Phil's PSS Jet Provost - Build Blog

[Phil Cooke]

hehe thanks Tim, my own fault that one, pushing the model beyond the wings capability - I'd put a meaty 1/4" ply spar 8" into each foam panel and then bandaged ontop of the centreline but the foam panel failed outboard of the spar - in hindsight I was daft and clearly the veneered foam panel needed a top and bottom 1/2 x 1/4 ramin spar from root to tip to endure the DS loading...

Gutted as it was going really well. Also gutted as it was only the models 6th flight, less than 10hrs on the airframe and I was just building a nice relationship with it, IYKWIM. Anyway there's another one on back order. With spars this time. It does show the extra loading involved in DS though, only the week before the crash I had flown on the Orme in 55mph winds and flew the skin off it, no signs of stress or flutter that day in conventional soaring...

And I must say thanks to Simon for making me look a numpty in the mag.

Anyway, back to the JP - are you joining us on this fun mass build effort? Should be a big squadron of these models up on the Orme in May/June!

[Phil Cooke]

Not a lot of progress here so far this week - I have made up the left and right handed fuselage sides with the triangular section and doublers around the wing root, I've also assembled the balsa tube that will accommodate the Rx Battery.

I've left the triangular section a little long at the front so it can be sanded flush with the former F2 once the nose is pulled in, otherwise a square cut flat on the board will end up being short of the former once curved. I've also elected to stop short the top triangular section under the tailplane, electing instead to drop a balsa block in flush with the tailplane datum surface once the fuselage is joined, as opposed to sanding the overhanging triangular stock back to this datum.

I've sourced both metal rod and carbon tube now to make the aileron torque rods, but struggling in both to get perfect sizes... I've found the 3mm carbon rod with a 1.5mm inner dia needs to be drilled out to accept the M2 metal end fittings, leaving in my opinion a very thin wall section. M2 rod alone seems a little flimsy. I'm also still to determine what best to use or make up as bearings for the rods, little ply blocks maybe?

Finally my vinyl decals arrived this morning from Pyramid models, tailored by Lee to suit my chosen scheme. This is the decal pack I've had altered - JP DECALS As well as the vinyl roundels, fin flashes, registration codes and warning triangles, Lee has also supplied a host of small detailed stencils - markings accurate to the full size JP - all for the list price!

[Andy Blackburn]

> 3mm carbon rod with a 1.5mm inner dia needs to be drilled out to accept the M2 metal end fittings, leaving in my opinion a very thin wall section.

3mm outer & 1.5mm inner sounds correct, you have to drill it out for the 2mm end fittings - do it exactly as it says on the plan (that is, aluminium tube completely cladding the carbon tube) and it will work really well.

Whatever you do, I advise you in the strongest possible terms not to use 3mm carbon rod on its own (you weren't thinking of doing that, were you?), because it's useless in torsion and you will then have comedy ailerons - been there, done that.

For the same reason, please stay away from M2 torque rods - far too twisty over that length.

 

Edited By Andy Blackburn on 13/02/2014 12:31:22

[Phil Cooke]

Thanks Andy, I wasn't going to use the 3mm carbon in its own, no, I appreciate its poor properties in torsion, but I'll be honest, I was considering using the M2 rod, as I cant seem to source any ali tube with a 3mm inner dia without it having a very thick wall section locally. My thoughts were the size of this model and the loads on the tiny ailerons would not cause the M2 rod any problems...

Anyway, I think weve cracked it, in that Matt J has today sourced some 12" lengths of 4-40 rod which is 2.4mm dia and in his eyes perfectly fit for purpose - hes sending some up in the post asap so I think this weekend we will have this layout all sorted.

In the meantime, Ill keep working on the fus.

[John A H]

With the amount of spar webbing you've put in your wing I can't see you having any problems with wing strength. The fuselage sides/wing saddle are fairly wide apart so that also helps to spread the load away from the centre join. I reckon you should be adding a ballast tube on yours and you'll be good for DS up to 120mph When the day of reckoning comes I suspect your JP wing spar will be the last thing remaining.

I've had a couple of cartwheel landings after been caught out by low level turbulence and only damage was tip tank snapping off the wing tip block. That's with ply brace as on the plan and the spar webbing installed as on Andy's plan (like yourself I did spend some time scratching my head about which way the grain should really run but decided to trust the plan ...... as a bonus it was easier to cut the bits to size with the grain running span wise).

[Phil Cooke]

I don't think I'll be breaking any PSDS records with this one John but thanks for your reassuring comment on wing strength.

I've done a little more on the fuselage construction this weekend, but only 1 photo to add.

The battery box assembly has been sorted but like others have commented, my former F2 and F3 (thin light ply in the Traplet kit) were warped - I wet them both out and 'pressed' them for 24 hrs under some weighted panels, which seemed to do the trick, but 24hrs later they had curled again! In the end I managed to straighten them by gluing some coffee stirrer lengths both vertically and horizontally on one or both sides of the former.

Dry fitting the formers against the fuselage sides and triangular stock showed a little trimming was needed on the 45 deg angled faces to allow the formers to seat fully against the fuselage. Checking the former against the plan it was perfectly formed, so this can only be variance in the triangular section dimensions, which is a notorious problem. Once I was happy all the formers could seat OK, F2 and F3 were fixed with cyano, F1 and F4 were glued with epoxy whilst the assy was vertical on the bench to ensure good alignment.

The photo also shows the triangular section with the sawcuts now added, allowing the fus sides to pull in where required.

[Phil Cooke]

Over the past couple of evenings I've completed the assembly of the battery box/former structure, and joined the fuselage, one side at a time, over formers F3 and F4 which are set square.

First, the nose guide formers F8 (x4), F11 and F12 were added, along with the cyparis stiffeners F13. Then, making sure everything was sat square with all joints showing good contact (I had to remove approx. 1/16th" off all 45 deg former edges to ensure a good 'seat' in the fuselage sides) the fuselage sides were glued to the box assembly with PVA, one side at a time over 2 nights.

With everything dry and looking square we can now think about pulling the nose in and adding the tapering formers going rearward.

[Phil Cooke]

It's been a quiet week for modelling but I managed to squeeze a few hours in today, a little more progress made on the fuselage with all the formers now fitted and the nose and tail pulled into shape.

The fuselage sides were pulled into F2 and clamped firmly - no water added at this stage, the saw cuts in the 1" triangular doing their job - although the squeeze needed was still significant - I used a pair of 1/4" ply load spreaders to ensure a uniform pull and to avoid damaging the fuselage sides.

Once F2 was dry, I added a little water to the most forward parts of the fuselage sides, and glued them around the F8 guide formers and upto F1. With a dampened fuselage side this proved easy. The 1" triangular section overhang could now be filed back flush with F2, leaving the nose complete ready for blocking in.

Moving rearwards I added F5, F6 and the wing mount former, which had been pre-drilled and fitted with the captive nut, secured with a little epoxy. The fuselage is pulled in around these and onto F7 at the tail, once each was aligned, and checking twice that everything was square and true, glue was added to all joints. The fuselage doublers F14 were added - they still need trimming to the wing seat in this view, and I added triangular stock between the wing mount plate and F6 to increase robustness - not shown on the plan but this IS mentioned in Andy 's write up.

Finally the 1/8th x 1/4" balsa spine was added between the tops of F4 and F5, meaning the next job is the planking!

[Phil Cooke]

I've put the fuselage planking off a few nights running, instead finding other jobs to do...

I've built up the nose in plan and side profile with 1/8th balsa to act as a sanding guide once all blocked in.

And I realised looking at the plan we had to cut out a section of the fuselage to create the cockpit floor, including the removal of the top section of F3. I felt this might be easier (more accurate) to do now, whilst the fuselage sides were still square - ie before I sand a lot of them away - so this was marked up according to the plan and cut out with a combo of scalpel and hacksaw. The section marked out in orange is removed, ready to be lined with 1/16th balsa.

[Phil Cooke]

The 1/16th" cockpit floor has now been added to the fuselage, and some work has been done around the jet pipe and tailplane seat.

I've deviated slightly from plan here, I fitted 2 pieces of 1/2" triangular square up to the tailplane seat, trimming them at the back and gluing them upto a piece of 1/8th balsa on the centre line.

I marked up the position of the tailplane/elevator hinge line (where the elevator horn will sit) and transferred the weight reduction scallop shape onto the wood with the pin pricking method. This was then cut and drilled out, then filed to shape.

I'm hoping the 1/2" triangular section at the very rear of the jet exhaust has enough material to sand round, I'm in the process of looking for a suitable tube to finish this part of the build off nicely...

[Phil Cooke]

I couldn't put the fuselage planking off any longer it's only a short stretch, but there is a marked difference of section between the rounded former F4 and the flat topped F5. I've used 3/16th balsa which can then be sanded back to 1/8th as required. Going thicker helps with the final finish but actually hinders the planking process.

I started on the top centreline, and in an attempt to minimise the number of joints, started with a piece as wide as I dared - 4cm wide proved ok, but this still required a little wetting of the balsa at the front for it to take the curve ok without too much loading...

Then on with the planking proper - I tend to use strips 1/4" wide, cut with angled sides to enable best fit both inside and out... working from both the top and the bottom until you end up with one horrible 'key stone' piece which has to be tailored carefully to suit both upper and lower faces.

I use a combo of aliphatic resin and cyano (at the ends) to 'grab' the planks whilst they are setting - this way no pinning is necessary and you can move from piece to piece quite quickly.

Once done I gave all the joints an extra squeeze of aliphatic inside and out, and rubbed it into any small cracks with my fingers... this is now complete, dry and ready for sanding...

[Phil Cooke]

But we'll do all the sanding in one session outdoors. Before that the nose needs blocking in with soft balsa.

That's the rear section done, between F1 and F2. Just the very front to complete. A lot of wood to come off here with the sanding block.

[Andy Blackburn]

Looks good - really neat building, that, Phil...

[Phil Cooke]

With the fus only needing the top and bottom sheeting rear of the wing before it could be considered structurally complete, I've given the elevator pushrod and radio installation a little thought today whilst access is still unhampered.

I'm not fitting a rudder on my JP, so I only have one servo to consider in the fuselage and as I'm using a conventional pushrod (as opposed to a snake, I really don't get on with snakes!) I've already opened up clearance holes in the formers rear of the wing bay.

Looking down the jet pipe you get a clear line of sight from where the elevator actuator horn would be (if it were fitted) and the area just above the battery box as it exits F4 at the front of the wing bay, so this is where I need to position the elevator servo. I'll fit a small Futaba Rx to the side, leaving the pushrod approximately central to run safely in between the aileron torque rod fittings when the wing is assembled.

I built a simple 'tray' from balsa, and fitted a pair of hardwood bearers. I cut a hole to suit the servo and 2 slots to enable a Velcro tie to be used to secure the Rx around the bearers.

The R/C gear was dry fitted to prove the fits were ok... I then removed the Rx and fitted the assembly permanently into the fuselage - the edge of the tray is glued to F4 and the doublers at the base of the decking behind the cockpit so was well supported. Thankfully, the height of the servo just enables the square 4-cell Rx battery to pass over the servo horn into its box so I'm quite happy with this simple arrangement.

[Phil Cooke]

The fus is now structurally complete, with rear sheeting added on top and bottom, allowing for the fin strake on top, and the nose is blocked up fully and sanded to a rough 2D section in plan view. This is now ready for the sanding session I SHOULD be doing this afternoon whilst the sun is out - but first I must mow the lawns!

I've also modified the fin and tailplane, as per Andys recommendation, to accept a hardwood spar and to alter the grain direction of the tips for stiffness. These can now be sanded to section, with a rounded L/E and tapered T/E, I normally aim for 1/8th" on my PSS models, any less and they get damaged too easily with general handling. The fin and tail will be glued onto the fuselage once that is fully sanded to shape.

[Phil Cooke]

Managed to get a little sanding done outdoors post lawn duty...

So its sanded to the correct shape now in plan and profile, time to round the corners. For this I quite fancy marking the 45deg angles up as tangents to the radius like RedBaron did so successfully to enable an accurate use of the razor plane before we start the rounding...

[Phil Cooke]

I decided to add the scale elevator balances to the modified tailplane tips tonight, will make covering a bit more tricky but these will add a nice scale feature once done.

[Fly_Boy_Rez]

Nice touch! Might try that myself!

[Phil Cooke]

Focus has returned back to the wing panels which still required the torque rods and top skins adding before we can join them and work the wing up to completion.

The torque rods are exactly as per Matt Jones' thread on the subject HERE - so 4-40 steel rod (2.4mm dia) ideal for standard SLEC end fittings, running in snake outer tubes as bearings.

First job was to mark up the required position from the aileron to the wing root and cut out the ribs locally to accept the bearings.

The torque rod was then positioned with the bearings seating in the ribs, they were then carefully glued in using cyano and supported with some small balsa doublers, again carefully glued into place ensuring the torque rod remained free of glue.

[Phil Cooke]

I'm still not quite decided whether to join the wings prior to adding the top sheet, or after. There's 2 issues which are causing me concern - Firstly, the ply wing joiner, as supplied by Traplet is only cut to the depth of the wing ribs, so it is not tall enough to stick right through the wing skins top and bottom, as per the design intent. Second, in cutting a thin slot through both top and bottom skins if we join after sheeting, Im left with little confidence in the joiner bond as it would be 'blind'.

For these reasons I'm more inclined to join the wings now whilst we can still access the wing joiner and ensure it is well bonded to the spar/webbing. I've cut a slot in both panels through the 2 inboard ribs and through the bottom sheeting into which the joiner can be slid.

If you push the joiner down to be flush with the outer surface of the bottom skin, then it ends up being 1/8th short in height and would never interface with the top sheeting, so I will add some extra material here (I'm thinking a hard balsa triangular section) to ensure a good bonding surface to the top skin. I will of course have to skin one wing panel at a time if I am skinning after joining, to enable the washout to be correctly added to each panel when flat mounted on the board.

I'm convinced this is the best solution to ensure a well bonded joiner.

[Steve Houghton]

Hi Phil,

I've been pondering about the wing brace as well. I'm not convinced that I could accurately make a blind cut into the wing skin after covering and have it end up flush against the spar webs without making an unsightly wide slot. I haven't decided on my solution, but yours is certainly a good contender.

Thanks for the rest of your blog also, it's really useful.

[Phil Cooke]

Hi Steve, the important thing is to get the joiner well bonded to the top and bottom skins, ideally it should go right through both and be sanded flush, but as described the supplied part isn't deep enough for this - this method has it through the bottom skin (flush mounted so no sanding necessary) and the additional supports I've added to the joiner will give it a good surface area to bond to the top skin without protruding through it.

So the joiner was glued into one of the panels flush with the bottom skin. Once that was set, it was fitted into the other panel again flat mounted on the board so the skin and the joiner bottom edge were exactly co-planar. So my dihedral is set by the joiner - I DID double check it with the dihedral supports as supplied and it's within a couple of mm..

Once the glue was dry I added 4 lengths of hard balsa triangular section such that these will give a good surface area to bond the top (and bottom) skins, effectively 'tying' the wing skins to the joiner. So I think this end result is just as strong as it breaking through the surface, with no nasty sanding to do on the top sides.

Edited By Phil Cooke on 16/03/2014 21:03:33

[Phil Cooke]

I've managed to get one of the top skins on before bed...

It's been one of those jobs that's challenging on the mind before you attempt it, but in actual fact once we were under way it really wasn't that bad... it IS worth having everything ready before you apply the glue, with the skin trimmed for any key areas like torque rod exit and servo bay.

I cut the washout wedge from 3/16th balsa, over 16" it ran out from 0.8mm to zero, I glued this onto the plan along the aileron hinge line then placed the wing on top pinning it to the board in a way that wouldn't effect the skin placement.

I used PVA, aligning the skin at the root around the features mentioned above, then pinned it out along the spar from root to the tip. I then found that pinning a piece of hard balsa along the L/E and T/E helped keep a nice constant pressure along the skins where they were curved over. With everything pinned down I double checked all the corners and edges to make sure there was nothing lifting, adding more pins as necessary, right through the wing and into the board.

Job done, one more to go...time for a well earned

[Phil Cooke]

Second wing skin now added, same method employed with the pinned battens in place along the L/E and T/E to ensure a nice even curvature and retention whilst the PVA sets overnight...

I thought whilst it was still pinned to the board I'd try and photograph the little wash out wedge in position - sorry for poor photo quality the light in my garage is not suited to macro camera work but you can see the 0.8mm wedge at the tip, mounted under and parallel to the aileron sub T/E. My wedge ran out to zero over 16".

Anyway, wing skins complete, once its off the board tonight we can trim all the overhangs and think about adding the L/E, T/E and tip material.

[Phil Cooke]

Not a lot of progress on the JP this week as I've been busy at work. I got a couple of hours in the garage tonight - really in an attempt to get all the major parts in a state ready for a good outdoor sanding session whilst the weather is good this weekend!

So the wing has been fitted with its balsa L/E and T/E, both in need of shaping, and I've made up some solid block ailerons which will now be tack glued to the wing to enable them to be accurately sanded too. I've added the wing bolt load spreader top and bottom, electing to put the thicker of the 2 supplied ply plates on the underside where the bolt head will react.

With the wing ready for sanding, I moved my attention back to the fus, which still needed a bit of work around the tail and exhaust area... a complex shape here, I used a coke can as a sanding block which seemed to do the trick in the concave areas. My fuselage sides have gone paper thin at the rearmost part of this area now, I'll insert a short plastic tube (30mm dia required) which I can then fill up against to build a bit of wall thickness back up whilst neatly representing a simple jet exhaust at the same time. The tube will need to be cut short, at the top at least - as any significant insertion would foul the elevator horn.

So we are ready for a good sanding session - Fuselage, Wing, Tail, Tip Tanks - all require some attention with the paper and block...with any luck the airframe may be ready for covering within the week. If I can get this model ready by 11th April I can take it to the Orme for our first PSSA event this season, and test fly it ahead of the Mass Build meet on 1st June.