DH9a (oz2503)


DH9a (oz2503) by Eric Coates 1975 - plan thumbnail

About this Plan

DH9a. Free flight scale power model, for 1-1.5cc diesel. Scale is 1/8 with 33oz flying weight. This is a Nationals winning model. Jan 1975 Aeromodeller.

Quote: "FIRST APPEARING in 1918, the DH9A was probably the best single-engined bomber of the First World War. With a top speed of 123 mph and a maximum bomb load of 750 lb, it was the culmination of the remarkable development of British bomber design in the relatively short (four-year) period since the commencement of the war. Like the much better-known Lancaster bomber of the Second War, it evolved from previous designs which were far from satisfactory - in the case of the DH9A these predecessors were the DH4 and the DH9. The DH4 was a very fast machine, when powered by the Rolls-Royce Eagle engine, but communication between the crew (who were separated several feet by the petrol tank) was almost impossible.

The DH9 was designed to rectify this problem; but unfortunately the BHP engine fitted was woefully lacking in power and subsequently the DH9's performance was much inferior to the earlier '4'. This fault was rectified by fitting the 400 hp American Liberty motor - the vast engine giving the DH9A, as it then became, its unmistakable bluff front end, so useful for hiding a long-stroke model engine! The extra weight of the Liberty necessitated the wing span being increased from 42 to almost 46 ft, and in this form it became one of the truly great aeroplanes of the Royal Air Force. Although only operational for the last few months of the war, it remained in service until the early 'thirties.

Now to the model. I built the prototype during the early months of 1972, completing it in time for the Nationals, but unfortunately it did not behave too well that year, apparently lacking longitudinal stability, and was easily induced into a stalling condition. In the winter of '72 I critically examined the model and decided that the trouble probably lay in the relatively short tail movement, so I moved the CG half an inch further forward. The model was then re-trimmed: test glides allowed the elevators to be brought back to the horizontal - previously, they had to be depressed about 5° to prevent a stall. More downthrust was then found to be necessary in order to keep the nose down with fall power on.

When re-trimmed in this forward CG configuration, the 'Ninack' has been one of the most consistent flyers I have ever had, and this is reflected in the results of the 1973 Scale contest season, when it was entered-in five contests - and won all of them! These competitions included the Super at the Nationals, the Eddie Riding at Woodford, and the Selby at the Northern Area Rally.

The model is 'bang-on' scale: tail areas are exact, the dihedral is correct at 3° providing ample lateral stability, while wing incidence and section are also exact scale. Although thin the spruce spars, assisted by the working bracing wires, are adequate to carry the loads. The design is the culmination of over twenty years of development of practical working-scale structures which look delicate, yet are strong enough for years of hard flying. The structure is designed to bend and 'give', in an awkward landing, rather than knock off.

The only part I have reservations about is the undercarriage - I normally fit a torsion-bar type on this class of model, but for the sake of authenticity I fitted a rigid job, allowing the scale bungoes to do all the work. This has performed well since the model has been re-trimmed, but some of the early, stally flights caused the rear legs to bend and splinter the fairing. I issue this as a warning to prospective builders!

Over the past few years, 1 have covered in great detail the construction of scale models, such as the 9A, in Flying Scale Column, and therefore do of intend to do so again, but will highlight one or two of the less obvious points.

Fuselage This is relatively simple, but incorporates several novelties. In full-size practice the fabric covering is not stuck to the structure, but is laced to the longerons and one or two uprights only, so in order to achieve this appearance on the model the longerons have 1/31 x 1/8 overlays to keep the covering away from the uprights. In order to impart torsional rigidity to the rear fuselage, 1/8 x 1/16 diagonals are fitted - formers 11, 12 and 13 are undersized to lie beneath the stringer/longeron line. These formers, and all diagonals and uprights, are greased with. a candle to prevent the covering adhering..."

Update 02/01/2017: added article, thanks to RFJ.

Supplementary file notes



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DH9a (oz2503) by Eric Coates 1975 - model pic


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DH9a (oz2503) by Eric Coates 1975 - pic 003.jpg
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User comments

A great flier and I added a small Tatone timer under the fuse and by it pulling back a pin (after about 10 to 30 seconds timer) the two balsa bombs on loops held by the pin and staggered so they dropped 1 to 2 seconds apart would drop in FREE FLIGHT mode. I could not get a rise off ground as it only had a MILLS 1.3 cc engine and 9X6 prop or 9X4 I recall, but perhaps a PAW 1.5 cc would have done the trick for R O G. But still from hand launch I won many free flight contests with it. It was a bit heavy as a free flight SCALE model tissue and silk outer cover and little thinned dope with hardwood wing spars by that method of designer Eric Coates, but from a running hand launch at the correct speed pretty fast it would just lift and FLOAT out of my left hand and right hand guiding the stern of the model near the rear sprung suspension with sheering elastic springing as per plan? Never chuck a free flight model, just let them go away at THEIR lift off speed. The profile publications did a colour picture as I recall of a WW1 type for my colouring with a grey rear fuse , I think. Many years as a robust model and so many flights, luckily only short fuel supply in the Mills tank PARTIALLY full so no thermal flyways, but I did have my name and address on it . Today it would easily take small r/c , and i/c but has to always fly minimalist VERY SLOW (again a big prop 9" or 9 1/2' with no rev head flights to stop torque affect and speed crashes!
Lyle - 02/08/2016
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