I'm a little skeptical about the claim of 'performance is the same at any altitude'... while yes, the engine performance is similar at altitude, the helo performance is not (what with the fewer air bits for the blades to move around)
Yes, it's my impression that helicopters have problems with thin air reducing lift long before they lose engine power.
That's why it's easier for them to fly forward than hover at high altitudes; the forward velocity of a moving helicopter provides more airflow over the rotors and thus more lift. I don't think it's a matter of jamming more air into the turboshaft engines, which presumably could be done using larger intakes.
> it's my impression that helicopters have problems with thin air reducing lift long before they lose engine power.
I believe this is generally true, but it's certainly possible to get into trouble.
The world record for the longest autorotation was set after an engine flameout, after an attempt at the world record for highest altitude reached in a helicopter. [0]
> the forward velocity of a moving helicopter provides more airflow over the rotors and thus more lift
It's more that a helicopter in a hover (assuming little wind) must produce additional lift (i.e. more power) as it's stuck in a downdraught of its own creation. Introduce some forward movement, and the helicopter is flying through undisturbed air (or more precisely, the air entering the main rotor disc is undisturbed), which takes far less power.
This is called 'translational lift'. It kicks in at around 30 knots, and occurs at any altitude. It might be more consequential at very high altitude though, or under very heavy load, where the helicopter might have enough power for forward flight, but inadequate power for a hover. [1]
The FAA have an interesting Power vs airspeed chart, p19 of [2]. If I understand correctly, it means that if your goal is to keep a helicopter in the air as long as possible, you want to stay at around 62 knots of indicated airspeed. Fairly slow, but nowhere near a hover. (edit 'Ground effect' might also have some bearing on that question, but that's another matter.)
Very interesting, thanks for this. Do you suppose a hybrid system might be useful in special applications, like providing a power boost to rescue helicopters trying to hover above mountains?
Rather beyond my knowledge but I'll take a stab at it:
I don't think tip jets (the name for this design, oddly absent from the article) [0] are generally thought to be of practical value in modern helicopters. They have the neat advantage that they don't require a tail-rotor (the main rotor isn't driven by a drive shaft from the main body of the helicopter so there's no torque trouble) but I don't think they're all that practical. This article dates from 2010 after all.
We already have a reliable means of increasing a helicopter's power: bigger engines. Alternatively, more engines.
Heavy-lifting helicopters tend to be powered by twin turbine engines. [1][2] Even if they use unusual designs [2] the power-plant is the same as for any other serious helicopter. The enormous Super Stallion military helicopter went even further: 3 turbine engines! [3]
I imagine a hybrid design would greatly increase complexity. Helicopters use a 'sprag clutch' to permit the rotor RPM to exceed the engine RPM, but not the other way around. This allows the rotor to keep spinning in case of an engine failure. (This is the reason helicopters don't drop like bricks when their engines fail.) Perhaps there would be a way to modify the design so that the conventional engine could still contribute power even as the rockets are firing, but I imagine it would be very high in complexity.
Also, tail rotors can suffer if they aren't working in clean air, and of course this can threaten the helicopter's safety. [4] It's not something I know anything about but I imagine rockets on the main rotor could be troublesome in that regard.
For yet another wacky (but likely impractical) alternative design with no need for a tail rotor, see [5]
Lastly, google tells me rockets have been used to assist fixed-wing aircraft in takeoff. [6]
They have various other such documents, they're each a chapter of their Helicopter Flying Handbook, or you can download the whole lot as one enormous PDF. Here's the hub page:
The thing is, there's less air for the blades to move, but there's also less drag on the blades for the same reason, so they spin faster with the same power input. This ends up cancelling out the loss of lift, meaning that the performance stays roughly constant with altitude until it is high enough that the blade tips start to approach the speed of sound.
> In normal operations, and design aims to achieve this, the rotor tips do not go supersonic since when they do, there is a sudden and large decrease in performance with more power required, higher blade loads, vibration and noise.
> Think about a helicopter flying forwards. The advancing blade at its most perpendicular position experiences a relative airflow which is equal (ignoring all kinds of minor side effects) to the forward speed plus the speed of the blade. The retreating blade is experiencing a relative airflow equal to the speed of the blade minus the speed of the helicopter.
> If the blades rotate so fast that the tips are supersonic, then the main lift generating part of the retreating blade, the outer two thirds of the span, would experience such a low airspeed, for some of the span it will even be negative, that the blades will stall causing a catastrophic roll into that side. It is this phenomenon which ultimately limits the rotational speed of the blades and the maximum speed of the helicopter.
