The test was done using a box-stock motor. I'm pretty sure the reverse crank allows the motor to run backwards without turning the front housing 90 degrees. Later engines with a one-piece crankcase will require the reverse crankshaft. I plan to buy an OS 46SF reverse crank and try running one of my 40SF's backward on the Duellist.

Ok, now I'm really confused. At the top of this thread DR-1 said to rotate the front housing 90 degrees to the right, viewed from the front of the engine.

Shortly afterward, Cobra1 said to rotate the front housing 90 degrees counter-clockwise (i.e., to the left) when viewed from the front.

Alright, which direction is it? Right or left?

JG26,

The effect of advancing or retarding the configuration doesn't matter. Turning, e.g., the front end of a 4-bolt front-end ENYA either way 90° results in the same reversed sequence of port timing.


Consider: Turning a "normal" CCW (viewed from in front) engines inlet 90° to the left (again, as seen from in front) delays opening and closing 90°, of course - in that rotation. The other 90° occurs because the inlet hole in the 'case meets the shaft opening 90° earlier the next time around. Summing these effects, results in a 180° swap of the porting sequence!

Any difference in timing "backwards" would come from any bias in the stock port opening arrangement. Long since, I tried this (when "pusher" props were more easily available), and it worked very much as here described. Engines were ENYA 35 III's when they were current mainstream items.

Problems? Other than fitting an engine with the venturii on sideways into a simple stunt model? Sure - theoretically. We didn't know as much about tank location vs venturii then. Today, with much more consistent, but still fussy, uniflow tanks de rigeur, it would take a lot of jacking around to get the same, dependable "tank height" relationship. (And then, too, there was learning a GOOD backwards prop flick...)

Back then, we took off rich, and the over/under vented tanks ran progressively leaner as the fuel burned off. MUCH more tolerant of tank height/ much less consistent from flight to flight. No mufflers, so no muffler pressure (case pressure was too much, and used for high RPM, high output events.) Jim Walker's pressure regulated tanks DID exist, and a rare few models used them, but for stunt? Very rare...

The only advantage I can claim for CW rotation engines is that many of the early stunt fliers flew clockwise when upright, and flying our conventional CCW upright direction forfeits the basic torque of the engine into the engine mounts. deBolt flew CW; his famous designs had the 'other' wing longer (the right panel, which was inboard when flying CW.) It takes wingtip weight, and occasionally 'warts' to fly e.g., an All American, Sr., level upright in our near-standard CCW direction. Mine flew CCW, but with a factory left-hand shaft Fox 35 Stunt.

On RPM difference between a CCW engine and a CW rotation engine - as seen from out front - may involve several things.

Shaft opening and closing time may be biased to favor the 'stock' rotation direction. Usually, for general usage engines, he difference is not very great.
Flow in the lower crankcase may also have been developed to favor 'stock' direction - things like the rod helping sweep intake flow into the 'case, then sweeping it into the bypasses,

Wear patterns may have formed if the engine was run much before reversing rotation. These will need some running to re-establish best fit. This process will wear the parts a bit more than they had worn; it's another "break-in."

"LH" props are not available in as many pitch/diameter forms, and a best-match of 'stock' engine and available props may not be likely. Even from the same maker, LH or reverse rotation props of identical pitch and diameter numbers may have different enough form (blade shape, airfoil, etc.) to cause a loss.

Then, too, unless you are seeking ultimate power, you can needle the faster of your opposed pair slightly richer to match the other, well within safe settings for dependable life for both. Check that throttle response stays consistent.

That doesn't sound right...

The front end governs the timing of opening into the crank and this should be moved 90° in the forward rotation direction of the engine. It will then end up on the other side of TDC, and the engine will now be correctly timed when turning in the opposite direction.

So seen from the cockpit the a regular engine turns CW and the front end should be rotated 90° in that rotational direction. With an uppright mounted engine the carb will then be on the right-hand side of the plane (as seen from the cockpit). The two positions are the only two possible positions where the engine will run, but it will run in two different rotational directions.

The trick works if the crank timing is symmetrical around 45° before TDC, it is then shifted to 45° after TDC and the engine needs to spin in the other direction.

Thanks for the thought! I found it does work, though.

The 'symmetrical 4-bolt front end' engines will also run - in the stock direction - with the front end rotated 180° btw.

Very good point that performance should be identical if the port timing is symmetrical about 45° BTDC.

Mr. Cox,
As it has been about 50 years since I used this method, to check my memory I hauled out a 1950's ENYA 35 III that has ever since given good, reliable, and dependable service in stock configuration .

It DOES work as I described it. Okay, I didn't run the engine in the three alternate configurations, but hand-rotated it through each to observe the sequence of 'case intake' shaft porting, compression and exhaust port sequence. (Plug removed, 'front end "loose" but held fully in place.)

This, of course, did not check for any bias in timing lead/lag that stock configuration may have had. The ONLY problem that appeared was interference of the piston skirt at BDC contacting the front end upper edge of the front end piece. I may have used a 29 III back then, as the stock 29 III's seemed to vibrate less than the 35 III's. The 29 III did not appear to have this problem.

The interference consisted of the piston skirt at BDC intruding about 1mm past the upper case edge. In 'normal' configuration, that edge of the bottom end was relieved to clear the piston skirt. To correct this would need only a slight touch with a fine file in my Mark 1 bare hands.

Back then, I, too overlooked the fact that the sequencing of the ports is reversed by the 90° rotation, but identical at the 180° rotation. The shaft rotation reverses the sequence of port functioning at 90°, but preserves it at the 180° rotation. I'm relieved that my memory serves. (The standing joke among people my age is that I can remember who sang: "Find a Wheel..." in about 1955, (Answer: Perry Como.) but sometimes I forget what I had for breakfast, today...)

Thanks for your comments, and that they led me to double-check.

Many thanks go out to Lou Crane and Mr. Cox for their thoughtful replies to my question.

I think it is easier to see how it works by looking at a timing diagram. The intake port is typically open for around 180° and it will close shortly after TDC in the direction that the engine runs. Below is an example that shows when this works particularly well. The diagram is for the MK16 which has a disc opening of 180° (in the backplate) and it is made symmetrical around 45° before TDC. This engine then has two working orientations out of the four "possible".

If one would rotate this intake timing by 180°, then the intake port would not even be open around TDC, so clearly that doesn't work...

Mr. Cox, surrender and greater thanks!

CONCLUSION: Rotating the front housing 90° in the direction of stock prop rotation works. 'Nuff said.

Again trying it with the 35 III, with more careful observation of shaft port timing, I concede my error. In the intake shift against stock rotation direction, there is an occasional similarity of port timing, but it is not near the 'stock' numbers. Thus not usable. At the 180° shifted position, : The shaft port was open on the downstroke. It would pump any 'closed-case' volume OUT, not draw flow IN.

I can only figure that the 'intake moved left' proved an easy and effective solution, so I did not scrupulously check out the other two possibilities. Glad to learn this, finally.


A possibility: the Fox factory 'LH Shafts' simply reversed the 'side' of the intake 'hole' to the other side of the crankpin. As stock, the RH shaft is cut so that the 'opening edge' is aligned with the crankpin centerline -to- shaft centerline, and the full opening "lags" about 90° after in the prop rotation direction.

The LH shafts have their 'opening edge' located the same, but the full opening is to the other side of the pin-to-center line. Mirror-image, done completely by location of the rotary shaft inlet port. No wonder it runs identically as the stock shaft, but turning the other direction...

Glad I finally have that sorted out.

Other reversible engines? We all know that reed-valve Cox engines run happily either way. Induction is controlled by positive and negative (gauge) pressure in the lower crankcase, NOT dependent on mechanical valving.

The old 3-port spark engines only ran reliably in the desired direction because the spark timing only worked in that direction. Reverse operation could be done by reversing the action of the breaker points to suit. The engine did not care.

The weakness of the 3-port layout was that the induction timing was symmetrical. We enjoy induction timing biased to provide longer 'pure intake' flow with 'capped-off' case compression.

ENYA offers certain of their engines in left-hand rotation form. Apparently the only difference is that used in the Fox factory LH shafts. I wonder if ENYA offers LH shafts as separate spares... Would be nice for certain of us CL fliers, and for RC fliers who want opposed rotation engines on a twin.