Autorotation Info

Dean_Dolph

Newbie
PRA Chapter 62 has a 30 -45 minute mentoring session before our meetings for people new to gyros. Those doing the mentoring are not experts by any stretch of the imagination but being an expert is relative to your audience.

We are fortunate to have a member with several thousand hours of helicopter time, and is a FAA certified ground school instructor, lead these sessions. However, we have been struggling to give a good explanation of autorotation and I would like to know if the following link is the best we can do.

http://www.pilotfriend.com/training/flight_training/rotary/gyro_aeronaut.htm

Thanks for any help since references on gyro theory are far a few between.

skier

Member
I would try looking at some theory on the aerodynamics of wind turbines. There seems to be many more publications on wind turbines than gyros and the phenomenon are very similar.

birdy

Newbie
being an expert is relative to your audience.
PRICELESS!!!!!!!!
Mind if i use that occasionally Dean.

birdy

Newbie
As if that wasn't unusual enough in itself, these manoeuvres are also performed without any of the gut-wrenching changes in G force "enjoyed" by fixed wing pilots
The G forces have nuthn to do with the rate of rotation or bank angle, but the speed your traveling at.
A 180 split turn witha 90* bank in 1/2 a second wont produce much G force at 30mph, but at 100mph, itll shoot you through the seat.

The windmill was probably the first human invention which used autorotation,
Incorrect.
Windmills dont auto rotate, they 'sail', at a negitive AOA.

The principle here is the same as with a sailing ship which can 'tack' close to the wind, meaning it can move forward against the wind,
This is an incorrect annalogy.
The sails ona boat are rigged to the boat, thats in water. If it wasnt for the resistance of the water, the only direction a boat would sail is directly down wind.
A boats sail is just a streight line mill sail, set at a - AOA.
Put a + AOA ona boat sail and it will pull the boat backwards.
Autorotation needs a positive AOA, outherwise its just a sail.

The easiest explination iv had to date wenever sumone asks me how it autorotates is, effective lift is always at 90* to the airflow. The blade is 'sucked' forward and up.
Then you just have to explain how air flows over a rotating gyro blade.

BTW, im assuming i have a dumb audience.

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choppergabor

Newbie
The windmill was probably the first human invention which used autorotation,
Incorrect.
Windmills dont auto rotate, they 'sail', at a negitive AOA.
Absolutely correct!!!!
Although I do not have thousands of hours of helicopter time..... I only survived a few bastards trying to kill me ROFL

skier

Member
I'm not sure what differences you guys are getting at between autorotation and the phenomenon that wind turbines use. Many modern wind turbines do have positive angles of attack with the wind.

Whatever difference you are trying to get at, in both cases the resultant force from the airfoil is shifted forward causing the blades to rotate into the oncoming wind. The may be optimized for different purposes, but to my knowledge the same theory can be applied in level flight. With wind turbines, the power output is related to the forces/moments causing the blades to spin, while with autogyros, the lift force we're interested in is related to the other component of the resultant forces on the airfoil.

That being said, wind turbines don't suffer from retreating blade stall or other issues with forward flight.

Sir Real

Member
Birdy's analogy is basically what finally gave me the breakthrough, after over a year of trying to figure it out. It's the vertical component of lift that does it. The problem is that I, like most people, was seeing the angle of attack as parallel to the plane of rotation. Try this; it's what finally made it click for me:

Draw an airfoil parallel to the bottom of the page. Draw a line towards it, representing the airflow. Then draw a line straight up from it, representing the vertical component of lift. That's the baseline, and most, if not all FW pilots get that. Reinforce that lift is ALWAYS 90 degrees.

Now do a second drawing, with the exact same airflow and lift lines parralel & perpendicular to the bottom of the page, but with the airfoil tipped up to represent the AOA of the advancing blade. Now you can see that the airfoil is pulled up AND forward by the vertical component.

If you want to be tricky, you can also make the second drawing with the airfoil level, but the airflow coming up at the same angle as the second drawing, and show the vertical component going off at a forward angle. Or just use the second drawing and tilt the page...

That's how I finally got it, anyway...

Passin' Thru

Newbie
What it is, is MAGIC!

What it is, is MAGIC!

Dean, here's something I swiped from Dr. Bensen years ago.

What turns the rotor in autorotation?”
This question is asked in million variations by the beginners, who insist that it should turn backwards since its blades are set at a positive pitch. This “common -sense” conclusion is not borne out by the nature’s behaviour. True enough, if a rotor were to start from a standstill in a vertical descent it would begin to turn backwards, and the airflow pattern through it would then fall in the category of “wind-milling” which we will describe later.
But if the autorotation had begun in the right direction previously, the rotor will continue to rotate in the same direction even in a vertical descent. One may say that the same kind of forces push the blade forward that enables a sailboat to make headway against the wind. More precisely, these forces combine into a vector diagram that is shown in figure 2. The important ingredient is the velocity vector Vr, rotational speed of the airfoil, which is considerably larger than the inflow velocity Vi. When they combine, they produce the total velocity vector Vt that acts upon the airfoil at a fairly shallow angle.
Constructing now the Lift and Drag force vectors (perpendicular and parallel to the impinging air), we see that the resultant force R lies ahead of the axis of rotation of the rotor. The bulk of the force is transmitted through the main bearing to the Mast as a part of the total lift LT, but the small vector component of it, F, remains. This vector F in fact is the force that acts upon the airfoil to drive it forward.
Not all sections of the rotor blade have the same vector diagram since the magnitude of the Vr varies with the radius of the blade. Thus toward the tip for instance the angle between Vt and the airfoil becomes so shallow that the driving force F becomes zero or even falls behind the axis of rotation. In the latter case then it tends to decelerate the blade and consumes the driving power supplied by the inboard sections.
Much further inboard, on the other hand, Vr becomes small compared to Vi, and the angle between Vt and the airfoil becomes large enough to cause the airfoil to stall. As you know, the stall is characterised by a sharp decrease of L and increase of D, which can again reduce F to zero and even make it negative. Thus the inner sections of the rotor blade may consume the power supplied by its middle sections.
Two more things are worth mentioning before we leave the subject of autorotation. One is the observation that Vi can be at almost any other angle than shown, including vertical, or parallel to the axis A. So long as it is in generally upward direction and its magnitude is relatively small compared to the rotational speed Vr, it will always combine into a Vt that will sustain autorotation. In practice, the rotor RPM of modern Autogyros shows very little variation when airspeed is reduced from cruising speed to vertical descent.
The second noteworthy observation is the equilibrium diagram included in the Figure 2. It shows the location of the centre of gravity of the gyro with respect to the lift vector and the propeller thrust. Ideally all three force vectors, Lift, Thrust and Weight, should intersect at one point, which is the craft’s centre of gravity. When the engine is shut off, its thrust T becomes zero, and in vertical descent the lift vector Lt must become vertical to directly oppose the Weight W.
Much more can be said about autorotation, but it has more to do with forward flight and we will return to it later.
5.WINDMILLING: The chief characteristic of the windmilling state is the extraction of power from the rotor. Windmills have been used by human beings long before aviation was born, to mill the grain, to pump water and do a host of other physical chores. It’s a wonder why nobody thought centuries ago of tilting a windmill rotor on its side and making it into a kiting gyroglider!
Blade pitches on windmilling rotors are generally set at a considerably negative pitch, and such rotors will not turn fast even when unloaded. Tip vortex vanishes completely. The rotor acts more as a perforated drag plate than a rotating body, although here too, the centre area offers less resistance to airflow than the tips.

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birdy

Newbie
but with the airfoil tipped up to represent the AOA of the advancing blade.
The blade tip sees the same AOA all the way round the disc. [ with very slight variations due to advancing and retreating, but still positive.]

Many modern wind turbines do have positive angles of attack with the wind.
You sure?
Iv never been up close to one of the megawatt ones, [ and if i remember correctly, they are indervidually featherable] but my little 1000W one has a [slight] -AOA.
Symetrical glass 2 bld that dose a zillion rpm ina dustorm.

Iv often thought of putn a +AOA gyro blade on it, but getn it to start in the rite direction would be hard, but not impossable.
Ill try it oneday, just i have more pressing issues rite now.

GraemeClarke

Member
Can anyone tell me how verticle axis windmills work? Here you have an advanceing and retreating blade simular to a gyro.
Graeme

skier

Member
The book I have on the Aerodynamics of Wind Turbines gives the Nordtank NTK 500/41 wind turbine as an example. It's a 3 blade 500 kW wind turbine and the blades are twisted 20 degrees at the root and 0.02 degrees at the tip. When in operation at 27 rpm, the turbine sees something like a 4 degree angle of attack from the root all the way to the tip.

C. Beaty

Gold Supporter
Autorotation is not complicated but we clutter it with too much extraneous stuff.

When we fly a kite with a lift/drag ratio of 1:1, it flies with a string angle of 45º (ignoring weight and aerodynamic drag of the string).

The string incorporates both lift and drag but its pull is in a single, solitary direction.

So it is with an airfoil moving through the air but with a much better L/D ratio than a kite.

An airfoil produces a thrust vector that is slightly greater than 90º to the airstream, depending upon its L/D ratio. It is 90º + atan1/(L/D). If the L/D ratio was 19:1, the rotor’s thrust vector would be at an angle of 93º. If it had an infinite L/D ratio, its thrust vector would lie at 90º to the airstream.

Portions of a rotorblade may have L/D ratios of better than 20:1.
I used L/D of 19:1 in the illustration so that the numbers stay simple.

Also, only vertical descent was considered in order to keep the trig simple. It is pointless to get bogged down in oblique triangles.

Choosing a vertical descent velocity of 21-fps (1260-fpm) gives an inflow angle of 4º. That gives whole numbers but it works for whatever reasonable inflow angle.

If the thrust vector points ahead of the axis of rotation, an accelerating force is produced. If it points aft, a decelerating force is produced.

An autorotating rotor always finds its equilibrium speed; a net accelerating torque speeds it up which in turn reduces the angle of attack and diminishes the accelerating torque.

Inboard portions of a rotor operate at a higher angle of attack and supply power to the rotor. Outboard portions operate at a lower angle of attack and extract power from the rotor.

The basic physics of autorotation is no different for sailboats, gliders and windmills.

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RotoPlane

Gold Supporter
I'm glad I stopped by to read this. That is the simplest, most straight forward explanation of autorotation I have yet read…and that includes the diagram! Saved it….thank you Chuck!

Sir Real

Member
Unfortunately, it was descriptions like Chuck's that slowed me down for a while. Absolutely correct and accurate, but when you start throwing Trigonometric functions & variable equations into it, it's not for beginners anymore. KISS. That kind of mathematical description can scare people away, or at least confuse them.

At least, it does me...

Dean_Dolph

Newbie
Unfortunately, it was descriptions like Chuck's that slowed me down for a while. Absolutely correct and accurate, but when you start throwing Trigonometric functions & variable equations into it, it's not for beginners anymore. KISS. That kind of mathematical description can scare people away, or at least confuse them.

At least, it does me...
I agree!

It seems that some are not aware that there was a time in this country when the education system wasn't uniform (still isn't!) and consequently not all math, science and physics classes were available for all. That is what happened to me. For younger people in our chapter, it is either they didn't take advantage of the opportunity to learn or didn't understand it on the first pass.

What I'm looking for is a simple non math explanation (using vectors is fine) that describes what the rotor blade forward propulsion derives from.

RotoPlane

Gold Supporter
The bottom sketch in post #12 shows that the thrust vector is forward of the axis of rotation by 1°, pulling the blade forward. Chuck's words below, I think, explains autorotation well….without math.

If the thrust vector points ahead of the axis of rotation, an accelerating force is produced. If it points aft, a decelerating force is produced.

An autorotating rotor always finds its equilibrium speed; a net accelerating torque speeds it up which in turn reduces the angle of attack and diminishes the accelerating torque.

Inboard portions of a rotor operate at a higher angle of attack and supply power to the rotor. Outboard portions operate at a lower angle of attack and extract power from the rotor.

The basic physics of autorotation is no different for sailboats, gliders and windmills.

birdy

Newbie
Cept for this line;
The basic physics of autorotation is no different for sailboats, gliders and windmills.
coz the average joe knows why a mill fan turnes, how a prop pumps air and how wind propels a boat.
Nun of the above are autorotating.
Joe can see how sails work, its logical.
But joe cant see vecters, coz you cant see things not phisical.
All flyn surfaces deflect air, and are energised by either restriction, gravity or HP, but extracting energy from the air its pumpn dont make sence, initialy.
Soon as you accept the fact that the lift is @ 90* to the airflow, it suddenly becomes logical.
And once you understand autorotation, you are much more confident in its abilities/limitations.

If you say gyro rotors are the same as 'sailboats, gliders and windmills', ol joe is go'n to be confused, coz he'd know that if he straped the fan off his windmill to the rotor head, it wouldnt fly.

RotoPlane

Gold Supporter
If you say gyro rotors are the same as 'sailboats, gliders and windmills', ol joe is go'n to be confused, coz he'd know that if he straped the fan off his windmill to the rotor head, it wouldnt fly.
Hmm....you are probably right Birdy....but I took that statement to mean the basic physics for lift vectors used in autorotation are also applicable for looking at lift on 'sailboats, gliders and windmills'. As you stated, "Soon as you accept the fact that the lift is @ 90* to the airflow, it suddenly becomes logical".

WaspAir

Supreme Allied Gyro CFI
sailboats?

sailboats?

I've never seen a sailboat autorotate (I'd love to see the keel on that boat), but I have put a gilder into a rotation rate-stable spin. That's the only flight mode in which you get any rotation in a sailplane.

birdy

Newbie
The only very basic similarity between a sailing surface and and automotive one is they all deflect air.
Lift and drag are common vecters, its how each dose it thats different.

You can prespin a wind mill to a zillion rpm, but soons you remove the drive, itll slow,stop and return to -AOA sailing.
Theres are no driveing and driven areas ona sail that balance for a given AOA, just drive.
If our rotors didnt hava driven area, theyd just keep acceleratn till they parted company.