R22 rotor decay below 50 kts
It is higher, by between 3 - 500 f/min, but it would need to be almost double to overcome the amount of drag suggested by the graph.
That is what I meant by significantly.
That is what I meant by significantly.
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Can anyone here produce the power curve for an autorotating helicopter?
It would be interesting to see it.
Just how different would it be to the power curve for straight and level that PC brings?
(a Rate of Descent against Airspeed would amount to the same thing if anyone has that, anyone? Nick?)
It would be interesting to see it.
Just how different would it be to the power curve for straight and level that PC brings?
(a Rate of Descent against Airspeed would amount to the same thing if anyone has that, anyone? Nick?)
They possibly are, but the graph needs to be drawn correctly.
The power required curve, as I understand it, is often taken from the fuel flow measurements in forward flight - easily measured and quantified. Every one I have seen has a much steeper curve than the one PC has shown - hence the need for the discontinuity in the vertical axis.
The Rate of descent equivalent is probably calculable with some physics and maths but may not be so easily compared.
The power required curve, as I understand it, is often taken from the fuel flow measurements in forward flight - easily measured and quantified. Every one I have seen has a much steeper curve than the one PC has shown - hence the need for the discontinuity in the vertical axis.
The Rate of descent equivalent is probably calculable with some physics and maths but may not be so easily compared.
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Crab I'm not sure what YOU mean by a discontinuity , and yes it would normally be steeper approaching the y axis , the gentle arrival at the y axis is typical for a power curve that includes ground effect (which probably should be considered as a 'special case').
LRP yup I have always thought that was a reasonable estimation. (Although probably unimportantly slightly different, curious as to how different, if anyone actually has a real graph of RoD against airspeed in autorotation). The reason I asked was that I was disconcerted that Crab would think that the rate of descent at zero airspeed would only be slightly greater than at 'bucket speed' (Vy) whereas the power required at zero would be significantly greater at zero airspeed (surely?). Since the power required at zero airspeed is so high compared to at Vy one would surely expect the Rate of Descent to be accordingly greater, since Rate of Descent is equivalent to power input for autorotation.
The rate of descent is of course directly equivalent to Power (supplied).
For each 330lbs that an aircraft weighs the VSI reading gives Horse Power directly.
ie "10" on a VSI (with units of 100fpm) IS 10HP per 330lbs.
so a RoD of 2500fpm is 25Hp per 330lbs, so for an aircraft weighing 3300lbs that would be 250Hp
LRP yup I have always thought that was a reasonable estimation. (Although probably unimportantly slightly different, curious as to how different, if anyone actually has a real graph of RoD against airspeed in autorotation). The reason I asked was that I was disconcerted that Crab would think that the rate of descent at zero airspeed would only be slightly greater than at 'bucket speed' (Vy) whereas the power required at zero would be significantly greater at zero airspeed (surely?). Since the power required at zero airspeed is so high compared to at Vy one would surely expect the Rate of Descent to be accordingly greater, since Rate of Descent is equivalent to power input for autorotation.
The rate of descent is of course directly equivalent to Power (supplied).
For each 330lbs that an aircraft weighs the VSI reading gives Horse Power directly.
ie "10" on a VSI (with units of 100fpm) IS 10HP per 330lbs.
so a RoD of 2500fpm is 25Hp per 330lbs, so for an aircraft weighing 3300lbs that would be 250Hp
Crab I'm not sure what YOU mean by a discontinuity
discontinuity (dɪsˌkɒntɪˈnjuːɪtɪ)
n, pl -ties
1. lack of rational connection or cohesion
2. a break or interruption
3. (Mathematics) maths
a. the property of being discontinuous
b. the point or the value of the variable at which a curve or function becomes discontinuous
n, pl -ties
1. lack of rational connection or cohesion
2. a break or interruption
3. (Mathematics) maths
a. the property of being discontinuous
b. the point or the value of the variable at which a curve or function becomes discontinuous
When you did your FI course you probably drew the power required curve and explained the different elements - the 3 elements are additive and that makes the graph very tall if you draw it correctly - therefore in order to get it on a normal page/board/slide, a discontinuity is inserted in the Y axis to compress the graph.
BTW - there isn't a 'gentle' arrival at the Y axis - if you include ground effect, the power required curve dips down again as it approaches the Y axis.
The 'standard' power curve being discussed here isn't really relevant, is it? Induced drag has to do with tip vortices and recirculating flow, surely, whereas in auto there is none of that, just some reduction in rate of descent airflow a la the 'parachute' effect Crab mentioned, unless I'm sadly mistaken (which is entirely possible of course).
In a 60 kt auto you might come down at 1500 - 1700 fpm, say, but there's no way you're doing 3400 fpm at zero speed, is there? More like the extra 300 fpm or so sounds about right.
In a 60 kt auto you might come down at 1500 - 1700 fpm, say, but there's no way you're doing 3400 fpm at zero speed, is there? More like the extra 300 fpm or so sounds about right.
AOTW - yes that was my only point regarding PC's use of the power require/drag curve.
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doesn't any one have a 'power curve' for autorotation ? It would be useful
i do not believe that it is only a matter of 300fmp in addition to the Vy RoD
Crab, no I don't draw it with a break, it's not too tall for my piece of paper, and anyway that would only be a break of pen to paper and not one in the underlying physics surely? (AND if it were THAT tall then it would represent a bigger RoD)
my idea of a discontinuity was as in Def3 (Mathematics), and there isn't one of those, there is no intstantaneous step change.
I rather thought your explanation was more in line with Def 1 "lack of rational connection or cohesion"
i do not believe that it is only a matter of 300fmp in addition to the Vy RoD
Crab, no I don't draw it with a break, it's not too tall for my piece of paper, and anyway that would only be a break of pen to paper and not one in the underlying physics surely? (AND if it were THAT tall then it would represent a bigger RoD)
my idea of a discontinuity was as in Def3 (Mathematics), and there isn't one of those, there is no intstantaneous step change.
I rather thought your explanation was more in line with Def 1 "lack of rational connection or cohesion"
i do not believe that it is only a matter of 300fmp in addition to the Vy RoD
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Aotw
No Offence recieved, I am not absolutely certain about it and you sound quite sure, I had always intended to do my own graph of this but have just never got around to it. You could well be right but I am just very doubtful, I did try and take a reading the other day but it was 'off the clock' and didn't have time to do it properly with a stopwatch and altimeter etc
No Offence recieved, I am not absolutely certain about it and you sound quite sure, I had always intended to do my own graph of this but have just never got around to it. You could well be right but I am just very doubtful, I did try and take a reading the other day but it was 'off the clock' and didn't have time to do it properly with a stopwatch and altimeter etc
I'm most often doing them in Robbies these days and therefore carrying some collective which may have an effect.
However, both with the speed either at 20 kt or less, or at 60 (and using the same rotor rpm), I'm sure there isn't that much of a difference in ROD. Once the VSI is stable you can read it off easily enough and I'm sure it's not hugely greater at airspeeds close to zero than at 50 - 60 KIAS. However I will check next time I'm doing autos.
However, both with the speed either at 20 kt or less, or at 60 (and using the same rotor rpm), I'm sure there isn't that much of a difference in ROD. Once the VSI is stable you can read it off easily enough and I'm sure it's not hugely greater at airspeeds close to zero than at 50 - 60 KIAS. However I will check next time I'm doing autos.
AnFI - so you are not absolutely certain, yet feel free to criticise those who are - and then regard yourself as a 'legend'...........................
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Crab your inputs are really painful. Aotw is being modest and conciliatory , as was I in offering the small degree of doubt required to accomodate the respect of aotw's opinion, I'm not really all that doubtful, aotw will maybe try it and come back with a revised figure I presume, then we'll (you'll) know...
perhaps someone here has a graph of Power vs Airspeed in Auto?
NL has a library of excellent graphs ... someone?
as for "regard yourself as a 'legend' " I don't think that could be further from the truth, perhaps you could pm me the evidence for that or retract it
perhaps someone here has a graph of Power vs Airspeed in Auto?
NL has a library of excellent graphs ... someone?
as for "regard yourself as a 'legend' " I don't think that could be further from the truth, perhaps you could pm me the evidence for that or retract it
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I apologize for the long reply: was busy getting ready for Christmas (and a broken N1 tach generator) so while I had time to read a couple people's postings, I didn't have time to put together a researched reply:
Yes, as I mentioned, this is the power required curve for the R22 as published by Robinson Helicopter Company.
I think it should be valid in autorotation, except maybe at exactly zero airspeed. I went back and checked Prouty's red book (Performance, Control, Stability?) to see if I had missed anything major, and the only thing I saw that jumped out at me is that he mentions that you are in Ring Vortex State when doing a zero speed autorotation. I hadn't thought about that, and I'm not sure exactly what the effect of that at zero airspeed might be, but by 10-15 knots I would expect rotor power required to be the same in autorotation as in powered flight.
As I said, this is Robinson's chart and a quick check seems to me to show that it is in fact showing total drag as the sum of the other three drag curves (I checked like... 4 points)...
I roughly measured the total drag curve, and I get a ratio of 1.4:1 between zero airspeed and bucket (53 kts) airspeed. That would mean from 1,500 ft/min at bucket airspeed to 2,100 ft/min at zero airspeed. That does seem slightly higher than my experience, but not as much as you're saying (it's not 2:1 on the drag curve). It's been a few years since I've been flying R22s but 1,800-1,900 ft/min is about right for a vertical autorotation in an R22 if I remember correctly. It's certainly close... One point Prouty makes is that in a vertical descent, parasite drag is in fact acting in your favor, i.e. it's having the effect of slowing your fall. So that might be one difference. At zero airspeed the power on curve parasite drag is zero, but the effect of the "vertical parasite drag" is that less thrust is required from the rotor and therefore less power. I suspect at the vertical speeds we are talking about that this is not a huge effect and could probably be ignored.
AnFI asks:
I was looking in the Prouty book for exactly that, and didn't find one. However he does have a rate-of-descent curve for autorotation and it looks very similar to the total drag curve. The rates were very high, but it was for a 20,000 pound helicopter which an R22 is definitely not! :-)
LRP says:
I agree with this. I'm sure there are some differences in autorotation because of upflow relative to the disk, but from a blade element standpoint I would expect the drag equations to be pretty much the same in powered and unpowered flight. I mean, the angle of attack is similar (according to Prouty), the airspeed is exactly the same if you run the RPM at the power on RPM... Therefore the relative wind the blade is seeing (at least the spanwise average) must be the same... Not really sure where the difference would be except maybe some differences in the wake; but even then I'm not really sure it would be different...
Crab says:
Like I said, this is from the Robinson Helicopter company and purports to be the drag curves of the R22 and I believe it to be accurate.
Crab continues:
Just a guess, but as you increase disk loading and solidity, you would expect the steeper curve you mention. The R22 almost certainly is near best case from an efficiency standpoint - it has very low disk loading, only two blades, and even the parasite drag is significantly lower than many helicopters of it's era. So maybe that's the difference compared to other drag curves you are familiar with. I tried to get the R44 drag curve from the company a while back and they said they would but then didn't - the engineer probably told the rep I talked to to go take a hike 
Crab continues:
I have no way of knowing whether Robinson considered ground effect when they produced this curve. I could go back and look at the original one to see if they mention that, but I'm too lazy! 
Arm out the window:
I think it probably is relevant. Average angle of attack across the disk is similar in powered flight and in autorotation according to Prouty. If the angle of attack is the same, and the airspeed is the same (RPM hasn't changed) then I guess I would expect tip vortices to be similar. As I mentioned, Prouty claims that at zero airspeed in autorotation you are actually in Ring Vortex State which says recirculation is taking place... I'll bow to any aerodynamicists who care to enter the fray, but from a first approximation I'm still of the opinion that the power required curve is basically the same power on or off (except for the VRS thing - that caught me by surprise).
Unless I did a bad job of measuring the graph, I actually think the 1.4:1 ratio of drag between 0 and 53 is probably pretty close. I'm not sure where the 3400 number came from - like I mentioned at the top, 1.4:1 means 1500 vs 2000 which is in the right ballpark...
Paul, I think your graph is a power required curve, usually used to demonstrate the different sources of drag that must be overcome with power.
I'm not sure it is valid in that form for autorotation since your rate of descent at zero speed would have to be significantly higher than at bucket speed.
That graph should be drawn with a discontinuity on the left hand axis since the total drag curve is additive (ie the sum of the other 3 curves at any point on the horizontal axis).
The drag according to your graph for a zero speed auto would be probably twice that at bucket speed because the curve would be more pronounced.
AnFI asks:
Can anyone here produce the power curve for an autorotating helicopter?
LRP says:
total drag, power required, rate of descent, should look about the same.
Crab says:
They possibly are, but the graph needs to be drawn correctly.
Crab continues:
The power required curve, as I understand it, is often taken from the fuel flow measurements in forward flight - easily measured and quantified. Every one I have seen has a much steeper curve than the one PC has shown - hence the need for the discontinuity in the vertical axis.
Crab continues:
BTW - there isn't a 'gentle' arrival at the Y axis - if you include ground effect, the power required curve dips down again as it approaches the Y axis.
Arm out the window:
The 'standard' power curve being discussed here isn't really relevant, is it? Induced drag has to do with tip vortices and recirculating flow, surely, whereas in auto there is none of that, just some reduction in rate of descent airflow a la the 'parachute' effect Crab mentioned, unless I'm sadly mistaken (which is entirely possible of course).
In a 60 kt auto you might come down at 1500 - 1700 fpm, say, but there's no way you're doing 3400 fpm at zero speed, is there? More like the extra 300 fpm or so sounds about right.
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P.C.
Uh?...
I went back and checked Prouty's red book (Performance, Control, Stability?) to see if I had missed anything major, and the only thing I saw that jumped out at me is that he mentions that you are in Ring Vortex State when doing a zero speed autorotation.
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PC "However he does have a rate-of-descent curve for autorotation " that would do since it is what we are talking about and is the equivalent of Rate of Descent
I agree with PC that a Power Curve should give a reasonable approximation to the Power (and therefore rate of descent) required in autorotation, it might not be exactly the same, but probably good enough (as many recognised design writers also state)
Crab, i think using fuel flow would give a slightly unhelpful error in that the engine whilst producing no useful power might still have a fuel flow (in the region) of 60% of it's maximum. That would not translate into Rate of Descent in terms of the proportionality you mention.
I'm not buying the vortex ring state bit in autorotation, (really?), even if it is Prouty (or you are sure that is what he's saying?). Vortex perhaps, but VRing, I doubt.
MOSTAFA quite correct
also I found a published curve here FWIW
I agree with PC that a Power Curve should give a reasonable approximation to the Power (and therefore rate of descent) required in autorotation, it might not be exactly the same, but probably good enough (as many recognised design writers also state)
Crab, i think using fuel flow would give a slightly unhelpful error in that the engine whilst producing no useful power might still have a fuel flow (in the region) of 60% of it's maximum. That would not translate into Rate of Descent in terms of the proportionality you mention.
I'm not buying the vortex ring state bit in autorotation, (really?), even if it is Prouty (or you are sure that is what he's saying?). Vortex perhaps, but VRing, I doubt.
MOSTAFA quite correct
also I found a published curve here FWIW
AnFi. - I don't recall you ever proving anything right, the only thing you appear to have a degree in, is obfuscating the issue - I certainly wasn't referring to Crab. Being another post graduate of, I still respect the Central Flying School for the knowledge and enthusiasm it imparts.