Hub Bars and Starter Motors

[asra]

Dave, you're a model of self restraint![^]Llewella

[asra]

and he hasn't even got John telling him he can't say that!! Some people have all the luck..

[NiqueNaque]

John,The business of separating engagement from rotation on a starter is a good idea. Though I would warn ahead of schedule that if it is used some thought must be put into making sure that the engagement function is definitely killed before take off and impossible in flight, I dread to think what would happen with a pinion trying to engage during flight.Cheers,Nick.

[Brian]

Sorry Nick, nothing much happens if the pre rotator engages in flight...except you can stuff up the starter if it stays engaged.

[birdy]

Not to mention the noise.

[asra]

[NiqueNaque]

Well,On that basis if a starter motor can friction weld itself how much resistance did that take?Enough to down a small gyro plane maybe, Brian...? Never mind the shock loadings imposed on the hub bar and rotor every time the bendix tries to engage and gets forced to disengage by the rotor RPM.Cheers,Nick.

[asra]

AES Mark. What amperage was the 6 volt battery that you used?[?]Was it a gell cell?[?]Aussie Paul. []www.firebirdgyros.com

[Brian]

No Nick, doubt very much that any damage is going to occur other than to the wallet!!Plus the chances of a starter motor going berserk are very minimal, put it this way Nick, there are a lot more relevant things to be concerned about.Whats AES Paul ?

[NiqueNaque]

AES = Albert E Sikorski.I believe you should reconsider your answer.Nick.

[Brian]

If we were to have a safety valve for every component on a gyro, then it would never get off the ground due to complexity and weight.The starter motor holds no fear to me in either pre rotating or "welding itself" imposing "shock loads" on the hub bar.If I had a 3/4 inch hub bar, you would get a difernt answer and if I didnt have a few hours of flying I might be a little concerned.There's enough BS out there now regarding non CLT gyros for example.

[asra]

Paul.........only about a 4 ah battery...out of a dt 175 yam i thinkjust nemember it only goes for about 10 secs before its flat ...but by then you are well and truly into the 12 volt hub bar snaping...starter motor welding.....gee some crap hypertheticals come up on here...we should be more worried about running out of fuel than hitting hippos on takeoff

[asra]

Thanks Mark, I can get 10, 12, or 14 amp/hr 6 volt gell cells. The 14 amp/hr is about $45.I had planned using two 6 volt car batteries is series but they are so heavy and expensive, so I had put it on the back burner for the time being. I am glad I did.I had some hard facing come of the small drive cog in the Hirth gearbox today, so I will have to put the 120 hp ej-20 and Sub 4 g/box, that I flew to Alice springs, back on. The box has around the 550 hours without laying a spanner on it, just the occaisional oil change, so I can't complain. Things always break when I am busy,[] never when it is sitting in the hanger!!!!!!![]Aussie Paul.[]www.firebirdgyros.com

[Tim]

The ring gear (rotors) out run the starter motor by a considerable margin in flight. For the pinion to engage in flight and remain engaged, the rotors would have to be turning backwards. In which case, the pinion being engaged would be a minor nuisance.Tim McClure

[Tim]

Jonh E. - your statement "a force in excess of 15,600 lbs is applied to each bolt. Their shear strength is listed as 3,650 lbs." - Concurrs with the calculation by Doug Riley below. "I'm not learned, just barefoot at the moment. 2600x12=31,200 lb. at the one-inch radius. If there are two bolts, then they each bear a load of 15,600 lb.The X-sec. area of a 1/4" bolt is pi x R x R = 3.14 X .125 x .125 = .049 sq.in.The theoretical shear load is 15,600/.049 = 318,000 psi. Ridiculous.Obviously, that's not what's going on. One thing that may hold things together is friction. Just as with a prop on a hub, a significant amount of drive force can bypass the bolts and be transmitted by friction between the rotor hub and the teeter block, if the bolts are snugged down. If they aren't, shear on the bolts is all you have."Doug Riley.A statement by Al Hammer is also relevant - "if the rotor is spun at 200 rpm and the reduction is 10:1 That means the rpm at the driving end is 2000 rpm.Hp= (torque X rpm)/5252, with torque in ft-lbs. so hp = 260 X 2000/5252 = 99 hp."Al Hammer.Even if the rotor was only spun to 100 rpm, the starter motor would still be developing 45 hp. We know that this is an impossibility. I guess my thoughts on this impossibility clouded my mathermatical ability - your calculation was obviously correct, it must be just the starting figures that are wrong.We certainlny have an enviable record of "history of safe operation" on such prerotator units, also testament to the reliability of the units. Doug Riley mentioned friction but we can discount a percentage of this because a lot of modern blades are pitch adjustable via thin shims under the teeter block, considerably reducing the friction area. The measurement of the 260 ftlbs is obviously the problem. There should be no impact torque because the starter is not powered until after the pinion is engaged. The only torque prior to this is from the slight helical twist on the shaft, designed to ensure engagment before the power is applied. Therefore most of the "impact" must come from slack in all the gears. Impact forces can be multiplied hundreds of times when acting on an immovable object, as compared to acting on a moveable object, where the force is reduced proportunately by the amount that the object moves. (Eg. if a force is spread out over 1 thou then the same force was spread out over 20 thou movement, the resultant would be 20 times less impact force. In our case the mast twists a considerable amount, simultanously as the rotors start to turn, reducing the impact forces considerably, possibly by ten, twenty or more times. The only true way to tell would be to do the torque measurement as in situ with a free mast and rotors installed.Tim McClure