Contra rotating Propellers

[ccheese]

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

Charles

[Graeme]

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

[Downwind.Maddl-Land]

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.

[Bruce R.]

Originally Posted by FLYBOYJ

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............

[FLYBOYJ]

Originally Posted by Bruce R.

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............

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.

[Bruce R.]

Originally Posted by FLYBOYJ

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………….

[SoD Stitch]

Originally Posted by Graeme

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.

[FLYBOYJ]

Originally Posted by Bruce R.

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.

Originally Posted by Bruce R.

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.

Originally Posted by Bruce R.

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.

Originally Posted by Bruce R.

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.

Originally Posted by Bruce R.

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

[Bruce R.]

Originally Posted by FLYBOYJ

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”.

Originally Posted by FLYBOYJ

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.

Originally Posted by FLYBOYJ

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.

Originally Posted by FLYBOYJ

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.

[FLYBOYJ]

Originally Posted by Bruce R.

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....

Originally Posted by Bruce R.

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.

Originally Posted by Bruce R.

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.

[Bruce R.]

"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.

[FLYBOYJ]

Originally Posted by Bruce R.

"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

[HoHun]

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)

[FLYBOYJ]

Originally Posted by HoHun

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

Originally Posted by HoHun

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

[davparlr]

Originally Posted by Graeme

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)