Basic Autogyro Theory 101

[Vance]

Originally Posted by bryancobb

Hey Fellas,

My 2 cents...

Any positioning of the blades, by pilot input, aircraft maneuvering, rigging, or any other way, THAT CREATES A TOTAL AERODYNAMIC FORCE VECTOR,TILTED REARWARD FROM THE ROTATIONAL AXIS WILL SLOW THE RPM.

Hello Brian,

I would be grateful if you would help me understand why you think; “Any Positioning of the blades, by pilot input, aircraft maneuvering, rigging or any other way, THAT CREATES A TOTAL AEROCYNAMIC FORCE VECTOR, TILTED REARWARD FROM THE ROTATIONAL AXIS WILL SLOW THE RPM.”

Thank you, Vance

[Vance]

Originally Posted by SGK

Hi Vance,

In my world, there are no forces that could force rotor to stop when relative wind comes from above. The profile is not so effective when so, but significant braking force is something I can‘t see. Also, the fact that most rotors have blades with positive pitched profiles makes me believe that rotor can autorotate in same direction even when it "blows from above".
Both my questios are related to the same phenomena.
What I‘m trying to do is to modernize our theory material and erase the statements based on myths.
Regards,
Roman

Thank you for making your questions clearer to me Roman.

In my experience; in a low G maneuver the wind through the rotor disk does not reverse, it only slows.

I feel what the rotor system sees in a low G maneuver is a lighter gyroplane and that reduces the airflow up through the driven region of the gyroplane rotor.

The on the ground positioning of the disk seems all together different to me.

It seems possible to have the airflow coming from the top instead of underneath.

I had a rotor brake fail and had only limited success with this technique. The wind was not steady so this may have been a flawed example.

I had trouble with managing the blade tip path.

I usually just hold the disk flat as practical and when the rotor slows to 50 rotor rpm apply the rotor brake.

More experienced pilots than I have recommended this technique.

Thank you, Vance

[kolibri282]

Any positioning of the blades, ...... THAT CREATES A TOTAL AERODYNAMIC FORCE VECTOR,TILTED REARWARD FROM THE ROTATIONAL AXIS WILL SLOW THE RPM.
Edit/Delete Message

The rotor disk plane is perpendicular to the axis of rotation. In forward flight the net rotor force referred to the disk plane is always inclined backwards as the attached picture from the book of Harris shows (Harris p. 52). This corresponds to the formula for the H-force where the dominant term is 0.25*mu*delta, where delta is the average profile drag coefficient, which is always positive. A positive H-force by definition points to the opposite of the flight direction.

The rotor keeps spinning nevertheless since the net moment about the axis of rotation is zero. The next two pictures explain this fact.

The first one from the excerpt of the "Rotorcraft Flying Handbook" that Phillip has posted shows that the driving region on the retreating blade is larger than on the advancing. My little (very simplified) sketch shows the net forces on the rotor. Let us assume for simplicity that the brown region of the advancing blade drives the magenta region on the retreating blade. The brown region has to be larger since the lever arm of the magenta region is much larger. This leaves us with the large magenta region on the advancing blade whose decelerating moment is balanced by the (slightly larger) brown region on the retreating blade. So torque is balanced and the rotor keeps spinning. If we now sum the two regions below the blue line (brown to the left, magenta to the right, where the forces point to the rear of the aircraft) and deduce the two regions above the blue line we have a net H-force pointing to the rear.

Harris' book can be found here:
http://www.rotaryforum.com/forum/showthread.php?t=32431

[cesphil]

The FAA publishes and excellent handbook - "Rotorcraft Flying Handbook" that covers both helicopter and autogyro technology, dynamics and flying.
It is available as a free pdf download from the FAA's web site.

Attached is an exert that deals with autogyro dynamics.

[bryancobb]

Originally Posted by Vance

...help me understand why you think any positioning of the blades THAT CREATES A TOTAL AERODYNAMIC FORCE VECTOR, TILTED REARWARD FROM THE ROTATIONAL AXIS WILL SLOW THE RPM...

Air hitting the leading edge is trying to slow down the rotor.
Energy to keep that from happening HAS to come from somewhere.
That energy comes the total force vector (The net sum of all) being tilted forward of the rotation axis.
If anything is done to tilt that total force vector (The net sum of all) closer to the rotational axis, RPM will decay.
If it's (The net sum of all) tilted aft of the rotational axis RPM decays very rapidly.

[kolibri282]

Whether a rotor is accellerated or slowed down has nothing to do with the net sum of forces (neither on the rotor as a whole nor on one single blade) but depends on the net sum of moments generated by the forces on a rotor. Moment is the product of the force times the lever arm that force is acting upon. As I pointed out in post #18 the net sum of forces on a rotor is pointing opposite to the direction of flight i.e. to the rear of the axis of rotation of the rotor.

[bryancobb]

OK...Let's try it this way...

I'm referring to kolibri's 2nd picture in post #2, this thread.

"A" & "E" clearly label the "Total Aerodynamic Force Aft of Axis of Rotation."
"C" clearly labels the "Total Aerodynamic Force Forward of Axis of Rotation."

The sum of all C's on the entire rotor disc MUST equal or exceed the sum of all (A+E)'s or the rotor will slow.
"B" "D" Neither slow nor accelerate the RPM.

If the sum of all C's on the entire rotor disc exceeds the sum of all (A+E)'s, it has the same effect as the cranking-rope does on a pressure washer! I.E. it adds energy and spins the mass FASTER.

The pilot can affect this in flight by raising or lowering the tip path at the nose. This is why RPM increases during the flare phase of a touchdown autorotation.

[kolibri282]

Let me explain the difference between force and moment with this little example. In the first picture below the accelerating force is 2.4 [lb] while the decelerating force is 2.0 [lb]. The net force is 2.4 - 2.0 = +0.4 so the net sum of forces is positive (=lying in front of the axis of rotation). Now let us consider equilibrium of moments, also called torque equilibrium. The accelerating torque is 2.4*7 = 16.8 [lb*ft] and the decelerating torque is 2.0*9 = 18 [lb*ft]. Although the accelerating force is much larger the rotor will slow down since the net sum of moments decides what happens. In this case +16.8 - 18.0 = -1.2 and the rotor decelerates.

It is the same physics by which a 51 lb girl can lift a 200 lb man. Her lever arm is 4 times larger so 200*2 - 51*8 = 400 - 408 = -8 [lb*ft] and the girl wins.

I feel that it is very important to make the distinction between force and moment since if you mix up the two you will perhaps never understand why the rotor in sustained autorotation (=torque equilibrium) generates a net force pointing to the rear.

[bryancobb]

When I use the term "sum of all C's," this INCLUDES moment-arms, all forward moments, all forward net forces, all 200# men, all 50# girls, etc.

The integral of all "f" between r=0 and r=d/2 for 0 deg azimuth through 359 deg azimuth.

Your sketches are 100% correct. This concept DOES affect RPM, and it is taken into account in my general comments.

[kolibri282]

Bryan,

it seems, we both mean the same thing. I just wanted to use the terms generally agreed upon to avoid any confusion and make it easier for the novice to read through a text covering the topic

[bryancobb]

It's nice to be on the same page of the score

[RotoPlane]

You are a very gifted teacher Juergen. Your explanations and use of sketches have helped to break down difficult subjects into their simpler and more understandable bits.

Although I believe I have always used force and moment values separately, your above example shows clearly why their distinct use in equations is important.

Thank you for the extra effort and knowledge you have been giving to this forum.

[kolibri282]

Ed,

thanks for the nice words! Many others have contributed so far to this thread and I do hope that many more will continue to share their experience and make this thread a great source of information for those who want to learn a bit more about rotary wing aircraft.

Cheers,

Juergen