|Can you imagine a modeller who doesn't like biplanes? I can't. Nor can I resist the beautiful lines and exciting look of a biplane performing aerobatics. Over the years I've built quite a number of bipes for pure fun. No effort was made to achieve good aerobatic performance. Two were capable of the full pattern, but they didn't reach the level of a 'serious' stunter. My experience was that most of these airplanes had lots of drag which reduced flight performance and resulted in a glide not unlike that of a rock. Also, the models tended to swing around the roll axis during manoeuvres, especially in the upper corners. While for competitive stunt flying the biplane seems to have too many drawbacks, I couldn't resist the challenge of giving this idea another try. To me, the design criteria were: 1) to keep the drag low, 2) to keep the weight down (which also calls for simple construction), 3) to find means to increase sharpness in corners.|
searched my modelling library for a pretty biplane. I found many exciting
designs with fancy fuselage shapes, exciting wire bracing, telescopic undercarriage,
wing struts, and so on. Alas, we’d better renounce on all these funny
things if we are looking for at least descent aerobatic performance. Low
drag is of prime importance. A more functional shape should give better
results. The looks of the airplane will be pretty enough, anyway. Since
we cannot do much about the fuselage - after all we have to have one - -
most thought should be concentrated on wing design. There are several aspects
to be considered.
(b) area distribution
(c) aspect ratio
There's no plausible reason to use a wing area very different from a comparable monoplane stunter. We should keep what we're used to and have found adequate. Usually that means a wing loading of about 13oz/sq.inch. You would think that two wings ought to be heavier than a single wing of the same area - and remember those wing struts. But in my experience “ it ain’t necessarily so”. In fact each of the two wings can be built lighter because we have the struts to restore rigidity.More on that later. Personally I'd prefer to use a little less area than a monoplane with weight and drag reduction in mind. But that’s purely my own view, and I don’t expect anybody to follow my thoughts. Most full-size biplanes feature a larger top wing. Maybe this serves quite well for their intended use. However, for our requirements a layout as symmetrical as possible is logical. So I’d suggest equal-span wings of identical area and shape.
One cannot talk about drag reduction without mentioning aspect ratio. Aspect ratio ( AR ) is the relation of chord to wing span. The smaller the chord (for a given area) the smaller the induced drag. There’s less induced drag on high AR wings. A lot of drag is produced by the wing tips ( vortexes ). And our biplane has four of them. Trying to reduce this drag calls for high AR wings. 'Normal' stunters have an aspect of about 5:1. For a biplane maybe we should consider a higher figure. Depending on airplane size I’d like to get closer to 6:1 . We must remember rigidity, too! For reasons of simplicity a constant-chord wing can be used - it really doesn't look ugly on a biplane, really. For yawing stability we use the usual wing sweep of around three degrees. Again, for symmetry I'd prefer exactly the same shape for both wings.
The wings should be located at an equal distance from the engine thrustline to give identical flying characteristics during inside and outside maneouvres. The distance between wings should be no less than one chord width; preferably a little more. The airflow around the wing (while producing lift ) will be influenced even at a considerable distance from the wing surface. If both wings are too close to each other the airflow from one wing interferes with the other, thus reducing lift and probably increasing drag. The bulk of pressure occurs at the front one-third of the airfoil. So if we use stagger (the horizontal difference in wing position) we effectively separate the wings even more and avoid airflow interference.
|On average, modern stunt airplanes have an airfoil thickness of about 18 to 20 % (flaps included). With higher thickness ( within reasonable limits ) lift will be increased - but so is drag! We know that the NOBLER is quite a capable airplane, and it’s airfoil thickness is 13 % ( including flaps, of course! ). With a very high aspect-ratio wing the structure might get just a little thin, causing rigidity to suffer. On the other hand we need wing struts ( at least for mounting our line guide). So wing thickness shouldn’t be a problem. However - from practical experience I tend to not use less than 15 %. The section shouldn’t have the high point further aft than at 25% chord ( Nobler = 23% ). Since we have wing struts we can afford to build the wings a little less rigid than what we would require from our monoplane wings. Clever design of the struts can add tremendous stiffness to the wings which might otherwise appear quite flimsy.|
some controversy about whether to use flaps or not. A high aspect-ratio
wing consequently has narrower flaps ( less chord ), so the dampening effect
of the flaps is less, and the wing is more effective. So we might be tempted
to actually forget about flaps. But we don't have enough information about
those angles of attack we need for flying corners. I fear that with a non-flapped
wing we need such a high angle of attack (to produce the necessary lift)
that it may produce more drag than a flapped wing with deflected flap at
a lower angle of attack. Who knows? Anyway - when using flaps I'd suggest
trying to get on with a little less deflection than what we normally use
in order to reduce drag. Of course flaps should be on both wings. The flaps
on the top wing should be driven by thin pushrods. These are connected to
small control horns which are installed at the trailing edge of the flaps,
preferably close to the fuselage - and not in line with the wing struts,
as is often seen. Close to the fuselage means close to the flap horn wire.
This is where the flap is driven, and this is where it should drive the
other ( top ) flap. Otherwise flaps can be twisted badly, and this is no
good for the flying characteristics only.
I see no reason for changing any other dimensions. We can use the same fuselage moment arms, CG location, and tailplane size (which is somewhere between 20 and 25% of wing area; the smaller amount for the non-flapped configuration, because we don't have the dampening effect of the flaps). Many builders are inspired by the beautiful lines of the Pitt’s Special and it’s stubby fuselage. However for our purpose these dimensions are not exactly what we need. From my experience I’d rather recommend those well known traditional proportions we’ve come to use on our monoplane airplanes. For those anxious aesthetes: that beautiful look doesn’t suffer at all!
CG location on biplanes causes some enthusiasts to shudder but it's not really a problem. Rather than drawing some fancy plans and desperately guessing or calculating the CG position, I use the opposite procedure. It’s not very scientific and the experts will have their hairs stand on end. But it’s practical and easy and doesn’t require a doctor in mathematics. Starting from a given monoplane design drawing, I draw the CG location at it’s known point into the fuselage side view. This is quite practical, since with another craft of identical size ( all other things being equal ) the CG should be in the same place again. With a rectangular wing planform things are quite easy. We simply use the same wing position. We find the position of the CG on the wing drawing in the place we prefer to have it. Now we simply draw the wing section into the fuselage drawing so that the CG position of both drawings coincide. This will be the horizontal location. Now we can arrange both wings at the desired positions above and below the thrust line. However remember that with a swept wing, the CG must be found on the mean average chord first. Having found the CG location on this MAC a line is drawn to the centre section of the wing ( in the wing top view drawing ). Again this CG “spot” has to coincide with the old CG location in the fuselage drawing. Now we simply draw the wing center sections ( upper and lower wing ) into the fuselage drawing . The CG locations of both wings have to coincide vertically with the CG location we had marked in the fuselage drawing.
wing stagger is used, we simply 'slide' both wings by the same amount -
the top wing forward, the bottom wing back. A happy side benefit of stagger
is that we get more room for the tank compartment, which is always a problem
with a short nose (which I prefer anyway ). Also, some people like the 'stubby'
nose look. With stagger this is easily achieved because in top view the
top wing hides part of the fuselage nose and makes it look shorter than
it really is!
I know this all sounds mighty complicated. It’s somewhat difficult to explain it with words, so I have included a few sketches. These should make things easily understandable.
|A very controversial topic is the angle of incidence of biplane wings. Over the years a lot of confusing information has been given by all kinds of experts. As for me, I cannot see a reason why wings of a biplane should have different incidence angles . We have just enough problems with trimming our airplanes which are already asymmetrical as they are now. I wouldn’t like to|
|have additional problems. Please note - I'm talking about control line high performance aerobatics as we fly it now, and not about some special cases and/or models which might need different treatment.|
the same wing area as with a similar sized monoplane, the biplane has a
much smaller wingspan. The distance CG to line guide is less, too. Actually
it's proportionally even less than you might think at first, since the line
guide is not mounted at the extreme wing tip. Usually it's mounted at the
wing strut, inboard of the wing tip (nobody wants to build a bipe with the
strut at the tip itself. Would you?). The distance 'CG to line guide' helps
to stabilize our airplane around the roll axis. The only thing we can do
here is to place the struts as near to the wing tip as our aesthetic eye
permits. And - this is another advantage of the high aspect ratio wing!
Also I think that the vertical location of the leadouts has not been designed carefully enough on many biplanes (including mine). What is not a problem with a monoplane can be quite tricky with a bipe. Since it's difficult to guess the vertical location of the CG before the airplane is finished , a line guide with provisions for horizontal and vertical adjustment is a big help. We can expect the CG location to be slightly - say about half-an-inch - below the engine thrust line (with an inverted mounted engine) but, of course, this depends on design. The line guide is then mounted on the strut in the same vertical place as the CG. With the line guide adjustable this shouldn’t be a problem. Horizontally we use the conventional amount of three degrees backward rake as a starting point. One additional idea about struts: if we spend a few thoughts about their shape and installation and carefully plan ahead, they can serve as a welcome structure to help precise aligning of the wings, and to keep control about wing incidence ( see sketch ). Those simple “N” struts (as on the Tiger Moth or similar old timers) look nice, but they are not easy to install and are heavy if built from wire. Plywood struts are simpler and lighter. Remember that fancy plywood strut shapes expose large areas to the air ( vortices ) which may result in unpleasant, uncontrollable motion when flying in wind. On the other hand carefully designed sheet struts might actually help line tension. So take your choice. No choice should be taken when it comes to rigging wires. Even if you make only modest aerobatic demands - forget the wires! We already have lots of drag, and our airplane will look very pretty even without them.
|I use struts not only to somehow mount the top wing. If shaped appropriately they make wing alignment quick and easy and can save complicated and time consuming measuring. The ribs which take the struts are cut from thicker wood than the other ribs. Then I use half ribs which go to the top of the top wing, respectively to the bottom of the bottom wing. They are installed during building the wing and have the thickness of the struts. These are first glued to the bottom wing, riding precisely on the edge of the half rib. When the top wing is mounted it is just pushed down "to the stop". The same method is of course used at the fuselage struts. Provided the bottom wing is built straight and rigid enough ( I said I build it a little stronger ) and the struts are shaped precisely, there's no way the top wing can have a mind of it's own. Of course it's possible to glue another rib right beside the half rib, thereby easily forming a slot for the strut - and we have some area to put on the Monoc ......errrrr, Silkspan !|
flyers have experienced more than acceptable “wobbling” in
corners. It’s especially true for those airplanes which were intended
to resemble a lovely “Pitts” or any similar beauty. But also
some more functional designs can suffer more or less from the infamous
“bipe wobble”. That’s no surprise. The more compact
layout of the biplane as mentioned above ( smaller span, line guide position
) is very often combined with a shorter fuselage shape. This results in
less tailplane moment arm, less fin moment arm, and less rear fuselage
side area. All this will reduce stability around the pitch axis and yaw
axis. This is why I prefer to keep conventional proportions. We also have
to consider an additional aspect. Since we decide our engine size depending
on wing area and weight, we now have a lot of power for our reduced wing
SPAN. And with the same propeller ( as on the monoplane ) the forces created
by the propeller are much more effective now. I don’t know enough
about P-factor effecting our airplanes ( Al and Ted should be the experts
who can answer these questions ). However from early experience I’m
absolutely convinced that Gyroscopic Precession can induce strong yaw
moments and plays a big role ( roll ?) in affecting the performance of
our small ( span ) toys. Again a good reason for high AR wings and not
too short a fuselage.
From my careful formulations you will have noticed that this is not a scientifically approved 100% save biplane guide. Rather it's a summary of my experiences mixed up with some strongly believed aerodynamic laws which I cannot simply change according to my desire. Good luck with your next biplane design.
P.S. You never stop learning. A recent discussion about bipe matters in the Stuka Stunt forum a Mr. Serge Krauss detected a mistake in my thoughts about biplane design. I want to make it quite clear that I'm not an expert in Aerodynamics. What I write is to the best of my knowledge. So if ever I'm in error I honestly accept corrections; actually I invite you to tell me so. I always want to learn and I don't want to give wrong information. Mr. Krauss comments about the efficiency of biplanes:
"Biplanes are not at a disadvantage regarding tip vortices. In fact their ideal-case induced-drag efficiency with infinite wing separation is 200% that of the "equivalent" elliptical monoplane wing (same area and span). When wings are stacked close enough to be practical, that advantage reduces to 136%".