Learned something today about twins

[Henning]

"In a P-38 if we were trying to escape an enemy single engine fighter, we would normally make a turn to the right, with counter rotating props we could turn equally well in both directions, but remember the enemy single engine fighter didn’t turn quite as well to the right as to the left. That is why we would turn right. "

Diary of a P-38 Pilot

Yep, I heard it from this guy.

That was him near the beginning of the war in his P-47. He also flew the P-38 & and 51 in combat as well as IP for Russians in P-39s (which he said was the best handling of the lot.).

He gave me my first stick in his 206 on floats and helm time on his boat. Still alive in Houma.

 

[simtech]

What is it that makes a piston engine ALWAYS rotate one way or the other? Is it something about the crankshaft?

Good question!

 

[simtech]

Subject line is misleading. Thought it was about 'twins', not twin engines..

 

[Henning]

Good question!

The direction of rotation the cam is ground for. Let's take an old Chevy V8 rocker arm engine with a chain drive. It will run one direction. If I replace the chain with a 2 gear set, the engine will run the other direction because I have reversed the direction of the cam rotation and reversed the sequencing of the intake and exhaust valves closing.

So with this change in intended direction, I now need to make a mirror of all my accessories that are rotation direction sensitive as well.

On thing many people fail to account for in typical maintenance of an "L" engine is the orientation of the alternator fan. This can lead to some interesting alternator issues.

This is where the cost premium comes from operating L engines, you lose a nearly full level of efficiency of scale, and that is why it is not common. IMO, the cost of CR engines is not particularly worth it.

The only difference I notice is I don't have to lead with the left throttle on the tip of my thumb and right in my palm as I advance them on the takeoff role, that's it.

 

[StinkBug]

The left turning engines are less common as fewer of them are produced but I don't see any reason they are drastically MORE expensive to make. In fact for most engines, there's hardly any difference in the L version.

The term used here would be "economies of scale". Most of the parts are the same, but not all of them. When you are doing smaller production runs of certain parts, they cost more per part to produce. Add to that the fact that you'll probably need a separate assembly line, or at least separate assembly procedures to keep everything separate and you end up with 2 very similar products with not so similar prices.

What is it that makes a piston engine ALWAYS rotate one way or the other? Is it something about the crankshaft?

It's mainly about the camshaft. You need to open and close valves in sequence. On top of that you also need a distributor (or magneto) that fires in the correct order, an oil pump that runs the other way, and any accessories driven by the engine either have to be designed to run backwards or have a system to reverse the drive rotation.

In some engine designs it becomes even more complicated. Generally speaking a crank doesn't really care what direction it spins, but if it's been designed with a certain shape for going one way (generally high performance engines) spinning backwards loses you any gains. Some engines also have balance shafts to reduce vibration, again gonna have to reverse the timing on that. The more complex the engine is the more complicated reversing it is.

 

[SoonerAviator]

Good question!

Well, usually starts with firing order, but the camshaft is designed to facilitate that firing order. In the marine world, sometimes the engine is counter-rotating, sometimes they do the easy thing and use a counter-rotating gearbox instead.

 

[Henning]

Well, usually starts with firing order, but the camshaft is designed to facilitate that firing order. In the marine world, sometimes the engine is counter-rotating, sometimes they do the easy thing and use a counter-rotating gearbox instead.

In the marine world, most of it is done in the gear set anymore with both engines identical. Some of the big engines are still direct reversing though.

 

[N801BH]

The direction of rotation the cam is ground for. Let's take an old Chevy V8 rocker arm engine with a chain drive. It will run one direction. If I replace the chain with a 2 gear set, the engine will run the other direction because I have reversed the direction of the cam rotation and reversed the sequencing of the intake and exhaust valves closing.

So with this change in intended direction, I now need to make a mirror of all my accessories that are rotation direction sensitive as well.

On thing many people fail to account for in typical maintenance of an "L" engine is the orientation of the alternator fan. This can lead to some interesting alternator issues.

This is where the cost premium comes from operating L engines, you lose a nearly full level of efficiency of scale, and that is why it is not common. IMO, the cost of CR engines is not particularly worth it.

The only difference I notice is I don't have to lead with the left throttle on the tip of my thumb and right in my palm as I advance them on the takeoff role, that's it.

Kinda... The profile of a cam is based on rotation.. The ramps leading up to max lift is slightly different then the decending ramps.... It is to optimizes, but it will run backwards.....

Also the 1-8-4-3-6-5-7-2 firing order will change.. And the distributor will not work backwards as the centrifigul advance plate is not right.... As the other guy pointed out.. Oil pump needs to be reversed and the crank is windaged the wrong way too...

Like Hennings said... most installations are reversed in the gear set...

 

[Henning]

Kinda... The profile of a cam is based on rotation.. The ramps leading up to max lift is slightly different then the decending ramps.... It is to optimizes, but it will run backwards.....

Also the 1-8-4-3-6-5-7-2 firing order will change.. And the distributor will not work backwards as the centrifigul advance plate is not right.... As the other guy pointed out.. Oil pump needs to be reversed and the crank is windaged the wrong way too...

Like Hennings said... most installations are reversed in the gear set...

Exactly, to actually make everything work correctly to run backwards is no a small undertaking, everything changes. I was just keeping it simple there to answer the base question of why an engine runs the direction it does.

I used to run an old tug with an Enterprise Deisel. That was a direct reversing 4 stroke. When you wanted reverse, you had to wait for it to swap camshafts.

 

[N801BH]

Exactly, to actually make everything work correctly to run backwards is no a small undertaking, everything changes. I was just keeping it simple there to answer the base question of why an engine runs the direction it does.

I used to run an old tug with an Enterprise Deisel. That was a direct reversing 4 stroke. When you wanted reverse, you had to wait for it to swap camshafts.

A buddy of mine ( Mike Rivera) used to run a tug boat up and down the Miami River... I rode along several times and the first time we needed reverse, I was SHOCKED to see him shut off the motor, push that lever over to switch cams and restart..... He said it is never a problem,,, Unless it doesn't restart... Then he is buying a NEW dock where the barge is parked...

 

[Henning]

A buddy of mine ( Mike Rivera) used to run a tug boat up and down the Miami River... I rode along several times and the first time we needed reverse, I was SHOCKED to see him shut off the motor, push that lever over to switch cams and restart..... He said it is never a problem,,, Unless it doesn't restart... Then he is buying a NEW dock where the barge is parked...

It's really not, you just have to do everything 13 seconds in advance. It's not much worse than the best clutch sets which are 7 seconds to reverse. The worse I had was a supply boat with 57 seconds to reverse due to a shaft brake system to deal with the big props. That was an interesting one to handle close quarters.

 

[kgruber]

The old direct reversing Mercury outboards were the best motors they ever made. Mark 75, Mark 78, Merc 800. They were light and fast. Single lever control, with a big red start button on the top.

Required a good battery, and good sense

 

[Henning]

Yep, 2 strokes make it simple.

 

[N801BH]

The old direct reversing Mercury outboards were the best motors they ever made. Mark 75, Mark 78, Merc 800. They were light and fast. Single lever control, with a big red start button on the top.

Required a good battery, and good sense

Loved them even more with "Quicie" lower units and tuning stacks ...

 

[schmookeeg]

"In a P-38 if we were trying to escape an enemy single engine fighter, we would normally make a turn to the right, with counter rotating props we could turn equally well in both directions, but remember the enemy single engine fighter didn’t turn quite as well to the right as to the left. That is why we would turn right. "

Diary of a P-38 Pilot

Careful, you're corroborating a Henning knowledge drop. You'll ruin your typecasting.

 

[flyingron]

In the marine world, most of it is done in the gear set anymore with both engines identical. Some of the big engines are still direct reversing though.

Especially since marine engines typically run both forward and backwards. To get a counterrotating engine you just need a prop pitched the other way around and to flip the reversing control around.

 

[Anthony]

You're on Pilots of America, not Parents of America or Fetishes of America. ;-)

lol! Sometimes I wonder!

 

[TangoWhiskey]

All this engine and twin talk, I'm surprised we haven't seen Ted.

 

[hish747]

I'm not surprised at all, there is MONEY TO BE SAVED by having both rotate the same way. It really is pretty irrelevant in practice. Don't get below Red Line and you will never realize the difference.

It's hard to agree that the difference is irrelevant. With the failure of the left engine, the right non-CR engine pulls the airplane more forcefully to the left. This means you have less time to apply correction than if it were a CR and more time for the plane to embark on extremely hostile territory. Additionally, planes that have smaller less effective rudders (Aerostar and twin comanche for example) are less able to overcome the adverse yaw and more likely to lose control earlier. I would not fly a non-CR TC and just a flat NO to the Aerostar with the two critical engines. While the difference may not be huge between CR and conventional, I would not say it is irrelevant.

 

[MauleSkinner]

It's hard to agree that the difference is irrelevant. With the failure of the left engine, the right non-CR engine pulls the airplane more forcefully to the left. This means you have less time to apply correction than if it were a CR and more time for the plane to embark on extremely hostile territory. Additionally, planes that have smaller less effective rudders (Aerostar and twin comanche for example) are less able to overcome the adverse yaw and more likely to lose control earlier. I would not fly a non-CR TC and just a flat NO to the Aerostar. While the difference may not be huge between CR and conventional, I would not say it is irrelevant.

I know quite a few people who would rather have a failure of either engine take them to the limits of certification. I’m happy to have only one side take me there.

plus, my right leg is stronger than my left.

 

[Tantalum]

Some of the Aerostars did also. Seems like the 700P when Piper bought the company the top of the props turned outward also.
In fact I can think of no other GA twins except the Piper Senaca and and Navajo that have counter rotating. Must be a Piper thing.

Duchess counter rotates..

 

[hish747]

I know quite a few people who would rather have a failure of either engine take them to the limits of certification. I’m happy to have only one side take me there.

plus, my right leg is stronger than my left.

To clarify, I also prefer a twin to a single, but I prefer a CR twin to a conventional.

 

[MauleSkinner]

To clarify, I also prefer a twin to a single, but I prefer a CR twin to a conventional.

To clarify, you’d rather require 150 pounds of force on either rudder in the event of an engine failure than 150 pounds on one side vs 130 pounds on the other.

 

[GRG55]

Duchess counter rotates..

As does the Piper Seminole, and the PA-39 version of the Twin Comanche, which Piper produced starting in 1970 until the 1972 flood. The PA-39 replaced the earlier PA-30 version that did not have counter rotating props.

But I'm with @MauleSkinner. CR, or lack thereof, in a comparatively low powered piston twin doesn't really make much difference. Losing an engine on one side or the other requires flying the airplane within its flight envelope limits...just like so many other aspects of successfully keeping from killing yourself in an airplane, any airplane.

The Aztec doesn't have counter-rotating propellers, but an engine loss on either side is a pretty tame affair, unless you really, really screw up. In which case it probably didn't matter which way the prop on the good engine was spinning.

 

[Pilawt]

The Aztec doesn't have counter-rotating propellers, but an engine loss on either side is a pretty tame affair, unless you really, really screw up. In which case it probably didn't matter which way the prop on the good engine was spinning.

On the Apaches and Aztecs I flew, lack of a hydraulic pump on the right engine made the left even more critical.

 

[GRG55]

On the Apaches and Aztecs I flew, lack of a hydraulic pump on the right engine made the left even more critical.

When I was shopping for an Aztec the dual hydraulic pumps was one of the items on my "must have" list. Mine is a 1979 'F' built by Piper with pumps on both engines.

Although theoretically possible I can't imagine trying to fly the airplane on one engine while climbing low to the ground and trying retract the gear with the hand pump...

 

[Tantalum]

The Aztec

I've started to develop a real sweet spot for the plane

 

[Tarheelpilot]

I wonder if henning will reply to the revival of this thread...

 

[hish747]

To clarify, you’d rather require 150 pounds of force on either rudder in the event of an engine failure than 150 pounds on one side vs 130 pounds on the other.

Why would a twin with CR propellers require different rudder forces depending on which engine failed? It should be the same in each direction.

 

[MauleSkinner]

Why would a twin with CR propellers require different rudder forces depending on which engine failed? It should be the same in each direction.

CR props would have the same on either side, and they will probably be equal to the higher on a non-CR airplane, not the lower. The manufacturer is most likely going to certify performance using maximum limits rather than something lower.

In other words, failing the critical engine on a non-CR airplane doesn’t make it harder. The airplane has to operate within certification limits. It makes failing the non-critical engine easier. If both engines are the same, neither will be easier.

 

[hish747]

CR props would have the same on either side, and they will probably be equal to the higher on a non-CR airplane, not the lower. The manufacturer is most likely going to certify performance using maximum limits rather than something lower.

In other words, failing the critical engine on a non-CR airplane doesn’t make it harder. The airplane has to operate within certification limits. It makes failing the non-critical engine easier. If both engines are the same, neither will be easier.

You may not be accurately describing what you are thinking, but sorry to say that what you are saying is illogical. Both engines on a CR engine are like failing the non-critical engine on a conventional twin. In other words, failing either engine on a CR is equal to failing the non-critical engine in a conventional. In other words the CR is clearly easier and requires less rudder forces which are essentially equal on either side.

 

[MauleSkinner]

You may not be accurately describing what you are thinking, but sorry to say that what you are saying is illogical. Both engines on a CR engine are like failing the non-critical engine on a conventional twin. In other words, failing either engine on a CR is equal to failing the non-critical engine in a conventional. In other words the CR is clearly easier and requires less rudder forces which are essentially equal on either side.

No, it’s like failing the critical engine. The manufacturer isn’t going to limit performance because they put CR engines on the airplane. They didn’t go from nonCR engines to CR in the Navajo to make it easier...they did it so they could bump the horsepower without changing the airframe. They went from needing full rudder at Vmc for loss of left engine to full rudder at Vmc for loss of either engine.

 

[benyflyguy]

“Learned something about twins today“.
Anybody else think this was going a different way. #disappointed.
I must be hanging out on reddit too much lately.

 

[hish747]

No, it’s like failing the critical engine. The manufacturer isn’t going to limit performance because they put CR engines on the airplane. They didn’t go from nonCR engines to CR in the Navajo to make it easier...they did it so they could bump the horsepower without changing the airframe. They went from needing full rudder at Vmc for loss of left engine to full rudder at Vmc for loss of either engine.

This conversation isn't specific to the Najavo. It's about the physics of CR vs conventional. But since you mentioned the Najavo, yes CR may allow higher horsepower without a change to the airframe because there is less asymmetrical force once you eliminate the critical engine and it's longer arm. Less stress on the airframe and less rudder required to counteract.

 

[Tantalum]

So why do most CR turn inwards? Wouldn't making them turn outwards make both of them not critical?

 

[GRG55]

So why do most CR turn inwards? Wouldn't making them turn outwards make both of them not critical?

P-factor. Asymmetric thrust from the downward going blade at high angle of attack, such as in a climb after lifting off the runway. Better to have the the downward going blade closer to the fuselage to minimize the moment arm creating yaw in the event of an engine loss.

 

[MauleSkinner]

This conversation isn't specific to the Najavo. It's about the physics of CR vs conventional. But since you mentioned the Najavo, yes CR may allow higher horsepower without a change to the airframe because there is less asymmetrical force once you eliminate the critical engine and it's longer arm. Less stress on the airframe and less rudder required to counteract.

Less rudder required to counteract the same horsepower, the same rudder required for more horsepower. Full rudder is full rudder, regardless of which way the engine turns.

And no, it’s not about just the Navajo...show me a non-trainer twin that the manufacturer advertises as “easy to fly—less performance”, or some equivalent.

 

[flyingcheesehead]

Holy necropost, Batman! Nothing like reviving a 5-year-old thread randomly. But it's a fun one I guess... I learned something about boats.

On the Apaches and Aztecs I flew, lack of a hydraulic pump on the right engine made the left even more critical.

But there were upgrades! The Apache I did my multi rating in had FOUR ways to get the gear down: The standard hydraulic pump on the left engine and the standard backup hand pump, plus an electric backup hydraulic pump and a CO2 blowdown system.

I was impressed with the Apache's hydraulic system, though... If the left engine or the hydraulic pump failed, requiring use of the hand pump, you could just select gear down and full flaps and then start pumping the hand pump, and it would first extend half flaps, then extend the gear, then extend the rest of the flaps. All you had to do was pump.

You may not be accurately describing what you are thinking, but sorry to say that what you are saying is illogical. Both engines on a CR engine are like failing the non-critical engine on a conventional twin. In other words, failing either engine on a CR is equal to failing the non-critical engine in a conventional. In other words the CR is clearly easier and requires less rudder forces which are essentially equal on either side.

No, the FAA certification requirements limit the maximum amount of pressure needed on the rudder pedals to keep the plane flying straight with one engine out and the other at max power. Generally, an engineer will design to be just within the given limits. Since the CR engines will both require the same amount of rudder, a CR airplane will probably be engineered such that the max amount of rudder pedal pressure is right near the limits and it'll be near those limits on both sides.

On the other hand, the non-CR plane will need to stay within that same limit when the critical engine fails, but since there is inherently less rudder required for the non-critical engine, the pedal pressure on the non-critical engine will be lower.

So why do most CR turn inwards? Wouldn't making them turn outwards make both of them not critical?

No, it makes BOTH of them critical since both of them have the descending blade (which produces more thrust in the climb) farther away from the fuselage.

 

[MauleSkinner]

No, it makes BOTH of them critical since both of them have the descending blade (which produces more thrust in the climb) farther away from the fuselage.

And on a CR airplane with both spinning toward the inside they’re also both critical, as either one would be “most adverse”.

 

[MauleSkinner]

“Learned something about twins today“.
Anybody else think this was going a different way. #disappointed.
I must be hanging out on reddit too much lately.

I figured if the thread hadn’t been locked in over five years it was probably safe.