Tail mounted engines question

[newmei]

On a typical twin those with engines on their wings, when engine fails they yaw AND roll into the dead engine. I was brainstorming the other day, on a jet liner like a 727, DC-9, MD80/90, if you had a engine failure, you would only get a yaw motion and no roll motion because you don't loose any slipstream over the wings to make it roll. Am I right in this assumption?

 

[Kingairrick]

newmie;
A good question, think it through. As the aircraft yaws, what happens to the speed of the air on the wing on the left? and the right? Essentially, one side of the aircraft is going faster than the other, producing more lift. You get roll because of the assymetrical lift.

I just made that up!

 

[newmei]

Hmm this is a good one to throw around. This is true, but what happens when you stop the yaw with the rudder? Thats half of the equation in piston twins we have accelerated slipstream. I say on a tail mounted engines you just have to use rudder and no 3-5 degrees of bank into the good engines because you don't have the loss of accelerated slipstream?

Remember, PAST, (for critical engine) p factor, accelerated slipstream, spirling slipstream, and torque. What happens to accelerated slipstream on tail-mounted engines? (I understand that jets don't really have critical engines like props, but its the example of accelerated slipstream).

 

[banned username 2]

Don't forget that when you get the Yaw, one wing will appear to have less sweep and the other wing more sweep. The "less swept" wing will create more lift than the "increase swept" wing.

 

[newmei]

"Don't forget that when you get the Yaw, one wing will appear to have less sweep and the other wing more sweep. The "less swept" wing will create more lift than the "increase swept" wing."

I think thats what the first poster said.....Albeit would'nt be it less than a wing (centered vertical) mounted engine? (no, loss of accelerated slipstream) Plus, once you correct for that yaw will you have to add some bank to get a zero side slip, or will this be non-existant because of the lack of accelerated slipstream?

 

[vclean]

Tail mounted engines produce their thrust close to the longitudinal
axis, additionally a dead jet engine doesn't produce the near the drag a windmilling prop does.

 

[Typhoon1244]

I've talked to a couple of DC-9 drivers who said they need darn near full scale rudder during the initial portion of a V1 cut. You wouldn't think so with engines so close to the centerline...but look how far aft they are from the C.G., particularly on the '80's.

I also recall an Air Force pilot saying the same thing about the F-4D and E...he was surprised how much it yawed when an engine quit.

 

[crosscut]

DC 9

NO banking – wings level at least until your single eng climb speed or so. In part due to the wing spoiler activation on the ‘inside’ wing caused by control wheel deflection of greater than 5 degrees. Spoilers, even if intermittent, cause drag.

At V1 a full scale rudder will be too much. Book says “apply rudder commensurate with thrust decay…” Even in the box I haven’t seen more than about half or a hair above half the scale. I heard in a real life V1 eng out scenario very little rudder was required for directional control.

How much input is required must also correlate to how big your rudder surface is. Somebody correct me, but doesn’t Aerostar (is it PA-60?), among other adverse eng out characteristics, have a small rudder surface thus requiring a greater rudder input? as opposed to other, bigger, hydraulically assisted, surfaces?

By the way, depending on the flap setting (5 or 15 degrees) full rudder deflection on the 9 is plus or minus 22 degrees. What is it on your airplane? and could the amount of the deflection also determine the amount of input?

 

[surplus1]

I've talked to a couple of DC-9 drivers who said they need darn near full scale rudder during the initial portion of a V1 cut. You wouldn't think so with engines so close to the centerline...but look how far aft they are from the C.G., particularly on the '80's.

The DC9 drivers you're talking to aren't wrong. If fact, darn near is perhaps minimizing it.

It is true that the engine placement reduces the yaw caused by assymetric thrust. It is also true that the moment of the assymetry is magnified by the greater arm from the c.g. Another thing that you haven't mentioned is a major factor and that is the physical size of the vertical fin and the rudder. Because the yawing forces are actually less than they are with wing mounted engines, the manufacturer is able to reduce the size and weight of the fin/rudder and they usually do this to the maximum extent permissable.

The certification criteria specifies the limits that are permissable (lbs. of pressure), including the angle of rudder deflection. As long as those standards are met the design is OK. That's keeping it simple but conveys the concept. As a result of this you may often find that more physical displacement/movement of the rudder and pedals is required in the fuselage mounted engine design than in an aircraft of similar size with wing mounts. The 9 is a good example of this particularly since the original design has been stretched several times and there hasn't been much of a change in the size of the vertical fin or the rudder. Douglas took it pretty close to the limits, which explains the need for a whole lot of rudder input from the pilot.

One thing I don't quite get is the original poster's (newmei's) reference to "loss of slipstream over the wings". This is a factor in propeller aircraft but doesn't appear to be relevant in jets, even when the engines are mounted (below) the wings. Maybe I'm not understanding what you meant? What Falcon said about effective change in sweep due to yaw and the associated changes in lift is quite accurate. Is that what you meant newmei?

Good question.

 

[mt2]

With the proper amount of opposite rudder to counter the adverse yaw,even a B757 with all that excess thrust, will not roll. Same is true of the B727. It takes less rudder but if used correctly neither airplane has a rolling tendency. It is the adverse yaw that causes that roll, keep it under control and you've got the roll beat.

And yes, a B727 will try to roll even with the engines near centerline thrust. They are a good ways aft of the yaw axis and thus a loss of an engine still requires a fair amount of rudder to counter that loss.