Bankin Towards Operating Engine

[uwochris]

Hey guys,

I have another question in regards to engine failures in multi engine a/c. My previous post (in regards to sideslipping caused by the engine failure) was helpful, however, after doing some more reading, I have developed another confusion.

Let's say that you are flying in a twin with counter rotating props, and the right engine quits.

The initial tendency would be yaw in the direction of the failed engine; this will cause a sideslip, as the a/c will momentarily continue in its initial direction of flight.

In this case, left rudder will be needed. This will prevent some of the yaw, but it will not eliminate the sideslip.

My question is: why must we bank towards the good engine?

Is not the force produced by the rudder sufficient to balance the asymetric thrust and adverse yaw?

I know that you must bank to elminate the sideslip; the bank produces a force which will balance the force produced by the rudder; however, I thought the rudder force already balanced the asymetric thrust forces. If this is the case, it seems that all forces are already balanced, and thus, no need for bank. Obviously this is wrong, but I do not see why.

Also, why is it that when sideslipping with both engines operating, you must use rudder and bank in opposite senses? ie) force of the rudder will be balanced by the force of the opposite bank? In this case (multi engine a/c with 1 engine inop), you use rudder and bank in the *same* direction. This is why I am confused- using them in the same direction seems that an unbalanced force will be produced in the direction of bank.

Well, I hope someone can clarify this misunderstanding for me.

Thanks in advance,

Chris.

 

[EagleRJ]

You need the bank to move the reletive wind back onto the nose.

When you are operating single-engine and are applying rudder to maintain level flight, the asymmetrical thrust force will combine with the asymmetrical yaw force to cause a flight path that is a few degrees off the aircraft's longitudinal axis (in other words, a sideslip).

That offset reletive wind will impact the side of the fuselage, the sides of the engine nacelles, and the side of the vertical stabilizer, causing additional drag. The fix is to bank 2-3 degrees into the operating engine. This horizontal lift vector you have added counteracts the sideslip, moving the reletive wind back onto the nose, and minimizing drag. That ensures that your single-engine climb performance will be all that it can be.

As for the differing control inputs, it just has to do with the forces that you are counteracting. In single-engine ops, it is an asymmetrical thrust vector, and in a sideslip, you are counteracting a crosswind.

Hopefully this makes sense!

 

[Bluto]

"Is not the force produced by the rudder sufficient to balance the asymetric thrust and adverse yaw?

I know that you must bank to elminate the sideslip; the bank produces a force which will balance the force produced by the rudder; however, I thought the rudder force already balanced the asymetric thrust forces. If this is the case, it seems that all forces are already balanced, and thus, no need for bank. Obviously this is wrong, but I do not see why."

You're thinking on the right track. The only problem is that these forces are acting on the aircraft in two different ways. I think of them as control forces and net forces. By adding the rudder to counteract the yaw you have equalized all the control forces. (No change in bank, yaw, or pitch.) However, the net forces on the aircraft (thrust offset from the c.g. centerline and rudder to counteract the yaw) yield a total force offset from the longitudinal axis. The equalizing force is the horizontal component of lift from a small bank towards the operating engine.

I made up some diagrams that help illustrate this, but I don't think I can post them here. pm me with an email address, or post it here and I'll get them right out to you.

 

[Mmmmmm Burritos]

The good engine actually causes the aircraft to yaw AND to roll. The accelerated slipstream from the prop of the good engine over the wing will cause that wing to increase lift, while the "blanked-out" area behind the bad, windmilling propeller will cause a loss of lift since the prop is blocking a lot of the airflow over the part of the wing directly behind it.

This makes a roll tendency toward the good engine. You already understand the yaw tendency... somewhat.

If you input enough rudder, you can counteract both the yaw and roll tendency but you will still be sideslipping. The reason for this is because the rudder produces not only a yaw force, but a sideways lift force. If the right engine is dead, you will need left rudder. This will cause a "total lift component" which is parallel to the airplane's lateral axis (aka sideways lift force).

The engine yaw force and the rudder yaw force cancel each other out (with enough rudder). The engine forward thrust and rudder "sideways" lift do not oppose and do not cancel. They are at approximately right angles and produce a force vector which happens to be the sideslip.

Therefore, the rudder sideways lift creates the sideslip.

The bank into the good engine will oppose the rudder sideways lift with the wings own horizontal component of lift, and you have coordinated flight.

This is why more weight reduces Vmc, but only in a banked condition. More weight required more lift to be created. When banked, the larger "total lift" vector produces a larger horizontal component of lift vector with a given angle of bank.

I am glad to see at least one student out there studying AND thinking about it at the same time. Usually it's one or the other. You should have no problem getting your CFI license.

 

[uwochris]

thanks for the responses.

Bluto, my email is: [email protected]. I would love to look at them,

 

[310]

Example : Right engine quits.
Without rudder input left engine would cause a yaw to the right.
So you add enough left rudder to prevent yaw.(for this discussion I will skip all the stuff about maintaining Vyse etc etc.)
So the rudder input prevents yaw... but the rudder is deflected to the left...so there is a net vector pushing the plane to the right because the rudder is deflected left out into the slipstream.
Without any banking to the left the plane is in a right side slip.
So 2 to 5 degrees of bank is needed towards the good engine(left in this case) to create a zero side slip.

 

[fulcrum]

Raise the dead

Raise the dead

 

[YaMama]

Banking toward good engine:

(1) improves SE climb performance

(2) lowers Vmc

Also, why is it that when sideslipping with both engines operating, you must use rudder and bank in opposite senses? ie) force of the rudder will be balanced by the force of the opposite bank? In this case (multi engine a/c with 1 engine inop), you use rudder and bank in the *same* direction. This is why I am confused- using them in the same direction seems that an unbalanced force will be produced in the direction of bank.

Imagine the aircraft from above, with the rudder deflected to the right. Can you see how this creates sideways lift that will try to pull the plane to the left? (The vertical tail is just an airfoil like the wing). Right bank is needed to pull the wing's lift vector to the right a bit and zero out the sideslip that the rudder wants to create.

 

[Mmmmmm Burritos]

Banking toward good engine:

(1) improves SE climb performance

(2) lowers Vmc

Yes, but not exactly. The more you bank into the good engine, the lower Vmc gets. Bank it 30-45 degrees if you have to.

Banking 2-3 degrees into the good engine will increase performance by reducing drag, but once you go beyond 2-3 degrees, you are decreasing performance. Less then 2-3 degrees will also reduce performance AND increase Vmc. Not good.

This is why the FAA sets the limit to 5 degrees of bank during the certification flight testing for the airplane. You can go more then 5 degrees if you want, and Vmc will plummet, but performance will suffer greatly.

 

[DGdaPilot]

Strap a yaw sting onto that airplane and see what happens in each situation.