Prerotator Question

[Alan_Cheatham]

This might be hard to describe so bear with me. Am I correct in assuming that there is a direct correlation between rotor rpm attained during prerotation and a minimum forward airspeed needed to flow air into the rotor to maintain that rpm once prerotator power is removed? In other words lets say a prerotator is designed so that it is only engaged when the stick is in the full forward position and the head is level, and disconnects when the stick is tilted back, removing prerotator power to the rotor. Lets also say this prerotator can get the rotor to 200 rpm, at which point the pilot would start his takeoff run. It seems to me that it would be unnecessary to tilt the rotor back at the beginning of the takeoff run and the pilot could wait until he had a certain forward airspeed, which would be related to the max prerotator rpm achieved, and then tilt the rotor back at which point prerotator power is removed but because of the inflow of air the rotor rpm would continue to build and not decay. Correct. Or is it really necessary to have the rotor tilted back at the beginning of the takeoff run no matter what?

Note that this would be on a tractor, taildragger gyro, so there would be no balancing on the mains during the takeoff run.

 

[bones]

It seems to me that it would be unnecessary to tilt the rotor back at the beginning of the takeoff run and the pilot could wait until he had a certain forward airspeed, which would be related to the max prerotator rpm achieved, and then tilt the rotor back at which point prerotator power is removed but because of the inflow of air the rotor rpm would continue to build and not decay. Correct..

Ohhhh boy hope you enjoy the rebuild, cause thats about whats gonna happen.
Question, why are you trying to reinvent the wheel, when from the above post ytou know very little(IMHO) about how a gyro operates?


[/QUOTE]
Or is it really necessary to have the rotor tilted back at the beginning of the takeoff run no matter what?
[/QUOTE]

What you descibed above, was exactly what happened to a guy over here, and he is in the middle of a rebuild now

 

[Alan_Cheatham]

Question, why are you trying to reinvent the wheel, when from the above post ytou know very little(IMHO) about how a gyro operates?

Thank you for your reply.

I'm not trying to reinvent the wheel, but make the wheel fit my particular gyro, and because my gyro is a tractor and configured differently than a pusher the prerotator is a particular challenge. One thing I don't want is to have any rotor rpm decay after prerotation and during the beginning of the takeoff run and to accomplish that when the rotor is tilted back will require a telescoping drive shaft with universal joints, and I'm trying to simplify the design as much as possible.

If you were to park a gyro on the runway with no headwind, prerotate to max attainable rpm with the rotor level, cut prerotator power and tilt the rotor back the rotor will immediately start to decay in rpm because it has no airstream to keep it powered. If this experiment were repeated in higher and higher headwind velocities it seems to me that at some certain airspeed, when prerotator power is cut and the rotor is tilted back that rpm will not decay, because the airstream has sufficient energy to power the rotor at that rpm. If this is true then it would also seem to me that the rotor could be kept level during the takeoff run until this airspeed was reached.

Now, if this theory is not operationally applicable in the real gyro world then I will adjust my design, with added complexity. I would also like to point out that pusher gyros are a "reinvented wheel" of the original tractor gyros, so not all reinvention is bad.

 

[Vance]

Note that this would be on a tractor, taildragger gyro, so there would be no balancing on the mains during the takeoff run.

Hello Alan,

I have not flown a tractor gyroplane and I was wondering why you wouldn’t have to balance on the mains with a tractor gyroplane?

What is the take off procedure with a tractor gyroplane?

Thank you, Vance

 

[scottessex]

Go with a hydraulic prerotator, no complicated mechanical driveshafts, slip joints, universals. The pump could be belt driven and could be engaged with an idler pully or with a clutch like on a Sparrowhawk.

Or even simpler but not as powerfull is a simple electric.

You do not have to re invent the wheel, these problems have already been adressed,
unless you are trying to do it for dirt cheap.

 

[jcarleto]

I'm no expert. Let me make that plain at the start. But, I don't think I would want to operate a gyro pre-rotator at high RPM that would not allow me to direct the rotor-disk-plane into a crosswind during pre-rotation. It sounds like you are trying to create something that will only spin up in one position.

Scott's solution is a good one. Hydraulic is easy and the pipes are nice and "bendy."

*JC*

 

[Alan_Cheatham]

Go with a hydraulic prerotator, no complicated mechanical driveshafts, slip joints, universals. The pump could be belt driven and could be engaged with an idler pully or with a clutch like on a Sparrowhawk.

Or even simpler but not as powerfull is a simple electric.

You do not have to re invent the wheel, these problems have already been adressed,
unless you are trying to do it for dirt cheap.

I have examined all the different types of prerotator mechanisms, either by doing research on the net or by examining my friends gyro, he has had four different types on his, so I know the pros and cons of each. I will absolutely not go electric, hydraulic would solve certain problems but also cause others and offers no advantage over my current system. Flex shaft also causes problems that I am trying to avoid.

I'm not asking how to design a prerotator but would like the answers to certain operational questions. I'm not trying to revolutionize the gyro world with a new prerotator design, but build the prerotator that fits the design requirements I have for my gyro.

If no one innovates, then technology stagnates.

 

[Alan_Cheatham]

I'm no expert. Let me make that plain at the start. But, I don't think I would want to operate a gyro pre-rotator at high RPM that would not allow me to direct the rotor-disk-plane into a crosswind during pre-rotation. It sounds like you are trying to create something that will only spin up in one position.*JC*

Not quite, the head would still be able to tilt from side to side but would need to be tilted to it's full forward position to mechanically link with the prerotator drive. Now, on my tractor which is taildragger and sits on it's gear at a 10 degree angle, full forward would be "negative 10 degrees" to get the rotor level.

 

[Alan_Cheatham]

Hello Alan,

I have not flown a tractor gyroplane and I was wondering why you wouldn’t have to balance on the mains with a tractor gyroplane?

What is the take off procedure with a tractor gyroplane?

Thank you, Vance

Tractor gyros takeoff and land in a three point attitude so don't need to do the balancing act on their mains that tricycle gyros do. From the videos I have of Ron Herons taildragger tractor the Littlewing, it is one of the smoothest ground handling gyros there is.

 

[ahs437]

I'm not all that well informed about operational issues with gyros, but unless I'm missing the original issue, it seems the question was whether the "decay" of the disk could be minimized by delaying changing the angle of attack to an optimal point in the take-off roll. That's is an interesting physics problem.

My gut feeling is that the transition could be optimized. In physics these things tend toward regression analysis solutions (O.R. math), but without dynamic feedback inputs from sensors into the process (drag, ground speed, angle of attack, rotor rpm, decay rates, disk load rate of change shift , etc.) and being able to react to them quickly enough, it is probably a problem best solved by empirical testing and then live with some solution that gives an acceptable and practical level of efficiency.

I'm interesting in talking if your interest is in a LW.

Andy

 

[Alan_Cheatham]

At this point it's really a non-issue as I incorporated a slip shaft into the prerotator so I could keep the rotor powered even when tilted back, so much for trying to keep the design simple. In fact, in order to meet certain design requirements I have I had to get "creative" with the prerotator, to the point that I have never seen a gyro with one like it before.

 

[Friendly]

Alan,
even if you do lose a small amount of rotor speed, it will be to your advantage IMHO that the rotors are at max loading during take off which will help keep your rolling speed to a minimum. It may not be as critical on a tractor design as it is on a pusher but the slower you can take off, the less chance of problems arising during the taxi. This is one of the things Bensen cautioned about in his manuel. Just a thought.

 

[ckurz7000]

The loss in rpm will be nothing to worry about if you do it properly. When I spin up my blades to 250 rrpm I pull back the stick, release the break and add full power. There is at most a drop of maybe 10 rrpm which I gain back in the initial 10 meters of the take off roll. That's with no headwind. In even a slight to moderate headwind the small drop in rrpm becomes even smaller.

-- Chris.

 

[Friendly]

Chris
250 is good rotor rpm, what prerotor set are you using