Biplanes vs. monoplanes

Why were early airplanes biplanes? What was it, aerodynamically, that made this configuration more successful than its alternatives?

And what breakthrough made the switch from biplanes to monoplanes possible?

Early airplanes were flimsy and underpowered. The extra set of wings gave the planes much needed extra lift. Also the structure was stronger with struts between the wings. The drag of the extra wings was not a big factor as these airplanes moved slowly.

Monoplanes became necessary and popular as engine power, design strength and materials used for building improved.

Take a look at some early monoplanes (like this), there were external wires to brace the wing both up and down. As long as you’re doing that, just build two wings with bracing between them.

Stringfellow’s steam-powered unmanned airplane from the 1850s was a surprisingly-modern monoplane. Possibly the tiny steam engine gave the equivalent of a proportionately greater push to the plane than the gasoline engines used for full-size flyers like the Wright brothers’ planes, so for the model stage they didn’t have to go through the biplane stage.
I understand that double or multiple wings give you not only greater lift but also greater control at low speeds, which is why von Richthofen’s plane was a tri-wing. Supposedly such multi-winged craft were much better at low-speed acrobatics than monoplanes were.

Not supposedly - that’s exactly why stunt planes today are still biplanes.

I hadn’t heard anything about multiplanes being inherently more controllable. In fact, von Richthofen’s triplane had a rotary engine (one where the crankshaft is mounted to the airframe and the engine and propellor rotate together). The large rotating mass made those very difficult to fly, and the lower moment of inertia of a compact design, like the triplane, would make it less resistant to changes in attitude.

The maneuverability would be an asset in combat, as long as you were a good enough pilot to get to the fight without an accident.

Monoplanes are still fairly widespread in aerobatics. The Edge 540, Extra 300 variations, the SU-29, SU-31, Staudacher, Cap 231. More I can’t even remember, and aside from the Pitts Model 12 or something like Sean Tucker’s modified Pitts, I can’t think of any biplanes in the top tier of aerobatic competition.

(of course, the real problem combat-wise with the triplane was the crappy visibility. It was also a problem on landing.)

The Wright brother’s first design was dependent for turning on actually twisting the wings so that one side had more lift and one less. This made the biwing design a necessity. Later aerilons were introduced, but as long as wings were flimsy structures of canvas stretched over a frame like a kite, the biplane design was necessary for sufficient rigidity.

From what I have read about dogfights in world war one and flying flight simulators of this era :), most fights came down to two airplanes chasing each other, spiraling downwards, trying to turn sharp enough to get the other plane in their sites. The plane with the tightest turning radius usually won as they had a definite advantage. To turn sharply in a plane you turn it on its side as much as possible and have full up elevator (which acts like a rudder in this position). In this configuration the lift of the wings help you turn (as they are "lifting"sideways). So the more wings, the more lift, the tighter the turn. Also a radial engine with a lot of torque would give you an advantage if you turned in the direction of the torque.

As I understand it, too, during WWI, there was a rather large debate over which was better for air combat: maneuverability or speed. In general, a bi-plane will have more surface area than a monoplane, and more surface area - this will have the effect of reducing the plane’s maximum flight speed, but at the other end of the flight envelope, it can maintain lift at a slower speed - making it able to turn inside a similarly power-weight ratio monoplane.

The way it’s worked out, however, is that while maneuverability is worth something, speed is the biggest factor for air combat. As an example - with only a few exceptions the idea of a Harrier vs an F-15 is a David vs Goliath match-up, even if both aircraft are armed with the same munitions. (There are ways, of course, that David can win, but the smart money will usually be on Goliath.)

So, as speed became recognized as the primary determinate for what made for a good fighting plane, the push for more powerful engines, and monowing designs dominated.

I know I’m offering only a slightly different view of what some other posters have mentioned, just saying that I think the emphasis was to push speed - which then lead to a need to get better monoplane designs in production. Rather than an automatic belief that monoplanes were better than biplanes.

Well, I’ve played Massively Multiplayer sims; what I found out there was that, in a
world where pilots have perfect information about enemy locations (and in these
games it is typically much easier to see enemies than it is in real life with haze and
the sun and clouds), the manueverable planes have a chance to see the faster
plane and react to it, but even then the faster plane controls the engagement,
including whether he will break it off or not.

In a world with imperfect information, it’s no contest-the faster plane will be able
to “bounce” a lot of unwary enemies using his superior speed. In modern air
combat, supposedly the newer “miracle” missiles will put the concept of dogfights
to bed-but they’ve said that before. Doubtful we’ll ever see two equal airforces
going at it in large enough numbers to really find out tho…

Another positive factor for triplanes is that three smaller wings can generate as much lift as two larger wings. The smaller wings made for a smaller aircraft, which in combat terms, made for a more difficult to hit target.

I’m more into WWII aircraft than WWI, but IIRC the Fokker DR.I was developed to counter the British Sopwith triplane. Not only did the DR.I have three sets of wings, it was also rather short-coupled, which made it very maneuverable (read: unstable). While it is associated with Richthofen, The Red Baron actually flew many types. He found as others had (Udet?) that the best way to win a fight was to shoot the other guy down from behind before he knew he was there. A diving attack also served the P-40 Kittyhawks well against the much more maneuverable Zeros.

Incidentally, I’ve read that the DR.I didn’t actually need struts; it was strong enough without them. But TPTB insisted it needed them.

Where to begin with the misinformation in this thread?

The short answer is that given engine power, material and engineering limitations of the day, it was much easier to make a biplane strong enough to support several times its weight (g loading, remember) by short wings, with inter-wing tension wires and compression struts.

With more powerful engines (to power the heavier structures to the higher speed needed to produce the lift to support thier weight) better materials (more strenghth, less weight) and more advanced engineering (lower safety factors, less dead weight) monoplanes became practical.

In general, though, short wings will always be less efficient than long wings. Using two or three of them doesn’t change that.

Essentially, you create lift by accelerating air downward. You can either accelerate a modest amount downward at relitivly high velocity using short wings, or you can accelerate a lot downward at lower velocity using long wings. The impulse (lift* time) is velocity times mass, but the work (drag*time) is velocity squared times mass, so the long wings win the effiency contest. cite: How may short winged sailplanes can you name (the space shuttle is a glider, but hardly a sailplane)

Now, short winged airplanes are stucturally stronger for the same reason that broomstick is stifer than a fly rod. You can get a fair amount of lift from a short wing by giving it lots of chord and a thick airfoil. OR, you can use two of them. Using two gives lots of opportunity for light, efficient structure to handle the loads.

Short wings have a low moment of inertia, and don’t have to transfer high g-loads through long thin spars, which is good for aerobatics etc.
As you increase airspeed, the drag from bracing wires becomes intolerable. One of the fastest production biplanes, the staggerwing (top wing behind lower wing) beech exchewed them, thus losing most of the structural advantages of a biplane. Early monplanes had kingposts, and extensive wire bracing, thus negating this advantage, which is why biplanes were still popular untill strut braced and finally canilevred wing monplanes were developed.

I don’t know about the Fokker, but I have read that the French Nieuport (also a rotary) was difficult to turn in one direction but snapped around quickly to the other. Pilots who picked this up used it to their advantage in dogfighting.

I do not remember reading anything conclusive about the Wrights’ choice of two wings over one. They were influenced by the designs of Octave Chanute, and he was a civil engineer who would have understood the inherent strength of the girder-like biplane design.

I don’t think that wing-warping requires a biplane wing, but the brothers had already decided on a biplane design before they understood wing-warping’s importance. The story is that Wilbur was twisting a cardboard box when he realized the solution; the sides of the box clearly mimicked the action of two parallel wings.

A biplane configuration provides great strength without a lot of weight, especially if you’re willing to live with increased drag. Even so, both small and large metal-wood monoplanes emerged by the late 1920s, only 10 years after the nearly all wood WWI models.

With the disclaimer that I am not an aerobatic pilot (yet) and my biplane flying experience is quite limited…

Early biplanes were flimsy by today’s standards. The biplane construction allowed for a more stable and ridgid wing than was possible with early monoplanes. These problems were, however, largely solved by the mid 1930’s. Biplanes continued to enjoy some favor due to inertia. Later biplanes were pretty solid machines, and a modern Pitts is pretty rock solid. Stength stopped being an issue by WWII, if not earlier.

Biplanes allow more lift for a given wingspan. This can have importance where space is limited, or if you don’t want a huge wing to lift a large mass. More powerful engines and more efficient designs largely eliminated this as a factor by (again) WWII.

Biplanes ALWAYS have more drag. This is in part because of necessary struts and bracing, but also because the two wings interfere with each other’s lift due to the airflows around them. A biplane with a given wing area will not generate as much lift as a monoplane with an equal wing area. Triplanes also have this problem, and triplanes are even less efficient in this respect than biplanes.

The faster you go, the more drag you generate. A significant portion of drag is generated by the wingtip vortices. (Airplanes can reduce some of this through the use of winglets - which is why more and more airliners are sporting those little jaunty upturned bits on their wingtips) Monoplanes have two such vortices. Biplanes have four. You do the math.

Biplanes do not inherently have a crappy forward view, although the vast majority do. This is because most of them were/are designed as tailwheel airplanes, and in tailwheels the nose rides high on the ground. There is no reason - other than custom and tradition - you can’t have a tricycle gear or pusher biplane with a great forward view. Tailwheel monoplanes also have a crappy forward view.

One advanrage biplanes have is that, due to the shortness of their wingspan relative to wing area, they have a faster roll rate than a monoplane of equal wing area. (Assuming I didn’t just blow that explanation) However, there is more to manuverability and surviving a dogfight than just roll rate.

A good example of a biplane with excellent visibility from the cockpit is the Airco DH-2 and its “relatives”, like the RAF FE-2 and a few other similar designs.

The vast majority of trully competitive modern aerobatic aircraft are monoplanes.

These two statements need some qualification. At the speed for best L/D, total drag is at a minimum - flying faster or slower will entail more drag. At high speed, induced drag (the type associated with wingtip vortices) is low.

But vortices are not all equally strong. Indeed, one way to reduce induced drag is to have multiple wingtips. Many soaring birds (e.g. the bald eagle) use this scheme, and there is interest in applying it to aircraft (cf. this link).