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The CH-47 Model 347, also referenced as BV-347, was a helicopter that utilized a wing for forward flight and to carry external payloads. Could the wings be extended and engines on the rear rotor modified to also provide enough trust to provide enough lift from the wings alone?

The wing would be up during takeoff and landing. Extending them with modern material should not be too hard, but modifying the turbine for thrust vectoring may be costly.

related: Rotor head plopper upgrade

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    $\begingroup$ If you add more weight, at some point you‘re going to lose hovering and vertical takeoff/landing capability, at which point it becomes an aircraft - and not a particularly optimised one either! I don’t see why it couldn’t be done in principle and if willing to (engineering-wise and money-wise), but why should you? $\endgroup$ – Cpt Reynolds Jul 7 at 21:42
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    $\begingroup$ I realize the proposed concept is nuts, but I don't quite get the downvotes. $\endgroup$ – AEhere Jul 9 at 8:23
  • $\begingroup$ The meaning of the phrase "engines on the rear rotor" is unclear and seems to suggest a misconception. In reality, both engines drive both rotors. $\endgroup$ – quiet flyer Jul 9 at 19:24
  • $\begingroup$ @quietflyer rivised thanks. $\endgroup$ – Muze the good Troll. Jul 9 at 20:27
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In an effort to find the most realistic solution to an odd proposal:

The wing: It does seem possible with current materials to extend the wing and keep it structurally sound, although the design would likely have to be altered a bit, probably with an external strut in order to have a wing root thin enough to keep the underslung weapons. Additionally, a much longer span would inevitably increase your root chord if you want to keep sufficient wing area, and I have a feeling that would start to block a lot of the area under your rotors, decreasing your vertical thrust and your payload capacity.

Without additional control surfaces (probably on the tail), the wing would also create large pitch instabilities, requiring the rotors to spin at all times to keep positive directional control. They could probably be spun at low power, but this would still limit your forward thrust component since some power would still have to go to the rotors.

The engines: While AEhere mentioned that the turboshaft engines aren't designed to provide jet thrust, but rather spin the turbines and thus the shaft, a dual-mode design could be feasible that would direct high-energy air proportional to the needs of each propulsion system: to the shaft turbines for vertical flight, and to jet nozzles for forward flight, including potentially into another set of turbines powering a turbofan for better efficiency. This seems to me like the most efficient way to keep the vertical lift system powered at some stages of flight while also providing sufficient forward thrust in others, as well as allowing mixing for partial thrust of each in between stages.

More ideas to come?

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In principle, if you have money, time and nothing better to do, yes, but it would be a Frankenstein's aircraft that would be a terrible plane and helicopter at the same time.

Wings on helicopters have been used on a number of models, most notably the Mi-24 family, to provide cruise lift. They work as expected for short low aspect ratio wings and serve a dual purpose to house the weapon hardpoints. The large wings you draw would require massive reinforcement of the roots and lead to stability complications, see below.

The engines would be a different story. The turboshafts on the Chinook are optimized to not do what regular jet engines do: provide thrust. Their exhaust has little momentum because on a regular helicopter engine that serves no purpose, the energy needs to go into shaft power to drive the main gearbox. On a turbojet engine, the energy is used to accelerate a mass of air out of the nozzle.

From a stability point of view it would also be terrible, as the wing by itself would be unstable in most weight and balance conditions and require the rotors to keep providing torque in flight to keep the airframe pitch-stable. And if you need to keep the rotors spinning throughout the flight anyway, then why add a wing large enough to lift the entire thing?

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