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If I can hover at 21"MP at SL will I still be able to hover at 21"MP at 7,000


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Look in the POH, under performance. Read the charts, and you'll see what power is required at different altitudes, temperatures, and gross weights. That will answer your question.

 

 

The performance charts tell you if you will be able to hover IGE or OGE at certain altitudes and temps, but not how much power is required to hover. There is also the Limit MP placard but that just tells you what MP your limited to pulling(R22). My question is will it always be the same MP to hover at any altitude given the weight and surface conditions are the same and the power is available.

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I'm going to try that next weekend.

 

Later

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It will take greater power to hover at altitude because you will have more collective and more pedal for a given weight. The greater angle of attack on both main and tail rotor is required to generate the same thrust (remember the lift equation - if you decrease air density you must increase something - v can't change, S can't change so CL has to change.)

 

Greater angle of attack means greater induced drag, means greater power requirement.

 

The amount of power the engine can generate decreases too (approx 2% per 1000 feet as far as I recall), this is the MAP limit you run into but it's entirely separate from the aerodynamics of the main and tail rotors.

 

Kris

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I admit to never having seen a performance chart for a Robinson, but every other model I've flown lets you determine the required hover power for various conditions. If the Robinson charts can't do that, they're deficient, and I tend to doubt that they are. They should at least be adequate for answering your question, to which the answer is no.

Edited by Gomer Pylot
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It will take greater power to hover at altitude because you will have more collective and more pedal for a given weight. The greater angle of attack on both main and tail rotor is required to generate the same thrust (remember the lift equation - if you decrease air density you must increase something - v can't change, S can't change so CL has to change.)

 

Greater angle of attack means greater induced drag, means greater power requirement.

 

The amount of power the engine can generate decreases too (approx 2% per 1000 feet as far as I recall), this is the MAP limit you run into but it's entirely separate from the aerodynamics of the main and tail rotors.

 

Kris

 

Yes it will of course take more "power" to hover the higher you go, thiner air, larger AOA, and the engine has to work harder to overcome drag. But is it really reflected in MP. Where as at SL the 21" its taking for me to hover may only be requiring 105hp of the 124 available (or limited to), then at altitude wouldn't that same 21" of MP now be requiring more "power" say 120 of the 124 available? Yet MP remains the same. Such as in the Robinson your limit MP decreases with altitude, such as at SL and 20 C you are limted to 22.6 MP continous (which would be using the full cont 124 HP) and at 6000 and 20 C you are limited to 21.3 (using the same full 124hp) meaning that at SL to pull that same 21.3 MP it may only be requiring 115 of the 124hp but at 6000 that 21.3MP requires more "power" (124hp) but its the same MP setting. I started to get a bit confused when I started reading this thread on manifold pressure:

 

http://helicopterforum.verticalreference.c...ure+at+altitude

 

and the user 500pilot was stating that it will always take the same mp to hover at altitude that it did when you tookoff given the same weight.

Edited by helifool
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I'm pretty sure I think I'm following your question? Let me re-state it and see if I got it right.

 

Your saying that at Sea Level (20*C) and hovering at say 115hp, your pulling 21.3" MP?

And at 6000 (20*C) and hovering at 124hp, your still pulling 21.3" MP?

 

is that what your asking?

 

I don't really want to comment on a R22 since I have no experience with it, but the 300 has a Lmp chart that will show specifically this, but I don't have it in front of me, so I'll see if I can't comment later.

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