SLEC Chipmunk build blog

[David P Williams]

Thanks Tim - hope you have as much fun with your Chipmunk as I have with mine.

[Tim Flyer]

Well horrible weather this week! I did manage to do some flying on Thursday though. I have started on the fuselage now. I’m not posting photos of the flat side as that would be dull. I think it is definitely worth doing a dry run on the fuselage assembly. The triangular balsa edging needs to be very carefully placed too. I initially tacked the two sides together via the radio tray with the help of thin cyano and activator. That makes the clamping less critical. Luckily my fuselage does seem to be fairly straight. I also epoxied the firewall in today . The kit provides a triangular firewall square made of liteply to check the firewall is square. What seems a little silly to me is that the triangle is totally square but the firewall is designed to have an offset to provide right thrust! This means that if the firewall is correctly fitted it will not meet the fuselage sides at perfect right angles . Anyway that’s just me being picky . Maybe the square is just used to prove it’s not at right angles and is left offset? Anyway my firewall is stuck down now and it wasn’t particularly difficult.
Today I ordered some more bits , a 16oz Sullivan flexi fuel tank which is 5 3/4 in long and should fit well at the bottom of the tank bay. The flexi tanks by Sullivan are really good and can easily be softened to fit into awkward spaces. I also ordered a metal 3 inch Aluminium Spinner which I will need later on.

Edited By Tim Flyer on 02/11/2019 15:53:12

[Tim Flyer]

I’m a little further on now. I have completed the wing bolt to fuselage fixing and the wing dowl fixings. I replaced the soft liteply dowl caps with my own stronger Birch ply caps . The fuselage is coming on a bit . All the lower formers are fitted and the shell for the air and exhaust outlet. I was wondering to keep the fuel bay access hatch which the instructions describe via the air exhaust. I probably will as it might be useful both for building and servicing. I fitted the supplied heavy duty M3 push rods and glued in the sleeves . Final assembly will be after covering.

[Tim Flyer]

Here is a picture of my tank bay and engine mounting ( the blind nuts will be put in after the bay is glassed and epoxied inside. In order to lower the 16 oz Sullivan tank to the correct height for the laser 180 I cut down the level of the former F2 . I then internally reinforced the lower half by laminating a birch ply cutout on the lower half and a spruce beam from side to side on the bottom edge of F2. I’m now happy with the yak height , To hold the tank down i epoxied two bolts as an anchor for a zip tie which will be added later . I will add the tank bay floor before epoxying the whole area,

[Applewood]

Hi Tim, I'm enjoying your build, great work.

I'm in the latter stages of the RAF trainer version, 2 piece wing (maybe 1 - jury's still out), & laser 150.

Are you levelling the centre line of the carb with the centre or bottom of the tank? Just trying to make sure I've got the right level tank.

[Tim Flyer]

Hi Applewood good to hear you are also building one and thank you for your encouragement . I have now fitted the stringers on the fuselage and sheeted the engine bay which I will epoxy tonight . Next main job is fuselage sheeting. I am going to keep the wing as 2 piece as I think it will be strong enough like that . I do have a small gap in the underside edge when the wings are joined. I was thinking of possibly eliminating it via car filler covered with cling film when joining. That will make a flush fit.
Regarding my carb level the carb centre is a tiny bit below the middle of the tank when the plane is horizontal , but once it’s on the ground and angled by the undercarriage the spray bar will be above the top of the tank( so it won’t siphon) . I have never found the 180 , or other Laser engines over sensitive to tank position unless it’s much too high ( e.g bottom of tank in line with carb). If you have middle of carb in line with middle of tank when horizontal that will be fine. By the way I lowered the tank by lowering the bottom of former 2 as described above also moving it back slightly away from the air outlet slope on the firewall which would have lifted the front. My tank is a nice relatively compact Sullivan 16oz tank ( just 5 3/4 inches long) 

Edited By Tim Flyer on 11/11/2019 12:32:12

Edited By Tim Flyer on 11/11/2019 12:37:24

Edited By Tim Flyer on 11/11/2019 12:37:54

[Tim Flyer]

I have started sheeting the fuselage now. While the glue was drying I painted the very good latex model pilot bought from Nexus models for about £11 . They delivered fast . It seems to be one of the few decent sport pilots available in 1:5 scale. I hope he has done is “A” Cert 😉 His reg number will be put on his overalls 😉😉😊. I’m starting to think about covering and the dreaded painting. If I film cover I’m still going to end up having to match painted areas on wing fairings and cowl. After speaking to Jon yesterday I think I might consider glassing this fuselage and using Solartex/glass on the tail , but possibly just use the film and Solartex on the wings. I will probably try Halfords car spray paint as it should be fairly easy to match film . Will have to cross my fingers for fuel proofer compatibility too. 

Edited By Tim Flyer on 13/11/2019 09:42:10

[Applewood]

Posted by Tim Flyer on 11/11/2019 12:31:32:

Hi Applewood good to hear you are also building one and thank you for your encouragement . I have now fitted the stringers on the fuselage and sheeted the engine bay which I will epoxy tonight . Next main job is fuselage sheeting. I am going to keep the wing as 2 piece as I think it will be strong enough like that . I do have a small gap in the underside edge when the wings are joined. I was thinking of possibly eliminating it via car filler covered with cling film when joining. That will make a flush fit.
Regarding my carb level the carb centre is a tiny bit below the middle of the tank when the plane is horizontal , but once it’s on the ground and angled by the undercarriage the spray bar will be above the top of the tank( so it won’t siphon) . I have never found the 180 , or other Laser engines over sensitive to tank position unless it’s much too high ( e.g bottom of tank in line with carb). If you have middle of carb in line with middle of tank when horizontal that will be fine. By the way I lowered the tank by lowering the bottom of former 2 as described above also moving it back slightly away from the air outlet slope on the firewall which would have lifted the front. My tank is a nice relatively compact Sullivan 16oz tank ( just 5 3/4 inches long)

Edited By Tim Flyer on 11/11/2019 12:32:12

Edited By Tim Flyer on 11/11/2019 12:37:24

Edited By Tim Flyer on 11/11/2019 12:37:54

Thanks Tim, you build is coming on really well. Are you going for twin flap servos or single?

I’ve measured my setup, the carb centre line tank is 18mm above the bottom of the tank. It’s also a 16oz Sullivan tank. Is the siphoning issue only a problem when static, or in flight too. Do you think the tank is low enough?

cheers

[Tim Flyer]

Cheers Applewood sorry I missed your post . I have a number of Laser engines and although ideally the spray bar is supposed to be at tank centre line , I have not had problems at all with centre line of intake at tank centre. The tall tanks (large tanks) can possibly lead to Syphoning on some planes but normally the fact that the undercarriage points the nose up stops ground siphoning . The other important matter is to properly tune the low speed needle setting. Many flyers leave their Laser engine low speed needle alone. It governs mixture up to around half throttle and does need occasional adjustment and should not be just left at factory setting. Jon at Laser is always happy to help with advice. A very rich low speed needle and tank a bit high can lead to cut out due to plug cooling.

[Tim Flyer]

I have moved my tank slightly back from the firewall so the middle of tank is level with the carb intake

[Tim Flyer]

The build is coming on now . The top is sheeted but cockpit cut outs need to be done . I have been also working on blending in the base of the wings with the nose floor. Quite a bit of sanding and grinding was needed to round the lower rear fuselage away from its box section

[Jon H]

Posted by Tim Flyer on 19/11/2019 14:46:44:

Cheers Applewood sorry I missed your post . I have a number of Laser engines and although ideally the spray bar is supposed to be at tank centre line , I have not had problems at all with centre line of intake at tank centre. The tall tanks (large tanks) can possibly lead to Syphoning on some planes but normally the fact that the undercarriage points the nose up stops ground siphoning . The other important matter is to properly tune the low speed needle setting. Many flyers leave their Laser engine low speed needle alone. It governs mixture up to around half throttle and does need occasional adjustment and should not be just left at factory setting. Jon at Laser is always happy to help with advice. A very rich low speed needle and tank a bit high can lead to cut out due to plug cooling.

I thought my ears were burning.

I tend to recommend the centre of the carb is no lower than the centre of the tank. Ideally the top of the tank should be in line as you then only have fuel below the carb and there is no change in head. A few mm higher is not normally a big problem though.

As Tim rightly says slow run adjustment is very important and often neglected. Dont be afraid of it, just lean off both needles as far as you can. Dont go 'a bit rich for safety', just lean it out. If the cooling is good enough and the tank position right then you will not have any problems. As a tuning guide, if it holds max rpm for 15 seconds then the tuning is fine. If it later stops its either overheated or gone lean due to fuel head changing. In this latter case, lower the fuel tank but leave the tuning alone.

[Applewood]

Posted by Jon - Laser Engines on 19/11/2019 15:00:07:

Posted by Tim Flyer on 19/11/2019 14:46:44:

Cheers Applewood sorry I missed your post . I have a number of Laser engines and although ideally the spray bar is supposed to be at tank centre line , I have not had problems at all with centre line of intake at tank centre. The tall tanks (large tanks) can possibly lead to Syphoning on some planes but normally the fact that the undercarriage points the nose up stops ground siphoning . The other important matter is to properly tune the low speed needle setting. Many flyers leave their Laser engine low speed needle alone. It governs mixture up to around half throttle and does need occasional adjustment and should not be just left at factory setting. Jon at Laser is always happy to help with advice. A very rich low speed needle and tank a bit high can lead to cut out due to plug cooling.

I thought my ears were burning.

I tend to recommend the centre of the carb is no lower than the centre of the tank. Ideally the top of the tank should be in line as you then only have fuel below the carb and there is no change in head. A few mm higher is not normally a big problem though.

As Tim rightly says slow run adjustment is very important and often neglected. Dont be afraid of it, just lean off both needles as far as you can. Dont go 'a bit rich for safety', just lean it out. If the cooling is good enough and the tank position right then you will not have any problems. As a tuning guide, if it holds max rpm for 15 seconds then the tuning is fine. If it later stops its either overheated or gone lean due to fuel head changing. In this latter case, lower the fuel tank but leave the tuning alone.

Hi Jon and Tim,

so my tanks is 55mm tall and the centre of the carb is 18mm from the bottom of the tank. This makes the centre of the tank 10mm higher than the centre of the carb. Would you recommend getting the tank lower??

[Tim Flyer]

Best I can do is show you what I have done . Here is a photo with ruler showing cut out I made on bottom of former F2. I cut it approximately 15mm lower. If necessary I will be able to cut a maximum 10mm lower . I’m not going to do that until I have test run the engine. It’s hard to measure exactly but I believe the centre of my carb is fairly close to the tank centre. As I said I also slid my tank slightly rearward to accommodate the sloping air outlet duct from the firewall.

Edited By Tim Flyer on 20/11/2019 08:12:39

[Tim Flyer]

I’m now making preparations for covering. I’m going to use Oracover and Solartex, and Halfords car spray for the cowl. Hopefully the car paint will allow Klass Kote fuel proofing without reacting . I’m painting the Balsa with diluted PVA to seal the grain and add hardness before covering. . I have painted the inside of the engine bay and front section with epoxy.

Edited By Tim Flyer on 21/11/2019 20:20:21

[Tim Flyer]

Today the fuselage got two coats of brushed “Clearcoat”. It is fuel resistant and helps film adhere well. It’s also very smelly so a mask was essential! It always seems rather like old fashioned cellulose dope.  I painted the tail fin stabiliser and rudder & elevator with two coats of diluted PVA. I think the wings will also eventually get the PVA treatment. My next job is attaching the wing fairings. These are ok but not very scale . I put foaming gorilla glue in them a few days ago to firm them up and give a better purchase when glued to the fuselage. I will make a stencil of the first before fitting so I can cut an exact size of red covering film around them . The fairings will be sprayed grey. I found some Halfords “Audi Laser Red” car spray which matches my Oracover Red film , and I will use that on the cowling. Hopefully Klass Kote fuel proofer won’t make it bubble. I will test first !

Edited By Tim Flyer on 22/11/2019 18:40:36

Edited By Tim Flyer on 22/11/2019 18:41:05

[Tim Flyer]

I have glued on the wing fairings now. Over the last few days I have filled them with foamy Gorilla glue which adds some strength to the soft abs plastic and provides a better surface to glue to the fuselage. I chose to glue them with wings removed after careful marking. This helps clamping and is easier to prevent messy seepage.

[Tim Flyer]

Preparation for covering is going well . The control surfaces have had a couple of coats of diluted PVA and sanding. The fuselage got clearcoat resin which gives a nice hard surface . I might give the wings the clearcoat treatment too. My new mask seems to filter the awful clearcoat fumes quite well so it isn’t too much of a problem using the cellulose based resin.

[Ron Gray]

You’re doing a grand job Tim, I like the idea of clear oat as a fuel proofer.

[Tim Flyer]

Cheers Ron . The clear coat resin was a bit of a discovery a year or so back . I bought a big tin. It was made by solarfilm I’m not sure what the current equivalent is . I see Guild paints make clear finishing lacquer which might be similar? . It seems to be designed to help hold film edges down. It’s also great at sealing grain and iron on film sticks very well to it . I have found it extremely good to stick down and repair the cheapo covering on my Acrowot ARTF. This Chipmunk is getting “ “luxury” Oracover and the hollow panels will be Solartexed . The clearcoat dries in a couple of hours too. I still painted the inside and firewall with epoxy ( old habits die hard !)😊

[Tim Flyer]

I have started covering some of the bits that I’m finishing “off the model” . The fin and tailplane are both covered and I have installed the tailplane. The tail section with integral rudders are always a fiddle on Chipmunks. I might add a bit of epoxy to the rudder control horn to prevent any risk of it turning on the shaft. I have already ground a flat spot to help the grub screw engage and used blue thread lock. A loose rudder isn’t fun !

[Nigel R]

Clear Coat was great stuff.

No idea what I'm going to use in its place.

Build is looking good Tim.

[Ron Gray]

I’ve got Clearcoat and only use it as a film edge sealer or as a base for film to stick to. Never thought of it as a fuel proofer though!

[Tim Flyer]

Hi Ron. I’ve really used it as a sanding sealer (thinned) on this plane and it’s great on balsa to help film adhere. I have given the whole fuselage two coats of it . I’m going to use PVA on the wings probably, but the clearcoat is much harder and resists dings. I think there are other cellulose clear resins around that could be similar. I will use it on the edges too. 

Edited By Tim Flyer on 26/11/2019 20:26:49

Edited By Tim Flyer on 26/11/2019 20:27:38

[ASH.]

Looking good Tim, I'm just surprised at the speed at which you build. You must be at it full-time!

I have been extolling the use of 'Clearcoat' for years now -- for fuel proofing and sealing film edges.

I use it on the firewall inside and out, tank bay and inside of fuselage, usually two coats. It not only fuel proofs but seals and strenghtens the wood. It does add on weight so I must remember to take it easy next time. It is a synthetic resin made for the job. Also excellent for film edges, a painstaking task - well worth doing. Shame it's not made anymore, maybe Guild Paints Finishing Lacquer is as good.

Tim, I know you can't help epoxying fuselage insides.. Did you use fibreglass cloth as well.? What weight cloth do you normally use?

BTW, I love the bubblegum smell of Clearcoat.

Tim, if you haven't bought the Oracover and Solartex yet - have a look at Solarfim Glosstex. A smooth strong heavy covering.