RRPM increase in the Auto flare...
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Nick, i know you background, but can i beg to differ slightly ??
the reason you feel the 1.2g is because the flare results in a braking or decelleration action that will disappear after the rotor speeds up - because of an increased airflow.
yes, you get the same result as suddenly increasing you weight, a breaking action against the air until the rotor speeds up, but as others have mentioned in other threads, you can get the same result as you hover over a ridge in an upflow of air to the point of flat pitch. you are not moving as the dynamic force, the air is.
weight only pulls you towards the earth faster, result, more air through rotor
the reason you feel the 1.2g is because the flare results in a braking or decelleration action that will disappear after the rotor speeds up - because of an increased airflow.
yes, you get the same result as suddenly increasing you weight, a breaking action against the air until the rotor speeds up, but as others have mentioned in other threads, you can get the same result as you hover over a ridge in an upflow of air to the point of flat pitch. you are not moving as the dynamic force, the air is.
weight only pulls you towards the earth faster, result, more air through rotor
Last edited by 407 too; 31st Oct 2005 at 18:30.
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Excuse the childlike analogy (must be 'cause I'm doing PPL), but previously trying to explain this to myself I wondered if it weren't like holding a child's toy windmill while running, with the plane of rotation parallel to the airflow, then turning it 90 degrees to face it. The airspeed will naturally increase.
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Si
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Si
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Johe02 - nice diagrams but, unless I am reading them wrong, in each case the rotor system is not in autorotation - as shown by the fact that the relative airflow is down through the disc not up through it. The rotor system is either under engine power or slowing down rapidly!
In the second diagram, although the rotor system is shown "flared", the relative airflow has a higher "down" component, because the aircraft starts to climb. This would not normally be the case in an auto flare (could be if you got it wrong I suppose!)
So I think the diagram is illustrating what happens when you pull the cyclic back in normal powered flight - is there another one that shows the same but in autorotation?
EC
In the second diagram, although the rotor system is shown "flared", the relative airflow has a higher "down" component, because the aircraft starts to climb. This would not normally be the case in an auto flare (could be if you got it wrong I suppose!)
So I think the diagram is illustrating what happens when you pull the cyclic back in normal powered flight - is there another one that shows the same but in autorotation?
EC
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407 too,
We don't disagree, you are discussing the effect, I am discussing the cause. Both correct.
The flare slows down our descent, and in the process, raises our rpm, because as platinum pure says, the energy has to go somewhere.
Be careful about more weight causing you to fall faster, Galileo had something to say about that, and he would disagree!
Simon853,
Your pinwheel analogy is correct, and perfect!
We don't disagree, you are discussing the effect, I am discussing the cause. Both correct.
The flare slows down our descent, and in the process, raises our rpm, because as platinum pure says, the energy has to go somewhere.
Be careful about more weight causing you to fall faster, Galileo had something to say about that, and he would disagree!
Simon853,
Your pinwheel analogy is correct, and perfect!
Ah Nick, but surely it is not the airspeed that increases (he is still running at the same speed) but the mass of air suddenly presented to the pinwheel that causes the wheel to spin faster as all those air molecules try and force themselves through the rotor and push the blades out of the way.
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Yes - more appropriately ice skaters - for the rest of you it is also changes in relative airflow, induced flow, angle of attack for same pitch angle, changes in lift/drag plus the ice skaters - work it out for yourself.
GAGS
E86
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E86
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Johe02
Not a flame just looking for clarification.
Induced flow, air entering the disc from above the POR.
ROD with IAS= inflow angle, air entering the disc from below the POR.
How does the diagram represent anything to do with Auto? Given thats what we are talking about.
EC is the only post that has actually explaned "inflow angle" which is responsible for rpm.
Of coarse other credits to Nick, Crab, talking about weight increase, what will this effect? ROD of coarse for the same given IAS increasing the inflow angle and so RPM, thats why we raise the lever to add drag pulling the TR back in line and so giving more lift to over come added weight as we hold rpm.
During the flare you increase in inflow angle by a massive amount, the weight of the machine apposing the direction change your trying to make forces this angle increase, or eat more air" as Nick said.
I also agree with an earlier post that Coriollis has only minnor effect.
take it easy on me... 
Not a flame just looking for clarification.
Induced flow, air entering the disc from above the POR.
ROD with IAS= inflow angle, air entering the disc from below the POR.
How does the diagram represent anything to do with Auto? Given thats what we are talking about.
EC is the only post that has actually explaned "inflow angle" which is responsible for rpm.
Of coarse other credits to Nick, Crab, talking about weight increase, what will this effect? ROD of coarse for the same given IAS increasing the inflow angle and so RPM, thats why we raise the lever to add drag pulling the TR back in line and so giving more lift to over come added weight as we hold rpm.
During the flare you increase in inflow angle by a massive amount, the weight of the machine apposing the direction change your trying to make forces this angle increase, or eat more air" as Nick said.
I also agree with an earlier post that Coriollis has only minnor effect.
take it easy on me...
The key point is (4) lift aligned more closely with the axis of rotation = less profile drag. .
I heard yesterday that Mikes son Quentin actually did the calculations involved in the Coriollis/ice skaters effect years ago and found them to be infinitesimally small. .
I heard yesterday that Mikes son Quentin actually did the calculations involved in the Coriollis/ice skaters effect years ago and found them to be infinitesimally small. .
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Coriolis Effect
Is it me, or would the panel agree with me that the Coriolis Effect has absolutly nothing to do with Conservation of Angular Momentum? (mentioed here as The Ice Skater effect) Why are we confusing the two?
[Google for Coriolis Effect, but there is a great explaination here ]
[Google for Coriolis Effect, but there is a great explaination here ]
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I always thought of its as an increase of inflow from below the rotor disc by exposing more of the disc to the airflow, thus dropping the relative airflow and increasing the angle between the RAF and the rotor tip plane path. As the lift vector is taken at 90 degrees to the RAF it brings it further forward from the axis of rotation thus increasing the autorotative acceleration and the NR.
could post a picture but can't remember how!
Will probably promote discussion, but that's what we're here for eh?
trying this link:
could post a picture but can't remember how!
Will probably promote discussion, but that's what we're here for eh?
trying this link:
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Leemind, to fully explain how the Coriolis Effect occurs, you need to conserve angular momentum. Are they the same? No, Coriolis is one little effect and angular momentum is an encompassing concept of physics.
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Autorotation
Actually I prefer the 'child's approach, keep a wind mill up and run and see what happens....
I will not get technical with the engineering graphs (already posted a while ago), but allow me the following remarks
First: as many said in this post, the inflow increases (from below) by the different angle of attack of the whole rotor disk, this starts to propel the rotor. This can be explained one level down by looking at detailed air flows around the blades (aerodynamic view point)
Second: the energy building up in the rotor is taken from the descelaration of the heli (as Nick suggests), assuming the flare takes you to horizontal, so the only way energy can come from is from the kinetic energy of the heli, so it will slow down (energy view point)
Third: by the desceleration, and the resulting increased G-forces and disk loading, the rotor will cone more. This coning will provoque a (very) small increase in RRPM, (secondary effect, coriolis)
d3
I will not get technical with the engineering graphs (already posted a while ago), but allow me the following remarks
First: as many said in this post, the inflow increases (from below) by the different angle of attack of the whole rotor disk, this starts to propel the rotor. This can be explained one level down by looking at detailed air flows around the blades (aerodynamic view point)
Second: the energy building up in the rotor is taken from the descelaration of the heli (as Nick suggests), assuming the flare takes you to horizontal, so the only way energy can come from is from the kinetic energy of the heli, so it will slow down (energy view point)
Third: by the desceleration, and the resulting increased G-forces and disk loading, the rotor will cone more. This coning will provoque a (very) small increase in RRPM, (secondary effect, coriolis)
d3
