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P.A.S.T Critical Engine / Vmc  
User currently offlineFr33fr0mlif3 From United States of America, joined Sep 2007, 5 posts, RR: 0
Posted (6 years 10 months 3 weeks 5 days 2 hours ago) and read 7426 times:

Hello, I am posting here to get some help on figuring out how P-Factor, Accelerated Slipstream, Spiraling Slipstream, and Torque define the left engine as the critical engine in conventional twins that have both props rotating clockwise from cockpit view.

Another factor apart of Multi-Engine flying I am interested in learning about is Vmc. What defines Vmc, what causes the plane to Vmc.

I appreciate all detailed explanations and I thank you very much!

-Proud Aviation Student.

21 replies: All unread, jump to last
 
User currently offlineFuturecaptain From , joined Dec 1969, posts, RR:
Reply 1, posted (6 years 10 months 3 weeks 5 days 2 hours ago) and read 7408 times:

I'll start and let others add info later cause I don't have alot of time...

What defines the critical engine?
As a prop is spinning it takes a bigger "bite" out of the air on the downward stroke compared to the upward stroke. The critical engine is the one which when lost will create the most adverse effect on your performance. If you lose the left engine in a conventional twin the right prop, spinning clockwise, is on the downward stroke on the far side of the fuselage. Because it is taking the bigger bite out of the air so far away it has a greater moment and affects performance moreso. basically.  Smile

Vmc?
Minimum control velocity. The airspeed you don't want to go below when you lose an engine. Many, many things affect Vmc including but not limited to CG location, total aircraft weight, bank angle, ect, ect, ect. When you lose an engine the rudder and ailerons have to take over to control the aircraft. Vmc is where you no longer have control of the aircraft on one engine because there is not enough air moving over the surfaces to keep the plane under control. When you practice with an instructor doing Vmc you will probably be told to say something like "at the first sign of yaw, roll, or stall I will recover." If you stall, can no longer control roll, or can no longer control yaw with one engine inop you have found Vmc.

If nobody else chimes in I can go into more details later.


User currently offlineAS739X From United States of America, joined Apr 2003, 6098 posts, RR: 23
Reply 2, posted (6 years 10 months 3 weeks 5 days 1 hour ago) and read 7389 times:

Please move to Tech Forum

ASSFO



"Some pilots avoid storm cells and some play connect the dots!"
User currently offlineFr33fr0mlif3 From United States of America, joined Sep 2007, 5 posts, RR: 0
Reply 3, posted (6 years 10 months 3 weeks 5 days ago) and read 7370 times:

So with P-Factor, with the loss of the left engine, you gain asymmetrical thrust. The left engine was produces the thrust closer to the centerline of the aircraft and right engines produces it on the right side of the engine, from the descending blade thus causing a lot more yaw? Is that correct?

User currently offlineKELPkid From United States of America, joined Nov 2005, 6346 posts, RR: 3
Reply 4, posted (6 years 10 months 3 weeks 4 days 23 hours ago) and read 7352 times:

Actually, in a propeller-powered multi, the single largest contributor to the critical engine phenomena is the corkscrew effect ("Spiraling slipstream" from your original post). The engine whose corkscrew airflow spirals into the rudder will have the greatest effect, so for traditional American aviation engines, that would be the left engine  Wink

Trivia time: name a twin-engined aircraft (and actually, it's only certain sub-variants of it  Wink ) that actually has two critical engines, and explain why that's the case  Smile



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineIFIXCF6 From United States of America, joined Sep 2004, 108 posts, RR: 0
Reply 5, posted (6 years 10 months 3 weeks 4 days 22 hours ago) and read 7339 times:

My guess (and it is a guess) is the P-38, due to counter rotation on the left and right engines.

Mike


User currently offlineFutureUALpilot From United States of America, joined May 2000, 2602 posts, RR: 8
Reply 6, posted (6 years 10 months 3 weeks 4 days 22 hours ago) and read 7338 times:

Quoting KELPkid (Reply 4):
Actually, in a propeller-powered multi, the single largest contributor to the critical engine phenomena is the corkscrew effect ("Spiraling slipstream" from your original post). The engine whose corkscrew airflow spirals into the rudder will have the greatest effect, so for traditional American aviation engines, that would be the left engine

Actually I think it is the loss of induced flow over the wing that causes the airplane to roll/yaw to the side of the failed engine, afaik, the slipstreams off of non-centerline powered twins doesn't contact the tail, or at least not enough to do anything.

Quoting KELPkid (Reply 4):
Trivia time: name a twin-engined aircraft (and actually, it's only certain sub-variants of it ) that actually has two critical engines, and explain why that's the case

Piper Seneca. Either both, or neither engine is the critical engine. They both rotate towards the fuselage, so losing one engine over the other doesn't create any worse of a situation. I beliebe the P38 had counter rotating props, did it not?

Quoting Fr33fr0mlif3 (Reply 3):
The left engine was produces the thrust closer to the centerline of the aircraft and right engines produces it on the right side of the engine, from the descending blade thus causing a lot more yaw? Is that correct?

Think of lever arms...would you rather use a short crowbar to pry open a door, or a longer one? Which one gives you greater leverage? The descending blade furthest from the centerline...so yes, you are essentially correct


I learned that there are 12 factors that affect Vmc...4 engine, 4 configuration, 2 weight and 2 pilot input...

1. Critical engine inop (greater yaw - more control input required to start - Vmc increases)
2. Prop on critical engine windmilling (unless autofeather installed) (Windmilling prop = more drag, Vmc increases)
3. Takeoff, or max available power on operating engine (Maximum power = more yaw = Vmc increases)
4. Sea level (More power at SL than at altitude = more yaw = Vmc increases)
5. Flaps in takeoff position (No change in Vmc - standardization item)
6. Cowl flaps in takeoff position (Same as above)
7. Airplane trimmed for takeoff (Same as above)
8. Gear up (Gear down adds keel effect = aircraft harder to displace sideways = Vmc decreases...gear up = Vmc incr)
9. Max sea level gross weight (More weight = more inertia = harder to displace = Vmc decrease)
10. Most unfavorable CG (usually aft) (Shorter arm to tail "crowbar" = Vmc decrease)
11. Airplane airborne and ground effect negligible (Standardization item)
12. Up to 5 degrees of bank toward good engine. (Vmc decreases in theory 3kts per degree of bank...Vmc decreases)



Life is better when you surf.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6346 posts, RR: 3
Reply 7, posted (6 years 10 months 3 weeks 4 days 22 hours ago) and read 7333 times:

Quoting IFIXCF6 (Reply 5):
My guess (and it is a guess) is the P-38, due to counter rotation on the left and right engines.

Mike

More info needed on the why part...  Wink

Quoting FutureUALpilot (Reply 6):
Piper Seneca. Either both, or neither engine is the critical engine. They both rotate towards the fuselage, so losing one engine over the other doesn't create any worse of a situation.

That would be neither...it is correct that the Senaca (Seneca II, III or IV that is  Wink ) has no critical engine. However, the twin in question has not one, but two critical engines.

Quoting FutureUALpilot (Reply 6):
I beliebe the P38 had counter rotating props, did it not?

Yes it did...but once again, what was unique about the P-38 that made it have two critical engines ?  Smile

Quoting FutureUALpilot (Reply 6):
Actually I think it is the loss of induced flow over the wing that causes the airplane to roll/yaw to the side of the failed engine,

The torque of the good engine also plays a role in the tendency of the aircraft to roll...



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineIFIXCF6 From United States of America, joined Sep 2004, 108 posts, RR: 0
Reply 8, posted (6 years 10 months 3 weeks 4 days 22 hours ago) and read 7330 times:

OK, I'll bite. The P-38 has 2 vertical fins and 2 rudders directly behind each engine. I'm not a pilot, and don't normally think of these types of things. Am I right?

Mike


User currently offlineKELPkid From United States of America, joined Nov 2005, 6346 posts, RR: 3
Reply 9, posted (6 years 10 months 3 weeks 4 days 22 hours ago) and read 7330 times:

Quoting IFIXCF6 (Reply 8):
OK, I'll bite. The P-38 has 2 vertical fins and 2 rudders directly behind each engine. I'm not a pilot, and don't normally think of these types of things. Am I right?

Mike

Nope, that's not it...it's something way more fundemental than that  Smile



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 10, posted (6 years 10 months 3 weeks 4 days 21 hours ago) and read 7321 times:

Quoting KELPkid (Reply 9):
Nope, that's not it...it's something way more fundemental than that

Well they both rotate outward from the fuselage as opposed to inward (i.e. PA-44). So both engines have a very long moment arm.

Or did I go too deep?

[Edited 2007-09-12 03:16:45]

User currently offlineKELPkid From United States of America, joined Nov 2005, 6346 posts, RR: 3
Reply 11, posted (6 years 10 months 3 weeks 4 days 21 hours ago) and read 7315 times:

Quoting FLY2HMO (Reply 10):
Well they both rotate outward from the fuselage as opposed to inward (i.e. PA-44). So both engines have a very long moment arm.

DINGDINGDING!!!! We have a winner, folks  yes 

Even more interesting is that in the P-38, both engines (the left and the right) are actually driven the same direction, but the engine with the left hand rotating prop uses gears to turn the propeller the other way. I understand this is possible because the Allison V-1710 was actually originally intended to be an airship engine, and was designed from the outset to accomidate a gearbox so the engines could be fully reversible in flight.



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineRwessel From United States of America, joined Jan 2007, 2316 posts, RR: 2
Reply 12, posted (6 years 10 months 3 weeks 4 days 17 hours ago) and read 7278 times:
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Quoting KELPkid (Reply 11):
Even more interesting is that in the P-38, both engines (the left and the right) are actually driven the same direction, but the engine with the left hand rotating prop uses gears to turn the propeller the other way. I understand this is possible because the Allison V-1710 was actually originally intended to be an airship engine, and was designed from the outset to accomidate a gearbox so the engines could be fully reversible in flight.

If I'm not mistaken, the reverse rotation version of the V-1710 was actually assembled with the crankshaft reversed. There did need to be gearing changes for the supercharger, and the starter had to be installed differently.


User currently offlineFr33fr0mlif3 From United States of America, joined Sep 2007, 5 posts, RR: 0
Reply 13, posted (6 years 10 months 3 weeks 4 days 17 hours ago) and read 7275 times:

I will make a new post since this one has strayed away from helping me.

User currently offlineFr33fr0mlif3 From United States of America, joined Sep 2007, 5 posts, RR: 0
Reply 14, posted (6 years 10 months 3 weeks 4 days 5 hours ago) and read 7228 times:

Can anyone help me with this?

User currently offlineFuturecaptain From , joined Dec 1969, posts, RR:
Reply 15, posted (6 years 10 months 3 weeks 4 days 5 hours ago) and read 7227 times:

Quoting Fr33fr0mlif3 (Reply 14):
Can anyone help me with this?

See Reply 1, 4, and 6. And I will try to dig some forgotten knowledge out of my books for you tonight. If Slamclick makes it in here you should get all the knowledge and examples you need though.


User currently offlineFr33fr0mlif3 From United States of America, joined Sep 2007, 5 posts, RR: 0
Reply 16, posted (6 years 10 months 3 weeks 4 days 2 hours ago) and read 7199 times:

Let me do another review.

P-Factor - Left engine is critical due to asymmetrical thrust. The left engine prop wash is closer to centerline of the aircraft, which has a greater moment, compared to the right engine producing thrust further away from the centerline of the aircraft. When left engine is loss, it most adversely affects the aircraft, thus left engine is critical.

Accelerated Slipstream - Still need help here.

Spiraling Slipstream - Left engine is deemed critical due to the fact you loose the air getting pushed against the rudder from the prop wash thus causing a great yaw. (Need more details)

Torque - For every reaction, there is an equal and opposite reaction. With both engine spinning clockwise from cockpit view, that means the torque is causing the aircraft to rotate counter-clockwise (to the left) If you loose the left engine, that me


User currently offlineAAR90 From United States of America, joined Jan 2000, 3466 posts, RR: 47
Reply 17, posted (6 years 10 months 3 weeks 3 days 16 hours ago) and read 7145 times:

From my days teaching E-2 Hawkeye powerplants systems to new students (and the one thing they _always_ remembered): "you're flying a two-engine airplane; when one engine quits... THE WORKING ENGINE _IS_ THE CRITICAL ENGINE!"

Sorry... couldn't resist.  Wink



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineFuturecaptain From , joined Dec 1969, posts, RR:
Reply 18, posted (6 years 10 months 3 weeks 3 days 4 hours ago) and read 7091 times:

Quoting Fr33fr0mlif3 (Reply 16):

Read up. Google is your friend.
And I'm tired and don't want to type much.  Wink

http://www.pilotscafe.com/articles/multi-OEI.pdf


User currently offlineBoeingOnFinal From Norway, joined Apr 2006, 476 posts, RR: 0
Reply 19, posted (6 years 10 months 3 weeks 3 days 4 hours ago) and read 7087 times:

Quoting Fr33fr0mlif3 (Reply 16):
The left engine prop wash is closer to centerline of the aircraft, which has a greater moment, compared to the right engine producing thrust further away from the centerline of the aircraft.

Correction: The right hand engine produces the greatest moment.

Moment: Arm x Force

The arm is the length from the longitudinal centerline of the aircraft to the centre of thrust of the engine.
Since the aircraft is flying at an angle of attack, the down going propeller blade will have a greater angle of attack than the up going blade, thus moving the centre of thrust a little to the right of the crank shaft on right hand engines. Thus engine number two (right engine) will have a longer arm than engine number one (left engine), and a greater moment.

So, loosing the left engine will give the greatest adverse yaw, thus the left engine as previously mentioned will be the critical engine.

Quoting Fr33fr0mlif3 (Reply 16):
Spiraling Slipstream - Left engine is deemed critical due to the fact you loose the air getting pushed against the rudder from the prop wash thus causing a great yaw.

On some aircrafts the vertical stabilizer is set at an angle of incidence to counter produce the force created by the spiraling slipstream, usually configured to cancel out any rudder input at cruising speed. This is most significant on single engine aircrafts with the engine placed in front. I can't say for sure if this applies to twin engines, and how severe the slipstream affects the rudder angle of attack, but assuming it has some significance;

The force produced by increased angle of attack of the vertical stabilizer due to slipstream from the left engine will counter the adverse yaw created by the left engine when flying on that alone, leaving the left engine, together with the factor of lesser moment, the preferable engine to be left with during an engine failure.
If you loose the left engine, you will have a greater adverse yaw due to greater moment of the right engine and due to loss of slipstream over the vertical stabilizer. And if the vertical stabilizer is set at an angle of incidence, this will give a force which will worsen the adverse yaw even more.

Quoting Fr33fr0mlif3 (Reply 16):
Torque - For every reaction, there is an equal and opposite reaction. With both engine spinning clockwise from cockpit view, that means the torque is causing the aircraft to rotate counter-clockwise (to the left) If you loose the left engine, that me

Newtons 3rd law: Every action has an equal and opposite reaction. The right hand propeller will try to bank the aircraft to the left. That and due to the fact that the right wing will have more lift than the left due to the slipstream going over it, this will need to be countered.


Important aspects during engine failure:

1. Reduce adverse yaw immediately with rudder!
2. Check that failed engine props are feathered (non feathered props will increase drag dramatically, and will increase adverse yaw.
3. Do not reduce airspeed below Vmc!

You can bank the aircraft towards the live engine. This will allow for less rudder deflection. No more than 5 degrees of bank should be practiced.



norwegianpilot.blogspot.com
User currently offlineKELPkid From United States of America, joined Nov 2005, 6346 posts, RR: 3
Reply 20, posted (6 years 10 months 3 weeks 3 days 3 hours ago) and read 7082 times:

Quoting BoeingOnFinal (Reply 19):
You can bank the aircraft towards the live engine. This will allow for less rudder deflection. No more than 5 degrees of bank should be practiced.

 checkmark  In fact, don't the manufacturers determine Vmc based on holding 5 degrees bank for certificaton? I seem to recall my commercial ground school instructor telling us that if you're not banking 5 deg., then the manufacturer's Vmc figures are invalid, and you're basically a test pilot at that point, and might find the aircraft's true Vmc in the configuration that you're flying it in  Wow!



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 21, posted (6 years 10 months 3 weeks 3 days 2 hours ago) and read 7071 times:

Quoting KELPkid (Reply 20):
In fact, don't the manufacturers determine Vmc based on holding 5 degrees bank for certificaton? I seem to recall my commercial ground school instructor telling us that if you're not banking 5 deg., then the manufacturer's Vmc figures are invalid, and you're basically a test pilot at that point, and might find the aircraft's true Vmc in the configuration that you're flying it in

Wow, I've been told that too, in the exact same wording. Interesting.  scratchchin 


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