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1/4.5 T-38C Talon scratch build

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1/4.5 T-38C Talon scratch build

Old 10-15-2019, 05:15 PM
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invertmast
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Default 1/4.5 T-38C Talon scratch build

A few friends (i’ll let those bad influences introduce themselves if they so desire) talked me into this project. I’ve always had an interest in the airplane due to it just looking fast.

So here’s a little bit with what I’m doing.

Dimensions:

1/4.5 scale
118” long
66” wingspan

Flying weight of 35-40 pounds. This is pretty light for an airframe this size, but im pretty confident it will be possible.

Power will be modular in that it can be twin 110-120mm EDF’s, twin 80-100 or a single 160-200 size turbines. Single turbine installations will utilize full ducting and bypass.

Landing gear will utilize Electron ER40 gear units, custom machined struts and wheels and electron electric brake units.

All of the control surface linkages are fully concealed.


With many people doing more and more composite molding for unique projects i will also try and fully detail All of the materials, layup schedules for the molds and parts and the majority of my sources for the materials. I’ll also do youtube video’s for some of the process as it will be easier to walk through the process instead of typing about it.


With my new work schedule, progress is slow, so because of that i am using alot of automation and technology to accelerate the progress and reduce the amount of manual labor needed. One of these tasks was designing, building and refining a custom large format 3D printer that will print anything that will fit in a 24”x24”x36” volume.


This project has started in the digital world. A “factory accurate” CAD model was purchased that was supposedly drawn using Northrop factory drawings. If thats true, it must of been drawn by one of the factory workers children because they were comically wrong.

Here, we have the Cad model as received with the only change being made that accurate airfoiled wings/tails were drawn.






At first glance, the model looks really good, but as it was scrutinized against photo’s of the T38, it was realized the Cad model was great in some area’s, but horrible in others.

Those area’s needing corrections ended up being:

1- fuselage spine, this meant chopping off the entire top of the fuselage from the windscreen all the way back to the V. Stab transition area.
2- bottom “corners” of the fuselage. These were too “rounded”
3- exhaust nozzle exit bulkheads. To rounded like #2
4- some other small odds and ends that popped up from the previous 3.
5- Inlets. The Cad model is an early T38 A/B and the inlets are different for a T38C

In all, it would of likely been far faster to just start from scratch, but the model was purchased, so I pressed on so as to not waste any $$$.


I then started making the changes to modify the inlets for the “C” model variant.






With the inlets mostly complete (i was leaving the refinement to be done by hand), i started work on fixing the issues with the rest of the model. Here’s a few photos in no specific order:






Old 10-15-2019, 05:36 PM
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Viper1GJ
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Hi Thomas,
Subscribed. Looking forward to seeing this one
Gary
Old 10-15-2019, 05:38 PM
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Nice!!

This should be awesome!!
Old 10-15-2019, 05:39 PM
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So with the Cad Solid model essentially “close enough” (i left some area’s to be refined by hand as it would be easier than dealing with the changes in cad), i started slicing up the model into individual components, wings, flaps, ailerons, stabs, rudder, hatches, etc.







With the model now sliced up, each individual “body” was saved as its own part file. Then i brought these into a solidworks “assembly” and started working out the internal structures for the composite airframes.

Fuselage:








The Photo’s below show a twin 110-120mm EDF and/or twin turbine installation. These are un-ducted installations:









For EDF installations, the lipo batteries would be installed just forward of the main hatch and would be accessible via the main hatch or the canopy. This photo shows Four 6s 10,000mah Lipo packs in position. For turbine installations, this would be the same location for one of the fuel tanks:





The photo’s below will show the fully ducted and bypassed single turbine installation as well as the three kevlar fuel tanks and there position. If my memory is correct, the saddle tanks make up a total of about 90oz of fuel and the main tank is around 80 to 100oz of fuel. I tried to keep the fuel as close to the CG location as possible to avoid trim changes as the fuel load was burned. I may still change the fuel tank layout though after airframe testing.









As mentioned earlier, all control surface linkages are internal. I like the use of aluminum “hex” stock that is pre-installed during the mold closure steps that mate directly to a female hex control horn for control. This makes an easy and zero slop linkage. The next photo shows the setup (minus attachment pins) for the rudder





Now we have the wing structure shown nearly complete:







That essentially made up about 90% of the cad work to be done with the last lingering elements being some equipment mounting plates and the nose gear retract structure to be determined.


If you noticed, there is Alot of CF internal structure. As it is planned all primary structure within the T38 will be a Carbon Fiber and Airex laminate, while secondary structures will be fiberglass and airex laminates. Any item that will be subjected to bolting or shock/vibration forces will be from Finnish birch aircraft ply. All wing and tail surfaces will utilize a airex core layup with significant amounts of CF applied due to there thin-ness. The scale airfoil was used with the only change made to its thickness so that off the shelf retract units would fit, this results in a wing just over 1 1/4” thick at the root and just under 3/8” thick at the tip, the tail section is even thinner.
Old 10-15-2019, 05:48 PM
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With all of the Cad work done and the model sliced up into individual pieces, Those “bodies” that were saved andcwere brought into the slicing program and printing began.

For some items, like the wings and v. Stab, they were cut into pieces to eliminate any overhangs and the need for print “supports”. The 3D printer has a max 36” build height, so the fuselage sections were cut to make full use of that and holes for a few 3/4” aluminum tubes were cut through the center to aid in alignment and future assembly.

Here are some photo’s from random times of the printing process:



















Old 10-15-2019, 06:00 PM
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With the entire airframe now 3D printed, i noticed a few things about the design i didnt like once i looked at it a few months later (it took about 6 months worth of printing and tuning on the new big printer before i got the T38 done). So bc of that, some changes were made to the cad model which required reprinting a few parts (flaps and rudder). Before i printed those parts, i was printing some other bits and came to the realization that the X and Y dimensions were off by a significant amount (7% on one axis and 13% on the other!). Once i fixed this inaccuracy and printing new flaps it was blatantly obvious that the entire airframe was going to need reprinted.... better to find out now rather than later right? So the entire print process was restarted. This actually ended up being a good thing because after 6 months of tuning on the printer, i was able to print the airframe in less pieces and with a better surface quality. 36 hours continuous prints were pretty common.










With all the printing done.. again... i assembled all of the pieces (1/4” and 3/8” diameter holes were placed through the flying surfaces for fiberglass rods to be placed to prevent warping of the surfaces) and began applying body filler to smooth and fill the joint seams.










And of course i had to assemble it all to see how it looked.






Old 10-15-2019, 06:04 PM
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Viper1GJ
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All I can say is WOW! The 3D printing and CAD have completely changed how everything is done. So fast and so accurate now. I could only dream of doing that the old way. I am really interested in this plane.

I have fond memories of flying it in the late 70's at Sheppard AFB. I even got to land one with no wing tips once. Once was enough! Keep the photos of your process and progess coming!

Gary
Old 10-15-2019, 06:13 PM
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invertmast
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Originally Posted by Viper1GJ View Post
All I can say is WOW! The 3D printing and CAD have completely changed how everything is done. So fast and so accurate now. I could only dream of doing that the old way. I am really interested in this plane.

I have fond memories of flying it in the late 70's at Sheppard AFB. I even got to land one with no wing tips once. Once was enough! Keep the photos of your process and progess coming!

Gary

Gary,
It is amazing how quick the project has been moving considering how little time (about 8 days a month) i am home each month. The 2nd print session was completed in late April and 60% of the airframe is ready for surface detailing currently. The main item needing the most work is the fuselage, but the current focus is getting all of the small parts ready for molding first. More on all this in future posts.

I have also designed the airplane so it has a high degree of prefabrication done to it before it even comes out of the mold, so much so that the only real “building” to be done would be hatch retention, gear door and speed brake hinging and gluing in some aileron servo mounts, everything else is basic component installation and paint.


I’ll post up more tomorrow, its bedtime.
Old 10-15-2019, 06:50 PM
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Yes! Yes! Yes!
Old 10-16-2019, 05:19 AM
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With the majority of the part seams filled it was time for some primer. Since the striations of the prints were reasonably deep (maybe .2mm) i wanted a primer that would build up thickness very quickly. This really only left a few options, the most popular being a polyester based product which i try to avoid polyester based stuff as much as possible, but this time, i conceedes and picked up some Duratec grey primer, mekp hardener, thinner and a hvlp gun with something like a 2mm nozzle tip size.






You can see in some of the above photo’s the primee didnt fill in the print striations very well and i ran out of catalysed primer. These parts were completely covered with a thin layer of Evercoat LiteWeight body filler. This was then sanded off with 120grit on a DA sander.









It was at this point i noticed the transition between the fuselage spin and vertical stab was off, so i drew up some filler pieces in cad, printed those out, attached and body worked then in to make a nice transition. I also sanded the primer in the fuselage and nose to hi-light any area’s that may need additional filling.












Then more primer was Sprayed. This totaled right at 1 gallon of Duratec and the only filling and priming left is on the bottom of the fuselage, inlets, hatches and some other odds and ends.





Old 10-16-2019, 05:37 AM
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At this point, we were around the middle of June and the amount of actual hands on labor was under 24 hours over the course of 2 or 3 weeks of working on it here and there. The plan of using technology to do all the hardwork was paying off, so i invested in a new piece of equipment. I had been wanting and researching SLA printers for months trying to find a budget SLA printer (around $500 or less) and for years, this was impossible. In early summer of this year, i came across the Elegoo Mars SLA printer. After a few months of research, i saw it was on Amazon for $250, so i purchased one and started using it to print some of the small surface detail pieces that would be near impossible to do on a conventional 3d printer.










Having purchased a 30” vinyl cutter a few years prior, i decided to draw out all of the panel lines for the flying surfaces in cad and have the vinyl cutter cut those out. This worked extremely well.









Then one day while sitting in Nyc i had the realization that it Might be possible to cut out out all of the panel line and rivet details all at once to be placed on the parts in a single application. I did some testing with varying style cutters, speeds and rivet sizes on the vinyl cutter and found out it was possible, so i went back to Cad and added all of these details to the vertical stab, rudder, wings and flaps.






I then cut out the vertical stab and rudder pieces and applied them to those plugs.









i also applied the details to the horizontal stab. Unfortunately i need to remove the center row of rivets and place 2 equidistant rows instead. I did this the old
fashioned way.





Last edited by invertmast; 10-16-2019 at 05:57 AM.
Old 10-16-2019, 05:56 AM
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With things progressing fairly quickly, i decided to take the time to refine the trailing edges of the wing components. The TE at the root was fairly thick at around 3/32 to 1/8” thick. Seeing as how the fullscale is razor thin on the TE, i decided to use some fiber reinforced body filler to build up the TE so that it could be sanded down to a nearly razor thin TE, in reality, i will likely keep it just under 1/16” thick to make closing the parts a little easier.







While that cured, i made up a stand using pink insulation foam so that the fuselage could be rolled upside to make finishing the bottom easier.





Now the work of correcting the bottom fuselage where the nose fairs into the main fuselage could begin. The main fuselage is curved inwards at the nose fairing area instead of flat, so it leaves a large divot. This area should be mostly flat (if you removed the nose fairing section) so the reinforced body filler was used to fill in the divot. Later i will use regular filler to fine tune the shape so it blends with the groove left between the inlets and the nose smoothly. This is one of those area’s that the cad model was completely wrong in and trying to correct it in Cad created more rebuild errors for the entire model, so it was deemed to be quicker to fix if by hand than in Cad. There is also very little complex structure here, so that make it an easier decision.








In one of the above photo’s you can see the outlines cuts that were printed with the model for the gear door and speed brake outlines. These are locating fixtures that will get mostly filled in to help locate the bottom fuselage vinyl cut surface detail piece.

This will also be a good time to bring up and explain one out of scale detail being implemented. If anyone has seen Brent Hecht’s 1/4 edf T-38, your familiar with how slow it lands and how the takeoff rotation is nice and smooth (unlike all of the jumpy T38’s from other manufacturers) and the landing rollout the nose is able to be held high for aerodynamic braking very easily.
I found out he moved the main gear forward a good few inches to make that possible on his. He made the recommendation to do the same on this model, but it looked to far out of position, so it was decided to split the difference and move the gear forward 1 1/2”. This will hopefully make for a smooth rotation at takeoff and nose high aerodynamic braking on landings



Well, that brings us up to the current status of the project. I’ve been short on time recently with fall sports for my son and other household duties, so time is pretty short to work on this thing. I also need to get my downdraft table design finalized and built so i can do more intense sanding in the shop without dusting up the entire place.
Old 10-16-2019, 06:11 AM
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Awesome! It would be really cool to be able to aerobrake on landing roll like the full scale does. I think I remember you held the nose up to about 100 kts for aerobraking.
Gary
Old 10-16-2019, 06:36 AM
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Originally Posted by Viper1GJ View Post
Awesome! It would be really cool to be able to aerobrake on landing roll like the full scale does. I think I remember you held the nose up to about 100 kts for aerobraking.
Gary

I agree! That is one of the coolest aspects of Brent’s T38 and one i think would make a fun addition to this model. Time will tell if its possible.

the current plan for now is to make about 6 of these, 4 of which are spoken for, after the 6th one, its really up to me on if i want to do anymore after that.


i’ve also got a friend working on the Cad design to make functional scale canopy hinging.
Old 10-16-2019, 07:59 AM
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Brilliant!!!
Thanks for sharing this build of one of my favorite aircraft. I would have the smaller SM version but the canopy area just doesn't look right to me.
Old 10-16-2019, 08:19 AM
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duplicate

Last edited by causeitflies; 10-16-2019 at 08:25 AM. Reason: Dup
Old 10-16-2019, 12:26 PM
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Very cool project. Nice work.
Old 10-16-2019, 05:22 PM
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Beautiful Build. Chic
Old 10-16-2019, 05:22 PM
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Beautiful Build. Chic
Old 10-31-2019, 08:35 AM
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Not alot of progress since soccer season for the son was in session, but i’ve gotten some additional CAD work done. I have elected to draw up all of the parting planes in CAD to have them laser cut to save me time as cutting parting planes is really time consuming.

The wing, horizontal stab, vertical stab and rudder parting planes are complete. I should get the flaps and some of the fuselage complete by the end of the month


Old 12-01-2019, 01:12 PM
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its been a bit, but ive been plugging along the last few weeks when the family and work lets me.

first up i rectified the overly thick and blunt trailing edges on the wings. They are now T38 like at around 1/16” thick. I also applied the vinyl cut surface detailings but ultimately was unhappy with the commonality of the top/bottom cuttings and pulled all it back off. My vinyl cutter software doesnt keep the sizing data when importing the DXF file the surface details are cut from, so getting the top/bottom imported files fo match was near impossible, and i tried... alot... cutting over 50’ of vinyl trying to get it “just” right. Oh well... i’ve got a new tool coming that will hopefully rectify that problem.






next up was some sanding and body filling to the fuselage. I filled in the rectangular cooling scoop openings using the previously 3D printed scoop defects.







i then brushed on some Duratec Base primer. I didnt really care about what it looked like, hence the plethora of runs and was to lazy to spray it because i wanted to be able to paint all of the surfaces in a single day. Ultimately this stuff sands super easy and it wasn’t a problem. I’m glad i built my downdraft table now!







at this point, it was just alot of sanding. Again, the downdraft table i designed and built became a HUGE commodity and im glad i took the time to make it.









With everything sanded, i glued on the previously 3D printed fuselage spine correction filler pieces to the fuselage and engine hatch.







Then began the process of body working the fuselage and engine hatch so they flowed well from one to the next.









With the hatch flowing well with the fuselage surface, it exacerbated the joint and i just wasnt happy with the fit, so it had to be fixed.

i placed HVAC tape on the fuselage to act as a release (i should of waxed it to.. oh well) then applied a significant bead of reinforced body filler onto the hatch flanges. I then










I also 3D printed the fuel tank plugs and brushed on alot of the Duratec base primer.




Old 12-01-2019, 07:41 PM
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Awesome!!! Pm Sent!
Old 12-02-2019, 06:27 AM
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I want to pose this question now while it is fresh on my mind.


For those who may be interested in one these. What type of completion level would you want?

Plug N Play
Plug N Play needing paint
Painted ARF
Kit w/ landing gear, wheels/brakes, tanks and tailpipe



From talking to friends who go to far more Jet eventa than i do, it seems the plug and play options are likely the two best options to go with?
Old 12-02-2019, 07:32 AM
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As a kit with gear, brakes, tanks, pipe and hardware unpainted.
Old 12-02-2019, 07:56 AM
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Kit with gear brakes tanks and pipe for single turbine unpainted. But would buy one that was painted.. Not interested in plug and play as much as I would take everything out to check work anyway. Love the project

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