Hi Jarrett,

  I am not doing this because I think this is a good way
  to make a twin pusher. As my old late friend Vance RIP
  used to say I am doing it because it is an interesting project.
  In the mean times I am waiting for slide throttle parts.

  The problem with spiral bevel gear sets is the separating forces
  between the gears. This amounts to a point load of about
  a thousand pounds at 200 foot pounds input. The traditional
  way of handling this is use big heavy bearings and a heavy
  thick wall spool. I am thinking of using a large diameter, thin
  section bearing to take the thrust load and a fabricated sheet
metal  spool. The 3.5 inch carbon fiber prop shaft will be bonded
directly to the ID of the spool.

  Consider the red device a rock solid buttress.
  What does an FEA look like in the spool.
  Material is 4130.

  Thanks

  Paul Lamar

  Paul, As I remember the detroit car companies mostly use hypoid
gear sets. Do they also have large separating forces. I remember
something from an applied mechanics course when I was at the
University of Pittsburgh in the late 1960's. The lecturer said that
gears were designed with  one of two curves. cycloidal was like  the
trace of a fixed point on the side of a rolling cylinder, and
involute was like the end trace of a string unwinding off a
cylinder. It was suggested that these two curves would make a
rolling contact point at the right center distance spacing  with no
change in distance to either of the shaft centers. Having the wrong
center distance would cause the driven gear to increase and decrease
speed at every tooth contact due to the small change in the contact
point to  shaft center distance.

I used white grease to get the contact point right and flank space
for lash  on the carrier bearings. There was considerable practice
with a chrysler hemi in a sprint car. In looking at the conceptual
design above, how about a pair of timken bearings on the small end
of the spool for radial and thrust loads and a large diameter
spherical roller on the larger end for the radial loads there. This
would reduce the  required size of the thrust bearings which seem
more rpm limited in the larger diameter.Will a universal joint be
necesary on the ford pinion carrier cartridge for access to the
shaft nut that retains the pinion bearings or could it be a flanged plate
with half round drive pins on bach that bolts to the yoke where the
universal joint caps would bolt up.

The flange might run smoother and allow
the drive shaft to be bonded to a flanged steel tube. One unrelated thought
is that if the engines can not be located far enough behind the
firewall for CG while keeping the drive shafts inside the wing, the
use of a full length gearbox to gearbox drive shaft tube with hyvo
chain reduction off each engine would keep both props turning with a
shut down engine. Also both engines could be right side up and
pointing in the same direction. I think the original sketch Jean
posted showed one engine turned 90 degrees for exhaust manifold
construction prposes. A full length shaft eliminates asymetrical
thrust issues and prop synchronization. I am a real fan of Tracy
Crook's PSRU but if an offset s needed to gear the drive shafts up
to a thicker section of the wing/and or CG isues are a deal buster
this could be a way out.

george grimes

The beauty of this design is the right angle drives do double
duty as prop speed reduction units. There are no U joints in the
prop shafts. Just the lateral shafts from engine to gear box have u-joints.
Those I suspect can have stock carbon fiber RX8 drive shafts.

Hyvo chain boxes are heavy. I used one in a mid engine Firebird
I built for Pontiac way back in 1968 :)

No problems keeping the lateral shafts inside the wing.

Paul Lamar


Hi Jean and Paul,
What is the length of the RX8 drive shafts? What is the approximate center
line spacing of conventional tractor twins?
To me the RX8 drive shaft looks to be about40 to 45 inches. If the engines
are centered in the airframe, the drive end will be about 12 inches from the
center and then add the 40+inches and the prop centers will be about 52
inches from the center line. A conventional twin engines should be about 76
inches. I figure that at cabin width 30", 12" space to prop tip and with a
68" prop radius of 34". The conventional air craft has a larger rudder to
control yaw. It could be hard to control on one engine. Can the drive shafts
be shortened and reglued? If the tail end of the fuselage could be
shortened, the props could be brought closer to the center. If one is
mounted a bit further ahead of the other, they could overlap, to reduce the
yaw.
As George stated in one of his notes to me, talking and throwing out ideas
for discussion is how we progress. Maybe I am full of it, but it is worth
discussing.
Another question; how does a propeller do at absorbing the horsepower at
altitude. As Rolf pointed out, a single prop can take about all the HP we
can give it. Only thing is with a canard, the ground clearance limits the
radius. A turbo charger spins faster a altitude to compensate a little for
the thin air. That only goes so far though. Does a propeller loose
efficiency in the thinner air?
Thanks to all writing here. I am learning a lot and enjoy communicating
with people that are intelligent. Very refreshing change from dealing with
cousins of chimps and the chimps are the smart side of the family:-)).

Dale Davies

I don't know the exact RX8 drive shaft length but I think you are about right.
You don't need to use them as you can get carbon tubing instead.
You can also make carbon tubing by wrapping tape around
a thin aluminum tube.

Paul Lamar