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Why make both engines critical ? - 8/27/2008 5:59 AM   
TexasAirBoss



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Status: offline 		I saw this pic and realized both engines were critical. Why do that ?

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RE: Why make both engines critical ? - 8/27/2008 6:32 AM   
TFF


 

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Status: online 		I believe both engines turn the same direction on the turbo commander.

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RE: Why make both engines critical ? - 8/27/2008 7:19 AM   
BMatthews



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Status: offline 		Um.... what do you mean by "critical"?

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RE: Why make both engines critical ? - 8/27/2008 7:41 AM   
TFF


 

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Status: online 		If you loose an engine, the critical one will make the plane yaw more than the non. There are two different engine out maneuvering airspeeds depends on which engine you are flying on. You have to fly faster to keep safe control on the critical one. It is better to have the counter rotating engines, but usually only the supper evil handling planes get them because of cost.

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RE: Why make both engines critical ? - 8/27/2008 7:52 AM   
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Status: offline 		I believe if you look into the history of the P-38, they changed the direction of both engines from the so-call non critical direction of rotation to the critical direction. This made the airplane fly better under power due to the way the slipstream interacted with the airframe.

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RE: Why make both engines critical ? - 8/28/2008 4:58 AM   
TFF


 

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Status: online 		I thought the P-38 did that. About 2 years ago local, three high time pilots were in an older twin comanche and they spun in on TO. The later ones, Piper changed to counter rotating and wanted owners to change the older ones over.

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RE: Why make both engines critical ? - 8/28/2008 5:54 AM   
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Status: offline 		if you loose either engine its critical

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RE: Why make both engines critical ? - 8/28/2008 8:36 AM   
Shoe



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Status: offline 		By definition, if they're both critical, doesn't that mean that neither is critical?

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RE: Why make both engines critical ? - 8/28/2008 9:01 AM   
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Status: offline 		I suppose it all depends on exactly what you mean by "critical" in my opinion for twins with the rare exception of an in-line tractor/pusher arrangement where each engine has the power to maintain air-speed above stall-speed at half throttle with the other engine dead then both engines are "critical". If I ever build a twin with "critical" engines I will make it an electric with the the motors wired in series so that if I will either loose both or none.

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RE: Why make both engines critical ? - 8/28/2008 9:26 AM   
BMatthews



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Status: offline 		As I understand it neither is critical if you maintain enough airspeed. The critical control speed being reached when the rudder no longer has enough yaw force to counter the offset thrust. This usually only happens when flying slowly under high output from the remaining engine... like in a climb right after takeoff when an engine quits.

But if you can keep flying at decent speed one engine out isn't a big deal. It's far from normal but it's easily handled. So the only time to worry is if you want to try to climb or fly level at a lower speed on one engine.

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RE: Why make both engines critical ? - 8/28/2008 1:27 PM   
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Status: offline 		Critical exists as law of physics and only applies when the loss of one engine has a different affect on performance than the loss of the other, as is typical when both engines rotate in the same direction. Loss of the critical engine is that which results in the worst performance. By definition then, there is no condition where they can both be critical. It can be argued than an inline twin has a critical engine. If the rear engine is lost, you have greatly reduced prop blast over the elevator, thus potentially lossing sufficient pitch control. I have seen it happen.

Bruce, your point is valid for keeping your aircraft whole and wallet sorta fat. Stay above critical minimums and it does not matter which engine is lost.

Bedford

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RE: Why make both engines critical ? - 8/28/2008 4:47 PM   
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Status: offline 		On the original prototype P-38, both blades travelled down by the fuselage. This is the conventional method of using the torque and P-factor to assist the pilot in engine out conditions. But test flight showed some flight conditions where severe buffetting occured. So thay swapped engines so both blades travelled upward by the fuselage. That would have raised the speed required to maintain control when either engine is out.

When both engines turn the same direction, you have the Critical engine, which is generally on the right side for the way engines turn. One of the reasons behind Rutan's Boomarang design, almost as strange as B&V 141 from WWII.

< Message edited by HighPlains -- 8/28/2008 4:50 PM >

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RE: Why make both engines critical ? - 9/5/2008 3:46 AM   
beenie



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Status: offline 		The Turbo Commanders are powered by the Honeywell/Garrett TPE331, I think the -8, although some have the -10. They turn "backwards" compared to what we are used to here in the states. They rotate clockwise when viewed from the front. I think the picture offered at the top of the thread makes it look like the engines are counterrotating, although they are not. The "critical" engine is now the right one, not the left one. As long as the plane is kept above Vmc (minimum single engine control speed), you will have control of the plane regardless of which engine is dead.
Ben

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RE: Why make both engines critical ? - 9/11/2008 6:24 AM   
TexasAirBoss



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Status: offline 		Thanks Ben. that pic had me stumped

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RE: Why make both engines critical ? - 9/14/2008 8:27 PM   
abelard


 

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Status: offline 		A propeller applies some torque around the yaw axis depending on the pitch angle of the airplane. If the airplane is in a nose-up attitude, the descending blade is at a higher angle of attack than the ascending blade, and produces a little more thrust. The formal name for this is "P-factor". If the rotation direction is "conventional" clockwise as seen from behind this thrust offset will make the airplane want to yaw left. That's why taildraggers need some right rudder on takeoff.

If you lose one engine on a twin, you have a BIG thrust offset, and the airplane wants to yaw toward the dead engine. You counter that by applying rudder. This is no problem at cruise speed, because the rudder can deliver all the yaw torque you need. But if you slow the airplane up which you're certainly going to do, because you will really want to land the rudder will progressively lose authority and you'll have to apply more and more rudder. Eventually the rudder will hit the stop, and if you slow up any more you'll get an uncontrollable yaw, followed by an uncontrollable roll, followed by a splat. So that determines the minimum speed at which you can fly the airplane.

At low speed, you're also in a nose-up attitude, so both engines will make the airplane want to yaw left. If the right engine quits, the P-factor of the left engine will be helping you on the rudder; if the left one quits, the P-factor of the right one will be fighting you...so losing the left engine hurts your low-speed controllability more than losing the right. Thus the left engine is called the "critical engine".

One of the performance figures for a twin is Vmc, the minimum controllable single-engine speed, and it's defined as the lowest airspeed at which you can maintain yaw control with the critical engine dead and feathered, and the other one at full throttle. You never, ever get below Vmc on an approach, and never lift off the runway below it, because if you lose an engine you're in deep yogurt.

If you build the airplane with counter-rotating props, there is no critical engine, and in the definition of Vmc you can substitute "either" for "critical". If the descending blades are on the fuselage side of the engines i.e., the right engine rotates "backwards" the P-factor will help out in a failure of either engine. If they rotate the other way, the P-factor will always hurt, and Vmc will be higher. But this really isn't a BIG factor: it's a matter of a few knots. Various airplanes, including the P-38, have realized benefits from "wrong way" rotation that outweigh a small increase in minimum speed.

Interesting to note that certain extreme single-engine airplanes, like the P-51, have what amounts to a Vmc: you can't do an unaccelerated stall at full throttle, because you will run out of rudder before you get down to stall speed.

rj

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RE: Why make both engines critical ? - 9/14/2008 11:38 PM   
abelard


 

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Status: offline 		Sorry if some of the sentences in the above message are hard to interpret. I normally use a double hyphen to make a dash, and the message editor apparently deletes them completely...

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RE: Why make both engines critical ? - 1/14/2009 8:16 PM   
jdeters79


 

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Status: offline 		Overall pretty good explanation abelard.

quote:

ORIGINAL: abelard
At low speed, you're also in a nose-up attitude, so both engines will make the airplane want to yaw left. If the right engine quits, the P-factor of the left engine will be helping you on the rudder; if the left one quits, the P-factor of the right one will be fighting you...so losing the left engine hurts your low-speed controllability more than losing the right. Thus the left engine is called the "critical engine".


This statement is, however, not completely correct. If the aircraft does not have counter-rotating props, then the torque from both engines will cause the airplane to yaw in the same direction. However, the P-factor from both engines do not cause the aircraft to yaw in the same direction. The P-factor from the RIGHT engine/prop will cause the airplane to yaw left (as is considered typical to single engine aircraft) and the P-factor from the LEFT engine/prop will cause the airplane to yaw right (due to the offset thrust centerline).



quote:

ORIGINAL: abelard
One of the performance figures for a twin is Vmc, the minimum controllable single-engine speed, and it's defined as the lowest airspeed at which you can maintain yaw control with the critical engine dead and feathered, and the other one at full throttle. You never, ever get below Vmc on an approach, and never lift off the runway below it, because if you lose an engine you're in deep yogurt.

If you build the airplane with counter-rotating props, there is no critical engine, and in the definition of Vmc you can substitute "either" for "critical". If the descending blades are on the fuselage side of the engines i.e., the right engine rotates "backwards" the P-factor will help out in a failure of either engine. If they rotate the other way, the P-factor will always hurt, and Vmc will be higher. But this really isn't a BIG factor: it's a matter of a few knots. Various airplanes, including the P-38, have realized benefits from "wrong way" rotation that outweigh a small increase in minimum speed.


This statement is exactly correct. Counter-rotating props = no critical engine (due to the same offset thrust centerline arm for both engine/props)

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RE: Why make both engines critical ? - 1/14/2009 9:32 PM   
G4guy


 

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quote:

ORIGINAL: jdeters79

Overall pretty good explanation abelard.

quote:

ORIGINAL: abelard
At low speed, you're also in a nose-up attitude, so both engines will make the airplane want to yaw left. If the right engine quits, the P-factor of the left engine will be helping you on the rudder; if the left one quits, the P-factor of the right one will be fighting you...so losing the left engine hurts your low-speed controllability more than losing the right. Thus the left engine is called the "critical engine".


This statement is, however, not completely correct. If the aircraft does not have counter-rotating props, then the torque from both engines will cause the airplane to yaw in the same direction. However, the P-factor from both engines do not cause the aircraft to yaw in the same direction. The P-factor from the RIGHT engine/prop will cause the airplane to yaw left (as is considered typical to single engine aircraft) and the P-factor from the LEFT engine/prop will cause the airplane to yaw right (due to the offset thrust centerline).



quote:

ORIGINAL: abelard
One of the performance figures for a twin is Vmc, the minimum controllable single-engine speed, and it's defined as the lowest airspeed at which you can maintain yaw control with the critical engine dead and feathered, and the other one at full throttle. You never, ever get below Vmc on an approach, and never lift off the runway below it, because if you lose an engine you're in deep yogurt.

If you build the airplane with counter-rotating props, there is no critical engine, and in the definition of Vmc you can substitute "either" for "critical". If the descending blades are on the fuselage side of the engines i.e., the right engine rotates "backwards" the P-factor will help out in a failure of either engine. If they rotate the other way, the P-factor will always hurt, and Vmc will be higher. But this really isn't a BIG factor: it's a matter of a few knots. Various airplanes, including the P-38, have realized benefits from "wrong way" rotation that outweigh a small increase in minimum speed.


This statement is exactly correct. Counter-rotating props = no critical engine (due to the same offset thrust centerline arm for both engine/props)


I believe the correct statement would be:

VMC is the airspeed below which directional control cannot be maintained
1. At Sea level
2. Critical Engine WINDMILLING
3. Other engine producing Maximum thrust
4. Flaps Up, Gear Up
5 Most aft C.G

If any of the above are changed, VMC will be lower.

Now I am not addressing Critical Engine, I believe each Aircraft manufacturer denotes "critical engine" for each particular aircraft.


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RE: Why make both engines critical ? - 1/15/2009 12:49 AM   
rmh



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Status: offline 		well boys I once called the sequences for Bob Hoover when he flew the Aero Commander at an air show in Utah - dead engine dead engines - you name it . Bob did rols etc..
he showed that NOTHING is critical if you know how to fly the plane
Also we had a P38 on take off at SLC #1 field - loose an engine years back - it went over and -in.

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RE: Why make both engines critical ? - 1/21/2009 6:02 AM   
TexasAirBoss



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quote:

ORIGINAL: abelard

Various airplanes, including the P-38, have realized benefits from "wrong way" rotation that outweigh a small increase in minimum speed.

rj


Thats my question. What are those benefits ?

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RE: Why make both engines critical ? - 1/21/2009 6:47 AM   
HighPlains


 

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RE: Why make both engines critical ? - 1/21/2009 4:04 PM   
TexasAirBoss



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Status: offline 		Can you be more specific ?

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RE: Why make both engines critical ? - 1/21/2009 5:19 PM   
HighPlains


 

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Status: offline 		I read about it 40 years ago. Happen to have a book on Dick Bong in the school library. Maybe it's still there, but who knows?

Try Google or Yahoo search.

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RE: Why make both engines critical ? - 1/21/2009 11:05 PM   
TexasAirBoss



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Status: offline 		I had heard about the Lightning's issues, vaguely. But didn't understand why it was germain to the Commander. Ealier versions had engines with conventionally rotating engines. Turns out Ben was right on. This pic shows that both did turn the same way. It was simply an engine brand selection.

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