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4 Answers

Altitude as a performance limiting factor

Asked by: 8435 views Aerodynamics, Helicopter

Hey there

Today I was talking to a friend about the limited service ceiling of helicopters. 

We know that helicopters are limited in altitude because the airfoils are at a certain altitude no longer able to produce enough lift (becuase of the lower air density etc). 

My friend suggested following thing: 

If helicopter manufacturers would implement a feature with which pilots could increase the lift producing surface of the airfoils (blades) it would allow them to compensate for the reduced air density (because bigger blades equals more lift). And since the air is less dense at altitude we would need the same amount of power even with those bigger blades!? 

Does that makes sense? And how is the engine power limited with increasing altitude (even with turbochargers)?



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4 Answers

  1. Kris Kortokrax on Apr 08, 2012

    First question would be why do helicopters need the higher service ceiling?  Other than to set altitude records or limited application for rescue operations in mountainous terrain, there is no need.
    The drag equation mirrors the lift equation except that the coefficient of drag replaces the coefficient of lift.  The surface area term has the same linear effect on drag that it has on lift.
    In reciprocating engine helicopters, the power is limited at lower altitudes, not higher ones.  This is done by observing a limit on the amount of manifold pressure that one may use.
    Other factors come into play.  The Robinson R-44 contains a limitation on flight above 9,000 above ground level.  This limitation is prescribed in case of fire (allows the pilot to be on the ground in a minimum amount of time).

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  2. Best Answer

    Nathan Parker on Apr 08, 2012

    I’m not a helicopter pilot, but I do have a couple of helicopter aerodynamics books.
    Helicopter performance unexpectedly has much in common with airplane performance.  An airplane’s service ceiling isn’t dependent on the quantity of lift, but on the quantity of power.  The same appears to be true of helicopters.  The problem of exceeding the critical angle of attack for all airfoils only becomes a problem when there isn’t enough power to move the airfoils fast enough to allow for a lower AoA.
    So the question really becomes, “How does an increased surface area of an airfoil affect the quantity of excess power?”  At low airspeeds, a large surface area would increase excess power, since induced drag would go down, at a slight penalty in parasite drag.  However, at high speeds, induced drag is low, so the increase in parasite drag would dominate and a greater surface area woulld decrease performance.  This is why airplanes have such things as Fowler flaps, which increase the surface area at low speeds, but reduce it at high speeds.
    In order to avoid a lot of time-consuming analysis, I would observe that airplanes intended for high altitude flight have long, thin wings, meaning high aspect ratio, so I would expect that helicopter blades would most benefit from the same planform, which they already have.  Increasing the chordline on those airfoils would reduce the aspect ratio, increasing the induced drag, which would tend to reduce the excess power.  If anything, a high altitude helicopter would benefit from longer rotor blades, but we eventually start to run into mach problems on the blade tips.
    But I will echo Kris’ observation that helicopters are intended for low altitude work, so it’s not clear to me that increasing the service ceiling is an important design objective.

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  3. Sam on Apr 08, 2012

    Hey there
    Thanks for the interesting answers. I was watching a documentary about helicopter rescue missions in the Himalaya Mountains. And for example the pilots were able to perform rescue missions up to 7000m elevation (with an AStar, which has as far as I know a max service ceiling of about 7000m anyway). I am sure that some mountaineers would like it if helicopters could safely rescue them from the top of Mount Everest. But I don’t wanna say that this should be the objective of helicopter rescue anyways. 

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  4. Christopher Arndt on Mar 12, 2013

    A larger airfoil generates more drag as well as lift.

    But your statement “it would need the same power to generate more lift” is not correct. The heavier weight/more drag would increase the power required to generate lift substantially.

    Engines often become the limited factor at higher altitude, at least in turbine aircraft. Torque required does increase with altitude and torque available decreases, because the aircraft may be “N1 limited/fuel flow limited” (the engines gas turbines cannot spin any faster, and/or the fuel control mechanisms have reached a physical limit in how much fuel they can inject into the combustion chamber).

    So, you can have a larger rotor blade with a larger engine and larger fuel tank, but all those things are heavier. You reach a point of diminishing marginal returns where it takes an impractically large and high powered aircraft to operate at very high altitudes and have any substantial purpose for being there.

    That being said, helicopters have overflown Mt.Everest, so the performance of many helicopters is extremely efficient. The next questions is why would you want to spend so much money going up there when operating at lower altitudes will gives you better results? 10,000 ft PA is pretty optimal for a non-supplemental O2 aircraft with a tailwind on a long cross country.

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