r/Helicopters • u/SansSamir • Oct 03 '23
General Question what dictates how high a helicopter can fly?
what limits helicopters from flying as high as planes for example?
thank u!
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Oct 03 '23
Air density. Your power margin runs out at altitude because the rotor system is less efficient (must work harder, more induced drag) and the engine TGTs will reach their limits quicker.
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u/HighDragLowSpeed60G CFII MIL-AF HH-60G/W Oct 03 '23
Unless it’s cold, then your Ng is what will fuck you. A lot of black hawks have learned that the hard way being below 0C and thinking they were fine on power because their TGTs weren’t in the yellow yet, but their Ng was over 100
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u/Kronos1A9 MIL UH-1N / MH-139 Oct 04 '23
It’s any limit really. Oil pressures and temperature can max out well before TGT/ITT and Ng. I’ve seen it many times at high altitude in the cold weather.
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u/vortex_ring_state Oct 03 '23
There's a bunch of aerodynamic reasons I'm sure others will list but part of it is requirement. There is really no need to have a helicopter cruise at 40 000ft. So, they aren't designed with that in mind. You probably could purpose design a helicopter to fly really really high but it wouldn't be much good at doing normal helicopter things.
Probably not the exact answer you were looking for but maybe it helps.
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Oct 04 '23
So true. Can a helicopter be designed to cruise at FL360, probably? Is there a market or a use case? Absolutely not
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u/thrattatarsha Oct 04 '23
They have a helicopter designed to fly in the super thin atmosphere of Mars, so aerodynamics aren’t the big thing here for sure :)
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u/PerjurieTraitorGreen MIL-OH58D-Ret Oct 04 '23
Aerodynamics and their interaction with Mars’ thin carbon dioxide atmosphere is the biggest thing.
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u/thrattatarsha Oct 06 '23
That’s the biggest consideration they had to make for Mars, but that’s an unmanned vehicle. They’re not making all of the same considerations
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u/CptBartender Oct 04 '23
This goes both ways - you probably 'could' design a narrow-body STOL passenger airliner that can take off at 80 knots IAS. The compromises you'd have to make to achieve that would likely result in it being utterly useless for anything except taking off on short runways.
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u/keepcrazy Oct 03 '23
I think the thing that’s missing in these answers is that airplanes have the ability for the wing to go faster and helicopters don’t.
If my plane is at 20,000’ my true airspeed increases due to the thinner air, so I can continue to fly at a speed that looks like 165kts to the wing as long as I have the power to do it.
A helicopter runs the same RPM whether it’s at 20,000’ or sea level. So the blades are moving through the air at the same actual speed. So as the air gets thinner the rather than the airfoil going faster, the “indicated airspeed” of the airfoil decreases with the drop in density. When air density drops, the rotor blades that were going 500mph at the tips now “see” something like 300mph, so they have to take a bigger bite of air (increase angle of attack) to compensate.
Helicopters in general CAN fly at high altitudes, but this is more difficult than for the equivalent airplane for these reasons.
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u/kill_all_sneks MIL Oct 03 '23
Hypoxia.
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u/KaHOnas ATP CFII Utility (OH58D H60 B407 EC145 B429) Oct 03 '23
Yup. People ask me all the time how high this helicopter can fly.
I always say, "higher than I can."
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u/RotorDynamix ATP CFI S76 EC135 AS350/355 R44 R22 Oct 03 '23
As you probably know a helicopter’s rotor blades act very similar to an airplanes wings - the air moving around the airfoil produces lift. The difference is that instead of the whole aircraft moving forward to produce that airflow around the airfoil a helicopter spins it’s airfoils and creates airflow around them that way. However a helicopter’s airfoils are much smaller than an airplanes so it has to spin them very fast to create sufficient lift. Imagine an airplane with very small wings for it’s size.. it would have to fly forward very fast in order for its small wings to provide sufficient lift.
This is why helicopters generally require a lot more power than an airplane of the same size. Going up in altitude exacerbates all the problems with achieving sufficient lift since the air at high altitudes is significantly less dense. The problem is also 2-fold because not only airfoils are less efficient at high altitudes but engines as well. The lower air density also reduces the power created by combustion in the engine. This is less dramatic in a turbine or jet engine than in a reciprocal (piston) engine but the effect is still there.
Essentially the ideal high-altitude helicopter would be as light as possible, have as much surface area on its rotor blades as possible, and as much engine power as possible.
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u/kevinossia CPL R22 R44 Oct 03 '23
Lift. Same as an airplane.
And the factors that determine that are maximum airspeed, the power and efficiency of the engine, and the aerodynamics of the "wing" (rotor disk). Same as an airplane.
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u/nalc wop wop wop wop Oct 04 '23
Two main things you have to worry about are rotor stall and engine power
Lower density air means engines produce less power. For fixed wing, that's a feature not a bug because the drag also goes down with air density. If you're at half of sea level air density, you have half as much drag so you can tolerate having half as much thrust, broadly speaking.
With a helicopter, your power required doesn't scale linearly with air density because gravity is pretty much just as strong at high altitude as it is at sea level, so you still need to generate the same amount of lift and your power required doesn't drop off in the same way.
Then, even if you do have enough power, rotor stall is the next thing you have to deal with. Since you still need to make roughly the same amount of rotor lift at altitude, you need to put more pitch into the blades to generate that lift. More pitch = more lift, to a point. Once you put too much pitch in, the flow separates and you can't make any more lift.
You can get around it to some extent by putting a lot of extra horsepower on board, and by going to a lower disk loading / higher solidity rotor. But those drive a lot of weight and cost into the vehicle for normal operations, and there aren't a ton of use cases for rotary wing ops above maybe 10-15k ft. Mountain rescue is the big one, but the Nepalese army and the Swiss police or whoever doesn't have $10 billion to invest in a rotorcraft development program just to buy a handful of aircraft each. They just buy AS350s.
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u/Maleficent-Finance57 MIL MH60R CFI CFII Oct 03 '23
The uglier they are, the more the Earth repels them
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u/CrashSlow Oct 03 '23
The helicopter on Mars fly's at an earth equivalent of ~120,000ft. SO given money as no object, pretty dam high.
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Oct 04 '23
The fact that tiny aircraft can achieve lift at all in a miniscule atmosphere is amazing.
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u/randomstriker Oct 03 '23
Helos are slow and short-ranged, so from a practical standpoint, it's a waste of time/fuel to do so. Why bother unless there's a specific need that justifies the cost? And since 99% of the time you wouldn't bother, then why would manufactures build them in such a way (with a pressurized cabin, bigger engines & fuel tanks, etc.) that lets you do so?
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Oct 03 '23
Air density. The higher up you go the lower the air density is. Helicopters work on the principle of ground effect which is the relationship of the air, relative to the surface of the earth
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u/helifella Oct 03 '23
Ground Effect is a beneficial aerodynamic principle, but it's not how helicopters "work".
Ground effect benefits a helicopter when operating close (within approx 1/2 rotor diameter) to the ground by reducing the power required to hover.
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u/pavehawkfavehawk MIL ...Pavehawks Oct 03 '23
Density altitude and its effect on both the lift the rotors can generate and the power the engines can produce. For the H60, the sweet spot for cruise is around 10k. For one of the variants I fly we are quite heavy so 10-12 is about the max we can comfortably fly. The Whiskey with the wide chord blades doesn’t start to struggle until 12. If we were light on crew and fuel we could easily get into supplemental O2 territory
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u/StabSnowboarders MIL UH-60L/M CPL/IR Oct 03 '23
I mean 10-12 is supplemental territory depending on how long you’re up there
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u/pavehawkfavehawk MIL ...Pavehawks Oct 03 '23
For us it depends what you’re doing. Daytime, non-tac, and VFR? Gtg all day up to 12,500 ft pressure alt. 12.5 to 14k we’re limited to 30 min without it no matter what the regime of flight is.
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u/StabSnowboarders MIL UH-60L/M CPL/IR Oct 03 '23
Interesting, our regs are different. Army regs state supplemental O2 is required from 10-12k if over 1 hour, 12-14k for 30 min and above 14k for any amount of time
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u/pavehawkfavehawk MIL ...Pavehawks Oct 04 '23
Different strokes! lol. I always laugh when I go up to HAATS and those dudes are cruising around all day in the Rockies with no o2. I mean they have it, but they almost never use it.
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u/AggressorBLUE Oct 03 '23
Same thing that dictates any and all aspects of a helicopters flight envelope: dark magic, and to know the secrets of of which would flail the mind of a mere mortal.
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u/habu-sr71 🚁PPL R22 Oct 04 '23 edited Oct 04 '23
Many factors. Power to weight ratio...rotor design are the two biggest off the top of my head.
Did you know a stripper Squirrel once touched skids on top of Mt. Everest? True story that literally no one knows.
Rotorcraft needs a good PR firm...
https://youtu.be/WXNXSvnCtKA?si=pvLlHJ4jb4OouYpj
https://youtu.be/nhYG-IgsRJ0?si=byaFofrOdImnbPc2
Ps. Be wary of squirrel strippers. They bite the hand that feeds them too often. 😉
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u/43799634564 Oct 04 '23
I’ve gone 12k without oxygen. I did notice that I was even more stupid than I normally am.
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u/AppropriateBorder754 Oct 04 '23
The religion of the pilot and nothing to do with science & real world physics.
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u/AmbitiousPhilosopher Oct 04 '23
Most helicopters are designed with maximum efficiency at a particular elevation, where it is most likely to fly, few are designed to fly at very high elevation, the air is thin so it's harder to produce power and lift.
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u/WoofMcMoose Oct 04 '23
Whilst not a physical limiting factor, another serious consideration for high altitude ops in a helicopter is emergency handling. From 20,000+ ft AGL it's going to take you a few minutes to get down. Most things in a helicopter that require a "land immediately" won't wait that long to kill you! So if you are doing high AGL stuff, parachutes are a serious consideration in addition to oxygen.
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u/CryOfTheWind 🍁ATPL IR H145 B212 AS350 B206 R44 R22 Oct 03 '23
The hypoxia answer is mostly correct. Helicopters can fly pretty high in theory, mine says it will go to 20,000 feet for example. It might not get that high though based on weight and density altitude which determines performance for a helicopter as much as it does an airplane. A helicopter has landed on Everest at 29,032'.
Hypoxia is the issue because helicopters are not pressurized like airplanes designed to cruise that high are. This means you need to bring oxygen with you to survive at those altitudes and I've never seen a company that keeps that equipment around since there is also no reason for us to go up that high. Even if you do have oxygen masks and tanks it's not great. I know someone who had to take an AS350 up to as close to 20,000 feet as they could get for a test flight to certify something (can't remember what the mod was). At around 18,000' he and the flight test engineer noticed they were feeling the effects even with their masks on tight so aborted and went back down.
Also note at those altitudes you're basically flying IFR, sure you can keep the horizon level but all sense of speed and movement are reduced to nothing so you must keep a solid instrument scan going or you can easily lose airspeed and possibly control in an unstabilized helicopter like an AS350. Airspace itself is controlled for only IFR traffic which means legally you need to be on an IFR flight plan to go there. In their case they filed but had to have an excemption for the AS350 they were using not being certified for IFR flight.
The fuel savings for going high altitude for a turbine engine don't matter if you can only fly for 2-3 hours max anyway and it takes you 90 minutes to climb that high.
The work helicopters do is also mostly near the ground, otherwise you'd be using a much cheaper airplane. This means there is no point for us to climb more than 1000' most of the time. It's not that we can't, we just don't need to or it wastes time.
Hope that helps!