Giant scale OV-10A

[daveculp]

Just about done with the nose gear, which includes the doors and mounting bits.  The idea is that to statically display the flyable airplane you first retract the landing gear, then you plug in the 3 scale gear as complete units.  To fly again you remove the scale gear, lower the original gear, and off you go.  That's the theory anyway.  I am a bit worried about the stability of the airplane sitting on three gear units that are not firmly affixed to the airplane.

 

The nose gear won't have to carry much weight, so it will be fine structurally.  The main gear might be a problem in this regard.

 

Here are the three components of the nose gear.  One part, printed on an FDM printer, is the well/doors part.  The other FDM printed part is the nose gear itself, printed all together for strength (shown here fresh off the printer).  The smaller part is printed on a DLP printer.  This one is the strut and strut door, which won't carry any load.

 

 

 

Assembled it will look like this:

 

 

There is still a lot of finishing to do - sanding, bondo, primer, paint.  There are plenty of surface artifacts on the FDM-printed parts, and not so much on the DLP-printed part.

 

Here is a photo of an earlier version showing overall fit:

 

 

I'm also working on the 230 gallon external tank now.  I 3D printed molds on my FDM printer - two mold halves for the front of the tank and two mold halves for the aft of the tank.  I tried using "Great Stuff" spray insulation to make the forward tank half, and it was a big fail.  The guy on YouTube said it works fine!  Actually it's awful stuff - very sticky even with petroleum mold release.  After reconditioning the molds I switched to "Foam-it!" brand liquid foam.  I chose "Foam-iT! 8" because it's supposed to come out of the mold with a smooth surface.  For mold release I used "Ease Release 2831".

 

Here is the mold ten minutes after pouring the two-component liquid foam into the mold:

 

I guessed 6 fluid ounces of the liquid (2 oz part B and 4 oz part A) would be enough considering the 8x expansion.  I guessed right.  After the two hour cure time I used a saw blade to cut off the excess and de-molded the part:

 

 

The part looks good, but it's too heavy for flying.  The original RC airplane tank is 150 gallon size and weighs 33 grams (without the magnets which I have removed to use for the 230 gallon tank).  The tank half I just poured weighs 108 grams.  I figure the completed tank will weigh on the order of 280 grams.  Quite a difference!  So, this is not a flying tank solution, but should be OK for static display.

[patricksparks]

Could you print a core for your tank mold to take the weight from the molded foam ?

Pat

Edited September 16, 2020 by patricksparks

[daveculp]

7 hours ago, patricksparks said:

Could you print a core for your tank mold to take the weight from the molded foam ?

Pat

 

 

Yes, this is a possibility.  It would complicate the de-molding process a bit, and there is the issue of keeping it centered even with the expanding foam pushing on it.  Still, it's probably better than using a hole saw to drill out the center.

 

I could use lighter foam, but this is my first experience with pourable foam, and I was worried about breaking the piece during de-molding.  I had to fight with the piece to de-mold it, so I'm glad it was strong.

[daveculp]

Got the second half of the tank de-molded.  Total length of the tank will be about 52 cm.  It looks huge, but the 150 gallon tank is 48 cm long, so this one is not much longer, but bigger around and less pointy.  Weight-wise we are at 243 grams already.  Sorry for the dark photo - the dining room is lit for romantic dinners, not airplane parts. 

 

[patricksparks]

13 hours ago, daveculp said:

 

 

Yes, this is a possibility.  It would complicate the de-molding process a bit, and there is the issue of keeping it centered even with the expanding foam pushing on it.  Still, it's probably better than using a hole saw to drill out the center.

 

I could use lighter foam, but this is my first experience with pourable foam, and I was worried about breaking the piece during de-molding.  I had to fight with the piece to de-mold it, so I'm glad it was strong.

Your tank came out really nice !!! Looking at the photo of your printed mold it looks as if you have a good amount of material on each half outside of the cavity, if you think about printing a core, maybe you could print it with "ears" on it that would sit on the flat ends of the existing mold and put some small screws through the "ears" into the end wall of your existing mold to keep the core from  moving while the foam cures.

Just a thought, looking forward to your progress !!!

Pat

[Out2gtcha]

Excellent work!  Really love the look of the A model Bronco. 

[daveculp]

4 hours ago, patricksparks said:

Your tank came out really nice !!! Looking at the photo of your printed mold it looks as if you have a good amount of material on each half outside of the cavity, if you think about printing a core, maybe you could print it with "ears" on it that would sit on the flat ends of the existing mold and put some small screws through the "ears" into the end wall of your existing mold to keep the core from  moving while the foam cures.

Just a thought, looking forward to your progress !!!

Pat

 

 

That's a good idea.  I really wasn't expecting that much pressure inside the mold.  I bought some more clamps, so I used three clamps on the next pour and had very little leakage.  One thing I would add to the mold design now is a few slots on the edges so I can insert a screwdriver to help pry the halves apart. 

 

Once the molds are separated the piece is still stuck in one of the mold halves.  Getting it out is not easy.  I've had to use a screwdriver to pry it out, which doesn't do the piece nor the mold any good.  If I switched to lighter foam there might not be a way to get it out in one piece (maybe put it in the freezer?).

[patricksparks]

On 9/17/2020 at 12:41 PM, daveculp said:

 

 

That's a good idea.  I really wasn't expecting that much pressure inside the mold.  I bought some more clamps, so I used three clamps on the next pour and had very little leakage.  One thing I would add to the mold design now is a few slots on the edges so I can insert a screwdriver to help pry the halves apart. 

 

Once the molds are separated the piece is still stuck in one of the mold halves.  Getting it out is not easy.  I've had to use a screwdriver to pry it out, which doesn't do the piece nor the mold any good.  If I switched to lighter foam there might not be a way to get it out in one piece (maybe put it in the freezer?).

I was thinking about the core idea, if a core was made with a straight taper on it , it could be wrapped with wax paper which might allow it to release more easily from the foam. Also I'm not sure how many different release agents you have tried, one that I used to use along time ago before anyone was using silicone for molds was PVA (poly vinyl alcohol) which we would apply with a spray gun onto our molds, you spray on light coats and let it dry a few minutes, spray again, 2 or 3 coats usually, put the mold together and pour your resin, the PVA is water soluble so you you take the part out and wash it off the part and the mold.

Good luck with your progress !!!

Pat

[daveculp]

Here's the 230 gallon tank with saddle.  Total weight so far is 275 grams without the magnet.  While I'm trying to solve bed adhesion issues with my FDM printer I printed the saddle on my DLP printer.  The piece won't be as strong as the FDM piece would be, but it's good enough for static display.

 

 

[daveculp]

230 gallon tank is finished and installed with a magnet.  The nose gear unit is "popped in" to the existing opening.  Here's a photo of the airplane sitting on its own three feet.  I'm amazed that no extra weight is needed in the nose!

 

 

 

So far each main gear unit will consist of four parts:

 

1)  The well/doors

2)  The main struts and wheels/tires

3)  The forward actuator and door

4)  The well cover (right side has blade antenna also)

 

 

The well/doors bit fits into the existing well.  It "pops in" to the existing well and doesn't replace the existing landing gear, which are retracted.  It is not firmly connected to the airplane, so there is a bit of wobble.  It'll be fine as long as you don't shove the airplane from the side.  This part was printed in one piece on an FDM printer (Anycubic Mega X -  you need a 300 mm print bed) using PLA filament.  The bit extending forward is a design cheat - the actual airplane has an extension of the well here that contains the actuating strut.

 

The main gear fits tightly into the well/doors part.  It is printed in one piece on the FDM printer.  As with the nose gear this piece requires at least two Bondo treatments and at least two thick primer treatments, with sanding between, in order to hide the surface roughness from the printing process.  Paint will be brushed-on Testors enamels, first coats flat white and flat black with sanding between.  Final coat of gloss white and flat black.  One design cheat used here is that the tire is connected to the trailing arm via a small "bridge" that will provide strength, sharing some of the load that would otherwise be carried entirely by the axle.  I used this same design cheat on the nose gear.

 

The forward actuator strut and door is printed on a resin printer (Elegoo Mars printer, Elegoo gray mixed with a bit of Siraya Tenacious).

 

The rear well cover is meant to hide the existing well where it extends too far aft.  In the actual OV-10 the main gear "curl-up" while retracting in order to fit into the well.  The RC airplane main gear don't do this, so they stay full length after retraction necessitating an extra long well.  The cover on the right side also has a large blade antenna on it (TACAN?  UHF?).  This part was printed on the resin printer.  The cover will be glued to the well/doors piece.

 

Some further parts to create - brakes, squat switches, the bit where the RBF flag goes, and the tie-down ring.

 

I'm curious to get the main gear units done and see if there is any sagging/bending over time.  So far they look strong enough.

 

 

[daveculp]

Still working in the main landing gear.  Before the final painting steps I printed up some small detail pieces on the resin printer - 

 

1.  "Squat switch" (air-ground switch)

2)  The part where the RBF flag goes

3)  Tie down ring

4)  Brakes (not shown here)

 

While printing the squat switch I noticed that the actual airplane only has a switch on the left main strut.  Every model I've seen (including the last one I made) has a switch on both main struts.  Ooops!  I used the brass gear on my last 1/32 OV-10A build, and removing the switch from the right strut would be a major chore, so I'll just pretend I don't see it.

 

My tire design is a little off - it should be a bit more round, a "balloon tire".  I thought about designing in a flattened bottom on the tire, but the problem is I didn't know where the exact bottom would be when all is installed.  When the weight is on all the wheels there will be some bending and leaning which could make the actual bottom a bit off from my calculated bottom.

 

 

 

 

From the flight manual:

 

GROUND SAFETY SWITCH

 

The ground safety switch is located on the left main landing gear strut.  When the landing gear is compressed (on the ground), the ground safety switch deactivates the angle of attack heater, pitot heater, yaw damper, missile fire, stall warning system; and activates nose wheel steering system.  The engine START switches are deactivated by the ground safety switch when the aircraft is in flight.

Edited November 6, 2020 by daveculp
additional info from the flight manual

[Out2gtcha]

Awesome!

I guess I forgot if you said earlier on the thread, is this going to be R/C or static?

 

[daveculp]

32 minutes ago, Out2gtcha said:

Awesome!

I guess I forgot if you said earlier on the thread, is this going to be R/C or static?

 

 

 

It's a hybrid at first:  flyable RC airplane with "dress up" parts for static display.  Once I'm done with that project I'll move into phase two, which is a fully static model.  The main difference between the two builds is the cockpit area.  A decent static display cockpit will require removing the RC nose gear completely.  So the phase one build will have the original cockpit.  The phase two build gets a whole new cockpit.  I hope that makes sense 

 

 

[REJ]

For what it's worth, we have an aviation museum, the Fort Worth Historical Aviation Museum (https://www.fortworthaviationmuseum.com/our-aircrafts/ ) that has three OV-10 aircraft on display. One is a mock-up, while the other two are an OV-10A (Air Force) and an OV-10A (Marines). The museum personnel are very friendly towards visiting veterans (yours truly) and usually allow for open cockpits and photography of any portion of the airframe you ask for. A friend and myself are planning a visit there next Saturday (14 Nov) and I can take detailed pictures of any/all portions of the aircraft you need. Just let me know specifically what you need and I'll be glad to get them for you. I can post them here, but would rather send them some other way so as to not take up so much space on your thread.

[daveculp]

Thanks REJ!   I'll take you up on the offer.  One thing I found out while working on this 1/9 scale airplane is that some placards that would normally be unreadable small black/silver squares are now actually big enough to be readable!  I could make up some words for them, like the Gettysburg Address or something, but maybe I should stick to the actual text.  In particular I'm looking at the placards on the main landing gear struts and the placards on the LAU-68 rocket launchers.  I can make my own decals, but I just need to know what text to write on them.

 

I haven't even started on the cockpit yet, although I did some research on that already for my 1/32 OV-10A build.  I'm sure I'll eventually have some questions about the cockpit area.

 

Thanks again!