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Andy's Partly-Retrospective PSS Jet Provost Build

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So, I designed and built this Jet provost back in late 2004;


happily, it turned out to be a very reasonable flying machine and I had several years of regular flying out of it, then a few years ago when the weather had been really bad all year, I sold it to Scott Edwards at one of the Ivinghoe Soaring Association bring & buy events. Of course, I regretted it within a few months, but Scotty refuses to give it back smiley.

I'd always planned to build another JP because it's very suitable for using small patches of lift and is generally good fun, but several years of mostly power flying got in the way, and coincided with a general disinclination to actually build anything with a traditional balsa structure (i.e. anything not an ARTF); I've no idea why this happens, but apparently it's quite common to have an occasional hiatus.

Luckily, it seems that over the last few months I'm much more inclined to sit down in front of a building board, or in front of a screen and design new models, the only things that get in the way are now motor sport and a requirement to spend time improving fitness following a recent health scare... sorry, I appear to have digressed.

I finally started on another Jet Provost during the early part of last year, and generally fiddled around with it on and off for most of 2013 until Phil announced his master plan for a PSSA Mass Build, so I thought I'd better pull my finger out. Needless to say, by that timeI was already committed to a Flair Fokker DVII build and this is still in progress, so there will be a certain amount of time-sharing of the building board going on, but I'm hoping to have the JP flyable by about April - we shall see.

Since part of this is retrospective, I'll be fairly brief in covering progress to date. There are a couple of things to note, though

  • Firstly, the wing structure here is NOT the same as that on the published plan because I re-drew the wing structure as an experiment to see if it could be improved. On balance, I think I prefer a variation of the original drawing with an additional bottom spar and 1/16" sheet webs.
  • And secondly, the parts are not from the laser-cut parts pack that other people are using, they were cut by Sequoia Systems in Wraysbury from some DXF files that I supplied; it cost a bit but it was worth it.

The wings were built with a 1/8" full-depth spar with 1/16" dihedral brace, which is possibly slightly quicker than the "normal" structure:


Torque rods were made from aluminium tube and blue snake outers as stated on the plan, but I couldn't find any 3mm carbon tube. However, it turned out that 1/8" dowel sanded down to about 3mm is nearly as good, and it still sounds like a pencil when tapped on the bench:


The only real downside to using 1/8" dowel is that drilling a hole in the end is more difficult because there isn't a ready-made pilot hole as there is with carbon tube.

This is the wing ribs slotted to accept the torque rods:


...and assembled just prior to adding the top wing skin:


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Tail surfaces are straightforward, but I added some 3/16" x 1/16" spruce reinforcement which can probably just be seen in this picture:


The fuselage just dropped together, although I was a bit blase about it and wasn't paying attention as it dried, so it ended up with a very minor twist that had to be corrected later:


Here's a picture of the wing with protective masking tape added when shaping the LE & TE, and the fuselage showing the planked top decking:


I know the plan says to sheet in 3/32" sheet, but on the two JPs that I've built I found that planking with 1/8" sheet and sanding down to 3/32" is easier and stronger.

This is the fuselage with all the bits of balsa block added:


I find it's usually best to use balsa cement for this because it doesn't leave a ridge after it's sanded to shape (cyano will, PVA might if the gap is big enough). Luckily, I was able to leave it in the airing cupboard overnight to speed up the drying process without arousing too much suspicion ( "What's that smell?" "Oh, nothing dear, just glueing something. I expect it'll be dry in the morning." ).

After a bit of razor-planing (didn't nick the fingers this time, thankfully) to match the side and top view, we're left with this:


...and after more sanding and razor-planing, the semi-finished structure can be revealed:


...OK, it's some way off being ready for covering (needs intakes, underwing fairings, wing root fairings, cockpit furniture and canopy), but it's getting there - should be a couple of months if I'm lucky, by which time I'm hoping that the rain might have stopped.

Some time ago I got hold of a decal set for XM384 (colour scheme is silver/daygo orange) from Pramid decals:


- which is the example that Terry Lidstone originally modelled. I'm hoping there won't be too many colour scheme clashes...

Edited By Andy Blackburn on 05/02/2014 14:10:56

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Posted by Andy Blackburn on 05/02/2014 14:05:20:

Some time ago I got hold of a decal set for XM384 (colour scheme is silver/daygo orange) from Pramid decals:

- which is the example that Terry Lidstone originally modelled. I'm hoping there won't be too many colour scheme clashes...

Edited By Andy Blackburn on 05/02/2014 14:10:56

Wrong! I will be there albeit with a different reg.

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Posted by Matt Jones on 05/02/2014 15:43:30:
Posted by Andy Blackburn on 05/02/2014 14:05:20:

Some time ago I got hold of a decal set for XM384 (colour scheme is silver/daygo orange) from Pramid decals:

- which is the example that Terry Lidstone originally modelled. I'm hoping there won't be too many colour scheme clashes...

Edited By Andy Blackburn on 05/02/2014 14:10:56

Wrong! I will be there albeit with a different reg.

Oh well, there you go... smiley.

Maybe Phil can provide a set of (removable) stickers so that we can decorate one of the tip-tanks with personal colours...

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Just got a few pictures today, been a bit busy with other things. The tailpipe was marked out with a soft pencil so that most of the excess wood can be removed:



And then after about five minutes with a drum sander attachment:


...and some careful work with some of the round Permagrit blocks, it looks like this:



It does look a bit fragile but the tailplane stub (the bit in between the elevators) has its grain at 90 degrees and will reinforce things nicely.

Also made a start on shaping the tip tanks properly, here are the corners razor-planed off at 45 degrees:


Planning to finish off the tanks next, then have a go at the intakes.


Edited By Andy Blackburn on 12/02/2014 20:41:02

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Went to the club "Winter Projects" meeting on Thursday, a fellow club member had a Nijhuis 72" Spitfire finished, a Brian Taylor bf 110 started and a Nijhuis P-51D being covered with glass cloth, all since the end of last year - Staggering. When asked how he managed to build so quickly, he said "Well, I don't watch much TV". I'm going to have to take a leaf out of his book, in spite of my dreadful weakness for 2 and 4 wheel motorsport...

This morning's job was the tip tanks; the story in pictures:

1. Roughly razor-planed by eye:


(yes, that *is* sunlight in the picture. I'm taking it as a good omen.)

2. After some work with a coarse Permagrit block, again by eye:


3. High spots identified using a card template:


4. High spots sanded off, smoothed off with hand-held sandpaper:


Quite gratifying, actually. They're not quite finished, not quite symmetrical and not quite the same, so I'm going to come back to them in a week or so and have another look.

On the original plan, I didn't think that the available drawings did a good job of representing the tip tanks so I measured them from a photograph on Jet Provost Heaven . However, (confession time) on looking at them again, I think that as drawn the nose probably needs to be very slightly more blunt - so before covering, I'm going to take a deep breath and sand a small amount off. Carefully, with fine sandpaper, after a stiff drink.

Edited By Andy Blackburn on 15/02/2014 14:26:54

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Made a start on the intakes this week; a couple of pieces of soft 1/2" sheet glued with balsa cement (works best if it's a rubbed joint) and luckily, it doesn't seem to have left a glue line. The edge that touches the wing has to be chamfered a bit to allow for the dihedral:


...actually, it turned out that this wasn't quite enough, it needs a bit more fettling. This where the intakes fit:


The top profile of the intakes were traced from the plan...


...and the intakes rough-shaped (by eye) with a razor plane:


...and then sanded to shape:


The front corners are at least as rounded as what's shown on the plan because sharp edges will cause turbulence = drag = lack of performance.

The intakes now need to be pegged in place so that they can be removed for covering, and I guess the next thing to do is the wing fix & centre section reinforcement, and I want to have a go at some wing root fairings, partly because it might look better, and partly because it should reduce drag.


Edited By Andy Blackburn on 21/02/2014 23:13:41

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Finished off the wing fixing and centre-section reinforcement this weekend; I decided that it was probably better to have at least one of the 1/32" ply wing bolt plates (changed from 1/64" ply on the bottom surface after the wing-bolt pull-through event described elsewhere) flat, so needed some 3/32" sheet infill around the wing bolt, sanded flat:


...so the 1/32" ply plate now sits flat:


The bottom plate is bent carefully after grooving with a biro and the the back of a modelling knife:


...which provides a gratifyingly-triangular-section ply/balsa/ply composite at the trailing edge:


the centre section was reinforced with gauze bandage and PVA - much easier to use than epoxy. Note that the inside of the servo bay has already had this treatment:


...and on the bottom:


Didn't want the wing bolt pulling through again, so added a 15 mm square of 1/32" ply which was then sanded (more or less) flat to provide a good surface for a steel washer under the bolt head:


...and here's what it looks like with the bolt and washer in place:


I'll try and sort out the wing root fairings this week, but it looks as though it might be quite busy at work. We'll see.

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Posted by John F on 24/02/2014 16:41:14:

Forgive me for asking but I would not have thought that PVA and gauze bandage offers any strength to the joint. Have you used this method before?

Edited By John F on 24/02/2014 16:41:38

Used it (and seen it used) several times, and never had one fail.

However, I'm curious - why do you think that PVA and gauze bandage wouldn't offer any strength to the joint?

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Many years ago 1982 IIRC Dennis Tapsfield had a plan published, Chilton DW1 (I known a power model) 84" span weighing in at around 10-11lbs my example powered eventually by a Laser 90, the wing was cut from Foam as per instructions the only difference to the plan was I covered them in 1/16" balsa, the tip panels were over 1/3 of the span and the plan called for them to be butt joined and a 1" bandage attached with PVA and that was it no braces etc.

I thought the expectation high! then I had the pleasure of meeting Dennis a few times starting around that time and asked if this was right? he assured me it was fine, I went to fly that model very regularly for 16 years before pensioning it off and it never once gave me any cause for concern.

So John I probably thought as you at the thought of it but in practise I would not hesitate to use this method.

Great model Andy I am building mine too but work is getting me behind with it, I may just have to take it into work if it carries on.



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Didn't get as much done this week as hoped, partly due to various car fiascos. But realised that I hadn't quite finished the intakes - there's a bit of a gap at the bottom edge of the intake that needs filling with thick sheet, sanded to fit, then the intakes need re-profiling to properly match the front view. Here's how it was done;

Some very coarse sandpaper was taped to the fuselage side:


then some soft 1/2" sheet was hacked/sanded to shape, then tack-glued to the fuselage side with some tiny spots of medium cyano:


...when set and the tape removed, it looked like this:


...which was then sanded flat to the fuselage side so that the intake fitted over the top, then carefully removed from the fuselage side - it should just pop off if a minimal amount of cyano was used.

One picture I don't have is the fuselage wrapped in clingfilm and the intakes + fairings glued (with balsa cement), taped and pinned; I'd removed everything before thinking to take a picture...

Anyway, the (almost) final intake looks like this:


It still needs a bit of a re-shape on the bottom edge, but it's basically there.

Now I can start the wing fairings... smiley

Edited By Andy Blackburn on 03/03/2014 13:23:55

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Please forgive the question Andy, I have been a medic for quite a few years and used gauze many, many times. I would never have thought of it as having any strength in such applications, even with PVA.

I am happy to be educated to the contrary.

I really like your sanding skills and the attention to detail that you're doing.

You mention that you designed this aircraft and have a published plan.  Do you have a link to the plan at all?

Edited By John F on 03/03/2014 13:37:56

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So, the wing fairings...

I've only just got around to this because a) I've been putting it off, and b) I need to move a whole load of CAD files from this computer (it's being end-lifed) and if I'm honest I've been putting that off as well.

I managed to find a really good picture that shows the wing root area, but to be honest I'm just going to make it up as I go along... smiley

The easiest construction at this scale is probably 1/64" ply base and soft balsa filling, carefully sanded - I don't really fancy rolling 1/64 ply when the bend radius is smaller than my fingers. Some card templates were cut and trial-fitted until they were about the right sort of size;


...which were used as patterns to cut a couple of 1/64" ply fairing bases. The wing seat was then rebated by about half a mm and the ply bases glued to the fuselage with 5 minute epoxy (because it's gap-filling!) with the wing bolted in place:


This seems to produce a decent fit if you tape the bases to the wing first (as shown), and - luckily - I had taken the precaution of sellotaping the wing centre-section so that the squeezed-out excess epoxy didn't stick to anything it shouldn't.

I added a brace that looked about the right size:


Then realised that it should have been a bit thicker so that the second part of the base could be added:


This is the second part of the base:


Bottom view of the other side:


I'm going to let the cyano go off properly (it's odourless, and takes a while for the glue fillets to set properly after the initial grab, and I don't like using kicker. Should have done by now though) then fettle it a bit with the sanding block (both sides are not quite the same) then find some really soft balsa to use as filler.


Edited By Andy Blackburn on 09/03/2014 17:01:52

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Finished the wing fairings, this is what happened. Added some shaped (very) soft 1/2" sheet:


Stuck a bit of protective tape on and sanded to shape with various Permagrit blocks:


Did a trial wing fit...


...but it needed a bit of adjustment, so:


...and this is how it looks from the top:


Not too bad for something cobbled together without any real plan - although it was a fair bit of effort.

On to the underwing fairings; I have some very light 1/2" sheet which was marked by highlighting the various wing features with 4B pencil and then pressing into place to leave an impression of the bits that sit proud:


- so, those bits need relieving so that the wood will sit flat on the wing surface, then it needs to be tacked in place for shaping - although there are a couple of potential problems:

  • The fairing is a little thin at the back
  • The wing bolt head is a bit thick

so the edges will probably have to be shaped first, then the fairing glued on permanently before being sanded to shape.

Edited By Andy Blackburn on 12/03/2014 20:54:46

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This is what I've managed to get done at the weekend, in between some important monitoring of motorsport events in Melbourne (pleased that Williams have finally built a good car, but don't like the noise they make...)

So, fairing halves glued together:...


...then glued to the wing. Of course, it'll be unsupported at the front...


...so a set of scrap filler pieces were cut from 1/4" sheet:


...then inserted and glued with PVA:


They don't have to fit 100% accurately because the excess is shaved/sanded off:


The wing was then bolted on and the usual protective masking tape applied:


After a few minutes with a razor plane, most of the scrap fairing wood was removed. However, the wing bolt projected below the surface of the fuselage; this was about as far as I dared go with the razor plane:


So I hacked around the head of a normal bolt so that it wouldn't get in the way:


...which seemed to fit OK and provided more headroom for the razor plane:


It *is* possible to use a scalpel to cut the edges of the fairing - this is one technique:


After a fair bit of work with razor plane, scalpel and Permagrit blocks, it looks like this:


...which isn't bad, really. Not perfect, but it's on the bottom so no-one will notice. smiley

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I haven't actually got much done this week because the impending demise of Windows XP (together with a number of incipient hardware failures) means that the computer on which I'm writing this is being decommissioned, so there's a vast amount of stored and generated "stuff" to be sifted through, tidied up and transferred to the new machine over the next couple of weeks or so. The task is only made bearable by sneaking off to the modelling room every so often, and quantities of red wine which - luckily - I am required to drink for medicinal reasons.

This next sequence of photos shows the construction of the under-wing fairings.

A prerequisite for this is that the intakes have to be pegged into place; a piece of scrap sheet 2" x 3/8" was positioned inside the fuselage on top of the cyparis/hardwood wing brace and used as a guide to make some pin holes from the inside of the fuselage, which were then enlarged to 1/8".

It's then relatively easy to sharpen a piece of 1/8" dowel, bolt the wing in place, position the intake in the right place and run the dowel all the way through the fuselage from the other side, making a dowel-hole in the intake in exactly the right place:

pegged-intakes- 001.jpg

Small pieces of scrap dowel are then cyanoed in place so that the intakes can be temporarily assembled whilst the rest of the fairings are shaped. I try and sand the inside of the intakes slightly concave so that gap-filling glue can be used for final assembly, allowing any excess out at the wing seat:

pegged-intakes- 003.jpg

Here are the rough-shaped intakes after drying overnight - the "extra" bits are there to cover up the gaps - had to do that otherwise it might have ended up with more filler than wood.

wing-fairing- 003.jpg

Wing bolted to fuselage with fairings taped in place:

wing-fairing- 005.jpg

One intake sanded to approximate shape, courtesy of Mr Permagrit...

wing-fairing- 010.jpg

...and the other side done to match. Looks a bit of a shambles, but when the tape comes off...

wing-fairing- 013.jpg

...it doesn't look too bad at all:

wing-fairing- 014.jpg

OK, there are a few dings and the left and right hand sides are not 100% identical, but that can be rectified later - blemishes become more obvious the day after. And in any case, I foresee a certain amount of lightweight filler being used in this area. The stuff that I have in stock is probably way past its shelf life though...

Edited By Andy Blackburn on 23/03/2014 21:16:08

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  • 2 weeks later...
  • 3 weeks later...

Doing the cockpit; wasn't sure which pilots to use, the Pete's Pilots ones are heavier, require no assembly, fit straight in but are possibly a tiny bit small for 1/12 scale, the Vortex Plastics ones are a little bigger, weigh almost nothing but will require trimming.

Decided to go with the Vortex ones, so the two halves get cyanoed to a scrap of 1/16" sheet:


Cockpit cut out with a razor saw blade and scalpel:


Lined with 1/16" sheet - rather than cut as on the plan, the cockpit cut out is a little lower to provide a bit more room for manoevre when fitting the pilots:


...which required some spacers on the bottom of the ejection seat parts. The pilot halves also required some internal reinforcement so that a few millimetres could be cut/sanded off the base:


...it's a tight fit, but it does all fit in:


...even with the canopy on:


Scrap balsa sides added to bring the cockpit edges up to the right level:


More to come tomorrow...

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I have actually done bits of model today but don't have much to show for it frown.

Tackling the tip fairings (that replace the tanks for light wind use); after a bit of thought, the simplest way forward seemed to be to cap the tips with 1/64" ply, cut to about the right shape with scissors and stuck with Superphatic, then tack another one on with medium cyano - which doesn't stick to ply very well - then drill through the whole lot to take a couple of 1/8" dowels:


The second ply cap is more obvious in this pic:


The 1/64" ply cap that was peeled off was then stuck to a tip block laminated from a couple of bits of 3/8" balsa; hopefully, there won't be a glue line:


The intention is then to use a couple of 1/8" dowels to position everything and then tack it in place for shaping.

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Carrying on with the tips; the tip blanks were drilled for dowels and then carefully glued with a bit of PVA at the bottom of the hole, inserted in the wing tips whilst everything set...


...then the tips were tack-glued in place (medium cyano again) and everything planed/sanded to shape. The tips turned out to be a slightly different shape to what was drawn, but the difference is minor...


I confess to a certain amount of stress when it was time to separate the tips, but in the end it all came off ok:


It seemed sensible to sort out the radio gear fitment at this stage, this is the switch holder that will be fixed to F4 so that the switch can be actuated by a bit of 20 guage wire:


...and the rudder and aileron servos fit into mounts that can be glued to the inside of the fuselage once the right position is determined - not my idea to do it this way, I borrowed it from Steve Griffiths.


Two aileron servos are fitted because it's then much easier to adjust the differential, and I want to pop-up the ailerons to act as spoilers for use in tight landing areas.


It seems that I've blundered a bit with the aileron horns - in the previous photo the torque rods project straight up out of the wing, but the position of the clevis hole on the torque rods is such that they are effectively raked forwards. Really, they should have been angled backwards about 20 degrees or so. There's nothing I can do about it at this point because unlike conventional wire horns, you can't twist these using pliers - there's no give. The best that can be done is to offset the servo horns a bit.

Ailerons were (carefully!) separated from the wing and chamfered, it needed quite a lot on the top surface:


...and they don't look too bad once installed:


...although I might have left a bit too large a gap at then ends, maybe some corrective action with scrap 1/32" sheet is called for.

I haven't got any M2 studding and the shops are shut, so had to make some aileron pushrods from the scrap box. Here's the ailerons at neutral - note the aileron horn offset to counteract the aforementioned torque rod issues.


Here's what happens when full spoiler is applied:


...and the is full spoiler and full aileron:


...it's not very comfortable but I think I've just about got away with it.

The plan for the next few days is to finish fitting-out the radio bay and then finish everything else off, working backwards towards the tail.

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