autorotating for range

[sen]

Hi there.
I apologize if this matter has been discussed already, but I failed to find a topic on it.
I have been trying to find out why the configuration for best range in autorotation, on R22 at least, involves reducing RRPM to 90%. I cant seem to figure it out in my mind, so now Im turning to you for help. I just doesnt make sense in my head, that reducing RRPM, and thereby reducing lift, should increase range.
So any help is appreciated
Thanks in advance, and good day.

[tombeeston]

i hope this is the answer (in not the most pedantic scientific terms):

the collective lever is in a higher position than it would be for a normal range auto.
ie you have increased the angle of attack of the blades.
this = more lift, so more range
but also more drag, so RRPM reduces.

(it is mainly the angle of attack that determines lift, rather than rrpm)

hope that helps

[sen]

thank you for your answer, but according to the lift formula, lift increases with velocity squared, whereas it only increases directly proportional with AoA. So reducing speed in order to increase AoA seems like a bad tradeoff...

Perhaps, despite the circumstances of the lift formula, the increase in AoA is so much greater than the decrease in speed, that the final result is more lift? I find that hard to imagine though.

L = 0,5 * density * V^2 * CL * S

[Heliwhore]

Straining my pea sized memory here. I think that 90 % is closer to the best lift/drag ratio for the R22.

So whilst your ideas on the lift formula make sense, Most(all?only singles?dunno) helicopters have their operating RPM set higher than their best lift/drag RPM, so that in an engine failure when the Rotor RPM is decaying, it is heading toward the best ratio rather than departing from it rapidly.

Happy for someone to correct me here if my memory serves me badly...
_________________
Generally wrapped in rubber, be it in the air or on the water.

[flip2]

Deleted - meandering and not particularly cogent

[Ascend_Charlie]

Remember that in a steady autorotation (or climb, or descent, or steady ANYTHING) Lift= Weight. You have not lost lift, you are just trading altitude (potential energy) for rotor revs via the rate of descent.

With lower RRPM, you are closer to the best lift/drag ratio for the blade (this is independent of airspeed) and with a slightly increased forward airspeed (and rate of descent) you are covering more distance for the height lost. Go too fast, though, and the extra drag causes a bigger ROD and you don't travel as far - hence the blue line on your ASI.

[sen]

okay, its starting to make sense now in my head, just need to look it through a couple more times to fully grasp it.
Thank you very much for your input.

/Sen

[paco]

The higher angle of attack simply gives you more time in the air during which you can go further, but you will have to get used to the increased rate of descent at the higher speed. The reduction in RPM giving you the range is a bit of a red herring, since it is actually the result of increasing the AA to its max value within the limits of the bottom red line.

phil

[Ascend_Charlie]

Errrr..... Paco, the higher rate of descent gives you LESS time in the air, but you get a bit further than normal.

[paco]

Nope. The higher AA gives you more time, which is indeed slightly offset by the extra rate of descent (see pic). The higher speed is obviously also a factor.


Phil

[Ascend_Charlie]

OK, if you are at 1000' and coming down at 2000fpm, you have 30 seconds in the air. You might travel 600metres

In range config, you are descending at 2200fpm, so you have LESS time in the air, but with the higher groundspeed, you might travel 700 metres.

You can't stay up there longer if you are coming down faster, simple mathematics.

[paco]

Granted, but we're looking at 2 separate effects. Anyway, the flare at the end is probably more effective.

Phil