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Reversing rotation of engines

Old 02-17-2017, 10:58 PM
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Default Reversing rotation of engines

Thought some here might be interested...

Over in the Glow Engines area, there's a long discussion about this concept. Some CL fliers may be interested as engine rotation direction affects several things we must deal with.

Engine torque, for one. Precession yaw and pitch loads are others.

A reply I was writing to clarify my earlier post a bit vaporized before I could post it.

Why do these matter? Think... On takeoff, prop torque loads into the engine mounts. An equal but opposite load tries to roll the model opposite to prop direction. With typical models, flying left-to-right as the flier sees it, that would 'roll' the model in to the center.

Much more "over there" that I found interesting, even some of my own blather. Try it, you might like it...
Old 02-18-2017, 04:40 AM
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Many years ago several clubs were doing "Golden Age Racing." Scale models of golden age racers with 36" span, .40 engines, and 11" props required. The big props made the engines run higher on the torque curve (lower rpm's) than normal. Take offs were hairy! I reversed the rotation of a rear rotor ST 40 and completely tamed this. Mew Gull in the photo was the easiest flying model of the bunch.
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Old 02-18-2017, 07:26 AM
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Some of my speed models I flew clockwise so they would not torque roll on takeoff. Oddly enough I still had a torque roll happen on the A speed model pictured on my avatar. It messed up the lines good. There was a left hand crank available for the TD .049s way back when. They needed a left hand prop, which were not too common, so that was not popular for long. On a stunt model, it is not such a wonderful thing, as it will tend to go slack while inverted which is not really desired, at least because you are not accustomed to it. (I am projecting this. I have not actually done it) The only other thing I can add is that the front case of the older motors can be clocked 90 degrees to give reverse rotation, like the K&B outboards did.

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Old 02-18-2017, 04:44 PM
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Lou
I am not sure I understand your post exactly, but reverse rotation produces a torque vector towards the out side of the circle. (based on the normal way we fly) I don't think there is a great advantage, but it is frequently used in carrier , speed and racing. With the advent of electrics we are seeing it more often. However, I always ran my reed valves in reverse rotation to improve line tension.
Old 02-19-2017, 12:11 AM
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XBY-1 and aspeed,

Gyroscopic precession is a strange thing. The details of how and why are beyond my total understanding, but the 'what' is easy to describe.

When I was much younger, simple gyroscope toys were available. A shaft with a flywheel weight was mounted in a frame of two circles at right angles to each other (to clear the rotating flywheel.) It was a "child's top" reduced to schematic. Such tops remain upright so long as RPM is high enough, remember?

If you've ever held the axle of a bicycle wheel and spun up some RPM, then tried to tilt the axle, you've SEEN that the axle resists any tilt you try to create. The wheel pushes strongly to tilt in the direction 90 later in the direction the wheel is rotating!

There are equations that calculate how much torque this creates, and which sense (direction) it acts in, using mass and speed of rotation as terms. On a 'standard' rotation engine in a CL model, e.g., a 'pitch UP' control input equals pushing the top of the prop disk back. Precessive load acts as if the model was pushed back 90 later, i.e., outboard tip pushed back relative to the model's forward motion direction.

This is completely reversible, and also applies to the "axis turning effect" of flying in a circle. When upright and level, DOWN control input causes a nose-in gyro load. (It is a 'couple', strictly.) BUT DOWN inputs sharp enough to matter usually occur when we fly "corners." When we're inverted. the basic direction of flight is reversed, thus so also is the resulting gyro 'couple.'

For stunters, this helps when we do sharp corners in pitch. LH engine rotation, as you've found, helps other high torque situations - racer take-offs and goosed throttle in low-speed Carrier mode. A stunter's maneuvering drag tries to slow the model, which increases the thrust load on the prop - which means the lower 'cruising' torque has to increase for an instant. (The model does NOT slow instantly - it enters a corner with the momentum it had when you popped in the input.)

Standard stunters today almost all use the reversed bellcrank setup. Sharp UP input from upright level flight creates a gyro couple trying to yaw the model nose-out. UP line forward moves the effective aim of line-pull further forward of the CG, That tends to oppose the gyro nose-out couple. So, our bellcrank's pushrod goes to the left (usually) of the bellcrank pivot, passing over the leadouts. Reversed engine rotation would reverse the odd effects, and the BC pushrod would go outboard of the BC pivot Simpler and less crowding over the bellcrank.

AND "UP line forward" wouldn't be necessary.

OBTW, re: old peoples' memory? The other basic comment is that:

"Memory is the second thing to go."

....."So, what's the first thing to go?"

"Er...uh..."
Old 02-19-2017, 12:34 AM
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XBY-1,

Forgot one thing in my (as usual) wordy earlier post...

Engine torque load on the mounts is equal and opposite the torque that turns the prop. With "standard" rotation, the prop turns clockwise seen by the imaginary pilot. Equal and opposite means the 'reaction' passed into the mounts trie to roll the model counter-clockwise.

Carrier and Racing events are all flown counter-clockwise, as seen from above. We can't turn the 'deck' around, or fly clockwise in a heat when everyone else is going counter-clockwise. (HOO-BOY!!) We COULD reverse the problem torque factor by running the engine "CCW" as the "pilot" sees it. The gyro couple from lapping in 3 sec or so is much less than the couple a stunter faces in a 90 corner in 1/3d of a second.

The torque situation is familiar: Ever jazz the throttle on a big V-8 and feel the car rock in response?
Old 02-19-2017, 04:11 AM
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With an engine that turns in the conventional direction, is it the torque that causes the take off dramas, or has it more to do with the affect of prop wash on the fin??

It seems to be cures on proto models with the fin below the stab.
Old 02-19-2017, 07:42 PM
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Sorry Greg, I wanted to dump this in first, so I wouldn't forget or lose it. Forgiven?

Mr. Cox, surrender and greater thanks! (As posted in the Glow Engines section.)

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.
Old 02-19-2017, 08:06 PM
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Now, Greg, to your Q:

I don't know enough about prop wash flow to estimate effects. I visualize it as an angled wash flow in the prop rotation direction . With a right hand turning engine, that would tend to raise the inboard wing panel and drop the outboard: - a right roll effect.

The wing and stab may act as a form of flow straighteners to a degree. I don't know why the same spiraling flow would act the other way when it reaches a sub-rudder.

The opposite-but-equal torque passed to the engine mounts, reflecting the effort of turning the prop, can or could be estimated. It is quite strong, strongest as the prop comes up to in-flight RPM. We say the prop unloads? It does.

In racing and combat, the load remains high, but not at the peak as early in ground roll, or on hand-launch release. In stunt and 'sport' CL, cruise RPM is not the ultimate the engine could generate on that prop, it is definitely unloaded significantly.

At flight speed, the leadout guides provide the strongest roll and yaw 'correction'. "Pull" aims through them at the CG, or tries to. Aerodynamics can affect that slightly.
Old 02-20-2017, 06:38 AM
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Greggles, I always thought it was just the torque, and the rudder on the bottom was just to lessen the angle of attack on takeoff? My plane came in on takeoff on CCW flight, but torqued out on CW flight and still rotated under the dolly. The plane should be a bit bigger I guess too. There is no rudder to go into the equation. I guess propwash is still there though.

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