Rotor profile power and induced power
Guest
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Induced power is te power required to overcome Induced drag. Essentially this is the drag created by the act of producing lift and includes the effects of tip vortices as well as the work imparted to produce the induced flow that creates thrust.
Profile power is that required to overcome profile drag. This is created by the physical act of dragging a body through a viscous fluid, with the attendant friction etc.
The former decreases with speed, the latter increases.
Profile power is that required to overcome profile drag. This is created by the physical act of dragging a body through a viscous fluid, with the attendant friction etc.
The former decreases with speed, the latter increases.
Guest
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No, I don't think that is correct. Profile drag is composed of form drag and skin friction. The form drag is the physical resistance to moving a solid object through the air and is reduced by reducing the thickness/cord ratio, the friction is self evident. That's why it's called 'profile' as it reflects the effect that the blade profile has on drag.
The other factors you refer to would come under the heading of installed losses.
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Another day in paradise
The other factors you refer to would come under the heading of installed losses.
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Another day in paradise
Guest
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I have heard of profile power having "installation" power included for convenience when drawing up the power / IAS graph. "Installation" power stays more or less constant regardless of speed. In fact if it includes the tail rotor power, it may decrease slightly with increased IAS due to the translational lift effect on the tail rotor.
Profile power typically increases gently until blade stall effects occur (on the retreating side) when the curve increase in steepness and then again very sharply as compressibility also affects the advancing blade side.
p.s. Don't forget parasite power!
[This message has been edited by ShyTorque (edited 19 March 2001).]
Profile power typically increases gently until blade stall effects occur (on the retreating side) when the curve increase in steepness and then again very sharply as compressibility also affects the advancing blade side.
p.s. Don't forget parasite power!
[This message has been edited by ShyTorque (edited 19 March 2001).]
Guest
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Posted above are very good explanations of both profile power and profile drag, induced power and induced drag. They are flawless, good accepted answers if someone has an exam to pass and holding onto them will do no harm all the way through a career so don't anybody get confused or ruffled as a result of what I have to say here, please!
Both standard and accepted explanations bother me. If you give power and drag a lot of thought many 'holes' appear in the reasoning. I believe that this can be put down to one major factor - That much fixed wing theory was 'borrowed' many years ago and superimposed onto the Helicopter. It is not suitable, although similar in some ways. Its use may be perpetuated because no significant study or research has been published (to my knowledge, although I'd really like to read Ray Prouty on it).
Fixed wing power and drag must be a great deal simpler, they only (the explanations) have to deal with two or more Aerofoils moving through the air at almost identical V2 values (unless turning or sideslipping) so drag (and therefore power) must be much the same for both, unlike a main rotor. They also have no TR to consider, just for instance.
A good case in point is the TR. Pick up one book and its included in profile, in another it is with induced, others side-step its inclusion completely, maybe because their Authors just don't know where to put it and have not the time or resources to re-evaluate the whole thing.
Let's take the power used to drive the TR for an example, give it a little fresh thought as it has been mentioned. Some may think it belongs with induced because you must have anti-torque to induce a flow on a conventional layout Heli.(not directly of course, but to sustain directional flight) It seems reasonable at outset but the idea collapses quickly.
The TR's power requirement and the resulting drag does not belong under a parasite heading either, as that deals with power and drag values associated with an increase in speed alone. Indeed, as has been said, TR power and drag are inversely proportional to both parasite power and drag due to several factors (not least TR translational lift and the 'offloading' afforded by the vertical stabiliser etc.).
So, it might get thrown in with profile, also incorrect as 212 has said. It does not appear to belong under any of these traditional headings and may well need one of its own, an 'Anti torque power/drag' heading.
Shy T's explanation recognises this in using a fourth denomination, 'installation' and I believe this is a path to proper definition of various types of power and drag when applied to the Helicopter.
As for engine ancilliaries, it seems reasonable to only consider shaft horsepower produced by the engine, and thus the ancilliary power used and drag resulting can be ignored - Can't it?
Well, no, it can't.
Consider the alternator's use of engine horsepower and the drag produced - If you make a high demand for electrical power they both rise sharply as a result, at any speed you like from zero to VNE. Putting this under a heading of 'installation' or 'ancilliaries' which is assumed to be the same across the speed range (a flat line across the graph) cannot be accurate (it is not suggested that Shy T has done this).
It seems there really should be at least three main groups of ancilliary power/drag, ie. alternator/generator, anti torque devices and the remainder of ancilliaries that use roughly constant power and cause constant drag (although not all of them actually do).
Moving on, the MR might need separate power and drag definitions in respect of the advancing and retreating blade because they do not increase and decrease proportionally with an increase in speed.Drag increases for the advancing blade at a different (greater) rate to the reduction of drag on the retreating blade as speed increases.
Just in my opinion, the whole subject of power and drag for Helicopters needs to be stripped right down and started again from scratch.
Are there any Universities or research establishments out there interested in doing that?
SPS
(No animals were harmed during the writing of this post).
Both standard and accepted explanations bother me. If you give power and drag a lot of thought many 'holes' appear in the reasoning. I believe that this can be put down to one major factor - That much fixed wing theory was 'borrowed' many years ago and superimposed onto the Helicopter. It is not suitable, although similar in some ways. Its use may be perpetuated because no significant study or research has been published (to my knowledge, although I'd really like to read Ray Prouty on it).
Fixed wing power and drag must be a great deal simpler, they only (the explanations) have to deal with two or more Aerofoils moving through the air at almost identical V2 values (unless turning or sideslipping) so drag (and therefore power) must be much the same for both, unlike a main rotor. They also have no TR to consider, just for instance.
A good case in point is the TR. Pick up one book and its included in profile, in another it is with induced, others side-step its inclusion completely, maybe because their Authors just don't know where to put it and have not the time or resources to re-evaluate the whole thing.
Let's take the power used to drive the TR for an example, give it a little fresh thought as it has been mentioned. Some may think it belongs with induced because you must have anti-torque to induce a flow on a conventional layout Heli.(not directly of course, but to sustain directional flight) It seems reasonable at outset but the idea collapses quickly.
The TR's power requirement and the resulting drag does not belong under a parasite heading either, as that deals with power and drag values associated with an increase in speed alone. Indeed, as has been said, TR power and drag are inversely proportional to both parasite power and drag due to several factors (not least TR translational lift and the 'offloading' afforded by the vertical stabiliser etc.).
So, it might get thrown in with profile, also incorrect as 212 has said. It does not appear to belong under any of these traditional headings and may well need one of its own, an 'Anti torque power/drag' heading.
Shy T's explanation recognises this in using a fourth denomination, 'installation' and I believe this is a path to proper definition of various types of power and drag when applied to the Helicopter.
As for engine ancilliaries, it seems reasonable to only consider shaft horsepower produced by the engine, and thus the ancilliary power used and drag resulting can be ignored - Can't it?
Well, no, it can't.
Consider the alternator's use of engine horsepower and the drag produced - If you make a high demand for electrical power they both rise sharply as a result, at any speed you like from zero to VNE. Putting this under a heading of 'installation' or 'ancilliaries' which is assumed to be the same across the speed range (a flat line across the graph) cannot be accurate (it is not suggested that Shy T has done this).
It seems there really should be at least three main groups of ancilliary power/drag, ie. alternator/generator, anti torque devices and the remainder of ancilliaries that use roughly constant power and cause constant drag (although not all of them actually do).
Moving on, the MR might need separate power and drag definitions in respect of the advancing and retreating blade because they do not increase and decrease proportionally with an increase in speed.Drag increases for the advancing blade at a different (greater) rate to the reduction of drag on the retreating blade as speed increases.
Just in my opinion, the whole subject of power and drag for Helicopters needs to be stripped right down and started again from scratch.
Are there any Universities or research establishments out there interested in doing that?
SPS
(No animals were harmed during the writing of this post).
Guest
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Cron, to try and put it simply, when you're sat on the ground at flight idle, the power/torque reading on your guage (seems to be around 18% on the types I've flown) will indicate the rotor profile power, ie the power required to get the rotor system up to flight idle.
Guest
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ok, here is my attempt to simplify the whole thing for my students.
Rotor profile drag/power - power required to run the rotor systems at normal operating speed while sitting on the ground with the lever fully down, this includes air resistance on the blades leading edges and all transmission and ancillaries drag from engine to final drives. in a robinson this accounts for 13-14" manifold pressure, in a 206 as i recall it was 15-20% torque (i cants actually remember this figure), rotor profile drag is generally constant over the speed range but the increase in drag on the advancing blade increases more than the decrease in drag on the retreating blade, so overall there is a small increase.
Induced Drag/Power - drag created by raising the lever and forcing air down through the rotor disc to create lift. sitting on the ground this will be theoretically nil with the lever down.
it will be at its maximum in the hover with no ground effect and induced drag reduces with an increase in airspeed
Parasite drag/power - drag of airframe through the air (excluding rotor blades as they are included in rotor profile drag),this increases exponentially in a similar way to cars as airspeed increases
add all these values together and you get the "u" shaped power curve we are familar with.
comments all this lot please !
as i assume nobody will agree with it.
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elpirata
Rotor profile drag/power - power required to run the rotor systems at normal operating speed while sitting on the ground with the lever fully down, this includes air resistance on the blades leading edges and all transmission and ancillaries drag from engine to final drives. in a robinson this accounts for 13-14" manifold pressure, in a 206 as i recall it was 15-20% torque (i cants actually remember this figure), rotor profile drag is generally constant over the speed range but the increase in drag on the advancing blade increases more than the decrease in drag on the retreating blade, so overall there is a small increase.
Induced Drag/Power - drag created by raising the lever and forcing air down through the rotor disc to create lift. sitting on the ground this will be theoretically nil with the lever down.
it will be at its maximum in the hover with no ground effect and induced drag reduces with an increase in airspeed
Parasite drag/power - drag of airframe through the air (excluding rotor blades as they are included in rotor profile drag),this increases exponentially in a similar way to cars as airspeed increases
add all these values together and you get the "u" shaped power curve we are familar with.
comments all this lot please !
as i assume nobody will agree with it.
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elpirata
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Elpirata,
I agree with most of it.
Just to add some other points not often considered -
Induced flow includes not only air that is successfully induced to flow through the disc.
Indeed, as speed increases more and more induced flow will 'miss' the disc altogether as by the time it has moved downward and reached disc level the heli (disc)has gone!
The induced flow that doesn't make it through the disc could rightly be described as a waste of power and may legitimately be added to induced drag, as any other relavant loss may be.
So, as speed increases, induced power must increase and so must induced drag.
If you look at the standard induced line on either the power or drag graphs this brings up an apparent anomaly, because they show induced power and drag reducing as speed increases, not increasing as speed increases.
However, the description 'apparent' is used for a good reason - As speed increases induced power and drag increase but their net effect is outweighed by improvements in rotor disc efficiency (many factors are responsible for this, not least the increasing amounts of 'free' air (not induced downward) going through the disc, responsible for translational lift).
This loss of induced flow does not only occur in directional flight. It also occurs (to a much lesser degree)in a ground relative hover with any 'wind'(an incoming airflow)at all, and in amounts proportional to the speed of that wind.
It really makes no difference whether the disc moves to the air (as in directional flight) or the air moves to the disc (as in a ground zero speed hover into wind), there is still an airflow relative or incoming to the disc.
Some induced flow will be blown away from the disc and not go through it, but travel downward on the downwind side of the disc. Again, the 'bonus' afforded by translational lift comes into play and offsets (outweighs) the loss.
Ah..., and this only scratches the very surface of what is going on, what COMPLEX machines these Helicopters are.....
That's what makes then so fascinating!
SPS
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Guest
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the idea that rotor profile power is represented by the manifold pressure or torque measurement while at flight idle on the ground is a good one, except for this: a rotor blade does not have a consistent angle of incidence/attack at any collective setting due to blade twist, therefore some portions of the blade will not be at 0 degrees AoA, and would be subject to small induced power requirements as well (at least I think).
[This message has been edited by lmlanphere (edited 21 March 2001).]
[This message has been edited by lmlanphere (edited 21 March 2001).]
Guest
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Cron, I am working on a concise reply to my earlier post. I will aim to finish by tomorrow, work permitting!
Imlanphere, good argument but rotor profile power is based on minimum pitch and not angle of attack. The basic aerodrynamics models for helicopter principles of flight rely on massive generalisation, the actual forces acting on a rotor blade can only be explained through complex mathematics.
Imlanphere, good argument but rotor profile power is based on minimum pitch and not angle of attack. The basic aerodrynamics models for helicopter principles of flight rely on massive generalisation, the actual forces acting on a rotor blade can only be explained through complex mathematics.
Guest
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The argument about ground idle power is only valid to a degree. The minimum pitch on ground (MPOG) with the lever fully down is not 0 angle of attack. The setting has more to do with autorotative performance than anything else (in fact most of the collective travel is for power off, power on is only a small range). As an aside, the MPOG torque is a useful guide to whether or not the autorevs are correctly set up (torque high, revs low).
I'm not sure why people feel it necessary to include factors that aren't really related to profile drag. The drag on the blade is a stand alone factor that will be constant for a given blade on a test bed or different a/c. The power required of the engines is something else, and it is true to say that there is absorbtion by the transmission and ancillary drives, as well as the TR but why corrupt the meaning of 'profile' drag just because there is no convenient title?
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Another day in paradise
I'm not sure why people feel it necessary to include factors that aren't really related to profile drag. The drag on the blade is a stand alone factor that will be constant for a given blade on a test bed or different a/c. The power required of the engines is something else, and it is true to say that there is absorbtion by the transmission and ancillary drives, as well as the TR but why corrupt the meaning of 'profile' drag just because there is no convenient title?
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Another day in paradise
Guest
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Well put 212.
Apples and Pears should not be mixed unless you are satisified with a lose definition (fruit).
Power and drag are well overdue for redefinition or classification.
"Some people are said to be 'pioneers', the ones that push boundaries. To be a true pioneer you must realise that boundaries are not just a line ahead of us, they are a sphere all around us. To really push boundaries you must be prepared to look sideways, downwards, backwards or in any direction, and if you find a boundary has been wrongly placed in the past you must be prepared to move it".
Apples and Pears should not be mixed unless you are satisified with a lose definition (fruit).
Power and drag are well overdue for redefinition or classification.
"Some people are said to be 'pioneers', the ones that push boundaries. To be a true pioneer you must realise that boundaries are not just a line ahead of us, they are a sphere all around us. To really push boundaries you must be prepared to look sideways, downwards, backwards or in any direction, and if you find a boundary has been wrongly placed in the past you must be prepared to move it".
