Parnall Elf

[Greyhead46]

Once again this is a retrospective thread because there is now a charge to fully view the original; so this a thread about the construction and flying of a 1/6th scale Parnall Elf.

 

To hopefully whet your appetite, I'll start at the end and post a photo of the finished model.



The project started about 3 years ago when I was given a pile of free plans from various mags one of which was for the Parnall Elf. I was hooked; a pretty biplane, upright engine, folding wings (no fiddly rigging to contend with) and last but by no means least, not a Tiger Moth!

A little bit of research soon revealed that the designer had been stretching the truth a bit when he described it as a scale model, so I set about redrawing the plans.

OK, so what was wrong with the plan? Don't get me wrong, I'm not a "rivet counter", but some things have to be right. The plan had top and bottom wing proportions wrong and the tip shape was wrong, as you can see from this photo it just wouldn't have looked like an Elf in the air!



After a trip to the Shuttleworth Collection, armed with the digital camera of course, numerous other less obvious errors were found. I'll not list them here but point them out as the thread progresses.

I intend to develop this thread in the chronological order that I built the Elf but if anyone is building a similar project and thinks I might have an answer to a particular problem let me know and I'll try to help.

I always start construction with the fuselage. Wings are usually fairly straight forward, but that didn't prove to be the case this time!

The first thing is the "heart" of the model, the engine bearers, cabane strut, bottom wing and wheel hard points. Birch ply is the material of choice here as basically everything that needs strength is attached to it.



Then the sides made from 1/16th ply and the rest of the formers.



The cabane brackets are made from 0.5mm galvanised steel, from B&Q, much stronger then brass for these high stress points, the extra weight is minimal. They are fixed to the fuselage using model railway track fixing pins.

·          

[Greyhead46]

The top of the fuselage was covered with 1/16th balsa leaving the luggage compartment open; I use this hatch as a convenient access point for the rudder and elevator servos, the bottom sheeting was left until after the cockpit detailing was finished. The flat just before the cowl was added later.

Next the tail plane, fin and rudder. Not a lot to say really, I used the popular “core and half-ribs method”, the rudder profile needed changing from a typical “Tiger Moth shape” on the plan to incorporate the distinctive straight TE. On a previous model I designed I got the tail incidence wrong so I decided to make this one variable as per full size, of course sod’s law prevailed and having gone to all that trouble it doesn’t need any adjustment.

Anyone who’s made a biplane knows what a pain cabane struts are. I usually end up with 3 or 4 sets in the bin before I get one that’s near enough to be able to fiddle the fixings to get the correct incidence and level wings. I decided there must be a better way, there is, the way the full size do it! But how to model them?

The answer proved to be quite simple. They are made from streamlined brass tube with a snake inner going right through the centre, in this case the ends are crimped and then a closed loop adaptor screwed in both ends.

 

 

The result is a fully adjustable cabane strut ready to be bolted to the brackets using 14BA bolts.

 

 

Any variation can now be adjusted out and they look a lot more scale.

 

Edited May 12, 2023 by Greyhead46

[Greyhead46]

Before I got to the stage shown in the photo above I needed to know how I was going to hinge the wings to the centre section. This is where the plan was seriously wrong; it had flat plate hinges. Whilst it was obvious using simple geometry that the hinges shown wouldn’t work, it wasn’t quite as obvious what would! Luckily the top hinges are visible and I was able to take a photo at Shuttleworth that gave me a good starting point but it wasn’t completely clear how they worked because of course I couldn’t fold the wings.

I made a jig to see what happens as the wings fold.
 

Wings fully extended

 

Wings half folded

 

Wings fully folded

 

 

This confirmed the design; the hinge has to turn in the fuselage fixing. I achieved this by using 4mm bolts with captive nuts in the fuselage and centre section.
 

 

The end result. No problem getting this in the car!

 

At this stage I’d made the hinges and satisfied myself that they would work but hadn’t started the wings so the photo of the hinge above is really out of sequence but it’s the only one I’ve got of a finished hinge showing the bolt.

[Greyhead46]

The next job was to furnish the cockpit; one advantage of these early planes is that they were very basic in the instrumentation department and with the front cockpit covered there’s only one to do. The throttle, fuel switch and all the other bits and pieces were made from scrap aluminium or plastic tube, balsa and litho plate.



As mentioned above the floor is yet to be added.



I hoped that it would be a simple job to use the photo of the full size panel for the dial printout but unfortunately because of parallax (hope that’s spelt correctly!) some of the dial faces are partly hidden by the bezels and whilst not obvious at first sight, perspective has made the outer dials oval.

 

After spending many a happy hour on Photoshop I produced acceptable dials etc. These were correctly spaced using Publisher and printed out on “Glossy Photo Paper”. The control panel itself is made from card covered with litho plate, the dial printout, a sheet of clear acetate and finally another layer of card. The bezels are cut from card and the “level flight” indicator is a length of Bowden cable outer.



The clarity possible using “cheap” technology is amazing, this photo doesn’t do the dial faces justice. I used a high-resolution setting when printing out and, although way “over the top”, it is possible to read the serial number on the rpm indicator using a magnifying glass!

Edited May 14, 2023 by Greyhead46

[Greyhead46]

The centre section uses dowel for the trailing edge

 

 

The outer ribs are ply as they have to hold captive nuts for the wing hinges and the cabane strut top mounts, once again fixed with track pins

 

 

It incorporates a fuel tank at the front …….

 

 

…… and a grab handle at the rear.

 

[Greyhead46]

Onto the cowl, my favourite part of any aircraft, they are so individual and have lots of lovely fixings, bumps, air scoops and other odd appendages to stretch our modelling imaginations.

First a photo of the finished article, it can appear a daunting task but taken step at a time it’s OK.



A decision has to be made as to whether to cut non-scale holes or use scale inspection hatches to gain access to the engine for choking and adjusting the needle valve etc. In this case the sides are hinged so the choice is fairly obvious although it does complicate the build somewhat. The first step is to make ply formers for the front and rear, with this cowl it was slightly more complex, because of the hinges there are actually 3 formers, top and left and right sides, for both front and rear. The formers are then joined with balsa “longerons” and covered with 1/16th balsa. Wetting the outer surfaces where there are tight bends helps considerably.

The next problem is the long hinge. Commercial “piano” hinges are just too big and won’t bend. You can make your own quite simply by soldering brass tube to brass strip and cutting into suitable lengths.



The hinges are glued using epoxy.

 

This shot shows the formers and the joining longerons.

 

The result is a very neat joint.



 

 

[Greyhead46]

A photo of the exhaust side of the cowl.

 

Reading through my last post I realise that I didn’t explain how to ensure a well fitting cowl, wonderful thing hindsight.

First of all make the templates for the formers using the model not the plans, remember to remove 1/16th for the covering, mark the hinge line and partly cut. Then fit the balsa longerons with the assembly in situ using cling film to stop unwanted adhesions, only then fully cut the sections free.

 

The cowl is covered with a combination of litho plate and printer paper. Where there are compound curves use litho but for flat plates printer paper is lighter and easier to stick, it’s also a lot cheaper, if you can’t find a friendly printer. The rivet lines are embossed from the rear using a dressmaker’s pattern copying wheel, the spacing may not be correct but who knows? At least they’re evenly spaced. For individual rivets use an old “Biro”.

The louvers are a simple job on the Elf as they are separate items riveted on. They are made from strips of litho bent to shape and glued through holes cut in the side panels. Don’t be tempted to just stick them on the outside; they’ll look terrible and won’t help to keep your precious engine cool!

 

 

The secret is to mark out all the holes, cut the first hole and glue the louver in before cutting the next hole. This maintains the curvature of the panel. I also strengthened the strips between the louvers with a few carbon fibre tows but this is not really necessary, it was just a bit of “belt and braces”.

The exhaust is from paper rolled around a felt tip pen.

 

 

Use medium cyano and then roll the pen on the workbench adding more glue as needed with an extra couple of wraps around the front section.

 

 

 I use kitchen foil, as it appears to be about the only thing cyano won’t bond to, to stop the lot sticking to the pen. Coating with a mixture of 30min. epoxy and micro balloons produces the pitted surface if you keep dabbing it with a “suitable implement” for the first few minutes as it cures.

The “suitable implement” I used was my finger but I’m sure the Health and Safety establishment would tell me I’m storing up all manner of ills for the future, so the choice is yours!

 

The cover is formed from lithoplate in a female mould using soft balsa for the forming tool, the two halves "welded"  together with 5 minute epoxy. A 14ba bolt and nut  finishes it off.

 

[Greyhead46]

The twin air scoops are made from litho with copper wire to form the front “roll” lip and really help the cooling by directing air down over the rocker box onto the cylinder block. The other bits and pieces use litho, copper wire, brass tube and washers.

You get a good view of the cabane struts with a coat of paint in this view and can just about make out the carbon fibre "tongues" that lock the wings in place.

The Elf logo was printed onto self-adhesive white paper, once again after a bit of manipulation using Photoshop. It's a good idea to seal the printout before you cut it out to prevent smudges, I used clear spray lacquer from an automotive supplier.

 

The fasteners are not functional, the sides are held in place by magnets, very useful items these, they also hold the luggage hatch and the rear section of the lower wings which fold down to allow the main wings to fold back. I prefer the ones originally designed to be used as electrical connectors, they are in a metal holder with a stiff wire attached; this helps with the positioning.

The windscreens are simple affairs cut from acetate sheet. The fairing is from litho formed on this jig and the “bolts” are cut down pins. To make the job of painting the fuselage easier they were not fitted until it was ready for the final coat hence the painting of the fairing.



When forming shapes such as this from litho it is important to cut it oversize to allow for the stretching etc. However too much excess and it tends to stop the litho holding its shape and forms folds, so as you work keep trimming the edges, it soon becomes obvious where you’ve left too much material.

Don’t try to do the job in one go, form it partly to shape, trim off excess, form some more, trim again and so on. It’s amazing how complex a shape you can get out of a flat sheet of litho.

[Manish Chandrayan]

Tell us more about your spinner please? That shape spinner is difficult to obtain commercially. 

And where can one still source that Solarbo? 😉 

[Greyhead46]

Hello Manish

 

Thanks for the reply; this model was built quite some time ago when Solarbo was still available so can't help you there.

 

The spinner is a simple modification to a cheap 2 part I.C. spinner, hopefully the photo below explains how it's done.

 

 

[Manish Chandrayan]

Thank you👍

[Greyhead46]

Hello Manish

 

No problem glad to be able to pass on these tips.

 

Grahame

 

Just the undercarriage left to do and the fuselage is virtually finished. This was simple as the plan was near enough for me, except for the missing rear strut but this is non-functional and made from plastic tube. The piano wire was bent to shape and silver soldered, soft solder really isn’t up to the job even if the joins are bound with wire before soldering. The fairings are made from balsa but as it is impossible to get a sharp edge they are covered with litho.

Now just stick on as many of the numerous brackets and strengthening plates as you can be bothered with, tape the joints and it’s ready for painting.

Remember to build light, having due regard for strength, because all these little bits and pieces soon start to pile on the weight. By the way I never bother to weigh my finished models, if its light it’ll fly that much better but if it’s over-weight it’s too late to do anything about it!

 

That’s the fuselage finished ready for painting.

[Greyhead46]

Now the wings. When building scale fabric covered wings we can’t use the normal modelling technique of top and bottom spars with or without sheeted leading edge (unless that’s scale of course!) as the fabric needs to sag in-between the ribs. Unfortunately this “D box” is what gives the wing most of its strength but I’ve devised a system which goes a long way to replacing the strength whilst retaining the scale appearance.

The wing has a front laminated spar and a spruce rear spar to which the hinge is attached, if the wings weren’t folding the rear spar could be missed out altogether or at least made from balsa to save weight.

The front spar is a spruce / balsa / spruce laminate with the balsa extended to go through the leading edge with the direction of the grain from LE to spar. The ribs are made in 2 pieces; the top front section is added after the composite spar with additional sections as “riblets”. The leading edge is 2 lengths of square balsa glued top and bottom of the balsa lamination.

Before covering, the ribs and more so the “riblets” are very vulnerable to damage whilst you do the other 101 necessary things to the wing. The answer here is to make a small hole in each of the pieces, it’s easier to do this before gluing in place, and then thread some strong cotton through the holes to make a “necklace”. After gluing in place, soak the cotton (button thread is the best) with thin cyano. It’s amazing how much this cotton also increases the torsional strength of the wing.

 

The rest of the main construction of the bottom wings was fairly straightforward. The hinges are pinned, again using model railway track pins, to the rear spar, glued with epoxy (see early photo) and finally given a few wraps with carbon fibre tows. If you haven’t used carbon fibre tows before I can highly recommend them where some extra strength is needed without adding too much weight. Just a few strands glued using epoxy, the best but a bit messy, or cyano, watch out for the fumes, will greatly increase the strength. I also moulded the tailskid from carbon fibre tows and epoxy; it has produced a very strong, light and flexible job.

 

 

 

The bottom wing is still to have the rear section attached, which is hinged using commercial flat plate hinges and held in place by 2 small magnets, the theory being that the air flow will keep them in position once the airspeed has built up and this has proved to be the case.

The photo below shows the mechanism for locking the wings in place. There is very little tension in the springs when locked, they’re just there to stop any movement caused by vibration. They lock into slots cut into free-floating tongues that pass right through the top centre section and fuselage.




I originally made tongues from 1.5mm paxolin circuit board but having given it some thought I came to the conclusion that the model would have better flight characteristics if the wings stayed in the non-folded position during flight, so I replaced the tongues with ones cut from carbon fibre sheet!

You will have noticed that I’ve said to thread the rib fronts to make a “necklace” before gluing in place but in the photos of the wing there’s no button thread to be seen! This is precisely why I’ve also said it is easier to do it that way. I built the bottom wings and then found out the hard way just how vulnerable the ribs were, i.e. I damaged a lot of them, that is when I worked out the idea of the thread and when it worked I incorporated it into the build for the top wings which was a lot easier. So here's one showing the thread.

 

[Greyhead46]

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 tube isn’t in line 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; there is quite a weight saving and surprisingly the result appears to be stronger.

The torque tube is made in 2 pieces, the ends are made first then the tubes 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 rear spar

 

In situ

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 tube had a join 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.

 

Edited May 18, 2023 by Greyhead46

[Greyhead46]

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.

I had previously soldered the crank to the torque tube 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 then cut the crank tube just before the yoke. To stop the crank soldering itself together the nail was tinned just at the very end where it fits into the yoke end of crank tube, and the yoke end heated to solder the tube to the nail. The result is a tube with a twisting joint, you can just see the joint close to the yoke in this photo.

 

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

[Greyhead46]

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 effects.

One disadvantage of choosing a later subject with fabric-covered wings is the making of serrated rib tapes; WW1 types used non-serrated frayed 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 in the above post 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,  I’ve hinged the lengths of pine together which ensures everything lines up easily, then 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 spacing template and a precision screwdriver.

 

 

Finally they are ironed into place being careful not to iron over the embossed stitching.

 

Edited May 19, 2023 by Greyhead46

[Greyhead46]

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.

 

[Greyhead46]

That’s the construction finished. 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, make sure it is well degreased after annealing to get rid of all that soap. 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 weathering is 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!!”

 

[Solly]

Beware of the Tufcote. My experience over the years tells me that this too will eventually lift the finish, especially with hot exhaust deposits. Clear KlassKote is the only product I’ve found to completely work.

[Manish Chandrayan]

For glow fuel with nitromethane content, only coatings that are 2K and catalyzes instead of cures by evaporation of solvents will be truly fuel proof. Every other coating will only be fuel resistance to varying degree.  

And many 2K will react with other non 2K coatings. I believe the solvents/thinners used for the 2K coatings are the culprits.  Yet to try the water borne 2K PU and see if that reacts with oil based enamels that I normally use for color coats. I suspect it may not react at all, but I may face challenge in terms of water borne coat adhering to the oil based enamel layer.   

[Greyhead46]

Hello Solly and Manish

 

Thanks for the replies. Personally I've never had any issues with Tufcote lifting but I use "straight" fuel, I find Lasers run perfectly well without any nitro.

 

Grahame

 

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 you can see one of the magnets which hold the lower rear wing section in position.



Now with the hatch open, once again a magnet is used to hold it shut.

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.

 

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 will carry the current for the remote glow (yet to be fitted). They also help secure the firewall as they go through brackets bolted to ply fuselage doublers (see photo in post 1, the "heart" of the model).

 

 

 

Edited May 22, 2023 by Greyhead46

[Greyhead46]

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


 

But if there’s a convenient hatch why no use it? This hides the switch and the remote glow.



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.

 

The engine compartment is rather more crowded then I’d first envisaged, the nose section 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 used silicone tube to join to the flexible pipe. (Turned out not to be a good idea!).

 

 

The engine with this, is 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!

Edited May 23, 2023 by Greyhead46

[Greyhead46]

One thing I have mentioned earlier in reply to Manish 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 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 probably 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!
 

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”!

[Greyhead46]

This is it, the weathers OK and the time is right, so pack up the car and head for the field. When I arrived the steady breeze had dropped to an almost flat calm, still this is better than going the other way.

First things first, a few photos just in case the worst happens, here’s one on the starting box and table…..


 

….and in the background of this photo the club aerobatic expert Bob Wasson is preparing his aerobatic model.


 

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 what I think is 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.

 

A couple more photos before the "moment of truth"

 

 

 

With the engine nicely warmed up a couple of runs to check the ground handling; no problems, straight as a die, but then it would be, the gyro’s doing all the work not me! Gradually build up the speed to see if the rear folding sections of the lower wings stay in place over the bumps and she’s nearly airborne, close the throttle and everything’s fine so the next one is for real.

This time a bit more throttle to ensure we reach flying speed and away she goes, as soon as the wheels leave the ground the left wing drops and it needs full right aileron just to keep level. Too late now to abort so gain height and see what we can do. A few clicks of right rudder eases the problem but when the throttle is closed a sudden right turn, it’s obvious that landing is going to be quite a challenge so I decided to cruise around at a safe height to let the nerves settle. Gradually reducing throttle and adjusting rudder trim line up for a landing, got to get this right as any sudden throttle change will result in an equally sudden turn! More by luck then judgement I hit the square and she’s down in one piece.

Time for the post mortem. First the ailerons are virtually useless so must use CAR (coupled aileron rudder). The trim changes with throttle indicate the need for more engine right thrust. Bob gave the model the “once over” and declared that there was in fact slight left thrust (note this is an expert not an “expert”!!) and he suggested I adjust the thrust line to 2º right thrust. At home careful measurements proved he was correct about the left thrust and I duly modified the bearers; could only get about 1½º right because of lack of room under the cowl.

So how did this fundament error occur? The only explanation I can come up with is that although I altered the construction of the fuselage nose section completely to accommodate the non-standard Laser engine bearers I used the original “F2” as a starting point, at some point during the tracing, altering and re-tracing onto the ply I must have turned the paper over so instead of the rear of the bearers being offset to the left they were offset to the right.

I mixed CAR and tried again a couple of weeks later, what a difference, the model was now completely controllable but still turned left as the throttle was opened, if I’d managed to get the full 2º as suggested I’m sure it would have “flown on rails”. I now also mix in a small amount of right rudder with throttle and she flies like a “good un”. A wonderful sight on a low, slow pass with the Laser burbling away at ¼ throttle, it doesn’t get much better than this!

Edited May 25, 2023 by Greyhead46

[Greyhead46]

There wasn’t much left of the flying season but the Elf had a few more flights before being packed away. After the winter recess, I got the Elf out for it’s pre-flight checks.
 
Of course the Laser fired up “first flick” and ran faultlessly but there were a couple of things to sort out, both involving 2mm nuts! When I checked the nut on the tail incidence adjuster it stripped the thread with only slight torque on the spanner.

 

The tail is mounted on brackets made from thin steel and attached via ball joints to allow for the fact that the brackets are fitted to the tapering fuselage sides. The closed loop for the rudder goes in the space between the fuselage and the tail plane. 

The adjuster consists of a 2mm rod threaded at both ends with a nut soldered at one end. The rear of the fuselage has a dowel, with a length of snake inner down the centre, into which the adjuster is screwed in or out to alter the tail incidence. A lock nut goes the other side of the rear tail brace to keep things solid.

 

The braces are attached to the tail and fuselage by steel brackets again using closed loop adaptors and 14BA nuts and bolts. There is very little movement at the fixing points as the tail incidence is altered and the bolts don’t need to be slackened.

In this shot you get a good view of the tailskid, made using epoxy and carbon fibre in a plasticine mould


 

It was actually the rod that had stripped; it was one of the 2mm rods from the model shop with a thread at one end. I needed a short rod; about 50mm long, with a thread at both ends so I’d tapped the extra thread. When I investigated the problem I found that the rod as supplied has a “rolled” 2mm thread but the rod itself is only 1.7mm diameter. All had seemed OK when it was made but obviously with vibration from the engine and the load when elevator is used, the undersized thread wasn’t good enough for the job.

 

The other thing is the aileron control arms; these originally used a 2mm nut and a closed loop adaptor.

 

Nuts are intended to be locked, either against another nut or some hard surface, but in this instance I’d used the nuts as a bearing and vibration etc. has caused considerable wear.

I’ve replaced the nut with a length of snake inner cyanoed into the tube and then tapped 2mm for the closed loop adaptor. I was a little worried relying solely on glue to hold the tube and snake together but when testing the result it was impossible to pull the closed loop adaptor out of the tube.

At least I found the problems before anything disastrous happened and it re-enforces the need for regular checks.