Am about to commence building of a Pusher Canard model (refer my post in the Scratch Building Forum). The power plant is pusher configuration & I have at least 2 different engines that can be made to run in reverse by rotating the front section thru 90 degrees. Alternatively, I can use the same engines in normal mode & use pusher props.

At this point, I'm favouring running the engine in reverse as I can use standard props simply by installing them on backwards. This will give me far greater variety in diameter & pitch sizes comapred to the rather rare pusher props.

Anyway, enough of the intro. My question is: do engines running in reverse have a tendancy to loosen the prop nut? I base this thought on the way the crankshaft threads are cut on our engines. With an engine running in the normal direction & apart from backfiring, the threads are such that the prop nut should remain tightened or even become tighter. But when running in the opposite direction & allowing for the RPM's varying from low to high, does the prop nut start to "undo"?

In theory yes but in practice no? I simply don't know, hence the question.

Well when you use a electric starter you are trying to loosen your prop nut or spinner. Your probably better off to use reverse rotation so you don't have to order in special props and if ground clearance is an issue a 3 blade can be used. I know my LHS doesn't stock pusher props.

In theory, yes the reverse rotation tries to loosen the nut while running but a lot of engines have been set up that way and I've never heard of any problems. As speedster said, most props come loose when starting so in fact you're a bit ahead in that respect .

The lack of available pusher prop choices certainly throws the advantage to reversing the engine.

Don't know the engine, but where are you going to get a reversed crank?

Paul

If you can reverse the engine rotation, that would certainly be the way to go simply because you have a lot more props to choose from if you can use normal tractor props. Not only are pusher props not always in stock but the selection of diameter and pitch is limited.

Many thanks for your comments & experiences. Paul, the engines I that I have that can be used via reverse rotation (by rotation of the front housing by 90 degrees) are the Irvine 40 & Royal 45, both being dual ball bearing. A reverse crank is not necessary under those circumstances. These engines, incidently, use the same muffler.

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Many years ago (very early Sixties) I was reading a treatise by Duke Fox on the designing of his then new Fox .15X. He used a technique of setting the crankshaft (viewed from the prop side of the engine) slightly to the left. This technique has a name and a french word associated with it. I think it starts with an "A". Darned if I can remember the word, but I'll bet you get the idea. It is supposed to help the engine favor counter-clockwise rotation during normal operation. You could actually see this offset while looking in through the Fox .15X backplate opening.

The reason I brought this up is that unless one is absolutely sure that the above technique has not been applied during the design of the candidate reverse rotation engine, it might be a good thing to measure the center of the bore alignment with the center of the crankshaft. I suppose that the worst that could happen would be disappointing performance when operating in clockwise fashion. But I would also be concerned with piston scuffing and wear.


Ed Cregger

DeSax, DeSaxe? I think the real advantage on baffle piston engines was being able to move the bypass over to the left as well, and thus have the engine looking symmetrical from the front. I don't right off know of any modern engines (non-Fox) with DeSax cylinders, but that may well just be ignorance on my part.

It seems there's a few ways to spell DeSaxe but it simply means "off axis", meaning the crankshaft centreline is off axis from the centre line of the cylinder. It does a couple of things, it can be used to reduce the load on the thrust side of the piston by giving less rod angularity on the power stroke and it gives a slight assymetric timing for the exhaust and transfer ports.

Fox used it on at least some of their engines and Merco used it on all their engines (29, 35, 49 and 61). Strangely, Merco used it on the opposite side to Fox so I'd assume it's a designer choice between either reducing thrust loading or the timing assymetry. I'm sure there are other engines that may still use it today but the only one I know of is the SC 1.08 and I only found that out when I was measuring the timings and kept getting a difference between the port opening and closing degrees. I figured at first I had the degree wheel set wrong but it wasn't...

All depends on the engine size. If you are building for 049s then get reed valve Cox engines as they run in both directions.

The typical .40 to 1.20 engine sizes are well provided with 'pusher' props.
See http://www.apcprop.com/cgi-bin/store...c=6a&product=+

Above 120 it can be difficult to find appropriate pusher props although you can get wood ones custom made.

Some large petrols with reed valve induction will run in both directions!