Thorp Air Command - T18.net

How light can a T-18 be built?

Most weights I've seen are over 900 lb. A web search found a claim of 701 lb for one, but it didn't indicate engine, etc.

I'm not looking for a comfy, cross country, IFR airplane. Something interesting for my son and I to build, that when done can provide a fun, day VFR flight is all I need. Let's say: a low compression O-235 without an electrical system but with a metal prop, no interior beyond seats and their cushions, single controls, truly minimalist panel (no electronics), no heater, and a 20 gal tank. In other words, fit out about as extravagantly as an early Piper Cub or Ercoupe. Judging by the CAFE numbers, such a plane might still make 150 mph, a cruise speed I've found adequate for my needs in rental planes. Combined with the T-18s reputation for fine handling, I think it would still put a smile on my face.

I'm not saying I'd build so spartan a plane, but how much do you think it would weigh?

Karl

There was a guy that flew out of Fallbrook, CA that built a bare bones T-18 . No interior other than seats and I think those were made out of straw (Boo YA !) ! Only a couple of instruments and used a clear hose as a fuel gauge . I can not remember his name ? Maybe Jon Levi can remember . If the aircraft were any lighter you would have to tie it down inside the hangar ! Pretty sure there are pics of the AC on the T-18 site . Stretch Balco comes to senior mind ?

RB

This is fun. My guess is 850#. I had an unpainted T-18 with an 0-290 with wood prop and a panel full of steam gauges and old heavy radios, Kentwood speakers, etc, etc. I also had a full interior with fabric covered side panels and a beautifully crafted panel. That plane weighed in at about 950#. Maybe with a bare interior and other attention to detail, my guess of 850# is feasible. I also have wing tanks. Not sure how much weight that adds to my plane....

Here is a pic of the old panel.

I think 850# would be "heavy" for Mark (Stretch) Batchelor. He & a buddy built very light T-18s with "Mouse Motor" O-290s I believe. One had only a carbon fiber "half" panel. I'm thinking that they were in the 700 to 800 pound range, but I might be dreamin'. I'm feeling pretty stupid right now, that I can't remember all the specifics... wish that Stretch was still active on this forum so that he could chime in.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Can or would the CG work out with an O-235 without extending the engine mounts a bit.

The original I believe was designed for no smaller engine than the O-290. I am not sure what the weight difference is between an O-235 and an O-290.

Has anyone ever built a thorp with a smaller engine than an O-290?

The American way is primarily is bigger and more HP, not smaller and less HP.

Today's glass or even ipad type electronic instruments I think can save a LOT of weight and show you as much if not more information as the traditional steam guages. Or at least that's my novice opinion.

Mine is pretty basic, but no super weight cutting "tricks". It has an O-360 & metal prop. The book claims 880lb.

O235 & O290 share case and crank so you are giving up a lot of CI's for not much of a weight advantage at all.

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http://www.dixiefriedfabrication.com/

I tossed in the O-235 because it was the smallest engine Thorp himself mentioned in the early EAA articles on the design, in Part 12. He may have been thinking about the high compression version, though. Also, Thorp may have beefed up the design as time went on. His original 600 lb proposal picked up a lot of weight.

I doubt an O-235 weighs much less than an O-290, but I suspect parts are easier to come by --that's only an assumption on my part. From Lycoming's current data, the O-235 weighs 28 lb less than an O-320; however, the plane I tossed up for discussion has no electrical system, and I'm not sure I'd care to hand flip a 320. A wood prop would save a lot of weight, but I've read the metal ones flip easier, and their weight would help with the CG.

The CG is definitely a concern. Do the O-360 planes need tail ballast? I recall Thorp mentioned a CS prop would likely need ballast. Perhaps as the plans evolved, he adjusted the wing position assuming the bigger engines would be common.

For what it's worth, I'll hazard a guess that a T-18 can be built at 700 lb, maybe a smidge less. I base that on a mental tally of weights I've seen over the years for the Tailwind and the T-18. The Tailwinds are usually around 850 and the T-18s more like 900. The lightest Tailwinds are down around 650 lb, with one as low as 625 I think it was.

Has anyone weighed, as construction progressed, a T-18's airframe (on gear) before the panel and firewall-forward stuff was added?

Karl

Just some further thoughts I had:

Is it REALLY worth the weight savings to not have a light weight starter and small battery with the additional cost of insurance that come along with hand propping?

Do you REALLY want to hand prop your plane? Insurance costs assuming you will insure it go up as well because of the accidents that happen if breaks fail or tail is not tied down etc. I previously owned an Aeronca Chief hand propped. It was easy to do but the process to do safely is a pain by yourself espeically if you fly somewhere other than back to your home airport. And even with 70 years of history and training of people, EVERY single year it seems like one hand propped antique airplane wanders off without a pilot after hand propping by himself, and sometimes the damage is only to that airplane, other times unfortunately the pilotless plane taxi's by istelf into another plane or planes, hangar, car or building or God forbid a person. And the insurance rates take those liabilities into account.

I understand the weight and complexity savings of a fixed wood prop. In my case I'm leaning towards a ground adjustable Sensinich (I think composit) as I plan to fly out of a 2,200' strip so want good take off performance, but would like to have the flexibility to when going on a longer X-C flight to increase the pitch, take off light and land at a nearby airport with a longer runway and load up bags and passenger there and take advantage of a higher criuse speed for the XC trip. Then change the pitch back to "local" flying pitch when I return. I got the impression that the ground adjustable prop is lighter than even a metal fixed pitch but I could be wrong on that. I just don't want the cost and weight of a CS prop although it would be nice from a performance standpoint in my situation.

It seems like your "mission" that you identified for an airplane will be mainly day, VFR and local. I'm all for keeping things as light as possible as light = performance. I love the Thorp, but I wonder if there is a more appropriate airplane for the specific mission you seek. The Thorp is really meant to be a go some place kind of bird. Do you really need a 150 mph airplane to puddle jump locally? I wonder if something such as like the Wittman "Buttercup" might be something that would meet your mission AND be Light Sport Eligible to boot. Still relatively fast and could use the O-200 or O-235 engines for economy. But that is just my thought and like I said the Thorp is an awesome airplane. I'd love to see a "Thorp Bantam" and see how it performs.

I did mention the plane proposed was not necessarily the one I'd build. I think I'd go for the light starter and battery combo.

No, I have not convinced myself I want to build a T-18. I'm just asking a lot of questions at this point to see what the limits of the design are.

I'm actually leaning towards the older Zenith design as a flivver. My son and I have gone to their factory and built the rudder kit. I was surprised I rather liked working in sheet metal. My son (like many young men) has a desire to go fast, so I threw in the T-18 as a possibility. As you might read between my lines, I'm curious if can be made up more like the W-8 Tailwind instead of the faster W-10.

Back when I was younger and had a desire to go faster, I bought from Wittman the plans to the Tailwind; I still have them. However, I've tried twice my hand at welding classes and didn't like the results. Besides, my son seems to think aluminum is the way to go, and it is hard to argue the point.

Karl

Big difference between the Thorp and a Zenith! I've never been that impressed with any of their designs. The quality of their kits however is impressive.

_________________
Lee Walton
Houston, TX
N51863,N118LW
KEFD

Lee, I've never flown in a T-18, and if built to plans, I couldn't fit (headroom). I have flown in the Zenith factory's current 650B, and I agree with you: I wasn't favorably impressed. The control harmony is poor, with a stiff rudder, light aileron, and very light elevator --quite out of balance compared to the planes I've flown. Also, their new wing structure has me scratching my head.

However, the older design (the 601) is even simpler in structure than the T-18, and at 575 lb on a big wing, it doesn't take much engine, even a VW would work. I suppose one would get used to the unusual control balance.

As I mentioned, my son is hoping for a faster plane, so I'm giving some thought to the T-18. For an aluminum homebuilt, I can't think of anything in between. It doesn't appear either the Pazmany or Davis plans are still sold. (Correction: The Pazmany plans are there.) The RV-9 would be a great compromise, but I don't fit and they don't sell plans. So, I'm curious if I might stretch (compress?) the T-18 design towards some compromise between my and my son's desires.

Karl

What about a sonex? Aluminum and fairly fast for the HP.

But it sounds as if you are a big and/or tall guy which might really limit your options. Is your son large/tall as well?

Thanks for the suggestion.

We're both about 6'5", much of it in the torso. I've yet to sit in a kit plane either of us could fly in, and I've tried most of them at OSH over the years. A plans built plane which also has a source for the more difficult parts is the best we can get. We would have to increase cabin height (lower the seat or raise the cabin ceiling) in any we select. On some designs, this isn't a difficult change, eg. the Tailwind. The T-18 would be more difficult, but I think it's possible. I don't have my heart set on doing a T-18; for now I'm asking questions, and I haven't yet posted the headroom question.

The Sonex would be about like the T-18: a taller turtle deck and front windscreen are needed. The Sonex is designed for a high output VW or the Jabiru, and I'm not confident about either engine. The older Zenith doesn't need as much horsepower from a VW (much lower wing loading), yet it's stressed to also take an O-200. For us, I think it the better of the two.

Karl

I know it sounds like you really want to build. But have you considered buying a factory plane?

Perhaps a Stinson or maybe a trips we or even a piper colt. A tandem seated plane may also fit both of you better. If you found a colt of pacer or PA12 or something like that may serve your needs and allow both of you to fit. Maybe a Citabria or older Champion. Although all those may be slower than your son may prefer. Finding a fabric plane with the fabric on its last legs will give you a project and ownership of sweat equity and lower the buy in price. Once new fabric is done the value of the plane will rise significantly too.

The Buttercup seems physically larger than a tailwind and I think is about as fast as a W8. I know you don't like welding but maybe you could find a project that most of the welding is already done.

Even the S18 which is 2inches wider than the standard T18 fuselage is a tight fit for my wife and I as my wife is pretty broad shouldered. But that is my only negative. She is not a big flyer anyhow so I'm not sure how much XC longer flights we will be doing together anyhow. Otherwise it seems the thorp fits my needs very well.

I truly enjoyed the Stinson, but I had to fly it with my head cocked. It was the only option in the area for getting tailwheel time. Actually, a Warrior with the adjustable seat height is a decent fit for me, and it flies nicely enough. They are not very expensive. An old friend and I considered getting one together, but he backed out, and I wasn't flying enough hours to justify the fixed cost of owning one --better to rent.

Building a plane is more motivated by a desire to get my oldest son some experience working with tools and off the PC. He likes the idea and enjoyed the Zenith workshop. I haven't stressed to him the odds of finishing and flying a plane are low. The other benefit of building is getting to be your own mechanic.

Welding is not my forte, but I haven't given up on the Tailwind nor the Buttercup. I could always tack the fuselage together and pay a pro to do the finish welding. My son seems to like the aluminum; however, I plan to get some tubing scraps and we can try building up some trusses out of it, so he can see how that goes. After trying on many planes at OSH this year, he now understands that for our height, a tube and rag design has its pluses. In other words, the old man wasn't daft after all.

To answer your question from the other thread, we are in Peoria. We were up to Madison last fall, piano shopping. Farley does sublime work.

Karl