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Phil's PSS Jet Provost - Build Blog


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Well it was only right I started my build blog on the PSSA 'Mass Build' launch date, having been banging on about it for a few weeks now... So here we are - 1st February 2014, 17 weeks until the PSSA Fly-In planned on the Great Orme, Llandudno (31st May/1st June) where with any luck we will see a healthy turn out of these pretty looking scale models...

wing 1.jpg

I didn't get as much work done as I had hoped today, but I've made a start with the wings. Last night I cut the plan into workable sections and taped the wing panel details to opposing sides of a new 15mm plaster building board.

I made up the lower wing skins from 1/16th balsa yesterday too, the joint was glued with PVA and left flat to harden overnight. So today they were ready for a light sanding prior to being marked up and used in the initial wing layup.

The ribs, very nicely laser cut from the Traplet woodpack, were quick and easy to remove from their sheets and each rib is etched with its number making assembly very easy. I pinned a chunky, slave L/E to the wing skin, positioned each rib up against this then ran round with the cyano as each rib was held vertical and down against its flat bottomed section.

wing 2.jpg

Once all the ribs were glued to the skins the 2 top spars can be added - all the slots in the ribs lined up perfectly. Then the Aileron hinge T/E was added - I cut this from sheet to suit the taper of the ribs from R6 to R10.

With cyano this was a very quick assembly process.

wing 3.jpg

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I've also done a little work on the pilots, as these needed assembly before they could be painted up.

These little 1/12th scale jet jockeys are available from Vortex Vacforms and weigh next to nothing. They are supplied in 2 halves, front and back, and are designed to be trimmed and glued to a piece of 1/16th balsa, which is then sanded carefully to the shape of the bust.

pilots 1.jpg

The photo shows one just glued and one sanded up. Once painted with Humbrol Enamels you wont see the balsa 'spine' and these little guys will populate the office very nicely indeed!

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The starboard wing panel I assembled last night has now had the shear webs added, the additional half rib R1A (at the root) and the slave L/E are also both fixed. Note the grain direction on the shear webs. Again parts were placed in position 'dry' then the wicking cyano was used to quickly fix all these pieces firm.

wing 4.jpg

Prior to fitting the slave L/E it was necessary to temporarily remove the panel from the board and curve the lower wing skin up towards the L/E of the ribs. I found the best way to do this accurately was to jack up the T/E of the wing panel gently (I rested it on an old piece of Aileron stock) whilst securely pinning the overhanging sheet flat to the board. There is far less L/E curvature towards the tip of the wing than at the root, so the wing needed to be raised far less at the tip than it did here at the root as shown. The aileron section makes this very easy to do, as shown.

wing 5.jpg

With the lower skin glued to the ribs at the front, I made up the slave L/E by cutting it from 1/16th sheet and once happy with its shape (and chamfer - remember the top skins will fit ON TOP of this at the front) I glued it in place.

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With the starboard wing panel at the point where we'd start to think about the torque rods, I put it to one side and assembled its matching pair. This went together even quicker than the first with the learner/repeat method improvement... a very enjoyable mornings work.

wing 6.jpg

Bringing the 2 panels together proved the angles of Ribs 1 were good for the dihedral thumbs up

wing 7.jpg

However, having removed the wings from the board I'm left with a question regarding the wing rib webbing. The plan has a clear note (actually covered by the wing when being built) showing the need to orientate the webbing grain SPAN WISE. Now in the 27 years I've been modelling I've seen a fair few built up wing structures and have always webbed the top and bottom spars with the grain vertically... and this is what I've done this morning, thinking I knew best...

Andy B - perhaps you can comment here - is the span wise grain selected purposefully to counter the fact there is no bottom spar in this particular wing??

If so, I'll have to admit my error at this stage of the build, and add another series of 1/16th webs in front of the top spar with the grain orientated as per the plan!

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The original had no spars at all and the grain of the webs was spanwise (relying on a medium-hard wing skin and medium-hard webbing for structural strength) and it seemed natural to build it with spanwise webs. However, the wing design was modified after I smacked it into the Ivinghoe south slope; there was no damage other than a couple of span-wise cracks near one of the wing tips where the inertia of the tip-tank had apparently twisted the wing a bit too much during the crash. The easiest solution seemed to be to modify the plan with a couple of medium-hard 3/32" spars in the wing top surface to handle some of the extra stress; unfortunately, this makes the wing look relatively conventional and it's very easy to just go into autopilot and add the webs in the usual manner.

Having said all that, I think it might be fine if the wing skin is made from sufficiently hard balsa, but if it bothers you then for peace of mind I'd consider adding the spanwise webs for at least the first three or four rib bays (just don't go to bay 5 and stop, because that's where the aileron is and there will then be a structural discontinuity), or possibly the whole wing. In the latter case, you will have quite a robust structure!

Edited By Andy Blackburn on 02/02/2014 16:31:57

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Whoops! enlightenedIt all makes sense now...

Thanks for the explanation Andy, as it is I'm not happy that the wing skins are anything more than medium density, so as you've recommended, for peace of mind, I'll add another row of webbing. I'll place the span-wise webs ahead of the rear spar - giving an effective ply-webbed effect.

In 1/16th balsa fitted with a few drops of cyano the weight gain should be negligible...

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Not your fault Andy, like you said, I was cruising 'happy on autopilot' with my vertical grain... easily fixed.

Earlier this afternoon I gave the wing panels a rest and assembled the push on Tip Tanks. There's a slight deviation from plan here, which shows a 1/8th hard balsa spine and 6 x 3/16th soft balsa laminates, the inboard 3 having aerofoil shaped holes cut into them to accept the wing. In reality, the wood supplied by Traplet is just very slightly thicker than 3/16th and this, multiplied by 6 in the stack means you can effectively do without the 1/8th central spine and still achieve the required width for the tip tank. So 6 parts per tank, not 7.

tip tanks 1.jpg

Take care aligning these and glue them together with PVA or aliphatic to enable smooth sanding later - don't use cyano or epoxy as you'll end up with ridges! My woodpack was one of the very first made by Traplet, and it had in it an omission in the laser programme where the 3rd inboard laminate had not received the aerofoil profile cut - so this had to be aligned and cut by hand to enable the wing to push on through the thickness of 3 laminates, not just 2.

I do recommend making one of these at a time and labelling the parts LEFT and RIGHT - take care to make a handed pair... with all the parts out of their wood sheets at once there's a lot of very similar looking sections to align. Next job for these is to sand to profile in plan view then make them round in section from front to back...

tip tanks 2.jpg

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Hmmm, well RedBaron must admit to not noticing this instruction either.. And like Phil my web grain is classically vertical.

For myself I don't think it matters a lot. If I was going to argue the point I would say that the grain of the skins provides the strength spanwise and the web with grain vertical holds the top and bottom skins together, thus completing the "D" section at the front of the wing and it is the "D" section that gives the wing its strength. If the web grain is horizontal possibly that might invite an unnoticed spanwise crack in the webbing and so destroy the integrity of the "D".

I think that it would be hard to aerodynamically stress these short wings to breaking point. Also models are designed to fly not crashsmiley. If one does crash one does want something simple to break thus absorbing the energy that does have to be dissipated.

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Thanks Phil for the info on where to find some lightweight pilots. I didn't fancy the 'Pete's Pilots' version as they sounded a bit heavy, and I'm trying to build as light as I possibly can. I was messing about trying to create something from a polystyrene ball and a few bits of foam sheet, but it wasn't going well! I'll probably just end up adding nose weight anyway so it's probably a waste of time, but worth a try.

Were these the PTS 4 pilots? I hope so cos that's what I just ordered!

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Hey Steve,

I think the Vortex website is a little out of date, and in fact the pilots I've bought from there don't seem to be listed on the website as far as I can tell - certainly the dimensions of the PTS4 are bigger than the ones I have here, my jet pilots are only 48mm high and approx. 45mm across the shoulders... making them much smaller at 1/12th scale than the PTS4 description.

I'd phone Vortex (Steve) and ask him to confirm, if you've only placed your order today I'm sure he will be able to correct the order and supply you the smaller ones suited to this model.

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Not too much progress to report on my JP build this past 2 days, I've added the span wise grain webs (in addition to the vertical grain ones added incorrectly) so there are webs now both fore and aft of the rear top spar in each wing panel, should be bullet-proof!

I've started to sand the wing tanks, although they are not pretty enough yet to justify a photo! - I've also assembled the balsa/ply/balsa wingtip laminates and begun to detail paint my pilots.

Next step for me is to build the aileron torque rods, for which I have some 3mm carbon tube on order.

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My wing build has been halted somewhat here whilst I await the delivery of the 3mm carbon tube for the torque rods, checking online shows my parcel due to arrive on Monday (frustratingly...)

So I've been working on some bits and bobs around the wing build (not wanting to start the fuselage until the wing is more progressed!)

I've made up a little elevator actuator from 2 lengths of bent M2 rod and a short piece of 5mm brass tubing. The 2 pieces of steel were bent to shape in the vice, tacked together with cyano on the board and once happy with the arrangement they were potted into a mixture of epoxy with microballoons inserted into the crimped brass tube. This was then crimped again to form the flat plat which the pushrod clevis will interface with.

elevator horn.jpg

A small hole is required still in the flat brass plate for the clevis to fit into.

The little plastic pilot is now fully detailed and ready to report for duty!! My chosen colourscheme is one of a famous RAF aerobatic team so I'm only having a single crew in my model as it would have been flown during displays...

pilots 2.jpg

I'm never very good at painting pilots accurately, but the colours are 'representative' of RAF flight attire - anyway I think he looks the part - "Tally Ho! Ginger!!"

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Phil,

look forward to seeing this build progress and the pilot looks good to me.

I remember our paths crossing years ago at work and that you mentioned that you were a keen aero modeller. I got back into this hobby a couple of years ago after building my last control line model back in the early eighties and it has been a great experience.

I think that I saw your name down for the bbmf visit in a couple of weeks time, so have I, or perhaps I have the wrong Phil Cooke.

cheers

dave

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Hiya Dave good to hear from you and great to know you have got back into the hobby!! As I recall it was golf that was eating your spare time back when we were working together on the Trent 700 EP?

Yes I'm due to visit Coningsby and the BBMF with the RAeS later this month, looking forward to it! - will have to have a catch up there on all things work and modelling!!

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Built up the little servo boxes in the wing and trial fitted the servos - simple 1/8th balsa box built behind the rear spar/web as per the plan - remember to leave enough room for your cables to exit without crimping. The top sheeting will be glued to the box, the height of which is tailored to suit its neighbouring wing rib.

wing servo 1.jpg

When the wing dihedral is added the servos get closer together than shown and would clash with these servo arms as shown, so its important to change these to a single arm for each (pointing outboard). The servos I'm using for reference are Hitec HS-65 with metal gears - well proven for slope models of this size/energy.

wing servo 2.jpg

Edited By Phil Cooke on 09/02/2014 11:51:52

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With no more jobs possible on the wing awaiting the torque rod material - (and some decisions on the final design I'm going to employ there), I've turned my building board over and commenced work on the fus whilst there was still some working time left this weekend...

Nothing too amazing here, but its worth pointing out that there are 4 pieces in the Traplet woodpack per fuselage side, as the total depth of the side just exceeds the 4" stock around the cockpit area and just behind the wing. The 'round fingered' joint has a degree of movement in it due to tolerances so its essential that with the front piece pinned down to the board accurately you ensure the tailplane datum is achieved exactly at the rear before that is allowed to dry.

fus side 1.jpg

Once the first half is dry its a good time to make the second one before you apply any of the doublers or triangular section to help ensure the 2 basic fus sides are identical in profile.

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Sorry to poke my nose in again, but...

On the original model I had a full-depth 1/32" ply dihedral brace that extended out to R3 on each wing, but for reasons of incipient paranoia I also had the back-end of the servo box in one piece, as a kind of mini dihedral brace, made from hard balsa with the grain running spanwise and the servo bay floor suitably reinforced (every little helps).

However, it doesn't show up that way on the plan because the wing centreline confuses matters, and I've always done it like that so didn't think to check; it probably doesn't matter much if you miss that additional brace off as it seems to be strong enough as it is (as long as the balsa used to construct the wing is quite hard), but I'd suggest an additional reinforcement on the wing joint with half an inch of gauze bandage & PVA (or similar) - just to be sure. It can't do any harm...

Edited By Andy Blackburn on 10/02/2014 18:24:19

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