On 9/24/2011 6:01 AM, Lloyd E. Sponenburgh wrote:
fired this volley in
news
And note, the photos DO show the trim tab failure.

You don't read posts before you answer them, do you?

Check back on what I said. I realize the trim tab failed. I can also see
the mode of failure. Duh.

Lloyd

Yes Lloyd, I read it.

If we are going to be huffy maybe I could reply that you don't know much
about aerodynamics and didn't bother to find out before you posted.

But I generally don't go into that kind of reply.
IT just leads to a ****ing match and a lot of hard feelings.

So, instead, let me offer the following...
Understand that this is very simplified approach to calculations of
aerodynamic forces, but it valid in subsonic flight.

Equation:
Lift = .001188 * (Coefficient of Lift) * (Velocity Squared) *
(Wing Area)

.001188 constant allows:
Sea level density altitude
Velocity in MPH
Wing Area in Square feet

And, of course, the equation can be rearranged to solve for any of the
included terms.

So...

Take off performance:
(given)
Stall Speed = 100 MPH
Wing Area = 235 SqFt

CL calculates to 1.66

Max Speed performance:
(given)
Velocity = 500 mph
Wing Area 235 sq ft

CL calculates to .0066 (!)

If, at high speed, the nose pitched up to the take-off angle of attack
(thus providing the take-off Coefficient of Lift)
(given)
Velocity = 500 mph
Wing Area = 235 sq ft

Lift calculates to 249,340 pounds.
For the 10,000 pound weight that means 24 Gs possible load factor.

Like I said earlier, the forces are tremendous!
The thing that modulates the wing's lift is the tail.
The long arm from wing to tail allows smaller forces to control the
pitch of the wing.

The trim tab in question is located at the very aft edge of the control
surface (elevator, aileron, whatever). The distance between the hinge
points (elevator hinge and trim tab hinge) define the trim arm.
That's how the small trim tab can deflect the much larger elevator, and
the elevator control the pitch of the much larger wing.

Now, yes, there are other approaches that can be taken.
Tailless (flying wing), tandem wing, and canards.
But they all have to face the same issues.

Tailless types have very short tail arms and are thus very limited in
their pitch authority. That usually means higher take off speeds.

Canards are kind of in the same category. The forward control surface
(the canard?) is designed to stall before the wing stalls. Has to be
this way to avoid the serious problems of a deep stall - where the wing
stalls before the canard - with an uncontrollable pitch up resulting.
Again that usually means higher take off speeds.

So the design of modern aircraft has evolved to the aft-tail arrangement
because it offers the widest range of performance.

Conclusion:
Directly related to the question at hand, this configuration also
offers higher speed potential since it can provide higher pitch down
forces at high speed.

I hope that perhaps this helps illustrate the "why" behind "this is how
it's done".