A good explanation for right tilt!

[mnowell129]

I found this while doing some online research. This explains the tilt needed in a fixed head gyro, or trim in one with cyclic. The answer is coning. In the diagram the local airflow is shown in the arrows. The forward facing blade sees a higher angle of attack than the rearward blade. Of course the angle is not in the blade airflow direction but if you add up all the vectors the front blade has a higher angle of attack than the rear one. It will thus try to flap up on the retreating side, giving the roll to the advancing blade.
I was wrong and stand corrected. Speeding up the rotor spin by lowering the pitch will help this situation because it lowers the coning angle and reduces this effect, but some trim will always be required due to the differential local angle of attack due to coning. It is still true that the clean air/dirty air problem will aggrevate this situation even further.
David, hope this satisfies the "seat of the pants".
mickey
P.S. Have you checked out my night flying video at
www.mickeynowell.com ->"hobbyworld"?

[David A Ramsey]

That was good Mickey. Pictures and diagrams help. I think I have a grasp of when and why the force on the blade is applied and when it is felt and what might affect the strength of that force.

For others looking at Mickey's explination, keep in mind this applies to CW, or CCW rotors. No matter which direction your rotor turns, think in terms of Advancing and Retreating blades.

Will have a look a video.

Thanks.

[mnowell129]

Good reminder.
Yes advancing and retreating blades. The trim needed is whatever
will reduce the pitch of the blade out in front, which becomes the retreating blade.

[JWarren]

Just for clearification, to everyone who reads this, is the trim you are refering to, left and right or front and back or both?

John

[JWarren]

Also,

Another question up for answers. On my model I tested my rotor at a rearward tilt angle of 10 degress. DT (motor downthrust) 5 degrees. CG forward the shaft. Hang angle of 7.5 degrees. I figured I would cut the recommended hang angle of 5-10 degrees in half for evaluation and place it at 7.5 during the initial testing. The model has a tendency to nose up. I decreassed the aft rotor tilt to 7 degrees and moved the CG forward to give a hang angle of 10 degrees. The noticable difference was a longer takeoff run and seemed heavy. The model did not want to take off. I am thinking now I should put the rotor back to 10 degrees move the CG back to 7 degress HA and increase the DT to 10 degress.

I have overcome the tendency to roll left with right trim on the CCW rotor. Which is, pull-pull, by the way. I have already replaced the linkage on the pitch servo to a heavier gauge push rod to eliminate the possibility of problems with the rotor bending the rod, as it is a push-pull setup.

Any thoughts?

John

[mnowell129]

Just for clearification, to everyone who reads this, is the trim you are refering to, left and right or front and back or both?

John
Left and right.

[mnowell129]

Also,

Another question up for answers. On my model I tested my rotor at a rearward tilt angle of 10 degress. DT (motor downthrust) 5 degrees. CG forward the shaft. Hang angle of 7.5 degrees. I figured I would cut the recommended hang angle of 5-10 degrees in half for evaluation and place it at 7.5 during the initial testing. The model has a tendency to nose up. I decreassed the aft rotor tilt to 7 degrees and moved the CG forward to give a hang angle of 10 degrees. The noticable difference was a longer takeoff run and seemed heavy. The model did not want to take off. I am thinking now I should put the rotor back to 10 degrees move the CG back to 7 degress HA and increase the DT to 10 degress.

I have overcome the tendency to roll left with right trim on the CCW rotor. Which is, pull-pull, by the way. I have already replaced the linkage on the pitch servo to a heavier gauge push rod to eliminate the possibility of problems with the rotor bending the rod, as it is a push-pull setup.

Any thoughts?

John
I always balance by having the hang angle be the same as the mast tilt angle. Essentially the CG is always along the rotor shaft. I.E. when you hang from the rotor the shaft hangs straight down. I never balance any other way. I get a fairly severe pitch up if the CG is further aft.
Also, ground attitude seems to be important for takeoff, don't have a pic to consider. Mine will stay glued, then pop off the ground with too low a stance.

[malt_master]

not to mention a law of gyros that a reaction occurs 90 degrees after the application of a force.

[Rhondas]

If make my blades to turn looking from the top CW, which way will I have to tilt my hub shaft right or left an how much to start with ? this is for an electric if it changes things email me ronplanes@optonline.net

[mnowell129]

I ran across this paper. It has some math in it but you can skim those parts.
Shows the empirical data that dictates the 15 degree tiltback angle.
Also diagrams the local angles of attack of the blades when coning is
present.
He points out that Cierva tried blade feathering before flapping and that
although this is a correct solution Cierva's implementation was not workable.
He seems to skip over the equivalency of flapping and feathering....

http://www.enae.umd.edu/AGRC/Aero/Leishman_giro_paper.pdf

An interesting link including how coning introduces roll.
http://www.dynamicflight.com/aerodynamics/flapping/

ANother link with a similar explanation.
http://www.w3mh.co.uk/articles/html/csm7_8.htm

A Quote from another interesting paper
http://naca.larc.nasa.gov/digidoc/report/tn/04/NACA-TN-1604.PDF

"At the time that the basic theoretical treatments of rotating wing
aircraft were made, the typical rotor arrangements involved the use of hinges to permit flapping but provided no mechanism by means of which
both flapping and feathering could be introduced relative to the rotor
shaft. The desired orientation of the rotor was achieved by tilting
the rotor shaft. Since that time, the mechanical arrangement in most
designs has been altered so that the rotor attitude is controlled by
feathering. A controllable amount of first-harmonic blade-pitch change
is thus introduced relative to the axis of the rotor shaft. The two
systems are aerodynamically equal; the blades follow the same path
relative to space axes, as regards both pitch angle and flapping angle,
for any given flight condition regardless of the mechanical means used
for achieving it. This fact may be confirmed by inspection and can also
be demonstrated mathematically."


mickey