Archie wrote:

Strictly a personal note,
Even though I have worked on many racing planetary
applications, I prefer the Powersport or the Marcotte
approaches for the aircraft application.
Simple, fewer parts, most effective.
(except for mfg.cost)
Archie

This is why I prefer the planetary for aircraft use.

Usually the limit to a gear boxes torque handling
capabilites is the length of the gear contact line.
With one contact line such as found in spur gears 
there is a limit to how long
you can make it for a given gear diameter.

The gest of it all is you can make a planetary with
a given torque capabilty lighter because you have
two or more gear contact lines. In my case there will
be six lines .85 long each for a total of over
five inches wide. A single spur gear set with a contact
line that long would be massive. The spur gear box in
the Stewart S51 is about four or so inches long and it
weighs 120 pounds.

Another factor is the separating forces in simple spur
gears are huge. This requires massive and stiff bearings
and housings. When you make a gear box that large the gear
mesh changes a lot as a function of temperature when 
the houseing is made of aluminum. In the planetary on 
the other-hand all separating  forces 
are contained within the ring gear. 

The Ford C6 set weighs about six pounds by itself. You 
would play heck getting a simple spur gear arrangement 
down to that weight with the same torque capacity.

BTW Ten pounds over weight in the engine compartment
is fifteen pounds directly in the airplane
and another three to five pounds in the structure of the 
aircraft all other things being equal. If the structure
remains the same the payload is reduced by fifteen
pounds or 2.5 gallons of gas.

Most all late large aircraft piston engines used planetary gear
boxes for their light weight. 

Pratt & Whitney just built a planetary box for
their new geared turbo fans that handles 40,000 HP as I recall.

The Pratt design removes the external thrust from the helical gears
by using a separated herringbone pattern. (The thrust design
problem I am dealing with as we speak.) This further 
reduces the weight of the housing and bearings. See below. 

The right hand and left hand helical
gears are separated because it is easier to machine them 
that way. Old style herringbone gears were back to back
belly to belly.

I would love to have a suitable rotary engine planetary set
with this feature. 

Also I might add for the rotary engine an offset spur gear box
waste's space in the cowl better put to use for a 
cooling radiator.

pratpsru.jpg

Paul Lamar
 
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