# Why can't helicopters reach mount everest?

Is there a reason why people can't just take the helicopter to mount Everest? Why is it that helicopters can't reach that high?

Apparently helicopters could reach mount Everest:

https://en.wikipedia.org/wiki/Mount_Everest#2005:_Pilot_summits_Everest_with_helicopter

Helicopters create lift by their rotor blades pushing an equal mass of air downwards.

The air pressure at the top of Mt Everest (29,000ft) is only about 1/3 as much as at sealevel so your helicopter can only generate 1/3 of the lift. In addition it's cold so you risk ice forming on the rotors, fuel and hydraulics freezing and finally the weather isn't always very nice.

The altitude record for a helicopter is 40,000ft. It was set in 1972 with a very light version of a simple helicopter. There isn't much reason to fly helicopters at the height of a jet airliner so there aren't many helicopters developed to fly this high.

• Is the oxygen needed by the engine an issue? – Bernhard May 28 '12 at 17:31
• Other than tuning for a particular role - no. Helicopters use the same type of jet turbine as other aircraft which fly at these heights. – Martin Beckett May 28 '12 at 17:49
• @MartinBeckett: Yes, but. With less air, the engines do produce less power and thrust. Due to less drag, an aircraft fortunately needs less of it. But that's not the case of helicopter, which always needs the same power. – Jan Hudec Apr 1 '14 at 20:02

Helicopter rotor is a rotating wing. It produces lift the same way aircraft wing does, but instead of relying on forward motion of the aircraft it has it's own motion.

The lift generated depends on coefficient of lift, air density and forward speed. Formally

$L = \frac{1}{2}\rho v^2\alpha C_L$

where $L$ is lift force, $\rho$ is air density, $v$ is forward velocity, $\alpha$ is angle of attack and $C_L$ is coefficient of lift, which is generally function of $\alpha$, but in the normal operating range can be considered constant.

At $8\ 900\ \mathrm{m}$, the air density is about $38\%$ compared to the sea level. But to balance the weight, the aircraft still needs the same amount of lift. The $\alpha$ is limited by the stall angle (above which $C_L$ is much less), so the only option to maintain lift is increasing speed.

Fixed-wing aircraft can simply fly faster, which is allowed by the fact that drag is also proportional to air density (until you get too close to speed of sound, which is why all jet transports curise at 0.8–0.85 of speed of sound). However making rotary wing spin faster, because the rotor blades can only be made so strong to withstand the centrifugal force and also because in forward flight the advancing blade could get dangerously close to speed of sound.

As was noted by others already, helicopters that can climb that high exist, but they need larger rotor and stronger engine while being light and since most uses don't require those high altitudes, most helicopters are not designed to reach them.

• Could one increase the amount of lift available at a given rotor speed by using stacked rotors, with the lower rotors set for a steeper pitch than the upper ones (since they would be meeting air that was already moving downward as a consequence of their contact with the upper rotors)? – supercat Oct 16 '14 at 17:23
• @supercat: Yes, but it would not be very efficient. And it would not have any advantage over having all those blades in single rotor. – Jan Hudec Oct 16 '14 at 20:06
• By my understanding, an optimal shape for a rotor should be curved so that the pitch angle at the rear is steeper than at the front, and the amount of difference between the front and rear angle should depend upon the pitch of the blade as a whole; I would think that having two sets of blades in somewhat different planes would be somewhat like having a blade whose front and rear edges had independently-controllable pitch. Would things not work that way? – supercat Oct 16 '14 at 22:19
• @supercat: Well, most likely rotors would be slightly more efficient if the blades were twisted like on propellers, but it's difficult to implement. See also this answer. – Jan Hudec Oct 17 '14 at 5:01
• @JanHudec Why would it be dangerous to reach speed of sound ? I guess overpassing this speed could break the wings ? – Andy M Oct 12 '15 at 7:42

Yes. Helicopters require a certain amount of air for lift. The lack of air near the peak of Mount Everest makes it impossible for most helicopters to get the required lift and therefore fly.

• A rotor is nothing more than a rotating wing, so if the air pressure was the only determining factor, aircraft would have trouble at that altitude, too. I suppose you should add that helicopter blades need to be much smaller than wings, and hence need more air, as rotating them faster is not a real option: the tips soon start going faster than sound. – hdhondt May 29 '12 at 3:50
• @hdhondt: Many aircraft do have trouble in that altitude too. The reason is somewhat more complicated though. I'll expand it to a separate answer. – Jan Hudec Apr 1 '14 at 19:58