# Thread: Low drag ultralight biplane

1. ## Low drag ultralight biplane

An interesting paper states that biplanes have less induced drag than monoplanes:
"The induced drag of a multiplane is lower than that of a monoplane of equal span and total lift because the nonplanar system can influence a larger mass of air, imparting to this air mass a lower average velocity change, and therefore less energy and drag. For a biplane, if the two wings are separated vertically by a very large distance, each wing carries half of the total lift, so the induced drag of each wing is 1/4 that of the single wing. The inviscid drag of the system is then half that of the monoplane." see paper here:http://aero.stanford.edu/Reports/VKI_nonplanar_Kroo.pdf

The paper also suggests building a biplane with cantilever wings instead of the usual strut and wire truss. This has been done before with Quickie and others.
But the paper also states that a biplane would have half the reynolds number of a monoplane of same span.

Could a cantilever biplane ultralight be almost as efficient as a monoplane with the same span and twice the wing chord (same area for both)?

2. ## Re: Low drag ultralight biplane

Holding the wing area (and so the wing loading) to be the same in both cases, the biplane would have to have wings of twice the aspect ratio to get the same span, and span is one of the two big factors in induced drag. Those wings would get pretty heavy if they're not mutually supporting each other with struts. The extra weight is likely to eat up most of the induced drag benefit in the cantilever case, so I'm not sure how much actual benefit you'd get from doing a biplane like that.

If you had a monoplane with the same wing area as a biplane, and the same aspect ratio, it'd have (according to Raymer, Aircraft Design, A Conceptual Approach, p.74) "...a span 41&#37; greater and a reduction of induced drag of about 31% when compared to the biplane. Thus, a biplane will actually provide a reduction in induced drag only if the aircraft's total span is limited for some reason to a value less than that desired for a monoplane."

The thrust of Kroo's paper is about aircraft serving commercial airports, where there is a distinct span limitation due to the size of taxiways and spacing of boarding gates. With a span limitation, multi-plane effects start to make some sense. For our kind of airplanes, there's no real reason to limit span unless you're bumping into the limits of your build-space. For the average garage and a two-foot-wide fuselage, you can get wings of up to 42 feet in span. That's enough for most airplanes we'd be likely to build. I'm pushing that limit with my sailplane, but induced drag is of particular concern to me.

3. ## Re: Low drag ultralight biplane

. . . stated by a glider pilot, of course.

4. ## Re: Low drag ultralight biplane

Darn tootin'!

Who's more paranoid about induced drag than a glider guy? Noooooobody!

Well, maybe airliner designers, these days...

(I was reading up more on sailplane design in Thomas' classic book the other day - modern practice is to also tailor the aspect ratio and area of the horizontal tail to minimize induced drag at various design points. Not just trim drag effect on the wing, but the actual induced drag of the tail itself. )

5. ## Re: Low drag ultralight biplane

orion said:. . . stated by a glider pilot, of course.

The mission and physics of a glider are different than an ultralight. A glider has unlimited climb power provided by the thermal and tow plane so the weight of the glider is not much of a factor. But an ultralight must climb with small engine power and weight IS a big deal. I built a 40 foot span motorglider back when I was younger in 1980 and it ended up heavier than expected and did not climb well. I learned a lesson about weight.
It also had poor roll response. This is another reason to limit span.

So the ideal span of an ultralight would be much less than a glider. If the span loading is low, I see no advantage in increasing the span, that would only add weight. A span loading around 20 to 25 is indeed low and that only requires about 18 foot span. But ultralights usually have much more span only to get the required wing area to be legal (about 3-4lbs/sq.ft.).
So the option of a biplane just to get the area seems to make some sense for an ultralight.

I am also considering the idea of designing so the top wing is optional. Fly it as a monoplane in the sport category or install the top wing for additional area to comply with FAR 103.
BB

6. ## Re: Low drag ultralight biplane

That optional wing idea is a good one - I ran into that about fifteen years ago when a gentleman wanted to build something simple (on the order of a PDQ-2) yet he too wanted the choice of an ultralight or monoplane, getting each with only a few changes, including adding a wing or removing it, depending on his whim of the day. His follow-up project then was going to be a Pitts S-1 that had the same ability.

I don't know how far he got - I helped him build an RC model of the Pitts idea and it worked great. But then he moved away and I lost touch.

7. ## Re: Low drag ultralight biplane

"The induced drag of a multiplane is lower than that of a monoplane... if the two wings are separated vertically by a very large distance..."
What's "a very large distance?" Large enough to eliminate interference effects, I guess... a distance which could lead to some undesirable handling qualities. You'd need to set the lower wing at a higher angle of incidence to insure it stalls first, which means one or both wings aren't operating at the most efficient L/D (or use some other method). If the wings aren't fully cantilevered, the "very large distance" means even more parasite drag from supporting structure. And so forth. I can't see it being particularly advantageous except in the case of an aircraft like the Quickie, where the design also eliminates the drag from a separate landing gear.

Weight matters for a glider just as it does for any other aircraft. It's true that it doesn't affect the L/D and therefore the glide ratio, but it does affect the sink rate, which can be an issue if you're trying to stay up in light lift.

Don't get me wrong; I love biplanes... but not for their aerodynamic efficiency.

-Dana

Aviation is like drugs: You go up, then you come down. You are out a big pile of money and have nothing to show for it but the experience.
And you can't wait to do it again.

8. ## Re: Low drag ultralight biplane

Originally Posted by BBerson
Could a cantilever biplane ultralight be almost as efficient as a monoplane with the same span and twice the wing chord (same area for both)?
bberson, it is indeed true that for a given span a biplane (assuming suitable gap & stagger) will have lower induced drag than a monoplane of the same span and area.

However, if you include wing weight in the mix, then it turns out in general ( not always, see below) the monoplane will be more efficient (lower power required) than a cantilever biplane- the lower weight compensates for the lower span efficiency. In fact if I am not mistaken there is paper by kroo that shows this result.

The weight increase is because for a given airfoil (and constant span and area) the biplane wings will be thinner than a single monoplane wing, as the monoplane has twice the chord. The load a spar can carry increases by the cube of the depth (solid spars). Hence monplane cantilever wings in general are lighter than biplane cantilever (not braced) wings.

That said, it all depends on the structural design of the wing and also the ratio of wing weight to gross weight. For example in solid foam wings (rutan style), the foam makes up a large part of the structural weight of the wing in light/ultralight airplanes. In this case the thinner wings of a biplane will be an advantage as they have a smaller volume per unit wing area. This results in less foam for the biplane wing and so may very well be lighter than an equivalent monoplane wing built using the same technology.

9. ## Re: Low drag ultralight biplane

Originally Posted by BBerson
...The mission and physics of a glider are different than an ultralight. A glider has unlimited climb power provided by the thermal and tow plane so the weight of the glider is not much of a factor.... So the ideal span of an ultralight would be much less than a glider. If the span loading is low, I see no advantage in increasing the span, that would only add weight....
Well, I would disagree about 'unlimited climb power' and weight not being much of a factor for gliders. A glider has the least climb power of anything (when off tow, of course), and has to make do with what little vertical velocity there is in a thermal - the net climb rate is whatever rate the thermal is going up, less the glider's own minimum sink rate. As such span- and wing-loading play hugely into performance of sailplanes, which is why you see such extremes of wing geometry to attain good values. Weight is very important to both values, which is another reason why carbon composites are so prevalent in sailplanes - lots of stiffness for very little weight.

I'm not suggesting that everything ought to look like a sailplane, but induced drag is induced drag, and there's a reason that the most induced-drag-sensitive aircraft out there have gone to monoplane wings with lots of span*. Induced drag can be cut by simply adding area as we've discussed here often before - it's more effective, as a matter of fact, if you're only looking at induced drag - but the added parasite drag will suck up power from what's available for climb just as effectively as induced drag.

If you want to look at it the other way, the lowest-power 'powered' aircraft (human-powered such as the Gossamer series or the MIT Daedalus), are all long-span monoplanes. They're probably the most 'extreme' ultralights ever built, if you simply must make a differentiation from unpowered aircraft. I don't make that distinction, myself, since I don't see any theoretical basis for it. The physics is exactly the same whether there's a powerplant of some sort on-board or not.

Roll rate is a function of well-designed ailerons. Span is just a factor in their design. Unless you're doing aerobatics, well-designed ailerons can get you useful roll rates almost regardless of the span of the aircraft.

Now, if you just like the look of biplanes, or want to do some aerobatics, then have at it. But if you're trying to improve climb rate for a given level of power, a biplane really isn't the best way to go.

*The current trend towards higher wing loadings in high-performance sailplanes has everything to do with increasing average speed cross-country (for racing) and nothing to do with minimum sink, which favors lighter wing loadings. There's been a bunch of attempts to make an effective variable-area sailplane, ranging from full-span Fowler-like flaps (that only track back, not down), to one design that telescoped the wing in on itself span-wise to increase wing-loading for fast intra-thermal flight.

10. ## Re: Low drag ultralight biplane

Dana,
Kroo has a graph that shows improvement increasing with gap and he recommends at least .2 gap to span ratio. I could build a streamlined pedestal to provide a large gap.
Ragflyer,
An ultralight as I propose could be a biplane with two 18 feet X 4 feet wings=144sq/ft.
Or a monoplane with one wing 18 feet X 8 feet= 144sq./ft.
The biplane wings have room for a 6.5" deep spar, the monoplane has twice the spar room but I don't think the weight saving would be significant considering the ribs would be larger. We don't see many 8 feet chord ultralights. I tried it on my X-Plane simulator and it works well, but it would be experimental in many ways. The biplane would have better visibility downward.
Topaz,
Sure, weight always matters, just not as important with gliders.
What I am saying is this: induced drag is related to span. If 18 feet span yields a favorable span loading for a 350 to 400lb gross ultralight, what is the point of increasing the span? We already have low induced drag.
I always thought monoplanes were best and probably are for most missions, but the ultralight category requires some rethinking, I think.

I am an old glider pilot as well and it is hard to shed ingrained ideas. But I also know that it's hard to build a light wing of large span. I have a Grob G109 motorglider that weighs 1320 lbs empty, 1820 lbs loaded. It takes a lot of power to climb with that much weight using the engine, yet it climbs easily in a thermal because the thermals lift is "unlimited". Thats what I meant by unlimited climb power in a thermal.
BB

11. ## Re: Low drag ultralight biplane

Originally Posted by BBerson
Topaz,
Sure, weight always matters, just not as important with gliders.
What I am saying is this: induced drag is related to span. If 18 feet span yields a favorable span loading for a 350 to 400lb gross ultralight, what is the point of increasing the span? We already have low induced drag.
I always thought monoplanes were best and probably are for most missions, but the ultralight category requires some rethinking, I think.

I am an old glider pilot as well and it is hard to shed ingrained ideas. But I also know that it's hard to build a light wing of large span. I have a Grob G109 motorglider that weighs 1320 lbs empty, 1820 lbs loaded. It takes a lot of power to climb with that much weight using the engine, yet it climbs easily in a thermal because the thermals lift is "unlimited". Thats what I meant by unlimited climb power in a thermal.
BB
I'm still not seeing how an ultralight requires 'different thinking.' Does it fly in different air than a glider or a 747? Does it obey different laws of physics? Not from where I sit. An ultralight, a glider, or a 747 are, at the most fundamental levels such as we're talking here, exactly the same.

Induced drag is determined by span and wing loading (and the shape of the lift distribution, but that's pretty much irrelevant to the discussion we're having here). Parasite drag, in all its various forms, is determined by the aircraft's shape. Climb rate is power available minus power required. Doesn't matter what kind of airplane it is. Every aircraft climbs at a rate determined by how much energy is required to lift that particular amount of mass upwards at that particular vertical velocity. How much energy is sucked out of the system by drag (and so has to be overcome by even more engine power) depends on how much induced and parasite drag is being produced at that airspeed. And that's it. Weight matters on any airplane, in terms of what it does to wing loading. Your Grob (nice airplane, BTW), may be 'heavy' in terms of gross weight, but if you calculate its wing loading you'll find that value to be quite low.

An unpowered glider's 'climb rate' is always negative, since it has no power available. There is never a point in unaccellerated flight where a glider can climb, ever. If it happens to be flying in a mass of air that's going upwards faster than its minimum sink rate it will physically move upwards away from the surface of the earth, but within that mass of air, the glider is still sinking at its minimum sink rate. Your Grob doesn't climb any 'better' in a thermal than upon the engine. In fact, no matter how fast you're moving up away from the surface of the earth in a thermal with the engine off, you're still not 'climbing' in the aerodynamic sense - you're sinking. The coordinate system for all of these calculations is relative to the air mass around the airplane, not the ground. When you power it up, the motor on the Grob has to first add enough energy to the system to overcome the aircraft's minimum sink rate in order to achieve level flight, then even more to physically shove the airplane 'up the slope' of the climb angle you're flying. If the glider weighed less, the motor wouldn't have to work as hard to push it up that slope. If the glider weighs less, all else being equal, the wing loading, and therefore the minimum sink rate, goes down. Since gliders fly or don't fly almost exclusively on their minimum sink rate and have no motor (when they're gliding), weight (and more specifically, wing loading) is more critical for gliders than almost any other airplane. Gliders have rediculously low wing loadings - the design wing loading on my sailplane is 3.80 psf.

It's exactly the same for an ultralight or a 747. For any given installed power, payload, range, performance parameters, and construction technique, there is going to be an optimum span and wing loading for a target climb rate goal. Saying an otherwise undefined airplane has a given span loading is only paying attention to part of the problem, and doesn't really answer anything. Wing loading counts, too, as does installed power. Since climb is dominated by (but not solely determined by) induced drag, any way to reduce that drag is 'good' from a climb rate standpoint. A biplane has a theoretical reduction in induced drag over an equal-span, equal wing-loading monoplane, but real-world factors (listed above by the others) involving weight, interference, and parasite drag reduce or nullify that advantage. If you're not limited to a particular span for any reason, a monoplane can have a much lower induced drag. A cantilever biplane throws away the one big advantage a biplane really has - structural weight. A monoplane of equal span and wing loading to a given biplane will weigh less, since the aspect ratio of the biplane will be twice as high. When you're thinking that 'long' wings are heavy, you really mean that high aspect ratio wings are heavy, which they are. By the time you stretch out the monoplane wing to be the same weight as that cantilever biplane, its span is so much more that the induced drag is quite a bit less than the biplane can accomplish.

Forgive the diatribe, but this is a pet peeve of mine. All airplanes are the same. None of them are 'magic' or 'different' in the physics they use to fly. If it's flying on wings, it's all the same, no matter how big or how many engines, or how fast or slow it flies. If, after all this, you still want to build a cantilever biplane, then absolutely fine. You'll know the compromises you're making. Whether or not you're doing the right thing with induced drag I'll still come out to sincerely applaud your first flight and buy you a beer for the fact you've built and flown an airplane, which is the final point, after all.

12. ## Re: Low drag ultralight biplane

Keeping on point about U/Ls... U/L design requires difficult compromises. Spacing the wings far enough apart that they actually become aerodynamically advantageous over a monoplane is structurally impractical, and can cause other problems. The wide spacing adds drag and weight with the long support struts and wires. And making a biplane's wings long enough, to have a good aspect ratio, sacrifices the advantage of the lighter spars that can be used with shorter wings.

It is easier to make the weight limit with the wings cantilevered on a biplane compared to a monoplane with similar wing area, just because they are shorter span. A good compromise that I'm using is having flying wires for positive Gs, but no landing wires for negative Gs. Those same flying wires keep the wings from moving side to side, relative to each other. At first I used more wires to address those needs separately. But the semi-cantilevered wings are stiff enough so the extra wires aren't really needed.

The shorter span of a biplane's wings, tends to improve roll rate a lot. But those shorter wings tend to yield a very low aspect ratio with lots of tip losses and a very low L/D. Be aware that it takes significantly more wing area to pass the stall speed limit because of the low aspect ratio (tip losses) and loss of efficiency from the wings' proximity to each other... about 20% more than the formulas and graphs in AC103-7... if you really care about being legal.

I would encourage someone interested in designing an U/L to consider a biplane. It is easy to make the weight and other limits with a simple structure. And the result is an extremely fun plane.

13. ## Re: Low drag ultralight biplane

Topaz,
I started this thread to explore the possibility that a biplane could make sense for an ultralight. I have not claimed that a biplane is superior, I simply noted that Mark Stull has had some success with his biplane and this has caught my attention. Bob Hovey designed several very light biplanes, his Wing Ding was 130lbs. I think.
In fact I still think a biplane is inferior and I have always believed that because of my glider background. But I am trying to get a better understanding of basic ultralight aerodynamics and the need for light structures, so I am open to the idea.
Now that the "fat" ultralight exemption is history, designers of the new breed of legal ultralights should consider all ideas.
Mark said:"It is easier to make the weight limit with the wings cantilevered on a biplane compared to a monoplane with similar wing area, just because they are shorter span".

Mark,
I was thinking the wing gap could be wider with cantilever biplane wings without much additional structure. The Kroo paper graph shows improvement with large gap.
BB

14. ## Re: Low drag ultralight biplane

BB,

Yes, the farther apart the wings are, the more efficient they'll be. Even if you make the wings fully cantilevered, there's still a very tall post in the middle (or more likely 2 or 4 posts) to support the upper wing, way up there. You can minimize this by filling part of that space with the fuselage. But still the extra spacing will mean longer posts.

Also, the farther up it is, the more leverage it can generate to break off from all the different possible forces. So longer posts mean fatter, stronger posts. So you'll find it will save weight, and maybe even drag, to triangulate things with some wires. So if you're going to have wires, you might as well let them double as flying wires so you can make your spars much lighter. Pencil out a few designs, and you'll see what I mean.

A wing spacing of around 1 chord is the common compromise. Less than that, you lose a lot of efficiency. Much greater than that, the above structural issues start to be disadvantageous.

Here's a couple other minor disadvantages of greater spacing. If the upper wing is very high, it moves your center of drag above your center of thrust. So adjusting power has a larger effect on pitch trim. Raising the upper wing also moves the CG of the whole plane up, which might necessitate wider main gear to keep it from tipping over on the ground. Wider main gear usually adds weight and drag. Consider how you're going to configure your aileron control system way up there.

Will you gain more efficiency with a higher aspect ratio, or greater wing spacing? Increasing the aspect ratio also decreases the chord, making the same wing spacing seem greater, measured in wing chords. If I was trying to make an efficient biplane, I would compromise toward a higher aspect ratio. As always though, the higher aspect ratio wings need heavier spars.

I chose to make my lower wing extra short so I could make the main gear shorter (and still allow a decent bank angle on landing). I love the resulting squat look. Looks count too :-) You'll find that you can save some weight if you make your wings in a single piece (rather than separate left and right wings). Everthing's a compromise.

15. ## Re: Low drag ultralight biplane

Originally Posted by BBerson
Topaz,
-sigh- Fine, see it as you will. Water ballast is used when you're looking for a higher maximum-L/D speed and don't particularly care about minimum sink because you're expecting very strong thermals. What do you do when the lift weakens and you need more 'climb rate'? Dump the ballast and so lower the wing loading. The ASW's wing loading is a bit over half that upper value without the ballast.

Being a glider pilot, I know you know that, so I'm not exactly sure what you're trying to say by quoting with-ballast numbers. That flight condition has nothing to do with maximizing climb rate - which was the original point of this discussion, category of aircraft notwithstanding. Your Grob's 8.3psf is not optimum for minimum sink/maximum climb rate either. It's designed to get you a higher speed at maximum L/D. Your Grob can get away with that because you've got the motor to save your bacon when the lift weakens.

I don't see any point in arguing this further. Moving on...