The main attraction of the Wankel for aircraft is no valve gear, extreme
mechanical simplicity not necessarily the RPM capability.  Gearboxes are
relatively expensive and all certified geared motors have had difficulty
with the
gearboxes. The certified engines were built with gearboxes for a while,

but

they
have almost totally gone to direct drive with more displacement.
I really think this is the sweet spot (low RPM direct drive) if you had a
funding
source to do a clean sheet of paper design.  If Wankels live well at
6500 RPMs just *think* how well they would live at 2800 RPMs!

Bill
Long EZ builder & pilot
BSME, MSME

Martin Morgan wrote:

I've been reading a number of comments about how much better an engine would
be "direct drive". While I agree that gearboxes are expensive I would also
say that many successful engines use them without problems.

In Australia the Rotax 912 is certified. This engine gives much better
thrust than equivalent engines because of the gearbox. They spin larger
props slower... I have never heard of a 912 gearbox giving trouble, and
these engines are regularly going to 2 tbo's (2400 hrs) before being worked
on (and then not the gearbox!).

Personally I am happy to use a well designed gearbox and let the engine spin
at a more efficient speed. I read somewhere a long time ago that propellors
were most efficient below 1700 rpm. I can't verify this but I know they are
certainly quieter at those speeds, and most current engines haven't a hope
in hell of being efficient that slow.

 So, bowing low towards Mr Crooks house, I join the "higher reduction ratio"
bandwagon. (more RPM, more POWER, bigger PROPS... ok I'm a petrol head.)

Martin
Pitts Pilot

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