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I bought a tow for my aircraft that was very poorly designed. In modifying it, I need a clutch between a 6HP gas enigne, and a forward/neutral/reverse transmission.
My problem is determining the size of the clutch plates. The clutch will be operated by a conventional bicycle handle bar brake handle and control cable. The pressure pad will be molded high strength urethane heavily loaded with copper fibers. The pressure pad will contact 7075 aluminum plates.

My basic problem is space. Ideally, the clutch should be no larger than 3.5 inch diameter, due to the transmission configuration. But will plates this size be capable of transmitting sufficient force to move the plane?

The clutch will be pushing a 4500 pound aircraft that is just a bit too heavy to be pushed by one person. Two people can push it, so I am guessing that it takes about 100-125 pounds force to move it.

BTW; the "poor" design is this; the tow currently has a handle bar type go/stop control, but to STOP the tow, the operator has to press the handle. A proper design would be the opposite, to GO, press the handle - to prevent run-away situations. Also, the belt drive can slip, also causing a run-away situation. Amazes me how people can design a product, and not think through the application.

I would greatly appreciate any help on designing this clutch.

George:

Complete engineering solutions are beyond the allowable scope of this forum. With that said here are a couple of things to consider.

Are you talking about the are brake the system or the clutch on this device? Most small engines have a centrifugal clutch unless they have more than on gear.

Clutches traditionally have passive engagements since you have to over come springs or other loaded mechanism to disengage clutch.

Brakes on the other hand can be dead-man where you have actively hold something to allow the device to move, similar to the blade break found on lawn most mowers or they can be active where you have to press them to activate them. The air brakes on trucks are also dead-man. They require air pressure to release them.

You might want to consider adapting the safety system from a lawnmower since the function you want is very similar. This would be easier than design your own clutch system from scratch. These systems already have demonstrated reliability and would already have a number of safety certifications which you would want to consider before putting your design on the market.

In looking at what is out there, I did find a company selling private plane tows. They have electrical and gasoline powered models. This company uses chain drives not belts.

Niel Leon
engineering. com

Niel, thank you for your comments. there are seveal companies building tows for private aircraft. I made the mistake of falling for the advertising of what turned out to be a small company that was just bought by new owners. The original owner apparently knew he was building sub-standard equipment, and when I contacted him, he wanted nothing to do with fixing the problems. New owners claimed I bought the unit from the old owner, and he says he wants nothing to do with it. So I am stuck making this thing usable.

This unit has a 6 HP B&S gas engine, directly driving a pulley. The pulley was connected to a second pulley which drove the input shaft of a 3 state transmission [forward,neutral,reverse - all fixed gear ratios], with an idler pulley in the loop.
The major problem was that to lessen tension on the belt and thereby stop the tow, the user had to squeeze the handle. Any first year engineering student knows that the proper way to do this is to REQUIRE pressure to get the tow to move, hence a dead-man effect.

I could not consider using the unit as it was, so I disassembled it, and started looking for a clutch that I could put in line between the tranny drive pulley, and the tranny. To my dismay, I was unable to find anything that would fit in the space I have available. So.... to the drawing board. I do not feel that a centrifugal clutch would be be satisfactory. I want to be able to move the aircraft at a very slow speed.

The clutch I designed is simple, two discs with a pressure plate inbetween. The lower plate is pressed upwards into the pressure plate upon application of force from the control handle. Mechanically, very simple. BUT, I have run into conflicting design examples in searching the net. One suggests that a 3 inch set of discs will be capable of transmitting sufficient force. Another example I found suggests that it will take a 6 inch disc. I do not have room for a 6 inch clutch.

My quandry is determining how much clutch area is required to apply the force needed. I am an electronics engineer, both hardware and software, and that does not give me the background to analyse this.

Moving the unit as slow speed does not mean the the engine would rotate slowly. Matter of fact you want the engine turning at its most efficient speed when you are at full tow speed. This is controlled with the ratio.

The clutch should be attached directly to the motor. There should be plenty out there. Finding it would take some time and effort.

The liability to me as a professional designer would be to high to consider. Couple of questions:

1. Have you considered taking both the previous owner and the current owner to court? If they are from within your state you could easily take them to small claims court. If they are from outside your state, you can still try it.

2. Also considering contacting the FAA since this has direct connection with use on airfields and they have jurisdiction there. See if they could help you get this resolved since use of this devices had direct bearing on the safety and welfare of aircraft and passengers of those aircraft.

Niel

The company I bought this from is out of state, and therefore using the courts is impractical - not enough money involved to hire a local lawyer.
FAA does not have jurisdication as tugs/tows of this nature are not used on active runways or active taxiways. They are normally used just for the purpose of pulling planes in/out of a hangar, and hangars are always located on pirvate, not FAA controlled property.
Not sure where liability comes in. No matter who I listen to, I will build the clutch; if it works, great. If not, I scratch my head, then try something else. If it moves the plane [meaning transfers sufficient power/torque to do that], then not much can go wrong. The only issue I can see is the control cable failing to release the clutch, and that is on my back, since It's my cable.
All I am really looking for is an equation that considers the surface area, hopefully the clutch material [is that even an issue, other than friction values?], force applied to the clutch to engage, and finally, the amount of force it can deliver.
I can't find a thing in my many electronics books that even comes close [kidding there].
George

Here is a link

http://thecartech.com/subjects/desi...utches.htm

Found searching the phrase - clutch design equations

Niel

Thank you, Neil. Great starting point. Now if I can remember my differential calculus!
George