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It seems in 1950's some countries investigated production of helicopters powered by ramjet engines on rotor tips - examples include American Hiller YH-32 Hornet or Polish JK-1 Trzmiel. The enthusiasm was short-lived - high fuel requirements, a significant load to the power transmission system, noise and high visibility on battlefield were significant disadvantages that ended the research programs which I know of.

Still, that's over half a century ago, and the technology has progressed quite a bit since then so maybe some technical problems of that epoch are now past? - The ramjets are small, lightweight and quite simple in construction, so despite the disadvantages the technology doesn't seem like a total dead-end. Was this line of helicopter propulsion investigated further? Could someone provide a progress report - if there is anything to report? Or is this research branch still dead, with no attempts to revive it?

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Tip jet rotors are still being developed, none of them are ramjet powered as far as I know. The Dragonfly DF1 is tip rocket powered, while the Pegasus uses air jets powered by a conventional engine.

Ramjet technology has not improved, so the cases for using them on rotor tips has not changed. They are still too heavy, thirsty and noisy.

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    $\begingroup$ Ramjets aren't particularly heavy, especially when designed for subsonic flight speeds. But at those speeds they are horribly inefficient. $\endgroup$ – Peter Kämpf Jul 6 '18 at 17:25
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Short answer: No.

Or at least, nothing that's been demonstrated that I'm aware of. Whether there have been experimental R&D aircraft or developments, I'm not sure - but I can't find any.

The simplest reason is just because there's no need for them - they offer no real advantages over a conventional internal combustion or jet engine, so as with so much else in aviation we stick with the tried and tested "we know it works" product, unless there's a good reason to change.

It's not necessarily that tip-jet helicopters are a complete dead end in terms of development, just that we don't see any advantages to be gained from them.

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I'm reading a book right now that talks briefly about flight testing the Hiller. As I recall, it was tough to get both engines started at the same time, and if one of them cut out, was a pretty violent affair. It was also an environmental disaster, with horrible fumes and noise. It was an interesting experiment, but it had no real world application at all. There still doesn't appear to be one.

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This particular idea has been abandoned for several reasons.

Tip-jets that put engines on the tips themselves - "hot" tip-jet, unlike "cold" tip-jet where a central turbine sends compressed air through the blades to be ejected at the tip - cause lots of drag, in addition to being very noisy. While the noise problem may have been lessened to an extent by further R&D, the drag problem is more or less unavoidable. This is why hot tip-jets have mostly been abandoned by now.

As an aside, cold tip-jets have been used to reasonable success with the Djinn helicopter: https://en.wikipedia.org/wiki/Sud-Ouest_Djinn

While it has much less autonomy than equivalent helicopters due to lower efficiency, it was also much lighter and much easier to pilot, and enjoyed some commercial success. With a heat exchanger to heat up the outgoing air with turbine heat, a newer helicopter could have had similar autonomy to a conventional one, the slightly lower efficiency being compensated by the lower mass. However designing an adequate heat exchanger would have been considerable R&D work, so no-one seems to have bothered with it as conventional helicopters were already doing the job.

But specifically for tip-ramjets, there is another, even worse problem: ramjets are efficient only above the speed of sound, and propeller blades are very inefficient and immensely noisy when the tip of the blade is supersonic. So you either loose considerable efficiency by having the ramjet operate below optimal speed, or you both loose considerable efficiency and make so much noise that it is physically dangerous for people nearby.

While not a tip-jet or a helicopter, the Thunderscreech showed what happens when you try to use supersonic propellers. It made mechanics literally sick, and could even knock people out from the sheer noise. https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech

The Soviet (and now Russian) Tupolev TU-95 bomber also has propellers that can go slightly supersonic, to get extra thrust at the expense of efficiency when going at max speed, and has been known to be detected by submarines due to the propeller noise.

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