Hub Bars and Starter Motors

[asra]

[Mark Horan]

Dear John,My instructor (god bless him) Taught me how to spin my rotors up by hand and I think that through my whole training we never used the pre-rotator once!I thought that he just liked to see me standing on the seat,in a semi crouched position,grunting and pulling to make the 28 ft gerrys spin past 45 or so rpm.Well after all that effort, neither of my two machines have pre rotators. Why? Because I dont NEED one! Even on those calm still days, with a bit more effort on the initial pat up to get them over the hump at a little over 50 rpm my blades spin up quite quickly with the correct amount of forward momentum.Ive been kicked by a horse and I wouldnt use that same force to start my rotors!Mark.

[NiqueNaque]

John,Thanks for this information.Cheers,Nick.

[birdy]

Ive been kicked by a horse and I wouldnt use that same force to start my rotors!I'v been kicked by a ^%*&^ nag too,and if my rotors would do the same revs as I did through the air it'd make for an effective prerotator I reckon.

[Tim]

"Now that 260 ft lbs is multiplied by a pinion to ring gear ratio of say 10:1, so in excess of 2600 ft lbs is applied to the hub bar. But the torque is then transmitted to the mass of the rotor by the bolts through the hub bar and then into the teeter block. If the bolts [often 1/4"] are 1" from the centre, a force in excess of 15,600 lbs is applied to each bolt."G'day John,Interesting figures. I can't help feeling that something is wrong though. Just to simplify the exercise, lets assume that the ratio of 10:1 is 1" drive and 10" driven. This makes the torque at the center of the hub bar 2600 ft.lbs as you correctly stated. This is where you lost me.If the bolts were 1" back from the center then I think that the torque at the bolts would be 9 tenths of 2600, which is 2340 ft lbs. if you divide this by the two bolts, then you have 1170 ft lbs shear on each bolt. Please tell me if I am wrong.This is assuming that the starter motor has 260 ft lbs of torque. This is another figure that I have a problem coming to grips with. At full throttle, a Subaru EA81 develops a maximum of 92 ft lbs of torque at 2400 rpm. That makes the starter motor nearly three times the torque of the Subaru. Perhaps I am doing something wrong here.There are many thousands of hours done here with electric pre-rotators, some with the high torque geared ones, with no problems. The ring gears are held on with 6 aluminium rivets.Tim McClure

[asra]

I'm no x spert on this issue by any means, is it possible to make the teeter block a little wider and then machine a U shape into the botttom of it so it was to snugly fit over the hub, this in itself would then take a majority or the sideways thrust/stress off the bolts for the initial burst of power from the starter ??Or would this start a whole new problem of stress to somewhere else, because the part of the teeter block that was machined would have sharp edges on the inside ??Mark

[Brian]

I stand to be corrected but doesnt the mast take some of the torque too ??

[asra]

[asra]

John is the resistor that you had along the lines of a voltage reducer, just been looking into them abit from what i can make out they can start from about 3-5v then with a turn of a knob you can open the voltage up to full. Would definately help with the problem i was thinking.Mark

[Greg Mitchell]

Butterfly/Monarch elect pre-rotators come with a "soft Start" reducing start up torque impulse.John actually helped sort out Allan's installation with his Monarch. After I solo, I'll be installing mine.Mitch.www.thebutterfly.info

[asra]

[Tim]

John, we seem to agree that if the starter had 260 ft lbs of torque with a 10:1 advantage, then there would be 2600 ft lbs of torque at the center of the head. As you come back from the center the force should now be divided, not multiplied, until you reach 1 foot again where the force will be back to the origonal 260 ft lbs. If you go even further out to the tip of the rotors the force is so small that you could hold it back with one finger. Where am I going wrong?Tim McClure

[Greg Mitchell]

John,Had Allan cut the coil wire down to 9" or so. Larry sent them out made up but later advised that the wire need only be a 1/3 of the original length. I'll see what I can do about getting one to you.I reviewed the GFLG Video last night and the soft start appears to come in very gently/slowly and is allowed to stabilise, then he gives it the rest and with a 20 to 25 knot headwind (sock is at the horizontal) he brings the blades up to speed, begins taxiing and at count of three nose wheel comes up another count of 2 and he's airbourne. I made that video available to the Bond Springs Event, so anyone who has it could review same. It looks like a real gentle start up but then I don't know much about all that, I'm playing pat, pat.Mitch.www.thebutterfly.info

[asra]

[asra]