The Defiant One


A Cox control-line P-51. Cox were making live fuel powered RTFs generations before the current boom in ARTFs.

Spring has sprung, the grass is riz. The blonde person has been having great fun with her brand new, shrieking-yellow strimmer. She’s become a dab hand at starting its 22cc petrol engine without endangering her manicure. Long ago she mastered the mysteries of carbs and chokes, and now looks down on me with that familiar disdain common to all ‘big petrol’ flyers down our flying field. But I don’t mind. At our spring Bring and Buy I snaffled up an unusual, brand new, sealed in the box, RTF flying model. This had a comfortingly tiny engine, too. In fact, I bought this particular  flying machine for my son, for old times’ sake. This RTF doesn’t look much like a ‘plane, since its supposed to be a spaceship. It really looks the part too. I’ll put you out of your misery – the model is a Cox almost ready to fly control-line Star Wars Trade Federation Droid Fighter, and it’s great fun. It has a suitably futuristic wing, and is powered by the famous Cox Babe B .049 cu. in. glow engine. That’s 0.8cc capacity for all you closet metricists.

I have a huge soft spot for Cox glow engines –  or ‘gas powered engines’ as the colourful Yank mags of my youth used to shout. I can remember when heaven was a flying field, a Cox control-liner and a can of Cox ‘Missile Mist’. In the ‘60s, Cox .049s were light years better than their classic British diesel counterparts, and they possessed an unbeatable marketing advantage: they started straight from the box! None of that forlorn flicking, setting, and desultory compression screw twiddling. No, if the glow and the fuel were present, then the Cox usually ran. And what a noise – politically incorrect beyond belief, a snarling open-exhaust 25k whine that can still set the hackles rising. Not an engine for use anywhere near habitation! However, in a better age for children, whole generations of kids had their first successful experience of power modelling with Cox 049s, and many still have the prop’ scars to prove it. I used to run mine on the billiock (building site / bombsite) just behind my house, deep in the inner city. No passing neighbour even bothered to comment, though they would give a cheery wave.
If you compare an early Cox glow engine with its diesel contemporaries, it looks startlingly modern. Cox engines are always beautifully finished, and very fine things to own. As a youth I bought a Cox engine after an uneven struggle with British diesels. It taught me all about fuels, props, fettling, starting – and safety – and began my lifelong love affair with glow engines. When the time came, I made damn sure that my own son learned all about glow engines and control-liners first-hand from flying his own Cox-powered PT-19 on the field behind our house. At the last count I had well over 20 of these little gems, of various marques, in various states of readiness, mostly culled from broken control-liners. Many were given to me by kind friends, who thought they would no longer run. They were wrong.


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One of my old but well-loved .049s which still works after numerous crashes over the last 35 years!

Cox have marketed their range of excellent ready-to-fly control-line models for decades. LM Cox Manufacturing celebrated 50 years in business in 1995, though I believe they are now owned by Estes, the rocket people. Most of Cox’ clever and innovative RTF control-liners are powered by variants of the trusty Cox .049, though a few, like their exquisite little Pitts Special, actually used an even smaller 0.020 powerplant. Cox invented ARTF in its earliest and purest form long before mass R/C could come along –  and their ARTF concept was always ‘engine included’. This lead to immense commercial success, spawned a whole new niche in the post-war leisure market, and prompted many imitators. Though nowadays many modellers may not realise it, the Cox .049 is effortlessly the runaway best-selling model engine of all time. It easily has the longest continuous span of any model engine (or for that matter any manufactured modelling product) with an unbroken production run of over 50 years. In fact, Cox production figures extend into the millions. Only balsa is older and more ubiquitous. This engine also has some of the finest precision tolerances of any mass produced model engine, with ground fits routinely being achieved down to a few microns, and this, decades before the days of CNC.

A Pee Wee .020 on the left and Cox Babe Bee .049 on the right. Note the metal accroutrements.


Do not let any engine ignoramus or sniffy know-all try to tell you that Cox engines are too small, too toy-like or too tricky. That’s just tosh. Keep ‘em clean, use fresh high-nitro fuel and a good battery, and they start almost instantly. I was once told by a so-called engine expert (and well known blockhead) that Cox engines were ‘irrelevant’. Irrelevant to whom? They are superbly designed technical objects, precision engineered, and despite their low cost and high revs, built to last. The Cox layout is a marvel of ingenuity, and its reed valve fuel induction is an elegant technical solution. Briefly: as the piston travels upwards in the bore, it creates an area of low pressure in the crankshaft. Meanwhile, atmospheric pressure outside is higher, so the reed valve is automatically pushed open by the air. This incoming rush of air is mixed with fuel by the carburettor. Once ignited by the glow head, the engine can continue to run. However, as if by magic, on the downwards piston power stroke, when internal pressure is higher than atmospheric pressure, the reed valve instantly closes. If this reed material is carefully chosen, it can open and close with minimal inertia, leading to smooth, torquey and responsive running. For many two-stroke applications, model and full-size, the simple reed valve has stood the test of time, and has outshone many less responsive, more expensive and finicky designs. There is another advantage to the reed valve, over rear disc valves and / or the typical crankshaft rotary valve on most glow engines: the reed valve does not have fixed timing. The other two forms of fuel induction have fixed opening and closing points, regardless of mixture, engine speed and load, so they are more of a compromise. Why? Well, in effect, they will need to be ‘timed’ for a particular projected engine speed. However, a well-designed reed valve system is much more responsive, and can open and close in direct relation to the engine’s needs. In effect, the reed valve can respond dynamically to its operating speed and the load applied. In this respect, it is a second cousin to fuel injection.

Cox moved on to employing some plastic parts for the later models destined for RTF control-liners.

Being a savage, I know from personal experience that a Cox can run for decades with only minimal care. I reckon most radio modellers of my generation just grew out of their Cox .049s, long before they ever broke them. It’s a pity there weren’t much larger Cox engines about, since, in the full-size world, there are successful reed valve engines in everything from outboards to motorcycles. Then there’s practicality; Cox engines with integral tanks / backplate mounts are just about the easiest mounting, self-sufficient little power plants you can bolt onto a model airframe. As an example, I used to fly a six-foot glider with only a Cox .049 to haul it aloft for thermal hunting. An engine run of about four minutes would make her a dot in the sky. Utterly practical too, since the after-market plastic Cox engine pod / fuel tank just slipped under the existing wing bands. For normal layout, 36”- 40” span, light, three / four function R/C sports power models, I’ve often used Cox .049s with extended tanks to boost their duration. I’ve even used these little motors on R/C airboats up at the strip, but that’s another story!


The Cox represents a very attractive power option for smaller full-function radio models and even some larger glider types. Many current electric ARTF kits could be reverse-engineered to take a proper Cox engine instead of a sad electric motor (tee hee!). Here are some of my reasons why you might consider messing about with a Cox:

● Very attractively designed engine.
● Cheap to acquire.
● Cheap to run.
● Runs best on 10% nitro (minimum) fuel.
● Ultra-low maintenance.
● Very practical ‘power package’ – a compact, self-contained, powerplant.
● Integral engine mount.
● Integral spring starter which really works well.
● Integral tank, that can be increased to extend duration.
● Engine can be orientated at 90 degree intervals in relation to its tank / needle valve assembly.
● Reed valve induction means that the engine can run equally well in either clockwise or anti-clockwise rotation, with no alteration necessary – just start it running in the direction you desire with the correct propeller. Ideal for torque cancellation on twins.
● Starts on a 1.2 volt starter cell, or even a 1.5V Duracell.
● Starts on a normal 2V glow battery with a long (six foot-ish) lead to limit current.
● The glow-head is very durable on a 1.2 volt supply, though commercial glow plug conversion heads are available.
● If you are lathe-savvy, you can easily convert the glow-head to a normal glow plug.
● Not fussy about props, though a well-boiled Keil Kraft bendy 6 x 4 Nylon could last forever if you don’t ding it too often.
● Ideal for small models, and especially nifty sport-scale models, with small nose areas.
● Simple to affix to models not necessarily intended for glow engines, such as gliders, hitherto electric models, etc.
● Most bits – including, amazingly, Cox pistons and liners – always seem to interchange quite readily, so keeping a Babe B running long-term is not a problem.
● R/C versions and Quiet-Zone muffled versions are available.
● Check the Estes internet website for current Cox models.

The neat little spanner supplied with each engine is used to loosen the integral glow head.


Running spares are most easily acquired in the form of old engines, though as a long-shot, model shops of long standing may have some Cox spares tucked away in a drawer. In my experience, wearing out of the Cox piston / liner assembly is rare, with most problems coming down to blocked fuel feeds and related issues to do with carburation. So, if your Cox engine starts well, but annoyingly just runs out its prime and then stops, you know it’s most likely to be a fuel feed problem.
In most cases these are easily fixed by thoroughly rinsing out the tank and cylinder with meths to break down any gelled fuel residue. In my experience, if a Babe B is a non-starter, old fuel congealed in the tiny supply tube inside the integral tank is the most common culprit, though also check that the head washer hasn’t been lost, thus bleeding away vital compression. If the tiny spring is missing from under the needle valve you’re in trouble, since the engine won’t be able to hold its tune when it is running. For such reasons, swapping a needle valve often effects a miracle cure on a dodgy runner.
The next most common problem seems to be a sticky, or blocked open, reed-valve assembly. A careful strip down, clean and reassembly usually works, though the reeds and holder do have to follow the correct assembly shown on the Cox instruction sheet. If these simple fixes do not work, you may still have problems with inducting the fuel, and you may need to fit a Factory spare reed valve assembly, which is a doddle. Alternatively, you could cannibalise another Cox engine for the bits. On one notable occasion I did this, and got the engine running. When I then reassembled the cannibalised engine (with rejected parts from the first engine) I was astounded, and delighted, to end up with two good runners! However, statistically, most of the non-working, so-called ‘scrap’ Cox engines given to me by mates only needed a soaking in meths overnight to sort them out.

I think many modellers look upon Cox engines as ‘expendable’ and therefore don’t bother to take a few minutes to clean them out and run them again. They’ll never be mainstream R/C engines since they’re too small, make a good bit of noise and don’t normally have a throttle, but that’s not the point. It’s too easy to take them for granted. Remember, to the dedicated .049 fancier they’re an engineering masterpiece, and a continuing delight to own and operate.

  • This installment of Alex's monthly Weekenders column was first published in 2005.


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