Thorp Air Command - T18.net

Greetings,

From the early Sport Aviation articles, the T-18 started as a 600 lb plane with no canopy, flaps, or electrical system and an O-290. Along the way, the plane became a 1000+ lb plane (CAFE test) with a big bubble canopy, an O-360 with CS prop, flaps, and an electrical system behind a big panel.

So, what plane is drawn in the plans available today?

Are you given different thicknesses to use for a given configuration? For example, I've read it's common to use thicker skins in some forward areas when using the big engines. I assume the original, full length turtle deck is long gone, replaced by the arrangement for the sliding bubble. I imagine no one today builds a T-18 light enough they'd want to go without flaps, so that portion of the original drawings must be out.

How about the CG shifts? An O-290 with a fixed prop and no electrical system has a much different CG than an O-360 with CS prop and old style electricals. Is this handled in the engine mount or via weights in the tail? Or was the plane's layout redone to accommodate a heavier engine as standard?

Enough questions for a first post,

Karl

Karl,
The first Thorp flew with an O360. The plans call for a light, strong airframe. A lot of us think we are better than the designer and beef up the plane needlessly. So, the plans I sell are S 18. There are some differences from the T18 plans that Richard Eklund sells. Most of the differences are in the wing design. The CG range has not changed.
Cubes

In the EAA articles, Thorp says the plane can be built with flush, pull, or brazier rivets. Do the plans still suggest this, perhaps offering different rivet spacings? As I recall, the planes I've seen at OSH have flush rivets.

Cubes, your availability of Thorp parts is one reason my son and I are considering the Thorp, even though I was hoping for a slower airplane.

Karl

I have only seen one T-18 that was built of pulled/round head rivets (at least on the exterior skin) and IMHO it looked really bad ! I used a lot of flush Cherry Max rivets on my AC mainly do to the lack of a helper to hold a bucking bar . I would not mess with the rivet spacing called for in the plans , in fact one of the mods is to ADD rivets to the nose ribs on the wing .

RB

Nor would I! Some forms of rivets aren't as strong as others. If the plans allowed for optional rivets, it might tell you to use a different spacing (for the weaker ones). I was just curious, after reading Thorp's early articles.

The plans only specify one type of rivet per installation. They vary per location. Many builders change to cherry max in some places for ease of construction.
Cubes

Rivets are a huge concern for me because I likely will not have a helper at least the majority of the time. I wanted to ask all of you with experience how much of the plane can I actually get riveted? I don't want to be stuck because my nephew is on the road welding. I love the Thorp, it is the first aircraft that I ever went up in. I hope the rivets wouldn't be what stops me from building what I want. Thanks for any input you guys can give.

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James G.

I did 98% of the riveting on my aircraft by myself . A good portion of the riveting was done with Cherry Max rivets which I got off of e-bay . They were flush Cherry Max and installed using a pneumatic rivet puller and a hand held manual rivet puller (the one with the rotating head ). I also used a pneumatic rivet squeezer on most all of the edge solid AN rivets . It did a beautiful job setting the shop end of the rivet . You can also use a hand held manual AN rivet squeezer . The Cherry Max rivets I got off e-bay were bought in the standard 100 piece factory packaging . Not sure what they cost now , but when I used them they were about 75% less than Aircraft Spruce . Just make sure they are Cherry Max and NOT some off the shelf "soft" aluminum rivets ! Other easy to access AN rivet locations were done with a standard rivet gun/bucking bar .

RB
NX115RX

Hello Rich,
That is very encouraging, I was wondering if a person could get it done using other methods. I figured that there had to be a quality, safe way to get it done. I still have alot of studying to do. I am trying to find some good books on the subject of aircraft building. Thank you for your time and for pointing in the right direction.

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James G.

I highly recommend you take the EAA Sport Air workshop class "Sheet Metal Basics" . Well worth the 2 day class . I took the class and it gave me the basic knowledge on how to rivet and work with sheet metal (aluminum) . BING , BANG , BOOM !

RB

Like Rich, I built my T-18 with the majority of solid rivets by myself. You can plan the order of assembly to make it easiest, and leave some occasions to get help where it's difficult to reach. Maybe just a few weekends including BBQ or something. There are certain cases most of us have had to use pop rivets, but only judiciously, with good forethought. I used aluminum pop rivets and narrow aluminum strips to secure rubber baffling around my engine against the cowling. That's not a structural element. Push out the mandrel, and they're easy to drill out when it comes time to replace the rubber. A few pop's were used at the rib's trailing edge of my horz stab. So narrow there, a bucking bar wouldn't fit.

If you need to use a pop rivet here are some considerations:

1. Rivets are intended for shear applications, not tensile loads. There is some amount of clamping effect, but don't depend on it. The advantage of solid rivets is that they swell to fill their hole. Some amount of clearance is necessary to first insert any kind of rivet to start with. Since pop rivets don't swell, there's a small residual amount of slip that may occur in shear. It's possible that only one pop rivet will fully line up at a time to resist the load in a series, and it could fail leaving the next to fail, creating a zipper effect. In a group or a row of rivets, the load can be shared by solid rivets because they all swell, leaving no amount of slippage. Not so with pop rivets, so higher ratings are prudent if you make the choice.

2. Dimpled sheets have some inherent shear strength due to the locking effect, so flush riveted joints are somewhat stronger.

3. Be certain what materials are involved. Instead of the hardware variety, search on-line at Grainger's or other where you can see what the spec's are. Solid rivets per plans are 2117 aluminum with the -AD suffix (that's important). Correctly driven, they possess 26,000 psi shear strength. Our typical 1/8" dia. solid rivet is good to about 350 lbs. in shear. Use this number to compare others types when reading the charts. Often, Cherry Max rivets are a good choice. I think some pop rivets are only good for half that value, but the application drives the requirement whether ok to use. It's not always obvious what the design engineer had in mind. Steel or Monel steel base is better than soft aluminum pop's. Steel is better than aluminum mandrels. Leave the mandrel in for better shear strength. Punch it out if you have to drill one away. Nevertheless, be mindful of consideration 1., above.

4. Manufacturers make washers to use for some varieties. As any washer does, these distributes the force on the back side of the joint as the shank is distorted during squeezing 'the pop out of it.' This helps make a good clean joint.

Hope this helps. I'm sure you can do the vast majority of a T-18 yourself with solid rivets. With good planning, only a few weekends with another hand may be all you need. Personally, I wouldn't attempt an all pop-riveted project. Best to stick to the drawings, they're really good. Around for quite a while now, the T-18 is a well proven design... Incidentally, it's not a low and slow airplane. Tony Bingelis' books are good too. If you come across any particular location on the T-18 that's troublesome, just ask the Forum. These guys may have a few differing opinions, but they're all good resources. These guys are great and reading all their articles, both old and new, helped me a lot.

Les Krumel
Albuquerque

Sport Aviation Sept. 2011 . 27 years of Determination by Rich Brazell . Building my S-18 .

RB