Colin Leighfield's Chipmunk build.

[Danny Fenton]

Mmmmm that's not good Colin, I have always used this stuff and found it more than strong enought for hinges as have many of my friends. Including the likes of Phil Clark. Any chance the material was nicked such that it was weakened?

The threads cutting through the material is normal, and I should have made allowances and shown how to sleeve the hinges with brass tubing. That is my fault for not saying do early on. but it just takes 1/16 sections of tube CA'd to the glass fibre board.

Cheers

Danny

[Silver Wolf]

Hi Colin,

I used this for the bush, ordered from Chronos

MBT19 Albion Alloys Brass Micro 1 £5.04 £5.04
Tube Round 1.9 mm OD x 1.7 mm
ID x 0.1 Wall - Pack of 3

Hope this helps

Regards

SW

[Colin Leighfield]

Thanks chaps, I don't have a problem with any of that. What I am saying though is that the tongues which the push rods connect to by z-bend or clevises do not have enough meat around them to be safe. The hinge areas are ok and bushing is practical, although I've done mine by super gluing thin steel washers onto the sides, does the same job.

However, the push-rod attachments are too flimsy and they're not safe. I hadn't nicked mine and pushing and pulling the push-rod back and forth while setting the aileron up was nothing I don't normally do. I've never had anything break like this in umpteen plus years, so I'm just saying don't trust them, its' not worth it.

In the meantime I'll find a solution.

[Nev]

That’s a bit of a downer Colin, especially as you looked like you were cracking on at the moment. I suppose as you haven’t covered anything yet it wouldn’t be too much of a job to fit a different type of hinge / horn at this stage if you’re not confident with these.

I have to admit mine have had quite a few knocks during building, I thought I had cracked the ones at the tips off a couple of times as I’ve caught them turning the wing over but all are still intact and strong. I’d still like to use the fibre glass ones, although will bush the ones in the wing as SW has done but will have a think about it.

Still following along.

Nev.

[Colin Leighfield]

Sorry chaps, it's just dawned on me, what happens when you put something down and come back six months later! This is the result of my own alteration to separate servos for ailerons and flaps. I cut the horns off the fibre glass hinge profiles and used the spare rudder hinges that I had decided not to use for the rudder, attached to the aileron at a different position one rib in. It's one of these that's broken out of the aileron leading edge, probably because I've had these things on and off so many times getting it all to work that I've probably worn it all out!

Panic over, hope I didn't cause you any indigestion.

[Colin Leighfield]

I'm still trying to get the flaps and ailerons sorted out. I thought I had but I hadn't. I think this is a better way of dealing with the joint pivot between them, it means you only need a threaded fastener at the inner end of the flap and outer end of the aileron. It's simply a piano wire pin epoxied in, with a steel washer on each side. I've got some work commitments getting in the way this week, so this is slow but I've got a couple of hours in tonight. At some point this build will start to look as if it is moving, but nothing ever seems to be right straight away.

[Colin Leighfield]

We are in Spain now for 19 days, so progress on the Chipmunk is once again suspended. When we get back my work commitments are less and I will have more time, also the shed environment should be a less intimidating as we move towards spring. I tend to spend some of the time out here drawing up design ideas, this is where the XB42 Mixmaster was dreamed up and last time I made a good start on the drawings for a large Wellesley, I will do some more on that now we are here again.

Edited By Colin Leighfield on 19/01/2016 08:01:19

[Colin Leighfield]

We arrived home last night and I got back onto the Chipmunk this afternoon. I've got the flap and aileron on one side set up in the right positions and working perfectly, I'll set the movements exactly later from the transmitter. A hiccough on the other side because the flap has warped, so it's pinned flat back on the bench after another coat of dope, in the hope that it will be acceptable tomorrow.

[Colin Leighfield]

It might not look much, but I've got flaps and ailerons sorted both sides now, working and lined up. Now repairing the bodges, finishing off the capping-strips and shrouds. Next I'll sand it and get it ready for covering, I'm not touching the fuselage again until I've finished this, but it's moving now at last.

[Colin Leighfield]

I've been working this week and had long days, managed to do some more on the wing last night but collapsed asleep in the chair tonight! However I completed the work on the capping etc last night, just leaving the underside of the centre section to sheet over before sanding and filling ready for covering. Also I will cut the slots behind the u/c legs to allow for rearwards movement. (There's a lot when I do landings). I will be at home tomorrow so will be able to do some more. I have stuck with the Depron and the next step then will be to carefully finish with Ezekote, take a deep breath and directly dope on the silk that I intend to use. This build is for me partially a project to see how far you can go in mixing Depron with conventional techniques. Depron and dope is seen as a no/no, but I think that might be wrong doing it this way, so you can save money and weight. However, I might be wrong and this weekend I will have it done or have a pile of mush on my hands. Watch this space and be ready for a laugh.

[Andy Muckley]

Good on you Colin, will be interesting to see how this pans out, hopefully all will go well

[Colin Leighfield]

Thanks Andy, fingers crossed. I've been at home today and done a bit more. I've started Ezekoting the Depron, back in the shed tonight with the heater on!

[Colin Leighfield]

Everything takes longer than I think it will. I'm still working on preparation for covering the wing, including putting a layer of 17gm glass cloth on the leading edge sheeting. In addition I'm working on the wing root fairings, another area where the plan detail is somewhat lacking, to say the least. I've decided to use blue foam instead of balsa. It's worked very well for the fuselage tail cone.

[Danny Fenton]

Looking good Colin, you are getting there

They always take longer than we expect......

[Colin Leighfield]

Thanks Danny. I'm trying to get my head around how to prevent that whippy wing warping when I dope the covering. I'll find a way, Dennis Bryant must have had the problem because I don't think for a second that he had heat-shrink material on his mind when he drew this one up.

[Colin Leighfield]

At last the acid test. Doping silk onto Depron. Coating with Ezekote first is the answer. No attack on the Depron at all. It means that if you want to, you could build a very light open structure with Depron and then cover it with tissue, silk or nylon using dope to stick it on and shrink it. We're motoring now.

[Danny Fenton]

Phew...... well done Colin

[Colin Leighfield]

Thank you Danny!

[Tony Richardson]

Colin you are certainly moving on with your build faster than I am keep it up looks really sharp.

[Colin Leighfield]

Tony, don't look too closely. This is not going to be in the same league as Danny's, but I hope it will be a reliable and relaxing, reasonably scale, flyer.

The main delay in getting this far was not touching it for about six months, because it was taking up so much of my limited time and I needed to finish off getting the Seafang into the air and I was keen to do the XB42 as well as try to get some flying in, practice time still being a problem.

Getting the "D-box" right is the critical feature of this wing. For anybody following this and perhaps interested in the continuing exploration of the scope for Depron in a composite build approach, before starting to cover the wing with heavyweight (a very limited term) silk, first I applied a layer of light (17gm) glass cloth to the leading edge "D-box" Depron sheeting, top and bottom. This makes it really strong. The capping strips and other areas of attachment for the covering are left as raw Depron with three coats of Ezekote, you can dope straight onto this.

Some may remember earlier conversations about webbing in the wing spars and my decision not to bother. The reason I wasn't concerned about it was because I know that the primary purpose of it is to prevent buckling of the top and bottom spars and to keep the tensile (lower spar in normal flight) and compression (upper spar) stresses in a direct straight line along the spars. In an open wing structure, that is more important because the primary loads are borne by the spars, plus a limited contribution from the leading and trailing edges. With a "D-box" like this, the curves of the upper and lower leading edge sheeting perform the same function as webbing between the spars and create a very strong girder. That is extremely unlikely to buckle, it is made even more rigid by direct attachment to the ribs, which ensure that internal spacing and profile are maintained, then to the spars as well, which is more than enough to ensure that they can't buckle anyway. I remember reading that the catastrophic structural failure of'the DH110 at Farnborough in 1952 was buckling due to inadequate internal support structure of the leading edge and the inability to hold up against the focused stress at the points of attachment of the tail-booms. The upper skin buckled inwards because there wasn't enough inside to prevent it doing so and as soon as that happened, the compression load on the lower skin lost its' direct line of tensile stress and simply bent upwards in milliseconds, leading to the disintegration of the airframe. This thinking sticks in my mind and sets basic principles for design.

I'm pretty confident in this these considerations. It will be hilarious if, assuming I get this to fly, the wings fold up at the first attempt to do a loop! However, I've always been prepared to open my big mouth and prove myself an idiot. Plus the fact that I blather on too much.

[Simon Chaddock]

Colin

A bit concerned at the lack of inter spar sheeting.

A wood spar under compression has a minimum cross section between supports that ensures it fails by crushing rather than buckling.

A continuous web ensures the spars are supported along their length so the critical compression cross section is kept to a minimum.

In a "D" box structure the spar webs are even more important as they complete the 'box' to resist twisting. An open sided box has greatly reduced torsional stiffness.

Of course your structure may have sufficient strength and stiffness anyway - most models do - but leaving out the spar webs does seem like "spoiling the ship for a ha'porth of tar!"

[Sean Tracey]

Hi Colin .Like your dope and silk job ,if I may be so bold as to ask you where you obtained silk. My reason for asking you is I am starting to build some Vintage American models that are supposed to be covered in dope and silk and as I like to have everything as near to original as possible I really need some silk.. Kind regards Sean Tracey.

[Silver Wolf]

Hi Colin,

Glad to see things went well with the dope. With regards to the spar webbing, there seems no mention of any on the plan. I was a little surprised how much the wing rotated when I was checking for washout on my wing. It was not possible to get an accurate reading due to the amount torsional twist. I think that I will add some form of webbing to mine though.

Regards

SW

[Colin Leighfield]

Hi chaps.

Sean, look at :- freeflightsupplies.co.uk. It's a treasure trove, you'll love it.

Simon, SW. It was a deliberate decision, I did think about it. The plan doesn't show it, because I'm sure Dennis Bryant didn't think it was necessary. When he designed and built it originally I bet that he wasn't thinking of covering with heat shrink material. He might have used tissue, but probably nylon or silk attached and shrunk with dope. That adds far more strength and rigidity to the airframe than Solarfilm, Solartex or any of the equivalents. I'm using silk doped on, closer to what he would have done.

From the structural engineering point of view, the ribs are very closely spaced, only 1.75"/43mm apart. They are attached directly to the leading edge sheeting and the spars, supporting the vertical spacing between the top and bottom spars and the sheeting, which in itself provides a rigid connection between the ribs and plays the same part to some degree that webbing would in making sure that the critical space between top and bottom can't reduce. I think that there are enough mechanical connections and load distribution there already without the need to stick the webbing in as well.

Having said all of that, I don't deny that I'm experimenting here. If it all goes pear-shaped I won't have a nervous breakdown. It's just providing ideas for the next build, which will be getting back to the Supermarine 317 project. It's just frustrating that it takes me so long to do things. That's also the question of using Depron in a plane powered by a throbbing four-stroke. That's going to be interesting.

Edited By Colin Leighfield on 28/02/2016 15:04:20

[Colin Leighfield]

One thing I forgot to mention in the earlier conversation. Despite my frustration at the fiddly build of this wing, some of it caused by my own deviations, Dennis knew how to engineer a light and efficient structure. Apart from the undercarriage attachment points on landing, there are no points of concentrated load on this wing to create stress raisers, the bending load will distribute pretty evenly along the span. I don't think there's much to worry about here.

Also I'm pretty sure that the crushing loads will be distributed between the upper spar and leading edge sheeting, so I don't think it should be a problem. A fully open structure would be a different kettle of fish though, no question about the necessity of webbing there.

Edited By Colin Leighfield on 29/02/2016 00:21:51