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SCEN Puma 3

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Nigel R06/01/2017 12:56:23
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3048 forum posts
475 photos

Crikey, it's been about twenty years since I last put a model together. Be gentle with me. This might be slow. A few weeks ago I was gifted an old kit by my dad who won it in a club raffle. I'm not entirely sure this strictly counts as a classic aerobat per se, but it does come from the 70's, and is aerobatic, and the box does state "for exhibition flying" (!) - so this seemed like the right place to put the build blog.

So this is what I have:

scen_puma_3.jpg

(not my photo)

Completely traditional build, all wood barring the plastic cowl. Kit contents vary from rather good (balsa) to some heavy, soft ply and heavy oversized doublers, so a few replacements to be made, but overall a good box of wood.

The low winger plan is on outerzone here: **LINK**

It's very much an easy build design. Box fuselage - dead flat top with rounded corners. Pre-cut and tapered (nearly) all sheet tail group. Straightforward standard D-box wing.

The kit I have has the fuselage sides cut for the shoulder wing version, so that has to change - I'll cut new fuselage sides.

I'll probably alter the wing section a bit - a semi symmetrical root was used and a very thick symmetrical tip (18% or thereabouts!) - as the model was really intended as a low wing trainer, and I'm after something I know will keep me interested when it's in the air.

My dad threw in a few standard servos with the kit (thanks!) which I will use, and I found some Hitec minis, in the BMFA classifieds, to go on ailerons and throttle.

Right now I'm putting together a build table and a fuselage jig (see **LINK**). All the bits for those have been cut and the jig table is assembled; the build table will be done soon, then there's a bunch of jig blocks to glue together. Hopefully I'll make a start on the kit proper in a week or two.

Ok, photos to follow!

Nigel R17/01/2017 15:18:50
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3048 forum posts
475 photos

Small update.

Build tables and jig are both varnished and need a quick sand. Half the 16 jig blocks are put together. I've ordered some wood and a bundle of hardware (wheels and control rods and all that good stuff) and a few bits to make up the airbourne pack. Wood will cover replacing the fuselage sides, the fuselage doublers, all formers, wing LE, ailerons, and some extra sheeting to deal with placing two servos in the wing.

What I have yet to figure out is where to get a bubble canopy from. 25 years ago there would be a selection in the LHS but even the mail order shops don't seem to carry many. Are there any good specialists?

Finally, I found this rather good photo for some inspiration to get movng!

Ace17/01/2017 18:42:49
265 forum posts
15 photos

SN,

Steve at Vortex would be a good place to start..

**LINK**

David

Nigel R18/01/2017 09:03:10
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3048 forum posts
475 photos

Nice one, thanks David

Martyn K18/01/2017 09:56:08
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4898 forum posts
3552 photos

Thant looks great. Of course it qualifies, its exactly what most of us do.

A thick wing section will help keep the speed consistent through manoeuvres.. Dont be in a rush to change it

Martyn

Nigel R18/01/2017 11:47:13
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3048 forum posts
475 photos

I'm not changing it too much - resulting thickness is mainly going to be determined by what profile I can fit within the original ribs, which works out about 14%, standard power model stuff, I think.

Nigel R23/01/2017 08:05:00
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3048 forum posts
475 photos

Although it isn't strictly part of the build, this is ready now:

 

20170122_222225.jpg

 

Which should ensure the fuselage is nice and straight, at least.

All 18mm MDF, 12" x 48" with some 3" wide stretchers underneath to allow it to be brought flat. Just don't look too closely at the varnishing. Jig blocks are 4" x 3.75" from 3mm MDF. Must admit, they were a bit monotonous with 32 identical slabs to cut out and true up. A disc sander would have made that much quicker.

 

By the way, is there any way I can rotate pictures in the albums here?

 

Edited By SuperNash on 23/01/2017 08:09:49

Edited By SuperNash on 23/01/2017 08:11:31

Martyn K23/01/2017 09:35:19
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4898 forum posts
3552 photos

Nice jig.. It may have been quicker to get them laser cut though

I don't think you can rotate the photos unfortunately.

Martyn

Nigel R23/01/2017 12:51:01
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3048 forum posts
475 photos

Thanks Good point on the laser cutting - it's not a facility I'm used to having.

Rotating, oh well. I'll have to edit before uploading then.

Nigel R27/01/2017 07:36:35
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3048 forum posts
475 photos

Some small progress, rough cut ribs -

20170126_230618.jpg

Nigel R28/01/2017 08:11:30
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3048 forum posts
475 photos

Tidier

20170127_205214.jpg

mightypeesh28/01/2017 08:46:37
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679 forum posts
892 photos

On the canopy front you could try the 'shrunken plastic bottle over a carved buck' technique - link to youtube video here **LINK** of somebody having a go. I have used it many times and although at first it is a bit experimental, once you get the hang of 'chasing the shrink' around your buck with your hot air gun it is really satisfying and cheap to do. Here is one I made for my Acrowot using a 'value' bottle of lemonade. I think it cost about 15p to make! The lemonade went down the sink though - it was vile....... The other is a much smaller model, a Jetex powered Firebird but you get the gist of whats possible.

Cheers, Simon

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Nigel R30/01/2017 08:01:22
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3048 forum posts
475 photos

That's a very professional looking result Simon. The video makes it look quite easy (and a few wasted bottles won't dent the budget much). I don't have a hot air gun though this might be a good reason to get one...

Nigel R01/02/2017 09:32:22
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3048 forum posts
475 photos

I like to get tail feathers done first. Also makes for an easy way to get my hand in with the knife & sanding block. Took longer than it should have and seemed trickier than it should be!

20170131_235203.jpg

This was one of the weak areas in the kit wood. Originally the tail was made up of a large triangle shaped L.E. piece of 5/16", some thin (1/8" ribs, and a single 5/16" square section T.E. The L.E. piece was rock hard, the T.E. piece was super soft. Also the pre cut and pre tapered elevator stock was thin at the L.E. and worse still, had a terrible odd shaped round formed, so that had to be fixed.

I cut the very front of the elevators off. The rock triangle section donated two short wide strips to glue on in its place. When the elevators are separated I will put some more strip running front to rear to help stop any warps appearing.

The front of the main tail section is now formed from the softer of the original wing L.E. strips (which I'm doing slightly differently with an 1/8" false L.E., and 1/4" L.E.), as are the ribs. The tail's T.E. is now made up of two strips, one harder 5/16" (the other wing L.E.), and the original soft strip.

All this bodging around should mean there is some better wood around the hinge area, and less dead weight at the tail's L.E.

The tail still needs some diagonals to be fitted, I think.

Nigel R01/02/2017 10:46:18
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3048 forum posts
475 photos

As an aside, I am making several detail changes to the wing design. Not that the original was bad, I simply have slightly different preferences.

#1. The kit has 1.5" strip ailerons. These are another unmentioned bad bit of wood, with poorly formed L.E. shape and one is a bit warped. Whatever else I did, I would be replacing them with some sheet which will be sanded to shape.

So while I am there, I have also altered the aileron width. They are now going to be wider - 2" - at the root, and narrower - 1.25" - at the tip. This taper means that the aileron is a larger % of the chord, at the root (17%), compared to the tip (15%). The theory being that this should make using flaperons a viable option - when flaps go down, the camber change at the root will then be greater than at the tip. Or in other words, lowering the flaps gives a bit of washout, reducing the chance of tip stall. If you have a constant width flaperons on a tapered wing, then lowering flaps gives wash in. With obvious drawbacks when you're landing! With the ailerons as specified by the kit, they were 13% root chord and 18% tip chord. But, the kit was only intended to be used with a single aileron servo.

#2 Section. As mentioned above, the designed wing uses a thick semi-symmetrical section at the root, and a chunky 20% symmetrical section at the tip. From previous experience I liked full symmetrical wings on an aerobat. I've used a 14% NACA with a slightly sharper leading edge - very similar to what was used on the mid 90s F3A machines.

#3 Leading edge. The kit came with a 5/16" square piece that was intended to go in at 45deg. My preference is a 1/4" leading edge and a 1/8" false leading edge. As I was changing the section, in for a penny...

#4 2 mini servos, instead of a central torque rod set up. Self explanatory.

Nigel R03/02/2017 09:43:30
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3048 forum posts
475 photos

That's better. I shall leave shaping/rounding until later. Time to move on to the wings next.

20170202_225203.jpg

Nigel R06/02/2017 10:19:13
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3048 forum posts
475 photos

No photos... but I have now cut the basic wing components to size.

To compensate for several more poor bits of wood selection (again!) I cut down the kit's TE strips (some straight grained hard strip) into spar size sections, and laminated two softer spars with some 1/16th to make the required TE width. I remember why I preferred scratchbuilding to kit work now! Anyway I have four decent hard spars now, and two medium TE sections. I also cut the 1/8 false LE, and a jig strip. The spar webs are all cut to size ready, I like to do that first and build up the wing in one go to be ready for sheeting. I've dry run the basic wing structure on both sides, which showed a tiny bit of fettling needed to get the spars sitting just so. The next couple of sessions should see some more visible progress as both panels get glued up.

edit - almost forgot, the spars had to be laminated too, the design calls for them to be doubled over the first three bays, tapering down over the fourth; about 10" total.

Edited By Nigel R on 06/02/2017 10:38:06

Robin Etherton06/02/2017 10:30:53
271 forum posts
41 photos

Enjoying the build.

Keep it up

Robin

Nigel R11/02/2017 10:25:49
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3048 forum posts
475 photos

Made a start on the left panel. All completely conventional structure.

20170210_211946.jpg

20170210_212027.jpg

Nigel R16/02/2017 09:46:47
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3048 forum posts
475 photos

Some updates;

I put together the right panel. Looked just like the left but the other way around.

In the middle, we need a couple of scrap block reinforcements for the wing dowel. Some leftover 1/4 and 1/8 provided these:

20170215_092315.jpg

You can also (just about) see the D box sheeting in progress here (see yellow contact adhesive, and small hole in spar). I always used contact adhesive for sheeting, its quick and easy. Got that one from my dad years ago. The hole (for a locating peg) is a recent addition to the technique. It goes like this:

Get wing panel jigged up and held securely in place.

Place sheeting over wing.

Drill a 2mm hole throught the sheeting into the spar at the root and tip. Poke a cocktail stick through the holes. This gives 2 solid locating pegs so that the sheeting goes back in exactly the right place.

Run a bead of contact adhesive over the structure.

Press sheet down in place, make sure you get good transfer of glue from structure to sheet.

Remove sheet, drink coffee while glue dries. Eat biscuit.

Use pegs (which are still in the sheet) to positively locate the sheet.

Press down gently to get good joint over all parts of structure.

I find this works best when you use a sub LE of 1/8". Then the real LE goes on after the sheeting has been sanded back to a nice flat surface. This gets a really good secure joint at the front. Rather than relying on a 45deg strip and edge joining the sheeting to it.

Here's the sheeting, with glue in all the right places and a peg poking through:

20170215_092325.jpg

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