R22 rotor decay below 50 kts
R22 rotor decay below 50 kts
It seems to me that there are a lot of knowledgeable people here that can help with a question that I have been struggling with. Working towards my CFI and I have a question about why NR decay of the R22 decreases below 50kts. It makes no sense to me since at lower airspeeds you would have more air moving up through the blades since your descent would be greater. What's the reason for this? Thanks Scientific equation? To clarify for some people that are complete jerks I'll rephrase the question. They make a book that would be great for reading it's called the fundamentals of instruction that describe things never to do to a student (it might be worth reading for some). Why does nr decay occur in zero airspeed autos. When I say zero airspeed I'm referring to a high decent rate and no forward airspeed. Thanks again
Last edited by Weads; 13th Dec 2016 at 12:18.
It seems to me that there are a lot of knowledgeable people here that can help with a question that I have been struggling with. Working towards my CFI and I have a question about why NR decay of the R22 decreases below 50kts. It makes no sense to me since at lower airspeeds you would have more air moving up through the blades since your descent would be greater. What's the reason for this? Thanks Scientific equation?
Just to be clear, on the information provided:
You are flying along and descending. Your NR is decaying so unless this decay is arrested, soon you'll be dead. You slow to 50kts (inferred) and the rate of decay decreases. So you will live slightly longer.
Brief over, lets go flying.
You are flying along and descending. Your NR is decaying so unless this decay is arrested, soon you'll be dead. You slow to 50kts (inferred) and the rate of decay decreases. So you will live slightly longer.
Brief over, lets go flying.
Sorry. I feel guilty for being harsh.
If you rephrase the question so that it accurately depicts the situation you need clarification on, I'll personally do my best to help you understand the phenomenon you are experiencing.
If you rephrase the question so that it accurately depicts the situation you need clarification on, I'll personally do my best to help you understand the phenomenon you are experiencing.
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Good on you Twist and Shout, for admitting that and correcting it.
I was about to post a comment on how people are being severely discouragd for asking a question.
And Weads, if you don't know something never be afraid to ask, as you have. Well done and keep your enquiring mind going.
I was about to post a comment on how people are being severely discouragd for asking a question.
And Weads, if you don't know something never be afraid to ask, as you have. Well done and keep your enquiring mind going.
I suspect you are talking about being in the flare of an auto possibly?
Below 50kts you are on the back side of the power curve. Is this what you are maybe hinting at?
Help us out a little and maybe explain your question a little better....
Below 50kts you are on the back side of the power curve. Is this what you are maybe hinting at?
Help us out a little and maybe explain your question a little better....
I think you're asking a question about rotor rpm in autorotation. If you review your books about autorotation you'll see that the rpm is governed by the relationship between the driving and the driven regions of the rotor disk. I know nothing about R-22's but generally in a properly rigged aircraft the maximum rpm will be obtained right around the max r/c airspeed (correspond to the minimum rate of descent airspeed for autorotation), as you slow below that airspeed rpm will decrease, as you go faster rpm will decrease. The rpm should stay in the green unless you exceed the max airspeed for autorotation.
Specific aircraft performance and limitations can vary significantly but generally most helicopters fall in these parameters.
If that wasn't your question, disregard.
Specific aircraft performance and limitations can vary significantly but generally most helicopters fall in these parameters.
If that wasn't your question, disregard.
Nr decay
If your nr decays then it means your aoa is too much therefore the drag created will slow the rotor continuously until either you lower the lever or you crash.
If the aoa is too high then you run the risk of stalling the blades/aerofoil. Once you stall it just goes from bad to worse and recovering the nr will become impossible beyond a certain point. I guess this is the point you are referring to.
Forward speed is irrelevant, once you stall the rotor it is quite often game over.
If the aoa is too high then you run the risk of stalling the blades/aerofoil. Once you stall it just goes from bad to worse and recovering the nr will become impossible beyond a certain point. I guess this is the point you are referring to.
Forward speed is irrelevant, once you stall the rotor it is quite often game over.
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Weads, you should know from your basic POF that the RPM are a function of IAS and weight. Higher speed or weight needs more disc loading. So, in an ungoverned helicopter in auto you demonstrate that the RRPM for a given weight at high speed then, at steady speeds, the slower you go the RRPM decrease. Show this to your student early in the flight then repeat at the end and note the difference with weight reduction. The important point from this is that the minimum weight allowed for flight is so that you will always have the minimum RRPM allowable for autorotation should you have the engine failure( provided you lower the lever promptly!)
Weads - do you mean that following an engine failure (or throttle chop) the Nr decays slower at 50 kts than at other speeds?
If so then it will be because 50 kts is about min power speed for a Robbie and you will therefore have least collective pitch applied to maintain straight and level at that speed - therefore Nr decay will be slower (but still bloody fast).
If so then it will be because 50 kts is about min power speed for a Robbie and you will therefore have least collective pitch applied to maintain straight and level at that speed - therefore Nr decay will be slower (but still bloody fast).
Weads
Are you starting to see the importance of being concise in your explanations, especially in instructional situations.
Correct use of technical jargon.
And avoidance of "double use". You say
Do you mean the NR is still reducing, but decreasing at a lower rate? (I suspect you didn't mean this.)
Try phrasing the complete question again.
You can see there are pleanty keen to help.
Are you starting to see the importance of being concise in your explanations, especially in instructional situations.
Correct use of technical jargon.
And avoidance of "double use". You say
the NR decay decreases.
Try phrasing the complete question again.
You can see there are pleanty keen to help.
Rotor rpm in steady state autorotation is the result of the balance of driving and driven regions of the disk. The actual value will vary due to density altitude and gross weight. For any given set of conditions you (again generally) will see maximum rpm at your best R/C airspeed (also best L/D airspeed), the steady state rpm will decrease as you go slower, and will also decrease you go faster up to your max airspeed for autorotation. The rotor rpm should stay in the "green" throughout the approved envelope. This is valid for steady state rpm, you may see the rpm go beyond limits while in transit between airspeeds.
If you want to go farther than that you're going to have to do a lot more "learnin".
Also for the R22, the practical situation is different to a theoretical steady state rotor RPM in auto because you will almost always be 'carrying' some collective, that is, having it raised a bit to stop the rotor RPM overspeeding.
Looking at your edited first post now, I'm still a bit confused about exactly what you're asking, and if you're open to a bit of advice, be aware there are some very smart, experienced and helpful helicopter experts around this forum and you won't get much help being rude (if that's not your intention, as it well may be by the looks):
Looking at your edited first post now, I'm still a bit confused about exactly what you're asking, and if you're open to a bit of advice, be aware there are some very smart, experienced and helpful helicopter experts around this forum and you won't get much help being rude (if that's not your intention, as it well may be by the looks):
To clarify for some people that are complete jerks I'll rephrase the question.
They make a book that would be great for reading it's called the fundamentals of instruction that describe things never to do to a student (it might be worth reading for some).
They make a book that would be great for reading it's called the fundamentals of instruction that describe things never to do to a student (it might be worth reading for some).
It also gives advice on being clear and unambiguous.
I've apologized for my original "attitude" (complete jerkiness?)
We are trying to help.
You seem to have gone from 50kts to 0 IAS.
"Rotor decay" implies a decreasing (changing) RPM
I think most will agree, in a constant IAS autorotation (autorotation wasn't mentioned in the original question BTW) the RRPM will remain constant.
When changing speeds the RRPM will vary first, and possibly dramatically depending on the rate of IAS change due to the loading/unloading of the disc which causes "flare effect" due to the law of conservation of angular momentum.
Secondarily, once a steady (different) IAS has been achieved and stabilized, the RRPM will stabilize. Potentially at a slightly different RRPM.
Thirdly gross weight and air density will affect what stabilized RRPM will result from a certain collective pitch condition/IAS combination.
My (distant past) experience of R22 autorotations. Suggests the flare effect to be dramatic, and therefore problematic if not taken into account, or beneficial if anticipated and used correctly.
While the difference in RRPMin steady state autorotation at normal speed range. (0-80kts) is undetectable. And masked by the small amount of collective required to maintain the RRPM in the safe range.
In case you haven't been exposed to it yet. The "auto revs" are setup during mtce by varying the "flat pitch" (collective fully down) pitch angle by adjusting the pitch link lengths.
This flat pitch RRPM is designed to be safe at the configuration that produces the lowest steady state RRPM - in this case a very light all up weight. This would result in a RRPM overspeed if a flat pitch autorotation was maintained in an AC at maximum gross weight. Hence the need to have some collective applied to reduce the RRPM to an acceptable level.
Have a great day.
Weads, I think I now understand what you are asking.
In a vertical autorotative descent, the air is slowed by the rotor disc (extracting energy from it) and the Nr will be say 100% with the lever probably on the bottom stop. The effective rate of descent (as experienced at the rotor disc) is less than the actual rate of descent as the column of air is slowed and can only pass through the disc at a certain rate - an equivalent parachute of the same size was a phrase used many years ago in Brit Mil teaching.
Now, when you increase speed, you are moving the aircraft into a faster moving mass of air (not slowed by the rotor) so the effective rate of descent goes up, increasing the Nr and usually allowing you to add a little collective to keep it in the green. However the actual rate of descent reduces.
Your text books may refer to factors A, B and C where A is the tilting of the disc that reduces its area to capture the moving air and so increases RoD, B is the change in inflow angle from increasing speed which also increases RoD but C is the effect of moving into faster air which increases rotor thrust and decreases RoD. So Factor C outweighs factors A and B.
This keeps happening until you get to your best auto speed (60kts I think for R22), beyond which the disc tilt and inflow angle outweigh the increase in airflow - lots of drag now comes into the mix requiring more nose down to overcome.
Some aircraft have a VNE in autorotation because the combined effect of factors A and B (and their effect on the inflow angle) means that the inflow angle gives too much rotor drag and not enough driving force to keep the Nr high enough.
In a vertical autorotative descent, the air is slowed by the rotor disc (extracting energy from it) and the Nr will be say 100% with the lever probably on the bottom stop. The effective rate of descent (as experienced at the rotor disc) is less than the actual rate of descent as the column of air is slowed and can only pass through the disc at a certain rate - an equivalent parachute of the same size was a phrase used many years ago in Brit Mil teaching.
Now, when you increase speed, you are moving the aircraft into a faster moving mass of air (not slowed by the rotor) so the effective rate of descent goes up, increasing the Nr and usually allowing you to add a little collective to keep it in the green. However the actual rate of descent reduces.
Your text books may refer to factors A, B and C where A is the tilting of the disc that reduces its area to capture the moving air and so increases RoD, B is the change in inflow angle from increasing speed which also increases RoD but C is the effect of moving into faster air which increases rotor thrust and decreases RoD. So Factor C outweighs factors A and B.
This keeps happening until you get to your best auto speed (60kts I think for R22), beyond which the disc tilt and inflow angle outweigh the increase in airflow - lots of drag now comes into the mix requiring more nose down to overcome.
Some aircraft have a VNE in autorotation because the combined effect of factors A and B (and their effect on the inflow angle) means that the inflow angle gives too much rotor drag and not enough driving force to keep the Nr high enough.
.
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Weads asks:
Here is the drag curve for the Robinson R22: 
(from Helicopter Aviation).
As you know, in the R22 the bucket airspeed (minimum drag) is 53 kts at 104% Nr. In both powered and unpowered flight, drag goes up as you increase or decrease airspeed.
The other thing you need to know is that rotor drag decreases as you decrease RPM. This is in fact why minimum sink in the R22 occurs at 90% RPM. That's the slowest you can safely turn the rotor system in flight according to RHC.
When you autorotate at speeds below or above 53 kts, the total drag increases. The result is that the excess drag will cause the Nr to decrease. As the Nr decreases, rotor drag also decreases as we mentioned above. At some point the decrease in rotor drag balances the increase in total drag, and the rotor will maintain that RPM in steady state autorotation.
I think there is also an effect due to the shift of relative wind to be more from below the aircraft causing an increase in angle of attack as you descend more vertically, and this has the same effect as raising the collective would. However, I'm not prepared to defend that statement in open forum!
Why does nr decay occur in zero airspeed autos. When I say zero airspeed I'm referring to a high decent rate and no forward airspeed. Thanks again
(from Helicopter Aviation).
As you know, in the R22 the bucket airspeed (minimum drag) is 53 kts at 104% Nr. In both powered and unpowered flight, drag goes up as you increase or decrease airspeed.
The other thing you need to know is that rotor drag decreases as you decrease RPM. This is in fact why minimum sink in the R22 occurs at 90% RPM. That's the slowest you can safely turn the rotor system in flight according to RHC.
When you autorotate at speeds below or above 53 kts, the total drag increases. The result is that the excess drag will cause the Nr to decrease. As the Nr decreases, rotor drag also decreases as we mentioned above. At some point the decrease in rotor drag balances the increase in total drag, and the rotor will maintain that RPM in steady state autorotation.
I think there is also an effect due to the shift of relative wind to be more from below the aircraft causing an increase in angle of attack as you descend more vertically, and this has the same effect as raising the collective would. However, I'm not prepared to defend that statement in open forum!
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.
That would mean you would need twice as high a RoD at zero speed compared to bucket speed. I haven't flown a helicopter yet where that is true.
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.
That would mean you would need twice as high a RoD at zero speed compared to bucket speed. I haven't flown a helicopter yet where that is true.