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From: mmallory@netcom.com (Mark Mallory)
Subject: Re: What makes an airlplane turn?
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Date: Tue, 16 Mar 1999 06:41:30 GMT
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Dave Mould (davem@airstrip.demon.co.uk) wrote:

: Andrew M. Sarangan wrote in message <36ED87EE.7AC0A1FA@unm.edu>...
: >After some thought I realized the following.  A banked airplane with no
: >rudder inputs WILL turn. Ron Natalie and a few others pointed out that 
: >a bank will only produce a sideways acceleration, and no turn. That is 
: not correct.  A sideways acceleration IS what causes an object to turn.

: No - remember that we not only have a change in direction of travel (which
: is what you are saying), but we also have a change in *heading* (the
: direction that the nose points).  After a bit of thought, I come to the
: conclusion that there is no good analogy with the rotation of the planets
: about the Sun.  The force in this case is external to the object it acts
: upon, and it doesn't translate.

Actually, the previous poster is correct: as soon as the airplane starts 
moving sideways, the direction of the "relative wind" changes; this will 
cause the direction of the Horizontal component of the Lift Vector to 
change as well (even if the *heading* of the airplane doesn't change).

Remember that Lift is always *perpendicular* to the relative wind (and 
Drag is *parallel* to it).  When the airplane is banked, both the vertical 
and HORIZONTAL components of lift must remain at right angles to the 
relative wind.  Therefore, as the airplane starts to accelerate at right 
angles to it's original direction, the horizontal component of lift 
*cannot* remain at right angles to that direction; this would mean that 
there would soon be a component of lift pointing TOWARDS the relative wind, 
which would cause the *speed* of the airplane to increase (an increase in 
Kinetic Energy, at no cost!).  No, the horizontal lift component *must* 
rotate so as to remain perpendicular to the changing direction of the 
relative wind as the airplane accelerates laterally; it will thus produce a 
*centripetal* acceleration and cause the airplane to follow a circular path.

: Think of the flying "rocket belt".  Tip it sideways, and it flies sideways,
: but does *not* turn.  (How the heck *do* you yaw those things, anyway?)

A rocket belt really isn't a good analogy, because it's "lift vector" 
doesn't depend at *all* on the relative wind, or the direction it's 
moving.  A "Flying Wing" with no sweep or dihedral is probably a better 
example: if the Wing were flying straight and level and it were *yawed*, 
the lift vector would remain *vertical* as long as the wing were not 
*rolled*, since there is no "Lateral Area" (fuselage or rudder) to 
produce a sidewards force.  In this case, it's seen that the HEADING of 
the wing doesn't change the *direction* of the lift vector (though 
obviously the Wing wouldn't work very well if it were yawed 90 degrees, 
any lift must still remain *perpendicular* to the relative wind).


