CFI Pop Quiz #108 - Dissymmetry of lift

[Chris Burgess]

What is dissymmetry of lift?

A) The difference in lift that exists between the advancing blade half and the retreating blade half of the disc area.
B) The difference in lift that exists between the rearward part and the forward part of the rotor disc during forward flight.
C) A term used to differentiate between air flowing downward through the rotor in powered flight and upward through the rotor in autorotative flight.

[chuter]

I'll go with A.

[ventana7]

A.

Keep em coming Chris -- this is the kind of stuff we need more of on the forum!

[Chris Burgess]

"A" is correct Mike and Rob. When our temperatures get back to 50 degrees, I may get too busy for this. But this helps keep me from causing trouble in the mean time.!!! I think the Secret Service is getting annoyed at me hanging around George's end of the airport, 15 miles from Camp David.

[Ga6riel]

yeah A
since it relates to the roll reaction when the dissymmetry can no longer be countered

this looks like helicopter stuff, (look at C) since the only way to power the rotor is via prerotor (dont try this at home)

[Chris Burgess]

You will find several questions are "tainted" with helicopter stuff. Several involve the Air and Space 18A also. Not many of us jump takeoff, not too many that is. Those questions are still around.

[mceagle]

Posted by Doug Riley "Well, the FAA apparently wants it to be A, so those studying for the test should note that fact carefully. Often the expedient thing to do with these tests is to find out what the testers want and give it to them... even if you know better. The questions are sometimes ambiguous or vague and selecting the answer means picking the "least bad" one.
This particular question boils down to definitions, about which even knowledgeable folk have to agree in advance before they can talk to each other".

I feel the same way about this question. Talking normal straight and level flight, there is no dissymmetry of lift - the extra lift created by the advancing blade is cancelled out by the pilots positioning of the joystick stick, pitching the rotors such as to make the lift equal on both sides (otherwise the gyro would turn). The only time that there is dissymmetry of lift is when the pilot is commencing a turn, and that is only temporary.
My opinion only.

NB. Chris keep posting questions, it is healthy to promote such discussions.

[ventana7]

Tim,

I believe the dissymetry of lift is equalized by one blade rising and one blade falling (flapping) on each revolution- not how you position the stick.

[pwendell]

Tim,

I believe that dissymetry of lift is always present in forward flight. It is caused by the fact that in forward flight the advancing blade is moving through the air faster than the retreating blade. As Rob stated, the dissymetry is compensated for by the flapping hinge (teeter bolt). The advancing blade flaps up, effectively decreasing its angle of attack and reducing lift. The retreating blade flaps down, effectively increasing its AOA and increasing lift. The rotor will flap as much as necessary to compensate for any inequality between the advancing and retreating blades. The only time dissymetry of lift is not present is when the gyro is in a vertical decent.

[mceagle]

G'day Rob, good to hear from you again.
The rotor blades do not rise and fall but rather "tend" to fly in a perfect circle on a slight lean. This lean would throw the rotor thrust vector off centre which would then bank the gyro into a turn. This reaction is counteracted by the pilot, to bring the thrust vector back to in line with the C of M, levelling the rotors and stoping the gyro from turning.
The lean that you see on a rotor disc from behind is another pilot joystick compensation for the torque of the engine, but that is another subject.

[Al_Hammer]

Tim,

Why would the rotor tend to lean to the left with dissymmetry of lift at the sides? All lift forces applied to the rotor are subject to precession.
When the pilot wants to tilt the disk up in front, the stick is pulled back, which increases angle of attack on the advancing side and the blade reaches max displacement at the front. (precession)
The dissymmetry of lift is also affecting the blades at the side position and will act like a nose up command from the pilot except that in this case, the rotor head doesn't have to tilt to produce a pitch change. The airspeed increase/decrease provides the input.
The rotor disk tilts up ,but the rotorhead doesn't, which "feeds back" a negative aerodynamic input in the form of reduced angle of attack until the dissymmetry is exactly neutralized and then the rotor remains at that angle of flight with respect to the rotorhead.
See the diagram below for a visual explanation.

The pilot will normally respond to the increased disk angle (flapping angle) by moving the stick forward to avoid climbing. This tilts the rotorhead forward and the rotor will follow, tilting the disk lower again, but note that the rotor will continue to fly at a higher angle than the plane of the rotorhead as long as it is neutralizing dissymmetry lift.

So, to maintain level flight, the pilot tilts the rotorhead to avoid the rotor from tilting relative to the flight path.

Seen from behind, the rotor will be tilted neither left nor right (ignoring any slight tilt needed for engine torque balance that you mentioned).

It will also NOT be tilted back from where it was at a slower airspeed, in fact it will be tilted somewhat FORWARD as speed increases. The rotor flapping angle will increase, however, which means the rotor head will increasingly tilt forward relative to the disk. Otherwise the gyro climbs.

So, I agree that the pilot does in fact move the stick (forward) as an indirect result of flapping. But there is no leaning tendency of the rotor.

[mceagle]

Yes of course Al. The extra "lean" that the pilot compensates for is in the forward and back plane due to precessive forces, not laterally as I alluded to. My mistake.
The fact remains that there is equal lift on both advancing and retreating blades - one is faster with less pitch and the other is slower with more pitch, and that pitch is controlled by the pilot.
Is it "flapping" that equalises the tendency for dissymmetry of lift or the pilot? We must consider that the rotor thrust vector is perpendicular to the tip plane axis, which must act through the C of M otherwise the Gyro will bank (turn).

[Al_Hammer]

Is it "flapping" that equalises the tendency for dissymmetry of lift or the pilot?

Tim, I'd say it's flapping. If the pilot doesn't move the stick, the lift will still be equalized.

We must consider that the rotor thrust vector is perpendicular to the tip plane axis, which must act through the C of M otherwise the Gyro will bank (turn).
Yes, but moving the stick only moves the rotorhead, it cannot directly control the flapping angle which is the angle between the tip plane and the plane of the rotorhead spin plane.

[Ga6riel]

one thing that has always confused the hell out of me is this
consider most ordinary sections, the lift centre is taken as 25% of the chord, the variance to this creates a moment around that point be it + or -.
Yet apparently a disc described by a helicopter rotor has its lift centre at 50%, the induced variations are somewhat eliminated by variations in pitch, while the control inputs are allowed to remain.

Therein are 2 differences,
the centre of lift of a described disc v/s that of a wing section
and while pitch variations work to even the dysmetry of lift they do not act on control inputs.

Why is it so....?

[Al_Hammer]

consider most ordinary sections, the lift centre is taken as 25% of the chord, the variance to this creates a moment around that point be it + or -.
Yet apparently a disc described by a helicopter rotor has its lift centre at 50%

I think centrifugal force and lift combine so that the total force on each blade can be considered to act outward and upwards at the attachment point, say 25% back from the leading edge. The blades are mounted so that the attachment points are exactly opposite each other and the center of force is therefore at the center of the hub.

and while pitch variations work to even the dysmetry of lift they do not act on control inputs.

Both dyssymmetry of lift and control inputs ultimately are aerodynamic inputs that cause the tip path plane to precess, or tilt 90 degrees later.

Dissymmetry of lift is a self cancelling force, because the upwards tilt of the disk(but not the rotorhead) removes the applied force. Otherwise the disk would continue to tilt.
Similarly with a control input, the disk precesses until the cyliic force created by the movement of the rotorhead is no longer present. In other words, the disk aligns with the rotorhead and restores equilibrium.

[Ga6riel]

many thanks Al
indeed that makes a lot of sense

[mceagle]

Tim, I'd say it's flapping. If the pilot doesn't move the stick, the lift will still be equalized.Al, if the blades flap due to airspeed differences, wouldn't the disc then fly at an angle? As I understand it, the force (airspeed) causing the tendency to flap reacts so as to lift the front of the disc. How could that equalise the dissymmetry of lift on the sides?

[Al_Hammer]

<Al, if the blades flap due to airspeed differences, wouldn't the disc then fly at an angle?
Yes.

>How could that equalise the dissymmetry of lift on the sides?

Good question , Tim. Take a look at the drawing below.

When you have disssymmetry of lift, the disc (tip path plane) tilts but NOT the rotorhead (the teetering hinge makes this possible), so the blades do not "point" in the same direction as the disc when they are at the side position. This changes the angle of attack.
Note that at the front -back position, the blades are no longer flying crosswise to the direction of the disc and no change of angle of attack occurs there.

[Doug Riley]

The lifting of the front of the disk results in each blade's experiencing a cyclic de-pitch as it sweeps throught the advancing position and a cyclic up-pitch as it sweeps through the retreating position. This is hard to sketch, but really easy to see if you have an actual rotor and rotor head handy. The effect is the same as if the pilot pulsed the stick forward each time a blade passed by the 9 o'clock or 3 o'clock position.

Tim McEagle is right about the fuzziness of FAA questions. You have to know JUST ENOUGH -- neither too much nor too little -- to get answer their questions the way they want. In fact, the flap hinges almost instantly wipe out virtually any dissymetry of lift as it crops up. It's not there to any significant extent in steady flight.

But... The old NACA wind tunnel test of a PCA-2 gyro rotor in the early 30's showed that the flap hinges didn't work perfectly. The center of thrust of the PCA rotor ended up to the LEFT of the rotor spindle by an inch or two on a 45-foot diameter rotor. Therefore, at a superficial level the FAA is right; at a deeper level they're wrong, and at a really picky level they're right again...

Good instructors know this and help you get it right for the test.

[robertstodaro]

For me the correct answer is "B" because I fly sideways most of the time.

[Butch S.]

Guys
If dissymetry of lift doesnt exist between the advancing & retreating blades why do we set up the rotor head so that when the stick is centered side to side the rotor head is tilted 3deg aprox to the right.
A newby recently corrected this on his new secondhand symes gyro & on a test flight by dave he had to hold the stick way to the right
Butch S.

[Doug Riley]

Butch: A couple things can cause that.

If the teeter hinge has significant friction, the rotor will tend to induce a left bank, requiring right stick to compensate. Many teeter bearings are plain or sleeve bearings, which aren't always perfectly friction-free.

Engine torque on a gyro that lacks a tall tail requires the airframe to ride slightly "won wing lo." For example, a gear-drive Rotax pusher turns the prop clockwise. The frame rolls counter-clockwise a bit and stays that way during level flight. With the frame rolled off to the left and the rotor level, the head must be tilted to the right relative to the frame. This will result in the stick being deflected to the right unless the pushrods are adjusted to center the stick in level flight -- by putting in a tilt of the head to the right.

[mceagle]

G'day Butch. The tendency for dissymmetry of lift from aerodynamic forces is almost negligable. As Doug said, engine torque is the main offender. The degree of offset required on the torque tube is approx the same but opposite for a gear driven Suby compared to a belt driven Suby. Something has to stop the airframe from rolling in flight due to engine torque.

[Butch S.]

Thanks Douge & Tim
That makes sense
Butch S.