High aspect ratio wings

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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.
 

BoKu

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...Why then would we continue to use low aspect ratio designs? Are they less efficient at speed?
Every airplane is a compromise between various competing demands, limitations, and requirements. For a given wing area, increasing the aspect ratio increases the wing span:

* Increased span increases the bending moment applied to the wing, which requires a stronger and stiffer wing spar, which increases the mass and cost.

* Increased span usually requires more structural mass to resist flutter, also increasing the cost.

* Increased span often results in operational liabilities like larger hangar and tiedown spaces, operational issues while taxiing, and issues with runway and taxiway width.
 

Vigilant1

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I can't add much to what Bob just provided. Minor points might be:
1) Available roll rate generally decreases as span increases.
2) The advantages provided by high aspect ratio wings decrease with airspeed (since induced drag decreases with speed, both on an absolute and especially in relation to form drag). So, there are just some use cases where high aspect ratio is less important and the factors Bob pointed out win.
 

fly2kads

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In addition to the above, packaging all of the "stuff" that goes into a wing can get more challenging as you increase aspect ratio. Control rods/cables, fuel tanks and plumbing, electrical wiring, landing gear, etc. all take up a certain amount of volume. As you increase aspect ratio, the cross-sectional area of the wing decreases, limiting how you can arrange everything.

Higher aspect ratio definitely make sense for some missions. Like most things in life, though, there is no free lunch, and there are tradeoffs to be made.
 

lr27

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That smaller internal volume can be an advantage, if you want to use a solid foam,wing core.

If power is limited, a larger span will help the airplane climb faster.
 

cluttonfred

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One interesting apples-to-apples comparison of low and high aspect ratio wings is the original Miles M.57 Aerovan (cantilever wing, 50' span, 390 sq ft area, AR 6.4) and the experimental Hurel-Dubois Miles HDM.105 (strut-braced wing, 75' 4" span, 277 sq ft area wing, 388 sq ft including lifting struts, wing AR 20.5, total AR 14.6). By going with a strut-braced solution the weight penalty of the long wing was only a couple of hundred pounds (on aircraft with a gross weight around 6,000) and the idea was that fuel savings over the life of the aircraft would be substantial in a transport role. The design led in a roundabout way to the Short SC.7 Skyvan (aka "the box the Twin Otter came in) with an AR of 11 and the general concept has been revived in some Boeing studies for economical airliners.

For our purposes, the takeaway is that moderately high aspect ratio (say 8 or more) can be useful in low-speed, low power situations especially if strut bracing is used. The goal is to increase climb rate and often economical cruising speed at the cost of reduced fast cruising speed and top speed for a given structural approach and engine power. So whether or not high aspect ratio makes sense depends on your mission and other constraints such as how big are your hangar doors.
 

Victor Bravo

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On low speed airplanes longer high aspect ratio wings are more aerodynamically efficient. As has been explained already, aerodynamic efficiency is only one part of an airplane's design. Looking at airplanes that really needed to be efficient to meet the primary mission, then you will find long, high aspect ratio wings. On airplanes that needed to do several things well, you will find modest aspect ratio. On airplanes where the mission was that it had to be cheap, or easier to build, or take high G loads, you will usually find shorter lower aspect ratio wings.
 

TFF

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You have to define the mission and structure. 15m sailplane flys at 1200lb with ballast. One person flying machine. A Beech Barron with about the same wingspan has to fly 8000 lbs. How big would a 8000 lbs sailplane style plane be? Simple in that one is more efficient, but could you fly a 300 ft span plane into any regular small airport? Could you build it strong enough at that weight? Mission is first stop for design. Now because details help, if you came up with a similar plane to the Barron with 5 ft more span, you can get some more efficiency without toppling the infrastructure. A local motoglider stopped in at my airport. They tried to taxi down the parking row at the airport. I could see that they were clear, but they had to shut down and push the plane out because the could not trust what they saw. It was close. It’s one of the reasons the Airbus 380 is a bust. Airports are not designed for handling people that it can haul. Boarding 150 people is miserable. 350 people, oh my.
 
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My question was particularly targeting low speed ultralight/light aircraft designs being made with 40+ year old wing designs. They have short stubby wings and in my mind step one is to drastically increase the span and shorten the chord. With the goal to achieve close to the same wing loading at similar weight for much less drag. Not gunna go crazy with 100' spans. Just push the design in the right direction.

Sounds like the biggest hurdle is structural which is just an engineering obstacle to overcome.
 

Riggerrob

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At the smaller/lighter end of the scale you also have to consider Reynolds numbers. Short chord wing ribs are very difficult to build accurate enough to maintain laminar airflow.
1 metre seems to be around the minimum wing chord for amateurs without exotic and expensive female molds.
Yes, we know that text books say that chords can be much smaller/shorter, but then you run into problems with dust and dead insects tripping smooth airflow.
 

cluttonfred

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Vigilant1

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My question was particularly targeting low speed ultralight/light aircraft designs being made with 40+ year old wing designs. They have short stubby wings and in my mind step one is to drastically increase the span and shorten the chord. ....

Sounds like the biggest hurdle is structural which is just an engineering obstacle to overcome.
We haven't learned anything new with regard to span/induced drag in the last 40 years that would invalidate the original designs. Maybe there are some new materials (e.g. wide availability of CF pultrusions) or technologies (e.g.3d printing) that change things a bit, but it's certainly worth taking a hard look at the original rationale behind a successful (in the air and/or in the market) design before making changes.

You'll want to consider all the operational issues, not just flying. For example, handling/folding/stowing a long, thin wing by yourself can be more challenging than if the wing were shorter.
 

wsimpso1

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My question was particularly targeting low speed ultralight/light aircraft designs being made with 40+ year old wing designs. They have short stubby wings and in my mind step one is to drastically increase the span and shorten the chord. With the goal to achieve close to the same wing loading at similar weight for much less drag. Not gunna go crazy with 100' spans. Just push the design in the right direction.

Sounds like the biggest hurdle is structural which is just an engineering obstacle to overcome.
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
 
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At the smaller/lighter end of the scale you also have to consider Reynolds numbers. Short chord wing ribs are very difficult to build accurate enough to maintain laminar airflow.
1 metre seems to be around the minimum wing chord for amateurs without exotic and expensive female molds.
Yes, we know that text books say that chords can be much smaller/shorter, but then you run into problems with dust and dead insects tripping smooth airflow.
That's definitely something I'd need to look into. Do you have more information/background on that? Places I could read up more? If I can't reduce the chord much it'd be hard to get significant gains. Is it possible to decrease the thickness of the wing much?

Thanks for those other posts as well. Good things to consider. I think my design would end up in LSA category and likely push the top speed there. I'd want it to be as light as possible and run on as low hp as possible while also having a very low drag profile and low stall speed. Two seater is preferred. I know I want my cake and I I want to eat it too, but the design is the fun part so might as well start with trying to achieve everything and I can make sacrifices as I go.
 

lr27

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TFF: The 8,000 lb sailplane wouldn't be anywhere near 300 feet. Try 127. Of course, if it was allowed the greater stall speed of the Barron, the span for the same aspect ratio would be much less.

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If ultralights aren't practical at higher spans and aspect ratios, then how does one explain the success of the Lazair? If the specs are to be believed, it was 210 lbs empty and had a 36 foot span. I should think that, with judicious use of carbon fiber, a somewhat higher empty weight, and a thicker root airfoil, spans above 40 feet ought to be acheivable.

Some kind of plug in tip section might be necessary to get it in a normal hangar, though.
 

TFF

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It’s not like I really put pen to paper for this thread. It’s an anecdotal representation of the items that might happen. Outcome the same.
 

lr27

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Sure. But I think you can find a bunch of airports that can handle a 127 foot wingspan. Very few for 300 feet. Especially since you'd need dollies to keep the wing tips from dragging. ;-p
 
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