DC motors on the rotors for a prerotator?

dabkb2

Dave Bacon
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Chapter 31 has a college class trying to design a prerotater and this is what they are currently thinking. I have never heard of this before so I thought I would ask the group


We were looking into some design ideas and wondered if we could get your input on some things. We were thinking of putting 2 DC motors on the rotor itself eliminating a drive shaft and balancing the two so as to keep it stable. We were wondering if that's been done before or if that's just not a good idea.

We were also wondering if theres anyway the pilot knows the rotor rpm while flying or if that is something that could be useful. For example, if we use an rpm sensor for calculations, we can also display it to the pilot in a gauge or digital display if that's something you think would be helpful or desirable. Something else we need to do is find the torque required for the rotor. That paper gave us a general idea but we would like to measure the torque ourselves if we can. I remember you or john said you've done that before and I'm not sure if you have the number we need or if it was for a smaller blade length.

Let me know what you think,
Any thought would be appreciated
 

Alan_Cheatham

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dabkb2;n1142492 said:
We were looking into some design ideas and wondered if we could get your input on some things. We were thinking of putting 2 DC motors on the rotor itself eliminating a drive shaft and balancing the two so as to keep it stable. We were wondering if that's been done before or if that's just not a good idea.

Ralph Taggart tested on his Gyrobee a prerotator first using two R/C aircraft propeller glow engines then went to electric propeller motors with battery packs mounted to the rotor blades. There are threads on the forum discussing these if you can find them.

There are many different rotor rpm instruments that can be installed on gyroplanes, almost every machine is equipped with one.
 

Jazzenjohn

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I set up my first electric prerotator with data collection and graphed the results. I posted a thread here: https://www.rotaryforum.com/forum/e...nents/35360-prerotator-power-required-vs-rrpm
It was brushless, had a 2 stage reduction, and I was using 23' Dragon Wing rotorblades. I compared my results with several other prerotator systems including the Go-ped prerotators from Dragonfly and Superfly, Ralph Taggarts results, and anything else I could find that had what I felt was reasonably good data and I believe the numbers I posted are pretty accurate from below 1 HP to at least 4 HP.
 

Jazzenjohn

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Jason, 200 RRPM will require about 2400 watts of power. That is a little more than 3 horsepower. You can look at the thread I started about power required vs RRPM and work either direction, how much power you will need to get a target RRPM, or what RRPM you can expect from a given motor/Battery/ESC combination, assuming you have a reasonably efficient reduction and correct ratio. At 12 volts, that means 200 amps to get 2400 Watts. I don't have any pics of that first system, that was 7-8 years ago. I'm on version 10 or 15 by now. I make my own rotorheads and build a ratchet and pawl system into them and use a chain for the final drive. The chain doesn't move in flight because it is freewheeling whenever the prerotator motor is off. Anything that has the chain spinning inflight is doomed to fail because it will almost certainly exceed the critical speed limit of the chain at flight speeds.
One thing you should know is that you will most likely need to give your blades a flip to get them started if you use a brushless motor because they have less torque at zero RPM than a brushed motor. Which Turnigy motor are you using?
 

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Jazzenjohn

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Yes Alan, It's too bad, this was a good place for those discussions. I have a couple new guys building near me now for the first time since I started in gyros.
 

Smack

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I would LIKE to build from a PLAN, but don't think that there are many out there of 'modern' design.
No Benson, KB3, or Hornet. I want to find a 4-stroke (Rotax or Subaru) single-seater plan.
Maybe the closest to what I want, but still not a complete plan, is the Tervamaki JT-5.
Anyone know of any 'plans' out there?
Brian
 

Kevin_Richey

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Smack;n1142545 said:
I would LIKE to build from a PLAN, but don't think that there are many out there of 'modern' design.
No Benson, KB3, or Hornet. I want to find a 4-stroke (Rotax or Subaru) single-seater plan.
Maybe the closest to what I want, but still not a complete plan, is the Tervamaki JT-5.
Anyone know of any 'plans' out there?
Brian

Brian: The PRA has the Termaki JT5 plans on their website. PRA members can access them after signing in.
I just tried the link w/out signing in and was able to see them...

It shows lots of info, such as dimensions, materials used, bearing sizes, and even shows drawings w/ explanations on how to build their fiberglass rotorblades!

The RB production process even explains what temperature to post-cure the resin at (70 degrees C.).
I think the three-way drawing of the JT5 is a work of art!

http://pra.org/Junkka/JT Downloads.html
 

Jazzenjohn

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JasonS;n1142548 said:
The 150cc that was suggested. I have not determined just yet what the next step in choosing a DC motor will be chosen.. I don't have the time now or the extra funds to experiment so I will need to be sure which motor to get. I am hoping to figure out through this posts to help me decide. Thankx for your research thus far. It will likely be a great help to myself and others looking for a solution.
Are you saying you have the Turnigy Rotomax 150 CC motor like this?

If so, You'll need a Huge battery that can supply 200 Amps at 52 Volts to get full power out of it. The ESC needs to be 250 Amp rated too. Is this the motor you have?
 

Jazzenjohn

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Jason, that motor is around 8.5 Horsepower. I believe your biggest problem will be the necessary torque required to get those heavy blades spun up from zero RPM. The reduction ratio plus the power of the selected motor would have a large effect on the number. I haven't done any data collection with heavier blades yet. I plan on comparing results from a set of Sport Rotors with the Dragon Wings to see how much effect blade mass has on my results. My expectation is that the largest effect of heavy blades would be on initial start up, 0-100 RPM. I'm confident you could get more than 150 RRPM using a large brushless motor. Are you certain you need it to go from ZERO RRPM? If yes, you need to consider going with a brushed motor since they have full torque at zero RPM.
 

Jazzenjohn

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A favorite motor from back in my Battlebot days was the Magmotor. It has since been renamed the Amp Flow motor. The A28-150 should do everything you ask. There may be others, but that would be my first choice for a brushed motor right now.
 

Jazzenjohn

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The Vyper ESC is a possibility, You can probably get away with running it at 10s Lipo with a 5 Ah battery good for 50c peak. I'd expect 4-8 prerotations/charge depending on how fast you ramped it up.
 

Alan_Cheatham

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The issue with a brushless outrunner motor at zero or low speed is really an issue with their controller and the timing of the signals to the motor, a sensored controller is probably best for pre-rotor application compared to a sensorless one.

Also keep in mind that most electric pre-rotators are using some sort of soft-start to limit torque during the initial application of power, the "inefficiency" in the starting torque of an outrunner motor may very well be desirable for that soft-start.

At one time we were building a gyro with a pre-rotator using the Turnigy 100cc motor but the project was never finished. My take on it was the cost and operational limitations weren't desirable compared to a good mechanical drive or go-ped system.
 

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Jazzenjohn

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Ebay for a used motor. Lower your power requirements by spinning up slower and accepting a lower top speed. Brushless is cheaper mostly because you have more choices, but the start from zero is problematical. You could go with a cordless tool motor. Smaller cheaper and lighter but less top speed. They are trial and error because you can rarely get good specs on them. David McCutcheon told me about someone that used a small scooter motor with reasonable success. There is a thread here: https://www.rotaryforum.com/forum/general/flying-photos/43248-electric-pre-rotor/page2
 

Jazzenjohn

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The Talon looks like a good controller, especially for the price. My only concern is whether it can handle the current draw of the Amp Flow motor. Same for the batteries. The specs are usually pretty vague about that stuff. If it can, that would be a pretty convenient package. If it can't, you will soon know what the inside of the Battle Box smells like.
 

dabkb2

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Thank's for all the great info we have some great minds in here, but they are talking about placing the motor directly on the rotor, one on each side to balance the weight, has anyone seen this before, or is it just a bad idea? I have never heard something like this, its definitely outside the box. Powering them would be a trick but not undoable. It is not my idea, but something the class thought might work.
 

Jazzenjohn

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Dave, if you are suggesting that they would use the motors, one on each side, to friction drive a disk or drum, or drive a large gear, then I personally haven't seen that configuration. There has been some working setups using props which provides a nearly torqueless prerotator. Offhand, the only problems I see with that setup are the ability to configure it to the different hub bars people use, and the wireless setup or commutator drive to supply power to the motors. Neither issue seems insurmountable.
 
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