Folland Gnat Build

[Mark Kettle 1]

Hi Steve hope you get well soon, yes covering in brown paper and fancy a blue camo finish seen, maybe it's a privately owned one.

[Peter Garsden]

Looking good Steve. Would like to see it maiden in 90mph winds with a brick tied to the bottom of it?

As for the Hawk - I wasn't up the Orme but went up to the Mermaid above Leek and maidened the Hawk. It flew OK but with a lot of up elevator, so I think the C of G may be too far forward. I had that problem before with the Alpha Jet. It is difficult to know where to balance a swept wing model. Anyway I have taken some lead out of the nose, so we will have to see.

Also there may be some reflex in the trailing edge of the wing due to the way in which I fixed it up. Anyway it did fly. Also the plan did not give you any movement limits and the aileron was quite twitchy so I have dialled in 40% expo which will help. I think it will roll very quickly. The Alpha Jet does with tiny ailerons.

I am planning to visit the Orme this Saturday as we are up at the Van this weekend. The problem with Leek is that it is not really fit for PSSA - it is a good slope but not as much lift as at the Orme. I worked out that the Hawk has a wing loading of 18oz per square foot so it needs some lift to make it fly.

I found an article which tells you how to work out the wing area of a swept wing model which was very useful

Span/2 x (Root Chord + Tip Chord) in inches then divide by 144 for square feet.

I did this but forgot to include the square area between the wings so I had worked it out at 22oz but with the extra middle area it is 18oz, which is slightly better.

It weighs 77 oz or 4lbs 12oz. So it is not a floater. I think it will go well once it is sorted out.

Edited By Peter Garsden on 29/10/2014 16:02:22

[Andy Meade]

Gents,

If you get stuck with your CofG's. here is a nice tool that gives you a reasonably conservative position :

http://adamone.rchomepage.com/cg_calc.htm

Worked well on the MiG and Me262. Remember a nose heavy plane flies badly - a tail heavy planes flies once!

[Steve McLaren]

I've put in a few hours on the build this weekend, but not a lot to show for it really. shaping the over-wing fuselage fairing in blue foam is a lot more difficult than I expected. I decided on a layering method - sanding each of the main curves as I went, and using a central balsa spine to define the shape. Here's an intermediate stage:

and then, this is as far as I got today:

One of the problems, is that you have to leave the PVA a long while to set, since the foam itself is an air barrier.

This gave me time to work on some other bits, so I managed to get the air intakes roughly fashioned from a few laminations of 1/2 inch balsa, and glued in place.

[Mark Kettle 1]

A bit more done Steve, will you make a plug to help mould and make the canopy ?

[Steve McLaren]

Hi Mark,

Yes the canopy mould is the next job. I sanded the rest of the upper wing/fuselage fairing this evening, and I think it's now approaching the right shape.

So, on to the canopy mould. Hmmm, this looks a bit tricky!

[Peter Garsden]

Canopy moulding sounds a bit like platting fog with non fog platting needles to me.

We have lots of fog up North if anyone ever needs some?

Last person I heard of making moulds for canopies was Job, and you know how much patience he had.

I always choose plans where you can get a ready made canopy, but hey, who am I - just a mere amateur?

And then, even when I get a canopy I try and fit the wrong way round, then glue it on with Cryano - school boy error that one?

[Matt Jones]

Canopy moulding can be a long and intensive project all on its own, ask me how I know..........

I'm about 80% through the process myself.

[Andy Meade]

I'm the same as Pete! But this winter I must persevere and make a canopy for the Fulcrum. The Gnat does have a strange bubble shape to it though!

[Andy Blackburn]

Starting to look really good, Steve. Have you put it on the scales yet?

[Steve McLaren]

Hi Andy,

Yes it's currently weighing 26 oz. And that's after hollowing the jet pipe ( see below) and before making the canopy. It includes elevator servo and receiver, but not aileron servos, battery and covering.

I think there are a few more grams to come off the nose yet - I haven't sorted the fuselage shape around the canopy area yet, and I might thin down the jet pipe a bit more. But I'm running out of weight reduction options.

I have started to make a dummy canopy mould out of lots of balsa off-cuts, just to help me get the fuselage shape right. After that, I'll make the proper canopy mould.

[Steve McLaren]

Well, I can confirm that Peter and Matt are right - there's a lot of work in this canopy moulding!

After listening to suggestions from quite a few people about the best methods, I went down to my local model shop (Mainly Models of Stotfold/Hitchin), whom I had heard offered a canopy moulding service, and asked for their advice. I had decided that I probably wasn't capable of achieving the right shape in a single piece of balsa or wood, so I was very pleased when they said that 'Isopon'P38' car body filler was an acceptable way of doing it.

So I hacked up a few bits of soft wood and epoxied them together to make a rough shape that would sit on the fuselage. Then I plastered it with Isopon. It was a bit like icing a cake (if you've ever done that!).

Then I sanded it to shape, and filled a few missing bits, and sanded it again, and filled a few more bits, and sanded some more.......etc, until I got something I was reasonably happy with.

Then I turned it over, and painted some red acrylic onto the bottom face at eh edges - hoping that this would allow me to see where the edge needed to be on the finished plug.

Then, they told me I needed to mount it on a flat base and radius all the corners so that there were no 'returns' or tight corners which could make it difficult to remove the canopy from the mould. So after glueing on a few more bits of wood, and after a few more Isopon mixes, this is it. There was enough red showing through for me to scribe a line which I hope will be the correct edge of the finished canopy.

As you can see, there are a couple of places where the wood shows through the Isopon. Apparently this could be a problem - the joints may show up in the moulding. But today the mould has gone to the shop for a trial. If it doesn't work well enough, I may have to re-work these bare wood areas and try again.

[Monz]

I'd shoot a few coats of primer over it sanding between each coat, that'll build up to a lovely uniform plug for moulding.

Great build too by the way!

[Mark Kettle 1]

Very nice Steve, looking good .

[Steve McLaren]

Thanks Monz. If the first attempt is no good, I'll try that. Any particular primer you recommend?

[Steve McLaren]

Well the first canopy is not bad at all! Its not perfect - as predicted, the bare wood edges seem to show a bit - but it will do for me.

A couple of little chips of isopon also came off the mould along with this first canopy, so I've got to do a little skimming before I can send it back for a couple of spares. So I'll take the bare wood areas out too, and spray a couple of coats of primer on it (as suggested by Monz) and see if I can get the next one perfect.

So not more than a bit of light sanding and some covering to do now. Oh, and I must not forget to give those pilots some eyes!

[Phil Cooke]

You've got to be over the moon with that Steve - it looks the business well done!!!

[Steve Houghton]

Steve,

I've not said much recently, but I have been following with interest. Well done on another great looking model.

I have particularly enjoyed seeing your canopy creation, as I've not done much of that myself yet, but am gleaning ideas where I can.

[Andy Blackburn]

That looks really nice, Steve. Looking forward to seeing that fly...

A.

[Steve McLaren]

Thanks guys,

Yes I'm pleased with it. They always look so good in bare balsa. Mine never look quite so good again. It's only when you start on the covering that you realise you should have done a little more sanding, in some areas, or else the hot iron on soft balsa tends to add all sorts of little marks and blemishes.

I put a few hours in on the covering this weekend. This is how it looks now. All those internal fillet radii were hard work. Hopefully the camera is far enough away not to show all the defects!

[Andy Meade]

Looking nice Steve - not long left now!

[Phil Cooke]

Great looking model that Steve - nice work!

[Andy Blackburn]

> a few hours in on the covering this weekend

I really don't understand how you can spend a couple of hours on covering and the thing is nearly finished. It's not natural...

That *is* Solarfilm, isn't it...?

Edited By Andy Blackburn on 10/12/2014 20:52:01

[Steve McLaren]

Ah well, when I said 'a few hours' that wasn't a strictly accurate measurement of time! More of a figure of speech! If I'm honest, there must have been about 8 hours of fiddling about with lots of small pieces of the nasty solarfilm!

One thing I learned from your experiences Andy, was to put the orange on first and then overlap the silver!

[Steve McLaren]

It looks like you've all been very busy while I've been distracted by Christmas duties.

I did pick up up my two spare canopies and the mould from the local model shop. These new ones are better than the one I've used - they look pretty much perfect to me. The primer (as suggested by Monz) seems to help fill all the minor scratches, and it seems to stay attached to the mould too! So if anyone is planning to build a Gnat and doesn't want all the hassle of making their own mould, just let me know.

Meanwhile I have attached the first canopy to the model using canopy glue. The canopy needs to be cut on the Gnat, as there is a small part which is permanently attached to the fuselage (the front 'windscreen' section), whilst the main part of the canopy forms a removable hatch along with the cockpit.

I glued the whole thing in place with a pit of non-stick baking parchment in place between the cockpit floor and the fuselage, to prevent it sticking where it shouldn't. Then, when the glue was dry, I was able to cut through the canopy and separate the removable bit from the fixed bit.

I then moulded a thin off-cut strip of acetate using a hot air gun on the correct area of the canopy mould to form an overlap between the 2 parts of the canopy. I have then glued this to the fixed forward part of the canopy - again with canopy glue..

I'm now wondering how long this glue might take to dry - since there's very little air exposure -especially after the edges have already gone off.

Anyway, if does ever dry completely, the idea is that the removable canopy will slot under this overlapping frame, and be secured with 2 magnets at the back edge. The frame itself will be covered with silver solartrim and should look like the real canopy frame.