SCEN Puma 3

[Nigel R]

More radio install work. Everything is still just held together in the jig at this point.

I added several 6 x 1/2" x 1/8" supports for the radio tray (not visible here as they're "underneath" the servo tray) and got the servos screwed in to the tray. A 1/8" former goes in at the end of the tray. The servos now live in their own compartment safe from any marauding batteries or cabling. Note, I'm hoping to mount the switches next to the throttle servo.

And another view showing throttle cable route, and the gap behind that former, for wiring to pass through. The bits of scrap are just that, they will be replaced by some neater supports for the throttle cable outer.

Intended connection to the throttle servo end is one of those screw-clamp clevises. The motor end will be a soldered extender and a regular nylon clevis.

I might get the rudder and elevator pushrods done next - as you may guess I want as much of the radio install done as I can, before anything is glued up, because I'll never have as good access to the fuselage interior as I do now - all the mistakes are easy to fix at this stage.

Edited By Nigel R on 06/06/2017 11:40:19

[Nigel R]

Pushrods made up (with masking tape for now, in a continuing theme of dry fitting stuff in the jig). A 12" threaded metal rod is long enough to make the threaded clevis end by the tail, and get a free Z bend from the other bit. 5/16" square hard balsa joins them up. When it comes time to glue up, I will use some heavy thread to wrap the ends (I don't trust heatshrink) and some epoxy to goop it all together.

Pushrods are a little too close to each other for comfort. I think the servos will need spacing further apart, by a 1/2" or thereabouts. We'll see, when I've figured out how much surface movement is needed and which servo horn hole is used. They might be OK, if I use a hole closer to the centre of the servo disc.

I like Z bends - can't come out, can't rotate, low cost.

Probably time to figure out the control runs for the nose gear steering & retraction next.

[Nigel R]

Ok, this is what I've come up with for the retract arrangement:

Two collets retain the main gear pushrods, nothing too clever there.

The nose gear is on a ball link. The thread on the ball is screwed direct to the horn. It has a couple of washers and an M2 nut to raise the ball by just enough to clear the collets. It feels solid enough. And as photo shows it clears both main gear pushrods and the wing surface:

And the nose gear rod itself, after some measuring & bending:

The tiny hole in the former will be enlarged by, well, quite a bit I expect, to allow easy removal.

Swing keeper holds it to the nose gear.

I'm anticipating making the nose gear "door" one big hatch so as to make connection easy on the field.

Steering link next...

[Nigel R]

Ok, not steering link. Instead, I tidied up the fuselage sides in preparation for glue-up, I added the block infill at the tailpost, and the 3 uprights.

The uprights are all from scrap 1/8". The kit supplied ply formers to go at these locations but I think that's complete overkill.

I also sanded the tailpost area to its proper taper so the sides meet nicely.

Ran out of workshop time before I got any doublers on around the tailplane cutout. These will be (probably) from 1/8" scrap again.

[TIM Shaw]

All looking really good Nigel.

Do you have any tips for how to shape the tail ends of the longerons to get such a good fit?

Something I've never managed in my life.....

[Nigel R]

Thanks Tim!

Once I had the sides and main formers in the jig, with the sides pulled together at the rear, lay a length of 1/4" sq (or similar), placed direct over the jig / fuselage centreline - mark off each side of that, and that gives a nice indication of where to sand the taper. I left about 1/16" on each side to do any necessary fine tuning of the angle. But you can get very close on the first go. It's a bit of a fiddle doing the last bit, a few times in and out of the jig so there's some room for improvement in my technique! There's a pictoral of the idea at the bottom of this page:

**LINK**

I wish I could claim credit for it!

[Nigel R]

Last few tidyups before glueing the basic fuselage structure.

At the very back, I added some vertical grain 1/16. Admittedly difficult to see in the photo.

What you can see are the horizontal pieces I made up to keep the sides pressed outward against the jig. They fit in at the same points as the formers would have gone. Without them, the differences in strength between longerons and fuselage sides mean you end up with an unequal taper.

[Nigel R]

Fuse is glued up and out the jig. Goes back in upside down just as easily - very happy with that.

Couldn't resist sitting it on the wing...

[Nigel R]

I thought now was a good time to get the tank floor and fuel lines sorted out, before the top goes on and while I still had easy access to the tank area. Also a good time to get some 1/4 sq reinforcement on the firewall / fuselage joint.

6.5mm holes just neatly fit the fuel tube when it is over the brass tank fittings. Only two lines to the tank, I use the carb feed to fill and drain the tank. I used a countersink bit to clean up the holes - I don't want any splinters or sharp edges near the tube. A couple of trial runs show that when the tank is installed for real, I can draw the tank into place using the fuel lines. The fuel lines can then be cut to length and some silicon sealant smeared round the holes where they pass through the firewall. Obviously I don't plan on removing the tank very often but my experience is that tank removal is rarely necessary.

A tight fit between mount and engine. I relieved the nylon mount with a pair of concave recesses to allow the fuel line to pass unrestricted (sandpaper wrapped around a small dowel). If the engine was a few mm forward that wouldn't have been necessary. Bad planning on my part there, but easy enough to work around.

[Jonathan M]

Good progress Nigel - and very instructive too!

[Nigel R]

Thanks Jonathan!

Got the top rear fuselage sheeting on. This is crossgrain mainly because the pieces supplied in the kit were too short for using lengthwise.

Fuelproofing for the tank area next, and time to get the fin/rudder sorted out.

I think I will leave the front half of the wing open for a while, so i can tweak the retract pushrod with the wing attached. I will need to get the wing and fuselage rigged up first of course.

[Nigel R]

Got a few jobs out the way this weekend.

First off, drilled the dowel holes in F2. Mental note, never ever again leave that job until the fuselage is glued together. I'm not particularly happy with the result - inevitably access was restricted and the drill wandered a little - one hole is higher than the other. I guess it matters not in the end. The holes in the wing were drilled when the wing was held in place on the fuselage, with drill passing through F2 into the wing, so at least everything lines up when the dowels are in. Still, a lesson for next time.

After the dowels were drilled I checked nose gear operation. I need to redrill the servo horn to get another 1/16" throw on the gear pushrod but otherwise all three units move nicely. I have a (poorly shot) short video showing operation, to upload when I can.

I revisited the tailplane: shaped the tips and joined and hinged the elevators. The joiner is 14g piano wire. Holes drilled very carefully with the elevator jigged level on the bench with the pin vice holding the drill at such a height to go in on the elevator centreline. Hinges were fitted in a similar manner using the dubro hinge tool.

Now that retract operation was checked, I finished off fitting the top sheeting, and got the rudder servo raised from the tray to help the pushrods to clear each other. Blocked up using 1/8" hard scrap and 1/8" liteply.

edit: nearly forgot, the tank bay was fuelproofed before the sheeting went on, although it'll need another go to do the sheet itself.

And lastly, I thought I'd get the lower sheeting on the rear. Fortunately, it occurred to me before I started that job, that I might well end up fitting the main battery behind the wing to get the balance right, so instead of doing the sheeting, I built a battery sized box just behind the wing opening. The plan is to leave the box open until the last minute, when I know if the box is actually needed or not. The box is from bits of scrap 1/8".

Edited By Nigel R on 26/06/2017 09:23:09

[Stuart Z]

Nigel

I know the feeling well, if only I could second guess those cuts and drillings before I stuck things together!!😳😳😳

[Nigel R]

Stuart, yes indeed, a crystal ball is probably the handiest shop tool I don't own!

[Nigel R]

Retract test...

I'm only powering the servo with a half flat 1/4 AAA, so it slows a bit during extension of the gear.

Anyway. It shows how I'm arranging the three linkages on the one servo horn without mechanical interference. The two main gear pushrods come in under the horn and are retained by a collet, and the nose gear is connected to a ball link screwed in from above the horn.

As mentioned before the throw is about 1/16 short, for the nose gear, so the horn needs to be redrilled a little further out.

Edited By Nigel R on 27/06/2017 10:12:40

[Nigel R]

Nearly forgot, I weighed the bits as they stand; wing complete with servos and gear, 680g, fuse with nose gear, 340g.

Total about 2 1/4 lb so far.

About 1/2lb more radio to go in, 1 1/4 lb of motor, plus tail (1/4lb?) and covering (1/4lb?) and a little more woodwork on the fuselage (1/4lb?).

I think its on track for about 5lb dry. I will be quite happy if that pans out, it will give a decent low wing loading of about 20oz/sqin (WCL of 10) and excellent power/weight.

[Nigel R]

Not much building lately - holidays instead!

Shop time has been limited to getting the underside sheeted:

and getting the cowl blocks in place, first step is using a couple of clamps to keep the nosering in the right place while two sides are fitted:

engine out, third side fitted:

and the thick chin block added:

Next up, fit the front underside, below the tank / retract bay. Then it is time to start sanding the nose (and the fuselage).

[Nigel R]

Got the retract bay cover cut and tack glued in place, then went to work with razor plane and sanding block:

Finally, the fuselage starts to appear

I need to tidy up a little around the nose ring (a job for some filler, I foolishly cut the ring to the same size as the spinner and of course it should really be about 1/16" larger all round). The cowl also needs me to cut out an aperture for the motor to go.

It does feel like the whole thing is properly taking shape now - very satisfying to carve and sand away at some big blocks and get to the real aircraft underneath.

[Martyn K]

Coming on very nicely. I like the retract mechanism video. What retracts and servo did you use?

Martyn

[Nigel R]

Thanks Martyn!

They are some "genuine retro" Goldberg mechanical units made in the 70s (I think) - very simple mechanisms and quite light.

The servo is the analogue Hitec retract unit (HS75) - it is quite a low profile which makes for easier fitting in a wing.

I would use the Goldberg units again. However, doing all three from one servo is "challenging"...

[Martyn K]

he he.. Yes I can see that it was challenging. A very neat solution though. How difficult is to engage the ball link when you remove or insert the wing?

Those goldbergs are very neat BTW

Martyn

[Nigel R]

Not sure on the ball link - the plan is to make the retract bay with a hatch, so I can connect the pushrod at the retract end (swing keeper, so very easy to do).

I've seen a few photos of setups using double layer servo arms, which I nearly did. I may yet have to, if the ball link doesn't stay firmly attached to the servo arm.

If I'm clever I might even be able to retain the hatch using the wing dowels - although I suspect that might be a bit too clever... and I'd be better off with a spring clip or something.

[Nigel R]

Lots of cutting and sanding and test fitting later...

Bit more clearance needed...

And then there's this which is just a bit too close for comfort. Anyone had any success bending headers? I need to get about 1/4" down.

[Bob Cotsford]

I've adjusted headers with a blowlamp and a bar that's a snug fit in the pipe. If it's an alloy header rub soap on the pipe, when it turns black it's at annealing temperature which should do the trick. Alternately, if you have access to pipe bending springs you could probably get 1/4" bending it cold.

[TIM Shaw]

I've only ever bent headers by accident, involving rapid deceleration from high speed on contact with the ground...

You could consider spacing the header off the exhaust port instead - if you could lay your hands on some 1/4" flat bar , drilkls, files and some longer mounting bolts.