Multi wing design rules?
 

Can anyone tell me the rules as far as the minimum spacing apart of the wings with any multi wing design and the reasons for this?
Also the amount of stagger needed ( if any) and for what reason this stagger in the wings exists.
Also any other rules as to the design of biplanes or multi winged craft, eg How to determine the balance point? etc,etc.[sm=confused.gif]

RE: Multi wing design rules?
 

You can go as low as .5 to .7 of the chord but at these numbers that the interference between the panels builds up a lot more drag and less lift as you get to the lower end of that range. Best is 1 to 1.3 chords spacing. As you can see from the numbers there's no set exact number. And generally thinner sections can get way with less than you can with thick sections.

RE: Multi wing design rules?
 

Induced drag increases rapidly as biplane wing gap decreases. A convenient yardstick is the equivalent monoplane aspect ratio, since as wing gap decreases, induced drag increases, very much the way it does as aspect ratio is decreased. Aerodynamically, gap to span ratio is the governing factor; gap to chord ratio is secondary.

At a gap of 0.2 times span, about as large as normally encountered, the Monoplane Equivalent Aspect Ratio correction factor is 1.49, making the bipe's induced drag equal to that of a monoplane with an aspect ratio 1/1.49 times lower than geometric. A bipe with geometric aspect ratio of 6.0 would have induced drag equal to that of a monoplane with aspect ratio of 6.0/1.49, or about 4.03.

At a gap of 0.15 times span, things get a bit worse. The M.E.A.R correction factor is 1.57, giving our bipe an equivalent aspect ratio of 3.82.

Cut the gap down to 0.10 times span, and the M.E.A.R. factor becomes 1.67; our bipe has a monoplane equivalent aspect ratio of 6/1.67, or about 3.59.

Since induced drag is inversely proportional to aspect ratio, a bipe with gap of 0.1 times span will have induced drag over 12% higher than one with gap of 0.2 times span. Maximum lift also decreases as gap is reduced. At a gap of 0.1 times span, maximum lift is just over 90% of that with gap 0.2 times span. Minimum drag also slightly increases as gap decreases. At a gap of 0.1 times span, minimum drag is about 2.2% higher than with gap of 0.2 times span.

Bipes can work reasonably well with smaller than usual gaps. There is no ideal gap, since structural weight and parasite drag increase along with gap, and little overall performance increase should normally be expected with a gap much over one chord, for a typical aspect ratio. Lowering the upper wing, which decreases the gap, can also improve aerobatic line holding, and promote a gentler stall.

Source: "Biplane Secrets" part 2, by Carl Risteen, from "How To's" Vol. 2, by Air Age Publications.

RE: Multi wing design rules?
 

Rotary, that's an interesting take on the topic. Up to now I had only read about gapping the wings based on chord.

Hobden, you asked about stagger and CG.

Stagger is not a requirement for a biplane and has minimal effect, if any, on the flying of the airplane. If you look through the history of biplanes you'll find that they stagger, both positive and negative, was added pretty much only for pilot visibility reasons or to achieve a good CG with minimal issues. Non staggered biplanes, like some of the racers of the 30's and the Dehavilland Gypsy Moth, flew just fine with no stagger or upper wing sweep. Here again the upper wing sweep is related to allowing the pilot easy entry and exit while shifting the wing area back at the tips to best achieve the CG rather than for any aerobatic reason. So the stagger, within reason, is up to you and your esthetics. Within reason means that the wings should overlap by at least 1/2 a chord or you may, or may not, find that it starts to have some odd effects.

The slickest CG determination I've seen is a trick that Tall Paul offered a while back in one of the RCU forums. This was that you "look" down on the top view of the plan and use the wing area that you can see as the equivalent "single wing" and use that view to determine the MAC and pick your CG at 28 to 30%'ish of that fake MAC. So a typical staggered setup would use the leading edge and tips of the top wing and the visible portion of the tips and the trailing edge of the lower wing as this CG determination wing. So a non staggered wing would just use the single wing to find the MAC.

And a last hint for bipes. Due to the interference effect of the wing gap biplane wing areas are often "derated" compared to monoplanes. If your gap is one or more chords you can take your effective wing area as being probably 90% of the true total area. For a gap of about 0.8C the wing should be derated to probably 80%. And for a gap of 0.7C the derating would be around 65 to 70%. I've got nothing scientific to base this derating on. It's just that it was mentioned in a few design articles by prominent model biplane designers over the 40 years of my magazine reading. But it's most likely related to that material in Phil's post where the maximum lift coefficient is lowered by the gap so the wing tends to stall at a lower angle of attack as the gap is made smaller. So to achieve the same stall speed with a bipe requires more area to lower the wing loading.

Or use my trick and just make the model come out lighter.

RE: Multi wing design rules?
 

Many thanks for all your hints and tips. If I ever get around to designing one of my own I will try to make it look a bit different, but at least it should stand a chance of flying reasonably well if I stick to the rules that you've laid out. Many thanks for all the advice. I will post some pictures of the machine if it ever leaves the drawing board.
Regards,
JH.

RE: Multi wing design rules?
 

This is a great thread. I have often wondered why the engine thrust seems to be so different in aerobatic bipe designs. From what I have seen/witnessed, I see that upthrust (rather than the usual downthrust) is actually required to attain a nice, nuetral flying aircraft. Could this have anything to do with higher velocity airflow from the prop underneath the centersection of the top wing?

RE: Multi wing design rules?
 

Regarding upthrust - I try to set up all my original design bipes at very close to 0-0-0. (Zero thrust offsets, zero wing incidence, zero horizontal stab incidence) About a half degree of negative incidence in the upper wing may help aerobatic line holding, particularly if the wings are positively staggered. BMatthews' comments on stagger are entirely correct - stagger is structurally inconvenient and tends to increase weight, and does very little to reduce induced drag, within the normal range of stagger. The actual numbers: wing tunnel tests revealed that three inches of stagger are equal to about one inch of additional gap in reducing induced drag.
Positive stagger (upper wing ahead of lower) was found to give a slightly softer stall in wind tunnel testing.

Upthrust might be required with a bipe with an unusually high thrust line, in combination with very fat, draggy tires, which could place the center of drag significantly below the thrust line, something I have not as yet encountered.

RE: Multi wing design rules?
 

I have no scientific proof of the following, only experience with about 11 bipes I've built and flown. I have found that the relative incidence of the upper to lower wings have a big effect on how well a plane flies. If you want maximum ease of flying at all attitudes (aerobatics) all of mine have required the upper wing to have about 1.5 degrees less angle of attack than the lower wing has. I also find that all required right thrust and downthrust to fly nicely at all power settings. Realize that these have all been typical bipes like Aeromasters, Phaeton90's, Phaeton40's, Moth's and several sizes of Chuck Cunninghams Lazy Ace; all slightly overpowered. If I were designing my own bipe, I'd make sure I could easily modify relative incidences until I had the design well tested. Just a half degree change can make a big difference in how well it flys.

RE: Multi wing design rules?
 

Rodney, Rotary,

Many thanks for your thoughts. I have thought of the drag/pitch issue as well. My instincts tell me that with the top with fairly high above the thrust line, it would likely need downthrust to counter the drag element above the thrustline. This is also dependant upon how far below the thrustline the bottom is located. There is also the consideration of the neutral pitch axis of the fuse. If a monoplane design is able to fly fairly neutral in K.E. (with nearly zero incidence in the wing) then perhaps that fuse design would be good place to start....??

I would like to create something that doesn't pitch in k.e. flight....but that will be difficult.

Thanks again,
Mark