How much will I have to modify my helicopter to have it fly supersonic?

Question

As explained in this question, current rotary wing aircraft cannot surpass the speed of sound because of retreating blade stall and the fact that the sonic boom would literally rip off the rotor. However, I want to have a helicopter-like search and rescue craft that can go supersonic. What is the least amount of change from a current-day helicopter that would allow my helicopter to go supersonic?

Parameters:

• My aircraft must be able to take off vertically
• My aircraft must not be broken after a single supersonic flight
• The 'current-day helicopter' that I am referring to is the Westland Lynx, which currently holds the helicopter air speed record at 400.87 km/h.
• The maximum change is the F-35B Lightning II, the only present-day V/STOL aircraft capable of supersonic flight.
• I would prefer for the helicopter to keep its rotors, and if not that, its shape, but if that is not possible that would be fine.

Answer 1

You basically have to turn it into a plane. Or have it turn into a plane.

Retreating blade stall is an inherent flaw that comes with the shape of a helicopter and its rotors; there's no way to completely remove it from the equation. The "shape" problem that I'm referring to is the hull shape - a helicopter does not generate lift from forward movement.

Planes generate lift through aerodynamics, whereas helicopters must rely on their rotors only.

So, the easiest solution is to have a fighter shaped "helicopter" (with the engines and everything), and design the plane such that its use of main rotor is limited to the VTOL portion of the flight. As soon as the craft reaches a high enough altitude, it disengages the rotors, and either ejects or using some other mechanism, folds and hides the rotor blades into the plane body (Which for the record isn't a very good idea, and would come with a slew of other complications), then engages its plane engines, allowing it regular flight.

But this comes with more complications. The flat body of a plane/fighter means that the rotor must be extremely strong and the blades much longer in order to generate the same amount of lift, since the body of the plane would block off most of the inner rotor - rotors generate lift by pushing air down, which doesn't work when there's something blocking it attached to the rotor. Thus, a larger rotor blade area would be needed to compensate for the shape of the craft.

Note: The rotor must also be able to withstand the force of the rotor trying to literally tear itself off the plane.

Assuming the issue above is okay, what we've really described here, is a plane with rotors to take off. But then why bother with the rotors in the first place? Planes don't have any issues taking off by themselves, and adding the extra rotor just adds weight and unnecessary complexity to the craft.

Let's now go into what these craft are used for. A plane is used to get somewhere/intercept fast, or to act as a quick, mobile fighter in the military. This combo craft sucks at that relative to a regular fighter because of the additional weight we've added, and the (possibly) hidden rotor blades.

A helicopter is used when stability is required, and you want to load objects into the body of the plane while hovering. In fact, the main reason to have helicopters could be boiled down to this hovering ability. This combo-craft can't hover because in order to hover, it would first have to redeploy its rotors. This means it has to slow down before doing so, but slowing down would stall out the plane and/or lead to certain death. Not feasible.

Essentially, this was a stupid idea. A regular plane or a regular helicopter both outshine this combination craft in its expected use cases.

Answer 2

Although an answer has already been selected, I'll offer up a historic project to do just that (and interestingly answer some of the objections Airy provided in his answer).

In the early 1950's, based on search and rescue experiences in the Second World War, it was thought that something like a "supersonic helicopter" was needed. The vehicle should be able to get to the crash site quickly (before the pilot succumbed to hypothermia if ditched in water, or enemy action if shot down over enemy territory), but needed to be able to hover for extended periods to effect search and rescue over the area where the pilot went down.

The Sikorsky XV-2 was a project designed to do just that. A slim roughly delta shaped aircraft mounting a retractable one bladed rotor extended the rotor for take off , hovering flight and landing, and retracted it for forward flight. While the aircraft itself was not supersonic (being an "X-plane" for testing the concept), there is no reason to believe that such design could not be developed over time to reach supersonic dash speeds.

1954 : S-57 (XV-2) Not a real helicopter as we know them today , was a proposed joint US Army/USAF development program for a radial engine powered convertiplane.

The system featured rotors that folded up for take-off and landing and folded away and stored in the fuselage for level flight. The 54-4403 serial number was assigned to the prototype but was cancelled before it was built.

https://www.helis.com/timeline/sikorsky2.php

More details can be found here: http://www.sikorskyarchives.com/S-57%20CONVERTIPLANE.php

Sikorsky XV-2 from "Fantastic Plastic"

Another view of the "Fantastic Plastic model*

Photoshopped image demonstrating what an XV-2 might look like in hover