Thorp Air Command - T18.net

As Bob Highley suggested in the FB group, I am finally starting a project thread here on .NET! I have been lurking this forum and the FB group for a while now but let me introduce myself. I'm Pete Geraci and I've been a mechanic for 25 years, starting in the Air Force, a few years at a GA flight school and then corporate jets for the last 18 where I've mostly specialized in avionics. I earned my private certificate in 2005 and have flown when I can although not nearly as much as I would like! That said, 2020 I was ready to start a father/sons project building an RV-14(boo I know!). Like many people, COVID changed a few plans and I decided that might have been a bad time for a major project. Talking with one of my friends at work, he told me the tale of a Thorp, how he came to own it, what his plans were, and how life events had taken his ability to really devote the time to it.

So, What started as helping a friend shed the cost of storing it, ended up with it occupying my garage for the last 2 years. As best as I can understand, This airframe was build by James Dewberry of Alabama and received its first CofA in 1972. According to the dataplate, it first flew with an O-290 and spent more time getting modified than flown. According to the airframe log it took nearly 21 years, 1993, to reach its first 40 hours! It then flew quite a bit for a while but was disassembled for storage in January of 2002 with 256 hours. The logbook is quiet for 11 years when suddenly it was fitted with a Lycoming O-360, went back to phase 1 for a few hours, and the last airframe logbook entry is the end of March 2012, when at 261.3 hours, it was returned to phase 2.

The owner at the time had trouble with ground handling and ran off the side of a taxi way (I'll go into my theory on that another day, no spoilers FB!). June of 2012, he removed the damaged Hartzell CS prop/governor, dialed the crank(.002), and installed a Sterba FP wooden propeller.

My friend traded his Cessna 152 for this, looking forward to the "Thorp Experience"! Unfortunately, a fuel leak developed in the custom built fiberglass fuel tank. He purchased the plans at that time with the intent of fabricating the fuel tank as drawn to install it and quickly discovered that the original builder had gone off the map with a custom tank, mounts, firewall supports, a set of modified Cessna rudder pedals and brake cylinders. The fiberglass tank was also smaller (I think 15 gallons), allowing for a full depth center avionics stack after the panel was relocated aft approximately 5 inches. So what started as going back to a plans built tank has tsunami'ed.

I was initially asked to evaluate it, see if it was even worth saving. The sad reality is, if the guys working for me weren't cheap, it would easily be beyond economical repair. For that fact, it would probably be easier and cheaper to buy the kit from Cubes and build! But, it landed in the lap of a nostalgic coot that has a knack for breathing new life into old stuff.

The tach, covered in dust sitting in a box of radios and gauges, reads 270.3 hours. The wings hang in racks from my garage ceiling, the tail feathers on the back wall. The engine found my homemade stand and the fuselage is casually hanging off of both ends of EAA 1000 inspired workbenches. A care package from Cubes showed up not so long ago with a new firewall and some misc. parts. So the question is, how good is an avionics guy at sheet metal!

The thread that got me started was things I found that were not in the drawings and I noticed were different from other T-18's out there. The first clue was the internal horizontal stabilizer balance weight. as drawn, is straight with the weight hanging in between the arms. Mine, has spacers where the weight should be, the weight hanging from the arms, and then the arms themselves are bent and reinforced. Looking at the current travel of the stab, the arms barely clear the upper skin leading me to believe this was modified to eliminate contact. The red markings on the angles are similar to others observed in the airframe so it would appear this was done at build.




The seat supports seem to be correctly located but the backs are extremely thin. The control sticks were bent and welded to move the grip approximately 4.5 inches aft of center as opposed to the straight sticks I've seen in most cockpit pictures. Shown is the removable copilots stick.


I also found shims nearly 3/8's thick that moved the wing aft. I haven't found any spacers for the aft spar fittings but according to some measurements I took yesterday, I would have to have them to mate the wing with the main spar shims installed.



Additionally, the instrument panel was moved approximately 4.5-5 inches aft which Bob Highley told me was a pretty common mod. This one however, had a full IFR stack with vacuum gyros and the most recent weighing showed 1363lbs @ 63.24. My theory is once a modern American and a sack of pilot stuff was thrown in the back that it may have created an aft CG condition. Locating the wing aft may have offset the flight characteristics a bit but I was curious how close I am to guessing an aft CG loading could make ground handling a little sporty. (I have no tailwheel time BTW so feel free to correct me)

Has anyone else seen anything like this or have any carnal knowledge of this airframe? Thanks again! -Pete

There are 2 items of concern on the 'bob weight'. The weight itself should be sandwiched between the angle stock. AND the part should be straight - not 'adjusted'. Both point to the collar on the stab being installed at the wrong angle. I've seen that on another Thorp I was involved with on a much smaller scale . The result of this mod will not allow you to get the full and proper angle of up and down horizontal stab movement . That also negatively affects aircraft handing.
- With regards to the front spar shims....were the rear spars shimmed as well?

Keep posting and asking questions - We'll get it correct!

I think it’s worth saving!!
But I would return it to plans configuration. I’m not an engineer, but I can tell you that unintended consequences often happen with even minor mods. Those are not minor mods!
Terry is right, the 510 might have been installed at the wrong angle. Not sure how to fix that.
I would move the wing back to the normal position. With a fixed pitch prop, you should be right in the sweet spot of the CG.
Cubes

I'll have to check the angle of the 510 arm, it may be what it is. I'm changing the trim to electric so once I get to re mating the stab I'll check the throws to make sure I'm getting good deflection. Hopefully it was remedy for an Oops that worked out. re orienting the 510 doesnt look like an easy thing to do but I'll address that when I get there.

I haven't found any shims for the aft spar but I measured the wing and compared it to the fuse and there would have to be something there to take up the slack. Hopefully that doesn't mess with the flap slot alignment but thats a future me problem for now!

I'm definitely going to try and get this as close to the plans as I can. The firewall change came into play once I realized the original fuel tank supports were installed several inches outboard and the number of holes punched in it would probably divert the fire into the fuel tank! Another oddity was the angle that held the fire wall to the lower structural brace was on top of the brace instead of the bottom where the fuel tank mounts were solely supported by the firewall and not riveted to the brace. Thankfully I was able to unbolt the brace and flip it with out having to remove and re-rivet the angle on.


good news is initial fitting of the new one is going well!

Awesome! You do quick work.
Cubes

A quick check of your stabilator and the placement of the 510-1 bracket may tell the tale. The 510 is to be installed 14 degrees 2 minutes slanted up from the horizontal axis of the stab. A not so uncommon mistake of the early builders was to install it at right angles to the stab. Many early builders were building ahead of John Thorp's final release of drawings. Several were being built in the Atlanta area and even had round fuselages. The fatal accident that resulted in the redesign of the horizontal tail was caused the bracket installation mistake and subsequent fix. The builder drilled out the rivets on 510-1 and correctly positioned it and redrilled the holes and re-riveted it. He essentially perforated the tube spar. A sharp pull up after a highspeed pass was all it took to fracture the spar. The spar doubler, redistributed bobweights, and the oh so important stainless steel straps on the tab were a result of this accident.

If you make your way to Sun 'n Fun this year, I would be glad to meet up with you and show you my finished plane and the one I have under construction. I am based in Lakeland Florida.

_________________
Bob Highley
N711SH
SN 835
KLAL