Fork and blade con rods

[GregLocock]

http://www.boddice.co.uk/bpic526.htm

These were pretty popular in V aero engines around WW2. Any idea why? C LF Taylor says it minimises the length of the engine, but I can only see a saving of about half a big-end's length, not much in return for the complexity and uglification involved.

My guess is that it drmatically reduces the loads on the mains due to the reduction in rocking couple from each pair of pistons, so you can have a smaller lighter crank.



Cheers

Greg Locock

 

[GraviMan]

I would imagine the real reason would be ideal balancing in each pair. A conventional V intoduces some 1st and 2nd order yaw or pitch since the pistons are not quite centred in the crankshft balancer masses. This is normally deemed acceptable, if you run a V12 the thing ain't gonna vibrate anyway - the crank masses just being to reduce crankshft bearing load. I'm a radial geek myself, and find looking at the conrod layout fascinting - this is probably the origin of the idea.

Mart

 

[evelrod]

Fooling around with the Harley guys, I had always assumed the issue was overall engine width. At least in this application "rocking couple is an oxymoron).
I helped in the engines on my cousins Cessna T-50 and an R-1340 once when I was a lot younger. I'm with Mart, I am totally fascinated by the big radials (I worked on the Hughes flying boat for a year or so).

Come to think of it, the big V-16 turbocharged methane burning Orange County CoGen units were fork and blade. Absolutely no good reason I can see. These engines are, like, 25 feet long and 10 feet tall with an 8' dia flywheel operating at a max rpm of something like 800 rpm --- if memory serves. Five of the biggest pushrod engines I ever worked around. I have no idea as to the overall balance problems on an engine of this size!!!

Rod

 

[EngJW]

I think another reason may be that for opposite banks the cylinders are in line- no offset between banks. Thus, it is possible for opposite cylinders to share the same bolt pattern and they can use through-bolts. These provide more clamping force for a split crankcase. This reason would apply to a horizontally opposed engine with separate jugs. There were some V engines that used fork and blade rods where this would not be important.

John Woodward

 

[ivymike]

AFAIK, Harley uses the fork-and-blade rods because marketing requires the twin to be twins. Offset cylinders would make so much more sense in so many ways, but their customers wouldn't go for it... That's what a Harley guy told me once, anyway.

I don't know why the aircraft guys did it, but I do know a guy with 30 years of aircraft engine design experience. I'll be sure to harrass him about it next time our paths cross.

 

[PJGD]

Not a definitive statement, but certainly supportive of the likely reasons expressed above, I notice that the history of Paxman Engines touches on this topic.

http://www.nelmes.fsnet.co.uk/paxman/paxeng34.htm

About 80% of the way down the page, the description of the Valenta engine notes it had fork-and-blade rods to minimise engine lenth. Keep on going, and the later engines had side-by-side rods to reduce cost!

PJGD