Rotorhead: Vibration, Stability & Controllability

[Rotor Rooter]

The following question relates to viability of an unusual modification to the design of the Electrotor's rotor and its gimbal hub. A hub that might also be used for a gyrocopter with a partially powered rotor.


Chuck Beaty mentioned a few years ago;

We have 3 interesting radii:

(1) CG- all mass is considered concentrated here to calculate centripetal force.

(2) Radius of gyration- all mass is considered concentrated here to calculate moment of inertia.

(3) Center of percussion- all mass is considered concentrated here when the blade swings as a pendulum.

Every time I try to look at this analytically, it seems, as Jean Fourcade said, that with flapping, if the teeter bolt is not on the CG plane, the rotor CG rotates in a 2/rev circle.

The following sketch shows the teetering portion of the proposed rotor. It will be noted that the motor and the planetary reducer teeter with the hub-bar and blades. The additional weight of this power-train will result in the center of mass moving from the red dot to the blue dot.

The question or point of speculative discussion is; Will the relocation of the teetering hinge from above the hub-bar to the new CG location (blue) below the hub-bar allow the above three aeromechanical plus aerodynamic requirements to be safely satisfied?



Dave

[Not Yet]

Dave
I’m not sure what you are showing (no callouts in drawing) but it looks to me that you are including the hub in your weight calculations.

I’m sure that you know that the location of the teeter bolt only takes into consideration the weight of the hub bar and the blades.

[Rotor Rooter]

Not Yet, thanks for questioning this rotorhead.

Yes. All the items that are shown on the sketch in the first post teeter with the blades. The electric motor, plus the planetary reducer that is between the motor and the 'teetering block', teeter with the blades.

The sketch on this post show the the non-teetering items. The assembly of teetering items fits inside the assembly of non-teetering items.




These two sketches show the rotorhead with a high teetering pin high and a conventional undersling based on the mass of the blades and hub-bar only.

However, this addition of the motor and reducer etc. will add to the mass of the teetering portion of the rotor. This additional mass will result in a lower center of the total teetering mass.

I speculate that lowering the teetering hinge to the new center of mass (blue location) will satisfy Chuck's points 1 and 2. However, point 3, plus aerodynamic activity may not be satisfied.

I am hoping that people with much more knowledge about gimbal rotors than me will be willing to hazard speculation as to potential problems with this rotor configuration.

Dave

[CopterCenturian]

Just a question.

I see the motor & reduction unit- does the reduction unit include a freewheeling clutch in the event of motor failure?

If partially powered what keeps the gears from being loaded and unloaded - handling fatigue?

Does the application later remove power- if so then consider the new cg when unpowered.

From what I see from the pics it looks like a good candidate for an electric prerotator but I think gear design would be critical in a powered application.

Very interesting concept.

Ken

[Not Yet]

Dave

Now I see what you are doing. Looks interesting.

As far as answering your question, I think we will have to wait for C. B.

[Rotor Rooter]

Ken,

The following sketch and comments are answers to your questions. Hopefully, they are sound ones.

I see the motor & reduction unit- does the reduction unit include a freewheeling clutch in the event of motor failure?

Yes there is a freewheeling sprag clutch located on the output shaft of the off-the-shelf planetary reducer. It transmits the torque to the Pitch Yoke.

If partially powered what keeps the gears from being loaded and unloaded - handling fatigue?

The gears will always be loaded when the motor 'is on'. For a gyrocopter, the thinking is to provide power to the motor from one or more large alternators on the engine. This electric drive would be used for pre-rotation. In addition, during flight it would provide some power to the main rotor, thereby reducing a portion of the aerodynamic transfer of power from the propeller to the main rotor.

Does the application later remove power- if so then consider the new cg when unpowered.

The CG will not be effected by the state of the motor (on/off). When the motor is on it will have rotational inertia and be subjected to gyroscopic precession. However, I do not think that these will be problems since the motor is co-axial with the rotor and they are both rotating in the same direction.

.....but I think gear design would be critical in a powered application.

Hopefully, this will not be a problem because the overrunning clutch will remove the power-train from the rotor and the rotor is already in an autorotative state.

There may be a number of pros and cons.

A potential pros might be the the lesser likelihood of a bunt-over. This is because the propeller is not providing all of the push (power) to rotate the rotor

____________

Not Yet,

I would love for C. B. to comment. This may be a new and unusual concept where no one can make statements with absolute conviction, but technically based speculation may collectively spot problems and develop solutions.


Dave

[CopterCenturian]

Dave,
I think you have a very cool design.

From my balancing experience with helicopters it looks like the flap rate would change with the cg located below the teeter angle of the hub/gimbal. From an aerodynamic perspective I have no clue if that would in fact be the case or if it would greatly affect the rotor dynamics.

An idea that may help in test your design... I have seen scaled rotor designs tested using a 3/4 hp electric motor to spin up the test rotor design. I'm talking big companies here not the average home builder but the same process could be applied here. The test pickups are expensive but the price of them has come down in the past few years.
The LHX (later RAH) rotor designs were tested this very way before determining a winning rotor design for production. Sorry I can't be of more help.

Best to you,
Ken

[Rotor Rooter]

Ken,

Thanks for your concern and comments.

The building of a scale model might be fun, if the theory doesn't first uncover something terminal . There are a couple of small wind tunnels at the local University. Perhaps the testing of this rotor might serve as a student project.

Dave

[Arnie Madsen]

Originally Posted by Rotor Rooter

Ken,

Thanks for your concern and comments.

The building of a scale model might be fun, if the theory doesn't first uncover something terminal . There are a couple of small wind tunnels at the local University. Perhaps the testing of this rotor might serve as a student project.

Dave

Now you are talking Dave ... I for one would like to see some of your ideas tested as scale models and in a wind tunnel. Who knows , the university may welcome someone with your ideas and accomodate the testing. It's worth a try. Here's hoping.....

Many thanks
Arnie Madsen
Bell 47 G2

[CopterCenturian]

Dave,

One quick precaution if you do test the design with the electric motor- be sure to shield the servos, use shielded wiring and shield the pickups. The motor gives off a lot of rf and this can cause signal interference with the servos and the test head be destroyed from improper signal inputs to the servos. Please understand a good test using this setup is about a two minute run. But the data that can be collected is huge for that 2 minutes of testing.

You have a great idea for a university project and you might even find government or private funding to help support the research. DARPA, NASA and others do sometimes fund these projects. If it is a major university with an engineering department the university may also fund it through alumni or other grants/funds.

Again best to you and your research,
Ken

[Rotor Rooter]

Arnie & Ken,

Thanks for the additional info and support.

Dave