Counter rotating engines

[piperboy84]

What set up or mods are needed to made standard aircraft engines counter rotate. For example a light twin with 2 lycoming 320's or 360,s , is it just a different crankshaft and cam?

[27/09]

Just a different cam plus any different ancillaries like starter motors etc.

I don't know why they were ever used on light twins. The safety argument was a fallacy in my opinion plus the non counter rotating versions performed better.

[India Four Two]

At least one:

PA-39 Twin Comanche C/RPA-30 with counter-rotating 160hp Lycoming IO-320-B1A engines and modified wing leading edges.

https://en.wikipedia.org/wiki/Piper_PA-30_Twin_Comanche

[27/09]

There's also the PA34, PA31, and PA44, Beech 76 to name a few others.

Not many had a non counter rotating variant but of those that did, the non counter rotating versions performed better.

In my opinion a the counter rotating option is a waste of time with no real benefits. The C/R option was really just marketing hype.

[piperboy84]

Originally Posted by 27/09

There's also the PA34, PA31, and PA44, Beech 76 to name a few others.

Not many had a non counter rotating variant but of those that did, the non counter rotating versions performed better.

In my opinion a the counter rotating option is a waste of time with no real benefits. The C/R option was really just marketing hype.

Is that because asymmetrical thrust on twins with non counter rotating props equipped with sub 200hp engines is easily held in check with the roll and yaw inputs even when it's a critical engine failure?

[9 lives]

A left hand prop would be useful too!

When I was a newer pilot I was hanging around the local small aerodrome, chatting with a private owner who was working on his C 177 Cardinal. Among the work he was doing was replacing the starter motor. I got it all ready to run, and told me to jump in and start it up for a ground run. I did my checks, and turned the key.

It turned over, but just did not seem right. It would not start. As the prop turned ahead of me, it was just wrong... I thought... I looked at him through the arc of the turning prop, and he looked back at me perplexed. Then he waved me off, as he realized the problem: He'd carelessly installed a left hand starter motor on a right hand turning engine. He started again....

[27/09]

piperboy84: Is that because asymmetrical thrust on twins with non counter rotating props equipped with sub 200hp engines is easily held in check with the roll and yaw inputs even when it's a critical engine failure?

The asymmetrical thrust is easily held in check on any relatively modern twin, I'm talking anything certified since the late 1950's early 1960's, no matter the horsepower. Sure some will be easier than others.

Even high powered twin don't have C/R props, I'm not aware of any turbo props with counter rotating props.

The only benefit of a C/R set up is a reduced minimum control speed when airborne (Vmca). An engine failure close to Vmca (since you're well below the best single engine rate of climb speed) pretty well means you're now flying a single that cannot climb (or most likely not even maintain height) therefore you're landing anyway.

You might as well treat it like a single that has had an engine failure, close the other throttle and carry out a landing while you still have control. Whether or not your Vmca is for example 70 knots rather than 75 knots is a bit irrelevant in my book.

I'd rather have the extra performance.

To give an example Piper had to increase the HP on the PA31 Navajo from 310 on the non C/R model to 325 on the C/R model to get the same performance.

[9 lives]

The only benefit of a C/R set up is a reduced minimum control speed when airborne (Vmca). An engine failure close to Vmca (since you're well below the best single engine rate of climb speed) pretty well means you're now flying a single that cannot climb (or most likely not even maintain height) therefore you're landing anyway.

The possible benefit of a left hand turning engine on a twin will be that there is no critical engine, which could be what defines a faster Vmca than with two right hand turning engines.

It is true that some twins have lackluster single engined climb, but they will have a positive rate of climb at speeds between Vmca and single engined best rate of climb speed. Or, they would not have passed certification testing.

Its an interesting study as to which way a designer chose to have the engines turning relative to the airframe - tips rotating toward or away from each other at the top. For most twins with opposing rotation engines, the props turn tips toward each other at the top. The exception to this was the P-38 Lightning, which (though I have no personal knowledge of this type) was said to have less than ideal single engined handling.

[oggers]

To give an example Piper had to increase the HP on the PA31 Navajo from 310 on the non C/R model to 325 on the C/R model to get the same performance.

Did they have to, or was it that they were able to take advantage of the lack of critical engine in order to increase power and still achieve the 1.2 Vs1 certification requirement for Vmc? I don't know....

It is true that some twins have lackluster single engined climb, but they will have a positive rate of climb at speeds between Vmca and single engined best rate of climb speed. Or, they would not have passed certification testing.

Most light twins are not required by certification to be able to climb with an engine inoperative. They only need to demonstrate what the performance is.

[eckhard]

The exception to this was the P-38 Lightning, which (though I have no personal knowledge of this type) was said to have less than ideal single engined handling.

I think that Lockheed found that there were undesirable pitch effects with the 'in at the top' configuration, so they switched to 'out at the top'.

I seem to remember Brian Abrahams posting something much more detailed than this on a previous thread?

[eckhard]

For most twins with opposing rotation engines, the props turn tips toward each other at the top.

That puts both thrust lines inboard at high AoA and therefore results in a lower Vmca.

The P-38 config put both thrust lines outboard, increasing the Vmca, which is maybe why the SE handling was tricky?

[9 lives]

Most light twins are not required by certification to be able to climb with an engine inoperative. They only need to demonstrate what the performance is.

There are some, yes. However, these aircraft are bound by other burdensome crashworthiness requirements, which probably make it more attractive, both for design and marketing, to simply design the aircraft so it is capable of climbing away on one engine.

[oggers]

There are some, yes. However, these aircraft are bound by other burdensome crashworthiness requirements

....not just some but most. If the aircraft has a Vso of 61kts or less they are not required to climb OEI nor are they "bound by other burdensome crashworthiness requirements". That is most light twins. Even though most can manage a slight climb.

[Brian Abraham]

I'm not aware of any turbo props with counter rotating props

A400?

The props on the prototype P-38 turned inwards at the top. The aircraft was written off after only 11:50 flying. Following wind tunnel tests outwards rotation at the top was found to be best, for the reason it gave the least pitch change from power on to power off, thus giving the pilot a more stable gun platform. The prototype rotation was the worst that could have been chosen, while rotation in the same direction lay between the worse and best. See graph below.

From the P-38 manual (abbreviated, but to give you a feel) Engine failure on take off - lift off is made at 100 mph, retract gear when certain you are airborne. Gear up the aircraft accelerates quickly to the single engine safety speed of 130 mph, the minimum airspeed at which the aircraft can be flown with gear up is 130 mph (gear contributes 60% of the total drag).

Failure with less than 130 mph cut both throttles and stop. Retract gear if necessary.

Failure with 130 mph or more and the gear up or starting up. Reduce power to gain directional control, then apply as much power as you can hold. While correcting jettison stores. Trim & feather. Excessive yaw will cause the vertical tail to stall and rudder forces will reverse. Engine power will need to be materially reduced and considerable rudder force applied to regain control. Situation can be prevented by keeping the aircraft flying straight by prompt rudder action and necessary throttling of live engine.

With one engine at normal rated power the aircraft can climb above 20,000 feet and have a TAS greater than 225 mph.