use a gas engine with a little more power than is required for level cruise, and use the batteries in addition to the gas engine for takeoff and climb.

The pictures come from this web page which sings the benefits of low AR: Low AR-planes
 

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Starman

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You're right, that lower aspect ratio wings are less aerodynamically efficient, and need more power/thrust.

Lower aspect ratio wings tend to be simpler, less expensive, and lighter, since the spars can be so much shorter and lighter. But they tend to need more power. The bigger engine will be heavier, more expensive, and burn more fuel, so can cost more in the long run.

I've designed and flown U/Ls with aspect ratios from as low as 3.3 to as high as 9.5. For a given wing area, higher aspect ratio wings will tend to climb better, glide better, stall slower, and burn less fuel. They are simply more efficient.

If you want to get into the air ASAP (and safely), buy a used U/L. You'll learn a lot that will help you design your own. Be sure to take your scale when you look at used U/Ls. If it's over weight, you'll have extra bargaining power.
Thanks, that's a good idea, to bring some scales along :) I might look into that, but if I bought a used one it could eat up my budget.

Concerning the efficiency of AR variations, I'm sure high AR is ultimately better, however, there's o e bit of efficiency I would like, and that is a very fast roll rate. Also low AR is less gust sensitive and less stall angle sensitive, ie, no snap stall.
 

BDD

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You can get a low enough stall speed with a low aspect ratio ultralight as long as you have enough wing area and can stay within the ultralight weight limit.

A delta or Arup configuration woud result in very deep ribs and spars. Might be possible that foam ribs with epoxied capstrips could be strong and light. I have always wanted to see one of those configurations tried as an ultralight.

Top speed could be limited with the prop design. I would think though that with it's naturally low drag at low angles of attack that the delta wing would tend to fly too fast for use as an ultralight. Of course, a powered Rogallo wing wouldn't be very fast at all. Unless you use your V-8.

I'd be a bit concerned about not having enough power when you are at a high angle of attack/ high drag configuration.

You could pack a lot of wing area into a relatively low aspect ratio biplane design and have a very light design. The trussed wing structure would be very light but strong and it would have acheive sufficient area easily. It creates a lot of drag though with the wires, struts and interference drag. That might be o.k. with you. I didn't really read your original question as assuming the same plane, just an ultralight. I am assuming that you would be starting with a new concept.
 

autoreply

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Also low AR is less gust sensitive and less stall angle sensitive, ie, no snap stall.
Which is the most underestimated benefit of Low-AR wings. According to some Mirage pilots the Mirage 2000 is capable of flying in turbulence that would tear apart every other plane (because of the low dCl/Dalpha)

The major concern is the power required close to stall, that can be full power or more if you have a very low AR. Some early jets are notorious for being unable to come out of that "coffins corner" without losing altitude, I.E. Even with full power they'll still sink.

You might know the Verhees Delta, but that plane proves all the benefits of a small low-AR aircraft, though it has it's disadvantages.
 

LittleBird

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How about the Milt Hatfield "Little Bird" ARUP style planes? I have the old original molds for these planes. The last one made was about 75% fiberglass and an ultralight.

Kevin in Missouri
tinyauto(at)aol.com
 
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