Jet powered biplanes ?

Is there any plausibility of the development of jet biplanes, obviously nothing like this happened in OTL so i wondered why.

Probably a very simple and good reason that im too ignorant to get:eek: Aware this is probably ASB but anyway ...
 
Is there any plausibility of the development of jet biplanes, obviously nothing like this happened in OTL so i wondered why.

Probably a very simple and good reason that im too ignorant to get:eek: Aware this is probably ASB but anyway ...

IMO I think the second wing would create unnecessary drag that would just hurt the performance of the aircraft. I could be wrong though I'm no expert on aircraft flights characteristics.
 
Is there any plausibility of the development of jet biplanes, obviously nothing like this happened in OTL so i wondered why.

Probably a very simple and good reason that im too ignorant to get:eek: Aware this is probably ASB but anyway ...

To a small degree is did happen in OTL. Consider the Coanda-1910.
 
However from a military point of view i suppose it would be impossible to make a biplane do anything a monolane couldnt do better.
 
However from a military point of view i suppose it would be impossible to make a biplane do anything a monolane couldnt do better.

It might have better low-speed maneuverability, useful in carrier landings for example (have a quick google for the Fairey Cuttlefish). I think most users would find the loss in top speed caused by the second wing too much of a disadvantage, however.
 
It might have better low-speed maneuverability, useful in carrier landings for example (have a quick google for the Fairey Cuttlefish). I think most users would find the loss in top speed caused by the second wing too much of a disadvantage, however.

One could theoretically make the lower wing retractable, able to be pulled back into the hull.

Can't see a reason to, though.
 
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Cook

Banned
development of jet biplanes, obviously nothing like this happened in OTL...
During World War Two the Soviet Union experimented with the Polikarpov I-153 augmented with ramjet engines. It also had retractable undercarriage.

fafbi.jpg
 

Delta Force

Banned
Ah, but it did. In the 1970s, Poland created the jet propelled biplane cropduster.

That makes me wonder if it is possible to make an efficent low speed jet engine. Most jets go quickly because that is kind of the point, but is there anything invalid about the concept of a jet cropduster or other slow aircraft? Jet engines are very easy to maintain and run on kerosene (easier to acquire than high octane aviation fuel for prop planes) so they do have a logistics edge, unless they are just doomed to guzzle fuel flying at low speeds.
 

NothingNow

Banned
A biplane version of a beegee racer is hardly what you'd call aerodynamic though.

It's still Best you can get for a Radial engine of that size, and actually it's Aerodynamic and agile enough that it could take down anything of comparable vintage in a dogfight.
 
That makes me wonder if it is possible to make an efficent low speed jet engine. Most jets go quickly because that is kind of the point, but is there anything invalid about the concept of a jet cropduster or other slow aircraft? Jet engines are very easy to maintain and run on kerosene (easier to acquire than high octane aviation fuel for prop planes) so they do have a logistics edge, unless they are just doomed to guzzle fuel flying at low speeds.

Define "jet!" In a sense all aircraft are "jet-propelled;" there's nothing to push against but air aloft, and they all grab various mass flows of air and shove them back to get thrust.

A sense that would exclude turboprops would also exclude turbofans. But most anyone would call a turbofan a "jet" and not a turboprop; we go by the look of the thing.

I suppose Williams International or whoever is the star of making small, cheap turbojets and turbofans these days could make a very high bypass ratio turbofan that is competitively efficient in the low speed ranges you are talking about. A simple version of such would have some advantages, avoiding the exposed spinning props might be nice for instance.

Basically, in the early days of jets, they were quite inefficient relative to their power output--but their power output was far higher than anyone could practically manage with piston engines. Gradually the efficiency of the basic combustion process has been improved in turbine engines, and meanwhile the turbofan has been developed, which mitigates the basic inefficiency of turbojets at low speeds, which is that a pure turbojet puts out its high power in the form of a relatively low mass of air being expelled at a very high speed, whereas the optimum power efficiency of any reaction engine (including props of course!) is achieved when the exhaust speed matches the airspeed--IE the exhaust is brought to a standstill relative to the free stream. With the air coming out of the tailpipe of a pure jet engine with no bypass augmentation at something like 2-3 times the speed of sound, clearly the thrust generated is inefficient at low speed. It becomes efficient at high speed so this is suitable for a fighter jet or the like, designed to operate above the speed of sound.

Running the fast, high-powered jet through some kind of system that slows it down and transfers its power to air not involved in combustion and power generation raises the thrust at low airspeeds and is therefore more efficient there; as the airspeed rises to approach the net exhaust speed it gets more so, but clearly efficiency starts to suffer if airspeed then exceeds that.

For low airspeeds, then, if one insists on what is commonly called a "jet," the amount of bypass thrust has to rise a whole lot. How slow is your cropduster designed to go? 150 knots? We are now at a quarter Mach; the turbofans used on modern jetliners are meant to operate at about three or more times that speed, we'd therefore have to raise the bypass ratio, already remarkably high, still higher by a factor of nine or more! The engine core is still a "simple" turbojet, and those are indeed much simpler in concept and execution than piston engines--the cost is, for efficient operation the materials involved have to operate in extremely demanding conditions, so they are expensive and difficult to machine. And if something goes wrong with them it goes wrong spectacularly and fast. But for one thing it is hard to scale down a jet engine--clearances get all the more critical for instance. Williams and its competitors do it all the time, but it's tricky, and a lot of their small turbines are meant to be used in missiles and the like so they don't have to be built to last, and often efficiency is not a paramount concern.

Meanwhile we have that massive bypass mechanism. What we'd actually have is a turboprop that we choose to gear to a ducted fan instead of a conventional exposed propeller.

On the one hand, we could just as well power that ductfan with a conventional piston engine of some kind.

On the other, conventional props have the advantage that the problem of varying their pitch has been pretty well licked since the beginning of WWII, whereas making a variable pitch ductfan is more challenging, if only because less effort has been expended on it. With simple constant pitch (which to be sure is often deemed satisfactory in small slow airplanes of this type) it is hard to match optimum engine efficiency with the level of thrust desired; the only way to vary the thrust is to throttle the engine up or down. With variable pitch props, the angle is automatically adjusted to get optimal thrust from the prevailing airstream conditions at a given throttle setting, so gunning the engine (which typically is kept running near an optimal RPM, the prop pitch variation varying the torque so a given throttle setting maintains that speed) results in a near-proportional surge of thrust, vice versa throttling back results in the thrust falling off neatly. With a fixed-pitch prop there is only one airspeed where the optimum engine RPM matches the thrust requirements, going slower or faster requires operation off optimum. For takeoff and landing, the thrust available is often poor.

These problems get more serious the greater the target cruise speed is, which is why the development of variable-props was so crucial for WWII era warplanes and transports even before that. But still today, small slow airplanes use fixed props because the cheapness and simplicity of the system offsets the moderately more mediocre performance off optimum.

However, I do believe all fixed-pitch prop planes run on piston engines, the reason being first of all that even today a turbine is not cheap, but more critically, regardless of the relative costs of the engines, a turbine engine is more sensitive to operating at the wrong RPMs than piston engines are. One can run a piston engine at half-speed, efficiency suffers, but that gives some range of thrust control even without pitch control of the prop blades. Try that with a turbine and efficiency suffers a lot more, and then the engine will stall out much more easily than a piston engine will.

So if we insist on using a turbine core instead of a piston engine for a low-power application, it becomes essential to have the complication of pitch control on the prop. In the same way, if we also insisted on getting rid of the prop and using a ducted fan system instead, it would again be vital to design that fan so it runs at pretty much constant RPM and somehow produces variable thrust from that. We might have other options than having to vary the pitch of the fan blades, such as varying the effective intake and output areas of the duct and the flow profile of the air in the duct, using irising ducts and vanes and so forth. So there might be some point in doing it this way I guess.

If the cost of the turbine core is low, and the variable thrust system we adopt is simple and foolproof enough, the advantages of such a system might conceivably balance or outweigh the drawbacks at such low power and thrust levels as you consider here. But the new system is up against long practice with old systems that have been shaken down over generations to work acceptably well, and are broadly familiar to people working in that field--pilots and mechanics alike.

I believe the Polish crop-duster was developed mainly because jet engines were being pushed from above, not because it was a more sensible approach than just making yet another piston-powered crop-duster.
 
I believe the Polish crop-duster was developed mainly because jet engines were being pushed from above, not because it was a more sensible approach than just making yet another piston-powered crop-duster.
Indeed, it sounds like the reasons were mostly economical or political. Perhaps needing to keep work up for government owned jet engine factories.
 
Really big canards

Does the Viggen count? Those canards could count as the upper part of a biplane layout...

Saab_AJSH 37_Viggen_001.jpg
 

Perkeo

Banned
I'm trying to give a short answer:

If your airplain is fast, you don't need to maximize lift but you do need to minimize drag, which is quite the opposite of what you get from a biplane.

If your airplain is slow, you don't need to maximize thrust, but you might want fuel efficiency, which is quite the opposite of what you get from a jet engine.

So the requirements are kind of contradictory.

Perhaps a kind of fold-up wing for starting and landing would make sense, but IOTL they have flaps and variable-sweep wings for this purpose.
 
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