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.
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 aps.anl.gov
Date: Mon, 08 Jul 2002 12:34:48 UTC
> >arry, 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 >