Hub Bars and Starter Motors

[asra]

Nick that type of teeter block design is a question i asked a while ago, because it is sitting over the sides there should be no stress on the bolts, but probably with no pic people could not under stand what i was talking about.Brain micro has am email picture reducer that works fine for this purpose also just go in and reduce the photo then save it in a file to use for here.. hope that makes senseMark

[asra]

Dear GentsThe channel locking teeter block "IS" going to have some affect in moving the stress point further out allong the hub bar, although desirable, and if you are reconstructing; why not move the bolts as far out as practical?, perhaps as far as 6 or 10 inches apart, this would spread the torque input loads considerably if using a prerotator, thus improving the fatige life considerably.Your coments please.CheersGyro Gearloose

[asra]

Great to have you with us Alex.As the bunny who started this thread I guess I will have to bear some responsibility for it. If you can stand to dredge though all the early stuff most of your questions will be answered. If you have a look at the 'Teeter blocks' thread you will see some info relating to this subject.I must admit I thought this thread had died, and haven't looked at it for a while, it's getting to be a monster!I think it was Ian Sganzerla who originally came up with that teeter block design, the trouble is it either requires a skinny hub bar or a rotor head with pillars further apart.John EvansThink logically and do things well, think laterally and do things better.

[asra]

Dear JohnIt seems we are cross topics in 2 forums, my thoughts were to move the bolts further out toward the blade tip as per existing hub bar design and dimensions, this would reuire a rework of the teeter block dimensions of course but it seems that was a part of the proposal anyway. Any increase that can be made in the pitch distance of these 2 bolts, will reduce the stress on the hub bar dramaticaly, if one crosses to the rotaor blades thread where you were last it is clear that ther is provision to go to a 3 inch radius of center ie; 6 inches appart, this has got to help.By the way what material are you using for the hub bar?Has anyone you know tried using a dual load path construction hub bar, to beef up and offer redundancy around the hub?CheersGyro Gearloose

[asra]

The usual material is 6061 T6, but the one that failed was 2024 T3. I am hoping these discussions motivate someone to revise their whole rotorhead and hub bar design to accomodate the improvements suggested. The trouble is that if you can't use the HOSO [history of safe operation] argument to support your design, you really need to provide the appropriate engineering analysis calculations and incorpoprate the usual safety factors, which have the effect of making something which many believe is overengineered, into a design which is really marginal.To me HOSO can be little more than a delusion as 'eye-ball engineers' stretch components to suit heavier machines with little concept of how much they are eating into safety margins.I know of a hub bar design using two pieces of 1/4" 4130, but that's still a long way off flying.You only have to consider the difficulties being experienced by Robinson with their helicopter blade to realise that this is a difficult area where sound engineering practice should be applied, and even then things can get tricky!John EvansThink logically and do things well, think laterally and do things better.

[asra]

Dear John As you suggested, and as I did also confess, the hidden 11 pages of forum held a substantial amount of interchange, running in cycles repeating itself interspaced with...... you know what. If I can anylize, everything has been said at somestage by some one, Did you know that 12 pages of forum = 51 PAGES of foolscap to read!!!!!. anyway I think Tim is on the right track, a lot of the initial impulse is taken up in the mast twistimg.I would like to suggest that a translating shock mounted motor might bear some development, and I have some Ideas that cant be put down in type at this stage, and will figure out drawings later.Pete Barsden and I have swapped some emails, he has done some impressive work, if any one were to email him and ask for some pics you would see that he has put his thoughts into action, albeit at a cost. I feel we can have a win with a resistor soft start and a shock mounted drive.CheersGyro Gearloose

[asra]

John,The Magni has a post hub and plates on each side conecting the blades.Fish.

[asra]

I'm not sure what that means Fish, can you do a sketch?Col and I were having our weekly 'show and tell'session yesterday,at this stage we concluded it would be best to have no stress raisers in the hub bar. It seems there are ways of doing this.Has anyone else noted the French rotors - Averso I think, which have a 'tracking bar' on each side. This may relieve the hub bar of the twice per rev force reversal that caused Courtney's crash in an early Cierva.[actually the Cierva would have a 4 per rev reversal as it was 4 blade].John EvansThink logically and do things well, think laterally and do things better.

[asra]

[asra]

I will have a look in all the pics I took of the Magni in WA.The rotor brake is a drum brake incorporated within the rotor head and pre rotator.Download Attachment: [img]images/icon_paperclip.gif[/img] magni head.jpg55.78

[asra]

Gee Pete, I'm always changing direction, who can I sue for 'stress!'?As I understand it, the force reversal through zero is a problem. Looking at the Magni picture, I notice that the hub plates would seem to flex with the 2 per rev, so smoothing it out. Do you know what the material is Paul? I wonder how fatigue prone they would be.Does anyone know about the Averso set up? If either compression or tension were applied by means of them, the force could be prevented from passing through zero.LlewellaThe law of Inverse Relevance: the less you intend to do about something, the more you have to keep talking about it.

[asra]

John, and others,The Averso blades have an aluminium tube, about 20mm, each side of the hub bar, connecting the leading edge of each blade with the trailing edge of the opposite. They also have a fixed teeter bolt and bearings in the teeter block. Wish I could post the pics I have taken of them but I can't. Sorry.Fish.

[asra]

I'd like to go and have a look at that French stuff sometime, it's a pity they choose not to talk proper[]. My schoolboy French petered out after about 6 months, I think we were a bit too much for the poor teacher, even if you figure out how to say something they might recognise, you've got no chance of understanding the reply.John EvansThink logically and do things well, think laterally and do things better.

[asra]

A lead/acid storage battery stores ~150 btu/lb. of which ~70% can be converted into mechanical energy.There are 20,000/btu/lb. in gasoline of which ~30% can be converted into mechanical energy.The advantage to gasoline, allowing for efficiencies, is ~57:1, which explains why battery power usage is limited to granny scooters, forklift trucks and the like. Power/weight ratio of granny scooters is mostly irrelevant and forklifts have to carry ballast anyhow.Starter motor prerotators are cheap and easy to rig but heavy and jolting to rotor and mast without soft start.Variable speed, battery powered drills are controlled by duty cycle modulators. The power is switched on for a few milliseconds and then off for several more milliseconds; the ratio of on/off times controlling the speed. There is an inductive kick when the power is switched off that must be snubbed by what is called a freewheeling diode. The freewheeling diode supplies current to the motor during the off interval (from the energy stored in the motor inductance) and keeps the flow of current through the motor fairly constant.The problem of applying that technique to starter motors is one of cost; neither the transistor switches nor the freewheeling diodes with the necessary current capacity are cheap. But the cost of these components is falling.The current draw of a starter motor with stalled armature, the situation when first starting the blades, is limited primarily by the battery capacity and wiring resistance. It can be several hundred amps.The torque of a stalled starter motor depends primarily on the current; I expect the figures quoted earlier in this thread are dead on.The most practical method of providing soft start seems to be a series resistor and two solenoid switches; one solenoid to turn the whole thing on and another to bridge across the resistor after a few seconds. Not very efficient but better than wringing off the mast. Nothing about electric prerotators is very efficient anyhow.The resistor could be made from almost anything except copper or silver.I expect a length of steel banding strap would work just fine. If it should melt in the few seconds required to get the rotor going, use parallel strands twice as long. Heating of the resistor depends upon watts dissipated/square inch of surface area.A strip sheared from a stainless steel sheet might work even better.Not much resistance is required. A 1 ohm resistor would limit startup current to 12 amps; 0.5 ohm to 24 amps, etc.

[asra]

Thanks Chuck, you are basically describing the design I put on Norm's old forum about 3 years ago. The reason I recommend Nichrome, which is cheap and readily available, is its almost flat resistance curve plotted against temperature, so that you don't get the heavier current through the cold stainless welding rod as in the Butterfly prerotator at startup, followed by a signifigant increase in resistance as it heats up, thus limiting the torque being applied when it should be the other way around. I used 80 strands strung on a 'harp' consisting of a wooden frame supporting 2 pieces of threaded brass rod, one of which was spring loaded to prevent arcing. This gave between one and half an ohm resistance.Perhaps I am being picky[], as anything which reduces that sledge hammer blow must be a good thing!I believe problems such as this are a result of evolution through 'eye-ball engineering', where people 'improve' what was basically a sound design to the point of eating up the factors of safety that should not be violated, often in the name of 'history of safe operation'.John EvansThink logically and do things well, think laterally and do things better.