Another Parnall Elf

[wonwinglo]

Great tips again,where do you get this carbon fibre tow material from Grahame ? locking those vulnerable ribs together with cotton thread like that makes good sense as well,it is flustrating to kick a few ribs whilst carrying out other tasks.

 

[duncan]

Try the "hi-tech materials" section of Freeflight Supplies in Norwich. The cap strips, ready made in a variety of thicknesses and widths are very useful for other uses. Longer lengths of ready made come in largeish loops. Used for edging Depron wings etc. There are a couple of suppliers on Ebay at the moment selling both tows and pultruded shapes. I`ll PM you later when I find them. Ironic that the oldest form of aeromodelling should be up there with the gas turbine boys in the use of modern materials. I`m sure somebody said that about balsa when it first appeared.

 

[Greyhead]

Barry

Our local model shop sells the carbon fibre tows; it comes wrapped around a piece of cardboard. It’s just lots of very thin strands of carbon fibre and am sure you should wear a respirator when cutting or sanding it!!

Grahame

 

[Greyhead]

At first sight the top wings appeared to be a fairly straightforward build; external pushrod from fuselage to wing then torque rod to the aileron. As is often the case, first impressions can be deceiving; the pushrod connection has to accommodate the folding of the wing and the torque rod isn’t inline with the aileron hinges.


The top wings were constructed using the same method as described early except that the rear spar is also a spruce / balsa / spruce laminate. In future I’ll laminate all spars as there is quite a weight saving and surprisingly the result appears to be stronger.


The torque rod is made in 2 pieces, the ends are made first then the rod fed through from both ends and joined with a short length of tube, when everything is lined up and slop free the joint is soldered.


First a general view.





I couldn’t figure out how the full size linkage worked so I decided to use a crank and slot, the photos are self-explanatory.





In this view you can also see one of the ply hinges, for strength they have a hole cut and are threaded onto the spar








At this point I’ll have to “do the time warp” and jump forward several months. Although all seemed satisfactory at this time, when I fitted the radio the ailerons wouldn’t neutralise. To this day I don’t know what was wrong or why what I did corrected it, but I made a new slot near the TE of the aileron, turned the crank through 180º and hey presto no more problem. Incidentally, the fact that the torque rod was joined made this modification easy. As you can see this is not quite as good from a scale point of view as the crank is now visible but only when the ailerons are raised or lowered.

 

[Gryphin]

It is good the that some modellers are not blinkered in the way that they build their models, the use of thread to strengthen the fragile sub ribs is pure genius, (who would of used thread). I am really enjoying reading this thread (excuse the pun) and look forward with anticipation for the next episode.

A big well done

 

[Phoenix]

yup yup great posting would likeyo see this model up close

 

[Greyhead]

This was the complicated bit, working out what had to do what!

The push rod to wing connection obviously has to be able to let the wing fold by approx 90º and this was easily done by soldering an M2 nut to the push rod for the ubiquitous closed loop adaptor. CLICK HERE FOR UPDATE. You can just about make it out in this photo. I hoped this would be it.

View attachment 106222


I had previously soldered the crank to the torque rod and as a “dry run” seemed OK I soldered the yoke. After several operations the M2 nut started to break the solder joint. Apparently during the “dry runs” the yoke had been moving on the crank. After unsoldering the yoke I checked and sure enough the yoke turned very slightly as the wing folded.

This caused a lot of “head scratching” eventually I solved the problem. A 1" nail was just the right diameter to fit inside the crank tube so I ground the head down and filed all the rough bits, re-soldered the yoke and cut the crank in half. The nail was tinned at the end to stop the crank soldering itself together the result is a tube with a twisting joint in the middle. You can just see the joint close to the yoke.

You can also see the front cockpit cover, this made life easy, only one to worry about fitting out!

 

[Greyhead]

The wings are covered using white Solatex and after heat shrinking I always give them 2 coats of very thinned down clear shrinking dope. This is not recommended by the manufacturers but I’ve found in practice that it stops any sagging during hot weather and to date I’ve had no adverse affects.

One disadvantage of choosing a later subject with fabric-covered wings is the making of serrated rib tapes; WW1 types used non-serrated tapes. Whilst not difficult once you’ve mastered the technique, I find it very boring and tend to make small batches as I go along and do something interested in between! The photo above clearly shows the difference rib tapes make to the finished model and surprisingly they are very noticeable when the model is flying.

The method I use is not original but I can’t remember where I heard about it. I use a hacksaw blade, with an appropriate number of teeth per inch, mounted onto a length of scrap pine, trap the tape material against it in a vice using another length of pine and tear it towards you. The best thing I’ve found to make the tapes from is a good quality tracing paper with a coat of Balsaloc applied to one side; always tear with this side towards you as it adds a slight downward curve to the serrations which makes them stick to the wing easier. Rib stitching is then embossed from the glue side using a precision screwdriver and finally they are ironed into place.

A couple of tips: join the 2 pieces of pine together with hinges to make “lining up” easier and be sure to let the balsaloc dry overnight before tearing the tapes.

See tutorial Making Serated Rib Tapes



The Warren Girders were made in a similar fashion to the cabane struts except that the snake inner this time is clad with spruce and 1/16th ply.The cross wires are made from 22swg piano wire and to avoid metal-to-metal interference they must be either insulated from each other or permanently joined. I decided to go the latter route so made small tinplate discs and formed a groove along the diameter for the wire to sit in. First one disc was held in place at the crossover point underneath the lower wire sandwiched between a piece of 1/16th balsa clamped to the wire and the wire itself then soldered, another pre-tinned disc was placed on top of the top wire and soldered to the lower disc.

 

[Greyhead]

That’s the construction finished, didn’t take long did it? Now for the finishing. All the wood parts are given at least one coat of clear dope and one of sanding sealer, this fills most of the grain but the finished surface is still obviously wood, parts that represent metal receive 3 or 4 coats of sanding sealer. Lithoplate is left as it is, you did make sure it was well degreased after annealing to get rid of all that soap didn’t you? Any “metal” made from printer paper, in this case the cowl side panels, get a coat of dope followed by a thinned coat of silver Solalac.

Matching colours is very difficult, even when you’ve taken your own photographs there seems to be many different shades. Using Photoshop I “sampled” several different parts from different photos and averaged the results. Armed with this information I had the colours mixed by an automotive paint supplier, not cellulose but brushing enamel as I don’t like spraying.

I always use matt paint as I find it covers a lot better, for a gloss finish just use gloss fuel proofer. A thinned coat first as primer/undercoat then as few full strength topcoats as you can get away with to save weight.

To be accurate the Elf should not be weathered much, as it is a museum piece and only flys on high days and holidays, but I like my models to look like the real thing and besides it’s such good fun. Leave the paint to really cure then rub gently with 1200 wet and dry using plenty of soapy water; rivet heads, panel edges and corners show first and with a bit more work worn areas around steps and cockpit edges come to life, brilliant!

The photos of the cowl earlier in this thread show the results of the weathering and in fact I like to include the odd dent and scratch to add to the “realism”.

I decided to use satin polyurethane varnish as a fuel proofer as it is recommended in all the model mags. This has proved to be a disaster as after only a few flights the paint, especially on the undercarriage and around the exhaust, started to lift. Re-painting is now complete and I’ll be giving the entire model a covering of matt Tuffcoat.

The moral of the story is “if you’re going to try something new don’t do it on a model that’s taken nearly 3 years to build!!”

For painting registration letters see "Hand Painting Registration Letters Etc." in tutorials.

 

[duncan]

Great stuff. How long did you wait before exposing the polyurethane to fuel? Any air-drying paint should have a couple of weeks to make sure, think the two-pack types are better. Any knowledge of water-based polyurethane ? Wonder if that is what Johnson`s "Klear" is.

 

[Greyhead]

Ducan

The varnish had plenty of drying time and it wasn't the water based variety.

To be fair to it the exhaust was quite powerful stuff, hot and mixed with the contents of the crankcase breather as you'll see later in the thread. I just hope that the Tuffcoat copes better and the exhaust is now not as hot.

Grahame

 

[Greyhead]

It’s now time to fit the radio gear. One problem with closed loop or pull / pull systems is “hiding” the cables as they pass through the cockpit, one solution is to use a snake from the servo to a bell crank fitted behind the cockpit to which the cables are attached but in this case I decided to fit the rudder and elevator servos in the luggage compartment; it also makes them easy to get at to adjust. No worries about ending up tail heavy with a large lump of Laser up front. First a shot with the hatch closed; you can see that the fuselage is clearly made of wood as the grain is still slightly visible as it is on the full size and one of the magnets for the lower rear wing section.





Now with the hatch open, once again a magnet used to hold it shut. You can also see the neat edges to the lettering.





The aileron servos are mounted to the fuselage side with the arms protruding through slots to attach to the push rods and a mini servo under the cowl operates the throttle.


One problem I’ve had with all my previous “early” biplanes is their tendency to ground loop i.e. they veer off course when taking off, as it’s important to take off and land directly into wind with these models this results in a lot of aborted take offs. The problem can be rectified by widening the under carriage and moving the wheels forwards but then it’s not a scale model is it?


I’ve found the perfect answer, a “heading hold” gyro to control the rudder. With the model pointing into wind the gyro is activated, via a spare Tx channel, and so long as you can resist the temptation and keep your hands off the rudder stick the model will track straight down the runway.

 

[Gryphin]

Duncan

I only use water based Polyurethane varnish, to fuelproof the tank areas and inside the cowl and if needed the whole model,and have never had any problems, just paint it on, two coats 24hrs between both coats and then fly 24hrs later.

The only time I used Tuffcoat and it lifted the paint, so you pays ya money and ya takes ya choice.

As for klear (a blast from the past) used that 30 years ago the give a nice deep shine to me boots dont know what effect this will have on a model.

 

[Phoenix]

i have clear coat for my plane when i get it finished

 

[duncan]

Gryph, I had to use the old spit and buff on mine but that was a bit more than 30 years ago !

 

[Greyhead]

Where to hide the switch etc. is always a challenge!





But if there’s a convenient hatch why no use it?





My home made hinges proved not to be up to the job at this small size i.e. only 3 section to each hinge so had to be replaced. The easiest way was to use thin rubber strip.

 

[Greyhead]

The engine compartment is rather more crowded then I’d first envisaged, the plan had the nose section non-tapered, very similar to an SE5a, whereas it is tapered in plan, side and front elevations. The standard silencer just wouldn’t fit so I decided to just use a flexy pipe, about 6" long, attached to the scale outlet. Having talked to other Laser owners we thought that this would give sufficient silencing.





The bent pipe is for the crankcase breather, saves having an extra hole in the under pan and blasts the oily mess away from the fuselage.





This system worked well, up to a point!


Laser engines normally sound very realistic and although it wasn’t excessively noisy, to me it sounded a lot more like a single cylinder.


Without any expansion chamber the exhaust is hotter, I’m sure we’ve all suffered from “frozen finger” when operating spray cans for a prolonged time, as gasses expand they lose heat. The hot exhaust mixed with the breather output may explain the paint problems as it is directed onto the undercarriage.


I’ve now made a remote silencer from brass sheet.





I’ve only test run, not flown, the engine with this, it’s certainly a lot quieter, I think more so than with the standard silencer, I just hope that it cools the exhaust sufficiently and that the engine noise is back to it’s original realistic self!





The throttle linkage caused some headaches, with the carburettor angled it wasn’t an easy job until I thought of the idea of making the linkage run back into the fuselage first. The Bowden cable loops around the fuel tank and then back into the engine compartment, a bit messy but it works.


The bolt through the firewall is one of a pair, the other one in a similar place on the other side of the fuselage, which carry the current for the remote glow. They also help secure the firewall as they go through brackets bolted to ply fuselage doublers (see photo on page 1).

 

[Greyhead]

One thing I should have mentioned earlier is the spinner. The shape on earlier planes is not like most of our plastic spinners that are more akin to the WW2 period. The required profile (see reply 28 page 3) can be approximated by using just the front section of a standard two part plastic spinner. The rear section, that usually covers the prop, can have the flange removed or be discarded and a back plate made from aluminium. The front section then has to have cut outs to fit the prop and the screw support pillars will need reducing to clear the prop hub.

One problem I’ve not managed to solve is the aluminium finish; the electric starter soon makes a ring on the spinner. The best I’ve come up with so far is a coat of silver paint covered with a skin of epoxy resin, this lasts a few starts at least.

I know the easy answer is to flick start the engine, but as I suffer from tennis elbow, strange that as I’ve never played a serious game of tennis in my life, it isn’t practical in my case!

All suggestions / ideas gratefully received.

 

[Greyhead]

Now comes the worst bit of any build!

You’ve spent what feels like a goodly proportion of your adult life building this model, the radio and engine are fitted and working a treat and now you have to DO NOTHING!! Just sit and twiddle your thumbs as you wait for perfect weather, a nice warm, steady breeze to help keep the landing speed reasonable. I can’t understand people who, after investing all the time and effort into building a scale model, test fly it in blustery conditions; apart from the obvious higher risk of damage it is very difficult to make meaningful trim changes.

I place another restriction on myself for a maiden flight; I don’t try when the site is busy. I feel enough pressure without numerous eyes watching your every move and /or a crowded sky. The worst culprit is the club “expert” who will point out all the things where you’ve gone wrong and explains at length how he’s solved the problems.

Talking of the club “expert” prompts me to ask 2 questions:

1 Does every club have one or are we just “lucky”?

2 Has anyone actually seen any of the multitude of models he professes to own, not to say having seen him fly anything?

The next post will chronicle the first traumatic flight and the subsequent changes which, made over a few flights, have transformed the Elf from a “dog” to a “pussy cat”!

 

[Bluewavestudios]

Hi Grahame,

An Excellent model with clearly many hours put in on it & the workmanship shows it. I hope the Test flight goes well for you.

As for the Club "Expert", yes we all seem to get one of those crop up wherever you go. Not that I mind useful advice and tips, I am all ears then. I see some really sound ideas coming up on here all the time.

I have noticed the so called Experts round here always seem to have just the one and the same model everytime we go out......surely they must have more than that...???!!!!

This is regardless of whether it is Cars, Boats, Planes or Heli's....!!!

Regards.........Mark.