High aspect ratio wings

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Aesquire

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Ultralights, in the U.S. pt103, are constrained by weight limits more than anything else. That's why all those draggy wires are still used, you get the lightest weight for the size with external wire bracing. ( although with carbon fiber you can get pretty light with a cantilever wing... at a cost )

To take the most common example of an American Ultralight, the Quicksilver in it's most common form has an A/R of about 5. A modern competition flex wing hang glider, about 8, and the high end carbon fibre ATOS rigid wing about 13.3 And the aspect ratios on hang gliders haven't changed much in years. They hit the compromise between weight and performance literally decades ago. ( about 70 pounds is the most you want to pick up and run with )

Keep in mind that the Quicksilver, and many other rectangular wing ultralights, use the "Ladder" or "Ultralight Style" construction on the wings, ( looks like a big ladder, leading and trailing edge tubes, chord wise tube bracing, not counting ribs & wires or strut bracing ) not because it's efficient, but for the light weight, and ease of construction.

There have been experiments with much higher aspect ratio Ultralight Style wings but the need for more bracing wires just adds drag faster than the improved efficiency reduces it. On a Quicksilver hang glider, it seemed easy, add another bay or 2. But when we tried that, going from an A/R of 7.7 up to A bit over 10, ( and more area! For bigger pilots ) that needed 8 additional bracing cables, and those the longest on the glider. Since the roll control was by rudder only, ( 2 axis control ) The additional span and inertia slower roll response to marginal for a soaring machine flown close to tree covered slopes.

I haven't done an apples to apples comparison with the same wing area & design & changing Aspect Ratio. It's always been a bigger/smaller or airfoil change too, with gliders in MY experience.

And I really wanted that bigger wing when we started putting marginal engines on & tried to fly from level ground.

More area, more span, cost top speed. But also makes a big difference in Angle of climb. The difference, for a big pilot between clearing the trees at the other end of the field.

Some compromises are important.
 

lr27

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Still, it's been demonstrated that you can have a very successful ultralight at a 36 foot span, without any exotic materials, not even carbon.

I think a lot of ultralights have unsophisticated design with short wings and draggy wires because it isn't necessary for sales, not because it's not practical. Part 103 specifically states that you can use a gadget to limit the top speed, so the drag isn't necessary.
 

stanislavz

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Some of mine thoughts - if you make wing with struts and go with carbon fibre - you can make it really thin and low tech, and with high aspect ratio. All spar would be embedded in a skin itself with layer of ud carbon.. (It was something like 0.5cm2-1cm2 cross section of spar for standard ultralight with strut )
 
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Aesquire

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I think a lot of ultralights have unsophisticated design with short wings and draggy wires because it isn't necessary for sales, not because it's not practical
It's also cheap, and a familiar construction technique for hang glider holders. Same tools and shop needs.

The "practical" part... It's a compromise. Price, weight, performance.

The hang glider version of the Quicksilver had a higher Aspect Ratio than the later commercial Ultralight powered version still sold.

The original powered ones kept the higher ( mid ) A/R wing. The current ones are the clipped wing models. ( hence my opinion that it's better to design for a bigger wing than the minimum ( chosen for top speed ) so you can absorb weight gains ( inevitable ) and make it a clipped wing version later. See Cubs, Spitfires, and Quicksilvers examples of success following that path. )

While the Ultralights were lowering A/R to fit in garages etc. The Glider guys were increasing it. Note that the pt103 weight limit for a glider is 155 pounds.
 

Norman

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Feels like I'm missing some piece of information.
Materials and money. Back when the only materials available were wood and steel it was found that spar weight started increasing really fast at AR>7 or 8 so most GA airplanes got that aspect ratio. The move to aluminum didn't change the weight to AR much so 7 or 8 is still typical. Fibreglass improved the AR to weight ratio a bit but carbon fiber pushed that point of diminishing returns out past 10:1 . In fact the strength to weight ratio of carbon is so high that the skin and ribs become the main driver of wing weight. So there's the answer to light high AR wings.

Then there's money. CF is quite expensive by the pound but it's also very low density so the cost of building with it gets a little complicated. Pultruded rods and prepreg uni_directional tape yeald the highest strength to weight ratio but you're paying someone else to do some of the work. If you can do reasonably consistent wet layup a carbon fiber spar can be cheaper than an aircraft grade spruce spar of the same strength. Depending of course on the market prices of Spruce and CF on the day you buy it but spruce has been going up for years while CF has been coming down so that may not be an issue anymore.
 

Shakeelsid

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Materials and money. Back when the only materials available were wood and steel it was found that spar weight started increasing really fast at AR>7 or 8 so most GA airplanes got that aspect ratio. The move to aluminum didn't change the weight to AR much so 7 or 8 is still typical. Fibreglass improved the AR to weight ratio a bit but carbon fiber pushed that point of diminishing returns out past 10:1 . In fact the strength to weight ratio of carbon is so high that the skin and ribs become the main driver of wing weight. So there's the answer to light high AR wings.

Then there's money. CF is quite expensive by the pound but it's also very low density so the cost of building with it gets a little complicated. Pultruded rods and prepreg uni_directional tape yeald the highest strength to weight ratio but you're paying someone else to do some of the work. If you can do reasonably consistent wet layup a carbon fiber spar can be cheaper than an aircraft grade spruce spar of the same strength. Depending of course on the market prices of Spruce and CF on the day you buy it but spruce has been going up for years while CF has been coming down so that may not be an issue anymore.
At 2020 prices, carbon is now cheaper
than wood in terms of strength to money ratio. Glues and resins are also on the way down, and if one is careful in fiber to resin ratios, uses multiple laminations to economize, and is super careful in fiber orientation and direction, it is possible to build a carbon structure cheaper than a wood and ply gusset structure.
 

cluttonfred

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Hmm, I'd love to see some hard numbers on this, say three equivalent airframes (wood/fabric, sheet metal, composite) and the cost to build each.

At 2020 prices, carbon is now cheaper
than wood in terms of strength to money ratio. Glues and resins are also on the way down, and if one is careful in fiber to resin ratios, uses multiple laminations to economize, and is super careful in fiber orientation and direction, it is possible to build a carbon structure cheaper than a wood and ply gusset structure.
 

lr27

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At 2020 prices, carbon is now cheaper
than wood in terms of strength to money ratio. Glues and resins are also on the way down, and if one is careful in fiber to resin ratios, uses multiple laminations to economize, and is super careful in fiber orientation and direction, it is possible to build a carbon structure cheaper than a wood and ply gusset structure.
I'm sure that's true for "certified" aircraft spruce, but is it true for stuff you can grade and select yourself from the lumberyard?
 

Pops

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Hmm. In the ultralight (US rules, 254 lb) camp, you are really up against weight limits, which says the airframe and the engine both have to be quite light. In LSA, you do have more latitude on structural weights.

Increasing AR is increasing wing span and decreasing chord simultaneously. When you increase span, you increase spar length, you increase shear over much of the span, and you increase bending moment on the spar. You are also making the chord smaller and the spar depth usually scales with chord. So, you have bigger loads over much of the spar and a less deep spar to carry it, which will almost always mean more weight in the spar to do the job. You can go to a deeper section wing, from 12% thickness to 15% or from 15% to 18%, which adds relatively little drag. Or you can put on more flying and landing wires which adds a lot of drag.

Four other points in the UL camp:
  • Your level top speed is capped pretty low, and drag helps you stay within that cap;
  • UL rarely need or want much altitude, so more climb rateis primarily of interest for obstacle clearance on takeoff;
  • Flight on really low power is a factor of having low enough wing loading, not having low enough drag;
  • UL usually have pretty lazy roll rates - adding span will reduce that even more, and is likely to increase adverse yaw too.
So, US rules UL can not really use a lower drag design.

Move into Ultralights as defined in other countries or into LSA rules, and while we again have max gross weight rules and speed limits, we have more room to work. We get to trade between level flight speed and field length limits. Yes, more span and less chord may give better takeoff and climb performance. But if you cut into your useful load because you are running a heavier wing, you may find the airplane's utility reduced instead of improved...

Choose carefully, analyze thoroughly, and make sure that you are making a better airplane for your mission.

Billski
On the JMR , the AR is 7.125. At first I was going to use the 2412 airfoil, but with the extra thickness I could go to the 2414 for almost the same performance and gain strength.
On the whole design, the hardest part to do was working out the layout of everything I needed in the 48" cord of the wing and still have fuel in the wings instead of the fuselage. In the wing there are fuel tanks, aileron control cables , flaps bellcranks and pushrods and torque tube and the fuel lines and spars, compression struts, all crammed in the 48". Still would like to have more fuel, but with the layout that I used, part of the fuel tank had to a valley built into the rear wall for clearance for the flap torque tube. The added bonus of going to thicker airfoil , was the fact it increased the amount of fuel from using the 2412 airfoil. Win/win.
 

Pops

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Hmm, I'd love to see some hard numbers on this, say three equivalent airframes (wood/fabric, sheet metal, composite) and the cost to build each.
Don't forget the difference in the shipping cost with the long wood spars.
 

cluttonfred

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I have sketched out many “UPS planes” with major components or subassemblies small enough to fit within UPS size limits:
  • Packages can be up to 150 lbs.
  • Packages can be up to 165 inches in length and girth combined.
  • Packages can be up to 108 inches in length.
It could certainly be done...an 8’ long x 18” diameter tube for spars, longerons, etc., smaller boxes for the rest.

Single-seaters are easy...a biplane with 8’ wing panels or a monoplane with three 8’ panels, for example. Two-seaters are also very doable if you don’t do subassemblies, rather use the small components to assemble a larger whole...two 8’ spar tubes nested together by 6” to make 15’ 6” panels for each side.

The idea would be to simplify kit sales and follow on support by allowing direct shipment of whole kits, sub-kits, or parts directly from the factory to the builder.

Don't forget the difference in the shipping cost with the long wood spars.
 

lr27

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Yes. For two reasons. The lift distribution will be a little further inboard, so the spar doesn't have to be quite as strong. Also, the wing will be much thicker near the root, so that the spar caps can be smaller for the same strength. (I'm assuming that the greater depth of the shear web doesn't add very much weight.)

I think tapering the wing may require a little more attention to stall behavior.
 

BJC

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From reading it sounds like the higher aspect ratio you can make your wing the better. Less induced drag and high lift/weight. Why then would we continue to use low aspect ratio designs? Are they less efficient at speed?

Feels like I'm missing some piece of information.
Several excellent comments above. This one falls into the “nit picking” category, but that is what we do here. It is not AR that that influences induced drag, it is span. And keep in mind that increasing AR by increasing span while holding area constant, reduces the Reynolds number for a given speed, leading to a significant difference in an airfoil’s performance at low airspeeds.


BJC
 

Lendo

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Just a Heads-up, there is a Carbon STRIP product, produced somewhere in Europe (1.2mm/ 1.4mm thick with widths 50, 100, 200). It's sold by Sika as a Reinforcement to Concrete Buildings Bridges etc. it's called CarboDur S512. If anyone in Europe is reading this could they tell me where and by who exactly, is making this product - it would be greatly appreciated. It may well be a Sika owned Company. I can't get much sense from the local so called experts in Sika Australia.

In my opinion this is ideal for Spar caps as it can be easily bent and stepped to cater for the elliptical distribution loads. It would be easier to manage and the Carbon Rod Voids (filled with resin) would be avoided.
The strength and resin content is similar to the Carbon Rods and is readily available here in Australia whereas products from Europe and US is much more expensive to procure. I awaiting response from Sika on my questions.

There is another manufacturer in China using German Technology and Equipment, that Company is called 'Horse', the product may be as good, research and time will tell.
George
 

Dana

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The Lazair has an aspect ratio of 9.3 which is a factor in its excellent performance. It works because it's very light, which also means delicate. Vne is only 55 mph, stall speed 17. It's the kind of plane you take out for a half hour in the dead calm around sunrise or sunset. Compromises. A friend owns a Stemme motorglider, which has an 82' wingspan. Some airports he can't land at, at his home field he has to pull off the runway, shut down, get out and fold the wings (37' folded) before he can taxi to his hangar. Compromises.
 

stanislavz

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Hmm, I'd love to see some hard numbers on this, say three equivalent airframes (wood/fabric, sheet metal, composite) and the cost to build each.
You can find a some carbon fiber on fleabay for fraction of shop prices - ie 8oz / 16 oz (300/600) ud for ~ 1.5/3 usd per square metre.

But even if buy new - pultruded carbon is a winner - i would not bet my life under locally sourced wood spar.
 
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