A very basic question

"Ramapriya" wrote in message
om...
I love jsd's site too, and have condensed all those chapters into two
Word files (in case someone is interested!). But I wish Denker had
also written the stuff for a non-aviator like me in mind. For example,
he's written loads on trim but till this day, I don't know what
exactly trimming is and how it physically works

Well, trimming isn't complicated. But there are numerous methods for
actually *accomplishing* it, so perhaps that's why Denker sort of just
assumes you're familiar with the concept and doesn't get into how it
"physically works".

That is, the basic concept is simple: "trimming" simply means to set a
particular control (the "trim"...generally you may have elevator, rudder, or
aileron trim or any combination of the three, though I don't doubt there's
at least one unusual aircraft out there that has yet another possibility I'm
not aware of) so that instead of the pilot having to hold a particular
control input, the "trim" holds it for him.

So, for elevator trim (the most common type), once the pilot has selected a
pitch attitude for a climb (for example), along with the desired power
setting (often full power), he can then set the elevator trim to hold the
elevator control input at that particular pitch attitude.

It gets complicated when you start talking about each specific trim
mechanism, since they all have subtle differences in exactly how they
accomplish that "hold the control input" action, as well as effects of power
or airspeed changes on the effect of the trim.

As an example, look at elevator trim:

Generally speaking, elevator trim can be thought of as "setting" an
airspeed, since for a given power setting, airspeed varies precisely with
pitch attitude. A further generalization is that changes in power do NOT
actually change the "trimmed airspeed". That is, for a given trim setting,
increasing power will result in the nose pitching up and decreasing power
will result in the nose pitching down, with the airspeed remaining constant
in both cases.

For some aircraft, the airspeed literally remains constant. For others, you
will actually see slight variations in airspeed. But regardless, any
variations are almost always so small that you can still think of setting
airspeed rather than a specific trim setting. Since trim doesn't really set
a pitch attitude either (except for a given power setting), it's not like
there's really a more useful paradigm to use.

Just as an example of one elevator trim mechanism: in most of the
single-engine Cessnas (and maybe their piston twins, but I don't know those
airplanes well enough to say), elevator trim is accomplished through the use
of a moveable "trim tab" mounted on the trailing edge of the elevator.
There's a wheel in the cockpit that moves this trim tab up and down. When
the trim tab moves up, it exerts a downward force on the elevator and when
the trim tab moves down, it exerts an upward force on the elevator
(aerodynamically, in the exact same way that the elevator itself changes the
horizontal stabilizer's up or down force). The trim tab is relatively
small, so that by pushing down on the elevator, the net effect is to cause
the horizontal stabilizer to create an upward force (nose down pitch), just
as if the pilot had pushed forward on the yoke to deflect the elevator
downward.

Anyway, other aviation references will provide much more detailed
information on that sort of thing. "Stick and Rudder" will give you a good
pilot's view of things, while the "Aerodynamics for Naval Aviators" delves
more deeply into the actual mechanics of flight (naturally).

Pete