Thorp Air Command - T18.net

John Thorp's series in Sport Aviation was done before the flaps were designed, and so there was never any article on building the flaps from scratch. I have a kit that was purchased from Richard Eklund many years ago. I am not satisfied with the fit of the LE of the skin to the ribs The radius is too sharp, and there is a gap between the nose of the ribs and the skin. If I try to "unbend" the radius, it probably won't work and in any case the pre-drilled holes would no longer line up. So I need to make new skins. I have been searching old newsletters and both forums, but so far I have only been able to find a contribution by Chris Fast on building flaps using pre-dimpled parts from Ken Knowles in newsletter #53. Actually, as I think about it, it might be easier if I also obtain new ribs without holes... Does anyone know if there is any info on building the flaps from scratch?

Also, is there any step-by-step info on installing the canopy? It really scares me of making a mistake in that expensive part by trimming too much or in the wrong place. How do you get the slope of the windshield to line up with the slope of the front of the canopy?

Thanks in advance for any ideas.

Bob

There's some good info on my project thread, page 3, about bending flap skins.

BOARD INDEX>>THORPT18>>PROJECTS>>c-FVBG AKA N886Y AKA MLM

viewtopic.php?f=36&t=7521&start=15

D

You don't line the windshield up with the canopy; you line the canopy up with the windshield by finding the right loft in the bubble and starting there to remove what is not "canopy" from the bubble. If it is any consolation, it took me five months to build the canopy frame and fit the windshield and canopy.

_________________
Bob Highley
N711SH
SN 835
KLAL

Flaps...pretty much used the wing building technique to get the leading edge radius . Don't rush the bend or you will "over bend the LE !" Canopy and windshield is a sticky wicket my friend . I started by fitting the WD frame and the canopy frame . Line up is very important as you want it to close tightly and have no leaks (I have no leaks and don't need the roll of Saran Wrap to keep the rain out !) One I had the frame pretty much where I wanted it I began the nail biting process of cutting of the canopy . Bob is right about cutting the "bubble" , but do so in stages . Remove enough to get a ruff fit with the WS frame . Once you remove it , it cannot be put back in place . I used a Dremmel cutoff tool with a THIN cutoff wheel to remove the bulk pieces and a Unibit to drill the holes after a very small pilot hole was drilled . Unibit leaves a very smooth hole with a de burred hole and chamfered edge . Use a slow speed with the Unibit . Poster board from the 99 cent store is also your friend when making the trim strips . You will go thru a lot of poster board ! When you make the templates you will be surprised how much aluminum it will take to make a long narrow trim strip !

I had no idea how to do this other than reading what articles I could find in the old NL's . Practice on some scrap acrylic BEFORE you begin on the big expensive piece . It took me a few months of patience to complete the process and I am happy how it turned out . FWIW I did crack my first canopy while cutting . Not sure of the cause , but it may have been a temperature thing ? I cut the second one at the beginning of summer and had no problems .


RB

If anyone is looking for a clear windshield and canopy, I have a set in a create that is new old stock I bought with some other parts. I am not going to use it (I like the smoke) $500.00 for the set. You must pick up. I live close to Sacramento Ca.

Thanks Phil
118 BC

Thanks to all for the suggestions and encouragement. I do understand that you want to install the windshield and have the front of the canopy line up with it, but doesn't the loft of the front also depend on where the aft end is trimmed? So where to you start? I am aware that hundreds of folks have done this job, and they all seem to have come out looking very attractive, so it is doable......I just can't seem to visualize in my mind the step by step process. One newsletter item said that the canopies for the T-18, S-18, and Mustang II all were pulled from the same mold. Perhaps the canopy is it flexible enough that you can lay it on the frame with the sides hanging down past the sides of the fuselage and the front overlapping the windshield and begin to get an idea of where to do the first rough trimming? And then just keep on find tuning by removing small amounts at a time? Bob

The T18 canopy is not the same as the S18. The cockpit area of an S18 bigger in all dimensions. The rollover bar is 3/4 inch taller too.
When I put new glass on my bird, I put the canopy frame inside the new glass and moved it fore and aft until it fit the windshield. There should be enough meat there to do it. Then I marked a line in grease pencil. I cut about an inch outside that line with a die grinder. Then I went through a process of putting it on the bird, marking it, taking it off, taking the frame out, using a belt sander to sneak up on the line and repeat. I did that about three times a day, every day for 2.5 weeks until it fit.
Cubes

I believe I have discovered a mistake in the plans!

On the 632 "Assembly Wing Flap" print John calls out "AN 526AD-4-3" rivets for all the flush riveted holes. I believe he meant to say "AN 426". I cannot find anything anywhere that indicates that there is any such thing as a " AN 526" solid rivet. I once found a thread that listed mistakes in the drawings, but I cannot find it now. Perhaps on the old forum? This must have been discovered 50 years ago by someone. Bob H.

That's kinda the idea that I had. I was thinking of starting with the large radius bend, using maybe a 4 inch piece of pvc pipe to wrap the skin around, and then doing the l.e. radius bend. Having some measurements that worked for John should be a big help. BTW, I re-skinned the rudder using .020 instead of .016, and I have about 7 feet left of the sheet, so I plan to use that for the flap skins. Seems reading through the newsletters there were enough folks who were told by J.T. that .020 would be OK that I feel comfortable. Thanks to all for the suggestions. Bob

There are several articles out there, including a recent one in Kitplanes, about bending skins with a shop vac. I've not seen this technique discussed on the Thorp Forum, but it seems to me an easier way to do it than using wood and towels.

Just a thought.

d

I made a nose brake, the first one I used was wood the second steel. Make a "V" using two pieces of plywood a little longer than the flap. Round a two x four and place skin across the "V" and press down with the nose of the 2 x 4. I think I got the idea from one of the old Sport Aviation years ago before they ruined the magazine.

Here's a YouTube video of the vacuum forming technique:

https://www.bing.com/videos/search?q=be ... ORM=VDQVAP

I am not quite sure if it can be adapted to the double bend that is in the flap skin. I may try a short piece of scrap and see what happens. Not sure if it would work on a full size skin either. Perhaps you would just need a larger hunk of plastic.

OK gang, one more puzzlement! The flap plans call out a .040 trailing edge strip. The Eklund kit came with four .025 strips. If I were to use two per flap, then the total would be .050. I have a piece of .040 material, so I can use .040 as the plans state. Why did Eklund supply four .025 strips? Anyone have any thoughts? Thanks in advance, Bob

My guess, Richard didn't have any 0.040" laying around so he subed 2 0.025". The strip is to add stiffness and thickness to the trailing edge. You want the thickness for flutter margin, a thin edge will flutter sooner than a thicker one.

_________________
Bruce Finney
N18JF T-18C #262
Yelm, WA USA

Wow Bruce, you're up early! So it's not a mod that was dictated due to problems arising in service experience? Would .010 be enough of a difference to be noticeable as different from the adjacent aileron? And is there really any concern about flutter with the flaps? To my knowledge you only worry about flutter with control surfaces that are free to move. The flaps do not move, and in any case are only deployed at relatively slow speeds. Bob H.