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Flight Test DHC-1 Chipmunk

PUBLISHED: 11:56 24 August 2016 | UPDATED: 11:56 24 August 2016

CHIPPY

CHIPPY

Darren Harbar Photography SINGLE USE PILOT MAG ONLY

A pre-production prototype, the world’s oldest airworthy Chipmunk still keeps turning heads

historic Chipmunk pix historic Chipmunk pix

As we approach Old Warden it occurs to me that someone just flying past might be a bit confused. Not only is the airfield crowded with aircraft, but they appear to be all the same type and they are! There are dozens and dozens of DHC-1 Chipmunks parked in ranks upon Old Warden’s rich green turf. It is of course the seventieth anniversary of the Chipmunk’s first flight, and the Shuttleworth Collection is hosting a big fly-in to celebrate. I’m flying a Chipmunk too, but not just any old Chippie−this is s/n 11, the oldest airworthy DHC-1 in the world and the undoubted star of the show. Even so, just as I’m about to make an initial radio call I realise I’ve forgotten the registration and it’s not to be found on the panel. “Er…” I begin, somewhat apologetically “what’s our call-sign, Dave?” “Look out the side,” laughs owner Dave Gillespie, “it’s painted on the wing.” Do I feel foolish!

One of the most-loved post-war light aircraft, the de Havilland Chipmunk is a pure thoroughbred. Designed in 1946 by Polish aero engineer Wsiewolod Jakimiuk as a successor to the immortal DH Tiger Moth, it was the first indigenous aircraft designed by de Havilland Canada, which had been given the job by the parent company as its English factories were extremely busy building Doves, Rapides, Vampires, Hornets and Mosquitoes, as well as both jet and piston engines. (Whatever happened to our aero industry?)

Test pilot Pat Fillingham flew the prototype, CF-DIO-X, on its maiden flight from DHC’s Downsview, Toronto factory on 22 May 1946 and, after evaluation by the RAF, production was initiated in both Canada and England. Eventually nearly 1,300 were built−some 1,000 in England, around 200 in Canada and a further sixty being built under licence in Portugal. With at least 500 still airworthy, the Chipmunk is certainly not a rare machine, but the subject of this air test, G-AKDN, is a bit special. It was the 11th pre-production prototype Chipmunk made at DHC’s Downsview factory−and is the oldest still airworthy. Furthermore and unlike most seventy-year-old aeroplanes−it is almost entirely original.

From a distance it may look essentially the same as all the other Chipmunks at Old Warden that day, but this test aircraft is most definitely not the Chipmunk that you, your father or even your grandfather flew. As anyone who has worked in military procurement (or indeed any government purchasing agency) can cheerfully confirm, the first things that an entity like the MoD does (and in direct contradiction to Kelly Johnson’s famous mantra to “add lightness and simplify”) is to add mass and complexity. The additional costs that this approach invariably creates are rarely mentioned, as is the degradation in performance!

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Anyway, Delta November was built before the Aeroplane and Armament Experimental Establishment (A&AEE) had finished its evaluation at RAF Boscombe Down and consequently has a much lower empty weight than any of the Chipmunks built at Hatfield or Hawarden. In fact, an impressive testimony to Jakimiuk’s genius is that the Chipmunk was designed to use the same engine (a Gipsy Major) as the Tiger Moth, and it outperforms it in every way, while still having very benign stall and landing characteristics.

Access to the air-cooled inverted four-cylinder engine is good, as the cowling hinges open on both sides. It is fed from a pair of wing tanks, and it’s good practice to check the oil as regularly as the fuel, because using up to three litres an hour is considered acceptable. Unlike the Menasco fitted to the Ryan ST-A featured in June’s Pilot, the Gipsy is a ‘left-hand tractor’ (the opposite of the ST-A’s) so the air inlet on the front of the cowling is on the port side of the nose bowl. This ensures that the cooling air flows from left to right across the cylinders. In contrast to the silly cartridge starter fitted to the RAF’s

T Mk10s, ‘KDN’ (as the Gillespies refer to her) has an electric starter because, although it originally flew in Canada with a Gipsy Major 1C engine from a Tiger Moth, almost as soon as it got to England it was upgraded with a Gipsy Major 10 Mk2 incorporating this device.

As it clearly has considerably less wing area than the Tiger it obviously has to be light−and it is. The fuselage is an all-metal semi-monococque structure built with the lightest possible metal skin with longitudinal stiffness supplied by myriad stringers. The cantilever wings use a single spar, with a stressed-skin D-section leading edge and a fabric covering aft of the spar. An auxiliary spar carries the slotted flaps and ailerons; both are metal frames covered in fabric. The fin, tailplane, rudder and elevators are constructed using the same methods and materials. There are trim tabs in the rudder, ailerons and starboard elevator, but only pitch trim can be adjusted in flight. Being a primary trainer, the Chipmunk boasts a usefully wide wheel track of 2.66m. The fixed main undercarriage features single cantilever struts, rubber shock absorbers and disc brakes. Interestingly, the Chipmunk was the first RAF aircraft to be fitted with discs, although the way they are actuated is−shall we say−a little Heath Robinson. More on this later.

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Spot the difference

Even up close the differences between KDN and the 48 other Chipmunks basking in the warm spring sunlight aren’t obvious. However, proud owner Dave Gillespie who, along with his pilot wife Karen, has brought KDN over from Canada for the seventieth anniversary celebrations, knows his aircraft inside out and back to front, and is keen to point out the myriad dissimilarities. “The engine and propeller are the same as most Chipmunks,” he begins, “a 145hp Gipsy Major turning a Fairy-Reed metal prop, however the exhaust is quite different as it was left over from the original Tiger Moth engine.

Far more importantly,” he continues, “KDN has a greater fuel capacity−27.7 imperial gallons useable, whereas the Mk10 merely has 18 and even the later Mk22s only carry 24. The undercarriage legs are the original, straight-down design, whereas the RAF added so much extra weight that the legs on British-built Chipmunks are canted forward slightly, to prevent the aircraft nosing over during heavy braking.

“It has two-piece wingtips, and the fairings on the tailcone and rear fuselage are handmade, as DHC was still working out how it was going to do things when it was built. The tailcone was replaced with a parachute for spin trials, and although KDN was retrofitted with anti-spin strakes while it was based in the UK during the 1960s, George Neal−DHC’s famous test pilot, who died earlier this year aged 96−told me that it doesn’t need them−and he spun it a lot! You may also know that the Chipmunks built in Canada for the RCAF do not have anti-spin strakes.”

I ask about the tiny pieces of wood built into the spin strakes and Dave explains that it is a tradition that “every DHC aircraft has some wood somewhere in the structure−even the Q400!

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“Take a good look at the canopy−although it may look like the British-built one, it’s actually a different shape. It’s the original built-up canopy, with the handles on the top and small top windows. The windshield has an exaggerated curve on the bottom and the windshield bow is a constant curve. Canadian-built Chipmunks use the same windshield design, but the RCAF had DHC design a new bubble canopy. KDN’s does have the bulged rearmost panels which were added after the evaluation trials, and these do improve the field of view for the instructor.

“Probably the most obvious difference−and it’s still fairly subtle one−is the rudder. KDN’s has a narrower chord. One difference you can’t see is that the battery box is right behind the rear seat. On RAF Chipmunks it’s much further aft, for easier access. This illustrates the light weight of KDN. It would never fly with the weight of the batteries further back.”

Much better informed (but probably none the wiser) I slip into the front cockpit. It’s been about ten years since I last flew a Chipmunk and it seems more Spartan than I remember. Dave explains there are no glareshields over the instrument panel as on RAF machines, and also fewer items of equipment.

The cockpit is typical of the period:

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by modern standards it is an ergonomic slum, riddled with inconsistencies and idiosyncrasies. The panel consists of black-faced dials in a black-painted panel with no colour-coding or even min/max indices for speed, rpm, temperature or pressure. The flight instruments are arranged in the traditional WWII RAF pattern, with (from left to right) the ASI, artificial horizon and VSI on the top row, and the altimeter, directional gyro and turn-and-slip beneath. The panel is dominated by the big Reid and Sigrist

T & S. The ASI is immediately to its left and, like the turn-and-slip indicator, is generic to many types of the era.

The sum of its parts

I begin to suspect that KDN was built using whatever was lying around the factory, the ASI is probably coming from a Mosquito as it goes up to 320mph (with the needle rotating through about 540° in the process). It’s rather unsatisfactory, as not only do you never need more than the first 270° (and then very rarely) and it can indicate more than double the Vne, but the lowest indication is sixty — still well above the stall speed. Consequently, not only is more than a third of the ASI’s range the ‘wrong’ side of the non-existent red line, but also the scale expansion at slow speed is poor. Indeed, the difference between ‘70’ and ‘75’ (typical approach speeds) is less than the width of the needle!

The fuel gauges are mounted in the wingroots, just next to the filler caps.

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On most taildraggers the gauges have two scales: an outer with large numerals for reading in flight, and an inner with the numbers in a smaller font and calibrated for the tail-down position, but KDN’s are calibrated for flight only, with quantity remaining shown as a fraction.

The control column and throttle look a bit like the Tiger Moth’s that I used to fly, and Dave confirms they’re both from a DH82. As mentioned earlier, KDN is an almost completely original prototype, and as Downview was still building Tigers when KDN was being made, this is another clue that they probably just used whatever they had handy. The controls also have their idiosyncrasies. For example, the throttle/mixture quadrant is nicely situated on the port cockpit sidewall, with the slightly too-low brake lever underneath and the wheel for the elevator trimmer immediately aft. The flap lever (strongly reminiscent of a vintage motorcar’s handbrake) is on the starboard side and has three positions: Up, TO (15°) and Land (30°). At 71kt, Vfe is usefully high.

The mixture works in the opposite sense to modern convention, with back for rich and forward to lean. The rationale here is that the throttle and mixture levers are interconnected in such a way that closing the throttle automatically enriches the mixture. I can see the logic in this system, but unfortunately it’s contrary to most other mixture arrangements−and another disadvantage is that if your sleeve snags the mixture knob you can accidentally lean the engine as you open the throttle!

Two other idiosyncrasies which, while not dangerous are certainly irritating when you’re flying solo, are that before you strap in you must check that the magneto switches in the rear cockpit are on and that the canopy has been slid to the half-closed position, as otherwise you can’t reach it. I bet every Chippie pilot will at some time have strapped in for a quick solo then remembered they’ve not turned on the rear mag switches or failed to half-close the canopy.

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Also on the starboard side (set uncomfortably far back and thus quite difficult to operate) is the carb heat selector. The Gipsy Major is quite an efficient ice-maker (perhaps DH should’ve called it the Gipsy Refrigerator) and Dave recommends that I just leave it in hot, which is where it is wire-locked on most ex-RAF Chipmunks. I would find it quite awkward to operate, even if I wanted to.

There’s no compass in the front−the classic P11 has given way to a small avionics stack (the sole concession to modernity), so I set the directional gyro from the runway heading. Also by the stick is the fuel cock, which is simply on or off−as it should be in a trainer.

Complicated braking

Now, although the Chipmunk has many sound design features, the brakes aren’t one of them. In fact the braking system is a pretty funky apparatus that consists of levers, cables and pulleys, as well as master cylinders and hydraulic hoses! It is the final clue that−in the spirit of post-war parsimony−DHC really did raid the existing parts bin, as Dave confirms that it is exactly the same system as a (Canadian) DH82C’s. And he should know; he owns one! How it works is that, for differential braking, you pre-set the ratchet-type brake lever and then apply rudder−the brakes are applied at the end of the rudder travel, which is controlled by a simple bar with a bolt through the middle. To apply both brakes evenly you simply pull the lever. I’m not a fan, particularly as the pneumatic tailwheel only castors, and whenever I’m taxying an aeroplane I prefer to hold the stick in one hand and the throttle in the other.

Having flown aeroplanes fitted with practically every type of retardation device, including heel, toe and hand brakes, and including skids, drag chutes and thrust reversers, I’ve never understood why the RAF stuck with hand-operated systems for so long (even the EE Lightning has hand-operated brakes), despite the fact that the toe-brake-equipped Harvard was the RAF’s advanced trainer for many years.

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Pre-setting the brakes when taxying, taking off and landing is a topic than can divide the room. Having spoken to many Chipmunk drivers, the factors that influence the decision making seem to be whether the pilot is ex-RAF or civilian, whether the brakes are the original (and not very effective) Dunlops or retrofitted Clevelands, the strength of the crosswind and whether the runway is tarmac or grass.

One possible ‘gotcha’ is that prior to performing either aerobatics or spins the brakes must be checked off because they restrict rudder pedal movement. Dave recommends not pre-setting.

The motor starts eagerly and a touch more throttle gets the Gipsy growling and we set off towards the runway. The view over the nose while taxying is fair and the huge rudder certainly provides plenty of directional control. The pre-takeoff checks are perfectly straightforward, although not having a trim position indicator seems odd. (On production aircraft the wheel is marked so that when the trim is ‘nose up’ the wheel is black and white when it’s ‘nose down’.)

Having set the first stage of flap (these also don’t have a position indicator, but you can see them from the cockpit) I note the strength and direction of the surface wind, and align the aircraft with the centreline. We roll forward a few metres to ensure the tailwheel is straight, then I ease open the throttle. A quick glance inside to check that the rpm are greater than 2,000 and we surge up the strip. There’s always the chance of a slight swing to starboard as the tail comes up, but this is cancelled out by the crosswind from port and there’s no difficulty keeping straight.

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Acceleration is adequate but not outstanding, although to be fair it is quite a warm day and we’re close to MAUW. We use about a third of the runway then the Chipmunk slides into the sky at around 60mph. I say ‘around’ as the ASI scale doesn’t go lower than that! At 500ft I raise the flaps, the nose pitches up as the aircraft settles slightly, then climb away at 80mph (70kt) and about 800fpm.

Pilots have always enthused about the quality of the Chipmunk’s handling, and as we close on the Magister cameraship carrying Peter Holloway and photographer Darren I remember why.

It really is one of the nicest-handling light aircraft ever made. Although the controls are all cable operated they feel like pushrods and are smooth and light with little ‘stiction’ and minimal breakout forces. Cleverly though, having provided the pilot with plenty of control, to prevent students over-controlling Jakimiuk made the stick ratios quite long. Consequently at times you do have to make quite large control inputs, although as the stick forces are so low this isn’t onerous.

Darren indicates he’s ready and I slide smoothly into position. Formation flying will always reveal any control inadequacies, and if the Chipmunk does have any deficiencies I certainly couldn’t find them. Manoeuvring around the Maggie is easy, although I have to keep looking around the windshield bow and canopy frame as they always seem to be in wrong place. Apart from this, the field of view is excellent.

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Darren soon has all the pictures he needs except for a ‘break’, so I roll the Chipmunk right up onto its port wingtip, then break down and away from the Maggie in a graceful sweeping curve.

With the photo session complete I can continue to assess the aircraft both quantitatively and qualitatively, starting with some 360o turns with varying amounts of bank in both directions. Even this simple manoeuvre confirms what all Chipmunk pilots know−taut handling and well coordinated controls mean lots of fun!

The ailerons, elevators and rudder are powerful, light and smooth, and also beautifully harmonised, with the ailerons being the lightest and the rudder the heaviest. In fact, people often compare the Chipmunk’s handling favourably to the Spitfire’s, but I think (with less than two hours in Spitfires) that this comparison is flawed, and that the Chipmunk actually handles better than the Spitfire.

Viewed quantitatively, the Spitfire’s handling is not ‘textbook’, as the elevators are lighter than the ailerons, which is the wrong way around. Interestingly, when I say this to Dave (who has also flown a two-seat Spitfire Tr9) he says “I agree 100 per cent!”

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Slow flight is as agreeable as every other aspect. It’s an old cliché, but the Chipmunk really does talk to you, and as the speed bleeds away and the ASI stops indicating (remember the lowest number is 60mph−and it stalls well below that) the stick starts to rattle gently. It’s trying to warn you that all is not well, and if you keep slowing down the warnings become more obvious and insistent. The buffet increases, the aircraft starts to mush and, as the stick hits the backstop, the nose pitches gently forward. Adding flap reduces the speed further, and just a bit of power makes the ASI give up completely. Recovery is practically instantaneous, just lower the nose.

The stability is just as good, being positive longitudinally and directionally, and just barely positive laterally. It’s also so nicely balanced that, once set up with power and trim, it’s almost a perfect ‘flying classroom’ in which to study the effects of power and flap on trim. Get it trimmed then add some more power and it pitches up and yaws to starboard; reduce power and it pitches down and yaws to port. Flaps down and it pitches down, flaps up and it pitches up.

So much fun

Tiring of playing test pilot, we decide to have some fun. Rolls are delightful−the trick seems to be to have an entry speed of about 120kt, as the aircraft does bleed energy quite rapidly−while taking care not to exceed 2,550rpm. Looping and rolling high above the beautiful Bedford countryside is just too much fun, particularly when a few chandelles, half-Cubans and stall turns are added! KDN is full of fuel and Dave’s in the back and it’s still wonderfully light on the controls. Flying it solo and with an hour’s fuel on a soft summer’s evening must be an absolute joy.

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All good things must end, and I reluctantly point KDN’s shapely spinner on a southerly heading back to Old Warden. During the brief transit we cruise at a comfortable 2,050rpm, which gives 95kt IAS for a fuel flow of approximately 35lit/hr. In fact the overall performance whether climbing or cruising is pretty good for only 145hp and the Gipsy is delightfully smooth, except for a curious rough spot at around 1,500rpm.

Back at Old Warden I can see that the wind is slightly stronger and has backed slightly to give a quartering tailwind, but I’d still rather land uphill on Runway 21. Again, Dave recommends not pre-setting the brakes, and as we slide into the circuit I sit up a little straighter in my seat. This is a unique situation: there are at least fifty current Chipmunk pilots on the field, most of whom are watching and critiquing every arrival.

Talk about being judged by your peers! I elect to land with only the first stage of flap selected as this will extend the flare. Despite a slightly protracted float the landing goes well, so I taxi back for another go. This time I leave the flaps up and can’t really discern if it significantly improves the takeoff. Abeam the numbers the first stage flap goes down, promptly followed by the second. We’re slightly high, so I close the throttle and commence a slightly slipping curved constant aspect approach. Crossing the hedge at around 55kt we still float a bit but the touchdown is again thankfully smooth, so I decide to quit while I’m still ahead. If I plant KDN in front of this crowd they’ll probably lynch me.

So, why is the Chipmunk special to so many aviators? For generations of military pilots and air cadets it was the first powered aircraft they flew, not just in the UK, but in literally dozens of other countries. You never forget your first love... but let’s take these rose-tinted goggles off for a moment. Why are so many still airworthy and why do they command such high prices? Well, even when the first airframes appeared on the surplus market pilots knew what the Chipmunk was all about−affordable flying, with delightful handling and also perfectly practical to operate−so long as you’re prepared to get your hands dirty.

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Replace the ridiculous cartridge starter with an electric one and you’re in business. The cockpits are reasonably comfortable and you can carry some baggage, while a cruise speed of 90-100kt isn’t bad and a fuel burn of around 30-35 lit/hr shouldn’t break the bank. The range and endurance of a TMk10 (which only carries 82 litres) isn’t that good, but at least the sliding canopy keeps you cool (both literally and figuratively). And, of course, KDN has a lighter empty weight for when you want to play, and a lot more fuel (44 litres more than a T Mk10) when you want to cruise.

Chipmunks are special, and KDN, being the oldest Chipmunk still airworthy (and also being almost entirely original) is just that little bit extra special.

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