Acro Image

Aerobatics Server

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

[International Aerobatic Club] [Communications] [Aerobatics Images]

Disclaimer: These aerobatics pages are developed by individual IAC members and do not represent official IAC policy or opinion.

[Usage Statistics]

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


Thread: [Acro] Re: Sealed my Gap...

Message: [Acro] Re: Sealed my Gap...

Follow-Up To: ACRO Email list (for List Members only)

From: klick at

Date: Mon, 08 Jul 2002 12:34:48 UTC



Good information, and very nicely written too.

Jim KLick
S1S.N9JT (with taped gaps)

>"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


© Dr. Günther Eichhorn
Email Guenther Eichhorn