Experienced pilots with prerotators need your opinion

Question: How long is the longest time period that you run your prerotator for before it pops the Bendix or you release it.

We need to know the battery capacity/life how minuets do you run a prerotator now to take off.

I need this for:
PRA Open Source Project is well on it way to developing a light weight electric state-of-the-art prerotator!
https://www.pra.org/forums/forumdisplay.php?71-Pre-rotator-Public-sugestions-amp-questions

Our goals:

Push one button forget about it. Unless you flip the emergency shut-down switch or you turn it over near or on the ground; then it turns itself off, and the electrical and fuel system off on blade IMPACT.

Our goal is to rotate 30 foot blades to 200 RRM's then release the brakes while it still driving it during the flapping stage. If you drive it then; the blades will not flap ever? This safety feature is a bonus as you keep driving it up to 350 rotor RPM's for the shortest take-off we believe. Until testing is done we will not really know.

All_In said:

Question: How long is the longest time period that you run your prerotator for before it pops the Bendix or you release it.

We need to know the battery capacity/life how minuets do you run a prerotator now to take off.

I need this for:
PRA Open Source Project is well on it way to developing a light weight electric state-of-the-art prerotator!
https://www.pra.org/forums/forumdisplay.php?71-Pre-rotator-Public-sugestions-amp-questions

Our goals:

Push one button forget about it. Unless you flip the emergency shut-down switch or you turn it over near or on the ground; then it turns itself off, and the electrical and fuel system off on blade IMPACT.

Our goal is to rotate 30 foot blades to 200 RRM's then release the brakes while it still driving it during the flapping stage. If you drive it then; the blades will not flap ever? This safety feature is a bonus as you keep driving it up to 350 rotor RPM's for the shortest take-off we believe. Until testing is done we will not really know.

Click to expand...

How much horsepower the prerotator has will govern how long it needs to be engaged to reach 200 rotor rpm.

The rotor blade weight, the blade chord and the rotor diameter will also affect time.

In my opinion the blades flap because a blade reaches too high an angle of attack.

In other words, too low a rotor rpm for too high an indicated air speed or gusts and the cyclic too far back will tend to stall a blade.

Whether or not the rotor is powered appears to me to make no difference on where the blade airfoil will stall.

I have not timed the Cavalon prerotator.

Regards, Vance

Hi Vance

Yes buddy; Team PRA all know and discussed this.

We purchased a 7 lb 7HP motor for our 1st test. The math indicates that it will do the job! But how long do we have to power it for? We would like our combo battery/capacitor pack to last 2 to 3 times that long.

We know the amps it draws at full power it easy math once we know how long you guys run them now. We will double that time as most are not running up to those RRPM.

The blade flap was formed as a question. It assumed that you are using best pilot technique and as soon as you are off the ground the pilot lower the angel of attack to back to almost 1.5 degrees blade angle to straight and level flight skimming the ground just after lift off to gain the proper airspeed. We suspect that you will not increase the angel of attack until after the prerotator has hit our target RPM and popped the Bendix as the torque/power has hit the motors max.

If the pilot does this then it would be hard to flap the blades because the RRPM will always be faster than anyone could accelerate over the same ground distance???

We used this pilot technique assumption of flying at almost the set blade angle to calculate the motor Horse Power needed to achieve 350 RRPM with the angel of attack and the mass of 30 foot blades. We used an angle of attack of 3 degrees until the relative wind start helping spin the blades as you increase the angle of attack. I have no clue what it really is never measured it but the HP is 1.2476234 over kill at 3!

That is what the question mark was for?

I will try again using different words.

I will try again using different words.

All_In said:

Hi Vance

Yes buddy; Team PRA all knows and discussed this.

We purchased a 7 lb 7HP motor for our 1st test. The math indicates that it will do the job! But how long do we have to power it for? We would like our combo battery/capacitor pack to last 2 to 3 times that long.

We know the amps it draws at full power it easy math once we know how long you guys run them now. We will double that time as most are not running up to those RRPM.

The blade flap was formed as a question. It assumed that you are using best pilot technique and as soon as you are off the ground the pilot lower the angel of attack to back to almost 1.5 degrees blade angle to straight and level flight skimming the ground just after lift off to gain the proper airspeed. We suspect that you will not increase the angel of attack until after the prerotator has hit our target RPM and popped the Bendix as the torque/power has hit the motors max.
This assumption is also of flying at almost the set blade angle to calculate the motor Horse Power needed to achieve 350 RRPM with the angel of attack and the mass of 30 foot blades.

If the pilot does this then it would be hard to flap the blades because the RRPM will always be faster than anyone could accelerate over the same ground distance???

That is what the question mark was for?

Click to expand...

I understood the questions and tried to answer them in post two.

When I take off in The Cavalon I pull the cyclic full back at 180 rotor rpm leaving the engine power where it was to reach 180 rotor rpm until I see 220 rotor rpm and add engine power. The nose comes up and I balance on the mains by moving the cyclic forward. At some combination of indicated air speed and rotor RPM she will lift off on her own.

If I pulled the cyclic full back at higher airspeed at 200 rotor rpm I feel I could flap the blades.

If I advance the throttle at 180 rotor rpm I can feel the blades trying to flap.

If I was taking off in a 35kt wind I would not pull the cyclic full back until 260 rotor rpm.

I feel a blade stalls (flaps) when the critical angle of attack is exceeded.

In my opinion this is from some combination of too much disk angle at too low a rotor rpm with too much airspeed.

I feel whether or not the rotor is powered at the time is not relevant to flapping the blades.

Regards, Vance

OK I was taught here that you can only flap the blades if you are accelerating forward faster than you rotor RPM's and the retreating blade doesn't flap but flies backward and hit the ground?

I really thought that if you powered them faster than you could accelerate over the ground they could not flap. Apparently I'm wrong!

I'll drop the safety feature but I do not really understand blade flap again... Back to the classroom for me?
Dang makes me wonder if I'm ever going to figure this out.

But my real question you did not anwser. How long do you run your prerotator for in minuets to achieve 180 PPRM?

All_In said:

OK I was taught here that you can only flap the blades if you are accelerating forward faster than you rotor RPM's and the retreating blade doesn't flap but flies backward and hit the ground?

I really thought that if you powered them faster than you could accelerate over the ground they could not flap. Apparently I'm wrong!

I'll drop the safety feature but I do not really understand blade flap again... Back to the classroom for me?
Dang makes me wonder if I'm ever going to figure this out.

But my real question you did not anwser. How long do you run your prerotator for in minuets to achieve 180 PPRM?

Click to expand...

In my opinion a rotor blade is an airfoil just like an airplane wing.

I feel if I exceed the critical angel of attack the airfoil stalls and stops flying.

This can happen from too little rotor rpm and too much disk angle combined with too much airspeed.

In my opinion it takes a combination or rotor rpm, disk angle and airspeed working together to make a gyroplane rotor work.

I answered how long by saying it depends on how much pre-rotator power, blade weight and rotor diameter.

The one horsepower Toyota starter on The Predator takes around 20 seconds for the 30 foot 8.5 inch blades to reach 150 rotor rpm with the help of the wind.

If I wanted to wear out the pre-rotator on the Cavalon it would take around 20 seconds to reach 180 rotor rpm. I like to be more gentle than that. I have not timed it, I will try to remember to time it tomorrow.

I am off to bed.

Regards, Vance

Thanks Vance that will help..

I do write badly.. to the rest of you!

Like the Predator anwser 20 seconds for the 30 foot 8.5 inch blades to reach 150 rotor rpm with the help of the wind.

But would like to know without the help of the win.

The formula the time is the unknown quantity! I can double or triple that to be safe!

This motor spins up to 7200 RPM so it will be geared low with 7HP to only reach 350 RRPM! = 20 to1 x 7HP?

It looks like a winner on paper with math only!

We cannot wait to see what it really will do and how close it matches the math..

So far this is much easier than I expected with the modern motors so many candidates today compared to when I first looked 4 years ago.

One piece price $149.75!!!

PRA will buy in volume with typical.electronic discounts of 20% to 80% depending on the quantity purchased! I don't believe state-of-the-art motors will have 80% discounts no matter how many we buy. But we will do this for all the parts and pass the saving on to our PRA brother and sisters!

We may make our fairly inexpensive goal on cost or reduce it. Depends on the cost of the two controller we found. The manufacture does not sell one for the motor we found and is one reason I believe it is so cheap right now compared to the other candidates.

Or they lie in there stat's!!

Also do any of you gurus know of an existing 20 to 1 gearbox for any light weight off the shelf solution. Else we have to make them?

John, if you want some data for an 8.6 m diameter rotor just go back and read the take-off technique thread of mine. I posted some rrpm trace vs. time and indicated where lift off occurred. It should answer your question as far as that's possible.

-- Chris.

John,
I am always careful to use a certain amount of 'Slippage' in the rotor start up procedure; I start out slowly at about 1800 rpms when engaging with the cyclic all the way forward. At about 60 - 70 rrpm's, I pull the cyclic back to neutral as it builds. I gently apply full pressure at about 100 rrpms to stop any further belt slippage as I slowly increase engine power and continue pulling the cyclic all the way back at about 150. The blades will not flap at that point and I can start my t/o roll. I allow the wind to take the blades from there. This saves a lot of wear/tear on the p/r. The initial forces of 'torque twist' need to be carefully considered; not only for the mast ( the possibility of unseen premature metal fatigue) but for the p/r shaft if one uses the Wunderlich type so as not to 'overtwist' with too much initial power input, especially in the higher hp engines.
A fine balance one can only learn from practice. Also, a small 'trick' I learned years ago is to apply a finger tip amount of corn starch as an inner belt dressing on the p/r drive belt. It allows for better slippage (no sudden grabbing or squeeling w/sudden unexpected torque) and will not cause any deterioration of the belt.
It's easy to put more horses to work to spin it up faster but what needs to be considered is the amount of adverse torque one places against the mast in a fast, non slipping prerotation. An electric p/r system does not (correct me if I'm wrong) allow for controlled slippage and if overly powered, will apply that destructive torque we may not see at first in the mast and other areas (unless one uses a 2 stage 'soft start' system). My blades are Sportcopter 8" X 27' and weigh about 100 lbs.
I can take my p/r well over 200 rrpm in about 20 - 30 seconds also but rarely stress the system as I have 8k' of runway to use.
On a second note, I still have not heard back from PRA once again regarding my check after leaving another message. Still hasn't cashed.??? Could you P/M me with an answer? Thanks in advance,

John, I wouldn't expect that it would take more than a minute to spin up with a 7 HP motor. I've built several brushless electric prerotators and all will reach max RPM before that, although I haven't built one anywhere near 7 HP. Which particular motor are you planning on using for the big prerotator?

motor and gearbox and battery....is it getting heavy yet?

ckurz7000 said:

John, if you want some data for an 8.6 m diameter rotor just go back and read the take-off technique thread of mine. I posted some rrpm trace vs. time and indicated where lift off occurred. It should answer your question as far as that's possible.

-- Chris.

Click to expand...

Hi Chris I'm glad you showed up..

You started this project with you posts and data. I've been using it...

But I could not understand the time scale of 0 to 14.
https://www.rotaryforum.com/forum/showthread.php?t=42321

How could it only take 14 seconds to complete?

Brother Mark... I know Mark now we are friends!

This info is very helpful to me learn a few things!!!

Also... the check NO WAY! I sent it the next day!!!

Dang I call and leave a message and get back to you!!!

I think that check has spell on it? Look if they do not get it forget! Just consider it a birthday present from me for all the grief you and I have gone through!! I'll get back to you.

Jazzenjohn said:

John, I wouldn't expect that it would take more than a minute to spin up with a 7 HP motor. I've built several brushless electric prerotators and all will reach max RPM before that, although I haven't built one anywhere near 7 HP. Which particular motor are you planning on using for the big prerotator?

Click to expand...

Thank john I think you are right..

So we really only need about 3 minuets of power for overkill!

That is great news...

When I watch video's it seem like I have to wait days while the pretotate before they do anything fun to watch = like start to move.

ultracruiser41 said:

motor and gearbox and battery....is it getting heavy yet?

Click to expand...

No we have to build the attach motor attach bracket and the steel gears, housing, and Bendix and weigh those... and two controller to really anwser your question! It could be as little as 10lbs up high on the mast but control we could put down low anywhere to try and offset the weight and balance?

We only need 3 min's of power we will be using super-capacitors with no battery then. They weight NOTHING = really feather light!! And can be recharged with even the Rotax low amp charger in less that a few minuets!

Friend is using them in an electric car and he paid for only one pack still cheap enough for him

PRA can buy in volume say one thousand and make it so much cheaper for all of us!

May be a total of 15 lbs or a little more. But that is a SWAG!!!

We will keep posting the weight as it grows...

John, You're right. I imagined different data than the one I actually posted. I will post a typical spinup time trace of rrpm from 20 rrpm up to 220. That should give you all the onfo you need.

-- Chris.

John, the energy density of caps is much poorer than lithium batteries. Caps are going to wind up being a dead end at this point in time. An RC or motorcycle lithium battery pack of about 4-6 amp hours, (4000-6000 mah) should be sufficient for about 3 prerotations before needing to be recharged. There are a few restrictions. They don't like cold weather so you may need to preheat the battery in really cold conditions and you really don't want to leave it discharged in the cold. (I mean Michigan cold not California cold). If you are going brushless, they don't like starting off from 0 RPM at all unless you add sensors which adds complexity.