DIYbanter - View Single Post - Fwd: Reno Air Race

Beryl[_6_]

wrote:
On Tue, 27 Sep 2011 22:28:05 -0700, Beryl wrote:

wrote:
... The elevator - and the trim tab - do a LOT of work on a race
plane at 500mph. And a lot at 200 too.
....
The trim TAB is a servo mechanism

Nitpick - there are servo tabs, too. They work like trim tabs, except
they're not independently controlled by a trim wheel. They move with
elevator movement, in the same direction a trim tab would move, and
reduce stick forces.

that causes a small amount of force to control a large amount of
force. The little trim tab is what makes the control input neutral.
At 500mph, the force produced by a couple degrees of "angle of
incidence" on a tab 20 inches wide and 2 inches long - just as an
example is VERY SIGNIFICANT - Stick your hand out the window at 50mph
and change the angle - feel the force. Now remember aerodynamic drag
increases at the cube of speed increse. The lift and drag work
directly in concert.Double the speed - 4X the force. You are going to
go 10 times as fast. What does that do to the forces? And that tab is
aerodynamically a lot cleaner than your hand. It is also
SIGNIFICANTLY more area - A few degrees of tab trim will input a lot
of force - particularly at the trailing edge - up to several feet
from the pivot. That trim makes the elevator (in this case) follow
along at the correct angle of incidence for straight and level flight
with no control input force (stick pressure). Now, let that trim tab
come loose at one end and start flapping in the breeze, 2 feet
farther back from the pivot than where it should be - or simply 15
degrees or more off from where it should be - and all of a sudden
LARGE AMOUNTS of control input are required to hold the elevator at
the right position for level flight. Several hundred pounds of force
on the stick would be required INSTANTLY to correct for the
separation - and if that correction is not made INSTANTLY, the quick
movement of the elevator control surface through a significant degree
of movement causes a dangerously violent change in attitude - forcing
the tail surface down - and on a LONG lever - the down force a LONG
way back from the center of lift - which acts as the fulcrum. It does
not take a lot of force that far back to really toss the aircraft out
of straight and level flight. The up-pitch of the plane cuased by the
quick drop of the tail in this case caused well over 12 G's of force
on the plane- and the pilot - making it virtually impossible for him
to correct and control the plane - particularly when that close to
the ground. The probability is VERY high that the 12 Gs of force
caused the (average)20 lb human head to weigh 240 lbs plus -
instantly snapping the pilot's neck in the process.

Anyone who doubts the effect of a trim tab at speed has never looked
seriously at aerodynamics or the flight characteristics of an
airplane. (and has likely never been at the controls of an airplane)

That was pretty good.
It didn't support "The elevator - and the trim tab - do a LOT of work on
a race plane at 500mph" one bit, except for the unfortunate case when
the tab comes off. If that happens, it does do a LOT of work.