"Too Hot for Helicopters to Fly"

Discussion in 'Aviation' started by foxs_marine, Apr 20, 2011.

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  1. Recently I've read Chris Hunter's book about Iraq & Toby Harnden's book "Dead Men Risen". Both these books refer to troops asking for helicopters, only to be told that it was too hot for them to fly.

    From a shameful past as an air cadet I recalled something about helicopters being modified for "hot & high" conditions. From this I presume that heat or altitude give thinner air, resulting in engines, rotors etc. having to work harder to achive a given power output / carry a given weight etc than they would in lower altitudes or cooler air.

    Within the bounds of opsec (if they apply), I'd be grateful if someone in the know could post the science behind this.

    Many Thanks,

    F-M
     
  2. rampant

    rampant LE Reviewer Book Reviewer

    Gases, Liquids, Solids expand when heated, therefore when a gas, ie atmosphere, is heated it will expand and the air molecules diffuse further away from each other and the space between them increases, the less molecules in a given area means there is less for the engines or rotors to act on. Simple
     
  3. Go with your presumption, although if you wait for Flash, as a QHI he can give you chapter and verse on why density altitude = pressure altitude corrected for temperature, and why running out of tail rotor authority hurts.

    Alternatively, jim24/tropper will stumble in, and give you chapter and verse about the time he told Igor Sikorsky about having to help resolve the troubles Leonardo da Vinci had with his early designs for rotorcraft.
     
    • Like Like x 7
  4. I'll wait for tropper to pop in and educate us.......

    Fox, youre pretty much spot on with your theory. Warm air = thinner air = more air required in the front of the engine to produce the same volume through compressor and combustion chamber = less efficient engine. Add to that a bit of altitude (less pressure = thinner air) and you have a jet engine thats working a lot harder than at ambient conditions at sea level. Seeing how most of our aircraft were designed for the cold war; stopping crazy Ivan coming through the Fulda gap, there wasnt a huge need to have engines that would work all over the world in every environment.

    Add to that thinner warmer air and the rotor blades dont have as much air to 'lean on' so the higher you go, the more pitch you require. More pitch means more power required. On the plus side, drag reduces slightly so you get a tiny amount of payback. Big limit though is engine performance.

    D=CdρV2S
    Drag= Coefficient of drag x (ρ) density x velocity squared x surface area. Density reduces so by definition velocity required reduces. Unfortunately, the power required does not outweigh this benefit in reduced drag. The answer is to make compressors more efficient at compressing the air before its delivered to the combustion chamber or ensuring you cool the air prior to chucking the fuel in and setting light to it. Cold air=thicker air=better burn ratio. Jet engines tend to work better at high altitude as the air is generally a lot cold (but less dense). In the case of Iraq and Afghan, the air at altitude is hot.

    :-D
     
  5. ..................also, there is an issue with Hydraulic oil, something to do with if OAT was over a certain value, oil cavitated in pumps or something. Recall not being able to fly Lynx at Ste Leacodie in the Pyrenees because of this problem.
     
  6. So what you're trying to say is that 'to hot, to high, it falls out of the sky'. No need for all that technical bollox.
     
    • Like Like x 1
  7. Very much so. And even more avionics. Things tend to melt when its hot. Good job theyve glued a pair of Maplin fans to the back of the 9A EPSIs.....
     
  8. Technical ? are you RLC? ;)
     
  9. With that description, I'd say he was a test pilot.....
     
    • Like Like x 1
  10. The air at ground level is hot too :S
     
  11. Poetry and test Pilot...Truely gifted.....
     
  12. 'Speshul' at any rate.....
     
  13. Such a shame his talent for spelling let him down, eh?
     
  14. My spelling is speshull don't take the pish.
     
  15. Bunch of ***** you lot.