RRPM increase in the Auto flare...

[TheFlyingSquirrel]

I'm interested to see what gets posted with the question. For what reasons does the RRPM increase during the auto flare ?

Thanks in advance !

TFS

[407 too]

rotor disk exposed to a larger amount of inflowing air

[Hilico]

Increased coning angle due to increased loading, RRPM goes up (converse also applies).

[R1Tamer]

The effect of the flare in autorotation reduces the forward speed, as it does in powered flight, by tilting the Total Rotor Thrust away from the direction of flight. This gives a horizontal component of total rotor thrust, rearwards, to decelerate the helicopter. The other effect of rearward tilt of the rotor disc in the flare is to cause the inflow angle to increase. This gives an increase in the angle of attack, which produces an increase in total rotor thrust. The vertical component of this increased Total Rotor Thrust will, during the flare, increase to a value greater than the weight, and consequently the rate of descent will decrease. The increase in Total Rotor Thrust will increase the coning angle and Coriolis effect will cause an increase in R.R.P.M. Also the increased inflow angle causes the autorotative section of the blade to move outboard, resulting in a substantial increase in Rotor R.P.M. It will be seen that the larger the flare, that is the greater the change of disc attitude, the greater will be the RRPM increase, rate of descent decrease and the more rapid the reduction in forward speed.

How did that sound?

[HELOFAN]

TEXTBOOK !!

:}

[R1Tamer]

Yep,

One thing for sure - it didnt just trip off the tongue

:O

[Eurochopper]

IMHO the coriolis effect caused by the increased coning angle is an insignificantly small component of the rrpm rise.

The main one is the increased angle of attack (resulting from increased air flow up through the disc). The angle between the relative airflow and the total reaction (total rotor thrust if you like) remains roughly constant for a given set of conditions - it represents the lift:drag ratio of the aerofoil. So if the angle of the relative airflow increases, this tilts the total reaction further forward (ie in the direction of blade travel) and this pulls the rotor blade along harder - ie the rrpm increases.

The converse is that in the hover, you are flying in a descending column of air - ie air is going down through the disc and that tilts the total reaction backwards, tending to reduce rrpm unless you have lots of power on!

EC

[eagle 86]

Conservation of angular momentum.
GAGS
E86

[Eurochopper]

E86

Conservation of angular momentum = coriolis effect. So what's your point?

EC

[eagle 86]

Just giving it another name - bit sensitive aren't you ec?
As I said -
GAGS
E86

[cl12pv2s]

For what reasons does the RRPM increase during the auto flare ?

Here's one which has caught instructors out.

How about...

Student inadvertantly rolling on throttle and *raising collective!!!! Arghhh!!!!

Particularly in R22, this can be very problematic. The correlator, govenor, increased disc loading (Conservation of angular momemtum or Coriolis - - whatever you call it) and low inertia can all work at the same time to bring up that RRPM, and before you can say,"Jack Robinson!" you have an overspeed to explain to maintainance.

I'll admit, it got me once some time ago, with a commercial student who had just joined to continue training with us/me. What it boiled down to was a difference in my timing and technique to what he'd already been taught. Very frustrating...but at the end of the day, as PIC (and instructor) my fault.

Lesson: Guard the collective and throttle like a hawk until you are sure you are familiar with a new student. (Even one with hours of experience.)

Hope this helps someone out there!

cl12pv2s

*Before anyone tries to flame me for this...(Of course I know raising would normally reduce RRPM)...if raising of the collective from flat pitch (full down collective) the correlator will raise RRPM faster than the increased blade pitch will have any effect in reduceing it. (This is demonstrated in the initial pickup and is the reason for the governor to be on then.)

[996]

If during the flare the RRPM increases and does so at a rate that the uppper limit is judged likely to be exceeded then a short sharp correction [control] by application of collective can be used to prevent the upper limit from being exceeded. However by carrying out this corrective action and following removal of the collective correction, the resultant RRPM will settle at a lower value than it was at prior to the flare being initiated. Lower RRPM is not desireable so how about use of secondry effect of cyclic control [push forward to achieve a higher ROD and fWD speed] ? Whaddayathink? Of course this is only a viable option at altitude.

[Schmitfire]

Something makes me think that Eurochopper doesn't know what he is talking about

[R1Tamer]

Eurochopper,

What! Come again! Eh! Care for a re-think!

R1tamer

[krobar]

Feeble attempt at scientific explanation, but potential energy (height) is converted to kinetic energy (falling outta sky, aka autorotating), and then back into potential energy in the rotor blades during the flare.
Its all relative... :8

[Eurochopper]

R1T and SchmitF

Give me a clue what it is you disagree with? Or to put it another way, why not contribute something more interesting than a snipe?

996 - sure, moving the cyclic forward reduces the disc loading and the air flowing up through the disc, so the rrpm will go down but only so long as the reduced g is held - so its only a short term solution and the longer term solution is to increase collective pitch.

EC

[helmet fire]

Lu, where are you mate? this must be a good giggle for you up there, and it s threads like these that we really miss you.

I can hear "centrifugal force" from here1;)

Prune fan loves this one too!

Try a search on this one fellas.

[NickLappos]

Actually, the rpm rises because the rotor thinks the weight of the helicopter went up - the positive G that builds because of the increased thrust is exactly the same as if the aircraft suddenly got heavier. If you pull 1.2 G as you flare, and you do, that is as if you added 20% gross weight, as far as the rotor is concerned. This means the trimmed rotor rpm must be higher, too (rpm goes up as weight goes up, for a given collective pitch position).

Another way to look at the same effect is to realize that the inflow velocity is raised considerably. It rises because we force the rotor to eat more air, which propels the system to higher rpm's.

[Johe02]

The gospel according to Mike Smith, Heliair. .



http://www.a2b-web.co.uk/pics/Flare.jpg


Oh, and if you are an instructor, listen to cl12pv2s and watch for those students rolling on the throttle with their 'death grip'. .


:ok:

[platinumpure]

Why not just read priciples of Helicopter flight. Great book which goes into quite a bit of detail. Excellent book for instructors and commercial students. Maybe a little to heavy for primary guys.

I used to explain it in terms of enery management to students to keep it simple. I do agree with Eurochopper and Mike Smith in relation to the airflow. However I feel it is easier to explain to new students in energy terms.

i.e. ; Your coming down in your glide with lots of airspeed and altitude stored. When you get to the flare you start to bleed off that airspeed (energy), and that rate of decent (energy) this energy has to go somewhere it doesn't just disapear. Therefore it goes towards the RPM (causing the RPM rise as it now has more energy than it had before). Blade coning really isn't that large a factor in RPM rise.

Now sure this isn't a scientific explaination with lots of vector and airflow diagrams. However in layman terms it is why the RPM rises in a flare and it is far easier to grasp when you are actually doing the auto. I have never flared and thought of a vector diagram, however maybe I'm abnormal. I'm always thinking about how much energy I have left to use to keep me unbroken.

[407 too]

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

[Simon853]

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.

?

Si

[Eurochopper]

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

[eagle 86]

Ballet dancers.
GAGS
E86

[petitfromage]

Dont you mean ice skaters?

[NickLappos]

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!

[[email protected]]

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.

[Johe02]

Eurochopper

Same principles apply. . the extra "induced flow upwards to counter the induced flow" comes from the forward speed. . etc

[eagle 86]

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

[skitzs]

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... :O

[Johe02]

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. .

:8

[leemind]

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 (http://www.physics.orst.edu/~mcintyre/coriolis/) ]

[wobble2plank]

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?

:ok:

trying this link:

http://www.helosim.com/images/Fig_2-85.gif

[Matthew Parsons]

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.

[delta3]

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