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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

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.

MaxBarraclough 6y ago
> 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.) [0] https://en.wikipedia.org/wiki/Jean_Boulet https://en.wikipedia.org/wiki/Jean_Boulet [1] https://en.wikipedia.org/wiki/Translational_lift https://en.wikipedia.org/wiki/Translational_lift [2] (PDF warning) https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch02.pdf https://www.faa.gov/regulations_policies/handbooks_manuals/a...
- liability 6y ago
  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?
  - MaxBarraclough 6y ago
    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] [0] https://en.wikipedia.org/wiki/Tip_jet https://en.wikipedia.org/wiki/Tip_jet [1] https://en.wikipedia.org/wiki/Sikorsky_S-64_Skycrane https://en.wikipedia.org/wiki/Sikorsky_S-64_Skycrane [2] https://en.wikipedia.org/wiki/Kaman_K-MAX https://en.wikipedia.org/wiki/Kaman_K-MAX [3] https://en.wikipedia.org/wiki/Sikorsky_CH-53E_Super_Stallion#Specifications_(CH-53E) https://en.wikipedia.org/wiki/Sikorsky_CH-53E_Super_Stallion... [4] https://en.wikipedia.org/wiki/Loss_of_tail-rotor_effectiveness https://en.wikipedia.org/wiki/Loss_of_tail-rotor_effectivene... [5] https://youtu.be/0Z2Rr39hiUs https://youtu.be/0Z2Rr39hiUs [6] https://en.wikipedia.org/wiki/JATO https://en.wikipedia.org/wiki/JATO

MaxBarraclough 6y ago

> 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.) [0] https://en.wikipedia.org/wiki/Jean_Boulet https://en.wikipedia.org/wiki/Jean_Boulet [1] https://en.wikipedia.org/wiki/Translational_lift https://en.wikipedia.org/wiki/Translational_lift [2] (PDF warning) https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch02.pdf https://www.faa.gov/regulations_policies/handbooks_manuals/a...

liability 6y ago
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?
- MaxBarraclough 6y ago
  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] [0] https://en.wikipedia.org/wiki/Tip_jet https://en.wikipedia.org/wiki/Tip_jet [1] https://en.wikipedia.org/wiki/Sikorsky_S-64_Skycrane https://en.wikipedia.org/wiki/Sikorsky_S-64_Skycrane [2] https://en.wikipedia.org/wiki/Kaman_K-MAX https://en.wikipedia.org/wiki/Kaman_K-MAX [3] https://en.wikipedia.org/wiki/Sikorsky_CH-53E_Super_Stallion#Specifications_(CH-53E) https://en.wikipedia.org/wiki/Sikorsky_CH-53E_Super_Stallion... [4] https://en.wikipedia.org/wiki/Loss_of_tail-rotor_effectiveness https://en.wikipedia.org/wiki/Loss_of_tail-rotor_effectivene... [5] https://youtu.be/0Z2Rr39hiUs https://youtu.be/0Z2Rr39hiUs [6] https://en.wikipedia.org/wiki/JATO https://en.wikipedia.org/wiki/JATO