Folland Gnat Build

[Steve McLaren]

After taking part in the Jet Provost mass build, and having attended the Great Orme event in June and discovered the thrill of PSS flying, I wanted to build another PSS model. The Jet Provost is fantastic, but having only one PSS model means that I have ‘all my eggs in one basket’ so to speak. With the PSSA events all requiring quite a commitment in terms of travel and time, I don’t want to rely on the serviceability of a single model.

So I looked through all the model photos on the PSSA website for something that would catch my imagination. I didn’t want anything too big, too complex, or too expensive - mainly because I want to get into service as quickly as possible, but also because at my level of flying skill, it may not last all that long!

So, after some deliberation, I settled on the Folland Gnat, designed by Alan Hulme, and which was apparently available as a plan from MHS. Like the Jet Provost, the Gnat features in some of my earliest memories of air shows, and so for me, represents something a lost ‘golden age’ of wonder and amazement.

When the plan arrived, I studied it for several hours (as you do!). I was pleased to find that the construction looked quite conventional and simple (albeit with a foam cored wing which I haven’t done before, but which I have been prepared for since late last year when I built a cutting bow in preparation for another project which never got started – only to be replaced by the Jet Provost build). But the plan is quite the opposite of the Jet Provost plan in many ways. It’s an old hand drawn plan, and Mr Hulme was clearly no stickler for accuracy! It’s more of a ‘construction suggestion’ than a detailed plan. There’s hardly a single dimension which is consistent in any 2 views! Where he’s drawn complete formers, the shapes are quite asymmetric! So I’m going to have to be very careful, and sort of re-create a consistent geometry over the top of the original plan.

But, just like the Jet Provost plan, I realised it was impossible to tell what shape the fuselage was really supposed to be (particularly at the rear end) from just 2 views. So, I went out to town and bought the 1/72^nd Airfix model to use as a reference.

Now, I must admit that when studying the photos of Alan Hulme’s model on the PSSA website, there was something about it which didn’t look quite right to me. On comparing the geometry of the plan with the airfix model it confirmed my suspicions. He’s designed it with absolutely huge wings! It really does look like a glider version of a Gnat! It turns out the plan is to 1/10^th scale, and the wing span on the plan is 39 inches. By my simple scaling it looks like a true scale span would be 29 inches. So 10 extra inches in 29, or 34% oversized. I’ve sketched the true scale tip position on the plan in the photo below.

I’ve decided that I don’t want mine to look so obviously out-of-proportion, so I’m planning to split the difference and cut the span down to 34 inches, in the hope that it will still look like a Gnat. I’ll just have to build it as light as I can, and see how it works.

I ordered all the balsa from the balsa cabin, and I’ve a made a start on those wings.

First I had to set up a jig on my bench to allow me to make some straight and square cuts of the foam black which I had been saving since last year (and was really supposed to turn into some wings for an electric mosquito, but I’ll get round to that one day). I made the jig with a pair of 90 degree angle brackets screwed to 2 pieces of wood, which were then carefully shimmed to the bench and screwed down, to make 2 true uprights.

I then photocopied the 2 core templates provided on the plan, and carefully checked that they were still accurate to scale, before pasting them to a sheet of paxolin and cutting them out.

I first cut the foam block to the wing planform shape, then marked up the edges to define the positions of each core template. I decided to allow for just 1/8^th inch of washout at the trailing edge of the tip (and not the 3/8ths suggested on the plan, and which seemed completely excessive to me – can that be right??).

I then set about cutting my first wing core. And I was quite pleased with result. So whilst I was on a roll, I moved the templates down the block a bit and cut the second one. They look alright to me (for a first attempt anyway).

Edited By Pete B - Moderator on 29/08/2014 12:06:23

[Phil Cooke]

Great job on cutting your first set of wings Steve - what type of cutting wire did you use for the blue foam?

Will be watching this one - will be nice to see another Alan Hulme Gnat on the slopes they do go well!

[Steve Houghton]

Steve,

I, too shall be following with interest. If the Gnat is finished half as well as your JP it'll be a cracker!

Nothing to worry about with your flying skills, from what I saw at the Lleyn - great flying. The mid-air was just an unfortunate coincidence!

Similarly to yourself, I have just sent for a Traplet plan. It's for a D/F jet that I hope to convert to PSS. Won't reveal details yet, in case I decide it's unsuitable for conversion and change my mind, although I suspect it's been PSS'ed before.

Happy building.

Steve

[Andy Meade]

Looking good Steve, what are you skinning the wings with? Don't worry too much about the smaller wings either - my Me262 is...ummm...6 inches under span after a slight faux pas with marking my foam out. Still flies well enough.

Any thoughts on colour scheme yet?

[Mark Kettle 1]

Looking good Steve and congratulations on your first hot-wire foam cut.

[Alan Gorham_]

The plan in the picture at the top of the thread is not a Traplet plan!

It clearly says Nexus Plans Service, which is now the myhobbystore plans service who co-incidentally seem to be owned by MyTimeMedia Ltd who publish RCM&E and own this forum...

Those of us of a certain age remember when it used to be the ASP plans service....

Good result on cutting the cores Steve, looks very tidy.

[Steve McLaren]

Sorry Alan, You are absolutely right. I forgot where I ordered the plan from. It is in fact from Myhobbystore Media. I will be more careful in future. Is it possible to correct that in my first posting?

To answer Phil's question, I used nichrome wire, for no better reason than I found a very useful website called something like 'nichromecalc' which allows you to work out the best thickness of nicrhome wire to suit your power supply voltage and length of bow. I had better get my references right this time, so its called http://www.jacobs-online.biz/nichrome/NichromeCalc. Then I went on ebay and found I could order wire of the right size by the metre.

I did read somewhere that nichrome is not very durable, but my first piece has survived a few cuts without any problem.

To Steve H: I'm intrigued. When are you going to tell us? Come on - spill the beans!!

To Andy M: Thanks, I just hope my Gnat flies any where near as well as your Me 262. I rather like the colour scheme on the Airfix box, which is the silver and orange trainer scheme, but I also like the red arrows scheme which is how I remember seeing them in the flesh. So I will think about that a bit more as I build.

To Mark K: Thanks Mark. Maybe one day I'll graduate to EPP? My JP is fully restored by the way. How is the white vulcan?

[Steve McLaren]

The wings are skinned with 1/16th balsa. I butt joined 2 off 4 inch sheets together along their sides (same as for the JP) and cut them to the planform shape of the wing with about 1/4 inch excess all round. I then buttered up both the 2 skins and the foam core for a single wing with PVA and squashed everything together with the outer foam profiles and a few heavy heavy weights, and left them to dry overnight. Due to the time taken to apply the PVA to the surfaces, I found I could only do 1 wing at a time. But I've done them both now and after trimming the excess balsa, they look like like this.

You can probably see that there is a slight under-camber developing as they dry out! I've got some weights on them right now trying to hold them in shape!

[Mark Kettle 1]

Nice work Steve, the white vulcan is all glued up with new wood in the nose, just needs sanding?and a dob of paint the mid air was just a scratch it didn't stop either of us flying after a quick patch up.

[leccyflyer]

What about the RAF Yellowjackets scheme? I always thought that was a really nice colour and the bees love it too!

Clubmate Derek Robertson's Gnat, nestling amongst the heather on the slope at the weekend. It;s all built up construction and possibly even from that very plan.

[Alan Gorham_]

Steve - no apologies needed re the source of the plans, I just think that the myhobbystore plans are the worlds best kept secret and need shouting about!

Thanks very much for posting the link to the nichrome calculator. I do a bit of foam cutting (mainly to make floats out of white polystyrene) and its all been done on a suck it and see basis. I'll look forward to trying the calculator next time to see what it says.

Incidentally, re core skinning; I know PVA is cheap and easily had but I use Ball Styccobond F1 to sheet my cores.

It's basically Copydex latex contact adhesive but available in 5 litre bottles. It's used by carpet fitters but it does speed things up a lot. Coat the core and sheeting with a thin layer, wait 20 mins and join and job done.

I think I paid £24 for my last bottle but it does plenty of wings/floats!

[Mark Kettle 1]

earlier in thread -

- " I did read somewhere that nichrome is not very durable, but my first piece has survived a few cuts without any problem "-

Steve my slicer wire has just broken this morning whilst slicing some blocks for the Vulcan - its has been slicing since March on the same wire yards and yards of foam stuff of size 27" x 14" foam and takes 6 mins a pass on a weighted draw table, so quite durable, it's a surprise when it breaks but I can't complain 5 + months on one wire and gets used est 3 days a week.

Enjoying your build post's especially the snippets / pictures done with the plan in the background - give's it a sort of ambiance - good work. Has the wing settled down and dried level and straight?

Edited By Mark Kettle 1 on 29/08/2014 15:25:01

[Steve McLaren]

"What about the RAF Yellowjackets scheme?" - well I'm too young to ever have seen that in the flesh! So I was going to discount it on that basis. But it does look good sitting there in the heather. Maybe.

On the wing skins - Thanks Alan, I did think about using a contact a adhesive, but I was worried that it would be a one chance process, and considering the flexibility of the cores, I just didn't think it would come out the right shape. Is there any 'workability time' which allows you to put it back between the outer foam pieces and apply weights? Or do you support the core on the lower outer piece to maintain the shape, and do one skin at a time? That would sound like a good method to me - not sure why I didn't think of it at the time - probably because I convinced myself I had to do both skins at once because I was using 'wet' PVA.

If you give me a bit more detail, I might try your method. Because I may have to re-make these wings. To answer Mark's question - I've still got a small amount of under-camber, and they appear to be fully dry now. Otherwise they look pretty good - i.e. they seem to have an even amount of wash-out at about 1/8 inch on each tip. I might as well finish them off with some leading and trailing edge pieces and then see what I've got.

[Alan Gorham_]

Steve - yes you guessed right contact adhesive is a one-shot deal.

Yes you do one skin at a time. Back in the old days when obechi veneer was more commonly used it was possible to use one sheet of veneer to do both top and bottom in one go, carrying the veneer around the leading edge, but that's not a great idea with thicker balsa sheet.

I would say it depends on your wing section if you need to use the outer off-cuts to support the skin while you apply the core to it.

You touch the core down onto either the leading or trailing edge of the skin and kind of roll it down onto the skin to ensure full contact between the skin and core.

If you have a flat bottom wing section then its easy to do the bottom flat on the bench.

I can recommend a book called Radio Control Foam Modelling by David Thomas which goes into great depth on many aspects of using foam - many of them very applicapble to glider models.

[Mark Kettle 1]

Tah-dah

.....doesn't the yellow gnat look good?

[Tim Mackey]

Posted by Mark Kettle 1 on 29/08/2014 05:40:49:
Nice work Steve, the white vulcan is all glued up with new wood in the nose, just needs sanding?and a dob of paint the mid air was just a scratch it didn't stop either of us flying after a quick patch up.

As the other pilot ( of the Vulcan ) I should apologise for not spotting you on that fast pass !

As Mark has said - part of the beauty of epp is its quick and easy to fix slopeside.

It was back in the air in about 10 mins or less

[Mark Kettle 1]

LOL

[Mark Kettle 1]

........pictures Steve of your black anti glare matt black paint on my the nose section and the white Vulcan is weight training for the next bout with your JP ( or any model for that matter) kind regards Mark.

[Mark Kettle 1]

.......fun isn't it.

any way back to the Folland Gnat build.........drat that has a really pointy nose 

Edited By Mark Kettle 1 on 29/08/2014 22:25:32

[Steve McLaren]

Tim,

No need to apologise. I did wonder later whether I was I was flying in the wrong place (wrong way round the circuit or something) but there didn't seem to be many rules that day! Do let me know if I 'm not following the etiquette.

Mark,

Although I've re-covered the nose of the JP, I've left some of the skid marks of your black EPP as a memento! (Actually I had planned to remove them later but it's strangely sticky and tenacious stuff when it's been welded to the solarfilm by the force of the impact!)

.... but back to the Gnat! Whilst waiting for the wings to dry out, I've done my fuselage sides. The construction is pretty much the same as the JP (for those who did that) just a different shape. Oh, and the main side pieces are 3/16ths rather than 1/8th. I was a bit worried that that may make it heavier than it needs to be, but it looks like most of it will be carved away because the fuselage will end up pretty much round for a fair bit of the length.

There's a 1/32nd ply doubler in between the triangular sections from the nose to just behind the wing. And I remembered to sand it down before glueing it in!

Yes it's a very pointy nose - maybe I should put a bit more ply in it! Maybe if I can spear a vulcan, the EPP will cushion the impact with the ground next time?

[Mark Kettle 1]

LOL , the gnat should be ready by the time of the next Orme meeting easy.

[Mark Kettle 1]

Steve have you seen the Gnat and Vulcan video?

[Peter Garsden]

Hi Steve - will this one be another competition winner? Looking forward to seeing it at the Orme.

I know what you mean about foam wings. I had to build a transformer, and two hot wire cutters as well as cope with a lost foam fibreglass fuselage for my Alpha Jet.

I found the same site as you, and ordered from Ebay the Nichrome wire just the same.

The mistake I made was to get a 6 to 12v Transformer instead of a 12 to 24v which is what I will need. There is enough power for my 30inch bow but not my 50inch bow. I will have to replace it for long wings.

I used copydex for the veneer on the foam which worked a treat. Didn't experience any warping. This is my transformer which has a light on the top to tell you how much current you have and a bodged ceiling fan dimmer switch on the front. Got all the parts off Ebay - works well.

[Peter Garsden]

This is the transformer I built

It uses a dimmer switch with a different knob - a 6 to 12v transformer which is not powerful enough. I need a 12 to 24v - if you are creating one please note. It will power my short 30 inch bow but not my long 50 inch one which is shown below. Not enough voltage. I got the transformer at huge expense from the Jacobs site but it cost loads in p&p from America. Easier to source the same thing on Ebay from England. I now know what I need.

[Peter Garsden]

This is my long bow which doesn't get hot enough - got the bungee cord idea from an Aussie You Tube video - works well.

Have also done a big article for the LMMGA mag, which I can send anyone that is interested.