Calling Nick Lappos - Blade Stall
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Calling Nick Lappos - Blade Stall
We have a gentleman (AnFI) who's posted the following on another thread,
and
Comment specifically on his "There is effectively a conning angle at which a disk is in effect stalled, regardless of RRPM" would be appreciated.
I've been invited by the poster to write my own technical paper, but thought I'd gain expert opinion first. We can discuss consultation fees by PM.
Glad to see up, about and engaged. 
Interesting youtube is littered with these 'high speed stall' accidents
The point about coning angle is really interesting. There is effectively a conning angle at which a disk is in effect stalled, regardless of RRPM.
RRPM 'cancels out of the maths' when you look for stall.
The point about coning angle is really interesting. There is effectively a conning angle at which a disk is in effect stalled, regardless of RRPM.
RRPM 'cancels out of the maths' when you look for stall.
cone is proportional to the ratio of Lift (L) to Centripetal Force (Cf)
both L and Cf are proportional to RRPM^2
take a helicopter in the hover at low load and low RRPM (such that it is very close to stall AoA)
measure the cone
if load is increased the RRPM has to be increased to increase Lift
the AoA remains just short of stall and the coning angle is unchanged.
that coning angle is the coning angle just short of stall
both L and Cf are proportional to RRPM^2
take a helicopter in the hover at low load and low RRPM (such that it is very close to stall AoA)
measure the cone
if load is increased the RRPM has to be increased to increase Lift
the AoA remains just short of stall and the coning angle is unchanged.
that coning angle is the coning angle just short of stall
I've been invited by the poster to write my own technical paper, but thought I'd gain expert opinion first. We can discuss consultation fees by PM.
Glad to see up, about and engaged.
Nick can no doubt provide numerous orders of magnitude better comment than me, but it seems on the face of it that the two quotes don't relate well to one another, considering the first is in reference to high speed stall and the other to the disc in the hover.
As soon as forward speed upsets uniform flow around the disc, I would say all bets are off regarding the direct relationship between RPM and coning angle, except in a very general manner of speaking.
As soon as forward speed upsets uniform flow around the disc, I would say all bets are off regarding the direct relationship between RPM and coning angle, except in a very general manner of speaking.
How unlike AnFI to muddy waters with confusing and conflicting explanations - Not!
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This could only ever be true for a fixed uniform static condition.
The important value is alpha. Anything that changes alpha independently invalidates the assumptions that the quoted relationship relies on.
Alpha can be influenced by any airflow changes perhaps due to translation or flapping so movement would cause changes independent of rotor speed. Then you have to consider that the rotor is inducing a flow itself and that with a real rotor this is not uniform.
In my opinion which I'm happy to be corrected on is that it is a far too simplistic. The real equation of lift for a rotor blade is not half.rho.v_squared.S.Cl, but needs to be integrated along the blade to account for changes in induced flow.
The important value is alpha. Anything that changes alpha independently invalidates the assumptions that the quoted relationship relies on.
Alpha can be influenced by any airflow changes perhaps due to translation or flapping so movement would cause changes independent of rotor speed. Then you have to consider that the rotor is inducing a flow itself and that with a real rotor this is not uniform.
In my opinion which I'm happy to be corrected on is that it is a far too simplistic. The real equation of lift for a rotor blade is not half.rho.v_squared.S.Cl, but needs to be integrated along the blade to account for changes in induced flow.
Something like this, ½ρv^2·S·Cl, but different? (I can't get superscripts/subscripts to work, sorry)
I think trying to educate AnFi on anything to be a Fool's Errand and not worth any effort.
That being said...the rest of us would greatly appreciate hearing from Nick about this or any other Topic although AnFi shall never accept what is relayed to us by Brother Lappos.
That being said...the rest of us would greatly appreciate hearing from Nick about this or any other Topic although AnFi shall never accept what is relayed to us by Brother Lappos.
Agreed Sasless
Not sure how AnFi can talk about a single coning angle where the disc is stalled regardless of RRPM - the differences along the length of the blade of V squared, blade twist and aerofoil section seem to make this a nonsensical argument.
What on earth this statement
is supposed to mean I don't know but seems to have his hallmarks of confusion all over it.
Not sure how AnFi can talk about a single coning angle where the disc is stalled regardless of RRPM - the differences along the length of the blade of V squared, blade twist and aerofoil section seem to make this a nonsensical argument.
What on earth this statement
RRPM 'cancels out of the maths' when you look for stall
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Nail on the head there Crab.
The problem with the original assertion is twofold.
1. Mathematical v Physical
2. Theoretical v Practical
You can replace the v in the original equation with theta dot x r. However the physics demands you specify a particular r or v. So only valid for a given point on the rotor.
The equation of lift is for an aerofoil which is a thing of theory. It doesn't give the complete answer for a wing and is far off the mark for a rotor blade which needs to account for variation of both tangential velocity and induced velocity along the blade.
So correct algebra applied incorrectly to a physics equation that is unrelated to the actual physics of the problem under consideration. Apart from that I'm sure it's OK
Just noted the use of the term Centripetal with Cf in the quoted post ... I would have used centrifugal myself.
The problem with the original assertion is twofold.
1. Mathematical v Physical
2. Theoretical v Practical
You can replace the v in the original equation with theta dot x r. However the physics demands you specify a particular r or v. So only valid for a given point on the rotor.
The equation of lift is for an aerofoil which is a thing of theory. It doesn't give the complete answer for a wing and is far off the mark for a rotor blade which needs to account for variation of both tangential velocity and induced velocity along the blade.
So correct algebra applied incorrectly to a physics equation that is unrelated to the actual physics of the problem under consideration. Apart from that I'm sure it's OK
Just noted the use of the term Centripetal with Cf in the quoted post ... I would have used centrifugal myself.
Last edited by dClbydalpha; 27th Sep 2016 at 19:43. Reason: Added note
Ties in nicely with his thrice reworded thesis and contradictory statement in Megan's new thread. I did suggest he stuck to multi-engine rants but sadly the ghost of Ray Prouty in him had to come out. Probably through the wrong exit.....
LMFAO - Nearly choked on my cocoa 212 Man
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differential of Cl by Alpha !!
you are close but wrong (to agree with Crab on that point)
you say: "So only valid for a given point on the rotor."
whereas I think you'll find it's valid for each point and therefore equally true when integrated for all r
(there are of course (always) second (of third) order effects that are not relevant to the essential thrust of the observation)
(Cf is Centripetal Force not Centrifugal acceleration, and you are being ridiculously nit picking. wrt an aeroplane wing folk talk about stall speeds, or even sometimes stall angles, but of course it isn't true everywhere on the wing, but it is nonetheless a useful concept in an aeroplane, similar to this concept in a helicopter. And yes I do mean 'Accelerated Stall' wrt FW FWIW)
you are close but wrong (to agree with Crab on that point)
you say: "So only valid for a given point on the rotor."
whereas I think you'll find it's valid for each point and therefore equally true when integrated for all r
(there are of course (always) second (of third) order effects that are not relevant to the essential thrust of the observation)
(Cf is Centripetal Force not Centrifugal acceleration, and you are being ridiculously nit picking. wrt an aeroplane wing folk talk about stall speeds, or even sometimes stall angles, but of course it isn't true everywhere on the wing, but it is nonetheless a useful concept in an aeroplane, similar to this concept in a helicopter. And yes I do mean 'Accelerated Stall' wrt FW FWIW)
Yeah, I think those of us that actually studied physics and aerodynamics and all that stuff, refer to it as centripetal force. Centrifugal is a colloquialism misused by many that didn't....
I find it easier to say that in the first place........
And yes I do mean 'Accelerated Stall' wrt FW FWIW)
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AnFI - You are correct that each point can be considered separately and then integrated. I suggest you try carrying out the integration, taking chord elements along the blade from root to tip, and post the equation that results. Sorry if I am telling you something you already know, but to start it is easier if you assume a constant chord/aerofoil and the ideal twist to give you a uniform downwash. Concentrate on the change in alpha along the blade as the tangential velocity changes with respect to the induced velocity. The equation you post, if correct, will tell you why I say that your overall assertion is only valid at a point and therefore why it cannot be applied to the rotor.
Centripetal v Centrifugal is not nitpicking, they are not colloquialisms, it is engineering terminology. Centripetal is a tendency to cause movement towards a centre. Centrifugal is a tendency to cause movement away from a centre. The definition is in the Latin name, centre-seeking and centre-fleeing. The two must not be confused, a movement of the blade CG toward the centre would add to and not oppose the lift. While physics tells us that centrifugal force is a "false" force, it is actually an inertia effect seen in a Newtonian frame of reference. It accepts that it is perfectly valid to calculate within a non-Newtonian frame of reference, such as one in constant rotational motion. In this case "false" forces become relevant and directly usable. It is much more convenient for certain rotor calculations to work in such a frame as it simplifies a number of equations, and as such the terminology is crucial to establish what is being worked with so a particular calculation doesn't cross frames of reference. Of course some who practice engineering experience this regularly, and some even have to work with a Newtonian, a constant rotation, and a relativistic view of the universe all in the one system.
But back to the original post. I assert that once you have calculated the lift along the blade it does not simplify to a direct relationship between stall and coning that can be applied generally, even for a simple rotor in hover, let alone one that employs varying aerofoils, varying chords, non-linear twist, that is translating as well as rotating.
Centripetal v Centrifugal is not nitpicking, they are not colloquialisms, it is engineering terminology. Centripetal is a tendency to cause movement towards a centre. Centrifugal is a tendency to cause movement away from a centre. The definition is in the Latin name, centre-seeking and centre-fleeing. The two must not be confused, a movement of the blade CG toward the centre would add to and not oppose the lift. While physics tells us that centrifugal force is a "false" force, it is actually an inertia effect seen in a Newtonian frame of reference. It accepts that it is perfectly valid to calculate within a non-Newtonian frame of reference, such as one in constant rotational motion. In this case "false" forces become relevant and directly usable. It is much more convenient for certain rotor calculations to work in such a frame as it simplifies a number of equations, and as such the terminology is crucial to establish what is being worked with so a particular calculation doesn't cross frames of reference. Of course some who practice engineering experience this regularly, and some even have to work with a Newtonian, a constant rotation, and a relativistic view of the universe all in the one system.
But back to the original post. I assert that once you have calculated the lift along the blade it does not simplify to a direct relationship between stall and coning that can be applied generally, even for a simple rotor in hover, let alone one that employs varying aerofoils, varying chords, non-linear twist, that is translating as well as rotating.
Well that's AnFi put back in his box
He will be back though, complaining you are being rude to him because you don't agree and then arguing some infinitessimal point to try and win his argument - then extending that to argue he was right all along
He will be back though, complaining you are being rude to him because you don't agree and then arguing some infinitessimal point to try and win his argument - then extending that to argue he was right all along
Anyone got some Duct Tape to seal that Box?
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that integration is fairly easy
and i think you'll find it proves my point
( and your explanation of why u use false forces is a laugh
" In this case "false" forces become relevant and directly usable."
thats ok with me )
and i think you'll find it proves my point
( and your explanation of why u use false forces is a laugh
" In this case "false" forces become relevant and directly usable."
thats ok with me )
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which is why I look forward to you posting the results of your attempt AnFI. With the starting point I've given you it's only 4 steps to the standard form of the equation that any rotorcraft engineer would recognise.Don't forget to state clearly any assumptions and particular notation forms you choose to use. I'd hate to think you'd got it wrong because of the use of different terminology, like centripetal being used to describe a force that acts away from the center of a circle.
which is why I look forward to you posting the results of your attempt AnFI