The JT 11

[Ga6riel]

Finding info difficult to procure on the engineering details/issues of the JT 11, PA 36 and others. The only info I can find on the shaft detail is scant indeed, and comes from 2 sources, the Mini Imp and from Martin Hollmann. But I heard Hollmans concept ran into difficulty.

Anyone care to discuss the pros/cons of the rear engine tractor concept ?
Or know of any flying examples

[karlbamforth]

Rear engined tractor has been done succesfully a few times.
Almost all attempts were relatively small scale.
There are a few Gliders and of course a WW2 fighter aircraft, the name of which has slipped my mind due to excessive red wine.:eek:
If I remember correctly shaft oscillations were the biggest problem.

[Ga6riel]

yes truly the shaft and the connections are the greatest weakness

[John Stahl]

This is the web site for the JT-11

http://www.icon.fi/%7Ejtki/jt11virtual.html


I think the gear reduction unit should be on the engine. I think it would be easer to work with the vibrations of a drive line that is going 2500 rpm instead of one that is turning 5000 rpm.
I don’t think there is enough VS in the concept drawing.
But that said I like the concept.

[PTKay]

shaft oscillations ARE the biggest problem

[Cobra Doc]

The most notables were the Bell P-39 Aircobra and P-63 Aircuda. The Russians loved the P-39. I think the US pilots were more concernd about the diveshaft proximity to vital body parts than any handling characteristics. On a combat aircraft, I can see where that could be a concern. One round in the shaft and the pilot gets the shaft.

[Not Yet]

Most car transmissions are 1:1 ratio in high gear. That would mean that the drive shaft turns at engine speed, so why would it be a problem?

Most car engine conversions are using the reducer and prop for the flywheel.
I believe this will cause problems with tonsorial resonance.

I think if a light flywheel and elastomer couplings are used that it should be very doable.

[karlbamforth]

The most notables were the Bell P-39 Aircobra and P-63 Aircuda. The Russians loved the P-39. I think the US pilots were more concernd about the diveshaft proximity to vital body parts than any handling characteristics. On a combat aircraft, I can see where that could be a concern. One round in the shaft and the pilot gets the shaft.

Thats the one.
A little off topic but I seem to remember that the coolant pipes ran thro the cockpit. Pilots were literally steamed to death if a leak occured for any reason. :eek:

[karlbamforth]

The JT 11 looks great, does anyone know the current status ?
Will any be built ?

[Ga6riel]

many thanks for the replies
John ~ sadly if the reduction drive is on the engine, the prop centre becomes rather low. I am familiar with that site, the pdf dwg has slightly more detail.

David (Not Yet) ~ welcome here, my thoughts were the same, that auto components greatly resemble the aircraft requirements

Karl ~ I have no idea what the JT 11 status is, in any event I have different requirements but I admire the designers ideas. More detail on the drive line would be usefull, I am reluctant to lean on Martin Hollmann for same.

[John Stahl]

My knowledge and understanding of tonsorial resonance is very limited. Would a rotary engine reduce the problem?

[Ga6riel]

John I think it would somewhat
but there would remain a dangerous zone that ought be avoided
and that is what concerns me, along with the practical fitments

[Not Yet]

Here are some sources for elastomer couplings.
Maybe you know of something better?

http://www.rw-america.com/elastomer_couplings/index.html

http://www.pyiinc.com/index.php?section=rd_marine&action=shaft-coupling&sn=2

https://www.svb.de/html/tn03e_421a424.html

[w.engels]

Some time ago I've been in contact with Mr. Tervamaki about the JT-11. He mentioned not having intentions to built one. He just wanted to present the concept and maybe someone will use it as a starting point for a design.

About the rpm of the propshaft: if one reduces the rpm by mounting the reduction unit on the engine, the propshaft has to transmit a higher torque. Such higher torque would lead to a heavier propshaft that has a lower bending frequency. Increasing the propshaft rpm might result in a lighter propshaft. In every situation, it is possible that, reagrdless of what the propshaft rpm is, it might rotate with a frequency too close to its bending frequency.

Maybe the best way to avoid propshaft vibrations is using an intermediate bearing. In that way the bending frequency of both shafts is significatnly higher than the one of a long propshaft.

[Ga6riel]

Maybe the best way to avoid propshaft vibrations is using an intermediate bearing. In that way the bending frequency of both shafts is significatnly higher than the one of a long propshaft.

but add the weight of the structure required to support it. Not that it would be a bad thing, as it could add to crash protection.

What I have been reading suggests, that higher shaft rpms is actually less troublesome than slower shaft speeds.

[Ga6riel]

I just found these guys, who make composite shafts with titanium flanges for subaru WRX cars. Those things spin like mixmasters.

http://www.gondwanaracing.com.au/drive_shaft.htm

for which and I quote "If you are offroad racing and you are breaking shafts due to harmonic imbalance, insufficient misalignment or inability of the shaft to cope with big horsepower and aggressive conditions, then Gondwana Racing Driveshafts are for you."

[w.engels]

You're right. When shafts turn at high rpm (above critical speed), they usually run smoother. But, to prevent damage to the shaft or surrounding structure, you have to avoid the critical rpm.

One has to increase rpm rapidly during engine startup and, if the critical rpm is between min engine rpm (idle) and max engine rpm, there is an rpm range that should be avoided during flight.

[Cobra Doc]

Wasn't it during the F1 race last Saturday that a composite shaft seperated from the metal flange? Seems like it was suspension, not the drive shaft. Maybe it was NASCAR on Sunday. I watched too many races over the weekend, including motorcyles in Spain (imagine that).

[karlbamforth]

I believe it was the composite lower wishbone that failed on the F1 car, although the driveshaft could be seen to seperate as the car came to a halt.

[Hognose]

Coolant lines on the P-39/63 did not run through the pit. The reason the planes failed in British and US service is the low-altitude engines they were saddled with (same reason the P-51A was not a success, and principal reason the P-40 flopped). They did better when used at low altitude as the Free French and especially Russians did. The engine in the P-39 was the Allison V-1710-35 and it did not have a 2-stage supercharger, unlike the Merlin.

That said, the P-39 was draggy and tended to lose lots of speed in maneuvers. That was the undoing of a British pilot flying a display a few years back, he entered a loop about 100kt too slow and wound up spinning in which did not help the plane's reputation. But most planes will bite you one way or another if you don't learn what the book says. (Or don't have a book).

Re: composite component failure... NASCAR does not use composites in suspension or drivetrain. The cars are basically a spec machine with all having functionallyidentical tube frames, double-A-arm front suspension with geometry from a 1965 full size Ford, and 9" Ford rear axles. The cars are pretty much for show; it's an entertainment product, like TV wrestling.

It's at the opposite end from F1, where the tech is so high you're never going to see it in a road car... in NASCAR, the tech is so low you're never going to see it in a road car, again.

cheers

-=K=-

[Ga6riel]

thanks Kevin
the composite shafts are in common use on the rally circuit it seems, I guess thats not as popular in the USA as it is here and in Europe.

Maybe the Airocobras Aspect Ratio, additional maintenance and weight complications held it back some too; most 'unsatisfactory' fighter aircraft were relegated to ground attack roles. The soviets certainly put them to work in the biggest possible way, becoming a favourite aircraft among many.