Is the theory of Autorotation,driving-driven region outdated?

[slowrotor]

This idea of driving the rotor from the inner region cannot be true.
After considerable thought on the matter, I say that the entire outer lifting portion of the rotor is a gliding wing section, and as such, provides for its own rotational thrust in the same manner as any glider.
I boldly claim that there is no such thing as a" driving" and "driven" region on a vertical autorotation.

Are the textbooks outdated?

[Hueymeister]

Why is it easier to overspeed the head in auto at hi Density Alts? Is it because more of the inboard of the blades are stalled therefore producing less lift and less drag, and allowing the outer dragging section to overspeed the whole chebang?

[SEL]

No, the textbooks aren't outdated but most of the texts used for PPL, ATPL etc are a tad simplistic on some issues. It is true about the different regions, its a balance between the blade section speed and the induced velocity distribution.

I think I know what you're getting at, a glider in descent, put simplistically, relies upon its downward and forward motion. The pilot can alter the angle of descent to maintain this balance. However, in a descending rotor, each section of a blade has a different angle, due to blade twist, rotational velocity and induced velocity. For the descent conditions mentioned, the combination of these variables means the inner portion gives the best 'gliding' results. Afterall, the pilot can only control the blade angle, which is constant along the span. This also means that due to the same variables, some portions are not in at the 'ideal' settings.

If this isn't what you mean, sorry!

As for density altitude, the only difference is that at altitude the air density will be less. In a broad approach, the profile drag is less thus allowing the blades to accelerate compared to the same conditions at a lower height where the density is higher.

Hope it helps.

Steve

[slowrotor]

This idea of driving the rotor with the inner region doesn't work in my mind, here is the logic.
The central hub area is completely stalled and the blade area some distance out from the hub is likely at a very high angle of attack close to stall as well. So it would be unlikely that the inner region could extract any energy from the airflow and then transfer that energy to drive the outer rotor. It just does'nt work that way, the outer rotor would need vastly more power because it goes twice as fast and therefor has the majority of the drag.
So how could the inner rotor drive the faster tip area?

[Jcooper]

Im not positive here but the inner portion does not drive the rotor, the outer portion at the tip does.

If I remember from Wagontok (spelling) correctly about the inner 1/4 of the blade is considered stalled, the middle half is considered driving and is creating much of the lift and moving the rotor, and the outer 1/4 is the driven region which produces lift and drag. Also a stalled airfoil will create much much more drag than a airfoil which is not which means the inner region would resist the movement create no lift, the middle would drive it and create lift, and the outer portion would make produce lift with its drag. That was only for a helicopter falling straight down and I believe if forward airspeed is used all the regions move forward and to the retreating side.

Again I dont have my aerodynamics book with me but that sounds more correct than the inner portion driving the rotor.

[Ascend Charlie]

What a waste of electrons this thread is!!

One person can't remember how it works, somebody else tries to remember what a book once told him, and you all have it wrong anyway. None of you could be bothered looking in a book, you just toss a bait into the ether and say "The theory is wrong because i can't understand it" and hope that Nick Lappos will write a page or two to remind you how slack you are.




Look in a book. And it's spelt Wagtendonk.


:*

[Dave_Jackson]

Any good Russian books?


The coaxial uses differential collective for yaw control. What are the aerodynamics at the rotors that caused Kamov to incorporate automatic pedal reversal for autorotations?

Just before the autorotating coaxial touches down, the pilot raises the collective to consume some of the rotational energy in the rotors. Does the helicopter get schizophrenia as its pedals try to decide which direction of rotation they are responsible for?


:D

[SEL]

You are right that the central part of the rotor is stalled. There may some confusion over terminology here. So here goes.. If we ignore geometric blade twist for now and look at the rotational velocity of an element and the inflow to the rotor, these factors combine to give the angle of attack, for a given blade angle. If, for simplicity, we consider a uniform inflow over the disk, then we need only look at the effect of the rotational velocity along the blade. At the hub centre it is zero and increases linearly to the tip. With the net inflow coming from beneath the rotor and the rotor speed horizontal, the resultant airflow onto the blade element is angled from below. As the blade element velocity increases along the radius, the angle of attack reduces. This means that as the centre region is stalled, as we move along the radius, the angle of attack reduces. As you pass through the mid sections of the blade the angle of attack allows the total reaction to be tilted forward driving the rotor. As you close on the tip, the blade element velocity increasing, the total reaction moves past vertical and therefore acts to slow the rotor.

Hope that makes it a little clearer. As for the negative comments above, well there is no such thing as a bad question. As for my knowledge of helicopter aerodynamics, well, I think I can safely say that I have read a one or two books on the subject. As for the suggested title, it is okay but does muddle a lot of different theories together and in some instances makes some serious mistakes. Look for any of the ‘Helicopter Aerodynamics’ books by Ray Prouty or ‘The Helicopter’ by John Fay. (If you’ve got good maths knowledge, at least A-Level, try ‘Fundamentals of Helicopter Aerodynamics’ by S.J. Newman , ‘Aerodynamics of the Helicopter’ by Gessow and Myers, its old but still good or ‘Helicopter Performance, Stability and Control’ also by Prouty).

Good question about co-axials. With co-axial rotors the yaw is via differential collective, increasing on one and decreasing on the other to maintain constant thrust but have different rotor torques producing yaw. In autorotation, with power off, the aerodynamics described above come into play. The yaw control is adjusted for the power on condition where the lift force on a blade section is tilted backwards and the torque has to overcome this and the other sources of drag. In autorotation, the lift force is tilted forwards, thus the torque is now coming from the lift. This means the yawing moment, via the pedals, is set for power on whereas in autorotation where the lift forces have changed direction, results in a control reversal. Does that make any sense to you?

Steve

[Flingwing207]

I was talking to a K-Max pilot who told me that the pedals are rigged to reverse controls when the collective is down (as it would be for an auto). To be honest, I don't know if this is true or not - I would think it could lead to a few interesting moments no matter what...

[Dave_Jackson]

SEL,

Thanks for the response. Yes, what you say makes sense.


Flingwing207,

A Kaman pilot and former poster on PPRuNe mentioned that things did get interesting when the collective was at low settings.
The Kaman helicopters use opposed longitudinal cyclic and differential torque for yaw control. Unfortunately, the coaxial only has differential torque to work with.

[slowrotor]

SEL,
Thanks for your interest and views.
I do have Prouty's Helicopter Performance Stability and Control, and his definition of vertical autorotation is: "At some rate of descent, the forward tilt of the lift vector is equal to the drag component. This is the condition of autorotation, since no torque need be applied to maintain rotor speed. In an actual rotor, some blade elements will have more drag than the forward component of lift; but on other elements the situation will be reversed".(ref. pages 96-97)

I note that Prouty has no mention of "driving-driven region" and hence my original question "is the driving- driven theory outdated"?

Your description is good up to the point when you said "as you close on the tip, the blade element velocity increasing, the total reaction moves past vertical and therefore acts to slow the rotor."
The forward lift vector would be maximum at the tip in my view, so I guess I disagree at that point.

As for the negative poster above, I feel that the discussion and debate of aero science is what this forum is all about.

[Capn Notarious]

Is the German pilot.



OTTO ROTATIONS.

I used that in the Pprune chat room last night. Even I a non pilot knew the spelling. With a shelf full of heli books..............

[SEL]

Most of the books I mentioned do not go into the regions of the rotor in autorotations. Newman mentions it, as I recall and you know what Prouty says. One point as to why most texts do not go into 'regions of the rotor', is that it suggests a steady-state situation. Most helicopters autorotate within Vortex ring/Turbulent wake states, not Windmill brake as I've heard mentioned. As such, the flow through the rotor is not as straight forward as to have certain regions doing the driving and others not. As a simple model, to offer an explanation, what I've said above broadly approximates what is happening. With that model, which is a blade element approach, drawing the diagrams shows that with an increasing rotational speed, the inflow angle reduces, the airflow becomes more horizontal and with lift perpendicular to it, it will tilt back. It is crude approximation bearing in mind the real changing flow state which is why it doesn't appear in most texts, in my humble opinion.

Lets just be glad autorotation works!! Most plank drivers think helis fly on black magic anyway, which isn't far off..

[Dave_Jackson]

slowrotor,

The following is extremely simplistic, but I think that it is basically correct, and it should support and supplement what SEL is saying;

One of the latest books on rotor aerodynamic is Principals of Helicopter Aerodynamics, by J. Gordon Leishman. He still uses 'Driving region' and 'Driven region' to explain autorotation. He says "At the tip of the blade where the induced angle of attack is low, these sections consume power because, as a result of the forward inclination of the lift vector, the propulsive component is insufficient to overcome the profile drag."


The key phrase above is 'profile drag'.
On a plane, if the wings are physically consistent from root to tip, they will have a consistent; lift, induced drag, and profile drag, from root to tip.
On a hovering helicopter, if the blades have a specific twist, they will have a consistent lift and induced drag, from root to tip. But, the profile drag will not be consistent from root to tip. It will be greater at the tip because the tip is experiencing a faster airflow. Therefore the middle portion of the blade must contribute some of the forward component of its lift to help the outer portion of the blade overcome this additional drag.

_____________________

Just for the fun of add verbiage;

The phrases 'Driving region' and 'Driven region' may be a little ambiguous, since what is driving and what is being driven, the air flow, the rotor, the craft? In addition, Leishman's statement "these sections consume power" could be superficially construed as 'the rotor consuming potential energy from the elevation' or as 'the craft consuming kinetic energy from the rotor'

Helicopters will have negative blade twist at angles up to 12 or more degrees. Gyrocopters, which are designed to operate only in autorotation, have a positive twist of about 1-degree.

_____________________

If any of the above is wrong, please beat me up.

[Flingwing207]

I would think for this scenerio (autorotation) driving = that which makes the rotor turn (or accelerate), and driven = that which would make the rotor stop (decelerate). Simplistic, yes.

[slowrotor]

Dave Jackson,
The quote from Leishman is right at the heart of my question, thanks.
But I still don't accept the explanation just yet.He uses the phrase"induced angle of attack", not sure there is such a thing. The "induced drag" is low at high speed. What he appears to be saying, I think, is that the tip region is going above terminal velocity,defined as: that speed at which profile drag exceeds the forward lift force of the airfoil.That could be the answer Dave!
I am unfamiliar with the dynamics of gliding wings at 450 mph, so that may be what I need to study. That may explain why autorotation performance is improved at the lower tip velocity.(550ft/sec according to Prouty)

The Leishman explanation is helpful but a little confusing.
I'll beat you up some other time, Thanks Dave

[Dave_Jackson]

Slowrotor,

My mistake.
Leishman actually used the Greek symbol phi, which Prouty defines as 'inflow angle'. I thought of using this popular meaning, and then felt that it was Lieshman's sentence, so I might as well use his meaning.

Should have just left the phi in the sentence. Sorry.

[SEL]

Slowrotor

That is the answer.

I finally understand, I think, what you've been getting it. The high speed outboard sections on the rotor have the highest profile drag. Now, you stated that at high speed the induced drag is less, that is the crux of the problem. The induced drag is that component of the lift in the horizontal. In autorotations, it is the same component that provides the driving force for the rotor. As you say, it is less at higher speeds, so, as you move outboard along the rotor, the profile drag increases while the horizontal component of lift reduces. I think my explanation, with the lift vector tilting backwards can be ignored. This may very well never happen. More important is the fact that at some point along the blade span, the profile drag will exceed the driving force. From this you can also see that the blade mid-sections driving force is bigger than the profile drag there. This 'surplus' is used to overcome the drag near the root and that at the tip.

Dave

I'd completely forgot about Gordon Leishman's book! Thanks for reminding me. Yeah, phi is always the inflow angle.

[DaveKnell]

Quick question, if I may. Consider a helicopter in autorotation; as we should all know, each rotor blade has, as we come away from the mast, a stalled region, a driving region and a driven region.

All other things being equal, does the boundary between the driving and driven regions move inwards or outwards along the balde when the pilot lowers the collective, and why?

--Dave

[griffothefog]

This is a rumour network isn't it..... :{

They got books for that stuff.

[Flyting]

GOOGLE is your friend.....!

By controlling the size of the driving region, the pilot can adjust autorotative rpm. For example, if the collective pitch is raised, the pitch angle increases in all regions. This causes the point of equilibrium to move inboard along the blade’s span, thus increasing the size of the driven region. The stall region also becomes larger while the driving region becomes smaller. Reducing the size of the driving region causes the acceleration force of the driving region and rpm to decrease.

and that took 0.14 seconds :D:D:D

[Rotorwashed]

The driven region is the part of the blade that is producing more drag then lift. as you lower the angle of attack, you are decreasing the amount of drag that the blade is producing.

So, as you drop collective, you are producing less lift, but also less drag. the driving region increases in size and rpm's increase because there is more force driving them then there is drag slowing them down. Of course, the opposite is true when you raise collective. More lift, and more drag. The driving region shrinks as the driven region grows resulting in rpm's being lost.

If you are autorotating with a constant rpm, then the driving region is spinning the rotors up at the exact same pace as the stalled and driven region are slowing it down.

(i think;))

by the way, welcome to the forums! (although I just noticed you joined before I did...oh well)

[rotorfossil]

When you raise the lever in autorotation, the inner boundary between the high AOA or stalled section and the driving section moves outboard. The outer boundary doesn't move much hence the driving section is smaller and the RRPM reduce. The next question that usually gets asked is why do we reduce the RRPM to the lower end of the band to reduce R of D and increase range. The answer is that the overall L/D ratio of the whole rotor system is usually arranged to be better at the lower end of the band, beneficial if you overpitch and regardless of weight, you can always achieve it in autorotation by raising the lever.

[DaveKnell]

So..

Griff - I have two books with two contradictory answers, so I figured that there was at least some rumour involved here ;-)

Rotorfossil says that the boundary stays roughly put. Flyting and Rotorwashed (from Wikipedia) that the boundary between driven and driving regions will move outwards as the collective is lowered, thus increasing the size of the driving area. Which makes some sense, as collective lowered->more RPM.

But I don't think this second view is necessarily correct. The angle of attack reduces along the blade from root to tip, and it is this reduction that produces the stalled/driving/driven regions. The point of equilibrium occurs at a particular angle of attack so, if you lower the collective, this point will move inwards along the blade. If that's not clear, consider the original point of equilibrium - its angle of attack is reduced once the collective is lowered, and this will push it into the driven region.

Given this, the increase in RPM associated with lowering the collective must be either the result of the reduction in size of the stalled region (which I'd not considered - thanks, RF) and/or an overall reduction in drag from the reduction in pitch along the blade.

What have I missed?!

--Dave