Could small ramjets on the tip of propellers allow for supersonic spin of turbine blades?

Question

Well, of course, if it was practical, someone would've already done that, but I'm still curious.

As you can imagine, I'm an illiterate on the matter of aerodynamics, and even though supersonic propeller blade driven aircraft is not feasible because of the tremendous forces they would suffer, I see it as a somewhat of a "barrier". "What would be possible if this weren't a problem?", so to speak.

Well, I can imagine that putting a ramjet at the tip of a propeller would increase complexity and increase the size of said propeller (so all components could fit in), which would increase the drag, but could allow for more air to be pushed.

The closest thing I could find was "Tip Jet", a type of helicopter that uses combustion at the tip of the blades. One of the various attempts had ramjets at the tips, the "Doblhoff WNF 342", but there isn't much information, and I'm more curious about the applications on turbines (which would be tremendously complex).

Answer 1

Spinning fast is not a problem. In fact, for a turbine-driven engine, it's easier: turbine is an inherently fast-spinning device. So much so that tips of the compressor fans are typically supersonic: slowing them down would require a heavy gearbox and/or a large multi-stage turbine. (Though both solutions are used to some extent).

There is no particular problem with "tremendous forces" (that could be helped by tip thrust) either. The centrifugal forces on the vanes/blades greatly exceed aerodynamic forces (which cause torque and could be negated by the tip jets). Witness foldable propellers you can see on many drones and airplane models: the blades usually don't have any lock and are held in place purely by centrifugal forces. On jet engines, the vanes often rattle when not spinning: they also have some freedom of movement. So, the blade must withstand "tremendous force" anyway, and more so if it's supersonic.

The problem is just the aerodynamic efficiency. Supersonic airflow is significantly less efficient to harness useful force from it, thus designers do everything to avoid it, instead of doing tricks to achieve it as you suggest. Still, as I mentioned, for turbines it may be difficult, and they might allow it.

The tip jets are only useful for helicopters. And there, they are not used to achieve higher speed, but rather to eliminate torque on the shaft, which reduces or eliminates the need for the tail rotor.