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  #1  
Old 08-31-2002, 01:33 PM
CC CC is offline
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why not modern biplanes?

This morning, while watching an old biplane putter along above the shores of Lake Michigan, I wondered why biplanes are no longer the preferred design. I presume two wings provide more lift than one, so there must be some other aerodynamic limiting factors that make them undesirable. But why not commuter prop biplanes and, in fact, why not jet biplanes?
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  #2  
Old 08-31-2002, 01:34 PM
Crusoe Crusoe is offline
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What advantages does a biplane design offer? I imagine that the time, material and assembly cost and complexity in building an extra wing section far outweighs any benefits.
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Old 08-31-2002, 01:46 PM
David Simmons David Simmons is offline
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Re: why not modern biplanes?

Quote:
Originally posted by CC
This morning, while watching an old biplane putter along above the shores of Lake Michigan, I wondered why biplanes are no longer the preferred design. I presume two wings provide more lift than one, so there must be some other aerodynamic limiting factors that make them undesirable. But why not commuter prop biplanes and, in fact, why not jet biplanes?
Just to reinforce Crusoe. If one wing provides all the lift you need, why use two? I learned to fly in an open cockpit biplane, the PT-17 Stearman. Helmet, goggles, the whole deal and it was a lot of fun. But aircraft design and engineering has long since passed up the two wing design as not needed.

Just one point. The structural "overhead" for two wings is high. The wings are cantilevered on a main spar, and maybe auxiliaries, and the fuselage must be reinforced at the point of attachment. These reinforcements add weight. With two wings you have four points of attachment with no more lift than is provided by a single wing with just two points of attachment. If the second wing isn't attached to the fuselage but is supported by struts from the other wing and the fuselage, then you have their added weight and drag and, again, no more lift.
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Old 08-31-2002, 02:04 PM
CC CC is offline
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since you both asked, I'll say that I see the greater lift that is provided by two wings an advantage over one. It seems that it would be easier to get a plane UP with more lift. Perhaps the amount of forward thrust that is translated into lift would also have to be increased, but then why did the Wright Bros build their plane as a biplane? I think I'm not satisfied with your posts and my own understanding of aerodynamics.
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Old 08-31-2002, 02:10 PM
kanicbird kanicbird is offline
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My WAG is that the biplane was a truss design. The 2 wings added structural stability to the plane. In the early days a single wing was too weak to lift a aircraft over a certain size weight. With better materials the strength advantage gave way to increased drag, innefficent lift and higher speeds.

Justa wag
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Old 08-31-2002, 02:12 PM
Satyagrahi Satyagrahi is offline
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One further note:

The reason, I think, that early designers didn't go straight to the monoplane is that two wings provide more lift at slow speeds.

The early engines had a low power to weight ratio and couldn't push a vehicle fast enough for a single wing to provide enough lift for the heavy engine along with the rest of the airplane. Therefore, two wings were necessary. I believe that most of the WWI biplanes had a top speed in level flight of about 50 or 60 mph.

Once stronger, lighter engines were developed that could push a vehicle to higher speeds, a single wing could provide enough lift and the second wing was dispensed with, for the reasons mentioned above.

Why go slow when you can go fast?
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Old 08-31-2002, 02:14 PM
Boyo Jim Boyo Jim is offline
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I believe the answer is found in "drag". Biplanes are inherently slower than monoplanes because of it. They do however, tend to have substantially greater lift, making them significantly more maneuverable than monoplanes.

The question of biplane vs monoplanes was resolved by combat in the 30's and in WWII. Faster monoplanes to choose when to initiate or break away from combat, and speed became more important than agility.

This decision largely set the tone for fighter design right through the 1970's. The F4 Phantom and "Century Series" AF fighters became vulnerable to smaller lighter designs like the Mig-21, and as a result the US developed the F-14, 15, and 16.

Better materials could make monoplane wings strong enough for tighter turns, but nothing could eliminate the drag imposed by a second wing.
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Old 08-31-2002, 02:44 PM
ski ski is offline
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There are modern biplanes, but they are used (as far as I know) only in aerobatic competitions. Pitts makes several. Biplanes are used because they are highly maneuverable, as mentioned before.

But then again, monoplane aerobatic aircraft such as the Extras are highly maneuverable too.

I know that's not the real point of the OP, though.
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Old 08-31-2002, 02:54 PM
Cap'n Crude Cap'n Crude is offline
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It may seem counter-intuitive, but most early airplanes needed two wings because of weak engines and construction materials.

Providing lift produces stress on a wing, and the wood/canvas designs didn't support a plane very well. A biplane design allowed each wing to produce less lift and still have a greater aggregate effect.

Engines weren't all that powerful either -- I believe the best aircraft engines of the early days of flight topped out around 60 HP -- so more lift was needed to get a plane into the air in the first place. The extra lift of a biplane design let you get more mass off the ground.

Also, not all WWI aircraft were biplanes and triplanes. The Germans had the Fokker Eindecker, a monoplane; the Allies had several monoplanes, the only one I can think of being the Morane Bullet, a French racing plane. All the monoplanes served early in the war, though the Eindecker survived into the later years. Improved engine design eventually allowed it to carry more machine guns, though it was slow and unmaneuverable.
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  #10  
Old 08-31-2002, 03:24 PM
Tranquilis Tranquilis is offline
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The very first pilot to successfully & sustainably fire through the propellor arc in WWI, Roland Garlos (IIRC), flew a monoplane, and was tremendously feared by the Germans until they learned how he was doing it, and went one better.
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  #11  
Old 08-31-2002, 03:47 PM
Rhapsody Rhapsody is offline
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they do still make biplanes for crop dusting. The do indeed provide more lift but at high speed lift isnt the problem it is drag. One wing = less drag. For low speed manuevering though, a biplane is ideal and they make great crop dusters becasue of this.
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  #12  
Old 08-31-2002, 04:33 PM
kanicbird kanicbird is offline
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Quote:
All the monoplanes served early in the war
The german main palin by the end of the way was the Folkner VII I beleive which was a biplane but they just came out with the Folkner VIII in limited number which was a monoplane - it was faster but fragile.
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Old 08-31-2002, 04:41 PM
happyheathen happyheathen is offline
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mostly, it is a matter of strength-of-materials.

the longer the wing, the stronger the spar(s) must be.

early airfoils were very inefficient, and really long wings were required - the materials available would not support a 60-80' run of wing, so it was cut into pieces (usually 2) and attached one-over-the-other.

Later airfoils were more efficient (lift/sq ft), so wings could be made shorter (which, when combined with metal instead of wood and fabric) made the monoplane the preferred design.

special use biplanes exist - aerobats (the shorter the wing, the faster it turns - so, if you cut the wing in two...)

and, the queen mother of biplanes, the AN-2:

http://www.boschaero.com/an-2.htm
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  #14  
Old 08-31-2002, 04:56 PM
Johnny L.A. Johnny L.A. is online now
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*sigh* Too late to add anything meaningful.
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  #15  
Old 08-31-2002, 04:58 PM
Broomstick Broomstick is offline
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Mostly, it's the drag factor.

Certainly, the early biplanes had so many flying wires and cross-braces that they were full of drag. But the wing-over-wing construction did make for a stronger structure with the materials and design limits of the time. Early flight being what it was, structural strength was more important than speed.

When they started to build monoplanes, they, too, had struts and crossbraces, which added to the drag even as they added to the strength.

Now, most of the new monoplane designs have no struts - we've learned to design wings that don't need external bracing, making them much more efficient.

There are some multi-wing designs being built today, the canards and tandem-wings, but they don't stack the wings - the airfoils are one in front of the other, and, again, are internally braced. And just because you see a biplane doesn't mean it's old - they're still being built today, either for aerobatics or because somebody just likes them. And, as pointed out, they have uses for things like crop-dusting, where precision and manuverability is more important than speed.

Theorectically, I'm sure a jet-propelled biplane could be built, but why? Jets work best at high speed, and better with relatively small, thin wings for their size. A guy in Ohio built a jet-propelled ultralight just to prove it could be done, but even he admits that a jet with a top speed of 53 mph that carries only one is silly. You might build something like that to prove a point, but there's no practical application
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Old 08-31-2002, 05:19 PM
Sam Stone Sam Stone is offline
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There are a number of factors here.

First, remember that a monoplane is capable of providing as much lift as any biplane. It's just that you have to make a longer, wider wing. So answers llike, "to get more lift" are inaccurate.

Second, longer wings are more efficient than stubbier wings. So one long wing that has the same lifting area as two shorter wings will do so with less induced drag. This is why sailplanes have very long, thin wings. Large jets also have very long, thin wings, and are actually very efficient designs. They even glide well.

Biplanes generally have struts between the wings, and those struts can create enormous amounts of drag. Even wing bracing with wires creates huge drag. You would be amazed at how much force is exerted on a 2' piece of piano wire cutting through the air at 100 mph. Try sticking a pencil out of your car window on the highway some time, and feel the drag force on it. This is because round wires are not streamlined, and create large amounts of turbulent, energy-robbing airflow behind them.

So why build biplanes at all? Several reasons:

1) Structural strength. A typical set of biplane wings is connected together with compression struts and tension wires. The fuselage in the middle acts as another compression strut. This structure is very strong. In fact, the Wright flyer was nothing more than a set of biplane wings that a man sat on. The structure transmits its own flying loads through itself.

In contrast, a monoplane wing is typically cantilevered, meaning that all the bending loads are absorbed by the spar running inside the wing, and transmitted back to the fuselage. That means you need a heavy strong spar inside the wing. If all you've got to work with is wood, you have to make a complex, expensive box structure in the wing to handle the loads. Modern materials have made this problem much less severe. But you'll still find that slower monoplanes like the Cessna 172 still use struts to support the wing, which again increases drag.

2) Manoeverability. A long, thin wing has a high moment of inertia. Making that thing move up and down quickly is not easy. This is the reason why some crop dusters and many competitive aerobatic planes like the Pitts special are biplanes. Now, you can make a monoplane really manoeverable as well, but to do so requires large ailerons, and that means lots of force to achieve the same thing. Again, modern materials have come to the rescue here.

3) Ground Handling. This may seem trivial, but you can park two biplanes in the space of one equivalent monoplane (well, not quite two, but certainly more of them). If we hadn't been able to build fighter planes with folding wings, you might have seen more biplanes on carriers in WWII.

4) Nostalgia. Many of the biplanes being built today are replicas of older designs.

Airplane design is an exercise in compromise. Every design decision involves numerous tradeoffs. The Biplane is fading away because the advances in technology have been such that the advantages almost never outweigh the drawbacks any more.

There is one class of airplane that's worth looking at, though. That's the canard, some of which are essentially biplanes with the forward wing being only slightly smaller than the rear wing. See, for example, the Quickie Q200, or the Scaled Composites "Proteus".

These airplanes are highly efficient, because they eliminate the conventional rear tail, which by necessity creates a downforce on the airplane, and replaces it with another lifting surface. Also note that they are all cantilevered designs. But still, they have two lifting wings, and are therefore biplanes.
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Old 08-31-2002, 05:21 PM
stockton stockton is offline
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<hijack>:
Can I just add how glad I am that we have David Simmons here?
You, sir, are a wealth of knowledge and a unique fountain of information. Not too many WWII vets on the 'dope.
Please stick around and keep us in line. Seriously.
<end hijack>
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  #18  
Old 08-31-2002, 07:24 PM
David Simmons David Simmons is offline
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Quote:
Originally posted by Sam Stone
There are a number of factors here.

First, remember that a monoplane is capable of providing as much lift as any biplane. It's just that you have to make a longer, wider wing. So answers llike, "to get more lift" are inaccurate.

A monoplane, such as an F-15, has enough lift to pull enough G's to render the pilot unconscious, even with a G-suit. And with hydraulic power to the control surfaces it can roll into a turn just as fast as any biplane ever built.

Yes, I can remember a lot of things about WWII but some of my memories are a little faulty. A few of them might even be a lot faulty. So you always have to check, but thanks anyway.
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Old 08-31-2002, 10:15 PM
KarlGauss KarlGauss is online now
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Quote:
Originally posted by Tranquilis
The very first pilot to successfully & sustainably fire through the propellor arc in WWI, Roland Garlos (IIRC), flew a monoplane, ...
Roland Garros, the first ace.
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