Contra rotating Propellers

[FLYBOYJ]

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

 

[chuter]

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


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

 

[FLYBOYJ]

. . . 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?

 

[chuter]

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.

 

[FLYBOYJ]

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....

 

[SoD Stitch]

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 . . .

 

[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.

 

[FLYBOYJ]

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.

 

[Seawitch]

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!

 

[FLYBOYJ]

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?!?!?

 

[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.]

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]

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.]

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]

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]

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.

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.

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.

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.

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.]

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".

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.

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.

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]

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....

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.

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.