Re: hydraulic problems with DC-10s??

From:         Keith Barr <barrk@ucsu.Colorado.EDU>
Date:         04 Dec 92 22:30:39 PST
References:   1 2 3 4
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Subject: Re: hydraulic problems with DC-10's??
Robert Dorsett Says:
> Normally, given asymmetric thrust, you bank into the good engine(s): rudder's 
> normally used to augment the ailerons as necessary to control sideslip.

Actually, you have this backwards.  Rudder is used to control the yaw,
and by controlling the yaw you introduce some sideslip that should be
counteracted by banking into the good engine (raise the dead is the way
I was tought to remember that :^)

The way this works is as follows....we will have to make due with ASCII

Normal Flight (Multi-Engine, Both turning)
     Left thrust  Right thrust    
             |     |              
             |  A  |              

Engine Out Flight (no correction)
        left thrust               
             |  A                 
                A          CW Moment
                A            |_   

Engine Out Flight (Yaw (moment) correction)
       left thrust                
             |  A                 
              --A--===>Rudder Force to counteract rotation
                |                 Now you can see we have fixed the
                |                 Rotation with rudder, but we have an
                |                 unbalanced vector diagram, so the aircraft
               Drag               will sideslip to the right

By raising the dead engine we tilt the lift vector to the left which balances
the force from the rudder.

> The second issue is the moment produced by the combination of the "dead"
> engine (with its drag) and the "good" engines.  This is generally a minimal
> issue, assuming the airspeed is there, and the pilot applies correct 
> technique.  Most transport aircraft can fly with all engines out on one side, 
> although I do not know if this is an explicit regulatory requirement.  As 
> long as the inherent longitudinal stability of the airplane (contributed
> by the vertical stabilizer, rudder, wings, and fuselage) is sufficient to 
> overcome the yawing moment, the airplane can be controlled.  So *correcting*
> for a lost engine is a near-instantaneous correction, applied by the pilot, 
> needing no altitude reserve.

Correct, but here is an added explanation for those who care:
There is really only one concern of the pilot in an engine out situation, that
is airspeed.  The pilot, if he has done an appropriate preflight, will know
whether he/she is able to climb on one engine out, so that is not a suprise.  
The biggest problem with an engine out is loss of control.  This airspeed,
called Vmc (Velocity Minimum Controllable) is the speed at which the rudder
doesn't have enough air flowing over it to create enough force to counteract
the moment from the good/dead engine.  As long as you are above this speed,
you should be controllable (ignoring the fact that one wing may stall if
the slat comes up, but I am not talking about that case in particular).

On the same thread, but different argument...
Michael Weiss writes:
>I have a hard time believing that an intact hydraulic system would have
>prevented AA191 from crashing.  Let's face it, a wing-mounted engine falling
>off produces such a rediculous unbalance that even full aileron wouldn't be
>able to counter it.

>After the third post with this answer, I figure it's time to clarify my
>statement.  I am referring to the unbalance of WEIGHT, not THRUST.  Nonetheless
>I suppose we should go on...

The change in weight from a lost engine is minimal.  A fully loaded DC-10-30
weighs 572,000 pounds.  A GE CF6-50C2B weighs only 8,731 pounds.  This means
that in normal flight each wing needs to support 286,000 pounds.  If each
wing supports the weight of its engine, now the left wing only needs to
create 277,269 pounds of lift, a 3.05% decrease.  I would imagine that 
ailerons easily can create a 3.05% increase in lift per side.

References:  Aviation Week and Space Technology 3/16/92 p. 102
             Illustrated Encyclopedia of Commercial Aircraft pp 148-157
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