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Re: [OM] IMG: B-25 from the Archives

Subject: Re: [OM] IMG: B-25 from the Archives
From: Chris Trask <christrask@xxxxxxxxxxxxx>
Date: Sat, 12 Dec 2015 07:59:30 -0700 (GMT-07:00)
>
>> Those aspects are true of most aircraft to some degree, Ken, but 
>>particularly of prop-driven aircraft with their propellors forward 
>>of the lifting surfaces.
>
>Maybe ChrisT can enlighten us on how variable the stall speeds are on
>the overblown wing of a C-130.
>
>The era is over, but I absolutely loved watching "Fat Albert" do a JATO launch.
>

     On the C-130 there isn't much variability in the stall speed (except with 
two engines out on one side), and the entire concept of "critical engine" 
performance was never mentioned.  I didn't learn about that until I got some 
instruction in a Piper Aztec.

     "Critical engine" refers to the difference in stall speed that occurs when 
you have a situation such as an engine out on a twin, such as a B-25.  The prop 
does not pull you straight forward, but instead pulls slightly to one side due 
to the "relative wind" on the prop blades.  If you have an appreciable angle of 
attack, the blade on one side will have a higher pitch than that of the other 
side, so it will take a bigger bite, resulting in pulling you to one side.  You 
can experience this in a single-engine aircraft by flying slow and advancing 
the throttle.

     The result is that you end up flying slightly cross-controlled to maintain 
a straight course, and the amount of cross-control will depend on which engine 
is out as you will have to apply more or less cross-control depending on which 
engine is out.  Consequently, there are different stall speeds depending on 
whether it is the left or right engine, and the one that results in the most 
adverse performance is called the "critical engine".  A very detailed 
explanation can be found here:

https://en.wikipedia.org/wiki/Critical_engine

     An interesting historical note is that the P-38 Lightning did not have a 
critical engine.  Since the props rotated in different directions the stall 
speed was the same regardless of which engine failed.  A nice concept unless 
you were a mechanic faced with two different props and gearboxes.


Chris

When the going gets weird, the weird turn pro 
     - Hunter S. Thompson
-- 
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