I recently watched a movie and in the movie it stated that the helicopter could not fly to the top of mt.everest to rescue the person stuck on the top. So why can't helicopters fly at such high altitudes?
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8$\begingroup$ Ask yourself this: Could a helicopter fly in a vacuum? Certainly not. So somewhere between sea level and outer space a helicopter is going to be unable to ascend further due to lack of air pressure. Where exactly that point is depends on the design characteristics of each model of helicopter. $\endgroup$– Greg HewgillSep 28, 2015 at 1:36
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6$\begingroup$ Actually this has been done on a AS350 B3, but this is exceptional, and was only a flight test. The highest rescue was by the same helicopter on the Annapurna (6,900 m - 22,640 ft). See Wikipedia. $\endgroup$– minsSep 28, 2015 at 5:03
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3$\begingroup$ This doesn't look like a duplicate. The other question asks for a technical term, while this one asks why helicopters have a certain limitation. I agree they are related, but they are not the same. $\endgroup$– Fabio says Reinstate MonicaFeb 7, 2019 at 0:20
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2 Answers
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Don't believe everything you see in the movies. Doubly so if the movie originated in Hollywood.
The world altitude record for a helicopter is 40,814ft (12440m), some 11,000 feet (3353m) higher then Everest, set in 1972 Reference: Wikipedia
In 2005 Didier DelSalle famously touched down on the summit of Everest in an AS350 Squirrel Reference: Wikipedia. See below for a video.
Both feats involved specially prepared aircraft and highly experienced crew.
As has already been pointed out, a helicopter, just like any other aircraft, has a ceiling above which they cannot fly. Since it's clear that it is possible to fly helicopters to the required altitude in certain conditions, this is just a red herring.
The real reason helicopters don't routinely fly to those altitudes is that they're not designed to. As with everything in aviation, performance is a trade off between engineering, aerodynamics and commercial pressures.
There's a big market for helicopters that can fly up to, say, 12-15,000ft (3600-4500m). The market for helicopters capable of rescuing climbers from the top of Everest is, at best, limited, and the technical demands of building a craft routinely capable of such a flight will push the costs up dramatically.
In short, they don't because nobody will pay for it.
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7$\begingroup$ I think another factor is that weather conditions have to be ideal for a flight to that altitude. The typical rescue is unlikely to be in ideal weather conditions. $\endgroup$ Mar 20, 2016 at 11:30
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2$\begingroup$ Technically, someone can install rockets onto a helicopter then it can even go into space, but that's just a stupid machine and nobody will fund it. $\endgroup$– NelsonJan 6, 2020 at 5:54
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2$\begingroup$ @Burgi Your money isn't going to be enough... $\endgroup$– NelsonJan 7, 2020 at 3:13
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The helicopter flies as long as the blades produce lift. Now,the lift is proportional to the density at a particular altitude. However, as altitude increases, the density goes down.
Usually, the helicopter compensates for this by increasing the lift coefficient by increasing the blade pitch (as the blade rotational speed, and hence V is constant). However, at a certain altitude, the maximum lift coefficient of the blades is reached and no further increase in lift is possible.
Hence, the helicopter cannot fly beyond this altitude. This is the absolute ceiling of the helicopter.
answered Sep 28, 2015 at 2:15
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4$\begingroup$ In most helicopters, the drag will exceed the available (or at least rated) power output of the engine leading to RRPM droop before max lift is reached. $\endgroup$– SimonSep 28, 2015 at 18:54
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3$\begingroup$ A deficiency I see in this answer is the same is true for planes. Planes need air for their wings to work too. Why helicopters need more air than planes is the real and interesting question. $\endgroup$– leorleorOct 25, 2018 at 20:02
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3$\begingroup$ @leorleor my guess would be the difference in surface area of a wing on a fixed wing plane vs the surface area of the blades on a helicopter main rotor make the difference between planes and helicopters. There's also the limit of the rotational speed of the engine (or motor) spinning the blades on a copter (tortional forces to be contended with and need for balance and ability allow blades to flap when advancing or retreating to keep lift on each side constant). Basically like others say the cost doesn't outweigh the benefit for use case of helicopters. $\endgroup$ Feb 2, 2019 at 19:48
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2$\begingroup$ Also helicopters don't maintain their forward momentum typically (they are stopping and going with regard to x/y coordinates) whereas a plane has inertia in the direction the jets are driving the plane and it is relatively continuous in direction and speed (stopping and going is a big "waste"/use of energy). Helicopters are good at short distance precision control and lifting power, but not good for long range or high altitude, it's just a trade off in the design, form = function. $\endgroup$ Feb 2, 2019 at 19:49