John Greenfield’s Horton

In 1931, following many experiments with models, the Horton brothers of Reimar and Walter started work on their first full-size aircraft, the Ho I sailplane, at their home in Bonn. Wooden construction with chiefly fabric covering was used. Elevons were used for pitch and roll control and yaw control was obtained from brake flaps (sometimes called drag rudders) above and below the leading edges near the wingtips. In 1935 the first examples of the Ho II were fitted with an engine to drive a pusher propeller. Over the next few years there were more developments in the design, one was even intended as a commercial aircraft for approximately 60 passengers.

The Ho IX was the first design intended specifically for combat, as a fighter with turbojets. The construction consisted of a welded steel tube centre section with a wooden structure for the outer panels, plywood covering being extensively used. With all the principal items of equipment mounted in the wing centre section the two BMW turbojets could not be installed through the spars of the V1. The Ho IX V2 was thus designed to take two Junkers turbojets (actually with larger diameters). This flew successfully, but was destroyed in a single-engined landing after logging only a few hours. The aircraft continued in its development over the next few years, but this is the version that John decided to model.


John had thought about the the idea to build a true flying wing for some time to learn about the aerodynamic issues involved and the flying of a model without any vertical fins. John’s previous model, a twin jet engined ME262 had suffered a bird strike at a show and he was propelled (good pun!) into action believing that as it was only basically a wing it would be relatively quick to build. This certainly did not prove to be the case.

Specification of the model

The design of the model was from a 1947 Royal Aircraft Establishment report of the work of the Horton brothers. In the report there were details of the design philosophy together with formula and sketches. John considered building a small test model, but decided that the ‘big fly better’ approach would result in more favourable aerodynamics and give a better chance of the model having flying characteristics similar to the full-size.

Weeks were spent plotting the complex aerofoil shapes and making jigs to accurately hold the wing in the required twist to achieve the famous bell shaped lift distribution. With no vertical surfaces, accuracy here was essential if the model was to fly successfully.


The main difficulty experienced was the accuracy required as everything needs to be symmetrical and in-balance. Differences between the sides could result in an aerodynamic twist that could not be trimmed out. To make things complicated there were almost no straight lines on the model, with the exception of the leading edge, and this meant that the whole model had to be built in stiff jigs to ensure it remained straight whilst the wing skins were added over the compound curves.

Once the number crunching had been done and the model building begun a major problem immediately had to be faced; how to keep a 2.1m long, 250mm high piece of 3mm lite- ply wing rib to stay straight. The problem of keeping everything straight gave John endless headaches during the construction of the model. Due to the accuracy required there were also problems with the wood moving and twisting slightly after it had been cut and fitted to the model. This was especially so along the trailing edge where the ribs are thin and have a reflex section.

After the basic wing had been framed up John was contacted from someone abroad who had seen some early pictures of the model’s construction and told him there were flaws in his design. John spent several months reconsidering and checking his work before pressing on with his design and ignoring the doubters. The model was skinned with balsa, which had to be put on in strips due to the compound curves. Control functions of elevons, flaps, drag rudders, retracts, wheel brakes were added. These required the use of 11 servos. Six batteries were used, the servos powered through a 6V supply. The fueal is kerosene and the total fuel capacity is 6 litres. The model was covered in glass cloth and G4.

The model has retracting undercarriage with sequencing doors and wheel brakes. The nose leg has quite an effect on the centre of gravity and much care was required in the calculations.


John decided to follow the full-size testing process and fly the model as a glider. The model had no engines or undercarriage and so no ballast was required. Steve Holland acted as the Tug pilot, Edie Barker, Chairman of the Swindon Club, provided access to the Wroughton Airfield and of-course the backing of the other members of the Ghost Squadron was invaluable. Four flights were completed successfully.

The concerns over the model were not totally extinguished though. John was particularly concerned about the centre of gravity. Calculations had suggested that the maximum range was +/- 2.5mm which meant that the weight of the noseleg retracting backwards had to be exactly counteracted by the weight on the main wheels moving forward. So, not only had the geometry to be correct, but so did the weight of the legs which caused problems in the material selection of the legs.


Following the initial testing as a glider, extensive testing of the airframe and complex control system took place in March and April 2002 finally resulting in a test hop late April. This immediately showed a weakness in the noseleg which bent. Steve Hall came to the rescue and offered to make a new noseleg outer tube from a piece of high tensile steel. The result was superb.

The actual test flight took place at Withernsea on 4th May 2002 during the LMA fly-in weekend. Withernsea was chosen because it has a good grass runway and plenty of open space around. Two flights were completed. The first flight was interesting as there was too much aileron differential and the adverse yaw caused the model to turn right when attempting to turn left!
The model incorporates drag rudders on each wingtip. To turn right, the right hand drag rudder is extended. For slow flight, all are deployed. Then to turn left, the right drag rudders are retracted.


John and the Ghost Squadron prepare the model for another flight at Cosford 2002.

John now reports that the model is ‘interesting’ to fly. The model has the same basic flaw as the full-size – it has no cross-wind capability as it cannot be yawed. Still, when the conditions permit, he puts on a good display and it certainly looks the part.

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