The De Havilland DH 60 Moth

A brief Chronology of the Moth thru the summer of 1928.

Too many words have been written by too many really knowledgeable people for me to attempt even a brief history of the type. Therefore I shall confine myself to the simplest of sketches, noting only the major milestones which led to the development of the two versions included in this package.

Captain Geoffrey de Havilland flew the first DH 60 (not yet named the Moth) for the first time on Sunday, 1925. The plane was a small biplane with a 29 foot wingspan and a maximum takeoff weight of 1298 pounds, although some sources quote 1240 or 1260 pounds. The engine was an ADC (Aircraft Disposal Company) Cirrus I, a 60 horsepower air cooled four cylinder inline which gave the plane a nominal top speed of 90 mph and a cruising speed of 75 mph. A 15 Imperial gallon fuel tank in the top wing center section allowed for a range of about 320 miles, and the wings were designed to be folded back (a feature which I did NOT include in this model) so that the airplane could be stored indoors without requiring a full sized hanger. The airplane was intended to serve both the private owner and the government sponsored flying clubs then being proposed, and the projected price was 885 pounds (which I shall now begin noting as L885, even though I don’t have the appropriate sign on my American keyboard).

The airplane was an immediate success and sales were brisk (for the day). An attempt to count serial numbers indicates that at least 62 were produced by the De Havilland factory at Stag Lane and others were produced under license in Finland and Australia. Total production is usually quoted as 113 machines.

In 1926 attempts were made to find a better and more powerful engine. In part this search was inspired by the fact that the Cirrus I used surplus cylinders from the World War I Royal Aircraft Factory (RAF) air cooled V-8 engine, and the supply of those cylinders was becoming exhausted. To fill the void, ADC developed its own cylinders and created the Cirrus II, an engine of greater displacement which produced 84 horsepower. De Havilland began offering this engine as a Moth option late in 1926.

The other engine adopted was the Armstrong Siddeley Genet I, a five cylinder radial engine that produced a nominal 75 horse power. This engine was installed on a small number of Moths, perhaps less than a dozen, most of which ended up serving the Royal Air Force.

January, 1927, saw the introduction of the 1927 Moth, also known as the Cirrus II Moth and occasionally as the Moth II. This model not only had the 84 horsepower Cirrus II engine but featured a slightly larger airframe with the wing span extended to 30 feet. Other less visible improvements were also made, but the “improvement” most prominently advertised by the folks at De Havilland was that the already low price of L795 was further reduced to L730. Between 31 and 40 Cirrus II Moths were delivered. More might have been built, but just three months later, De Havilland introduced the DH60X.

DH60X featured two readily apparent changes. The first was the installation of a larger 19 Imperial gallon fuel tank in place of the smaller unit on previous versions. The second was the lowering of the engine by several inches, a change which significantly improved the pilot’s forward visibility. There were other less obvious changes, the biggest probably being an increase in the stagger between the lower and upper wings. This became the standard Moth configuration for the next year.

By serial number count, at least 302 DH60X aircraft were built by De Havilland at Stag Lane. A few dozen more were built overseas under license, mostly in Australia, and the most often quoted total production figure is 338 airframes.

Things remained fairly static for the next year, until April, 1928. That month saw the first installation of the split axle landing gear which quickly replaced the “spreader bar” landing gear previously used. At the same time, the first set of Handley Page “autoslots” was installed on a civilian Moth, although the Royal Air Force had been operating them for some time. Apparently they had not been offered previously because in addition to the cost of the autoslots themselves, Handley Page received a license fee for each set, a fee which was paid by the customer and which frequently exceeded the cost of the autoslots themselves. It was rumored that Captain de Havilland was so irritated by the arrangement that he insisted that only one set of autoslots be owned by the company for testing purposes.

The small change introduced in May was the first three-piece windscreen which replaced the “blown” Plexiglas units previously used. However June brought the biggest change when the first Gipsy engine was mated to the airframe. Initially developed as a 135 hp racing engine, this four cylinder inline engine was de-rated to 85 horsepower for use in the Moth. With this engine installed the airplane became the DH60G Gipsy Moth, and this version of the Moth outsold all Cirrus powered Moths combined. When the last Gipsy Moth was delivered in the late summer of 1933, the factory at Stag Lane had produced 595 Gipsy Moths. Nearly 100 more examples were built under license in France, the United States, and Australia. The type was used by flight schools and flying clubs world wide and the list of famous record flights is far too long to recount here.

Late in 1928 one more version emerged. In response to requests by operators who used their airplanes in more primitive areas where seemingly minor damage could keep the airplane on the ground for extensive repairs, the company introduced a metal tube framed fuselage. Produced in tandem with the wooden framed DH60G and X airplanes, this airplane was designated the DH60 M “Metal Moth”, and it became the most popular version of all, with nearly 900 being built worldwide.

The Planes

This package includes two planes. The first is DH60X G-EBUZ, c/n 478, as it might have looked if delivered in the late summer of 1927. I found one picture of this airplane taken in December, 1927. The picture is in black and white, so the choice of colors is a guess on my part. The aircraft was stricken from the record in December, 1934.

The Gipsy Moth is G-AAHI, c/n 1082, as it might have looked when delivered in 1929. This aircraft is still active today and photographs are available showing it with green fuselage and silver wings and red fuselage and white wings. I chose the green livery.

I based the instrument panel on vintage rather than modern photographs and for that reason they are basic to the point of being stark. There were no electrical systems, starters, lights, radios, or gyros. Most of these conveniences were available at the time as options, but the vast majority of Moths appear to have been used in daylight visual flying conditions, so that is how I set them up. For those who insist on talking to people when they fly, I did put the default ATC popup in the 2D panel.

Performance and Specifications

As is common when studying really old airplanes, I found that the published specifications varied from one source to another. Where I could find them, I quote figures given in contemporary reports. Where I could not find such reports, I used an “average” figure based on the sources I do have at hand.

Perhaps the biggest disconnects are in the area of weights. In his excellent book The De Havilland DH. 60 Moth, Stuart McKay notes that the early DH60X weighed 853 pounds empty and 1370 pounds fully loaded, but most published sources quote the much higher figures of

955# and 1750# respectively. Since I intended my Moths to be as they were originally configured, I elected to use the lighter weights quoted my Mr. McKay.

I had a stroke of good luck when it came time to figure the weights for the Gipsy Moth. As noted, G-AAHI is still active, and as such, it’s C of A is available on line listing the approved gross weight. I wish everything I did was that easy!

I encountered one other puzzle as I created the flight dynamics of these airplane. Most sources agree that the Cirrus II delivered 84 or 85 horsepower at 2000 rpm, but output for the Gipsy I is not so well defined. U. S. CAA specifications dated 1950 state that the Gipsy delivers 85 hp at 1900, and that figure is also quoted in some of British sources I have. However, several other sources claim that the engine produced 98 – 100 hp at 2100 rpm. In the end, I elected to use that latter value. It does not really matter in the grand scheme of things, but if there are purists out there who wonder what I was thinking, now you know. By the bye, I am always seeking to enlarge my data base, so if any of you have more definitive data on any version of the DH60, I would be pleased if you would share it. If I discover that I have made a gross error, I am always willing to post corrected updates.

Some notes about flying the Moth

Flying does not get much more basic than it is in the Moth. I included a very rudimentary even though it appears that the checklist as we know it did not actually come into common use until sometime in the mid 1930s. Prior to then, a “real pilot” was expected to know what to do and when to do it without outside aid. I often wonder if that attitude didn’t contribute to the rather high accident rates experienced in those bygone days ….

There are a couple of items that may catch the unwary by surprise. The fuel tap is the lever mounted on the left rear cabane strut. Vertical is ON, 45 degrees counter-clockwise is OFF. The switches are on the forward decking on the left side of the fuselage. You cannot reach through the glass with the tool tip to flip them ON any more than you can reach through the Plexiglas on a real plane. Slide yourself to the left until you have a clear shot at the switches to turn them ON.

When you check the oil pressure after engine start you will see absurdly low values. Normal oil pressure for the Cirrus is 5 to 15 psi with a maximum limit of between 25 - 30 psi where a pressure relief valve opens. (The normal operating pressure for the earlier Cirrus I was lower at 5 to 10 psi!) When I first encountered these numbers in an article about the airplane I thought it was a misprint, but I later found a Cirrus II engine manual and confirmed them.

The pressures for the Gipsy engine are only about 10 psi higher.

The takeoff run is short. I recommend lifting the tail just enough to get the skid out of the sod and letting the plane fly off in a tail low attitude. Best rate of climb speed appears to be about 55 mph, but for better visibility and engine cooling 65 mph works just fine. At this point you will discover one of the two major differences between the two versions of the Moth. The Cirrus is a right hand engine and the Gipsy is a left hand engine. While neither is going to pull the nose around like the Merlin pulls a Spitfire, you need to remember that the Cirrus requires right rudder on takeoff and climb whereas the Gipsy requires left rudder.

The best word to describe cruise flight in the Moth is “leisurely.” In March, 1928, De Havilland ran fuel consumption tests which indicated that the most efficient cruise speed for the DH60X was 79 mph where the engine burned 4.3 Imperial gallons per hour. Similar tests run on the Gipsy Moth later that year indicated that the Gipsy was most efficient at 76 mph where it burned 4.35 gph. However it appears that the average pilot not engaged on a world spanning record flight simply set 1800 rpm and let it go at that. This usually yields a true airspeed in the low 80s and a fuel burn around 4.5 Imp. gallons an hour. Note that the fuel is read in U. S. gallons in the simulator, so remember to multiply your average fuel consumption by 1.2 to prevent yourself from running dry over embarrassingly hostile terrain.

Landing is pretty much a non-event. I fly downwind at cruise power and as I pass abeam the touchdown point, I close the throttle and begin slowing. About the time the speed hits 65 I turn base and I hold that speed till turning final. Once established on final the other major difference between the two versions becomes obvious. The DH60X has a “plain” wing, but the later Gipsy Moth is equipped with the Handley Page Autoslots. I fly the X down final no slower than 55 mph, but with the autoslots, I can comfortably fly the final at 50 mph indicated (the slots extend at about 53 mph) and still have the energy and elevator authority to make proper three point landings.

As is typical of the era, the long nose effectively blocks your forward vision on final. At the same time, with all that drag, the airplane comes down at a pretty good rate. Both conditions argue for what later became known as the “Spitfire Curved Approach,” which basically means a fairly tight pattern with a continuous shallow turn from down wind to short final. As always, the best way to find out what works best for you is to experiment and then practice.

There are no brakes on these Moths, but even so the plane rolls to a stop in short order. It takes a certain amount of power to keep the plane rolling when taxiing and the best way to change direction is to give a quick blast of power while applying full rudder into the turn. Do not leave the power in for any length of time because the plane accelerates quickly, especially when light.

Once more, this is flying at its most basic. No radios, no clock. Navigation is done by following a pencil line on a chart. With no lights you must you land with the sun and spend the evenings at the local pub. It doesn’t get any better than this. Enjoy!

jcdouglass

31-12-08

SPECIFICATIONS

1927 De Havilland DH60X

Span: 30’ Chord 52.4” Wing Area 243 square’ Airfoil RAF 15 Modified

Length 23’ 8.5” Height 8’ 9.5”

Empty Weight 859# Useful Load 511# Gross Weight 1370#

Vne (no one called it that in those days) 160 mph indicated airspeed

Vh * 98 mph true airspeed at sea level

Cruise speed 85 mph true airspeed

Stall speed 40 mph indicated

Service Ceiling 14,500 feet

Fuel Capacity 19 Imp/ 22.8 U. S. gal

Oil Capacity 2.4 Imperial gallons

1929 De Havilland DH60 G Gipsy Moth

Span: 30’ Chord 52.4” Wing Area 243 square’ Airfoil RAF 15 Modified

Length 23’ 11” Height 8’ 9.5”

Empty Weight 920# Useful Load 730# Gross Weight 1650#

Vne (no one called it that in those days) 160 mph indicated airspeed

Vh * 102 mph true airspeed at sea level

Cruise speed 85 mph true airspeed

Stall speed 40 mph indicated

Rate of Climb 550 fpm

Service Ceiling 14,500 feet

Fuel Capacity 19 Imp/ 22.8 U. S. gal

Oil Capacity 2.4 Imperial gallons

*Vh is the airplane’s maximum speed at maximum gross weight at the altitude at which the engine produces maximum power – sea level in this instance.

All these performance figures were achieved or calculated on a standard day at maximum gross weight.