# Contra rotating Propellers



## Seawitch (Jul 21, 2007)

When I first noticed these on aircraft like the later Seafires I didn't think thats what they were, I rather thought that it was a way of mounting that many blades without weakening the mount/shaft.
Now i know different and wonder was this very complicated to do, why do it?
Did this produce more power? Neutralise Torque?
Over to the experts!


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## mkloby (Jul 21, 2007)

I would imagine that it would produce more thrust than a single prop. However, it will add more weight, and no doubt complicate design and maintenance. It should also serve to aid in offseting torque-effect, p-factor, and gyro precession.


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## ToughOmbre (Jul 21, 2007)

Technical question...

Contra rotating...that the same as counter rotating? I've seen the terms used inter-changeably. When talking about the P-38 I say counter.


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## k9kiwi (Jul 21, 2007)

Contra rotating is what the US Government did around a South American guerilla force.  

They are interchangeable expressions, however the correct usage is Contra Rotating.


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## Graeme (Jul 21, 2007)

Certainly reduced torque. Many of the powerful single prop Schneider Trophy racers had great difficulty taking off due to engine torque. The Macchi-Castoldi MC.72 solved its torque problem with the counter-rotating propeller.


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## pasoleati (Aug 5, 2007)

They are not interchangeable terms!!! Contrarotating means a propeller that has two props rotating in different directions on the same axis while counterrotating described two separate props like the P-38 case.

Their primary benefit is elimination of asymmetric torque. Read the Sptfire 21 handling reports with both prop types. The contrarotating prop made flight possible without even touching the rudder.

Had the F4U been fitted with one, accident rates would have plummeted.


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## Graeme (Aug 5, 2007)

pasoleati said:


> while counterrotating described two separate props like the P-38 case.



Maybe..But not according to Derek James-The Schneider Trophy Aircraft.


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## 130fe (Aug 6, 2007)

Mr James misspoke, pasoleati did not.


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## Graeme (Aug 6, 2007)

130fe said:


> Mr James misspoke,



Disappointing, coming from the 'esteemed' Putnam range. Oddly, he corrects himself in the captioning.


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## pasoleati (Aug 6, 2007)

The topic was discussed in Torque Meter, the journal of Aircraft Engine Historical Society and indeed the terms are defined as I previously wrote.


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## HoHun (Aug 6, 2007)

Hi Pasoleati,

>The topic was discussed in Torque Meter, the journal of Aircraft Engine Historical Society and indeed the terms are defined as I previously wrote.

Years ago, I read in the context of a Fairey Gannet discussion that "contra-rotating" not only included co-axial propellers, but actually one common drive train for both.

I'm not sure about the Macchi, but it might be that each of the two coupled engines has its own independend drive train. If we follow the above definition, it would have to be called "counter" rather than "contra rotating".

(From an linguistic point of view, I disapprove of the attempt to make a distinction by using two different variants of a synonym. We don't have a thread "The Greatest Etymological Blunders of WW2", but if we had, I'd nominate this one 

Regards,

Henning (HoHun)


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## SoD Stitch (Aug 6, 2007)

My understanding as to the main reason for contra-rotating props is that they are more efficient; the self-cancelling torque feature of contra-rotating props is a side-benefit, not the main benefit. Contra-rotating props eliminate the rotational airflow around the airframe which causes energy loss, making the propellers more efficient.

Quote:

"When airspeed is low the mass of the air flowing through the propeller disk (thrust) causes a significant amount of tangential or rotational air flow to be created by the spinning blades. The energy of this tangential air flow is wasted in a single propeller design. To use this wasted effort the placement of a second propeller behind the first takes advantage of the disturbed airflow.

If it is well designed, a contra-rotating propeller will have no rotational air flow, pushing a maximum amount of air uniformly through the propeller disk, resulting in high performance and low induced energy loss. It also serves to counter the asymmetrical torque effect of a conventional propeller. Some contra-rotating systems were designed to be used at take off for maximum power and efficiency, and allowing one of the propellers to be disabled during cruise to extend flight time.

The efficiency of a contra-rotating prop is somewhat offset by its mechanical complexity. Nonetheless, coaxial contra-rotating propellers and rotors are moderately common in military aircraft and naval applications, such as torpedoes, where the added maintenance is not a concern to government budgets."

From: contra-rotating propellers: Information from Answers.com


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## ccheese (Aug 6, 2007)

Lemme see..... If I were sitting in a P-38, the left prop spinning counter-
clockwise and the right prop spinning clockwise, I would be witnessing
counter-rotating props. Now.... the Seafire (and there was a Reno
racing P-51 so configured) has two props on two shafts (one within the 
other) turning in opposite directions. Aha .....Contra-rotating.....

And of course, you have the Dornier 335.....push-pull.

Don't you just love it when a plan comes together ??

Charles


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## FLYBOYJ (Aug 6, 2007)

And with the P-38s propeller rotation, this eliminated a "critical engine."

Wikipedia did good on this one...

Critical engine - Wikipedia, the free encyclopedia


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## Graeme (Aug 6, 2007)

HoHun said:


> (From an linguistic point of view, I disapprove of the attempt to make a distinction by using two different variants of a synonym. We don't have a thread "The Greatest Etymological Blunders of WW2", but if we had, I'd nominate this one
> Regards,
> 
> Henning (HoHun)



Very true!

And you're right about the Macchi. A unique engine installation that has 'fused' the definitions.


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## ccheese (Aug 7, 2007)

Joe: That's a very good explanation of the "critical engine" I had heard
the term before but Wikipedia explains it so even I can understand it.

Is that why they say, "never turn into a dead engine" ?

Charles


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## Parmigiano (Aug 7, 2007)

Counter (or contra) rotating prop on the same axis, either driven by 1 or 2 engines, also have the advantage of allowing shorter blades and so to reduce the tip speed (vs a single prop configuration designed to handle the same power)


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## FLYBOYJ (Aug 7, 2007)

ccheese said:


> Joe: That's a very good explanation of the "critical engine" I had heard
> the term before but Wikipedia explains it so even I can understand it.
> 
> Is that why they say, "never turn into a dead engine" ?
> ...



Yep - and as in WW2, today the biggest killer of (GA) pilots is lack of proficiency when performing emergency engine out procedures on twin engine aircraft - I've ranted about this on earlier threads.


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## mkloby (Aug 7, 2007)

FLYBOYJ said:


> And with the P-38s propeller rotation, this eliminated a "critical engine."
> 
> Wikipedia did good on this one...
> 
> Critical engine - Wikipedia, the free encyclopedia



Losing your critical engine sucks. I would always hate it when they failed my left engine (critical) in the C-12B... standing on the damn rudder gets old REAL quick.


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## chuter (Aug 22, 2007)

FLYBOYJ said:


> And with the P-38s propeller rotation, this eliminated a "critical engine."
> 
> Wikipedia did good on this one...
> 
> Critical engine - Wikipedia, the free encyclopedia



Well, the P38 didn't have a critical engine because it didn't have a better performing engine (inboard turning) of the two. Pfactor is the result of an an effective AOA change of a tilted prop disc. Tip a prop disc back . . . say, ten degrees . . . and maintain its horizontal direction and the down going blade has a higher angle of attack (pitch) than the up going blade. This has the effect of moving the thrust line from the prop axis horizontally towards the down going blade. Whether this is towards or away from aircraft centerline determines how much rudder (drag) is required to maintain neutral yaw. With both engines running the inboard wing sections would have a higher effective AOA than the outboard sections (spiral slipstream affect) which would help stalls start inboard where they should. With the surplus of power the P38 had as well as reasonable yaw control this was seen as a good compromise. The P82 adopted the same approach initially but stall performance was so benign North American switched the engines so both engines gave superior single engine performance.


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## FLYBOYJ (Aug 22, 2007)

chuter said:


> Well, the P38 didn't have a critical engine because it didn't have a better performing engine (inboard turning) of the two. Pfactor is the result of an an effective AOA change of a tilted prop disc. Tip a prop disc back . . . say, ten degrees . . . and maintain its horizontal direction and the down going blade has a higher angle of attack (pitch) than the up going blade. This has the effect of moving the thrust line from the prop axis horizontally towards the down going blade. Whether this is towards or away from aircraft centerline determines how much rudder (drag) is required to maintain neutral yaw. With both engines running the inboard wing sections would have a higher effective AOA than the outboard sections (spiral slipstream affect) which would help stalls start inboard where they should. With the surplus of power the P38 had as well as reasonable yaw control this was seen as a good compromise. The P82 adopted the same approach initially but stall performance was so benign North American switched the engines so both engines gave superior single engine performance.



OK...........

As stated, the P-38 did not have a critical engine.............


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## chuter (Aug 22, 2007)

. . . uhhh . . . wow . . . it was the middle of the night. 


-- I was near the end of my work shift (A320 C-checks). --


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## FLYBOYJ (Aug 22, 2007)

chuter said:


> . . . uhhh . . . wow . . . it was the middle of the night.
> 
> 
> -- I was near the end of my work shift (A320 C-checks). --



been there, done that - 737s, DC-9s - where do you work?


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## chuter (Aug 24, 2007)

SFO

I work at a "Major Carrier's" maintainance base on the field (the only one). Continuing rumors (rumors are now hitting the press) we are to be sold . . . happy days :/ . . . are taking their toll here. On the flip side I start a one week vacation today. WOOHOO.


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## FLYBOYJ (Aug 24, 2007)

chuter said:


> SFO
> 
> I work at a "Major Carrier's" maintainance base on the field (the only one). Continuing rumors (rumors are now hitting the press) we are to be sold . . . happy days :/ . . . are taking their toll here. On the flip side I start a one week vacation today. WOOHOO.


I did heavy iron maintenance for a while - I hated it. SFO - probably United? My father in law retired last year from United...

Have a great vacation....


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## SoD Stitch (Aug 24, 2007)

FLYBOYJ said:


> I did heavy iron maintenance for a while - I hated it. SFO - probably United? My father in law retired last year from United...
> 
> Have a great vacation....



Small world . . . my father-in-law also retired from United, but about 5 years ago. Got out just before they declared Chapter 11 . . . used to fly 747-400's international (mostly Hong Kong, Beijing and Okinawa). My brother used to work at the MOC in Oakland, and my wife used to work at Central Res in SF. Weird . . .


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## Graeme (Aug 25, 2007)

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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## FLYBOYJ (Aug 26, 2007)

SoD Stitch said:


> Small world . . . my father-in-law also retired from United, but about 5 years ago. Got out just before they declared Chapter 11 . . . used to fly 747-400's international (mostly Hong Kong, Beijing and Okinawa). My brother used to work at the MOC in Oakland, and my wife used to work at Central Res in SF. Weird . . .


Wow! pretty weird! Are you guys on the "buddy pass" gravy train as my father in law likes to call it? I use his passes more than any one in the family - he doesn't care, as long as they all get used up before the end of the year.


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## Seawitch (Aug 31, 2007)

Hi Graeme
Thanks for posting that, I never heard of the Aircraft itself, but it's interesting that the engine is also behind the pilot in a layout like that......tough on him if he has to go through anything though.
I heard it said that with a propeller plane the engine makes a hole and you follow through, with a Jet YOU make the hole, the engine follows you!


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## FLYBOYJ (Aug 31, 2007)

Seawitch said:


> Hi Graeme
> Thanks for posting that, I never heard of the Aircraft itself, but it's interesting that the engine is also behind the pilot in a layout like that......tough on him if he has to go through anything though.
> I heard it said that with a propeller plane the engine makes a hole and you follow through, with a Jet YOU make the hole, the engine follows you!


And with a turbo-prop?!?!?


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## ccheese (Aug 31, 2007)

Interesting.... The engine behind the pilot... ala Bell P-39. And..... prop
shaft between the legs..... scary !

Charles


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## Graeme (Sep 1, 2007)

Two separate engines with props swinging in the opposite directions..technically, is the Do 335 an example of 'counter rotating propellers'?


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## Downwind.Maddl-Land (Sep 2, 2007)

The P-38 was not the only twin-engined fighter to have counter-rotating props of course, the DH Hornet (aaaaah – de Havilland!) also used the feature to obviate torque effect and reduce drag (no need to put an offset in the fin). However it complicated logistics, as one needed 2 types of engines for a single aircraft type, in this case Melin 130/131s.

The Griffons in the Avro Shackleton were fitted with contra-rotating propellers that suffered from frequent failures of the gearbox translation units. This is not a highly desirous state of affairs when 8 hrs out over the North Atlantic. At one point the Shacklebomber was, without peer, the world’s best 3-engined maritime patrol aircraft.


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## Bruce R. (Nov 6, 2008)

FLYBOYJ said:


> And with the P-38s propeller rotation, this eliminated a "critical engine."
> 
> Wikipedia did good on this one...
> 
> Critical engine - Wikipedia, the free encyclopedia



I hate to be a newbie and for my first post disagree with an "old timer", but I guess I'll have to.

The P-38 was a unique plane, it didn't have one critical engine, it had *TWO *critical engines. 

I didn't read the Wiki post, but I know a bit about this. The idea is that the down turning blade of a prop has the aircrafts AOA added to its pitch angle, while the up turning blade has that AOA angle subtracted; therefore thrust is asymmetrical on the prop disc. If the higher thrust is outboard of the nacelle the thrust arm can overload the effect of the rudder and cause loss of control at lower speeds. (I hope that makes some sort of sense.)

In the case of the _EARLY_ P-38’s both engines turned inboard (at the top of the prop disc), this limited the asymmetrical thrust from the centerline of drag (the fuselage), and this is typical of counter-rotating props on most aircraft. 
Problems with elevator and horizontal stabilizer flutter caused by this set up caused Lockheed engineers to reverse the rotation of both engines and if you look carefully at the aircraft, you will notice that the tops of both props rotate outboard. This would cause BOTH engines to be considered to be critical by any normal standards.

Since I wrote this post, I have read the Wiki article, and it in fact backs up my contention............


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## FLYBOYJ (Nov 6, 2008)

Bruce R. said:


> I hate to be a newbie and for my first post disagree with an "old timer", but I guess I'll have to.
> 
> The P-38 was a unique plane, it didn't have one critical engine, it had *TWO *critical engines.
> 
> ...



Read the article and understand why an engine is labeled "critical." I don't know if you fly or have flown twins. The critical engine of a twin is the one whose failure would effect performace the most. In the P-38's case a left engine failure was the same as a right engine failure, the aircraft performed the same regardless of which engine was inoperate, therefore no critical engine.


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## Bruce R. (Nov 6, 2008)

FLYBOYJ said:


> Read the article and understand why an engine is labeled "critical." I don't know if you fly or have flown twins. The critical engine of a twin is the one whose failure would effect performace the most. In the P-38's case a left engine failure was the same as a right engine failure, the aircraft performed the same regardless of which engine was inoperate, therefore no critical engine.



I'm a multi-engine rated instructor pilot, if that means anything, and to say that just because both engines effect the aircraft the same way, therefore neither is critical just doesn't make sense to me, sorry. 

In the case of the P-38, a failure of either engine at less than Vmc is a serious condition, and by any standards must be considered "critical".
As I mentioned at the very end of my post, I finally did read the article, the last paragraph is below. Please note the last sentence.

"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, 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."

The very pronounced yaw caused by the offset thrust of outward turning props is VERY difficult to control unless you have sufficient airspeed. That is the primary definition of "critical engine".
The secondary definition is if one engine has equipment required for flight, equipment such as generators, hydraulic pumps, or vacuum pumps.
Check out the FAA definitions, see if I’m right………….


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## SoD Stitch (Nov 6, 2008)

Graeme said:


> Two separate engines with props swinging in the opposite directions..technically, is the Do 335 an example of 'counter rotating propellers'?



I would have to say "yes", since they are turning in the opposite direction to each other. 

The other advantage the Do 335 layout had was that the engines were identical, in that they both turned in the same direction (unlike the "left-handed" and "right-handed" engines of the P-38 ), but since one engine was pointing in the other direction, it's rotation was opposite in relation to the other engine; this lessened the logistical problem of having two different engines for the same a/c, especially critical for Germany in 1944-45.


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## FLYBOYJ (Nov 6, 2008)

Bruce R. said:


> I'm a multi-engine rated instructor pilot, if that means anything, and to say that just because both engines effect the aircraft the same way, therefore neither is critical just doesn't make sense to me, sorry.


Well that's the way Lockheed and the USAAF advertised it as the engine out procedures were the same regardless of the engine that failed, simply put you did the same thing regardless of side.


Bruce R. said:


> In the case of the P-38, a failure of either engine at less than Vmc is a serious condition, and by any standards must be considered "critical".


Agree but again in the case of a P-38 you’re getting the same result regardless of the side that failed.


Bruce R. said:


> As I mentioned at the very end of my post, I finally did read the article, the last paragraph is below. Please note the last sentence.
> "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, 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."


What is not mentioned is the P-38 was actually "overpowered" in an engine out situation (if there could actually be such a thing) where the good engine had to be throttled back to prevent yaw, this was especially critical during takeoff with the gear down.


Bruce R. said:


> The very pronounced yaw caused by the offset thrust of outward turning props is VERY difficult to control unless you have sufficient airspeed. That is the primary definition of "critical engine".


See above - if the left or right engine failed, you were powering back the good engine to offset the yaw and one side didn't behave differently when compared with the other, hence the reason for saying the P-38 did not have a critical engine. Powering back on the good engine is everything contrary to what we’ve learned in flying twin GA aircraft, but it was procedure on the P-38.


Bruce R. said:


> The secondary definition is if one engine has equipment required for flight, equipment such as generators, hydraulic pumps, or vacuum pumps.
> Check out the FAA definitions, see if I’m right………….


You're right and again the only difference I know of in early P-38s was the left engine carried a heater shroud over the exhaust ducting, I think there were other accessories that were only on the left engine in early models, but I think later J models everything found on the left engine was also on the right – but then again the P-38 wasn’t built under FAR 23 either


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## Bruce R. (Nov 6, 2008)

FLYBOYJ said:


> Well that's the way Lockheed and the USAAF advertised it as the engine out procedures were the same regardless of the engine that failed, simply put you did the same thing regardless of side.
> Agree but again in the case of a P-38 you’re getting the same result regardless of the side that failed.



That doesn't matter in the strict definition of the words “critical engine”.



FLYBOYJ said:


> What is not mentioned is the P-38 was actually "overpowered" in an engine out situation (if there could actually be such a thing) where the good engine had to be throttled back to prevent yaw, this was especially critical during takeoff with the gear down.



The fact that the aircraft was "overpowered" just makes the situation worse. The stronger the asymmetric thrust, the quicker the plane will roll on climb out, or, if still on the ground, spin off into the boondocks and end up in a ball. 
The reason aircraft like the Duchess are so easy to handle, particularly for low time pilots is the fact that they don’t have a critical engine. The thrust is close enough to the center of drag that there is relatively little yaw induced even at low airspeeds and high power settings.

As to what the USAAF and Lockheed advertised, well you couldn’t expect that they would say : “This plane is a Lieutenant Killer so you better be paying attention to what you’re doing.” 
They also said the M-4 Sherman was the best tank on the battlefield, when they knew the Tiger was out there.



FLYBOYJ said:


> See above - if the left or right engine failed, you were powering back the good engine to offset the yaw and one side didn't behave differently when compared with the other, hence the reason for saying the P-38 did not have a critical engine. Powering back on the good engine is everything contrary to what we’ve learned in flying twin GA aircraft, but it was procedure on the P-38.



I see and understand your reasoning, but that is not the way either the FAA or your own Wiki definition sees it.



FLYBOYJ said:


> You're right and again the only difference I know of in early P-38s was the left engine carried a heater shroud over the exhaust ducting, I think there were other accessories that were only on the left engine in early models, but I think later J models everything found on the left engine was also on the right – but then again the P-38 wasn’t built under FAR 23 either



Please understand that I am not here to start a fight, but when I found the site, and read this thread, I couldn't let it alone. Words mean things, and to say that neither engine on this plane is critical, just isn't right.


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## FLYBOYJ (Nov 6, 2008)

Bruce R. said:


> The fact that the aircraft was "overpowered" just makes the situation worse. The stronger the asymmetric thrust, the quicker the plane will roll on climb out, or, if still on the ground, spin off into the boondocks and end up in a ball.
> The reason aircraft like the Duchess are so easy to handle, particularly for low time pilots is the fact that they don’t have a critical engine. The thrust is close enough to the center of drag that there is relatively little yaw induced even at low airspeeds and high power settings.
> 
> As to what the USAAF and Lockheed advertised, well you couldn’t expect that they would say : “This plane is a Lieutenant Killer so you better be paying attention to what you’re doing.”
> They also said the M-4 Sherman was the best tank on the battlefield, when they knew the Tiger was out there.


I hear ya on all accounts but then again a dutchess to a P-38, two different animals....



Bruce R. said:


> I see and understand your reasoning, but that is not the way either the FAA or your own Wiki definition sees it.


If you consider the definition of the FAA with regards to accessories on the engine, yes, but in terms of performance on one engine...

BTW this is out of Wiki...

"The critical engine of a multi-engine, fixed-wing aircraft is the one whose failure would result in *the most adverse *effects on the aircraft's handling and performance."

Note "the most adverse." I think the thought process is since the aircraft performed the same on either engine the term critical engine went out the window, but I think it would be "adverse" to loose any engine on a twin regardless on how it will perform on one engine.



Bruce R. said:


> Please understand that I am not here to start a fight, but when I found the site, and read this thread, I couldn't let it alone. Words mean things, and to say that neither engine on this plane is critical, just isn't right.


No fighting there Bruce, just a discussion on a differance of oppinion.


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## Bruce R. (Nov 6, 2008)

"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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## FLYBOYJ (Nov 6, 2008)

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.


 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_


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## HoHun (Nov 6, 2008)

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)


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## FLYBOYJ (Nov 6, 2008)

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.


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)


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


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## davparlr (Nov 15, 2008)

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.


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)


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## davparlr (Nov 15, 2008)

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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## kool kitty89 (Nov 16, 2008)

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.


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## bruno_ (Nov 16, 2008)

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.


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## machine shop tom (Nov 16, 2008)

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



tom


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## machine shop tom (Nov 16, 2008)

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.



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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## FLYBOYJ (Nov 16, 2008)

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



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.


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## Bruce R. (Nov 17, 2008)

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....


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## FLYBOYJ (Nov 17, 2008)

Bruce R. said:


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


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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## Bruce R. (Nov 17, 2008)

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.



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…


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## FLYBOYJ (Nov 17, 2008)

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.


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).


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…


Agree...


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## HoHun (Nov 17, 2008)

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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## FLYBOYJ (Nov 17, 2008)

HoHun said:


> Hi Flyboyj,
> 
> >the FAA definition is based on FAR part 1 and 23 considerations
> 
> ...



*"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


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## HoHun (Nov 17, 2008)

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)


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## FLYBOYJ (Nov 17, 2008)

HoHun said:


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


*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)



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!


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## Bruce R. (Nov 19, 2008)

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.”.


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## FLYBOYJ (Nov 19, 2008)

Bruce R. said:


> 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.


Agree to a point but as stated folks like Tony Levier showed that the aircraft indeed could be handled at the most extreme conditions with one engine out if the pilot knew what he was doing.


Bruce R. said:


> 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.”.


That's correct so with that said does the definition of a critical engine in FAR 1 apply? Again I vew it as having the glass "half full or half empty."


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## FLYBOYJ (Nov 19, 2008)

I work at the USAFA and work with the 98th FTS. They operate Twin Otters. Many of them have tons of twin time and some are test pilot school grads. I asked 6 of them "if a twin engine aircraft has the same engine out performance characteristics regardless of the engine you shut down or loose, does the aircraft still have a critical engine?"

3 said yes, 3 said no.

I asked my father in law, a retired USAF LtCol, former test pilot and retired United Airlines Captain. Without pause he said "no."

His explanation was based on a training mentality stating that "the plane will fly just as crummy on the right engine as the left, so why confuse the issue"?

Not my opinions. I'll field more folks for their opinions.


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## ccheese (Nov 19, 2008)

I'm not into the technical stuff like Joe, but I have flown in quite a few twins (B-26, B-25, A-20, R4D. P2V and the TF. Without exception, I've been along when an engine was shut down for whatever reason. I've talked to the pilots, and except for the R4D (C-47) the guys up front had their hands full of airplane. I remember the B-26 jockey saying both the pilot and the co-pilot had their feet on the rudder pedal. It can't be fun !

I also flew in the C-119, and fortunately never lost an engine. I do not think the C-119 will fly on one engine. If it will, I don't want to be on board.

Charles


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## FLYBOYJ (Nov 19, 2008)

ccheese said:


> I also flew in the C-119, and fortunately never lost an engine. I do not think the C-119 will fly on one engine. If it will, I don't want to be on board.
> 
> Charles



A engine out on a C-119 would be a scary thing indeed!

I got more of an explanation out of my father in law - In his opinion when assuming you have a twin with a critical engine, there is a comparison made in performance between the right and left engine. As he simply put it if the aircraft flies the same regardless of which engine is out then how can you have a critical engine with no performance comparison?


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## Bruce R. (Nov 20, 2008)

FLYBOYJ said:


> Agree to a point but as stated folks like Tony Levier showed that the aircraft indeed could be handled at the most extreme conditions with one engine out if the pilot knew what he was doing.



Test pilots like Tony Levier, Chuck Yeager, Boone Guyton, and Scott Crossfield, to name just a few are and were exceptional people with exceptional skills, the fact that they could handle a plane with "different" characteristics isn't surprising, nor should it be. I knew Scott Crossfield, and I saw what Bob Hoover could do with an F-100 when Air Force pilots were running off the runways at Clark AFB with burned up brakes and drag chutes deployed early. Guys with those skills are not close to average pilots by any means.



FLYBOYJ said:


> That's correct so with that said does the definition of a critical engine in FAR 1 apply? Again I view it as having the glass "half full or half empty."



Actually, yes it does. The FAA is the governing body on all things related to aircraft since its inception. They have stayed out of the military arena because the pilot training and aircraft requirements are different from civilian needs. Since there are no longer any P-38’s in military service, and all remaining aircraft are owned by civilians, they (P-38’s) are now under FAA jurisdiction, so Part 23 as well as any and all other regs. also have full weight of law.


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## FLYBOYJ (Nov 20, 2008)

Bruce R. said:


> Test pilots like Tony Levier, Chuck Yeager, Boone Guyton, and Scott Crossfield, to name just a few are and were exceptional people with exceptional skills, the fact that they could handle a plane with "different" characteristics isn't surprising, nor should it be. I knew Scott Crossfield, and I saw what Bob Hoover could do with an F-100 when Air Force pilots were running off the runways at Clark AFB with burned up brakes and drag chutes deployed early. Guys with those skills are not close to average pilots by any means.


Agree - but at the same time you had your Bongs, McGuires, Lynchs' and Johnsons' who were able to do the same things with the p-38, but agree, those type of P-38 pilots were the exception rather than the rule



Bruce R. said:


> Actually, yes it does. The FAA is the governing body on all things related to aircraft since its inception. They have stayed out of the military arena because the pilot training and aircraft requirements are different from civilian needs. Since there are no longer any P-38’s in military service, and all remaining aircraft are owned by civilians, they (P-38’s) are now under FAA jurisdiction, so Part 23 as well as any and all other regs. also have full weight of law.


FAR 23 would apply to a type certificated aircraft as it would be deemed "airworthy" after certification and required inspections, the first line of FAR 23 specifies applicability - *"(a) This part prescribes airworthiness standards for the issue of type certificates, and changes to those certificates, for airplanes in the normal, utility, acrobatic, and commuter categories."* 

Former military aircraft like the P-38 that doesn't have a TCDS would be granted an "experimental" air worthiness certificate (you probably know all this) under FAR 21.191, 193 and 195. An aircraft lacking a TCDS and given an "experimental" airworthiness certificate is allowed to fly based on the issuance of a program letter (where its specified under what type of experimental catagory the aircraft will fly in, Exhibition, R&D, Racing, demostration of compliance, amature, kit built and certain training) and a maintenance program. The FSDO and in some cases MIDO will determine if the aircraft is "safe for flight" and it's operating area based on those documents. Additionally it is within those documents where operational flight parameters are spelled out (usually a standard flight manual developed by the manufacturer and any supplements for additional equipment, ie. radios, GPSs, etc). In the end an "Experimental" aircraft that is a former warbird will never be in compliance with FAR 23 as it was never certificated under FAR 23 requirements, in fact an experimental aircraft of this type could never be signed off as "airworthy" - instead the term "safe for flight or operation" is the nomenclature used to issue airworthiness certificates and to sign off condition inspections. Additionally there are FAR 23 requirements where an aircraft like a P-38 could never meet (placement of instruments, proper markings, landing gear and flap handle configuration and location, etc. just to name a few)


I've applied for and gotten about 20 experimental airworthiness certificates for various warbaird owners that included recips, jets and helicopters. I know this process better than I sometimes wish to!


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## HoHun (Nov 20, 2008)

Hi Flyboyj,

>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!  

From the FAR perspective, I'd say that "critical engine" is just a definition device to arrive at a conservative Vmc figure. If both single-engine flight variants arrive at the same figure, there is no critical engine from that point of view.

However, you could apply FAR criteria twice to a twin and determine "Vml" and "Vmr" (I made these up - minimum single engine speed on the left respectively right engine). For a P-38 with conventional engines (I believe the British export variant didn't have counter-rotating propellers), Vml and Vmr would be different. For the US variants, they would be identical, coinceding with the historical Vmc - and match the higher of the two British figures!

That's why one could say that both of the P-38's engines were "critical" ...it's not true in the sense of the FAA definition, but you really end up with the "worse case" minimum single-engine speed figure no matter which engine fails.

If you think it through, the direction in which the *stopped* engine used to turn determines if the engine that is still turning is considered to be critical or not!  

If a British Lightning would fly alongside a US Lightning, both having their left engine stopped, you'd have two identical aircraft with the right engine turning in the same direction. The British one would be flying on a critical engine and the US one not - but the pilots would find their handling to be completely identical ...

I think it's a healthy reaction to consider this a bit confusing 

Regards,

Henning (HoHun)


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## davparlr (Nov 20, 2008)

FLYBOYJ said:


> I work at the USAFA and work with the 98th FTS. They operate Twin Otters.



Twin Otters don't fly, they just taxi in the air! 

The only twin engine aircraft I have flown were basically centerline thrust, T-37 and T-38, but for non centerline thrust twins, I think procedures would be for worst case and applied to either engine out. Good basic aircraft control should suffice. You don't want average pilots thinking too hard in stress.


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## pbfoot (Nov 20, 2008)

davparlr said:


> Twin Otters don't fly, they just taxi in the air!
> 
> The only twin engine aircraft I have flown were basically centerline thrust, T-37 and T-38, but for non centerline thrust twins, I think procedures would be for worst case and applied to either engine out. Good basic aircraft control should suffice. You don't want average pilots thinking too hard in stress.


They are in production again after 20 years and with possibly the Beaver and Otter to follow on. Even the US Army has ordered them 
Viking Air - 03/18/2008 - Viking Lands Three Orders for New Twin Otter Series 400 Aircraft from U.S. Army


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## FLYBOYJ (Nov 21, 2008)

davparlr said:


> You don't want average pilots thinking too hard in stress.


And I think that's where the thinking was to say that the P-38 had no critical engine.


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## FLYBOYJ (Nov 21, 2008)

HoHun said:


> Hi Flyboyj,
> 
> >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!
> 
> ...



Great perspective, your last comment nails it!


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## mkloby (Nov 21, 2008)

FLYBOYJ said:


> I work at the USAFA and work with the 98th FTS. They operate Twin Otters. Many of them have tons of twin time and some are test pilot school grads. I asked 6 of them "if a twin engine aircraft has the same engine out performance characteristics regardless of the engine you shut down or loose, does the aircraft still have a critical engine?"
> 
> 3 said yes, 3 said no.
> 
> ...



No degredation in handling characteristics w/ either eng out is true, since that is the basis for determining which would be a critical engine. I would answer no also.


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## FLYBOYJ (Nov 21, 2008)

mkloby said:


> No degredation in handling characteristics w/ either eng out is true, since that is the basis for determining which would be a critical engine. I would answer no also.


Thanks for the input - maybe you could field this questions to some of your fellow "devil-dogs."


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## davparlr (Nov 21, 2008)

pbfoot said:


> They are in production again after 20 years and with possibly the Beaver and Otter to follow on. Even the US Army has ordered them
> Viking Air - 03/18/2008 - Viking Lands Three Orders for New Twin Otter Series 400 Aircraft from U.S. Army



Hard to keep a good plane down!


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## Wingscrubber (Jan 14, 2009)

Machineshoptom, what kind of engine is that? Looks like a Tupolev bear engine...


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