Debra
01-04-2007, 11:43 AM
Hello Mr. Lee,
Your column has always been very helpful to us modelers. My questions/comments concern an area that I don't recall ever being discussed. I have had several models that were pushers and plan a few more of this configuration. My questions all pertain to glow engines being used to push the model forward by turning a prop that directs the prop blast away from the engine.
Should an engine designed for helicopter applications be considered for a pusher because it has a better heat-sink head to allow installation with minimal airflow for cooling?
Should an engine intended for pusher use be broken in with a standard puller prop, a pusher prop, or some time with each?
Can a glow engine be expected to endure the change from a pusher to a puller or vice versa, or should it forever either push or pull?
Would a bushing equipped engine be less suited for this change than a ball-bearing engine?
Most two-stroke engines have exhaust systems that discharge forward when installed as pushers. I don't like this arrangement and have usually rigged some way to direct exhaust gases either up or down and outside the propeller arc. I think the exhaust from four stroke engines is more easily directed aft.
Are four-strokers well suited for pusher application?
Will a three-bladed pusher prop provide better engine cooling as it could effect increased airflow in a smaller diameter around the engine?
Are there any advantages in reversing engine rotation and using a standard puller prop other than the price of the prop?
Thanks, Walt M
Dear Mr. M,
Taking your questions in order:
Cooling a pusher engine is not that big a problem as long as the cylinder is exposed, i.e., not cowled. Keep in mind that the propeller is a fan and draws air pass the cylinder. Although the cooling effect might be a little less, it is still ample. You shouldn't need a helicopter engine.
It would be best to break the engine in with a tractor prop where it will receive maximum cooling, especially when the metal surfaces are still seating and friction (and heat) is higher.
As long as the engine utilizes a ball bearing supported crankshaft it doesn't matter whether it is running as a tractor or pusher.
A bushed engine should not be used in pusher configuration.
Directing the exhaust residue outside the propeller arc is a good idea. Just be sure that the exhaust system is not too restrictive causing more than a 300-400 rpm loss over open exhaust. Restriction increased the engine heat.
If 4-strokesare your thing, then no reason not to use one.
I can't say for sure whether a 3-blade prop would provide more cooling or not having not conducted any experiments such as this, i.e., taking head temperature readings, but it does seem logical.
None that I can think of.
Regards,
Clarence Lee
(to write to Clarence, please email man@airage.com)
Your column has always been very helpful to us modelers. My questions/comments concern an area that I don't recall ever being discussed. I have had several models that were pushers and plan a few more of this configuration. My questions all pertain to glow engines being used to push the model forward by turning a prop that directs the prop blast away from the engine.
Should an engine designed for helicopter applications be considered for a pusher because it has a better heat-sink head to allow installation with minimal airflow for cooling?
Should an engine intended for pusher use be broken in with a standard puller prop, a pusher prop, or some time with each?
Can a glow engine be expected to endure the change from a pusher to a puller or vice versa, or should it forever either push or pull?
Would a bushing equipped engine be less suited for this change than a ball-bearing engine?
Most two-stroke engines have exhaust systems that discharge forward when installed as pushers. I don't like this arrangement and have usually rigged some way to direct exhaust gases either up or down and outside the propeller arc. I think the exhaust from four stroke engines is more easily directed aft.
Are four-strokers well suited for pusher application?
Will a three-bladed pusher prop provide better engine cooling as it could effect increased airflow in a smaller diameter around the engine?
Are there any advantages in reversing engine rotation and using a standard puller prop other than the price of the prop?
Thanks, Walt M
Dear Mr. M,
Taking your questions in order:
Cooling a pusher engine is not that big a problem as long as the cylinder is exposed, i.e., not cowled. Keep in mind that the propeller is a fan and draws air pass the cylinder. Although the cooling effect might be a little less, it is still ample. You shouldn't need a helicopter engine.
It would be best to break the engine in with a tractor prop where it will receive maximum cooling, especially when the metal surfaces are still seating and friction (and heat) is higher.
As long as the engine utilizes a ball bearing supported crankshaft it doesn't matter whether it is running as a tractor or pusher.
A bushed engine should not be used in pusher configuration.
Directing the exhaust residue outside the propeller arc is a good idea. Just be sure that the exhaust system is not too restrictive causing more than a 300-400 rpm loss over open exhaust. Restriction increased the engine heat.
If 4-strokesare your thing, then no reason not to use one.
I can't say for sure whether a 3-blade prop would provide more cooling or not having not conducted any experiments such as this, i.e., taking head temperature readings, but it does seem logical.
None that I can think of.
Regards,
Clarence Lee
(to write to Clarence, please email man@airage.com)