Flight test: DHC-1 Chipmunk Mk 23
- Credit: Photos: Keith Wilson
Adapted as an agplane, now enjoying a second life as a glider tug, the only airworthy Mk 23 is a fine flying machine | Words: Dave Unwin - Photos: Keith Wilson
Truth be told, I’m a little anxious. It’s the bleak mid-winter, the Chipmunk hasn’t flown for a month and the battery is far from the first flush of youth. I prime carefully, cross my fingers and turn the key to ‘start’.
The prop lurches arthritically, pauses and then morphs into a shimmering blur as the engine roars into life Hoorah! Generator Flow on, Generator Supply on, Instrument Supply on, fuel pump off then lean the mixture slightly, and while photographer Keith and his pilot Al wedge themselves into the long-suffering EuroFox, I taxi out.
It’s a stunning December day with not a cloud in the sky and the sun a great orange ball barely twenty degrees above the horizon. Bumble Bee is trembling with suppressed energy, it wants to fly?and so do I!
Generations of RAF pilots and Air Cadets had their first flights in a de Havilland Chipmunk. It’s possibly?if not probably?one of the most recognisable British light aircraft, even though it was designed in Canada by a Pole, Wsiewolod Jakimiuk.
Intended as a replacement for the DH Tiger Moth, it was the first indigenous aircraft designed by de Havilland Canada, and the prototype took to the air from DHC’s Downsview, Toronto factory on 22 May 1946.
After a thorough appraisal by the RAF, production was initiated in both Canada and England and ultimately around 1,300 were built?approximately 1,000 in England, 200 in Canada, and 60 in Portugal, under licence.
As there are at least 500 Chipmunks still airworthy, the aircraft is far from uncommon. But this particular one, Golf Alpha Oscar Tango Fox, very much is. It is a rare Mk 23, in fact the only Mk 23 still flying.
Bathed in soft light from the low winter sun it practically glows in its striking yellow and black livery. As I start the preflight inspection it is obvious that the fuselage and wings are essentially the same as an RAF T10, being an all-metal semi-monocoque fuselage, while the single spar cantilever wings feature a stressed-skin D-section with a fabric covering aft of the spar.
The ailerons, elevators, rudder and slotted flaps are all metal structures, covered in fabric. There are trim tabs in the rudder, ailerons and starboard elevator, but only the elevator trim tab can be adjusted in flight.
As it was designed as a primary trainer, the fixed main undercarriage has a usefully wide wheel track of 2.66m. It features single cantilever struts, rubber block shock absorbers and disc brakes, while the large pneumatic tailwheel is fully castoring.
Access to the air-cooled flat-four is excellent, as the cowling hinges wide open on both sides?which brings us neatly to a significant difference between it and a T10 (interestingly this is the third aircraft I’ve tested in the last year where the Gipsy was junked and an O-360 substituted).
As with the Beagle Terrier and SAAB Safir, to preserve the C of G the lighter Lycoming was moved forward on a redesigned engine mount. This also improves access to the magnetos and accessories.
By the 1950s the advantages of treating crops via aerial application were well understood, the most popular aircraft for ‘dusting’ outside of the US and USSR being the ubiquitous Tiger Moth. Nevertheless, for several reasons the Tiger wasn’t great (in fact, the primary reasons for its popularity were the low purchase price and excellent spares availability).
However, the potential market for a more advanced aerial applicator had caught the collective eye of DH’s management, and a decision was made to produce a ‘duster’ based on the Chipmunk. Having no experience in the field (see what I did there?) the company liaised with Fison-Airwork Ltd for assistance with the preliminary design and the fitting of the hopper and spray gear. A Chipmunk stored at RAF Aston Down was purchased and taken to Leavesden. Converted and registered as G-APMN, it became the prototype Mk 23.
The conversion was relatively straightforward. The front cockpit was removed and replaced with a large hopper, the rear seat raised slightly and a new canopy fitted, while spray gear was installed beneath the wings. ‘Mike November’ began flying with Fison-Airwork in June 1958 and was kept busy with evaluation and demonstration flights until crashing the following month.
As DH planned to display a Mk 23 at that year’s Farnborough show in September it had to move fast, so another stored Chipmunk was bought, registered as G-APOS and converted to Mk 23 configuration using the hopper from Mike November. Somewhat surprisingly it failed to find favour with agricultural aviators, and G-APOS was in storage by June the following year.
The story could have ended there, except for Bill Bowker. He’d started his aviation career as an apprentice at DH and was very aware of the Tiger Moth’s limitations. He was also a Director and pilot at Farm Aviation Ltd, and in 1962 he purchased G-APOS, along with the rights to produce more Mk 23s. Leading edge slats were fitted, the original hopper redesigned and the structure reinforced, enabling the Mk 23 to carry 8cwt of granular fertiliser, an improvement of around 50% over the Tiger’s carrying capacity.
Farm Aviation operated a small fleet of Mk.23s for several years, but towards the end of the 1960s Piper’s Pawnee (which had been designed from its inception for agricultural work) was the preference of most aerial applicators and the Mk 23s were quietly retired, Farm Aviation having been the sole operator.
Along with the four agricultural Mk 23s there were several other curious Chipmunks constructed over the years. The most numerous are probably the so-called ‘Supermunks’ built by the British Gliding Association and RAF Gliding and Soaring Association as glider tugs – engineer Dick Stratton being the project’s principal. The conversion primarily entailed replacing the 145hp Gipsy Major 8 with a 180hp Lycoming O-360, and around eight aircraft were adapted.
Another Chipmunk-based project was the civilian Mk 22A, an attempt to overcome the small fuel tanks of the standard military Chipmunk T10, which constrained range and endurance. In the Mk 22A the tankage was increased by a third, from nine to twelve imp gal (55 litres) a side. The chairman of the Bristol Aircraft Company, Sir Peter Masefield, took this idea a step further with several enhancements to his personal Chipmunk.
These included wing-mounted baggage bays, faired undercarriage legs and wheel spats, and a single-piece blown canopy, similar to those fitted to Canadian Chipmunks. Fuel capacity was substantially increased, from 109 litres to 291, and Bristol eventually converted four or so.
A similar scheme was tried in Australia by Aerostructures of Bankstown Airport, Sydney. Its ‘Sundowner’ featured a 180hp Lycoming O-360, tip-tanks, a blown canopy and metal skins instead of fabric. A company called Sasin also converted three into single-seat agplanes similar to the Mk 23 and called them ‘Spraymasters’.
Another Chipmunk curiosity was the machine created in the UK by Bill Bonner. This was a Mk 22A fitted with a three-litre liquid-cooled V-6 of Bonner’s design, loosely based on the Ford engine fitted to the famous Capri Mk.III 3.0S (a car much favoured by Bodie and Doyle in the 1980s TV show The Professionals), but with the block cast in aluminium.
Called the Super Sapphire, it produced 200hp and utilised a big belly-mounted radiator, which made the Chipmunk look a bit like a Hawker Hurricane. It flew at the 1980 Farnborough Air Show and the RAF expressed an interest in re-equipping its Chipmunk fleet with Sapphires, but the project fell victim to the defence cuts of the early 1980s.
Probably the most famous DHC-1s are the Super Chipmunks conceived by famous aerobatic pilot Art Scholl. He ultimately fitted a retractable undercarriage, clipped the wings, changed the cockpit to a single-seat configuration and doubled the power by fitting a 280hp GO-480 turning a constant-speed prop. Scholl, who was killed while filming Top Gun in 1985, is also remembered for flying a modified, ‘wire braced’ open-cockpit Chipmunk in the infamous outside loop stunt of The Great Waldo Pepper.
Several of Scholl’s modified Chipmunks were built, and they were as glorious as our last ‘Chipmunk Curiosity’ was gormless, for the most poorly considered version was undoubtedly the Rover Turbine Chipmunk.
This aircraft, a Mk 21 that had the original 145hp piston engine replaced with a 90hp turboprop (basically the same single-shaft unit fitted as an APU to the Armstrong-Whitworth Argosy and Avro Vulcan), appeared as at the 1966 Farnborough Air Show. Because of its prodigious thirst an additional 114 lit flexible fuel tank was installed forward of the front cockpit, but even though the engine was considerably lighter than a Gipsy, the advantage was offset by the requirements for extra fuel and a constant-speed propeller.
Of course, it also had 55 fewer horses, and to quote a fictional Scottish starship Chief Engineer “ye cannae change the Laws of Physics!” Contemporary accounts record it was woefully underpowered. Unsurprisingly, cold commercial reality soon prevailed, and the idea rapidly faded into obscurity.
The fuel is carried in wing tanks, and as the fuel gauges are mounted in the wingroots just next to the filler caps, it’s easy to verify while filling that the indication is correct.
Although on most Chipmunks the gauges have two scales (white numerals for reading in flight, and red numbers calibrated for the tail-down position) in this aircraft they are calibrated for flight only. The gauges show capacity in imperial units, and with only nine gallons (41 litres) each side, the range and endurance are far from impressive.
Having climbed up onto the wingroot and slid the slab-sided canopy right back I stand the seat cushion up (to avoid getting mud on it), then step onto the seat and down into the cockpit. With both elbows resting on the canopy rails I look around with considerable interest.
The cockpit is as spartan as that of a WWII fighter, and this impression is enhanced by the long nose – in fact the single seat is located almost halfway along the fuselage, and there’s about three-and-a-half metres of Chipmunk between my eyes and the spinner.
A distinctly non-standard panel
The instruments consist of mostly black-faced dials in a black-painted panel, the dials mounted in two rows and in a far-from-standard arrangement. In the top row, and from left to right, there are a compass, an overly ambitious ASI, tachometer, CHT and voltmeter.
Underneath I find the VSI, altimeter, Turn & Slip, a curious instrument combining oil temperature, oil pressure and fuel pressure, and finally a carb temperature gauge. A few things piqued my interest here.
The oil temperature indicator starts at an Arctic -50°C, while the ASI goes up to 300 knots! I find this unsatisfactory. You’d never need more than the first 150° of the dial (and then very rarely) as it can indicate more than double the very conservative Vne of 120, yet at the same time the lowest indication is 40 (greater than the stall speed).
The controls also have their idiosyncrasies: for example, the throttle is a surprisingly small lever but the mixture is operated by a Vernier (although, to be fair, this is part of the Lycoming conversion). Both are nicely situated on the port cockpit sidewall, with the brake lever underneath and the wheel for the elevator trimmer below and aft.
The carb heat?another addition?is not so well situated, as it’s on the starboard side, below the panel near the fuel shut-off. The flap lever (which is strongly reminiscent of a vintage motorcar’s handbrake) is also on the starboard side and has three-positions - ‘Up’, ‘Take Off’ (15°) and ‘Land’ (30°).
Once strapped in with the robust four-point harness I find it a bit of a stretch to reach both brake and flap levers, then remember that the seat was raised when it was converted to Mk.23 configuration. It is clear that at some point in Tango Fox’s career somebody got carried away with the red paint, as the mixture, cable release, flap and brake levers are all red, and only the mixture should be.
As for electrics, there is a row of very rugged looking toggle switches on a sub panel below the instrument panel, as well as some circuit breakers. Oddly, there’s a ‘Master’ switch and a ‘Battery Master’, along with switches I’ve never seen before for ‘Generator Flow’, ‘Generator Supply’ and ‘Instrument Supply’.
It all actually seems somewhat over-complicated. The reasons for these switches and whether they were installed when the aircraft was converted to a Mk.23 or later, when the Lycoming was fitted are lost in the mists of time.
With the engine growling contentedly I slide the canopy to the mid setting and roll along the taxiway, which, with a long hangar on one side and a row of trees on the other is not as wide as when it was a part of USAAF Station 538 (yes, we are at Saltby). Tail down, the view over the nose is poor, which makes taxying in a series of S-turns prudent, and I’m quickly reminded of the Chipmunk’s worst facet; its bizarre brakes.
These are completely standard and, in my opinion, worthy of ridicule. On some aircraft the brakes are operated by cables and pulleys, while others use master cylinders and hydraulic hoses. Similarly, on some types you use your feet, and on others your hands.
Well, the Chipmunk covers every base by managing to utilise cables and hydraulics, operated by a combination of hands and feet. I don’t rate this system, especially as whenever I’m taxying an aeroplane (and particularly around a crowded launch point) I like to have the stick in one hand and the throttle in the other. Oh, and did I mention the tailwheel only castors? At least the huge rudder provides plenty of directional control…
Out at the runway I run through my personal pre-takeoff checks, which includes fuel on, pump on and sufficient quantity remaining. The first two are easy: the fuel shut off is near the carb heat plunger and held closed by a spring clip, while the pump switch is by the large, old-school T&S.
However, the fuel gauges simply cannot be seen from the single seat (positioned where the rear cockpit used to be) and knowing full-well just how tiring a busy day can be for a tug pilot (my personal record is 57 launches) I’m a little surprised that no-one has seen fit to install a ‘low fuel’ light.
The checks are otherwise straightforward, and the trim wheel is marked so that when the trim is ‘nose up’ the section visible is black, and white when its ‘nose down’. There’s a useful raised screwhead on the wheel which, when it’s at twelve o’clock, gives a tactile indication of neutral trim.
Having set the first stage of flap (there’s no position indicator but you can see them from the cockpit) I roll forward a few metres to ensure the tailwheel is straight, then ease open the throttle. A quick glance inside to check that the gauges are all good then it’s eyes out and concentrate on keeping straight.
Being on tarmac with a castoring tailwheel means there’s always the chance of a slight swing to port (the opposite of a Gipsy-powered Chipmunk) but the combination of a hard runway, very cold air and a fair breeze on the nose sees the Chipmunk airborne practically before the throttle hits the stop. Flaps up and we’re climbing away with the VSI showing well over 1,000fpm at 70kt.
While waiting for ‘Salt One’ to launch I crack on with evaluating the control and stability. The air is as smooth as glass, perfect for an appraisal, and I rapidly appreciate that although the handling is as good as ever, the directional stability is significantly ‘softer’ than a stock Chipmunk.
Some people claim that the fin has 2.5º of offset, and of course as the Lycoming turns the other way this would exacerbate matters. Others say the offset is a myth. I suspect its more likely that the culprits are the canopy and the aft C of G, so I spend several minutes experimenting and I notice something I’ve never seen before?the large slip ball is constantly revolving!
The longitudinal stability is also barely positive (again, that aft C of G) while laterally the spiral stability is positive to port but divergent to starboard. However, I’m much more interested in the control than the stability, and can confirm that the handling is as good as ever.
Not a real bee – life as Tango Fox
Built in 1950, Tango Fox was the fifteenth Chipmunk made at DH’s Hatfield Factory and joined the RAF as WB563, but only served for a few years. By 1953 the Air Council was becoming increasingly aware of the irrelevance of flying piston-powered taildraggers from grass fields, when jets and tarmac runways were increasingly the norm, and began to close the Reserve Flying Schools.
A considerable number of Chipmunks, including WB563, were placed in storage in 1953, and when the last RFS closed the following year they were declared ‘NES’ – Non Effective Stock – and many including WB563 were eventually sold as surplus. Bought by Farm Aviation Ltd in 1963, this aircraft was registered G-AOTF and converted to Mk 23 configuration.
By 1967 ‘Tango Fox’ had left the agricultural industry and soon found gainful employment as a glider tug with Air Tows at Lasham. It then joined the RAF Gliding and Soaring Association (again as a tug) and eventually had its original Gipsy engine replaced with a Lycoming O-360.
At some point it was painted in its striking black-and-yellow colour scheme and acquired the sobriquet Bumble Bee. (Incidentally, I can’t claim to be an expert in entomology, but I can tell that it’s not actually a bumble bee painted on the cowling. It’s a feisty-looking little bastard, the smooth sting being the giveaway - bee stings are barbed. Even more bafflingly, this wasp, or hornet, appears to be wearing a ‘FitBit’ on its left wrist).
After a long and peripatetic career, this aircraft finally ended up with Richard Ellingworth, who plans to use Bumble Bee to tow banners, as well as gliders. Bumble Bee moved to Saltby from Spanhoe towards the end of last year.
I like my flying machines to possess crisp, coordinated controls with plenty of authority, and that’s the Chipmunk. Powerful but light ailerons, an effective elevator and influential rudder that are all beautifully harmonised make Bumble Bee the finest-handling tug I’ve ever flown. It’s also got an excellent power-to-weight ratio, and really climbs, so it’s a real shame that (unlike a stock Chipmunk) it’s not aerobatic.
A few loops, rolls and chandelles would simply be too much fun on a morning such as this (I think that the ‘no aerobatics’ limitation is possibly an LAA thing, as is the Vne, which is down from 173kt to 120kt).
At the other side of the speed scale, slow flight is amazing; it just doesn’t want to stop flying, and that’s with the standard wing. (It must’ve been incredible as first converted by Farm Aviation Ltd, when it had leading edge slats.) If the speed is reduced slowly it never really stalls, just buffets as the sink rate increases.
Get the nose well above the horizon and it will stall and fall through, but in a very half-hearted manner. Drop some flap, add power and you can practically hang it off the prop as the ASI gives up with its needle flickering pointlessly below forty.
‘Salt One’ now appears, and I slot quickly into echelon port as the EuroFox’s door opens. It must be viciously cold for Keith, and he won’t appreciate being made to wait! Once all the pictures are in the can I head back to Saltby so we can shoot some aerotows, and on the way back briefly examine the cruise.
Ninety knots and 2,400rpm seems comfortable, but with a (guesstimated) fuel flow of 40 lit/hr it’s no tourer. It’s a tug, and I think rather a good one. I’ve only logged fifteen tows in it so far, due to the diabolical weather, but I really like it.
The field of view is good, and while the Chipmunk is not as strong as a Pawnee, you must remember that the handling makes the Pawnee feel like the tractor it is. I’ve said it before and I’ll say it again, the Chipmunk really is one of the nicest handling light aircraft ever made.
I’d swear the controls are all operated by pushrods not cables, as they’re so light and smooth with practically no ‘stiction’ and low breakout forces. Because it was designed as a primary trainer the stick ratios are quite long (to prevent students over-controlling) but even large control inputs are easy since the stick forces are so low.
Circuits are as straightforward as everything else (although I’ve not flown it in a crosswind of any strength yet). The view over the long nose on final is fine and the speed stability good. Doing 55kt on short final and bleeding back to fifty ‘over the fence’ gives a brief float and a smooth three-pointer.
I love flying Bumble Bee. It’s great fun to tow with and I’m very glad Richard has chosen to base it at Saltby. I just wish the summer would hurry up and get here!