The Oxalys pictured above with 10 year old Alice Mellor (now 11) is the model featured in this article.
ZN Oxalys by Stuart Mellor
On this build I was determined to find out exactly where all the weight in a model comes from and to identify exactly where the extra pound sneaks in when you're not looking, and also to see where significant weight savings could be made, if anywhere. To this end all components were weighed to 1 gram before and after construction (where I remembered, that is) and the results shown in the attached table.
Also, I intended using all the light ply materials supplied with the basic kit without using nomex, carbon or other lightweight materials. Weight savings from these materials, although useful, won't in themselves deliver a light model. I know it all adds up but are they just the icing on the cake or does the weight that really matters come from elsewhere? Engines used were to be either the YS 160 or OS 140 RX and I modified the ply crutch to accept either engine.
I had to use my own balsa for the facings, since the supplied material was too narrow so I used some really light wood of my own. I don't like the in vogue carbon undercarriages. For me, they just ruin the front end of an otherwise nice looking model, not to mention their biggest drawback: damaging your wonderfully finished fuselage, unless you happen to fly from an aerodrome or putting green. So, I let into the wing some old Rhom-air retract units, locked down and faced these over with balsa. Then I could use fixed 8g wire legs, plugging in after covering and easily removing for transport if required with 1/2 turn of an Allen key. Incidentally- a pair of 8g wire legs are lighter then the ZN carbon and are invisible in the air.
I always join the wing with white glue, after sanding the roots flat on a board. Epoxy is heavy and not necessary for joining since you are simply gluing foam to foam -epoxy will not be stronger - only heavier. Simply smear the roots sparingly with white glue, bring the wing halves together briefly to check 100% contact. Stand vertical until the glue starts to set, check incidence on a meter if you like and leave to set. Checking incidence at this stage is difficult and can bring on unnecessary nagging doubts about the accuracy of the wing! I have never had any problems simply
aligning the root ribs on ZN and PL wings. The only slight worry I had was that when joined as supplied there was slight anhedral on the top surface; previous French models were either flat on top or shows a slight dihedral. I've since joined another Oxalys wing and that was the same, so assumed that was how it was meant and unfortunately no dihedral measurements are given. The wing was then prepared for covering by an ancient C20 technique by doping with 50/50 dope/thinners and lightly sanding off the fluff. The reasoning behind this old-fashioned approach is that this method ensures that film will stick properly and will not slacken later and only added 14 gms. (If you stick film directly onto wood, the film only adheres to the balsa fluff and this can lead to lifting later).
ZN instructions call for 3 layers of glass cloth reinforcing, but I always extend this as follows: the 1st piece extends 50mm past the aileron roots, angling the edge of the cloth at a diagonal across the chord; the 2nd piece approx half way to the ailerons and the 3rd extending about 25mm from the fus sides, also angled from front to back. Also make sure the pieces of cloth on the reverse side of the wing are about 25mm shorter or longer - don't give the wing any natural line of vertical weakness where a break could start. To date I've never had a wing failure using this method (now clutching the desk top). The cloth is initially doped in place with 50/50 dope/thinners and scraped flat with an old credit card. You don't use much as the wing has already been doped. I find this makes a far superior job than a 1 shot attempt using epoxy resin and you won't need as much resin either. The resin is dribbled on with a spoon and scraped off with a credit card. In this case the epoxy only adding 6 gms. Pre-doping also allows the cloth edges to be feathered off to a hard surface and thus won't show through the film. There is no need to completely fill the weave of the cloth. A visible weave is better and prevents ballooning of the film if you're careless with the iron and won't show though the film.
The wing was filmed on top with Solar film polyester (40gms) (yes I know Profilm is flavour of the month but I much prefer solar film products.) For weight comparison, I covered the bottom with some old Solarspan I had in stock. (57gms) Interestingly, the Solarspan was 40% heavier (17 gms) - due to its multilayer construction. Trim added a surprising 42 gms. and was a mixture of Solartrim and Pro trim. Amount of trim can be seen on photo on front of newsletter. Aileron servos are Futaba metal geared types, so perhaps slightly heavier than the norm.
All up weight of the wing finished - 1157gms.
Conventional construction using parts supplied, including ali tube. Finished weight including 2 Futaba 3002 mini servos, polyester Solar film - 314gms.
Fuselage as received weighed 583gms (20.5oz), belly pan 106gms (3.7oz).
The light ply and balsa former set as supplied was used, except I modified the motor crutch plate to accept the standard YS mount and my own design 2 stroke mount, which is basically 2 tufnol plates which bolt onto the engine lugs and the plates bolt to the crutch with 4 gator rubbers - weighs about 25gms! Motor mount parts and former were tacked in place with epoxy and bonded with 25mm strips of 50gm cloth soaked in epoxy resin. This gives a good strong and light joint to the fus - much better than simply epoxying the formers in place.
A tank tray was made from a balsa/thin ply sandwich - which came out heavier than I thought at 50gm., so was lightened after installation with a drum sander attachment. A separate glass "air scoop" is also supplied which is let into the fus aft of the wing to enable the current long pipes to be used. (20gm. installed). The fuselage seam was perfect as supplied and after a light rub down was handed over to Tom Bootyman of www.aeropaint.co.uk for painting. I asked Tom to disguise the seam with a thin strip of primer/filler and to finish the model in base coat and 1 coat of 2 pack clear lacquer, leaving the white areas unpainted, but lacquered. Also, I couldn't see the point of a second coat of lacquer just to cover over the paint ridges (when a finish is so good that you can only feel the ridges - that's good enough for me!). Also, if fuel attacks the finish the second coat won't make much difference. Thanks to Tom's good work the finish only added a total of 34gm! 23gms on the fus and l1 on the underbelly - quite remarkable. It really does pay to have your model painted by an aerobatic flier who understands the requirements. We have all seen ZN and PL models where the modeller has used all the latest equipment and accessories but the model is over 5kgs., with no apparent explanation. So either the fuselage was very heavy to begin with or, as I suspect – the model has been plastered with filler & paint to achieve a showroom finish.
Fuselage finished less gear etc. 993gms.
OS 140RX = 820gms.and O.D. mount: YS DZI60 = 920gms and YS mount.
Although the YS is apparently heavier than the OS, use of the new carbon pipes and light weight accessories for both engines will reduce this difference to a bare minimum - but it costs. However I did find that the heavier YS pushed the CG forward a lot compared with the 2 stroke. Also with the OS you can get away with a 16 oz tank, compared with 24 oz for the YS - another additional 8oz at take off.
Completed model weighs 4390gms rtf with the OS 140 used for initial flights and though I really liked the set up, I felt the model was running away with me at times with the 17 x l2 and 10% nitro. (too much power - perish the thought!) Substituting a 4 blade 15.5 x 12 transformed the model - the extra drag of the 4 blader gave me much more control, although I had to increase nitro to 20%. Throttle response and tick over is excellent on the standard OS carb and I never have to touch the needle. Even with 2 rear mounted elevator servos I still had to move the battery and Rx as far back as possible before the model felt right in pitch inverted, but now flies wonderfully. It's without a doubt the best French model I've ever had – and have I had a few! CPLR has definitely worked his magic again.
Once trimmed, I substituted the YSI60 and Hattori muffler and Baz fuel. (Coolpower 30%) Performance was again excellent with the added bonus of the nice 4 stroke sound. I'm convinced that having flown power plants in the same model, the sound of the 4 stroke induces a feeling of relaxation and therefore flight appears less hurried and apparently smoother without any input from the pilot. This is simply a sort of noise seduction but probably means more points! The same will happen with electrics, I believe – the model will be perceived to be slower and more controllable, but in actuality it's simply that noise levels are lower.
I certainly enjoyed flying the YSI60 and never noticed the weight increase, but unfortunately the CG was now too far forward for my preference and there was little I could do, save adding weight to the tail, which is an anthemia to me after all the hard work reducing weight. Also I think on the next one I'll eliminate the anhedral since there is a very slight tendency to roll opposite to rudder input. This is easily mixed out of course, but it probably does indicate a flat top wing may be better.
In the end I decided to reinstall the OS and enjoy the benefits of reduced fuel costs and no engine maintenance for the rest of the season and apply any lessons learned to the next Oxalys.
So to sum up the few potential weight savings from the build:
1. Wing trim added 42grms. Overlapping the base colours would save this - but tricky to do. Potential saving . + 40grms..
2. Wing covering all Solarfilm polyester. Potential saving over thicker films -17grms per side.
3. RX battery (fat sub c type nicad) 1800mah -186grms.. Substitute smaller battery - potential (get it?) saving - 50grms..
4. Tank tray - ply/ balsa sandwich - 40grms after lightening. Substitute Nomex tank tray - 9grms. Potential saving - 30grms..
5. Ali Tail joiner -19grms. Carbon joiner - 9grms. Potential saving – 10grms..
Total potential savings on this model -147grms..
Thus model could have weighed 4390 - 147 = 4243grms..
Fliers will no doubt draw their own from the table - but based on the above, weight savings from nomex and carbon are indeed useful, but will easily be cancelled out by over indulgence in the finishing department.
Next time I will continue to use polyester S.film but probably overlap to save trim weight; keep the ali tail joiner for CG reasons; use a nomex tank tray; keep the large battery and 2 switches for safety, probably use carbon/nomex crutch for CG reasons, again have the fus sprayed by Aeropaint.
For any passing 2 stroke devotee this is my current set up:-
OS 140RX, carb. as supplied; ancient long Aldous ali manifold, Azano long 2 stroke carbon pipe, probably 30-50rnm longer than optimum; model car type silicon stinger + 50mm ali extension.
Needle - OS "long" needle -2 turns open, not stubby type as supplied with engine - too peaky; OS four stroke plug - essential; fuel - 20% nitro ( ix of 1/2 Coolpower 30% and Technics 10%); Tank 20 ozs, plenty for 1½ schedules. Prop 15.5 x 12 APC 4 blade or 17xl2 2 blade. Spinner 80mm Graupner (Ripmax) carbon and lightened backing plate.
See you on the line
Table of Initial and Finished Weights
|Left panel as received||223|
|Right panel as received||230||
|All woodwork added||
|Wing doped (2 thin coats)||14||704|
|Glass cloth in centre||56||760|
|Epoxy resined and rubbed down||6||766|
|Top filmed - S.film Polyester||40||806|
|Bottom filmed - S.film Solarspan||57||863|
|Profilm trim / Solar trim||42||908|
|8g wire U/C and aileron servo||218||1126|
|Wheels/collets/wing bolts||33||Ready to fly|
|U/belly - air ducts cut out||106|
|Bolts and fittings||13||Ready to fly|
|2 Elevator servos/hinging/horns||93||Ready to fly|
|Motor mount and former (light ply)||157||740|
|Ply wing mounts||30||820|
|Rear balsa crutch/tail skid mount/tail post||40||860|
|Glass tunnel moulding (scoop)||20||880|
|Canopy added and filled||90||970|
|Rx battery (1800mah nicad)||186||1179|
|2 switches and charge socket||25||1207|
|Rudder servo and cables||52||1259|
|OS 140RX engine and manifold||923||2059|
|Prop APC 17 x 12n||97||2606|
|Spinner 80mm carbon||57||2663|
|Bolts and nuts etc.||19||2682|
|Azano pipe etc.||107||2789|
|Tail plane complete||314||3103|
|Underbelly and fittings||130||3233|
|Wing complete||1157||Model ready to fly|