Calling Nick Lappos - Blade Stall
Joined: Apr 2003
Posts: 3,012
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From: USA
There will come a time when you good ppruners will realize that AnFI is able to counter any argument with excellently phrased pap, and make it sound scientific.
That Greek Apache was flown into the water with virtually NO pitch rate, and therefore no maneuver-induced load factor. Blade stall is certainly not a factor. A large collective pull reduced the descent and the RPM and it almost worked.
AnFI has "analyzed" it to prove his crackpot theory of how rotors work, and the result is a sausage casing of misapplied theory and mismeasured "facts".
There comes a time when you are wrestling in the mud with a pig and you realize the pig loves it.
That Greek Apache was flown into the water with virtually NO pitch rate, and therefore no maneuver-induced load factor. Blade stall is certainly not a factor. A large collective pull reduced the descent and the RPM and it almost worked.
AnFI has "analyzed" it to prove his crackpot theory of how rotors work, and the result is a sausage casing of misapplied theory and mismeasured "facts".
There comes a time when you are wrestling in the mud with a pig and you realize the pig loves it.
Joined: Dec 2008
Posts: 843
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From: N/A
oh well at least I now know that coning angle is not the ratio of Lift to Cf apparently
and that neither lift nor Cf are proportional to RRPM^2 which is why apparently they don't cancel out (!)
furthermore courtesy of PPrune I have learnt that Vortex Ring a a condition that can be inescapable for 4000ft ! apparently
and I have learnt from Crab that if you crash then you are too low, seems obvious now, silly me.
PoF courtesy of PPrune
and that neither lift nor Cf are proportional to RRPM^2 which is why apparently they don't cancel out (!)
furthermore courtesy of PPrune I have learnt that Vortex Ring a a condition that can be inescapable for 4000ft ! apparently
and I have learnt from Crab that if you crash then you are too low, seems obvious now, silly me.
PoF courtesy of PPrune
Last edited by AnFI; 15th October 2016 at 23:55.
Joined: Apr 2003
Posts: 3,012
Likes: 1
From: USA
AnFI, this drags on. Cf falls with RPM, but lift must be a constant, because the pilot raises the collective and increases the blade angle to compensate, therefor as RPM falls, the coning angle increases immensely. Please, consider that this is just a bit more complex than your simple analysis, and that some of us have a bit of knowledge about this.
Joined: Dec 2008
Posts: 843
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From: N/A
it does drag but there is a serious point in there Ascend approximately understood it in post 33
The Cl will hit it's max at the same coning angle regardless of the RRPM.
eg increase RRPM but add weight to keep Cl at max and the coning angle will be the same (as good as damn it)
get it yet?
(last go)
Originally Posted by AscendCharlie
But I suppose that, as the revs decay, the CL would be increasing with the pilot pulling in pitch to slow the descent and cushion on, which from your equation would increase the coning angle. Apparently the change in angle between low RRPM (pitch high, CL high) and high RRPM (pitch less, angle less) is in fact due solely to CL and not RRPM.
eg increase RRPM but add weight to keep Cl at max and the coning angle will be the same (as good as damn it)
get it yet?
(last go)
Joined: Apr 2000
Aviation Qualifications: ATP+Mil
Posts: 10,959
Likes: 1,814
From: EGDC
(last go)
But you follow a predictable path AnFi - you come up with an off-piste notion, get challenged on it, move the nub of the argument from place to place, confuse everyone, refute the opinions of those who have an esteemed track record in the discipline and finally claim no-one else understands you and flounce off.
And then in a few weeks you will be back to do the same again.
I was accused by Canute of not bringing anything useful on this thread - he hasn't dealt with you before so that is understandable.
However, in the final analysis, your confusing, unproven ideas are actually damaging to those seeking to further their knowledge of helicopters - fortunately we have our resident experts to guide them back on the the true path.
Joined: Jan 2011
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From: on the cusp
Sincerely, AnFI your fundamental premise relies on Lift being proportional to RRPM squared and only to RRPM squared so that you can "cancel" it out. I've posted for you the simplest equation, based on BET, for lift on an individual blade. In calculating α you need RRPM, if you choose to dismiss α as a second order effect when calculating CL then that neatly sums up my concern with your approach.
Rotor design is fascinating because each section along the blade radius is experiencing significantly different conditions due to rotor speed, then each blade experiences significantly different conditions each cyclical rotation. The lift of each blade not only contributes collectively to the total lift, but individually has to cyclically influence the orientation of the disc. Rotor design, and therefore the selection of aerodynamic qualities, has to take this and many other things in to account. The concept of a "stalled" disc is interesting, but is so much more complicated than the concept of a stalled wing - it certainly doesn't distill straight to the CLmax of each blade independent of RRPM which your initial premise implies.
Rotor design is fascinating because each section along the blade radius is experiencing significantly different conditions due to rotor speed, then each blade experiences significantly different conditions each cyclical rotation. The lift of each blade not only contributes collectively to the total lift, but individually has to cyclically influence the orientation of the disc. Rotor design, and therefore the selection of aerodynamic qualities, has to take this and many other things in to account. The concept of a "stalled" disc is interesting, but is so much more complicated than the concept of a stalled wing - it certainly doesn't distill straight to the CLmax of each blade independent of RRPM which your initial premise implies.
Joined: Oct 1999
Aviation Qualifications: ATPL
Posts: 7,373
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From: Den Haag
Sincerely, AnFI your fundamental premise relies on Lift being proportional to RRPM squared and only to RRPM squared so that you can "cancel" it out. I've posted for you the simplest equation, based on BET, for lift on an individual blade. In calculating α you need RRPM, if you choose to dismiss α as a second order effect when calculating CL then that neatly sums up my concern with your approach.
Rotor design is fascinating because each section along the blade radius is experiencing significantly different conditions due to rotor speed, then each blade experiences significantly different conditions each cyclical rotation. The lift of each blade not only contributes collectively to the total lift, but individually has to cyclically influence the orientation of the disc. Rotor design, and therefore the selection of aerodynamic qualities, has to take this and many other things in to account. The concept of a "stalled" disc is interesting, but is so much more complicated than the concept of a stalled wing - it certainly doesn't distill straight to the CLmax of each blade independent of RRPM which your initial premise implies.
Rotor design is fascinating because each section along the blade radius is experiencing significantly different conditions due to rotor speed, then each blade experiences significantly different conditions each cyclical rotation. The lift of each blade not only contributes collectively to the total lift, but individually has to cyclically influence the orientation of the disc. Rotor design, and therefore the selection of aerodynamic qualities, has to take this and many other things in to account. The concept of a "stalled" disc is interesting, but is so much more complicated than the concept of a stalled wing - it certainly doesn't distill straight to the CLmax of each blade independent of RRPM which your initial premise implies.
Joined: Apr 2000
Aviation Qualifications: ATP+Mil
Posts: 10,959
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From: EGDC
Was it Simon Newman? I attended a short university course at Southampton courtesy of the RAF a good few years ago and he was one of the lecturers - absolutely excellent.
Joined: Jan 2011
Posts: 217
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From: on the cusp
It's a good book, mine sits on my shelf at work with many post-it notes.
It reads like he lectured. Each chapter is an individual topic and starts with an explanation. For many uses the explanation is sufficient without having to dive into the maths. I always recommend it to young graduates who join the team.
It reads like he lectured. Each chapter is an individual topic and starts with an explanation. For many uses the explanation is sufficient without having to dive into the maths. I always recommend it to young graduates who join the team.
Avoid imitations
Joined: Nov 2000
Aviation Qualifications: ATPL
Posts: 15,111
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From: Wandering the FIR and cyberspace often at highly unsociable times
Asking two highly experienced, world renowned, chief test pilots of a major helicopter manufacturer if they "get it yet".....priceless.
Thread Starter
Joined: Mar 2005
Aviation Qualifications: Military
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From: Aus
Nick what you say is very high quality and is mostly correct.
Joined: Aug 2009
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From: Texas
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