Many of you know that I fly "unusual" airplanes, some of which are of the "Pusher" configuration. After doing some
research I became concerned about the cooling of model aircraft engines used in a "pusher configuration". I wrote the
following letter to Mr. Clarence Lee who many of you will remember, authored a column entitled "Engine Clinic" for quite
a few years. My letter appears below, with Mr. Lee's answer to each question shown in Red.
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?
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 past the cylinder. Although the cooling effect might be a little less, it
is still ample. You shouldn't need a helicopter engine.
- Should an engine intended for pusher use be broken in with a standard puller prop, a pusher prop, or some time with each?
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.
- Can a glow engine be expected to endure the change from a pusher to a puller or viceversa or should it forever either
push or pull?
As long as the engine utilizes a ball bearing supported crankshaft it doesn't matter whether it is running as a
tractor or pusher.
- Would a bushing equipped engine be less suited for this change than a ball-bearing engine?
A bushed engine should not be used in pusher configuration.
- 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.
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 increases the engine heat.
- Are four-strokers well suited for pusher application?
If 4-strokes are your thing, then no reason not to use one.
- Will a three-bladed pusher prop provide better engine cooling as it could effect increased airflow in a smaller diameter
around the engine?
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.
- Are there any advantages in reversing engine rotation and using a standard puller prop other than the price of the prop?
None that I can think of.
Thanks, Walt Moore
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