Auto RPM

[Kota]

Could someone explain to me why,.... once you are established in an autorotation,and you apply forward cyclic,why does the rpm decrease?, and does it remain so until you then apply another input into another control?.

Thanks to those who reply.;)

[Hilico]

Now I'm no test pilot (unless you count those eight years I spent as an instructor at ETPS (no, only kidding)), but as I reckon it, during auto, the weight of the aircraft acts downwards and as a result the airflow through the disc is upward. Increase the weight, more airflow; decrease the weight, less airflow. The airflow is proportional to the rpm. The effect of moving the stick forward is to reduce the g, that is the weight, so the airflow is reduced and the rpm drops. Conversely, pull and watch the rpm rise.

These effects will be the initial ones. After a few seconds, the speed will have changed and the loading will come back to 1g and the rpm should go back to the previous figure.

Now Lappos, Zuckerman, Coyle et al can shoot me down, but I don't care - I've already lost the engine and how much worse can it get?

[helix47]

The change in "G" causes a change in coning angle of the rotor and then coriolis effect (I think ) occurs , as it does with the ice skater. The ice skater is spinning and pulls in "her" ( I don't watch male skaters) arms and the spin rate increases. She puts out her arms and the spin rate decreases.

Put in forward cyclic, the "g" force decreases, the coning angle decreases ( blade C of G moves outwards from the hub) and the blades slow down. Pull aft cyclic, the "G" force increase, the coning angle increases (blade Cof G moves closer to the hub) Rotor RPM increases.

You can get a demonstration by sitting in a swivelling office chair. Start in spinning - pull in your arms & legs and you will get a faster spin rate. Stick your arms and legs out and you will slow.
Most probably, until you get the hang of it, you & the chair will fall over and amuse your audience.
The effect is temporary until the "g" force stabilises again.

[Whirlybird]

In both powered flight and autorotation, unloading the disc, ie moving the cyclic forward, causes the rotor RPM to decrease initially. It will then increase as you pick up speed. If you then flare, you'll get an initial increase in RPM, then a decrease as you slow down. In powered flight the governor usually masks the rotor RPM changes. The reasons have been explained as clearly as I probably could, though I'm sure someone else can add more detail if you want it.

[212man]

I always thought it was (predominently) a function of the autorotaive section of the blades moving spanwise as a result of inflow angle changing.

With flare effect the inflow angle increases so the autorotative section moves outboard, and vice versa. Obviously the corresponding movement of the rotor drag sections combined with the autorotative section will have a dramatic effect on the rpm.

In low inertia head types such as the S-76 or EC-155, abrupt bunting can result in eye watering Nr droop.

[SASless]

Now 212man....abrupt bunting conjures up other concepts that might lead one to forego concern over decreased Nr....particularly in a 212 type aircraft. Bunt too abruptly in one of them and it becomes an issue of having a Main Rotor at all. Is not smoothness in all things to be savoured.....whiskey, female epidermis....control touch ???

[ATPMBA]

In the autorotation if you push the cyclic forward the RPM will decrease.

I believe this is caused by a decrease in the angle of attack. There is now a less component force pulling the blade forward (driving region) and RPM will decrease. Conversely, aft cyclic will cause an increase in the angle of attack and an increase in the component force to pull the blade forward thereby increasing RPM.

Perhaps another way to look at, in an autorotation if you pitched down 90 degrees how much airflow would be coming up through the disk, what would the angle of attack be? I’m sure the RPM would drop way off in that situation.

[212man]

SASless,
I can't argue with that. Unfortunately one isn't always at the controls!

[vorticey]

i think an r22 has the most efficient speed in auto at 53 kts. faster or slower will be a faster decent because of induced and parasitic drag.
initialy pushing the collective forward will reduce the rpm because the airflow through the disc is offset. as the decent increases the airflow through the disc will build up as will the rpm but still some power will be lost to parasitic drag.
flairing will increase the airflow through the disc increasing rpm momentarily but losing speed at the wrong time could be hazardous .

[[email protected]]

Vorticey - have you been drinking? pushing the collective forward? airflow throught the disc offset? que?

Kota - when you flare the aircraft by pulling the cyclic back you reduce the induced flow by changing the angle at which the air meets the blades (relative airflow). This increases the angle of attack and thus the lift across the disc - the blades want to cone up due to the extra lift produced and as they do so the C of G of the blades moves inwards and conservation of angular momentum (the skater analogy) increases the RPM as a result.
The rate of change of the angle of attack determines how much pitch up and increase in rotor thrust (and NR) you get and the faster you are going the more effective it is (ie at low forward speeds a larger cyclic movement is required to produce the same increase compared to high forward speeds where a small aft cyclic input will produce large changes).

When you push the cyclic forward in auto, the reverse is exactly true - the angle of the rate of descent flow is reduced(inflow angle), therefore the AoA of the blades is reduced, they 'uncone' (for want of a better phrase - flap down is probably better), the C of G moves outwards and the blades slow down. As with flare effect the more aggresive the cyclic input the greater the change.
Once you have made the cyclic input and stabilised the attitude, eventually the Nr will recover - you have increased you speed in auto and will experience an increase in RoD which will gradually increase the inflow angle again, restoring the AoA.

[212man]

Crab,
surely when considering autorotation, the effect is far more to do with the movement of the braking and driving sections along the blade, than with c of g movement due to coning angles?

[[email protected]]

212man - the driving section of the blade certainly moves inboard as the inflow angle reduces due to disc tilt but the high AoA in the inboard section reduces which compensates for the increased area of 'dragging' section outboard. Overall I don't think there is sufficient change in the balance between dragging and driving sections to make a difference when all you do is move the cyclic forward.
Your explanation is very pertinent to autorotation at differing AUM or DA where the need to apply collective pitch to contain the Nr will affect the relationship between driving and dragging sections.
However if you believe the theory of flare effect, and that it works the same in auto as well as in forward flight, the converse must be true with forward cyclic producing 'negative flare effect'.

[vorticey]

instead of offset maybe " reduce induced flow" may be corect but i thought offset would be easie to think of. and yes ive just had a couple a tooheys.

kissmysquirrel & [email protected]
of cause there is more than one reason the blades speed up in a flair: one is the c of g changing (wieght moving closer to the centre) and two is the extra driving region area by changing the relative air flow. you can also make the same changes with a heavier weight in the auto.

:ouch:

[[email protected]]

KMS I don't think there is much to disagree with regarding conservation of angular momentum - it is very well documented in the world of physics and it is why a skater speeds up when they move their arms inwards and vice versa - it is also the cause of precession in a gyro.
I would love to hear Mike Smith's alternative explanation for Nr rise in the flare, perhaps you can get him to post it here.
I'm sure it is very small distance that we are talking about the C of G moving but when something has a great deal of momentum ie a heavy blade travelling at high speed then small movements can create large speed changes. Try asking a skater to spin holding 20kg dumbells outstretched and the see how far they have to move their arms in to achieve their normal high rotational speed - a lot less than without the dumbells.

Vorticey - all the changes are a result of changing the inflow angle and thus the AoA of the blade - the AoA change moves the driving section and the flapping resulting from the AoA change moves the C of G.

[Head Turner]

From what I was taught and subsequently discovered and taught, the explanation that Crab gives is the most logical and easily understood. So you guys, go for that.
In all these things, just make sure you have the RRPM there ready for the landing.

[vorticey]

you dont seem to be dissagreeing with me but wouldnt the changes be made to induced flow (inflow angle is relitive to the rpm)? and coning would be the result of the greater angle of attack. you agree that the driving section gets bigger, so it must also increase the rpm (along side preservation of angular momentem).:ok:

[[email protected]]

Vorticey - no I said the driving section moves, not gets bigger. The sequence is cyclic forward - disc tilts - inflow angle reduces - AoA decreases - less lift so blades flap down (decone) - C of G moves out - blades slow down.

Admittedly this is a simplification as the advancing and retreating blades will be experiencing different inflow angles but is an acceptable explanation of what happens.

bye the waiy verticee is yowur spill chocka forked?

[Kota]

Thanks for all those who replied to my question on Auto Rpm.I am now a little more clearer on what happens.I have my Instuctors(CFI) Flight test Monday the 19th,so if my examiner asks me that one I now have enough info to get me through.Thanks everyone. :O :ok:

[212man]

I'm afraid, rightly or wrongly, I can't believe that coriolis effect is the prime cause.

Consider the following: If one is simply referring to conservation of angular mamentum, then a 10% rise in rpm would require 25 degrees of coning angle. A 10 degree coning angle would increase rpm by only 1.5%. Clearly these figures don't tally with reality.

If you look at a rigid headed a/c there is practicaly no coning o speak of. Even the 155 with an articulated head has very little coning, yet has lively Nr in Auto. The 212 has pre-coning bulit in and again shows little variation.

The effect of coriolis must surely be primarily of interest when considering the relatively smal lead lag forces rather than on those required to vary rpm.

The variations in spanwise location of driving and braking elements will have a huge effect though, as there is a square law involved, rather than the linear law for angular momentum.

I think I'd prefer to see more definite proof before becoming converted!

[[email protected]]

212man, where did you get your figures from? If you are correct then there may well be more to it than just CoAM, however I think you are wrong about rigid rotors not coning - the Lynx has a 2 piece titanium head with a massive effective hinge offset and the blades certainly do cone (I have lots of photos of it in flight).

CoAM is the cause of precession in a gyro and they do not cone at all - the movement is produced by the mass of the rotor trying to move relative to the spin axis.

[212man]

Crab,
if you consider the disc to be flat to begin (I know not correct but easier to explain and should not affect the figures), and the centre of mass of each blade to be at radius R from the centre.

As there is a linear relationship between angular momentum, rotational speed and polar moment of inertia, then if the rotaitional speed increases by 10%, the polar moment of inertia must decrease by 10%. As the mass does not change, this requires the effective centre of mass to be at 0.9R from the centre of rotation (okay, 0.909).

To achieve this, the blade must rise by anticos(0.9) degrees, which is 24.6 degrees.

Similarly, if you raise the blade by 10 degrees, the affective radius becomes 0.985R which would give a 1.5% rise in rotational speed.

This could be entirely flawed logic of course!

I don't dispute the Lynx has coning; of course all a/c have it to varying degrees, but these angles are very large.

What I actually believe is that there are a range of influences simultaneously acting to varying degrees. However it is far too complicated to explain them to student pilots, and so the CoAM theory is used as a simple explanation.

[SASless]

I always heard that one teaches best what one knows the least about. If an instructor cannot understand nor explain what he is putting forth.....should he do so in the first place? Also....is there not a point beyond which "one does not have a need to know" and thus such philosophical discourses could best be left to the bar at the purchase of the nineteenth pint of the evening? This discussion is about as useful as memorizing the number of rivets in a UH-1 (Bell 205) tailboom for use on checkrides.

[212man]

Mr. Selfish,
angular momentum is the product of the polar moment of inertia (sigma mass times radius) (sorry, can't do funny symbols here!) and the angular veocity in radians per second. Therefore the relationship between angular velocity and effective radius should be linear for a given angular momentum. We are not talking about airspeed and drag.

SASless, I take the point; call it idle curiosity!

[[email protected]]

I think this is where I make the standard CFS disclaimer that P of F is not an exact science, rather it is an acceptable explanation of something that can be seen to occur eg Nr rise in the flare.
To prove or disprove the CoAM argument will probably take pages of greek flute music (algebra) that would only make my brain melt. Equally to prove or disprove your theory of spanwise variation of driving and dragging sections would require flight test data, knowledge of aerofoil section, washout, taper, lock number, AoA, pitch angle, air density etc etc etc. ..... so I suggest we ask a test pilot - Nick are you there?

[212man]

Crab, I agree!

just to add a further dimension to the topic: why should there be conservation of momentum anyway? In autorotation there is a total energy concept, with rotor energy being one of the elements. By flaring or bunting you are changing the energy distribution between the rotor, airframe velocity and rate of descent, so there should be no real reason to assume that the roror enegy stays constant. Indeed, surely this is one reason for the flare at the bottom; to arrest the descent by transferring the enrgy to the rotor? (again, not exactly true, but generally so).