Contra rotating Propellers

"Aircraft which have counter-rotating propellers rotating toward the cockpit on the top side (such as the Beechcraft Duchess) do not have a critical engine, while both engines are critical on aircraft with counter-rotating propellers turning away from the cockpit.
The Lockheed P-38 was an example of the latter."

All I can say is that the above exerpt came from the article that you sited.

Bruce R. said:

"Aircraft which have counter-rotating propellers rotating toward the cockpit on the top side (such as the Beechcraft Duchess) do not have a critical engine, while both engines are critical on aircraft with counter-rotating propellers turning away from the cockpit.
The Lockheed P-38 was an example of the latter."

All I can say is that the above exerpt came from the article that you sited.

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Agree, the article does say that, but then if both engines are critical, doesn't it contradict the definition of a critical engine?

Check this out...

"The P-38 Lightning had two critcial engines with counter-rotating propellers which turned in the opposite way to the usual directions. (a) Something odd must have been happening. (b) This article is clearly not the whole truth. 86.141.175.242 12:36, 8 February 2007 (UTC)

Please quote a reliable source that claims a P-38 has two critical engines. With contra-rotating propellers, it had no critical engine at all (or, to put it another way, both engines were equally critical).--chris.lawson 23:30, 8 February 2007 (UTC)
Chris- you are exactly correct! The P-38 DID have counter-rotating props, meaning, neither engine was critical- sustained level flight was achieved with either engine inop. My uncle is a WWII Vet and has numerous hours in the 38. Although, the artical is lacking, it's a good start. The post about the 38 having TWO critical engines is not correct."

Talk:Critical engine - Wikipedia, the free encyclopedia

Hi Flyboyj,

>Agree, the article does say that, but then if both engines are critical, doesn't it contradict the definition of a critical engine?

Someone mentioned that there was an FAA definition of the term "critical engine", and I think this would be interesting in this context.

I think we're all in agreement that the direction of the rotation of the P-38 engines made it harder to control the P-38 on a single engine than if it had been reversed (as it actually was on the first few aircraft).

However, if the "critical engine" is just a concept to determine whether the procedures have to differ depending on which engine is lost, this might make the P-38 an aircraft with "no" critical engines, even though the impact of propeller rotation is "bad" regardless of which engine is lost.

I believe the definition for critical single engine speed for example makes some assumptions like a bank limited to 5 degrees with one engine out, while the procedure for the WW2 Tupolew Tu-2 required some 20 degrees of bank in single engine flight. Likewise, critical single engine speed seems to be defined for "full power" (my inaccurate term here, I'm sure it's more accurately defined in the actual FAA document , so pulling back power might be a good move in real life, but not acceptable for determining the critical single-engine speed according to the definition.

Regards,

Henning (HoHun)

HoHun said:

However, if the "critical engine" is just a concept to determine whether the procedures have to differ depending on which engine is lost, this might make the P-38 an aircraft with "no" critical engines, even though the impact of propeller rotation is "bad" regardless of which engine is lost.

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My point

HoHun said:

I believe the definition for critical single engine speed for example makes some assumptions like a bank limited to 5 degrees with one engine out, while the procedure for the WW2 Tupolew Tu-2 required some 20 degrees of bank in single engine flight. Likewise, critical single engine speed seems to be defined for "full power" (my inaccurate term here, I'm sure it's more accurately defined in the actual FAA document , so pulling back power might be a good move in real life, but not acceptable for determining the critical single-engine speed according to the definition.

Regards,

Henning (HoHun)

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Actually pulling power back on the good engine for most twins is very much the exception than the rule. IMO the FAA considers most "normal" GA twins for its definitions

Graeme said:

Found this interesting schematic showing the contraprop workings of the Koolhoven FK 55 in a 1944 Dutch aircraft encyclopedia. Maybe someone can translate some of the labelling.

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I can guess

Dristributie-Kast – Countra - rotation gear box

Snelvuur-Canon - Cannon

Lange Drijfas – Drive shaft

Tunnel – Tunnel

860 P.K. Lorraine Petrel – Engine type

aandrijf achterste propeller – rear propeller gear set (thanks to babelfish)

aandrijf voorste propeller – front propeller gear set (thanks to babelfish)

I seriously doubt that countra (counter) rotating props were ever implemented to counter torque, not worth the complexity and weight. Improve efficiency, definitely. Reduce propeller diameter, most likely not, less risky to build longer landing gear.

In the case of counter rotating (handed) engines of a twin engined aircraft, this was usually done to counteract torque. No complexity or weight was added in the P-38's case, though it meant the props were not interchangable. (and the engines weren't in the general sense)

In most engines different reduction gearing was used for opposite rotation (done with several german engine types for multi-engine aircraft), this usually adds a very small amount of weight (an extra gear added to reverse rotation) and meant engines were not interchangable. (without replacing the gearbox)

In the case of the Allison V-1710, the design allowed the rotation to be revered by installing the crankshaft backwards, with the reduction gears being the same but the crankshaft turning in the opposite direction. This also meant that an engine could technically be rebuilt in the field (with the crankshaft layed end for end) to rotate in the opposite direction if there was a shortage of replacements of the required type, but I don't know if this was ever done in practice.


And in the case of contra-rotating props (2 on the same axis), this was often done to eliminate torque on single engine aircraft wich would normally experience a massive amount of torque. (the ellimination of torque would its self reduce drag as there was no need to trim the rudded and add drag)

This was a significant problem on the Griffon engined Spitfire with the massive torque for a relatively small aircraft, the addition of contra rotating props significantly improved stability and ease of control, particularly on the ground.

This was also the case with the P-72 with the massive R-4360.

The elimination of torque also reduces the necessary size of the vertical stabilizer (fin) for adequate lateral stability.


The improved efficiency (in terms of actual thrust) shouldn't be that much more than a similar prop configuration with a many bladed single propeller. (like the 5-bladed version on late spitfires) A wider diameter propeller also acomplishes this, but the limiting rpm for the prop is less due to the higher resulting tip speeds. Another posibility is a larger chord propeller instead od a many-bladed one. (hence why the later Fw 190's used a 3-bladed paddle prop instead of a 4-bladed propeller)



On a different note, I believe the P-38 switched from the more stable inward rotating (at top) prop configuration due to problems with the resulting airflow over the center section.

It would be worth rememebring the case of Macchi Mc 72, the plane designed for the schneider trophy that was unable to participate to an official edition, but later, got the speed record for its category (still unmbeaten). I know it's a "particular case" but it could be interesting anyway from the technical viewpoint.
MC 72 had a couple of counter rotating two blade propeller for the following reasons:

a)to counteract propeller torque (big issue for that category of planes);
b)to give to "the propeller set" a good efficiency during the take off phase: since propellers had no variable pitch mechanism, they were designed for giving their best at high speed. Thus, in the low speed range, their performance were poor. In the MC72 arrangment, however, at zero or low speed, the first propeller provided an air flow to the second prop. This gave the second prop the possibility to work in far better conditions.

The counter rotating solution adopted in Macchi MC 72 was also a "simple" and logical consequence of the way the engine was designed. In fact the Fiat AS 6 engine was obtained basically by placing two engines in line.

All other contra-rotating propeller engines are sissies compared to this one:



tom

davparlr said:

I seriously doubt that countra (counter) rotating props were ever implemented to counter torque, not worth the complexity and weight. Improve efficiency, definitely. Reduce propeller diameter, most likely not, less risky to build longer landing gear.

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Tell that to the jet pilots that flew Mustangs in the Korean war. More than a few flipped over on take-off because they were unused to the torque from the prop engine. Contra-rotating props would have made a big difference.

No, I am not saying the Mustang should have had contra-props.....

tom

machine shop tom said:

Tell that to the jet pilots that flew Mustangs in the Korean war. More than a few flipped over on take-off because they were unused to the torque from the prop engine. Contra-rotating props would have made a big difference.

No, I am not saying the Mustang should have had contra-props.....

tom

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Most of the "jet pilots" who wound up in Mustangs during the Korean War would of trained in T-6s, including any "new-bees." Many of the "vets" would of had plenty of Mustang or T-bolt time before going into jets and reverting back to recips and I think those instances were far and few. I don't think Mustang P factor (or loosing control of the aircraft due to poor rudder coordination) was ever or could ever be documented as an attrition problem during the Korean War, especially with the P-51.

While I can agree with your theory, it still goes counter to FAA definitions and their direction on the subject.

The combination of adverse yaw caused by P-factor and airspeed, or lack of airflow over the rudder(s) is what determines critical engine and the proper reactions to it. Just because both engines will require the same reactions, doesn't mean they both can't be considered "critical" when they might fail.

Please don't take this the wrong way, but the governing body over this definition is the FAA, not you....

Bruce R. said:

Please don't take this the wrong way, but the governing body over this definition is the FAA, not you....

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I understand what you're saying but the FAA definition is based on FAR part 1 and 23 considerations. I don't think any "Normal, Utility, Acrobatic and Commuter Aircraft" configured like the P-38 would have the same performance under one engine as the P-38 and when Lockheed (and or the USAAF) made the claim that the P-38 doesn't have a critical engine, the definition wasn't even defined, as a matter of fact I doubt the old CARs even defined Critical Engine. Additionally, in today's world the definition of a critical engine would also technically only apply to an aircraft with a type certificate, again something that a P-38 can never achieve in it's production configuration.

Again, I think we're going to agree to disagree, like calling the glass half full or half empty.

FLYBOYJ said:

I understand what you're saying but the FAA definition is based on FAR part 1 and 23 considerations. I don't think any "Normal, Utility, Acrobatic and Commuter Aircraft" configured like the P-38 would have the same performance under one engine as the P-38 and when Lockheed (and or the USAAF) made the claim that the P-38 doesn't have a critical engine, the definition wasn't even defined, as a matter of fact I doubt the old CARs even defined Critical Engine. Additionally, in today's world the definition of a critical engine would also technically only apply to an aircraft with a type certificate, again something that a P-38 can never achieve in it's production configuration.

Again, I think we're going to agree to disagree, like calling the glass half full or half empty.

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It would appear that you are allowing the performance category of this particular aircraft weigh in the definition of what is critical and what isn't, and that just isn't the case.
I think we both know that the P-38 would never be able to get a type certificate because of its poor single engine handling characteristics, and this may be another reason why the FAA calls both engines critical.

This exact subject came up at an Instructors Seminar, and I spoke about it with one of the VERY few FAA licensed pilots that had an unrestricted license, (He could fly anything that was airworthy, from a J3 Cub on up.) his opinion was inline with mine (or more correctly mine was in line with his).

I believe further discussion is pointless, as I can see that I am no more likely to change your opinion than you are to change mine…

Bruce R. said:

It would appear that you are allowing the performance category of this particular aircraft weigh in the definition of what is critical and what isn't, and that just isn't the case.
I think we both know that the P-38 would never be able to get a type certificate because of its poor single engine handling characteristics, and this may be another reason why the FAA calls both engines critical.

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There are other configuration issues in the cockpit that would prevent a P-38 TCDS. As far as poor single engine performance - that was a matter of opinion. I knew Tony LeVier (The same guy who did low level aerobatics over bases with one engine out) very well and in his opinion the P-38 had excellent engine out performance if you knew what you were doing.

Bruce R. said:

This exact subject came up at an Instructors Seminar, and I spoke about it with one of the VERY few FAA licensed pilots that had an unrestricted license, (He could fly anything that was airworthy, from a J3 Cub on up.) his opinion was inline with mine (or more correctly mine was in line with his).

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I had this discussion with a former FAA examiner in my area. He flew B-25s but did have P-38 time. He felt that this was also conflicting but in his recollection "critical engine" wasn't really thought about in the same nature as we do today, especially in the civilian world. He went on to say that he felt the term "critical engine" was avoided with regards to the P-38 because it would just add more complexity to an already complex aircraft (for it's day). Remember, at the start of WW2 there was little or no comprehensive twin engine training. My former neighbor flew P-38s and P-51s in the ETO and he said if it wasn't for the fact that he had a few hundred hours in a B-25 prior to transitioning to the P-38, he probably would have killed himself.

Bruce R. said:

I believe further discussion is pointless, as I can see that I am no more likely to change your opinion than you are to change mine…

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Agree...

Hi Flyboyj,

>the FAA definition is based on FAR part 1 and 23 considerations

Do you happen to have the text of the actual definition, or a link where it can be found? I already tried searching the FAA site, but not being familiar with their documentation structure, I ended empty-handed.

Thanks in advance!

Regards,

Henning (HoHun)

HoHun said:

Hi Flyboyj,

>the FAA definition is based on FAR part 1 and 23 considerations

Do you happen to have the text of the actual definition, or a link where it can be found? I already tried searching the FAA site, but not being familiar with their documentation structure, I ended empty-handed.

Thanks in advance!

Regards,

Henning (HoHun)

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"Critical engine means the engine whose failure would most adversely affect the performance or handling qualities of an aircraft."

Electronic Code of Federal Regulations:
It's under FAR Part 1

Hi Flyboyj,

>"Critical engine means the engine whose failure would most adversely affect the performance or handling qualities of an aircraft."

Ah, thanks a lot! I had seen this definition before in this thread, but hadn't realized it was actually the verbatim FAA definition I was looking for.

Armed with your link, I also managed to find the requirements for the Vmc demonstration (there are several, this is for the "normal" category):

Electronic Code of Federal Regulations:

It includes the 5 degree bank limitation as well as the requirement to maintain maximum take-off power on the operating engines.

The Pilot's Flight Operating Instructions for the P-38D through G Series warn to reduce power on the operating engine if the speed is below 120 mph, and to reduce power on the operating engine to "normal rated power as soon as practicable". The use of bank is not mentioned.

Regards,

Henning (HoHun)

HoHun said:

Hi Flyboyj,

>"Critical engine means the engine whose failure would most adversely affect the performance or handling qualities of an aircraft."

Ah, thanks a lot! I had seen this definition before in this thread, but hadn't realized it was actually the verbatim FAA definition I was looking for.

Armed with your link, I also managed to find the requirements for the Vmc demonstration (there are several, this is for the "normal" category):

Electronic Code of Federal Regulations:

It includes the 5 degree bank limitation as well as the requirement to maintain maximum take-off power on the operating engines.

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Very good homework there Henning!

HoHun said:

The Pilot's Flight Operating Instructions for the P-38D through G Series warn to reduce power on the operating engine if the speed is below 120 mph, and to reduce power on the operating engine to "normal rated power as soon as practicable". The use of bank is not mentioned.

Regards,

Henning (HoHun)

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And you hit on the point I've been trying to make - most aircraft engine out procedures incorporate what is mentioned in the FAR as norm - the P-38's engine out procedures did not incorporate any bank and actually reduced power on the good engine, something that is probably outside of the norm for 99.9% of the aircraft certified under Part 23. With a degradation of performance the same on either side when an engine is inoperative, I can't see how the FAR 1 definition would apply...

"Critical engine means the engine whose failure would most adversely affect the performance or handling qualities of an aircraft."

Again if the failure of either engine give you the same end result, how can you have a "critical engine?" Or as stated by Bruce, both engines are critical, but then again you don't have one that brings greater adversity to the situation!

I think the problem we have here is the fact that there are no other aircraft (that I am aware of) that have two outward turning engines as the P-38 does.
In most aircraft the engines all turn either clockwise or counter-clockwise; some aircraft have counter-rotating engines that make things easier for the pilot to handle in the case of an engine failure (inward turning). The "critical engine" definition becomes quite easy.

The P-38 is unique (I believe) in that in the case of failure with either engine you have a serious control issue, particularly at lower airspeeds. The fact that the control problems are similar no matter which fails doesn't change that fact. The reduction in power tends to reinforce the point that there isn't enough stabilizer or rudder to overcome the asymmetric thrust the engines can develop. That alone could make either engine critical.

You have mentioned specific wording in the FAR's as relevant, I would suggest that hanging your hat on that particular hook doesn't cut it because in regulations as well as any other documents every specific instance or aircraft will not be called out, you have to remember you own words, " The P-38 was not built under part 23.".