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Are there any, or have there ever been any commercial/military jet bi-planes?
What would be some of the major advantages vs disadvantages if one was built?

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Electric radio controlled ducted fan model.

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University of Tokyo concept model.

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Yes

I present to you, the Belphegor, a jet powered, crop-dusting, biplane. I think I need not say that this is a Soviet Bloc design, from Poland in particular.

M-15 at the Muzeum Lotnictwa Polskiego

Note the braces between the wings acted as tanks for whatever chemical agent it would be dispensing over the kolkhozy. It was not very successful due to a host of disadvantages, including:

  • Jet engines are not efficient at slow speeds, which are in turn required for crop dusting operations. The designers may have hoped to do away with this requirement by using it over the large collectivized farms of the USSR, but the straight wing attests to the top speed being low by design. The jet engine was apparently mandated by soviet planners wanting to simplify the logistics of fuel and maintenance by reusing the AI-25 engine.

  • Biplane configurations are less efficient than monowings of the same total area due to interference between both lifting surfaces.

  • For crop-dusting applications in particular, @Harper points out the fact that the shorter span of biplanes is detrimental to effective spraying, by limiting the width of the spray system, which needs to end short of the wingtips. Otherwise the agent will get picked up by the wingtip vortexes and spread in unintended directions.

On the other hand:

  • One advantage of biplanes is low roll inertia due to the shorter wings, which is mostly useful in dogfights and thus not highly-sought nowadays.

  • Another advantage of biplanes is the lower bending moment at the root of the individual wings, due to them being shorter. This allowed earlier designs to get into the air in spite of poor building materials and QA procedures; this solution was gradually abandoned as aircraft moved away from wood and fabric and towards aluminium alloys.

  • The lower span of biplanes might be useful in application that are span-limited, like modern airliners, which are limited by airport parking and runway restrictions. If you absolutely need to obtain more lift out of an airframe that is already at its maximum span, adding a second wing is not unreasonable.

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    $\begingroup$ @RobertDiGiovanni do you have a source on the helicopter conversion bit? Sounds weird because the wikipedia article does point to this being a clean(ish)-slate design. $\endgroup$ – AEhere supports Monica May 1 at 9:17
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    $\begingroup$ The short biplane wing works against cropdusters in a way. In America at least, you are only allowed spray nozzles across 2/3 of the wing width, that is to keep chemicals out of the wingtip vortices where they will be kicked up and carried by the wind to places that should not be sprayed. Shortening the wing means more passes needed. $\endgroup$ – Harper - Reinstate Monica May 1 at 13:05
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    $\begingroup$ @GdD there are a couple advantages to using a jet engine; one of the biggest (and probably the reason this plane exists) is that turbines can run off of almost any fuel, unlike piston engines which generally run on a rather narrow range of fuels. If you're out in the country and don't know what fuel will be readily available, a jet might be a reasonable choice. $\endgroup$ – Skyler May 1 at 15:28
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    $\begingroup$ "Soviet", without the "Union" refers to a council of the type that Poland found itself under for nearly half a century. $\endgroup$ – Jim Horn May 1 at 19:14
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    $\begingroup$ @Skyler yeah, and on the collective farms the fuel most readily available would be the diesel for the farm machinery, which is very similar to Jet A. $\endgroup$ – jwenting May 2 at 3:43
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A military jet-powered biplane was the I-153DM Chaika, which was adapted by strapping a couple of ramjets underneath each wing. (DM = Dopolnityelnyi Motor – supplementary engine).

I-153DM Chaika picture source

The I-153 was a development of the Polikarpov I-15, an advanced fighter when it debuted in 1933. The I-153 sought various improvements, including retractable undercarriage, but even so by the start of WW2 it was no match for its opposition.

The Russians were pioneers in ramjet technology, and in December 1939 the designer I.A. Merkulov tested the addition of DM-2 and later DM-4 jets to a ski-equipped I-153.

There was little immediate advantage: the jets yielded only an extra 50kph at best. The disadvantage was that in standard flight the extra drag made the little plane rather slower. However, it is notable that this was the first ramjet powered flight.

Over 70 flights were completed with the I-153 variants. It is reported that Merkulov also tested the DM-4 in a similar configuration on the Borovkov-Florov I-207 biplane prototype. source

I-153DM Chaika picture source

I-153DM Chaika picture source

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    $\begingroup$ This is fascinating! I'd heard of the Belphegor, but not this one - and it is neat to see ramjets combined with a radial propeller engine. $\endgroup$ – Skyler May 1 at 19:11
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    $\begingroup$ Technically the chaika could not propel itself on the jets alone, but that's beside the point. I'd heard of this thing but didn't register the fact that it was actually tested and that those were ramjets of all things. Nice one! $\endgroup$ – AEhere supports Monica May 2 at 7:25

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