Bertie the Bunyip wrote:
nospam wrote in
news:bYydndxV96btLr_VnZ2dnUVZ_vCdnZ2d@internode:

Bertie the Bunyip wrote:
nospam wrote in
newsbmdnXqirNejAr_VnZ2dnUVZ_sednZ2d@internode:

wrote:
On May 5, 5:55 pm, WingFlaps wrote:

Does the elevator lift force and stall angle reflect trim setting
at
all?
Cheers
Probably to some rather minor degree. The government just demands
that the airplane behave in certain ways in various configurations
and
maneuvers, so the designers have to build their airplanes to fit
within those specs. An elevator should never stall before the wing,
for example, or the whole machine could flip over onto its back.
The
rising tail, rising because the stab/elevator stalled, would
experience an even higher AOA as it rose and things would get very
nasty. The certification guys want the nose to drop gently as the
wing
stalls, which couldn't happen if the stab let go too soon. Some
airplanes (I.E. Ercoupe) had limited up-elevator to prevent wing
stall
and therefore the stall/spin scenario that killed so many in the
'40s
and '50s. The nose didn't drop because the wing stalled but because
the stab/elevator ran out of nose-up authority. It could easily
have
been modified to get the stall. There was plenty of area there.
Only
problem was that guys would get slow on final and pancake into the
ground and break their backs with compression fractures. Don't
necessarily need to stall to get killed.
The Cessna Cardinal had a problem early on with the
stabilator
stalling in the landing flare and smashing the nosewheel on pretty
hard, and they fixed that with a slot in the leading edge of the
stabilator. IIRC the ground effect had something to do with the
stab
stall problem. I never had any such thing happen at altitude in the
'68 (non-slotted) Cardinals.

Dan
Usually, in conventional aircraft, the tailplane force is a
download.
When this download is suddenly reduced, as in a tailplane stall,
there
is a sudden and probably fairly violent nose down pitch. How you
determine whether it is an elevator stall, or tailplane stall,
without
special instrumentation, is beyond me.
Cheers

You can't, and the reason you can't is because it's all one unit.
There's no difference because you can't seperate their functions.

Bertie
Well, even without instrumentation, one can determine if the elevator
power is sufficient to do a landing flare at say 1.3 Vs minus 5kts at
forward CG. Increasing elevator area may be one method of increasing
elevator power. Also you cannot treat the elevator and tailplane as
one
unit where elevator hinge moments are needed to be of a particular
(algebraic)sign ie stick free longitudinal static stability
measurement.
Cheers



Sure you can, one without the other is notreally much of anything. they
work together.

Bertie
They only work "together", as you put, after a lot of careful
engineering of the individual components and the interaction between
them. Even then, testing often shows that further refinements are
necessary.

Take, for example 4 tails all of the same planform and aerofoil section.

1. An all flying tail hydraulically operated.
The pivot point can be almost anywhere, hinge moments don't matter much
if sufficient hydraulic power is available. No tabs are required and
control feel can be as simple as a set of springs.

2. An all flying tail manually operated.
The pivot point position has to be placed to achieve correct control
feel throughout the tail range of motion at all angles of attack the
tail will "see" in service. A anti-balance tab will be required - this
will affect the tail lift curve. A trim tab will be required, depending
on the aerodynamic problems this may or may not be incorporated in the
anti-balance tab operation.

3. A fixed tail with an elevator.
The hinge positions can be comparatively easily calculated to achieve
the correct hinge moments for feel and stick fixed stability. To have
the same power as the two above more area is required. A trim tab is
required and an elevator down spring may be necessary to achieve the
same stable CG range as the above 2.

4 A fixed tail with an elevator which requires a geared balance tab to
either increase or decrease elevator hinge moments and therefore control
feel.

Similar to above but will be more or less powerful depending on the
direction of operation of the geared balance tab.


Sure it all works together but has to be designed to do it.

For those who were wondering about tab effect, or indeed elevator effect
on total tail lift the following may help;
Each item can be considered separately.
There will be a basic tail camber lift component which in many cases is
zero.
Then find the tailplane AOA and from the lift curve slope find tail Cl -
put that into the normal lift equation.
At that particular tailplane AOA, select the elevator AOA and again find
the Cl from the lift curve.
Then do the same for the tab.
Add the 3 solutions to get total tail lift.
Do this for the complete range of angle of attack for each component and
you will know the total range (and direction) of tail lift.
Cheers