"Peter Stickney" wrote in message
...
In article ,
(Eunometic) writes:
In discusing the characteristics of how the Me 109K should be
flown
against the P51 Mustang and P47 the issue of control reversability
came up. Would someone be able to expand on control
reversability.

The Me 109 G10 and Me 109 K4 (G14 was a stopgap due to engine
delays
in the G10)had a powerfull engine that allowed them to do a speed
of
458mph and outclimbe all allied aircraft. However the old crate
had
an old wing section that created enormous aileron forces for the
pilot; also becuase the the small Me 109 cockpit a pilot could
generate only 40lbs of joystick force could have generated 60lbs
of
force in a P51. As a result only 2-3 degree of airleron
deflection
was possible at 400mph the 109 had a roll rate of 45
degrees/second.
A FW190A and even a P47 could have managed nearly 180 degrees in
that
time.

That's only part of the story. Stick forces are a result of
balancing
aerodynamic forces on the control surfaces, force feedback (feel) to
the pilot, and the ingenuity of whoever designed the control
system. (Oh, yeah, and the ability of the airplane to stay in rig -
Folland Gnats were notoriously poor in that regard).


It's possible to have a control system that will give small forces
at
high IAS - but the lack of feedback at low speeds leads to
overcontrolling at best, and Pilot Induces Oscillation at worst.
And then there's flutter to worry about, as well.

There ways to mitigate this. The F4U COrsair and F6F-5 Hellcat used
balance tabs and servo tabs on the ailerons to add aerodynamic force
to help control deflection at high speeds. The A6M Zero used a
setup
where the gearing in the aileron system changed when the landing
gear
was extended. But these systems come at a cost of complexity and
have
less tolerance of manufacturing imperfections.

The issue of control reversability then came up. If power
ailerons
were fited to the Me 109 they would have allowed a greater
deflection
but would this have caused control reversability at some point as
the
wing twisted and the ailerons acted more like trim tabs?

That's another issue. When you're talking about "reversal" in these
contexts, you have to be very careful. There's Control Force
Reversal, where teh feel of the stick in your hand changes while you
hold the controls in a fixed position,

Are you saying that in some cases that the stik can be, for instance
in the extreme left, and the stick forces could be pulling the stick
further to the left?



and the Aeroelasticity related
reversal, where you're actually bending the airplane at high speeds.
I suspect that the 109 would be extremely poor, in aeroelastic
terms.
The torsional frequency (stiffness, if you will) of the wing was so
poor that it wouldn't have been acceptable in either the USAAF or
teh
RAF (In fact, the P-47 nearly didn't get accepted for that reason,
until they measured a captured 109 and found that it was about 60%
worse.)

That would be understandable: the Bf 109 was probably the first modern
all metal enclosed cockpit fighter evern built (in 1935) and it was
built to opperate in the 300mph range at most.) It stayed
competitive because of the philosophy of the small size and light
weight allowed it to overcome its dated aerodynamic inefficiency
through high power to weight ratio.



What causes reversability? Why is a slab elevator sometimes
used?

Stabilators (slab tails) are one of those tradeoff things. What
elevators do is change the lift component of the horizontal tail
(THe
same, of course, goes for rudders & fins, or ailerons & wings). At
subsonic speeds, the elevator acts to change the flow feild over the
entire surface. As teh flow over teh stabilizer becomes transonic,
adn later, supersonic, the elevator only affects the flow over
itself.
This reduces pitch control greatly, and can lead to the tail being
not
able to properly balance the wing's pitching moment. (Nose tuck or
pitch up). A slab tail doesn't have that problem. You're moving
the
entire surface. The drawbacks are that its not as effective at
subsonic speeds, and it's a heavy surface, both in structure weight
and the aerodynamic load on it.

It overcomes shockwave impingement as well I believe. Delta wings
seem to fly OK.


I've also heard of WW2 pilots using trim tabs to pull out of a
dive or
get an aircraft rightway up. What were they doing?

A trim tab is a small aerodynamic surface that acts to deflect the
control system that it's attached to to "Zero out" the force felt on
the stick at a particular IAS. (It can also add a force component as
well - when flying formation, I liked to put in just a bit of
nose-down trim so tha it took a small amount of back pressure to
stay
level. It made things feel a wee bit tighter & more responsive)
Remember that an airplane in flight is a balancing act. In the
pitch
axis, for example, the stabilizer-elevator system is balancing the
pitching moment of the airfoil in the wing. This moment depends on
IAS, AoA, and the location of the CG. The trim tab allow the
zero-force postion of teh control system to be set so that it
balances
that pitching moment at some particular combination of those
factors.
(In other words, if you trim the airplane to fly in a condition
corresponding to 200 mph IAS, it'll want to maintain that condition.
Add power (thrust), and as the airplane accelerates, it'll try to
pitch up, to restore the balance of forces.

Note that among those nations in WW 2 who had made a serious study
of
the transonic behavior of their fighters, (The Germans, BTW, aren't
among them. It's a curious blind spot - they certinly put a lot of
effort into supersonic aerodynamics, but failed to explore the high
speed behavior of their exixting airplanes. Their solution was to
pring a Big Red "Thou Shalt Not" in the Pilot's Handbook and
continue
on) using the trim tabs to help recover from a transonic dive was
very
vigorously discouraged.

As Early as 1940 Messerschmidt was considering a 37 or 45 degree sweep
for the Me262 based on swept wing research. They settled on a
straight wing (conservative I suspect) but when the engines turned
out bigger than expected the expedient of a slight sweep to correct
the center of gravity problems seemed natural. The HG I,II,III
series of research study/versions was to return the high sweep angle.

Clearly of they thought swept wings was the way to go then expending
effort on their dated conventional fighters would be wasted. Erhardt
Milch was targeting the entry into service date for the Me 262 as
middle of 1943.

This was becasue the ineffectiveness of the
elevators in pitch had nothing at all to do with the high forces
involved - it was the small amount of flow affected by a deflected
elevator that was the problem. If you tried to trim out of the
dive,
you'd over-G the airplane as you descended into the warmer air at
lower altitudes, and the elevators became effective, and the trim
tabs
made them pitch the nose up. The best transonic dive solution was
the
inclusion Dive Recover Flaps, which were small surfaces on the
unserside of the wing that would induce a nose-up pitching moment.
They weren't Dive Brakes or Speed Brakes, but they did pitch the
nose
up.

Which allied aircraft had these Dive Recovery Flaps? When were they
introduced?




The P38 had a smaller turning circle than the Me 109 (presumably
at
lower speeds of around 300mph) but its roll rate was even worse
than
the 109 and this is how 109s escaped P38s and I note that some
late
war P38s received power controls.

Well, stuff like wing loading & Clmax giving a superior turn rate
occurs at low speeds, when you're going to run into aerodynimic
limitations, rather than structural ones. So when you're talking a
smaller turning circle, it definitely will be at low speeds. (Of
course, this is affected by things like control forces - not only
would a 109 not turn inside a P-38 at 350 IAS, but its high stick
force/G at those speeds makes it very hard to crank it in tight.
Early P-38s did have somewhat lower roll rates - and that isn't too
surprising. Not only were you trying to roll a 52 ft span wing vs.
a
32 ft span, but you had the extra mass of the engines & other such
stuff to accelerate. Hmm... sfx - pages being flipped diffing
through my reports of test data, the early P-38 wasn't that bad in
roll, with a peak roll rate of about 80 degrees/sec at 300 mph IAS.
Roll acceleration wasn't that great, but isn't quite as much of a
factor.
Roll performance dropped off above that speed, becasue of high
control
forces. The powered Ailerons woldn't help at low speeds - full
control deflection was available up to 300 IAS, in a typical case,
but
would be a big help at high speeds.

--
Pete Stickney
A strong conviction that something must be done is the parent of
many
bad measures. -- Daniel Webster