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Jul 05, 2019, 09:26 AM
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pete914's Avatar
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Discussion

One 96" long T-38 Talon Build


After much rumination and reading through the fantastic 144" T-38 build thread as well as the WOW! Hobbies build(s) thread I decided a few weeks ago to take the scratch built plunge.

https://www.rcgroups.com/forums/show...44-long-T-38-s

There's something about the lines of this jet that are irresistible. I decided I must have one for myself. I love my HSD Jets F-16; but I want something bigger than can handle grass landings better. I want a 12-14s setup and a 120mm fan for simplicity. Can I keep the weight down to make this work?

The T-38 checks all the boxes at 1/6 scale. I considered copying the 1/4 scale setup but I wanted to avoid running dual 12s setups and I figured I was already taking a big bite, perhaps that can be build #2...more on that later.

I used the below three views and it's not easy. Blown up to 8-ft long the lines are nearly 1/4 wide and broken. It's hard to get the fine line detail so interpretation is required at every step. I also noticed the former's scale don't match up to the body 3-view. There's only 9 formers too...
I've spent a lot of time staring at the ruler and the three view trying to determine where to cut. This has been a project of measure 10-times before cutting and then end up redoing that cut too...hahah.


It's confusing such an incredible, important Jet like the T-38 hasn't been done by MotionRC, etc.
The EDF jet market has exploded but the T-38 has been ignored? It's about to be retired...we need a manufacturer to step up.

This is my first scratch built plane. I have experience with foam and building wings and fuselages, repairs, etc, but this is my first honest scratch-built plane.

So, I'm a beginner essentially and armed with the above thread as well as Google and a 1/32 scale kit, I hope to get this thing airborne. Hopefully, this inspires others to build one and we can finally get some formation flights

I hope this thread becomes an update to the original thread that's approaching 7-years old.
I'm going to try to synthesize the build techniques used there, Ramy RC, etc.

Constructive comments are welcome and hopefully Falcon5 and Corsair Nut will chime in...I'm going to need some help on this one friends.
Last edited by pete914; Jul 05, 2019 at 11:45 AM.
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Jul 05, 2019, 09:27 AM
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pete914's Avatar
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Step 1: Kinkos!

Holy hell they were not helpful. First clerk told me flat out can't be done---manager please.
Manager appears annoyed and begins 2-hr process of trying to figure out how to scale up the line drawing.
I had already done the math for him but he was confused about nothing. He kept printing in different scales and was confused why I kept measuring the print.
On his fourth and final attempt I got what I needed but at a slightly different scale than planned. Oh well, keep it moving...

Officially the 1:5.7 plans are done with a final cost of $125...ouch.

Step 2: First cuts

The first step is to trace and cut I chose to trace the outer most part of the lines but interpretation is needed. I leaned heavily on the few photos Falcon 5 provided of this setup. In retrospect, I'm glad I did a first attempt and then trashed it. I learned a lot.
Things to remember:

1. Use a sharp box cutter-style blade and focuse on 90 degrees to the table! Hard to do
2. Mate the two sides and sand to make exact copies
3. Don't forget to compensate for width of the board
3. Super 77 spray glue is awesome...but not perfect. Wait around 30-90 seconds before joining and don't spray too close; it will eat the foam
Last edited by pete914; Jul 05, 2019 at 10:02 AM.
Jul 05, 2019, 09:53 AM
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Getting the box built is so satisfying...but humbling..

I would have like to use 3/4 or even 1" foam. but it would have complicated the 120mm fan unit; there would have been no room and required extensive hogging of the foam...I'm going to try to keep this build lighter than 30 lbs so 25mm or roughly 1/2" board was selected. I think it should work well. I still had to add about 1/4 inch to the width to allow the 120mm fan to fit where I'm planning to put it
Last edited by pete914; Jul 05, 2019 at 11:48 AM.
Jul 05, 2019, 10:00 AM
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Styrospray 1000

Falcon5 intro'd this on the original T-38 thread. WOW! hobbies sold it too. This product is probably what made me consider taking the plunge.

Glassing, filling, sanding is a process! For some reason the allure of just painting on a coating to the fuse that will be plastic-hard and weigh about as much as 2 oz glass seemed REALLY appealing.

I ordered two quarts for about $80 shipped and Did some tests.
With no primer and two light coats the strength is impressive but I'm not convinced. I think it will need 4 coats to be strong enough with the thinner foam...

If anyone has experience with this product on their builds I'm all ears...
Jul 05, 2019, 10:22 AM
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The moment of truth: first set of curves to sand.


I cut out formers using yard-sign material from Lowes. This again required some interpretation as the print-out does not capture the subleties whatsoever. Having the scale model to reference really helps...

tip: mark a line on the fuse so you know where to finish your sanding...this will help keep things symmetrical.
Last edited by pete914; Jul 05, 2019 at 11:10 AM.
Jul 05, 2019, 12:31 PM
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Dirty Dee's Avatar
Subscribed!

Pink foam is fun to work with, and yea I learned the same things when I tried 3M 77 spray
Jul 05, 2019, 02:31 PM
Team EJF >>>WHOOSH>>>
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Noyce
Jul 07, 2019, 07:09 AM
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Joining the two fuselage halves was a mental challenge! Other threads did not specify how they did it.
I'm trying to keep this jet mostly foam. So, I bought $5 "craft" foam board from Lowes. It's thicker and basically EPX but feels a bit stiffer and is 1" thick.

Using the central line I marked on all the boards at the start of the build I did my best to line up the two fuselage halves and then tacked them together with Super 77. It would have been better to use a laser system or build a jig. I just used my eye and a ruler and I'm sure I'm off by a few degrees...

I then used a shape gauge to transfer the fuselage "dihedral " to the 1" foam. I shaped the bottom of the foam "spar" to mate well with the bottom board of the rear and forward fuse. I then cut a relief slot in the upper rear panel of the fuse this way the vertical foam spar mated to both upper and lower boards of the rear fuse. I then shaped the second piece of 1" foam to intersect with it AT 90 degrees and grab the side walls of the forward fuse. I then cut a foam stringer for the forward fuse that will run from the 1" joiner to the nose mounted landing gear. All this was glued in overnight with Gorilla glue "clear gel" no foam. I plan to do some glassing of the forward stringer.

I guess a picture is worth a thousand words....
Last edited by pete914; Jul 07, 2019 at 07:16 AM.
Jul 08, 2019, 04:41 PM
Dont forget the velcro straps
corsair nut's Avatar
so cool man! will be following for sure!
Jul 09, 2019, 09:31 AM
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Corsair,
Thanks man! You and Falcon5 are the inspiration to try this out.
I have questions for you:

1. Did you move your rear landing gear forward to get those nice lift off's and landings? I want that scale "wheelie"! I assume the closer the rear LG is to the CG point, the sooner I should be able to get the nose off the ground right?

2. Where did you set your CG for maiden? 30% aft of the leading edge?

Here's how I tackled the fuselage arch form. I used a modified approach RamyRC does for his huge jetliners.

I cut 1/4" 5mm strips of DTFB (dollar tree foam board) and laid them a bit fat over the formers.
On the rear hump, there is almost no "wave" like there is on the forward canopy. So on the rear hump, I just made scores rather than cuts as the foam just needed to curve in one plane. I painted 5 min epoxy on the back and held it in place with tape. Then sand away...

I'm not sure if this technique is faster than just shaping a huge chunk of foam. This was a PITA although it felt faster before starting and I feel like it will end up a little lighter.
If it were to do this again, I may go with the big chunk of o foam
Jul 09, 2019, 09:41 AM
Dont forget the velcro straps
corsair nut's Avatar
yeah I moved the gear fwd from scale a good amount. id have to measure the cg and chord...I think its a little less than 30% of the chord though. I want to say the cg is about 10.5" back (with lipos installed.......lolol) and the overall chord is around 36" but its have to double check those numbers.

ps....nice e36 m3 saloon you got there
Jul 09, 2019, 09:51 AM
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Great info! Thanks corsair. I'm glad your an M3 fan too...

Oh, as Dirty Dee said, Pink foam is so cool to work with. It sucks CA glue doesn't work with it as I like to work fast and waiting for glue to dry sucks...but otherwise, it ain't bad

I repaired the area I cut out of the nose (to original piece got too thin and was flexing when I was sanding). I got as close as I could, Super 77 tacked into place and sanded to foam. A little foam filler and it should be all good...
Last edited by pete914; Jul 09, 2019 at 10:46 AM.
Jul 10, 2019, 08:44 AM
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I'm not thrilled with the canopy; especially the forward part, but I'm calling it basically shaped

I'm ready to move to the next challenge...

I'm thinking about glassing the canopy so I can remove the foam and do some final fill and sanding....or I could try to StyroSpray the whole thing...
Thoughts?

I'm worried that there is going to be some warpage of the canopy if I glass it first?

I'm getting to the point where the fuse is getting pressure dings faster than I can fill them. I think it will be best to just get it "covered" and then do a few rounds of filling/sanding on both
Jul 10, 2019, 10:35 AM
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turbonut's Avatar
It looks good from here!
Latest blog entry: In flight
Jul 12, 2019, 07:23 AM
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Thanks Turbo. This is going to be a 15-footer type build for sure


Ok, finishing up last bits on the fuse before installing the vertical stab and hopefully finishing the fuse and canopy for covering this weekend.

Cut out pieces to seal up the intakes to the forward fuse. This adds some lateral stiffness. I rounded out the intakes but I think they need more rounding....


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