Weight vs ROD in Autorotation
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Only sometimes true....
.... yes 100 ft for a heavy helicopter is more potential energy than 100 ft for a light helicopter
BUT the efficiency of the helicopter generally gets better with bigger pitch angle - and then it gets worse again - so the ROD will increase again when the power required to generate thrust equal to weight starts to increase faster than the energy available as a result of the increased weight...
as for choppabeefer - i beg to disagree - the one place where extra weight does matter is at the bottom where the energy in the rotor either has to hover a light helicopter or a heavy helicopter - it can hover the light helicopter for longer (=easier, the energy lasts more time so cushioning is easier with greater margin, surely? am i missing something?), (as for 800EOL - is that a lot?)
BUT the efficiency of the helicopter generally gets better with bigger pitch angle - and then it gets worse again - so the ROD will increase again when the power required to generate thrust equal to weight starts to increase faster than the energy available as a result of the increased weight...
as for choppabeefer - i beg to disagree - the one place where extra weight does matter is at the bottom where the energy in the rotor either has to hover a light helicopter or a heavy helicopter - it can hover the light helicopter for longer (=easier, the energy lasts more time so cushioning is easier with greater margin, surely? am i missing something?), (as for 800EOL - is that a lot?)
Prouty gives some assistance but only highlights that in certain weight bands (quite small) in certain conditions (lower altitude rather than higher) you can theoretically show that the potential energy goes up faster than the power required and gives a small reduction in the RoD as weight increases. He does say that this data has been observed in flight tests in some aircraft.
However, at higher gross weights, because the induced power is proportional to the square of the weight, the RoD increases again.
His findings are based on the reasoning that the power required for autorotation is almost the same as in level flight and the approximate RoD needed to produce this power is found by multiplying the horsepower for level flight by 33,000 and then dividing this by the gross weight.
We all agree that to achieve a steady state auto, the heavier aircraft must fall faster than the light one in order to generate sufficient rotor thrust to balance the weight which will result in a higher Nr. I don't think anyone's flight test schedule will show the the autorevs expected will go down with increased AUM.
So it all comes down to whether or not raising the lever to contain the Nr( to a nominal 100% for the sake of argument) will reduce the RoD back to the same as/more/less than the lighter helo. Mr Prouty doesn't cover that question but the subject of drooping Nr to reduce RoD (and increase range) does involve taking the blades into a slightly more efficient L/D area of operation. You still need the higher Nr for the EOL however.
It appears that there is no absolute answer to this question and different helicopters will provide different results in tests - just as not all helos have a recommendation to droop Nr to increase range - it depends on the type of aerofoil section, what the helo is optimised for, and each helo may well produce different results in differing conditions of DA and AUM.
Would I rather be in a lighter helicopter to do an EOL - yes please. Like others, I have taught and demonstrated many, many EOLs and there is no doubt that a lighter aircraft gives you more margin for error and that what works on a 206 would kill you in a Robinson.
However, at higher gross weights, because the induced power is proportional to the square of the weight, the RoD increases again.
His findings are based on the reasoning that the power required for autorotation is almost the same as in level flight and the approximate RoD needed to produce this power is found by multiplying the horsepower for level flight by 33,000 and then dividing this by the gross weight.
We all agree that to achieve a steady state auto, the heavier aircraft must fall faster than the light one in order to generate sufficient rotor thrust to balance the weight which will result in a higher Nr. I don't think anyone's flight test schedule will show the the autorevs expected will go down with increased AUM.
So it all comes down to whether or not raising the lever to contain the Nr( to a nominal 100% for the sake of argument) will reduce the RoD back to the same as/more/less than the lighter helo. Mr Prouty doesn't cover that question but the subject of drooping Nr to reduce RoD (and increase range) does involve taking the blades into a slightly more efficient L/D area of operation. You still need the higher Nr for the EOL however.
It appears that there is no absolute answer to this question and different helicopters will provide different results in tests - just as not all helos have a recommendation to droop Nr to increase range - it depends on the type of aerofoil section, what the helo is optimised for, and each helo may well produce different results in differing conditions of DA and AUM.
Would I rather be in a lighter helicopter to do an EOL - yes please. Like others, I have taught and demonstrated many, many EOLs and there is no doubt that a lighter aircraft gives you more margin for error and that what works on a 206 would kill you in a Robinson.
Join Date: May 2012
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Agree - somewhat
That is true, but only for very high weights, density altitudes or configurations in which the induced power (the power required to create thrust) dominates the profile power (power required to rotate the blades) and the peer required to overcome the drag of the fuselage.
Going back to the original issue, the RoD will decrease with increasing AUW in most conditions. As in most cases, there is so simple 'one size fits all' answer because rotorcraft (and their physics) are complicated.
Going back to the original issue, the RoD will decrease with increasing AUW in most conditions. As in most cases, there is so simple 'one size fits all' answer because rotorcraft (and their physics) are complicated.
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what works on a 206 would kill you in a Robinson.
As for the Horse Power reference you may be interested to know that for each 330lbs you weigh the VSI is in Horse Power
Last edited by AnFI; 14th May 2012 at 06:21.
