Jet Provost 1.5metre Andy Blackburn PSS Plan

[Alan Gorham_]

I wouldn't normally expect to have to use Ammonia to bend balsa sheet over a typical leading edge radius. I suspect that, looking at the grain patter of the wood you have used, that the sheets you have selected are too stiff for wing sheeting Pete!

[Peter Garsden]

Posted by Alan Gorham_ on 11/09/2019 11:57:21:

I wouldn't normally expect to have to use Ammonia to bend balsa sheet over a typical leading edge radius. I suspect that, looking at the grain patter of the wood you have used, that the sheets you have selected are too stiff for wing sheeting Pete!

I think I could have glued the sheeting in place without the ammonia, I had just received it in the post so I thought I would give it a whirl and it made life a lot easier.

[Alan Gorham_]

No doubt it dissolved some troublesome nose hairs at the same time! Its pungent stuff...

[Peter Garsden]

I think I have a problem. I am becoming addicted to ammonia. I am using it for everything - cleaning, aircraft, etc. I must get some help. I decided to bend the leading edge round the surve on one side, as the scarf joint as per the Hurrican on the other side was a bit tricky, and it works well. The other option is of course a butt joint. The plan doesn't show a join though I seem to remember there was one on the smaller version. I will check - still not sure as I gave my plan to Steve when I sold my smaller version.

So the choice is yours. Actually a butt joint is probably the simplest and the one I have not tried

Andy recommends cutting strips from 3/8" sheet which is probably the best. I had some 3/8 x 1/2 strip lying around so used that but it is actually about 5/8" deep so I had to add some 2mm scrap to make the depth up which is not ideal.

Edited By Peter Garsden on 13/09/2019 06:19:43

[Peter Garsden]

Have also made the wing tips from 3/4" then 1/32" ply then 3/16" sheet. Make sure you make a right and left hand version with the layers reversed. I remember making two right hand versions when building the smaller one. You can see I marked them T for Top and B for Bottom (sorry moderator). They have different angles each end to match the wing so again make sure they are correctly lined up.

Edited By Peter Garsden on 13/09/2019 09:26:45

[Andy Blackburn]

Posted by Peter Garsden on 10/09/2019 08:38:20:

...

Thought I would show you some pictures of the servo installation. Incidentally I was wondering if the wing needed the beef of an HS85mg - I think you could get away with something lighter and metal geared as the drive length is short and the torque should not be enormous.

I put it to m'learned friend that there are some recommendations for minimum servo torque based on the size and deflection of the control surface and an estimate of how fast it will be flown, positioned near the title block of sheet 1. The recommendations are:

• Aileron: 2.7 kg/cm minimum
• Elevator 3.2 kg/cm minimum.

You could risk smaller and less capable servos but if you run out of servo torque it'll be horrible to fly - it does happen...

A.

[Peter Garsden]

Have now put together my first tip tank BUT beware, make sure the 1.5mm ply template fits over the carved wing tip far enough. I had to enlarge the hole on mine BEFORE you make up the sheets of balsa which make up the tank, as it is very difficult to adjust the hole afterwards. The hole in the template needed enlarging in some parts by as much as about 8mm.

The tips narrow toward the tip, so whilst the template may fit over the tip it won't slide all the way to the end rib, which is where it should rest.

I think I am going to attach these with magnets - something I wouldn't have attempted when I made the JP back in 2016. I didn't have Dremmel to make the holes for one thing.

The first 3 sheets - 1/8", and 2 off 3/8" have holes in, the remainder not. So all glued and clamped with aliphatic

[Andy Blackburn]

If you think about it, given that we only have one template it has to be a smaller size - if it was larger so that the template could slide all the way to the end rib, that wouldn't be any good because the tip tank would then be a sloppy fit and would fall off.

What the builder is expected to do is to make the tip tanks to that size and then use a Permagrit flat sanding tool (or similar) to sand the inside of the airfoil shaped hole until it's a good fit on the tip and fits all the way on - obviously, you can only do this properly after the wing is covered. The tip tanks are then just a friction fit, none of mine (2 models) have ever come off unless the landing has been a bit traumatic.

[Andy Blackburn]

I've added a section to the plan that should hopefully show how the aileron bay fits together. Here are the new bits:

[Peter Garsden]

Excellent Andy thank you - just what is needed. Mine is slightly different but it will do. I realised that yours is the correct version and that mine is wrong as I stuck a piece of 1/8" balsa to the ply plate and stuck the servo to that. Never mind.

I have now got on with sanding and carving the tip tanks and the alternative tips for light wind. I have decided to use magnets for both fixings, so my order of play is

1. Mark the position of the holes for the light wind tip securing dowels - I use bamboo skewers.
2. Fit them onto the tips and mark holes, then drill them out being careful to line up the holes square..
3. Glue in the securing dowels and push fit the tips in place
4. Carve to shape.
5. Glue in some 1.5mm x 10mm neomodium magnets having dremmelled some recessed holes for them in the tip balsa - make sure they aren't too deep (I used cyano)
6. Attach some identical magnets to those.
7. Paint them with some black paint.
8. Offer them up to the tips and remove. It should leave a black circle, which you then dremmel out again not going too deep.
9. Glue the opposing magnets in place and you have your attached tips
10. Follow the same procedure with the large tip tanks and use the same magnets. This is easier because you only need to dremmel holes inside the tip tanks

In the next photo you can see the ailerons which have been cut out using the pin hole mark I made in the bottom skin before attaching the top skin (DON'T FORGET!) I used a combination of a balsa knife for the in grain cuts, a balsa saw for some of the cross grain, and to finish off a handy small Tamiya tiny saw. I can't show the picture but this is a link to Amazon - **LINK**

[Peter Garsden]

Next error - forgot to insert balsa infill either side of the R1 central rib to reinforce the centre of the trailing edge. Didn't realise this until the top balsa sheet was on, so I decided to overlay a piece of 1/64" ply on the outside instead. Then I laid 2 inch wide carbon tape over the top with epoxy resin.

[Peter Garsden]

Andy recommends 2 inch wide bandage soaked in PVA. I couldn't find a chemist that sells conventional bandage so bought crepe bandage which is not the thing. I soaked it in PVA and it took 2 days to dry - so don't use it. This is why I used resin and tape underneath

[Andy Blackburn]

> ...forgot to insert balsa infill either side of the R1 central rib to reinforce the centre of the trailing edge.

Oh dearie me... However, looking on the bright side, you probably won't notice unless it hits something hard enough to pull the wing bolt out...

Other than that minor contrafibulation, though, your wing looks pretty good.

> ... so bought crepe bandage which is not the thing

I think this is the sort of gauze bandage I normally use (picture's not that good):

**LINK**

- single layer, non-stretchy. If you can't find any of this, you're better off using 2 oz or 4 oz glass cloth.

[Peter Garsden]

Indeed so Andy - it is available but I think it was probably used in the NHS by Florence Nightingale - God bless her courage.

So The Fuselage

Phil points out that for each fuselage side, all you need is 1 x 48" x 4" x 3/16" sheet, and 1 x 3" x 36" x 3/16" sheet to save on wood. Having joined it and laid it over the plan, I concur with my learned friend. You must make sure, however, that you line up the sheets correctly. I am using carbon paper, and it will be a fiddle to line it up. Might be easier for the 3 inch wide piece to be 48" long rather than 36" but you would end up with more scrap.

[Peter Garsden]

The Battery Box

The parts for this are not included in the short kit but are oblong so a doddle to cut out from the plan. I remember getting into a stew about this 6 years ago because the sides weren't lined up properly so I think the best way is:-

1. Cut out the square holes in the formers (F1 to F4) which the box will sit inside.
2. Make sure the battery box sides fit inside the holes snugly.
3. Line up the sides inside F1 and spot glue each side to the next with cyano. Do the same with the opposite end into F4. Put the spot glue about 1 inch from the end to avoid sticking the box to the former.
4. Remove the spot glued box then run thin cyano down the outside and inside. Easier with a narrow capillary tube (always use this for cyano as you can leave the top off and it won't weld itself shut to the screw on top resulting in you have to throw it all away.
5. Ease the sides of the formers where the box fits to make sure it is a sliding but tight fit.
6. DON'T glue the box to the formers yet - you have to line it up with the finished fuselage sides.

[Peter Garsden]

Well, the sides are both cut out and some of the doublers glued to each side. NB. there are more doublers on the larger version than the small, in particular between F2, F3 and F4.

There is an issue with F16 bottom doubler, which may be resolved on the revised plan which we are going to release when Phil and I finish our beta build. Phil noticed this on his fuselage build. The issue is that it is necessary to cut a slot in F16 to accomodate the full depth of the spruce/cyparis longeron as in the pictures

One can see how effective carbon paper is at marking wood.

Hopefully we will be able to email those who have the same version of the plan as we have, the amended plan, as you all have the short wood kit already. The plan has many more notes and the above drawing of the servo well.

 

Edited By Peter Garsden on 27/09/2019 20:45:26

[Peter Garsden]

This is the fully loaded fuselage side with all the doublers attached.

You can see that the spruce longeron has to be paired down each end to follow the curve of the fuselage. I have yet to make the saw cuts to the front 1 inch triangular to help it curve. There is an awful lot of wood, but, as Andy says, you carve most of it away.

Edited By Peter Garsden on 28/09/2019 17:16:48

[Peter Garsden]

Revision to Former F5

There is a change needed to F5. The plan shows the correct section with it relieved to fit round F14, the doubler immediately above the wing bay. The woodpack, which will be revised, however does not provide relief for it, so you have to cut away 3/16" at the bottom. Easy enough with 3mm liteply. I have marked on the plan the wood you need to remove on the former

[Peter Garsden]

When offering the formers to the fuselage sides, I found that one has to adjust them slightly to ensure they fit flush to the sides. This is owing to the variations in the cut of the triangular stock, and the way in which the doublers are attached and cut out.

It is important that this is all done before we glue up the battery box round F2,F3, and F4.

[Andy Blackburn]

FYI, what I've done on the latest version of the plan is to remove the top (thinner) part of F16 (the part above the spruce spar) because, on reflection, it's just not required. All the other changes that people have identified have already been incorporated, so I'm hoping that'll be it.

[Peter Garsden]

Like I have been saying, line up the battery box parts with the fuselage sides. I do remember not doing that last time and having difficulty making everything fit properly. I had to do a lot of adjustment - nothing major - but enough to put the work in before gluing up. It is so much easier to line up the formers before even part of the fuselage is glued up as you can work flat on the building board with the formers.

I have decided to follow Phil's preference and make up an old fashioned 1/4" balsa control rod for the elevator. So I have opened up some wider holes in all the formers to make way for it.

I glued F2 and F3 with thin cyano, and F1 and F4 with epoxy mixed with microballoons

[Peter Garsden]

And the next job is to glue the spruce longerons to the side of the battery box in readiness for attaching the sides. The straight pieces have to be paired down and rounded off at each end to follow the curvature of the fuselage to front and rear

Edited By Peter Garsden on 29/09/2019 22:38:05

[Peter Garsden]

What a struggle attaching F2 to F4 was - not because of parts issues at all, but because I didn't set up the parts and do a practise clamping before I mixed the glue. I panicked because my biggest clamps were just too narrow to cope with the width of fuselage (6.5 inches) and the clamps would only stretch to 6". So BEWARE, you need clamps with a 12 inch spread.

So I went to Screw Fix and bought some and they work fine.

Problem was, somehow i had to remove the glue from F2 where I was unable to bring the fuselage together and it was already stuck in the open position!! Damn! All drying now - epoxy doesn't go off properly for 12 hours, so will leave it overnight. Witness how many clamps I used. I have done this before and got into the same pickle with the first version.

I also used some Ammonia to help with the bending.

I have also set in position F1 which bent fine again with Ammonia.

[Peter Garsden]

What a struggle attaching F2 to F4 was - not because of parts issues at all, but because I didn't set up the parts and do a practise clamping before I mixed the glue. I panicked because my biggest clamps were just too narrow to cope with the width of fuselage (6.5 inches) and the clamps would only stretch to 6". So BEWARE, you need clamps with a 12 inch spread.

So I went to Screw Fix and bought some and they work fine.

Problem was, somehow i had to remove the glue from F2 where I was unable to bring the fuselage together and it was already stuck in the open position!! Damn! All drying now - epoxy doesn't go off properly for 12 hours, so will leave it overnight. Witness how many clamps I used. I have done this before and got into the same pickle with the first version.

I also used some Ammonia to help with the bending.

I have also set in position F1 which bent fine again with Ammonia.

[Peter Garsden]

More part adjusting I am afraid.

1. F6 and F7 have a slot in the top for the fin, which is marked out on the plan at 3/16 whereas the fin is now 3/8". It just needs easing on each side by 3/15 to give it the correct

1. The hole for the wingbolt nut is too small for the nut and bolt speciified on the plan namely.at 8mm for the hole and 6mm for the wing bolt it is small Very easy to drill out with the correct diameter hole.
2. The point is that the plan specifies a 6mm bolt, but you could argue that you could get away with 5mm, which would mean that the hole was the correct size at 6mm. I had to enlarge mine to 8mm

Edited By Peter Garsden on 02/10/2019 17:09:37