Saturday, August 19, 2023

Classic balsa RCM Trainer Junior electric: Build Log and Flight reports

This is my scratch-built electric version of an iconic 1970s i.c. trainer - the Radio Control Modeler magazine (RCM) Trainer Junior. The article on this 52” span model appeared in the June 1974 edition and opened with the remarks: "Another reliable, good flying 60 becomes a reliable, good flying 40. An aerodynamic design that makes for slow, stable flying when you want it but, will loop, roll, spin, fly inverted, or…[sic]."


Designed by Joe Bridi and Don Dewey, the RCM Trainer Junior became the “Trainer 20”, kitted out by Great Planes. As far as I can see, the Great Planes kit version differed in having a split elevator instead of one continuous sheet and framework to hold the wing dowels instead of solid balsa block. Both plans are available from the Outerzone website. These trainer type designs were ubiquitous and became a staple. I suppose the modern equivalent would be foam trainers, like the Durafly Tundra, E-Flite Timber, FMS Kingfisher, Max-Thrust Riot, etc, although of course, they are tail-draggers, as opposed to tricycle undercarriage. 

I was curious to see how an old-timer balsa trainer would compare to the modern stuff. Also, this would be my first tricycle undercarriage RC plane. With photos, I’ll show you the build, the mods I made and describe the first few flights.


Building the RCM Trainer Junior

Rather than messing about with pdf files, I conveniently obtained 2 copies of the 1974 original plan from eBay seller “mrhobby” in Maryland, USA. I left one plan intact, and used the other one. Thanks to mrhobby for continuing to help the modelling community. While I'm at it, thanks to folks at 4-Max UK, DuBro in the USA for very helpful email correspondence, and for some inspiration. I cut the plan into sections to fit on my building board. Adhesives were primarily aliphatic resin (EvoStik exterior), epoxy (Devcon 5min and Araldite precision) e.g. for the fin-fuselage joint, and just a little CA, e.g. thin Zap for Great Planes CA hinges, and on a few bits and pieces where CA made sense.  

Tail Feathers 

The elevator was solid balsa, as per the plan. To try to keep the tail end light, I departed from the plan and constructed a built-up fin, rudder and stabiliser.

You can just see the thin ply ‘biscuit’ brace to strengthen the centre joint. Why did I bother with this? Well, many experienced RC modellers told me that i.c. engine aeroplane designs require lots of nose weight when you convert them to electric, because the i.c. motor and tank are heavy compared to a modern brushless electric motor and LiPo. 


Fuselage construction followed, and my next mods to the plan were to change the second bulkhead to an open frame and add a ply bulkhead for the electric motor ahead of the one for the nosewheel steering mount (DuBro nylon for 5/32” (~4mm) wire).

For access, there is the hatch above. Underneath, I made another hatch out of 1/16” ply, eventually painted white and held on by 4 screws. That needed an egg-shaped cutout to allow for movement of the sprung nosewheel steering arm.

It is important to have the nosewheel steering arm spring centered on the bottom edge of its bulkhead. It's simple physics of moments and indeed, it is shown on the plan. That said, I can't count the number of times I've seen that spring dangling way below a model!


I used the pinhole method to make templates: hold the plan over a piece of card, use a pin to prick holes around the desired outline, e.g. the wing rib, then join the dots on the card, and cut out a template. I ended up using thinner ribs outboard and thicker ones inboard, just because my balsa sheet stock was limited.


Spars were 6mm bass wood, rather than balsa. Another departure from the plan was to angle slightly the thick innermost ribs on each wing half and then sand them for the dihedral joint. To me, this was easier than the additional wedge shaped centre rib and dowels shown in the plan.

To glue on the LE and TE, I used this technique with pole elastic and clothes pegs. It worked very well.

Similarly for the pine blocks in the middle and pins for the tips.

As you can see, I also made wing servo mounts, for individual wing servos. I think this is more modern and easier than the torque rod single servo setup.

It means short control rods externally, but that itself is useful when it comes to setup and adjustment. Further, it gives me the option of programming differential or flaperon.

The plan specifies wing dihedral as 1 ¼” (32mm) under each tip rib. When joining the wings, I reduced this to 20mm and added short ply braces across the central joint. Generally, I had kept an eye on the weight of each wing half as I built it and chose materials to keep them balanced. This was very useful because at the end all I had to do was adjust my choice of sheeting balsa to even out the lateral wing balance - no balancing weights were required! As per the plan, after sheeting, I applied glass cloth over the centre joint.  


In addition to the wire sprung nosewheel described above, I made the undercarriage legs out of aluminium sheet and longer than the plan, as I needed clearance for a 12” prop that suited the recommended electric motor (advice from 4-Max). Since it is not “Dural” and therefore prone to bending, I decided to also add suspension: a central binding eyelet, springs and wire connecting rods. 

Servo Mounts and Connections

I added the rudder snake outer at this stage, but here I made a mistake: I built all the fuselage servo mountings at this point - Rudder (snake), Elevator (pushrod) and Nosewheel (pushrod). This eagerness bit me later on, as I’ll explain below in the sections CG and Balancing, and Radio Installation. 


The obligatory naked model photo:

For the Oracover covering scheme, I deliberated for quite a while. My aims were to show off the framework in the tail feathers and wing, aid in visibility - distinguish underside from top and model heading forwards or backwards - and to look nice!

Underside is all white except for some red stripes (see photo in the Undercarriage section). I allowed myself a single stripe of black trim, and a couple of side panel stickers. I applied thin white foam around the inside of the motor bay to clean up the look and perhaps provide some sound damping. The hardest parts to cover were the front corners of the wing. I could not get them wrinkle free. It was awkward because I was using one sheet for the main bay and tip. I suppose I could cut off the film from the outside edge of the last rib to the tip, and try to stick on new smaller sections without wrinkles, but I’ll leave that as a rainy day task.

CG and Balancing

To my total irritation, it came out nose heavy. I guess the motor is pretty chunky and far forward. As I’d built the servo mounts, pushrods and fitted the snake already, I couldn’t easily move the battery back enough to balance it out. And even if I’d ripped out all the servo mounts and moved the battery back, it would have been very awkward in use as I’d need to take the wing off to change battery. So, having considered all the options, I moved the battery back as far as I could while still being able to access it from the main hatch and added tail weight. Generally, I prefer not to add tail weight, but here, the convenience won out. I fixed lead buttons (for curtains) inside the rear of the fuselage, which I reinforced. I also added a tail skid, which would save the elevator in an awkward tail-down landing.

Altogether, I added about 35g to the rear. So, I would have been ok building solid balsa tail feathers after all!

Radio Installation

Here are photos of the fuselage-mounted servos and receiver. First, a top plan view, looking right and finally looking left:

The best bits are the paper tubes that allow me to orient and fix the antennae tips at 90° to each other. They help to ensure that the working tip portions of the antennae are kept straight. As a reminder, in 2.4GHz kit, the strongest connection between transmitter and receiver antennae is when they are parallel to each other. I made the paper tube by rolling thin paper over a cocktail stick (using squared paper helps with alignment), and gluing the long edge with pva adhesive. Leave some paper over as a flap, to help with mounting. I mounted the tubes with paper masking tape, in case I need to reposition things after experimenting.

What’s it like to fly?

It was a lovely day for the maiden, sunshine and 7mph wind. With the CG at the forward end of the range on the plan, transmitter on low rates, I did all my checks. Nerves mounting, the take off was exciting and easy! I just moved the throttle open, picked up speed and lifted her off! She seemed nicely balanced fore-aft and laterally, just two clicks of down trim required. Changing the throttle setting, I was surprised that I didn't feel the need for any thrust line or balance changes. She glides really well, can fly slowly - so wing loading must be low - and there is loads of power available if I need it. A slightly bumpy landing - because I didn’t align for the cross wind very well and didn’t hold off properly. Plane was fine, except that the right undercarriage leg had straightened a bit, making the right side sit low. Decided to stop there, quit while I was ahead and return another day! The colours were magic for orientation and my wife said it looked great in the air. Very satisfying. 

At home, I removed the alloy undercarriage, straightened it and refixed it with a slightly longer and wider central spring anchor. I also changed the springs to much stronger ones. This may help to prevent it deforming on a heavy landing. I found myself wondering what she would fly like on mid rates!

A few weeks later, I flew again and discovered the answer: the plane feels fabulous on mid and high rates - simply brilliant. I tried some aerobatics: inside loops, Immelman turns, humpty bumps, rolls, reverse shark tooth, cuban eight. I could hold inverted too. I tried a spin, but wasn’t sure if what ensued was a spiral dive, although it looked pretty. I have not yet tried outside loops. It was so much fun to fly, and I nailed these landings, properly lining up and holding-off for a gentle touchdown.  

After just a few flights, I love this plane. It’s not as “flippy” as the modern foam trainers mentioned above. To explain further what I mean, consider the BMFA ‘figure of eight’ manoeuvre that is used in the A and B tests. This should be thought of as two level 360° circles, not a squashed 8. Compared to a modern foam trainer, the Trainer Jr was much easier to control during this - I found it easier to adjust the bank, turn radius and height. I’d imagine that the unmodified original, with its greater wing dihedral, would be even more stable. I’d describe the response of my version as immediate, smooth, predictable, and very rewarding. I also enjoy the tricycle undercarriage as it seems to steer well on the ground and lessen the influence of a crosswind after touchdown. 


Span: 52”

AUW ready to fly: 1.56kg

Balance point: 81mm from LE

Motor: 4-Max Professional Brushless Outrunner 3547, 960kv 

Prop: APC Electric 12x6

ESC: 4-Max 40A 

Battery: Overlander Sport LiPo 2200mAh 3S 35C XT60 connector 

Receiver: FrSky Archer R6 ACCESS

Transmitter: Taranis QX7 ACCESS, Open Tx

Rudder servo: EMAX ES3104 - 19g

Nosewheel servo: EMAX ES3104 - 19g

Elevator servo: JX 1109MG - 9g

Aileron servos: EMAX ES08MAII - 12g


A self-build model nearly always provides enjoyment during construction, but you never know whether you’ll enjoy the flying until you try. This one definitely ticked both boxes. Overall, the project has been hugely satisfying and worthwhile - it seems that the original designers’ opening remarks were correct! 

Friday, July 28, 2023

Build a field box for RC aeroplane flying - DIY project

Being organised on the flying field is important. It reduces the chance of accidents and makes your life easier. Recently, I acquired my first ever i.c. powered RC plane. Flying i.c. requires you to carry fuel bottles, fuel pump, battery, starter motor, glow starter, and other odds and ends, such as glow plug spanner, spray bottles and cloths for cleaning any residue off the plane. Searching online, field boxes seemed very expensive. I just wanted a basic one for i.c. that worked for me, so I built one out of bits and pieces that I had lying around. 

Materials were ply, dowel, a few bits of softwood and even some balsa spar - which I used to frame around the base of the battery, to make a tray. The battery is held down in the tray with the top ply plate and three screws - there's a pad of stiff foam on top of the battery. This arrangement is easy and keeps the battery in place.  

Unusual colours because I had leftover small sample tins of a water-based outdoor garden paint (Sadolin Superdec, Dark Dusty Rose internally, Gull Grey and Cloverleaf externally). The handle had 4 coats of a linseed/carnuba/beeswax and the other ply parts around the battery box and electrical panel were varnished.

Below is a close up of the panel. There are two safety features: 1) a physical barrier (wood) between the 4mm binding posts; and 2) a fuse (20A). I thought the triangle was a quirky design choice!
The battery is sealed lead acid 12V, 7Ah. I could also use it to charge LiPOs on the airfield. Below, side view, there is enough space to hold what I need.  

I didn't include space for transmitters as I have a separate alloy flight case for my radio gear. I particularly like the hand-operated fuel pump. It has a filter on one end of the fuel line that drops into the fuel bottle. The other end is attached to the filler nozzle on the plane. Cranking the handle pumps fuel into the model's tank. 

 All in all, this didn't cost me much, except time. I enjoyed designing and building it and am certain it will be useful. A great way to occupy days of unflyable weather!

Friday, September 2, 2022

GWS SlowStick Re Build Log, LiPO battery conversion


This is my recently renovated GWS Slow Stick, now sporting a Batman theme! Bought new in 2007, it flew only a few times, on 35MHz and puny NiMH, and crashed a little. It was still airworthy when I stored it in my garage. It had been fun, but over the years I'd moved on to other aircraft, even cannibalising the SlowStick’s ESC and servos. 

While clearing out recently, I 'found' it. Abandoned for 15 years, covered with dust and rust, it was filthy. The stab had separated from the bent and dented square section alloy fuselage. Steel wire undercarriage and pushrods were rusty, wheels jammed solid, one small tear in the wing, gummed up tape all over the leading and trailing edges and wing centre. For comparison with the refurbed plane above, here is how the standard stick model looks:

Searching the net for parts (there aren’t many here in the UK!) I saw a new old stock GWS ESC ICS-400Li 25A. This is for brushed motors and LiPO batteries - an unusual piece of electronics! So I decided to start with that, as after all, the geared conventional motor was hardly used. I believe that this is the GWS EPS-300C-D-BB motor, but I’m not certain of that. Perhaps a brushless motor could come later, but how good was the original motor?

A description follows of everything that I did to restore it (where I mention glue, it’s UHU Por unless stated otherwise):

  1. Using steel wool and 3 in 1 oil, remove rust from wire UC and tail wheel legs. Clean axles with Al-Ox paper, and ensure that the wheels are aligned and rotate smoothly.
  2. Remove rust from steel pushrods, ensure they move smoothly.

  3. Clean motor gears and very lightly grease.
  4. Remove the EP1180 prop, clean, inspect, balance and refit it. 
  5. Straighten alloy stick fuselage. Although it has a dent, I left it as is for now. If we have another urgent visit to Terra Firma, then I'll see about replacing it. 
  6. Remove the German Imperial cross stickers. Remove tape residue from wings, leading and trailing edge carbon rods and their alloy bent tube connectors. “Goo Gone” worked ok. Clean the whole wing, carbon rods and their connectors, stab and fin - microfibre cloth wet with soapy water, another microfibre with clean water, and finally kitchen paper towels to dry. Miraculous colour improvement! 
  7. Mend small tear at the wing centrefold with glue and Tesa white cloth tape. 

  8. Use red Sharpie to colour the bits of the wing where the top shiny red coating had come off. 
  9. Make a large card protractor for: 160 and 170 degrees (dihedral should be within those limits, i.e. 5-10 deg under each wing), and the R E control throws according to the GWS manual (35deg each way and 20-25deg each way respectively).

  10. Check angle of the alloy tubes and carbon rods with the protractor. They were at around 7deg and within the 5-10deg range. Left them as is. 
  11. Cut a 2mm blue foam canopy profile, to sit between the wing halves. Dry fit together, allowing three 50mm wide spaces at the LE, TE and through a slot in the depron foam. These are to allow tape to be run spanwise in these three places, and for wing rubber bands to cross.  
  12. Glue blue canopy profile and wing halves together with dihedral as per the spars, run tape through the 50mm spaces and slot. Canopy profile was a bit floppy, so I glued on some carbon capping strips that I had lying about to stiffen it a bit and provide detail. Glued on a foam Batman pilot to both sides of the canopy profile. 
  13. File a channel in the LE and TE foam, a cutout for the alloy joining tubes. Glue the carbon spars into the alloy joiners using 5min epoxy. After they set, glue and strategically tape them to the wing LE and TE. See this build info for details. 
  14. Tape chordwise both sides of canopy, through the slot in the canopy, and fore and aft of the canopy, overlapping LE and TE spars.
  15. Fin seems fairly secure, but not square. Apply a balsa triangle support piece with 5min epoxy, to keep it square and strong. 
  16. Glue a 30 x 1.5 mm balsa mounting plate (grain running spanwise) to stab. Then glue the mounting plate and stab to the alloy fuselage. Mounting plate improves both the joint and rigidity.  

  17. Red Sharpie marker over the balsa plate and touch up the LE of fin. Apply Batman stickers to wings and fin. I think the yellow and black logos look great on the red!
  18. Fit servos, ensure correct direction and mechanical centre. As I had some handy, used 9g Tower Pro SG92R digital, nylon with carbon fibre gears. These are 4.8V, not rated for 6V, but seem to be ok on a 5V ESC BEC. One was jittery so I replaced it with a good one. I noted that the horns had not been fitted with their pushrod holes directly above the hinge line. "Amateur me", back then! The horns were still fixed securely and the 3M Blenderm hinge tape was in great condition. For stability, add a couple of small fresh pieces of 3M Blenderm tape to the rudder hinge above and below the horn. 
  19. Cut out grey 6mm depron fuselage profile and glue to alloy fuselage stick. Glue small balsa blocks on each side of alloy fuselage, just behind the motor mount. Wrap soft 2mm foam sheet around the nose to form a cowl, using double sided foam tape to fix it to the balsa blocks. It was minimal and basic, just to give the impression of a Fokker Eindecker. Later I swapped this for a darker foam.

  20. Fit remainder of radio gear, including solder 2mm bullet plugs for motor to ESC, battery, Rx. Set control throws using the protractor. Set failsafe and throttle arm switch. In view of the horn placement, I had to program the throws to ensure the same amount of travel in each direction.

  21. Balance point was set to 100mm behind the leading edge. My thinking behind this is as follows. The instruction booklet says 105mm for the EPS-300C motor, which is what I think this is, and 95mm for the EPS-400, which was a more powerful unit. Previously, I had been flying this plane with NiMH batteries at 105mm without any issues. So as I’m now using LiPOs, I’ve moved it forward a bit, but not as much as 95mm. Hmm, let’s see how it goes - it’s just a starting point as they say. 
  22. With a 2S 1300 mAh LiPO battery, and balanced at 100mm from LE, it weighs 408g, which is the lower end of the range stated in the instruction manual (405g-440g).

I waited for a day with little or no wind for the re-maiden. It needed a bit of down trim. I’m not yet sure whether the CoG needs to be moved forwards, or the thrust line needs altering, but in any case, it was a lot of fun, and SLOW! It loops really tightly, even from horizontal flight.

Tuesday, August 9, 2022

Durafly Tundra v2, fixing broken undercarriage


The Durafly Tundra (here in v2) is an excellent trainer. Understandably, Hobbyking, the origin of these planes, generally has low stock as they keep selling out. However, the scarcity of replacement bits and spare parts is sad. 

I broke a small part of the plastic undercarriage bracket. Annoyingly, it did not happen during a flight or landing, but later at home. I was trying to straighten the wires a little, when "bang"! A message for me to learn: always remove the wire U/C parts from the plane BEFORE trying to straighten them! Here's a picture of the bit that broke and also my solution. A small piece of plate steel, shaped, drilled and bent a little. 

You may notice that it broke at the weakest spot, through the centre of the bolt hole. So I cut the remaining plastic square, to enable the new steel part to butt up to it. The steel is quite thin and I cut it out of an angle bracket from an old shelf. I used a small nylon washer for the bolt, to help press it down in view of the spring. Let's see how long this will last. Hopefully, long enough for Hobbyking to get more plastic brackets back in stock! Another photo below:


Another simple thing that I think is worth doing is to put M3 nylon washers on both sides of each wheel, such as here:

And also on the other side of the axle under the retaining nut, here:

These two washers on each wheel make a massive difference to how smoothly the wheels roll. They also seem to help with how they react on impact - seem to be a bit more damage resistant. 

Happy flying all!

Friday, September 4, 2020

Flying a small glider off a High Start: Lulu wing on Crofter

The Peterborough Model Flying Club specify a high start for small gliders as 7.5m of rubber (they recommend 1/8" strip) attached to 22.5m of line. This works beautifully. Here are some details of mine. First, this is how I store it, on a 1cm thick Depron foam board stiffened with a piece of wood (a bit of old fencing!):

This works very well. Simply remove the split ring from the knob of Depron, fix it to the ground with a decent stake, and unwind the foam board as you walk back downwind. To put it away, start by putting the glider end split ring onto a Depron knob, then wind it up as you walk forward to the stake, making sure that the rubber goes on slack. Use a hand over hand movement, otherwise the line will twist.  Knots wise, I use a Palomar knot to tie the rubber to the stake ring, a Figure 8 (Flemish) bend to join rubber to line, and an Anchor Hitch to join the line to the ring at the glider end. I also add half hitches and overhand knots on top of these to prevent slippage. If the kite line at the Figure 8 bend cuts into the rubber, then I'll do something else, but so far, it's held up well.  

Following on from posts about the CROFTER, I had a 24" wing that was made by my son for a school project. It's based on the LULU wing, scaled down.  Here it is attached to the CROFTER fuselage:

If you look carefully, you can see that all I've done is add a couple of 1/16" tabs in front and behind the wing platform, before banding the wing on. It means that the band pegs are a bit awkward to reach, but it's not a big deal. 

No idea why my son went for a German/Belgian flag look with the tissue! 

Balanced at 49% of chord (90mm, so at 44mm behind the leading edge), all up flying weight was 36.7g. That means it's about 10% lighter than the CROFTER with roughly 20% more wing area. Wing loading hugely improved. 

A quick shoulder glide, showed it needed a 1/32" shim of up on the stab, and it had a slight right turn which I encouraged with an Al foil tape trim tab on the fin. Then on to the high start. First flight was over 1min. I had a go at trying to take videos myself, but it was pretty lousy trying to do it myself and launch. First a 4s clip of the end of the high start launch. 

Now a longer video of part of one of today's flights. It starts off doing right hand turns, but then goes downwind in a straightish glide before turning a little and landing. 

It's abundantly clear that the LULU wing on the CROFTER fuselage is far better than the original CROFTER wing. A tribute to John Barker, who originally designed the LULU. RIP Hepcat.