I am attempting to construct a small glider that will stay in the air for the longest possible time. Can a biplane style work for this? The information I'm getting doesn't say anything negative about a biplane.

The max wing span on it is 40 cm with a max wing chord of 10, so I thought having an extra wing beneath the first would have an advantage.
Also, there is a weight restriction, will the benefits of a biplane counter the negatives of the extra amount of weight?
Would the rear horizontal stabilizer also need an adaptation for this?

Any help would be wonderful, just keep in mind that this isn't my main hobby so I may get a bit confused.

Interesting question. The biplane configuration is seldom used for gliders, since it tends to have considerably higher induced drag, due to mutual wing aerodynamic interference, which steepens the glide angle. However, if minimum sink rate is the sole criterion, doubling the wing area, if weight was held constant, would reduce sink rate. However, I would be inclined to keep it simple, and use a larger area monoplane wing, unless there is a limitation placed on wingspan, in which case an externally-braced biplane wing cellule might be worth trying.

If you double the wing area, you should also approximately double the horizontal and vertical tail areas, in order to maintain the same stability.

It sounds to me like ur doing Science Olympiad. I did it as well last year and the top five contetstants were all monoplanes (single wing). I also have seen some videos of the most incredible gliders in rubber band powered competitions, and the ones that win stay up for 35 minutes. Those are all single wing as well. I don't know the physics of all of it, but it seems that biplanes are not quite as good. Hope this helps.

Unfortunately, the answer is "it depends". Sink rate will depend on both the wing loading and the ratio of Lift to Drag that you can achieve. Adding more wing area should lower your wing loading (as long as you don't add too much weight in the process) and therefore lower your minimum sink rate. However, you have to be careful about HOW you add the wing area. If the extra wing area reduces your Lift to Drag ratio too much, then you might actually increase your minimum sink rate.

Generally speaking, the only time that a biplane would make sense in terms of Lift to Drag ratio is when your span is constrained (as it is for your design). Suppose your design is already at the maximum span limit (40 cm) and you want to add more area. As long as you are not yet at maximum chord, you could add area in two ways. You could add more chord, or you could "grow" another wing. I think your question really boils down to which would give less sink rate. Once again, the answer is "it depends". Crudely speaking, if your design has more "induced drag" than "profile drag" at minimum sink airspeed, then another wing would probably make sense. I suspect that if you are careful to keep your design light, then you would be in the other category where you have more profile drag than induced drag at min sink. In that case, adding more chord would probably be the answer.

Seems that if you add a second wing, and then do all the other things (increasing wingspan, increasing chord to increase area without increasing span, etc) you still have induced drag, which gets greater with wing area, be it wider chord or longer span, and weight, which increases whenever you add anything. Take a lesson from the soaring crowd: how many biplanes do you see in the ranks of sailplanes?

Fact of aerodynamics: biplanes are less efficient flyers than monoplanes (or gliders). History has borne this out.

Two airplanes have equal weight and equal wing area. One is a biplane. The biplane has 4 wingtips that cause tip losses. The monoplane has only 2 wingtips causing tip losses. To simplify the example, lets assume that the tiploss for a tip on the biplane wing is equal to the tiploss for a tip on the monoplane wing. In fact, they will be different, but not by very much. So if we compare the two airplanes, the biplane is suffering the effect of 4 tips versus the monoplane's 2.

That's one major reason that for best efficiency, monoplanes beat biplanes when the wing areas are the same.

ok, Biplane vrs Monoplane is simple. "HAHA" There is no current data for biplane drag numbers except stuff that you can find from say 1929. No I am not joking, at least published data. Obtain NACA reports. I used to know the numbers back when I was doing SAE heavy lift +/- study between bipe/mono for a maximum 72" wingspan heavy lift plane.

In short build your excel spreadsheet to be able to move different aspects around to obtain the best Lift/Drag ratio for your competition. This is the ONLY number you care about. Naturally of course you have to factor in the added weight of a biplane over monoplane etc. With a biplane one can in general use thinner airfoils since a biplane is stronger structurally.

Da Rock is completely wrong about induced drag(wing tip losses). The MAIN ADVANTAGE of a BIPLANE over a MONOPLANE is its reduced induced drag for the same span and required lift. History only did away with the biplane with the advent of better runways and more powerful engines/sturctural materials allowing higher takeoff speeds. Also the main reason was the NEED FOR SPEED in fighters/bombers and biplanes are limited in speed due to FRICTION DRAG because of more AREA 'visible' to the air. This is the main limiting factor to high speed flight along with profile drag.

Main detractors to the biplane is in the micro scale where the main weight is the covering, is its added area aka weight. Structurally a biplane is lighter than a monoplane when designed CORRECTLY. Extremely hard to do on the micro scale. Reason is that you have 2 weight bearing spars seperated by a distance (G = gap) which effectively increases the moment of inertia of the coupled beams. This last fact really only comes into play on much larger models since volume is a cubic multiplier.

In general where DA Rock said that for same area monoplane is more efficient is partially true for 70% of the cases, but naturally is limited by aspect ratio which is a squared function of induced drag.

Get Martin Simons Model Aircraft Aerodynamics book for the equations and rule of thumbs. For stability, yes it requires a larger tail in a dynamic event. For a basically static glider, this is not the case. You don't need to turn on a dime in fact a nice steady turn is much preferable.

Enjoy trying to read Gap/chord ratios and induced decreases in lift etc

B

A biplane is a good way to put a lot of wing area into a compact space. DARock is right that if the two have the same wing area that the monoplane will be more efficient. The losses from the 4 tips DO matter.

But if your competition event is span limited but not area limited then going for a biplane will allow you to have more wing area and thus a lighter wing loading and thus a slow sink rate and thus longer duration from any given altitude. But it'll only help if you can keep the weight of your biplane down to very close to what a monoplane would be.

Super light indoor duration competition style materials and building techniques are what I suggest. For example the world indoor microfilm class models are a 50 cm wingspan and have a minimum weight of 1 gram. Modelers had to overbuild to get the weight UP to that minimum by going for rather large wing chords or even biplane designs. Now I would not expect you as a beginner to manage to build this light but I'd suggest you google for "indoor microfilm model airplanes" and get a feel for what they are managing to do.

Bmathews, the point of a biplane is the fact the wings wingtip losses interfere with eachother, thus they have a lower total induced drag for same span and lift in comparison to a monoplane. The amount of interfernce cuased by a biplane is based on the Gap to Chord Ratio. It is definetly not a straght forward problem. At low speeds a biplane is superior to a monoplane when constrained to span. If one is not constrained by span, then no, the monoplane wins out easily. The only reason my bro and I did not go with the biplane, theoretically it could pick up more than our monoplane design, was becauase of a dearth of information on the interference effects of a high lift airfoil such as the S1223 or the Eppler 423 or say a ClarkY with slats and compound flaps. We did not have access to a large enough wind tunnel to test properly so we had to modify the data from 1929 era NACA white papers. As it turned out, if the interference between the wings was only 84% or highger the biplane beats the monoplane. When competition arrived, the only reason my brother and I won is because our plane was so much lighter than the biplane designs at the competition that we picked the same weight minus a pound and won because we were able to accurately predict our payload whereas the biplane guys were not able to do so. In the end the competition showed that a biplane lifted more, in fact if they had made a wider Gap/chord Ratio wing they would have been able to pick up an extra 5-10lbs!!!! in comparison to our monoplane, but obviously they did not have an excel program like I made that allows them to modify their parameters easily. Oh yea with the biplane heavy lift design the maximum Cl needed to achieve this extra lift was only 1.7 in comparison to the 2.2 of the S1223, thus lower profile drag. We had tall winglets which allowed us to pick an extra 2-3lbs. Biplanes can also have winglets which also allow them to cut their induced drag just like a monoplane.

The whole point of the origional post was a span limited design.

In short, ONLY on SPAN limited airplanes: BIPLANE >> MONOPLANE @ low speed
Otherwise Monoplane blows biplane away

B

always the theories about airfoils etc..
The reality is that structurally the one with most area and lightest weight (lowest loading )-is usually the victor
as the weight drops , the airfoil can become less of an issue .
look at what has been proven in the extremely low wing loading but fast and maneuverable models using really modern materials - a wing that is for all purposes flat - works great .
because the loading is so low.
wingloading controls structural needs . and that always means thicker or braced wings .
I designed some Buckers in 1989 for TOC and this was really caused by a bonus being offered for being a bipe entrant .
We ended up pickin a constant 11% airfoil simply because it was as thin as I dared go without adding rigging .

I had 1790 sq in models weighing 16,9 lbs that stood up (some are still flying) under the advanced aerobatics.
In doing monoplanes of same wing area - I never got under 20 lbs .and these were done after doing the bipes so I had already seen where structural weight could be saved . The bipe configuration was better for stresses.
In the end tho - not many guys wanted bipes as they are a pain in the behind to assemble and don't look "speedy".
More power available also played a huge role in abandoning bipes - the lessened drag overcame the lessened wingloading advantage

ditto in full scale stuff.

It would be very interesting to see some windtunnel studies of a biplane with wide vortex controlling interplane struts at the tips of the wings. I'm wondering if it would not be possible to gain a lot through that sort of control.

And for Dick interplane tip panels like that would allow for easy knife edge flight...

silver_moon_beam’s design faces a span constraint. Everybody here knows that a monoplane with a span constraint will have minimum induced drag when the span is pushed right up to the limit. If induced drag is your concern, you can "work around" the span constraint by adding another wing (biplane). The most efficient way to do this is to separate the two wings vertically as much as practical (i.e. without adding too much weight or profile drag). The reduction in induced drag for a biplane does not come from interference between the tip vortices as BFoote suggests. In fact, the less interaction between the wings the better.

Here's a simple "thought experiment" that might explain... suppose you are given a fixed amount of area for your wing (or wings) and you want to design a plane with minimum induced drag. Design A has a span equal to 8 and a chord equal to 1. Design B has a span equal to 4 and a chord equal to 2. Everything else being equal, design A should have lower induce drag. Why?

Induced drag comes from putting kinetic energy into the air. If you want to minimize the kinetic energy you are putting into the air, it is better to "push on two air molecules once than the same air molecule twice". If you think of design B as two "closely coupled" wings, this design is essentially pushing on the same air molecules twice to achieve the same effect that design A achieves by pushing on different molecules once (crudely speaking). Geese fly in a V rather than in a column for the same reason (the efficiency they gain is slightly higher because the outboard geese actually take back some of the kinetic energy that the inboard geese put into the air).

Separating biplane wings vertically (increasing the gap) minimizes the extent to which the wings are influencing the same air molecules. Design C with two wings of span 4, chord 1 and infinite gap is more efficient than a biplane with less gap, but it is still not as efficient as design A (geese don’t fly in a “stack” either).

How about adding vertical struts between the tips? Suppose you are given not only a span constraint, but also a height constraint for your design (i.e. your design has to fit inside a box of specified size). It can be shown that the design with minimum induced drag is a biplane with maximum allowed span and height (gap), with vertical struts between the wingtips (incidence of the strut panels is important). Why is this "box" design not more popular? Because the induced drag benefit of the vertical struts is usually offset by the profile drag and weight penalty.

Ah, but profile drag and weight take a back seat when unique single goals are the issue. For a model where nothing matters but minimal sink it's possible that joining the wings in this manner would prove benificial.

It's one of those areas where it's really hard to say since so many things in aerodynamics interact. Without some windtunnel or practical and carefully controlled model testing it's hard to say for sure what the outcome would be but fairly easy to ask the questions.

well-- in the "been there " file --- the boxed wing is terrific and allows for a couple of hings
1 --in high alpha flight the tip spill is greatly reduced. The interplane panels do this . Guy (son) and I did some foamy bipes using this idea - which we picked up from a foreign ,Internet associate. (see pic)terrific indoor model

on the Buckers, we took another approach -
holding the top wing tightly to the fuselage (actually at scale interval) made a huge and effective channel in that section of the fuselage side -- much like increasing lateral area there . these planes fly at extremly low AOA upright/inverted anyway due to low loading.
When I did the first one - there were concerns about the small (relatively) fin n rudder . "can't you fudge it bigger?"
But I figgered that the effective greater fuselage lift up front, would reduce the need for rudder authority.
It worked -extremely well
the Buckers would (will) do hammerheads that are within a wingspan on the 180 pivot from vertical to vertical. and knife edge is almost with a horizontal fuselage attitude
The interplanes and the cabanes are simply minimal streamline sections.
Most model bipes get really heavy and the advantages are lost.
they are terrific lower speed designs but again, big engines and need for speed changed the criteria for "best setup".

A nit pick point Shoe. You are correct that increasing the Gap increases the efficiency of the wings because there is less interference for Lift. But, you are wrong as you spread the Wings on a biplane apart the interference decreases for Induced drag and thus you then have your "4" wingtip losses that are closer to 100% as the gap increases for whichever aspect ratio that you have. All of the old NACA reports show this, they weren't dumb, thus just didn't have our gagetry we have today. Think of whirling vortices on both ends of the wing. The top one interferes with the bottom one because they spin the same direction, thus the bottom wing vortex comes up and smacks the top wing vortex. Thus, for this reason a biplane with a staggered wing where the top or bottom wing is about 10-20% chord in front and about 10-20% longer has a lower induced drag and an increase in lift efficiency.

Yes, Box wing works great and decreases induced drag. A perfect winglet on a biplane is a bit harder to make as the twist angle from the top wing is opposite that of the bottom wing as they meet. But then who needs perfect?

My bro made an acrobatic bipe, weighed 17lbs with a heavy chainsaw engine and fuselage, where the wings were made of 1/4 balsa sticks for the main spars on a 78" span upper and a 72" lower. Both wings together + struts with canvas covering minus paint weight in at 1 3/4lbs if I remeber right. The paint to make the biplane look cool weighed more than the wings did together!!! Most modelers as Dick Hanson points out make their airplanes like bricks and lose the advantages of the biplane.

I would have to say that in a minimum sink competition that the monoplane is superior since they are working at VERY low speeds, Very low RE thus friction/stiction of air, since its laminar, on the surface is probably the driving factor in the competition.

Cheers
Brian

Oh yea, my bros Bipe was a pain in the arse to put together at the flying field, thus it wasn't flown much and a 60" funfly was made instead which required 0 assembly time.

B