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ACRO E-mail Archive Thread: [Acro] Sealed my Gap...



                


Thread: [Acro] Sealed my Gap...

Message: [Acro] Sealed my Gap...

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From: Larry Lowe <webmaster at airspacemag.com>

Date: Fri, 05 Jul 2002 19:32:49 UTC


Message:

  

"Robert E. Fry" wrote:
> 
> Do you know if anyone has played with gap seals on 
> the rudder and elevators of  a Super Decathlon?
> Make a useful difference? 
> What was used and where to get it?

There is no question that sealing the gap in any conventional steel tube
control surface will increase its effectiveness and reduce the stick
forces, sometimes more than you might actually like.

Back when I had more hair (on my head), I owned and operated a
morphodite 180 HP Clipped Wing Cub with T-craft wings known as the
Dutton-Lacy Special, a concoction of a variety of airplane parts,
including the elevator/stab assembly from a Tri-pacer on a shortened,
single seat fuselage.

The tail group was prolly closer to the downwash from the wing than the
original designer might have liked, and despite a long, fat tube of a
control handle, elevator pressures seemed high.  

Or at least so I thought after I was given a chance to fly a Buecker
Jungmiester--still the finest handling aerobatic mount ever produced.

Ramp legend at the time had it that if you sealed the gap between the
elevator and stab, the stick forces would go down, so I got out a roll
of 3 inch wide, 200 mph certified Duct Tape, tore off enough lengths to
cover the distance between all the hinges on the stab/elevator joint
twice and set to work.  The first step was to overlap a pair of equal
length strips of tape about an inch, face to face so the adhesive melded
them into a single length with a couple inches sticky on one side and a
couple inches sticky on the other.  Then the length was fished down into
the appropriate slot and one side was pressed down onto the top of the
stab while the other was pressed onto the bottom of the elevator,
creating a slant that covered the gap.

A little ugly, perhaps, but I was just testing to see if the rumor was
true.  Plus which the rest of the plane was fairly ugly to begin with.

After copiously sealing every inch of the elevator/stab gap, I crawled
in prevailed upon one of my airport colleagues to pull the prop through
to give me an engine start.  (Anyone remember how to prop an engine?)

As was my habit, I reached down and ground the 1949 Hudson window crank
that had been pressed into service as a trim tab crank all the way to
the stop and then backed it off to what I had come to expect was takeoff
trim--there was no trim indicator.

With the curious crowd from the gas shack looking on, I twirled around
in a circle in the run-up area to make sure no one was trying to land
where I was about to take off, lined up with the runway and launched.

The 180 hp shot me off the runway like it usually did, and (in the only
display of proper common sense of the day) I just let the airplane fly
off and climb out.  Moments later I was through 1500 AGL feet and--for
whatever reason--elected to use the trip to level out, cranking slowly
on the former window crank until the nose settled in the right position
below the horizon, the airspeed wound up to about 165 and the altimeter
said we were no longer climbing.

Out of the pattern but not sight of the field, it was time to execute my
well planned flight test program.

I thought, off hand, that a 45 climb line might be a good enough place
to start, so I checked the horizon on the wingtip and gave the stick a
crisp yank.

I got a sudden and unforgettable lesson about the effectiveness of a
fully sealed elevator gap on an oversized tail on a short coupled
airplane cruising at full throttle.

The G came on so suddenly my arm was weighted down and the stick came
back more than I wanted and the nose popped up off the horizon like a
manhole cover after a gas explosion.

As my socks rolled down and the sweat ran into my eyes, I realized this
was way too much pitch rate and mustered enough muscle power to
overpower the G and ram the stick forward past neutral.

Which gave me instantaneously the same, but opposite effect, about 4
negative G where the positive had been, forcing my arm and the stick to
get stuck well forward of neutral.  Dust, dirt, straps and my socks all
made a vivid lunge for the plexiglass ceiling while the horizon went
through the windshield in an upward blur.

Instinctively, I reacted with an adrenaline charged pull on the stick,
which overcame the neg G and popped it back in my lamp.

Everything on the ceiling hit the floor and the horizon went through the
windshield in a downward blur.

For the next several minutes, my airshow special looked like a
patriotically painted bullfrog, lunging up and down through the sky,
essaying a abrupt vertical S curve along the line of general level
flight while I hung grimly onto the tube I use to get extra leverage for
outsides, alternately pushing and pulling against the forces generated
by the absurd pitching, and overcoming historesis only to get locked too
far in the other pitch direction.

No amount of conscious management of pressures on the stick could settle
things down, as I was in a pilot induced oscillation that was magnified
by the G delivered by the almost pressureless control stick and a big
tail surface bobbing in and out of a lot of down wash.

Finally, I accidentally did the right thing when I let go of the control
stick to prepare to jettison the door in case I had to jump out.

After a brief settling down, the Cub resumed level flight, blissfully
devoid of any control input from the pilot's seat.

Very tenuous fingertip pressures on the stick confirmed that it was WAY
too sensitive to use in aerobatics, but that--given you did not really
try to use it--could be induced to control pitch.

A much better option seemed to be the trusty 1949 Hudson window crank
and I only used the control stick to provide roll input while I flew a
long, hot approach to a wheel landing using mostly the elevator trim.

After a certain amount of somewhat more responsible investigation into
the issue of elevator gap seal effectiveness, it was determined that
sealing the gap from the airframe about half way out the hinge, leaving
the outer half of the gap unsealed, provided some approximation of the
Jungmiester elevator and allowed me to maintain control of the airplane
during abrupt pitch excursions.

I therefore recommend that anyone considering sealing the gaps on their
acro mounts proceed with caution, perhaps doing so in stages and
examining the results only once well above ground.

My two cents worth...

Larry Lowe


                


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