My weight assessment has already taken into account the fact that the ring gear is half as light as that of a car , thanks to the absence of overtorque relatively to car starter.
For safety reasons, the pinion must be released from the crown during flight. That's why I've considered a bendix system (without the solenoid) and not a belt.
The Wunderlich bendix i just put on the scale was north of 1500 grams, more than 3 pounds by itself. the ring gear was another 925 grams. The total for the Wunderlich system came out to about 20-25 pounds with the very heavy flex shaft and all the other parts involved. That is the big reason i switched out from it to other alternative systems. It works great and is a durable and long lasting prerotator system, but it is terribly heavy for those interested in keeping weight down. The NANO hydraulic system by comparison is about 11-12 pounds. Part of the lighter weight is that it doesn't have any heavy castings and trades out the bendix for a much lighter sprag clutch. All in all it is an excellent system. The manually activated bendix by Tango gyro seems lighter and the bendix used by Denis from Gyrotechnic seems like a very good, lightweight alternative. https://gyrotechnic.com/wp-content/uploads/2019/06/W-Flex-A.jpg
Thanks for the picture and the weights information .The advantage of the electric pre-rotator, despite the high cost of the battery, is that since its operation is independent of the propulsion engine, it doesn't require a powerful wheel brake like the others.
Thanks for the picture and the weights information .The advantage of the electric pre-rotator, despite the high cost of the battery, is that since its operation is independent of the propulsion engine, it doesn't require a powerful wheel brake like the others.
That is a great point, I had not thought much about it. And it would also allow more controlled acceleration during the spin up to get airflow though the disk started to prevent the transition phase of the rotor going from a fan to establishing auto rotation.
I think having the propulsion separate from prerotation would result in a safer take of profile, like taking off in a controlled head wind.
Whatever the prerotation (i,e at rest), there is always during the run a drop before the rrpm resumes, but where is problem?
With 230 rpm of 24' rotor, the run at immediately full throttle is possible (stick full back, of course !)
But with the electric system, propeller keeping to idle in grass requires no wheel braking.
230 RRPM on a 24' rotor would need approximately 5000 watts or 6.5 HP. However, with electric you can impart force into the blades during the takeoff which minimizes or eliminates the RRPM drop and doesn't require the full power to achieve it. There is potentially the problem with the prerotator turning the body of the gyroplane to the right if balancing on the mains without brakes. This has happened to me several times when the nose wheel lifted since i only have nose wheel brake and steering. I now add power before the nose wheel lifts and apply left rudder to counteract the prerotator force. I don't have much experience on grass.
Im taking that from the actual numbers i got years ago when I first did my brushless prerotator experiments. They were put into a graph in the thread Prerotator power required vs RRPM. They tracked well with the other systems ive built since. all my numbers are with 23' Dragon wing rotors. Arco recently also did some brushless experiments and put his data on that thread as well. He used a 28' Rotor that weighed twice as much. I took his data and added the curve from my experiments to basically see the difference between the different rotors. The curve matches up fairly well and I just extrapolated the power needed for a 24' rotor by going in between the 2 curves.
What power in watts do you expect you'll need for 230 RRPM and where are you getting your data from?
John
Your measures seem indisputable to me, but I would like to raise the following points:
- Your base is a DW rotor whose aerodynamic pitch adjustment I've often mentioned is greater than that of most other rotors in use.
So, in static, that it absorbs more power than most, all other things being equal.
-To extrapolate, we need to take into account that mechanical power is proportional to Diam.^4 and Ω^3, and chord, which follows from fundamental aerodynamic laws (Drag = ½ ρ S Cd V^2).
So, your measurement at 220 Rpm for 23' indicates 3000 W.
Good, then 24' instead of 23' , with the same aerodynamic pitch setting and the same chord, the power consumption becomes 3000 * (24/23)^3 = 3600 w, and with a more standard pitch setting it would be more like 3000 W.
Finally, for 230 rpm instead of 220, mechanical power consumption will be: 3000 W ( 230/220)^3 = 3430 W
That's why I think the 5000 W you're predicting seems wrong due to bad extrapolation.
Now, the Arco's measurement for a rotor with standard pitch setting and 8.5m x 0.22 m rotor gived 4000 Welec at 210 rpm.
Reported at 230 rpm, 7.32 m and a 0.18 m chord, this gives:
4000 W *(7.32/8.5)^4 * (230/210)^3 * (0.18/0.22) = 2400 Welec And with the same efficiency as you assumed ( i,e 0.92) we obtain that the absorbed mechanical power will be 2400 W*0.92 = 2200 W
I wouldn't presume to debate you in math. I might debate you in actual results. In all the experiments i've done over the past 12 years, my experience tells me that it is unlikely you'll get that favorable an outcome with the small additional power you suggest. I agree that my extrapolation my be off some because it is exactly that, an extrapolation from a graph. When discussing the power required to get the prerotation performance at these levels, 500 watts is almost a rounding error. In any case, the kind of power you are proposing should give you a very good result presuming the rest of the system is up to the task (battery, ESC, wiring, control).
Of course! my formula only deals with the power absorbed by the aerodynamic pressures on the rotor. (With International units) Protor = 0.005* ρ* c* R⁴ * Ω³ = about 2700 W
Then we have to add all the other losses: chains, belts, electrical engine, cables, battery, speed controller...and the inertia of the blades if you don't wait until the end of acceleration...
I wouldn't presume to debate you in math. I might debate you in actual results. In all the experiments i've done over the past 12 years, my experience tells me that it is unlikely you'll get that favorable an outcome with the small additional power you suggest. I agree that my extrapolation my be off some because it is exactly that, an extrapolation from a graph. When discussing the power required to get the prerotation performance at these levels, 500 watts is almost a rounding error. In any case, the kind of power you are proposing should give you a very good result presuming the rest of the system is up to the task (battery, ESC, wiring, control).
Just from working with these motors, I have had a rough estimation about this application. I have always thought it would take around 8hp to 10hp
to do this effectively, but Jean Claude's numbers seem to indicate less. The readily available inexpensive motor/controllers top out in the 2700w. range.
Both of these are about 4hp., the outrunner weighs 1.5lbs. the Hacker weighs about 1/2lb. and has 6.7 to 1 reduction.
The outrunner on the left powers a 25lb. aircraft to over 100mph. The purple hacker powers a 14lb. flying wing that can cruise at 55mph. on less than 125w and can fly on solar power while charging the batteries. It was chosen for efficiency, not power. They no longer make this model...
This discussion has had me hopeful that less power was required and maybe this range of motor would work, but I still have doubts.
The 5hp. and above motors are out of the RC bell curve and into the UAV territory and cost a lot more.
You can run multiple motors on the same speed controller, so using 2- 2700w motors gets you into the 6hp+ range. but now you have a weight penalty.
No free lunch...
Running 2 motors side by side on the same cog belt would reduce the gear engagement so if something did lock up the belt would fail earlier. I really don't think this is an issue though, the belt will always fail if it is not over sized.
Another possibility is another 2stk engine. This one is 7hp. and weighs less than 7lbs. with prop and mufflers...
This could use a belt and tensioner, or a clutch bearing that is not locked on a threaded shaft, so if it locked up, it would immediately unscrew from the system and throw the belt.
You would hand pat then engage the tensioner to start it, or use a pull cord on a lower pulley if you had a clutch bearing. These start with a couple revolutions and are very reliable. I can usually start these with 3 hand flips of the prop.
I'm throwing out options to get more input, all of these are very viable and a lot lighter than Wunderlich.....
Now to find some inexpensive 6hp+ motors......
It isn't necessary to prerotate to 230 or higher. You can do fine going up to only 130 or so, which requires a far smaller motor, but it is nice to go higher. My gyro does well with 3000 watts / 4hp. I get 185+ with batteries that have half a charge and i get many prerotations if I elect to just go to 170 or so. The highest number I've gotten with my current setup was 215 with new, freshly charged batteries. The outrunner in your pic would do a great job if set up correctly.
It isn't necessary to prerotate to 230 or higher. You can do fine going up to only 130 or so, which requires a far smaller motor, but it is nice to go higher. My gyro does well with 3000 watts / 4hp. I get 185+ with batteries that have half a charge and i get many prerotations if I elect to just go to 170 or so. The highest number I've gotten with my current setup was 215 with new, freshly charged batteries. The outrunner in your pic would do a great job if set up correctly.
Have you come up with a rolling procedure that get the airflow engaged before the rotor becomes a "Fan" and thus lowers the power requirement for prerotating?
It seems like there would be a perfect ratio of a rolling start combined with powering the rotor up to the point of take off....
I get that it's not going to be a particularly short take off, but some of the high powered gyros in the past had fairly short take offs and the ability to pre rotate without doing the throttle/brake/prerotator dance would seem to be more efficient.
Note: Although the outrunners have great torque right out of the box, the inrunners with planetary gear boxes are actually more efficient.
Not sure how the gear box would hold up to the repetitive high loading though. They were never designed for that....
The lightness of electric motors comes with high rpm. So you have to add a gearbox that weighs more and costs more.
I have no experience of the service life of these accessories, but if the variable speed drives had a current regulation loop, then the torque applied would remain constant throughout the launch and there would be no premature ageing.
Of course, the battery is also a critical element.
35- 40 volts seem necessary, which would limit the current to 80 - 90 Amp. Since a launch would take about 35-40 s this requires 80*35 = 3000 A.s or about 1 Ah per launch. So you'll need a 10-12 cell LiPo and more than 10 Ah to supply the electric power swallowed by the motor.
Probably more 2 lbs. Do you have an idea of a possible model? Weight and price?
Recharging by the rotor via the electric motor as générator seems to me to be out of the question, given the mechanical irreversibility of the high-ratio transmission (35-40). But since the autonomy would reach 10 launches, it doesn't seem useful.
Don't hesitate to tell me if I've said the wrong thing!
Have you come up with a rolling procedure that get the airflow engaged before the rotor becomes a "Fan" and thus lowers the power requirement for prerotating?
It seems like there would be a perfect ratio of a rolling start combined with powering the rotor up to the point of take off....
I get that it's not going to be a particularly short take off, but some of the high powered gyros in the past had fairly short take offs and the ability to pre rotate without doing the throttle/brake/prerotator dance would seem to be more efficient.
Note: Although the outrunners have great torque right out of the box, the inrunners with planetary gear boxes are actually more efficient.
Not sure how the gear box would hold up to the repetitive high loading though. They were never designed for that....
In the case like Johns even at only around 170rrpm, it is perfect for what can be a very short take off with correct technique.
A pre rotater separate from the main engine is a great way to go. Pre spin close to the max and leave spinner engaged all through the take off roll will accelerate the rotor very fast. With that relatively low power spinner you could also leave that on until airborne. ( Not as a beginner though).
Like Wolfy says, the electric prerotators can have the ability to add power to the rotor during takeoff. I use a push button system that keeps power on during takeoff all the way up to 250 RRPM and then shuts itself off automatically. I use some form of 1 way bearing, like a sprag or ratchet and pawl, or one way needle roller bearing built into the rotorhead to do this. It saves the weight of the bendix and makes control simpler, but it makes it nearly impossible to simply add onto someone elses rotorhead without extensive modification, or complete reengineering. There is some danger if the motor is still driving and you are balancing on the mains. It can send you off the runway to the right. It's happened to me several times. main wheel brakes can fix this, a smaller rockback angle can help some, putting in full left rudder and applying power before it rocks back works. I'm sure there are other ways.
The lightness of electric motors comes with high rpm. So you have to add a gearbox that weighs more and costs more.
I have no experience of the service life of these accessories, but if the variable speed drives had a current regulation loop, then the torque applied would remain constant throughout the launch and there would be no premature ageing.
Of course, the battery is also a critical element.
35- 40 volts seem necessary, which would limit the current to 80 - 90 Amp. Since a launch would take about 35-40 s this requires 80*35 = 3000 A.s or about 1 Ah per launch. So you'll need a 10-12 cell LiPo and more than 10 Ah to supply the electric power swallowed by the motor.
Probably more 2 lbs. Do you have an idea of a possible model? Weight and price?
Recharging by the rotor via the electric motor as générator seems to me to be out of the question, given the mechanical irreversibility of the high-ratio transmission (35-40). But since the autonomy would reach 10 launches, it doesn't seem useful.
Don't hesitate to tell me if I've said the wrong thing!
Using the motor as an alternator probably would not work with the geared inrunner, will work with the outrunner.
I have done it, but used higher voltage due to the aircrafts engine RPM range. Everything was regulated downward for different aspects of the aircraft.
Here are a couple batteries and motors in our range:
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You might need 2 in series., so this configuration, but you can get an idea of what is available.
2.5lb. $80. I don't think you need this much and I would probably be looking at something like this:
I would be shooting for 2 to 3 prerotations with onboard charging and maybe even building a brushless alternator that could power charge, or the rotor in flight. This one is on sale for $89, weighs 2.4 lbs and is discontinued, but there are other variations. I only spend 3 minutes looking....
Here are 2 motors rated at over 7000w, they weigh about 4lbs. and are around $450-ish....
Like Wolfy says, the electric prerotators can have the ability to add power to the rotor during takeoff. I use a push button system that keeps power on during takeoff all the way up to 250 RRPM and then shuts itself off automatically. I use some form of 1 way bearing, like a sprag or ratchet and pawl, or one way needle roller bearing built into the rotorhead to do this. It saves the weight of the bendix and makes control simpler, but it makes it nearly impossible to simply add onto someone elses rotorhead without extensive modification, or complete reengineering. There is some danger if the motor is still driving and you are balancing on the mains. It can send you off the runway to the right. It's happened to me several times. main wheel brakes can fix this, a smaller rockback angle can help some, putting in full left rudder and applying power before it rocks back works. I'm sure there are other ways.
Tall tail, or short tail?
Just wondering if eliminating some of the P factor would help, or maybe the P factor does help depending on which way you prop is spinning..
![[RotaryForum.com] - Prop wash as prerotator?](https://data3.rotaryforum.com/attachments/133/133725-e32462468e2df536746f74e98448d415.jpg?hash=nmMzBVNR0A "[RotaryForum.com] - Prop wash as prerotator?")
![[RotaryForum.com] - Prop wash as prerotator?](https://data3.rotaryforum.com/attachments/133/133726-f9152a625ca5fb63eb68d7bac6d25653.jpg?hash=bcUeOZ138y "[RotaryForum.com] - Prop wash as prerotator?")
![[RotaryForum.com] - Prop wash as prerotator?](https://data3.rotaryforum.com/attachments/133/133724-e385c68f637c3c6eb01afa37422cbf24.jpg?hash=RsL1t3skuz "[RotaryForum.com] - Prop wash as prerotator?")
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