Flight test: De Havilland DH60 Cirrus Moth
- Credit: Darren Harbar
By the end of the 1920s most of Britain’s light aeroplanes were de Havilland Moths of one kind or another. Today BAE Systems’ Heritage Flight operates the oldest DH60 Cirrus Moth still flying | Words: Dave Unwin - Photos: Darren Harbar
The wind in the wires sighs and soughs, and the eager roar of the engine subsides to a muted muttering as I ease the throttle back. It's a profoundly visceral experience, enhanced by the beautiful view.
Off to starboard the two huge airship sheds at Cardington dominate the landscape while, to port, the famed Old Warden airfield basks in September sunshine.
It's a timeless scene, and one that could have occurred at any time over the last ninety years, as the aircraft I'm flying is even older than the airship sheds. It's an old cliché (which we all know should be avoided like the plague) but "ah - de Havilland" was never more apposite.
There is something particularly special about flying open-cockpit biplanes. Watching the world go by through a pair of sturdy, wire-braced wings is a wonderful experience, and on this occasion it's not any old biplane but one of the most influential and iconic aircraft of all time: the immortal de Havilland DH60 Moth.
And not just any old Moth (if there even is such a thing) but the oldest and most-original in the world!
By the 1920s Britain was becoming an increasingly air-minded nation and de Havilland realised that the market needed a two-seat biplane that was not only rugged and possessed passable performance, but also affordable.
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Rather than start with a clean sheet, DH decided to build an aircraft loosely based on the 1924 three-seat DH51, but slightly smaller. Interestingly, not only was the DH60 a scaled-down version of the DH51, but its motor was derived from the DH51's engine.
The DH51 was powered by an Aircraft Disposal Company (usually referred to as either ADC or Airdisco) air-cooled V-8 that produced 120hp and was built using many parts from war-surplus 70hp Renault V-8s.
For the DH60 engine, Major F B Halford (ADC's chief designer) took one bank of cylinders and many other components from an Airdisco V-8 and mounted them on a new crankcase. Named the Cirrus I, it produced 90hp and played a major role in making the Cirrus Moth an immediate success from a performance perspective, and the type soon cemented its reputation as possibly the world's first practical light aircraft. All de Havilland had to do then was ensure it was competitively priced, and at around £650 it was.
Orders flooded in, and by the end of the decade the factory at Stag Lane in North London was making three aircraft a day. At that time 85% of the UK's GA aircraft fleet were Moths of one kind or another, which probably explains why (in much the same way that the generic term for a light aeroplane in the US was 'Cub') most Britons of the time referred to all small aircraft as 'Moths'.
Incidentally, the reason the name 'Moth' was chosen was due to Captain de Havilland's formidable reputation as a lepidopterist. Ultimately, more than 2,000 DH60s were produced, including the subject of this air test.
Based at Old Warden and owned by the BAE Systems Heritage Flight, G-EBLV was built in 1925 and is the oldest airworthy Moth. Intriguingly, even though it is almost 95 years old it has only logged 317 hours!
One of the first things I notice is that, unlike the Tiger Moth I have flown, the engine is not inverted. Although most DH60s were powered by a DH Gipsy engine Lima Victor is fitted with a Cirrus III, which produces 90hp at 2,100rpm.
There's an Automobile Association badge on the starboard side of the cowling. Why it's there seems lost in the mists of time, but one connection might be that the AA formed an Aviation Section in 1929, which helped Amy Johnson plan her 1932 flight to Cape Town and (in association with the Royal Aero Club) produced 'Flying Maps'. Alternatively, it's quite possible that one of Lima Victor's previous owners was also an AA member.
Before moving in for the preflight I stand back and admire the aircraft from a distance. The de Havilland Company always designed handsome machines, and even if I hadn't known it was a DH product I would have guessed?the extraordinarily curvaceous rudder would give it away!
Lima Victor is finished in the classic DH scheme of 'Moth Silver' for the wings and tail, while the fuselage colour was the owner or club's choice. As many Moths were bought by clubs, specific fleets had specific colours.
The London Aero Club had its Moths painted yellow, Midlands' were green, and Newcastle's red, while the Lancashire Aero Club had Lima Victor finished with a blue fuselage. One of the most famous aviators of the era, Sir Alan Cobham, delivered it to Woodford.
As you'd expect of a flying machine designed in the 1920s, much of the material used in its construction is organic; mostly spruce, plywood and cotton. The fuselage is essentially a box built around four square-section longerons made of spruce, braced by cross-members and skinned with stress-bearing plywood sheets.
Already I was beginning to understand why the aircraft had been such a success. Not only is this a very cheap and simple yet robust design, but any competent carpenter equipped with only basic woodworking tools could make quite major repairs to the airframe.
The wings are made mainly of spruce, covered with fabric (the wingtips are aluminium tube), separated by wide-chord interplane struts and braced by streamlined flying wires.
The upper wings are mounted on a centre section, which is basically an aerofoil-shaped 68-litre fuel tank carried by six streamlined hollow steel struts (four vertical and two sloping), which are braced fore and aft by more streamlined flying wires. A 45-litre auxiliary tank situated forward of the front cockpit was an option.
As the upright engine is bolted to the top longerons, the forward field of view is somewhat impeded by the tall cylinders. This would be especially noticeable when taxying, and I begin to appreciate why de Havilland inverted the engine on the Tiger Moth. The design of the exhaust is also noteworthy, as it runs along the port side and ends well aft of the rear seat.
De Havilland's patented differential ailerons are fitted to the lower wings only. An excellent attribute of the Moth is that it possesses one of my favourite features: folding wings. Once the spring-loaded quick release bolts in the leading edge of the wing root are removed, the wings pivot about the rear spar to fold aft.
A temporary jury strut helps support the wings which, once folded, reduce the aircraft's span by two-thirds from 9.14m to 2.98m. When not in use, it is secured to the fuselage by leather straps next to the exhaust. Much was made of the folding wings, and some period advertisements show a Moth being towed by an Austin 7.
The undercarriage features large diameter, narrow wheels, fitted with relatively high-pressure tyres and mounted on a single axle. Although the prototype had an unbalanced rudder, all production aircraft featured a horn balance and, like many aircraft of this era, the trapezium fin is tiny and the shapely rudder huge.
The strut-braced tailplane and large two-piece elevator are also curvaceous, and I note with considerable interest that the trailing edges of the elevator are extremely close to the ground and that the tailskid is non-steerable. As the wheels are not fitted with brakes, taxying could soon get tricky in anything stronger than a gentle zephyr.
Fascinatingly, and in common with many other DH designs, as delivered from the factory there is no carb heat. Instead, as the carburettor is mounted quite close to the cylinders, it was assumed that they kept the carburettor sufficiently warm to prevent the formation of carb ice.
Pete generously offers me my choice of seat and I'm sorely tempted by the rear cockpit, but I know that he'll want to do the formation flying so I take the front.
Before climbing in I study the rear cockpit, which is a delight of period instrumentation, including a single pointer 'height meter' and a P11 compass. To my mind, the P11 always looks more like a ship's compass than an aircraft's, though it complements the Moth's cockpit perfectly.
Access to the front cockpit is reasonable for an aircraft of this type, as there is a small door on the starboard side. The front cockpit is very sparsely furnished with only an ASI and height meter (it's not an altimeter; it doesn't have a 'Kollsman' window), and has only a stick, throttle and rudder pedals?not even a trim lever!
The two pairs of brass magneto switches are mounted externally, on the starboard side and just in front of each cockpit. They bear a strong resemblance to large Edwardian light switches, while the unusual seat belt is the worst I've ever seen in an aeroplane. It's almost a chest strap!
Starting a machine of this vintage is always something of a ritual, but the engine is soon running smoothly and, after gently warming it up, Pete eases us out of our parking space and passes control to me.
If there were any real wind, wing-walkers would be a necessity, but there isn't and I manage to get us to the runway unaided. However, almost as soon as we began to roll, it's obvious the Cirrus Moth is quite a bit different to a Tiger.
There seems to be more engine vibration, probably because the engine does not sit in rubber engine mounts but is bolted directly to the longerons. The DH60 is also not as benign to handle on the ground as a Tiger as the field of view is significantly worse, making S-turning not only prudent but essential.
This is due mainly to the upright engine, although the much larger diameter wheels do make the aircraft sit somewhat higher than its younger sibling. Furthermore, while a DH82 provides a relatively comfortable ride on its fat, low-pressure tyres and nice soft, coil spring undercarriage, the DH60 feels much more skittish on its thin, high-pressure tyres and rather crude undercarriage, which uses only rubber blocks in compression for shock-absorption.
While taxying you need to be very cautious, yet conversely occasionally bold. Remember the tailskid doesn't steer, so in the absence of a wing-walker you must blip the throttle with the stick well forward (to unload the skid) while applying full rudder in the desired direction.
With such simple systems the pre-takeoff checks are minimal, so I line up and very slowly open the throttle. My initial impressions of the machine being somewhat 'fidgety' are reinforced as we accelerate.
Lift-off occurs at around 45 knots (in common with most aircraft of this era the ASI is calibrated in mph, not knots, but I'll use knots for the sake of consistency). However, flickering once or twice the ASI's needle flops back lifelessly to zero.
I report this to Pete, who calls out the speeds over the intercom. I know 60-65 is a good climbing speed and, when Pete confirms we've attained sixty, I note the attitude and try and hold it.
In fact as the name suggests the Moth does not so much fly though the air as flutter upon it. This is a function of its very low wing loading, which is barely 5.5 lb/sq ft, even when flown at the MAUW of 1,350 lb.
Our ascent is leisurely at somewhere around the 500fpm mark, as I try to maintain a constant attitude and accept the speed and climb rate that this produces. This is the best way to fly most aeroplanes, and is especially true of aircraft that have a low wing loading.
Such light machines have very little inertia, and there is nothing to be gained (but a great deal to be lost) by chasing the airspeed. All those wires and struts hanging out there in the breeze are generating a lot of drag, and you've only got ninety horses doing the pulling.
I also get the distinct impression that, although it will clearly climb or turn, it is not especially enthusiastic about doing both simultaneously. As mentioned previously, there's not a huge amount of surplus thrust but plenty of excess drag. If you incline the lift vector out to one side, the rate of climb suffers accordingly.
While we wait for the Miles Magister flown by Bob Morcom and carrying photographer Darren to join us, I begin a preliminary assessment of the controls and stability. The handling seems somewhat better than a Tiger, although anyone who has ever flown a DH82 will readily admit that this isn't saying much!
Indeed, although using the ailerons without a coordinated rudder input does generate a significant amount of adverse yaw, it doesn't seem as bad as a Tiger.
This is because the DH60 has relatively straight wings and limited dihedral, while the DH82 has sweptback wings and more dihedral. This arrangement tends to produce yaw-to-roll coupling, and is quite pronounced in a Tiger Moth.
Another aspect of the general handling significantly different to the Tiger's is that the DH60 seems less speed-stable. As for its lateral and directional stability, both are positive, although just barely so.
The directional stability is quite 'soft'; if you skid the aircraft, put the rudder to neutral and then hold it in place with your feet it's not so bad, but if you let the rudder 'float' the yaw oscillation is almost divergent, but to be honest this isn't surprising?look at the size of the fin! As for the field of view, because the front cockpit is situated between the two wings it's somewhat constrained down either side, and immediately above.
On the plus side, you get less of a draught in the front cockpit than the rear, as long as your turns are nicely coordinated. If they are not, and you are either slipping or skidding, it soon becomes obvious when the airflow strikes your head!
You can also definitely feel it 'in the seat of your pants' if the aircraft is being mishandled. But when it all comes together it is most gratifying, and there is certainly a sense of profound satisfaction to be had from executing a well-coordinated 360° turn in a vintage biplane.
Unsurprisingly, although you can maintain either the same speed or same altitude, you will not be able to do both. Something's got to give, and you must either add power or descend (to maintain speed) or slow down (to maintain height).
The 'Maggie' appears. Pete takes control and slides us smoothly into formation. It's a bit bumpy and he does an excellent job, leaving me to keep an eye out for traffic.
While we wheel and swing high above Old Warden, with the bright yellow Magister framed between the silver wings and with the gigantic airship sheds in the background, the whole experience has a certain air of timelessness.
Once all the pictures are in the can we break away. Pete returns control to me and I continue my evaluation. Slow flight is very benign, with the stall somewhere around 34kt.
As the stall is approached the Moth starts to mush, there's a little buffet and then it breaks with a slight wing drop. It's nothing dramatic but not quite as gentle as a Tiger, which has slats, and an upper wing set at an angle of incidence a few degrees greater than the lower wing (so that it stalls first and 'nods' the nose down).
On the Cirrus Moth, both sets of wings are set at three degrees. Just as with the Tiger?and indeed most aeroplanes?do not try to pick the wing up with the ailerons post-stall. This will only exacerbate the situation and may even incite a spin.
For the brief transit back to Old Warden Pete recommends setting 1,500rpm, which gives an IAS of around 60kt, for a fuel flow of about 22 lph. Consequently, the still air range (no reserves) is around 180 nautical miles.
Back in the circuit, the classic curved glide approach all the way round from the end of the downwind leg to short final seems appropriate, with Pete calling out the speed.
I can see that the wind has freshened slightly, and that we have what is best described as a 'cheeky crosswind' at about 45° from starboard. As I arc around base onto final Pete calls that I'm about eight knots faster than the briefed speed, but doesn't suggest I slow down.
I can see from the windsock that it's a little gusty and decide that a bit more speed would be no bad thing! I also know that the Moth is light and draggy with little inertia. When I close the throttle and flare the speed will soon wash off and it won't go far.
The landing isn't at all bad and I convert it into a touch and go, confident that the next landing will be even better. Well it isn't, and we hop, skip and bounce in a somewhat undignified fashion to a stop. Pete is very nice about it and says that "it was one of those landings that felt worse than it looked".
However, it still felt pretty bad! As we slow I can feel through my seat that the Moth will cheerfully ground loop if I let it. Remember, this is an aircraft that was built when runways simply didn't exist.
Instead, an airfield was exactly that?a field for aeroplanes usually consisting of a large, often vaguely circular, area of grass. Therefore, you always landed into wind, (which is just as well, as the demonstrated crosswind component is only six knots) and the reassuring drag of the tailskid kept you straight during the rollout.
Conclusions? Well, I hugely enjoyed flying this iconic aircraft which, despite being almost a century old, is a real fun flying machine.
It was both an honour and a privilege, and I'd like to thank Peter Kosogorin, John Hurrell and the BAE Systems Heritage Flight for this wonderful opportunity.
While writing this report I thought a lot about pioneering pilots such as Jean Batten, Francis Chichester and Amy Johnson, who all flew DH60s vast distances.
Like most other wood-and-fabric flying machines, the Cirrus Moth is an aircraft that flies upon the air, not through it. Consequently, you choose the days you fly it carefully, for though it is a delightful machine in calm air, the whole experience would be much less enjoyable on a bumpy, blustery day and would quickly become quite tiring.
Accordingly, my profound respect for these brave adventurers has increased still further. Johnson flew her Gipsy Moth Jason from Croydon to Darwin between 5 and 24 May 1930.
Even today, in our world of ELTs, GPS and SatPhones this would be an epic flight, but ninety years ago...? Incredible!