Tom Harper
Aug 03, 2005, 07:29 AM
I like the pusher configuration. Seldom have a broken prop, safer in a crash, generally nice to work with.
However there's a couple of problems. Glo engines are designed for the crankshaft to bear against the front of the crankcase. This provides for the bearing surface and the lubrication seal. Use of a pusher prop moves the crankshaft back a few thousnadths. The only bearing surface is the back of the prop washer.
One manufacturer states that there will be some raw fuel forced out through the main bearing. Some plain bearing engines have a shallow spiral cut to allow this. However if the engine is a bit worn, moving the crankshaft back will allow a LOT of fuel to flow out through the main bearing. And, the needle setting wil be unreliable.
It varies with engine type. The OS .15 handles it very well. K&B sportsters do not. The worst is the Norvel .25 BB. The Norvel still puts out the most power but only about 60% of what you would get with a tractor prop.
Another problem is overheating on the ground. The Norvel will run at full RPM for about 30 seconds and then begin to lag. It will not hold a needle setting once it gets hot.
Electrics are not home free. The thrust bearing problem may be similar. The brushes have to be adjusted for reverse rotation. I don't know what the rules are for brushless.
So, I think the pusher configuration is very desirable. But, there is more to it than just turning the engine around.
However there's a couple of problems. Glo engines are designed for the crankshaft to bear against the front of the crankcase. This provides for the bearing surface and the lubrication seal. Use of a pusher prop moves the crankshaft back a few thousnadths. The only bearing surface is the back of the prop washer.
One manufacturer states that there will be some raw fuel forced out through the main bearing. Some plain bearing engines have a shallow spiral cut to allow this. However if the engine is a bit worn, moving the crankshaft back will allow a LOT of fuel to flow out through the main bearing. And, the needle setting wil be unreliable.
It varies with engine type. The OS .15 handles it very well. K&B sportsters do not. The worst is the Norvel .25 BB. The Norvel still puts out the most power but only about 60% of what you would get with a tractor prop.
Another problem is overheating on the ground. The Norvel will run at full RPM for about 30 seconds and then begin to lag. It will not hold a needle setting once it gets hot.
Electrics are not home free. The thrust bearing problem may be similar. The brushes have to be adjusted for reverse rotation. I don't know what the rules are for brushless.
So, I think the pusher configuration is very desirable. But, there is more to it than just turning the engine around.