Mig 21 Rocket Tube Scratchbuild for EDF64

[Bill G]

Been tossing around the next jet subject to model for a while. Meteor, Hunter, various Lufts, etc. In the past I've used model rocket tubes for nacelle frames with success, so why not try a fuse core with one. The only issue is that you need a relatively straight subject, as the curves are a bit difficult to get. Could do an F-104, but I'd end up cheating on enlarged wings so much that it would look like a Mig 21, so Mig 21 it is.

The basic frame is made of 3 sections, with the ends having tapered V-cuts to create a gentle curve. Added a bit of balsa sheet to the end sections for sculpting, to get the core shape more accurate, as it didn't come out quite as curved as desired. Strategically placed lightening slots and holes were cut for weight reduction. I arrived at the pattern by several considerations. First, it needed reasonable width rings to hold shape. The slots could be longer that wide, as the balsa has more strength in the grain direction. The slot width was kept small, to keep the 1/32 sheeting from "sinking" into the slots. The additional circular holes were cut from the "intersection" areas of the web, as the effective ring thickness is largest there, thus making a consistent ring thickness. Its all an educated guess, but there is some common sense to the scheme. The ribs are thicker at the sides for wing attachment strength, and at the top for tail attachment strength.

The completed fuse core looks like a machine gun barrel skin, with cooling holes. The dense laminated cardboard is actually quite strong stuff. Used CF spar rings are in the ends to maintain circular shape. The sheeting went very easily, with small relief cuts added at the nose for the curved shaped, overlapping the fuse core ribs.

The fuse will probably be cut at the seam directly behind the cockpit for EDF and equipment. The mating sections will have some method of attachment, like an Alfa Mig or Sabre. Probably a cheater hole in the fuse bottom.

Before continuing, I need to determine the cg and experiment with EDF and battery location. The canopy will probably be the battery door. One good variable is that the EDF can be located anywhere in the center section. The EDF 64 housing ring only requires a small amount of meterial to be sanded off, to create a perfect slip fit in the fuse. I'm envisioning slide rails in the fuse, and mating key cuts in the EDF ring. Maybe just a small screw to secure location.

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More bizzarre building techniques with each build
Bill

[Bill G]

Finished building the wing panels. Using 2 CF rods in each wing, which will install in tubing, permanently fixed in the fuse. The front rods on the wings will insert in short sections of fuse mounted tubing, so that the EDF can be installed and removed past them.
Started the perimeter frames in the first pic with the CF rods temporarily installed, for proper alignment. Next, the panels were separated and built separately, in the second pic. The rods were temporarily placed in the frames, as they were laid out in on the separated panel plans. Again, to ensure alignment.
The CF rods pass through the wing formers, and along the sq balsa spar aileron hinge line, where they will be permanently glued. Also have CF spars along the wing's LEs. Should make strong panels. The completed wings will plug into the fuse center.
Slowly starting to have a little more faith in this project, as many scratch builds start with, "Do I really want to build this?".

[watt_the?!]

the mig21...my favourite plane of all time...been waiting and wanting a plan for edf for aloong time..

i see you are doing the early variants with bubble canopy?....

Tim....will be watching.

[Bill G]

the mig21...my favourite plane of all time...been waiting and wanting a plan for edf for aloong time..

i see you are doing the early variants with bubble canopy?....

Tim....will be watching.

I like early jets, and this one is not quite vintage, but early enough for my tastes. I also like undermodeled planes. This planes is sush a good subject to model, I'm not sure why there are so few kits, ARFs, or plans.

[Bill G]

Finished sheeting the wings with 1/32 balsa. They plug into the fuse and align well. Had to locate the fuse dowels precisely, as there is no error allowed. The EDF is shown in the fuse, which will be slid much further backward to its permanent location. Thinking of using CF spars inside the fuse as slide rails. The EDF will have small mating notches. The front wing plug-in fuse tubing is only 2 small pieces glued in the fuse sides. The idea is that after the EDF is installed past them, a second piece of tubing is inserted through them before installing the wings, for a strong plug-in socket. Probably will extend the CF spar slide rails past the fuse mating cut. Thinking of gluing locking tabs on them, to latch on the lip of a fuse locating ring, glued inside the front fuse half. The CF spars will provide spring action for the locking tabs to latch on the lip of the fuse mating ring. Will need small holes in the fuse to insert a wire, to release the latches.
The aileron torque rods are also installed in the wings. Tough to see in the pic, but the torque rods have a drag link on them, which be be outside of the fuse. Hey, can't have it all. The drag links are a ball socket link on one end and an e-z nlyon snap link on the other. Will need to locate another servo driven crank arm through the fuse.
Starting to think it may be wise to build the tail section at this point, and try as a chuck glider, before adding internals.

[Bill G]

Well it happened at last, got a $38 USB interface card at Wal Mart, and I can now take and download digital pictures with a digicam. The camera has been laying around for 3 weeks collecting dust. Polaroids were getting expensive, and the last 2 pack I just bought would have almost paid for the card.

Have the elevator servo mounted in the backbone on the Mig, which hides all but the servo horn. Carved a block for the servo mounting postion of the backbone, at the rudder fin front. The rest of the backbone will cover the servo cable. The aileron servo fits inside the bottom rear fin, which is on a real Mig. Hides it a bit, and protects it. You have to get a little creative mounting servos, with the constraints of a closed tube fuse, not wanting to obstruck airflow, and removable plug-in wings. Probably will have cheater intakes.

The nose will be a tab-lock quick release design, with the battery velcro attached to the main fuse floor. The battery seems to have plenty of slide room to accomodate for cg setting, and possible use of lipos from 1320ma to 2100ma.

Pretty much determined the locations for everything, just need to make a bit more building time than the last few days. I'm not happy, as things have cut into my plane building time. "You know how it is".

[Bill G]

Made the ailerons from 1/32 sheet, with a 3/32 former in front, and a 1/16 spar about 2/3 of the way back. A good bit lighter than solid balsa aileron stock.
The aileron and elevator cables route through the backbone. Since the slip fit EDF has to pass by them where they reenter the fuse, I'll need to cut a small notch in the EDF housing for clearance. Could also temporarily stuff them back into the backbone from inside the fuse, with a retrieving string. The front-load design has determined quite a bit of this design.
Still thinking away on nose latch options. Must be quick release, as I don't want to be removing screws every time the battery is installed/removed.

Don't know how to act with this digi-cam. Pic #3, no more Polaroids!

[roccobro]

Looking great Bill, both the camera and plane! Keep them comming. :)

Justin

[Bill G]

Roccobro, thanks, good to hear from you.

Got the nose release mechanism working well. Used carbon fiber spars as latches (sticking out of the rear fuse). 1/2" carbon fiber spar sections are glued to the latching arms, and inside the nose cone, for the arms to latch onto.
The radar cone is mounted on the wire fork shown in the photo. The fork arms slide inside tubing, and are spring loaded, to keep the cone sticking out. The tubing sections are glued inside the front fuse half. This part is not yet glued in place, in the first photo. When the radar cone is pushed in, (see 2nd photo) the wires slide underneath the latching arms and release them, allowing the nose section to be pulled away. The removable nose section will be used for battery access.

[Bill G]

Monokoted the tail surfaces while watching the Super Bowl. Guess its pretty obvious I'd rather build than watch tv. This build has been interesting, due to the use of different linkage setups, than I've used on most planes. I didn't use a standard tie rod for the elevator, since it doesn't work will with the swept geometry, and it interferes with the thrust tube.
In the picture, the elevator servo is mounted just in front of the rudder, in the fuse backbone. Previous posts show the servo location a bit better. Sullivan gold cables route through the fuse and back out to the elevator surfaces. The cables run close to the inside fuse wall, to minimize thrust interference. Since the two cables move in different directions, the control horns are mounted on opposite elevator surfaces.

[St. Martin]

Just saw this thread, Bill. Very nice wood working! Good subject, too.

Steve

[Cyclops]

Bill. You can go the motor with just a fan blade on it. Add 4 or 5 airflow straighteners and done. I too like a 21 for the massive weight reductions and straight thru design. Made for cold air EDF's. :)

A couple of others fall into this layout . British Electric over and under Lightening

Form follows function in any time span.

[Bill G]

Thanks Steve. I still have this project going, but started a mini ME262 (mini me). I seem to have a habit of getting a model near completion, starting and finishing another one, and then going back to the first one.
Cyclops, I like the Lightning to. The EDF actually fits perfectly inside the tube, so it works out well, considering the way this build is addembled. I may make a cheater on the bottom for intake, as the exhaust is cleaner, and shorter distance from the fan.

[Cyclops]

Bill you and me have the same building genes in us. :D

I need a cheap L-4 foamy. Got a great plan from the AMA, 30" WS, $8. Rolled !!

Have a 6' piece of blue foam left over. :p

Plan place, blows it up, :) :) :) .

Where is my respirator ? Happy belt sander.

[Cyclops]

Judie raised my allowance to, 2 blue foam billets, because I got a Hot Flash, and finshed all the unfinished jobs. Great wife. :D

[Bill G]

Bill you and me have the same building genes in us. :D

I need a cheap L-4 foamy. Got a great plan from the AMA, 30" WS, $8. Rolled !!

Have a 6' piece of blue foam left over. :p

Plan place, blows it up, :) :) :) .

Where is my respirator ? Happy belt sander.

To make the plan enlarging real interesting, choose a reference point, get out the old calculator, and rescale by plotting a million data points. There's always some pride in doing things the old fashioned way.:D
On the materials subject, I'm trying a number of different things now. There's so many possibilities. Was strictly balsa for a few years, and then these EDF guys got me into the foam thing. Now I'm experimenting with using different materials in combination with the foam. I'm trying to get away from sticking to the same old building techniques. I think we're going to see a number of new construction techniques developed in the next few years in the model industry, such as the composite ARFs that have been out for a while now.

[Bill G]

One thing I like about this site. I haven't posted on this thread since 2/24, and I only had to go to page 2 to find it.

Back on this thing. Forgot how easy it was to cover tubes with just 1 piece the whole way around, versus ME262 compound curve fuses. Pretty much just needs assembled and covering finished at this point. Still need to devise latching mechanisms for the wing panels. Thought about magnets, since the panels stay in place pretty well as is, with the plug in rods. Only need enough force to keep them from vibrating out.

The spring loaded radar cone push-in nose release is working well. Ok, so if it takes a nose hit, it may bounce everything back into the air with the nose separated. Probably get to use up this gyro that's been layijng around for a while as a roll stabilizer. Never built a delta before, so I'll have to see what happens.

[Ed Waldrep]

If you are looking a canopy that more closely matches the original, you might look at the Sig Kougar canopy, although its probably too big. On my scratchbuilds the past few years I've been using blue foam to cut canopies, sanding to shape, the glassing and painting it matt black since I don't have a vacuum forming machine. I never get around to adding cockpit detail anyway so the black isn't blocking anything but the battery view! ;)

Keep it up! Can't wait to see how it flies.

[Bill G]

Thanks Ed. The vacuum forming machine is the one piece of equipment I'd like to have now. I guess there's no way to pull them over a mold with a heat gun, without vacuum. I toyed with the idea about 5 minutes once, enough to see it wasn't going to well. I'd also like to make them, so I could use plastic the weight of GWS canopies, or even as light as the Guillow's ones. The Sig canopies are both heavy and useless weight, other than looks. The 5" WWII canopy weighs around 8 grams, after trimming down. Way to much weight. 2 grams would be more acceptable.

Covered the nose tonight also. Used 1 piece. Had to pull it tight to shrink around curves, so it slightly flattened the sheeting, where the lightening holes were cut out of the tube frame behind it. Still looks pretty good. Just makes all the time spent shaping and filling to perfection a bit of a waste of time.

[roccobro]

Keep busy Bill. I'm "building" vicariously through your posts. :D

Justin

[Bill G]

Yeah, tonight was a bad night. I had to go to some group I meet with once a month. Takes way to much time away from my building schedule.

[wireit]

Hi Bill,
Nice Plane
if you want to bump the 64 up later on you can use a mini fan or power fan 400 rotor cut down to fit the 64 housing and a B20-15L / Razor2500 on 3s... 20 amps and much more thrust. use a heat sink

joe

[Bill G]

Hi Bill,
Nice Plane
if you want to bump the 64 up later on you can use a mini fan or power fan 400 rotor cut down to fit the 64 housing and a B20-15L / Razor2500 on 3s... 20 amps and much more thrust. use a heat sink

joe

I have a Himax 2015-4100 in it now, that was laying around. I may try it on 4s. I'll probably go the 2025-5300 route, if I like the plane. (May have to anyway, just to get it flying) I not sure what rating the B20-15L is, but a bit more efficiency than the 2025-5300 would be nice. Getting the draw down from mid 20's to lower 20's would be nice on a 3s-2100.

[wireit]

The kv of the 15L is about the same as the RZ2500, 3800~3900 both are pretty efficient... on 3s the power will be up about as far as the rotor blades can stand. Thats the real issue with the 64mm rotor once you get them turning 30K + changing the power setting up and down is all it takes to send the blades flying.... at least that was the end result I got.

joe