Firefox Mig-31 twin 70mm EDF

[noahangel11]

This build is for a Firefox Mig-31. The fictional plane that Clint Eastwood stole from the Russians in the 80’s move Firefox, and flew by thinking in Russian through the brain interface hemet! I have always been a big fan of the movie and this was my chance to fly this futuristic fighter! (without having to learn Russian)

Please read the entire guide before deciding to build, so you know what’s involved and how it goes together.

I would love to see another one besides mine at FF 2023!

Here is some info on the Mig-31 designation:
https://theaviationist.com/2017/12/...nal-advanced-high-speed-interceptor-ever/amp/

Specifications:
Wingspan: 1210mm (~48”)
Length: 1445mm (~57”)
Powerplant: twin 70mm EDF
CG: 57mm (2.25”) back from angle in wing edge.
Weight of model, no electronics: 25 ounces.

Electronics:
4 channel receiver minimum
3 servos. qty2-17G for ailerons, qty1-9g for canard elevator.
Battery 6S 4000mah
EDF: qty2-70mm

Supplies:
Hot glue gun and glue
5 minute epoxy
10 sheets of 20x30 Dollar Tree Foam Board or FT Foam board.
1/8” (3mm) ply and a way to cut shapes out of it.
qty1 20x30 poster board (black) for thrust tubes.

Plans: Update: Tiled files sheet 1 and the ply and card stock sheets have been fixed to be the correct size.

Rc-skunkworks

rc-skunkworks.godaddysites.com

Flying characteristics:
Tracks straight and true, the delta wing makes it a slight rolly at slower speeds, and I added 40% expo to the elevons. But bring it in straight and it lands smooth and slow. Don’t try to bank hard at slow speeds, or you may stall.

video Maiden:

 

[noahangel11]

Start the build with the engine nacelles. Remove paper from the nacelle sheets on the side that has the score lines. Add some shape to the outermost sections of the sheet only. For each score line, take a BBQ skewer blunt end and widen out the scores, allowing the sheet to bend at the scores.

 

[noahangel11]

In the following photos, I am working on the Left wing engine nacelle. you can tell by placing the sheet on the table score side up with the wing notch facing away from you and the larger outer edge segment is facing to the right.

Glue Qty2 E1 formers together for one nacelle. The E1 formers have notch tabs that fit into the nacelle. Insert the former into the nacelle sheet notch as shown in the picture, with the 3 holes at the bottom and the curved side facing to the right. When happy with the fit and orientation, glue it on the bottom side onto the nacelle sheet.

 

[noahangel11]

On the most recent plans, I added guide lines for where to glue the nacelle skin around the formers. These pictures do not show that.

Glue together 2 E2 formers. They have no notch tabs on them. Glue it at the rear of the nacelle skin as shown, with the curved side facing the same way as E1 and the 3 holes at the bottom.

Slowly fold and glue a side at a time of the sheet onto the 2 formers, working on the non-curved parts of the sheet first. Do not glue down the curved sheet parts yet.
I also run a bead of glue down each score line so the sheet will be stronger and hold its shape.

Be sure to follow the former guide lines and keep everything square.

 

[noahangel11]

Before gluing the ends of the sheet that you shaped, we need to create the thrust tubes and install the EDFs.

roll the card stock thrust tubes up. One side has a tab shape that is a guide for where to glue it to the other side when rolled, overlapping just the tab area. But be sure to try this with your brand of EDF to ensure the larger end of the taper in the sheet fits onto your EDF. Glue to fit your EDF.
Tape thrust tube over end of EDF, overlapping just enough to secure with tape. If you pass the wiring you went too far, there is no notch for wiring.

 

[noahangel11]

Put EDF in place as shown, with wires facing up through open area. Secure EDF with glue.Attach ESC and confirm motor direction with a servo tester or receiver. Tidy up wiring. Add an additional servo extension wire passing in the wing notch, through to the shaped nacelle notch where it and the EDF wires will exit once you glue the nacelle sheets down in place. See pics.

 

[noahangel11]

Repeat for right wing engine nacelle sheet, but make it a mirror image, where larger sheet shaped edge faces left, wing notch away from you, and former curve faces left.

 

[noahangel11]

Next, let's build the main wing. It consists of sheets glued flat together for strength. I found that doing this did not require a carbon fiber spar or rod to strengthen the wing.



Notch out the servo wire path in the bottom wing sheets (the ones with the servo cutouts). The paper remains on the outside, just notch the foam out where sheet was scored. Also not out the edge of the wing sheet for the plywood spars that hold the outer wing tips at their downward angle. Like the servo wire path, only remove the foam not the paper, from the screw sheet side.

 

[noahangel11]

Pass the servo wire through the wing sheet servo cutout and place the servo in the opening where the aileron clearance is cut out. Glue the top wing sheet onto the bottom wing sheet, do not put glue on top of servo wire notch out, so wire can slide freely.

Fold the aileron back at the score and bevel cut the aileron edge to allow bending. Make a glue joint out of hot glue, smoothing off the excess glue with a piece of scrap foam board.

 

[noahangel11]

Repeat for other wing half, mirroring the side you have assembled.

Butt glue the 2 wing halves together, reinforcing the joint with some packing tape.

 

[noahangel11]

The engine nacelles slide on to the wings where the nacelles are notched out. When installing the nacelles from the rear of the wing, the nacelle curved surfaces face in towards the center of the wing. Routing the wires from the EDF and the wing sevos is important. Make sure wires pass through the notch in the nacelle and are guided under the wing surface. The wires face in towards the center of the wing when the nacelles are on their correct sides.
Once happy with the position of the nacelle, run beads of glue to glue them to wing surface, making sure nacelle spans the cutouts for the intake evenly on both sides. Use push pins if needed to hold nacelle into proper place while gluing. See the photo looking into the intake towards the EDF blades, where nacelle sits evenly along wing cutout. Lining this up is important for fitment of other parts.

 

[noahangel11]

Repeat for other nacelle.

 

[noahangel11]

Next let’s make the battery tray. It serves double purpose as the main fuselage support structure.

Use the dull side of a razor to follow the scores in the battery tray. Remove the foam from the trench area where the battery tray will fold. This is a B fold, where the sides are beside the bottom piece. The end of the battery tray with the 2 holes will face the front of the plane.
See pics.

 

[noahangel11]

Collect all the former pieces, stack and glue together the following:
Qty2 F1 formers
Qty2 F2 formers
Qtyd F3 formers
Qty2 F4 formers
Qty2 F5 formers

Next find the F1 to F2 fuselage skin. Remove paper from the scored side, and run a BBQ skewer down the scores to widen them and make the sheet bend at those scores. Pre-bend the sheet to get it flexed.Add some curve to the center section, which is the top of the fuselage. See pics for reference.

 

[noahangel11]

Attach F1 former to the end of the battery tray that has the holes in it, making it flush with the end of the tray. Next add the 1/8” ply reinforcement plates on the outside and inside of the tray holes. This is where the front stabilizer pivot axle passes through.

 

[noahangel11]

Take the 1/8” ply stabilizer control horn and slide it onto the cut to length carbon fiber pivot axle. The length and size of the CF rod is specified in the plans. Feel free to make substitutions for the materials. You could even use an old servo control horn and drill it out to fit the CF rod.

Once satisfied with the fit, slide the CF rod through one side of the battery tray, through the control horn, and then through the other side of the battery tray. Center the CF rod equal distance from the outside of the tray on both sides, then center the control horn and glue it in place. I recommend thick CA or epoxy for the joint.

 

[noahangel11]

Next attach a servo pushrod and mount a servo on the bottom of the tray. It only takes a rod of about 2” to make the connection. MAKE SURE SERVO IS CENTERED WITH A SERVO TESTER OR RADIO PRIOR TO GLUING SERVO IN PLACE. Make servo arm centered and control horn on axle centered.

NOTE: Extend the stabilizer servo wire with an extension long enough to reach out the back end of the servo tray.

 

[noahangel11]

The F1-F2 skin narrower end of the sheet faces to the rear of the model, away from the front of the battery tray and F1 former. Also, the plans mark which end of the skin goes on which former. See pics.
Slide Skin F1-F2 over each end of the stabilizer axle and glue it to the F1 former, leave 1/2 the thickness of the former exposed so you have a surface to attach the nose piece to. Then place F2 former 1/2 a sheet into the back side of the F1-F2 skin.

 

[noahangel11]

Prepare the F2-F3 skin in the same fashion as the first skin. Remove paper, widen scores, add curve to center section. Put it in place on F2 and use it as a spacing guide to place F3 former on the battery tray. F3 former sticks out 2 layers of the 4 from the skin, leaving a surface for the next sheet. When satisfied with the fit, glue F3 to the tray and the glue the skin between the F2 and F3 former, leaving 2 of the F3 layers exposed.

 

[noahangel11]

Repeat these steps for the F3-F4 skin, and the F4 former. Leave 1/2 of F4 former sticking out of the skin.