Thorp Air Command - T18.net

OK It's been some time getting this project started but last weekend obvious progress was finally seen on our project.
A new T18 fuse started to take shape
We used machine cut skins as I had mentioned we were planning to do in some previous forum posts.
The machine cut approach appears to work well. At present one side is alclad and the other side is the trial piece we did in plastic, the floor and deck are also plastic at this stage which was probably not needed given the accuracy we are seeing however it has been helpful as a sacrificial temporary construction aid and gives us a chance to change or fix any errors we might have madein our CAD drawings. I am not sure how long it normally takes to do by hand but it took the machine we used about 40 minutes to cut a complete side skin which included all cutouts and 3/32 pilot holes for every rivet.
From now on my questions might be of a slightly more practical nature!

Stuart

Pretty cool guys.

Thanks for the pictures. We all like pictures. You'll make noticeable progress really quick with the fuselage.

Since you're being so careful with it all I'd like to provide a heads up using templates or any large items in plastic. My shop isn't heated (unless I take the time to get the wood stove going). If you're like me you could have temperature swings of 50F. On a 10 ft. piece, you could have as much as 3/16" difference between aluminum and plastic, depending on what kind, etc. If you're gonna drill holes, etc. there should be no problem if you're careful about doing it at the right temperature, or doing both L & R sides at the same time. Just something to consider. I see bare feet so it must be pretty nice there. Watch out for the sharp little shavings! ha, ha.

Les Krumel,
Albuquerque

We had the temperature issue in the back of our minds but haven't made any specific allowances for it, other than avoiding cutting any of the material on extremely hot or cold days.
I wasn't so much worried about the differential expansion of the plastic compared to the alclad but was more concerened about cutting the alclad side skins on different temperature days and ending up up with slight differences in them. Fortunately our climate here is near perfect especially so at this time of year so no need for shoes and never a need a heater in the workshop!

What type of equipment are you using to cut your parts?
Inquiring minds want to know.

Looks good.

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

Stuart:

Thanks for the pictures and up date.

What are the tolerances that you are working too?

Not familiar with the machine that you are using?

How sharp are the edges after cutting?

Is there any finish filing required?

Do you use plazma cutter attachment?

40 minutes per side, are you planning to mass produce, as setting up and cutting just one set of sides seems a shame.

Being in Canada at this time of the year I envy your warm hanger.

Cheers.

_________________
Victor J Thompson
C-GIRQ
In memory of Dad

The machine we used is an Australian made CNC router similar to these. http://www.multicam.com.au/wood_series.html
I'm not sure what tolerance the machine achieves but we have drawn our drawings to match the original plans (4 decimal places) it achieves pretty good results. I will take a close up picture of the of finished edges and post that soon pretty sure they will require a light deburr.

It was a lot of effort for one set of side skins but the intent was to make it production quality rather than production-able having said that production could be an option.
Stuart

Some pictures of the cutting machine and close ups of the tail area. It's hard to a really good picture up close. At this point we have only wiped off the cutting fluid and swarf, no other clean up of the edges at this stage .

Wow, Stuart,
A pre punched T-18.
I presume you love both CNC and Airplanes?
Kind of a double your fun project?
Certainly the first airframe would take a long time with all the set up, but are you keeping the files to make "kit parts" if someone asks?

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

Scott CNC is just a means to an end. If I had more skill with tin snips I don't think I'd be doing it this way.
Not sure there is much of a market for kits, of course if you were to call it an "RV T18" that might get more interest. I'll be keeping the files in case it can help another builder in the future.

A question for any past builder regarding the 601 beam, 602 doubler and the C580 modifications to that area.
In the attached photos you may notice that unless we do something different, there is going to be a small gap near the top of the 601 beam and the side skin, it's where the blue masking tape is .
I think I have interpreted the plans correctly but thought it strange to see a gap in this location, Is this gap normal should it be packed out or possibly have I misunderstood the plans?
Stuart

Stuart,

The small gap you are seeing is normal. When John Thorp first designed the plane, the reinforcements did not exist. They were designed in afterwards based on ealy cracking in the area. You will not notice the gaps after the area is riveted up. Upon closer inspection, I see that you have placed the wrap reinforcement inside the 580 skin. That needs to go on the outside. The "pork chop" piece has a joggle in it. See DWG 580-16 for details. Project is looking good!

_________________
Bob Highley
N711SH
SN 835
KLAL

This is as much as art and technology can go together !

/VB

Thanks Bob that explains it.
I will need to check the plans more carefully before asking the dumb question next time.
It should look more like this.
Stuart

Stuart: Thanks for the shot of your CNC machine and the close ups.

CNC Machine looks like it makes pretty decent edges/ cuts.

My dad and I used a step up from tin snips, a 12" Hand Operated Metal Shear, (similar to the one below) but to cut out a side in 40 minutes, with cut outs and pre drilled very impressive.

Great work.

_________________
Victor J Thompson
C-GIRQ
In memory of Dad

Hi all.
While all the Thorp formation flyers were out and about enjoying the weekend I came across a bit of a problem.
The side skin rivet holes I had pre drilled for the landing beam angles didn't line up as can be seen from the photo. Sure enough it turns out I had made an error in the drawing and had them too high however even after carefully checking, double checking and and correcting the drawing there is still a discrepancy I can't explain. So I am not sure I am interpreting and coordinating between the side skin and 527 drawings correctly.

So we now have 2 "issues" to be resolved.
Do I trash the side skin with this error or can I fix it?
I could just drill 2 new rows of rivet holes and somehow make good the unused holes, it might be possible to use a 3/4" * 1" angle to to pick up the top row of rivets but it looks like there is no way of avoiding a second row of holes on the lower side so either way we end up with 1 or 2 row of unused holes in the skin which will need attention. I will be looking for options to make good this error rather than scrapping an otherwise good side skin.

For the second side skin which is still to be cut we should be able to fix the drawing and get it right but I'd like to know it is 100% right before proceeding further. If I can't be 100% confident that we can get this right then I will have to avoid pre drilling the holes and waih to do it in situ which I assume most builders would actually have done.
Thoughts suggestions!
Stuart

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Stug

I'm at a loss to see how you miscalculated the rivet lines. Looks as though they are on the line of the tops of the bracing angles. I do however, have a method for "erasing" rivet holes. This goes back to my friend John Shinn.

First, punch a bunch of holes in some scrap aluminum the same size as your mistake holes. In this case it appears to be .040. Take the slugs from the punching operation and tape them into the holes in error. I usually install them dimple side in. Then, with a mushroom set in my rivet gun and a large surface bucking bar, I simply hammer them in like a miniature freeze plug in an automobile engine block. Practice on some scrap first. Don't hit them too hard or you will streach the sleet you are trying to repair.

_________________
Bob Highley
N711SH
SN 835
KLAL