2fer1: why no jet biplanes? and another German jet Q

I’m also an aerospace engineer and the topic of biplane tradeoffs has been pretty well covered. The only biplanes still in production are specialty aircraft where the configuration offers an advantage in their particular niche, such as the Antonov An-2, which is optimized for slow flight, or aerobatic aircraft such as the Pitts S-1/S-2.

I’d also like to add that those German designs were very real, but one of the major factors holding them back (besides the strains on resources caused by strategic bombing) was that metallurgical technology wasn’t quite where it needed to be. The main factor that delayed the entry of jet fighters into service was setbacks in engine development. The Junkers Jumo 004 engines (which powered the Me-262), for example, only had a rated lifespan of about 25 hours.

But back to the OP, there was in fact at least one jet biplane, the little-known Coanda jet plane of 1910. It flew briefly only once, catching fire and crashing on it’s first test run. The inventor, Henri Coanda, was testing the engine in a ground test when he noticed the exhaust was sticking to the sides of the fuselage. Distracted by the flames, the aircraft started moving. When he noticed he was rolling forward, he saw he was heading straight for a hedgerow and approaching takeoff speed. He lifted off to avoid the hedges, lost control, and crashed. The plane was never rebuilt due to lack of funding and general interest in the aviation community. It would be another 25 years before anyone started investigating jet propulsion again.

There is some debate though about whether the Coanda could be considered a pure jet because the compressor was powered by a conventional piston engine, but motive power was provided by reactive thrust produced by burning fuel in a turbojet-style combustion chamber, and the thrust was much greater than could be produced by the piston engine alone.

Right, it’s called “induced drag” - the simple act of creating lift creates drag in proportion to it, simply because you’re moving air.

Right, the increased air velocity over the lower wing’s upper surface pulls the air under the upper wing along with it, increasing its velocity and reducing the upper wing’s lift. That’s why most biplanes are built with some stagger, so that the highest-velocity portion of the lower wing is some distance away from the maximum-thickness, maximum-lift section of the upper wing. But that can only go so far, since one of the main reasons for using 2 wings is the structural stiffness they can give each other, and the more the stagger, the more the structural weight needed. Also, although most biplanes have the upper wing more forward for better visibility from the cockpit, that isn’t necessary - the Beech Model 17 Staggerwing of the '30’s had the top wing further aft, and it’s one of the prettiest designs ever.

To the OP, the key is “propulsive efficiency”, which is a function of the ratio of the propulsion system’s discharge velocity to the aircraft’s airspeed - ideally, you’d like them to match, with the jet exhaust or propwash in effect just sitting there from the external air’s point of view, with all the energy produced going into moving the airplane. A propeller produces the lowest average “exhaust” velocity of all common propulsion systems, making it up in volume, and is the most efficient match for a low-airspeed aircraft. Then, as airspeed goes up the scale, come turboprops, high-bypass turbofans, medium- and low-bypass turbofans, straight-pipe jets, and rockets. Biplanes are slow because of their high induced drag so they want propellers, which in turn require them to be slow - it’s a package deal.

Drag is a function of the square of velocity. So you just added one extra to the exponent. Just like you added an extra letter to my username. :wink:

Power required is a function of the cube of speed. That’s because power = drag times velocity.

The main reason for biplanes in early aviation has been mentioned briefly and then ignored.

It was a structural consideration. SSgtBaloo mentioned truss bridges, but that’s not quite right. What they were building were box beams.

I can build a nice, long monoplane wing that’s strong enough to support the vertical loads of lift and light enough to only need a low horsepower engine to go fast enough to fly. The trouble is the other loads generated by flying aren’t going to be merely vertical. There will be a significant rearward drag component, but I might be able to live with that.

What will undo my little effort are torsional forces that will twist that sucker right off the fuselage. Torsional deviation from the proper alignment in the airstream tends to be unstable: That is, the greater the amount of twist the greater the twisting force. So in order to get something light and torsionally rigid they used box beams.

The box beam is formed between members on the fuselage and the interplane struts out near the wingtips, both forming rough rectangles. Between the corners of those rectangles I run four wooden spars. On each of the four faces of the rectangular box formed I run diagonal bracing wires, including the two surfaces that will form my wing. That’s right, most early bipes had internal wing bracing wires. The tension in all the wires puts the struts into compression and I’m left with a light, torsionally stiff box beam.

All I have to do now is put reasonably aerodynamic shaped wings on the two horizontal surfaces and I can go flying.

Well, the order of construction is, of course wrong, but that structure is what keeps lightly constructed biplane wings from twisting off. And if you’re constricted by low horsepower your airplane better be light, baby.

Wow. I mean, wow.

Thanks, Dopers. I am very well answered, I’ll tell you whut.
If anyone wants to keep discussing this, go right ahead. My curiosity is sated. :slight_smile:

Since the OP has been answered very well I started an MPSIMS thread to continue the discussion.

If anyone’s still interested, I happened across this today. The M-15 Belphegor is a Polish jet biplane from the 1970’s. It was an agricultural aircraft designed for aerial spraying of large farms. The combination of jet engine and biplane configuration gives it a high maximum payload and excellent low-speed flight characteristics. More information here.

Thanks, J.J.

I was offline for a bit. That’s an odd plane there.