Prop shaft

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Primaris22

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I’m working on a design that would need a long prop shaft ( Aircobra/ BD 5). Having trouble accessing any info on how these are designed. The shaft would be approx. 5 feet and the HP is 100. Any help would be appreciated.
 

TFF

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Where you start is studying cars. Except for electric. They all have drive shafts. Front drive , rear drive, all wheel drive. You are going to do the same thing output from engine to input to propeller. No different from output from transmission to input to drive wheel.
 

Tiger Tim

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The Taylor Imp would probably give some inspiration as well, though you would still need to understand the conditions you’re designing it for.
 

Bigshu

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More likely to find info on the Taylor Mini-Imp. It's the single seater with both a drive shaft and a soft start device that decouples the prop at idle revs. The Imp was side by side two seater that was considered too difficult for homebuilders. Still used shaft drive, but not a lot of info out there to research.
 

Bigshu

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You might research the Lesher Teal.


BJC
Lesher got some help from Molt Taylor on the drive train. I think Teal might also use the mechanical soft start device that the Mini-Imps use. Hard to find and expensive. You might be able to see it up close at the EAA museum if you ask nicely, for measurements and such. The two place Lesher pusher design is on display at the University of Michigan in the Engineering building lobby.
 

Vigilant1

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Molt Taylor used the Dodge Flexidyne coupler in all of his pushers to prevent destructive torsional vibration. Several articles by Molt in the Sport Aviation archives.
I got a tour of the Wright brothers museum a few years ago. They have a fully functional, practical, but not-meant-to-be-a-true-replica of the Wright B Flyer that uses one of these shot-filled Flexidyne couplers in the drivetrain.
As Molt used them to provide some torsional isolation between the driveshaft and the engine in continuos running (not just as a soft-start), I wonder if they got hot? There are a lot of little friction spots inside that mass of steel shot.
 

BJC

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We used Flexidyne couplings on screw conveyors. They had no slip when running normally and being driven by induction motors in the 40 to 60 HP range. Don’t know how they would deal with a four cylinder aircraft engine’s power pulses.


BJC
 

Bigshu

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They can get hot if you're not careful with the shot charge. There's some experimentation involved to get grip and slip just right. When it locks up is the what you're adjusting with the shot charge, and the manufacturer isn't interested in helping you find out. The coupler is designed to connect electric motors to a load like a conveyer system. The max RPM listed in their tech specs is 1760. So, direct driving it at 2200 to 3200 rpm is beyond the specs, but on the other hand, you aren't running 24/7 in a factory. There's a lot of solutions designed for aircraft now, so maybe a better one is already out there. I can tell you that finding a Flexidyne 9C is something that all us Mini-Imp builders really seem to have trouble with.
 

Bigshu

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I've often thought that industrial couplings have merit in aviation applications, and that a coupler with a resilient "spider" between the metal parts on one end, and a sprag or over-running clutch on the other would help a lot with both the resonance issue and the gyroscopic forces from the prop. My chief complaint about them is weight, since they're designed for loads and duty cycles that an airplane doesn't see. If there were light weight versions, they might be an elegant solution to a lot of problems in direct drive setups.
 

Vigilant1

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I can tell you that finding a Flexidyne 9C is something that all us Mini-Imp builders really seem to have trouble with.
And I know they can be pricey if you can find them. Have builders in the Mini-IMP community had any experience/results in fabricating their own Flexidyne-type couplers? The concept is simple enough (but, so is an Otto-cycle engine, and few folks successfully make their own).
The idea of a DIY coupler seems attractive. Conversely, the idea of a few pounds of hot steel shot being flung out of a ruptured 3000rpm housing seems unattractive.
 

Dan Thomas

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Where you start is studying cars. Except for electric. They all have drive shafts. Front drive , rear drive, all wheel drive. You are going to do the same thing output from engine to input to propeller. No different from output from transmission to input to drive wheel.
Except that the propeller is a flywheel at the other end of that shaft. If the engine's power pulses aren't taken out before the power is sent down the shaft, something's gonna flex and get hot and fail. In a car, the clutch plate has springs that do that, or in an automatic the torque converter does it. Billski can elaborate on this.

In a helicopter, the tail rotor is not expected to be a flywheel, it weighs almost nothing, and that shaft is slender and winds up some during flight. It's designed that way. The piston helicopter's engine has a flywheel and a clutch of some sort.
 

TFF

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The tailrotor of a helicopter is about 20% of the horsepower usage. On an Enstrom that’s about 45 hp at full throttle. An Enstrom tail drive shaft is only a piece of .056 4130 on hangar bearings. You can definitely feel the shaft twist if you move the tail rotor to spin the blades to clock for putting in the hangar.

There is a VK30 project in our hangar and it has a big carbon drive shaft from a Combine and at the engine there is an elastomeric dampener15-18” round and 4-5” thick. It’s the updated design not the one that comes in the kit. The drive mount for the prop is pretty substantial much less the tail being able to handle the thrust.
 

Bigshu

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And I know they can be pricey if you can find them. Have builders in the Mini-IMP community had any experience/results in fabricating their own Flexidyne-type couplers? The concept is simple enough (but, so is an Otto-cycle engine, and few folks successfully make their own).
The idea of a DIY coupler seems attractive. Conversely, the idea of a few pounds of hot steel shot being flung out of a ruptured 3000rpm housing seems unattractive.
Yeah, that's the rub. I'm unaware of anyone trying to fabricate there own. Back in the day, some early builders bought more than one, so now when people get to the point they need one, there's a secondary market that's more reasonable. There are some builders who age out or lose interest (it's not a super intuitive plan set), so if they bought a coupler, they have no use for it, so up for bid it goes. I'm so far from needing the flexidyne, I suspect there might be some floating around when I'll need it, or another solution might be more viable.
 

Hephaestus

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As we wait for billski to say "It's not that easy"

Note all the homebuilts with driveshafts have driveshaft problems... Not 100% sure, but yet to see mentions of one running an extension that didn't have issues.

I really wish there was an easy way to get a 3-5' extension that's reliable. But even the short ones are problematic.
 
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