Re: Engine rotation

From:         kbarr@nyx10.cs.du.edu (Keith Barr)
Organization: Nyx, Public Access Unix at U. of Denver Math/CS dept.
Date:         07 Apr 94 13:55:17 PDT
References:   1 2
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In article <airliners.1994.1099@orchard.chicago.com>,
Terrell D. Drinkard <drinkard@bcstec.ca.boeing.com> wrote:
>Making them all turn the same way has more
>than economic implications.  It also eliminates the 'critical' engine

I imagine further follow-ups should be directed to sci.aeronautics...

Actually, a critical engine only exists when the engines turn the same way.

Numerous light twins, such as the Beechcraft Duchess BE-76, have counter-
rotating propellers to eliminate the critical engine.

First some background.  Before one can understand why there is a critical
engine, one must first understand P-factor.  Basically P-factor is a non-
symmetrical distribution of thrust over the rotating disk formed by the
propeller due to varying speeds and angles of attack of the prop blades
at various positions.  More simply, you can just take it as gospel that
while in any sort of nose-high condition there is more thrust on the
side of the propeller that is travelling down.  I could try to ASCII-art
it, but I think it would be too difficult.  If you are interested in the
fine points, find an aerodynamics text.

When viewed from the top, a propeller attached to a plane in a nose-high
configuration will have a lift distribution as shown below:

Normal Engine...rotates clockwise as viewed from behind.

( | represents the thrust vectors )

                              |||||||||
 ||||||||           ^      |||||||||||||
|||||||||||||||||| / \ ||||||||||||||||||
==================/hub\==================
  Ascending               Descending


This can be more easily represented/simplified to:

                           .
                          /|\
                           |
                    ^      |
                   / \     |
==================/hub\==================

With that said, let me give a quick introduction on flying a multi-engine
aircraft with a dead engine (not counting the Cessna 337).  When an
engine dies there is a yawing moment created that tends to "move" the
nose away from the good engine.  This yawing must be counteracted, if
the pilot is to retain control of the aircraft, with rudder, which yaws
the nose back into the proper direction.  In this configuration,
the aircraft is pointing in the right direction, but it is
slipping slightly in the direction of the dead engine, so normal
procedures call for banking away from the dead engine slightly (in
pilot circles we "raise the dead" so to speak).

On with the explanation...
So, if you have a twin with both engines rotating the same way, you can see
that the right engine creates a larger moment than the left engine.  This
means in an engine out situation, you will have to counteract more moment
with the rudder.  The critical engine is the engine you do not want
to lose, so the left engine is the critical engine.

             |     ^       |
             |    / \      |
          -----  |   |  -----
 __________+++___| C |___+++__________
|                | G |                |
|________________|   |________________|
                 |   |
                 |   |
                 |   |
                 |   |
               ___| |___
              |____.____|

In a well designed aircraft with counter-rotating props, you have the followng
situation, and neither engine is considered to be critical.

             |     ^     |
             |    / \    |
          -----  |   |  -----
 __________+++___| C |___+++__________
|                | G |                |
|________________|   |________________|
                 |   |
                 |   |
                 |   |
                 |   |
               ___| |___
              |____.____|

Note:  There have been some aircraft designed with the counter-rotating engines
switched, so both engines are effectively critical engines.

Enough rambling...hopefully this wasn't too confusing.
 _____________________________           _____
| Keith Barr                  \           \   \__      _____                  
| barr@ncar.ucar.edu           \___________\   \/_______\___\_____________    
| Commercial/AS&MEL/Inst/A&IGI /           ( /_/   .....................  `-.
|_____________________________/             `-----------,----,--------------'
                                                      _/____/