1/2A Pylon Racing and 'Dick Ohm Special'

Written for 'RCM&E', February 1975.

Dick Ohm Special'

The Model - Motors - Propellers - Fuels and Tanks - Courses - Conclusions -
Construction - Flying - BMPRA 1/2A Pylon Racing Rules

'1/2A league onward...' or 'TD or not TD?'

The original purpose of 1/2A pylon racing was to establish a cheap, simple class of pylon racing in which anyone could compete who couldn't afford the more expensive forms such as Formula 1 or FAl Racing It was Started in this country three or four years ago by members of the Southampton MAC who formed their own set of rules based loosely on the American rules published in 'R/C Modeler' magazine. In February 1972 the writer was persuaded by friend Mike Oagg to attend one of the Southampton contests. Following this and also following discussions with members of Southampton MAC, a set of rules for 1/2A pylon racing were drawn up and presented to the BMPRA for ratification. These rules were approved at the AGM early in 1973 and were used during the 1973 season. Apart from one small modification allowing hand launching (which the writer personally opposes), these are the rules still in use. They will be found in an Appendix to this article. Originally, the fuselage size was governed by a cross sectional area rule of 8.5 sq. in. but this has been changed to a size of 4.5 in. high x 2.25 in. wide in the interests of simplicity of checking. This has, however, resulted in a certain number of weirdos similar to those seen in FAI pylon racing. The early American rules restricted the wing planform to constant chord type, but this was felt to be rather restrictive on model~design and, therefore, the requirement was altered to 7/8 in. constant thickness with no limitation on planform. This means that several of the more attractive scale type models may be built. There has been a move afoot recently to delete the undercarriage, mainly by those flying off rough fields, but this does in fact, seem to defeat its own obiect since it causes damage to the model rather than saving damage and also tends to break the rather fragile Cox propellers which are used. At the moment, a compromise has been reached by allowing undercarriages to be removed, but a competitor cannot qualify for BMPRA championship points by using a model in this condition.

The Model

There have been several kits for 1/2A racers available in the past, most of these tending to suffer by having foam wings and being rather heavy. There is at the moment one excellent British kit available with a fibre glass fuselage as reviewed in 'RCM&E', November '74 issue. The actual layout of the model does not seem to be too important, the main thing being to ensure a fairly clean nose entry to enable the use of fairly small propellers in order to achieve the high RPM at which the motors realise their peak horse power. Virtually all models are now flown on ailerons and elevator - originally several models appeared with rudder and elevator and while take-offs were mandatory, this did in fact, still result in a fairly competitive model in that the models with ailerons usually lost a considerable amount of ground following take-off while establishing themselves on a course to the No. I pylon. As band launching is now allowed, this advantage by the rudder model has now been lost and it would appear that in order to be competitive it is necessary to fly on ailerons and elevator Experiments with coupled ailerons and rudder have not shown any great advantage and in fact it is usually very difficult to establish the correct set-up between the rudder and aileron in order to ensure that the model will fly and handle nicely. There are of course, no restrictions on radio gear so it it is felt necessary to have a rudder, there is no reason why another servo should not be used together with three function gear. However, while it is possible to build a model down to the minimum weight with this arrangement, it would be far easier to do so by using only two servos. Incidentally, it is quite possible and practicable to build a model down to the minimum weight without using miniature radio gear. The gear used by the writer weighs approximately 7.5 ozs., whereas it is possible to obtain gear weighing 5 ozs. or less. (I cannot tell you where without plugging my employer). What in the World can he mean - Ed!


The motors used are almost invariably the TD 049 or 051, although anyone who possesses one of the very old Holland Hornet motors would do well to try it. The whole secret of performance with these motors is mainly in the piston-cylinder fit. Attempts to improve performance by modifying the ports, etc. invariably result in lower performance. It is said that the best vintage of TD is 1968, which was apparently a very good year. It remains to be seen whether the very latest motors with an air filter on the intake and various other modifications are in fact better.

Opinions are divided as to whether the motor should be run on suction or pressure. It is certainly possible to be competitive while running on suction, in fact the writer cannot make pressure work at all. However, most of the really fast models are run on pressure and this is undoubtedly a factor in their speed.

Modifying the motor for pressure can he done in two ways, one of these is the method shown in the instructions which come with the motor and does in fact give a timed pressure which is taken from a small pip moulded on the side of the front housing. This method is probably over critical and it is far better to use untimed pressure which can be taken from the backplate of the motor by means of a simple modification See Fig. 1

Silencers are mandatory. At the moment there are two silencers in use, one of these being made specially by a member of the Cotswold Club and this is not generally available. It is said to produce gains of anything up to 1,000 RPM, mainly due to the fact.that it retains the sub-piston induction on the motor. If the writer can possibly manage to lay his hands on an example he will try it and see whether the claims are justified. The other type which is generally available is the Tatone silencer. This, if slightly modified by removing the restriction in the outlet pipe, appears to give a little or no power loss. It does have the advantage of directing all the exhaust residue in one direction and tending to reduce the amount of oil which gets plastered over the aeroplane.

Pressure mod and new course


At the moment, the propellers made by Cox specially for the motors appear to have an advantage. However, very few people have done any real experimenting in this direction and it is felt that some of the Tornado nylon propellers, or better still some home made wooden props., would show a real improvement in performance. Of the Cox propellers, there are basically two sizes, each of which is made in a soft black nylon and a hard glass-filled grey plastic. The two sizes are 5.5 in. x 4 in. and 5 in. x 3 in.

The 5.5 x 4 in. in the black nylon appears to be reasonably competitive, whereas the grey nylon version seems to be slightly inferior. However, in the 5 in. x 3 in. size, the grey nylon prop. is considerably superior to the black one and is in fact the propeller used at the moment by nearly everyone racing 1/2A. A good motor would show approximately 21,000 r.p.m. on this propeller although some people are claiming considerably more. It should be noted that the grey nylon or plastic propellers break very easily and it is a fairly easy matter to break one on a landing approach or even by putting the model down into grass. The writer's early experiments with the yellow Tornado nylon propellers in the 5.5 in. x 4 in. and 5 in. x 4 in. size, tended to show that these were perhaps a little faster in the air but rather sluggish on take-off. As hand launching is now permitted they may well be worth trying again.

Fuels and Tanks

The TD engines thrive on nitromethane. There are no restrictions on fuel formula in 1/2A rules and at one point last season, the writer was using 60 per cent - this may well be too much in fact as under British weather conditions somewhere in the region of 40-50 per cent would appear to be the correct amount. However, on a very hot dry day, considerably more may be used. Without wishing to get involved in the synthetic oils argument it has been found that Ucon LB.625 may be used as a lubricant without any problems appearing, although it is probably better to use equal parts of Ucon and Castor oils. MSSR has been used by some people, again without any apparent problems. The correct tank size is approximately I fluid ounce. Most people have their own ideas as to the best tank set-up to use - it is quite possible to use a polythene bottle, although rather fiddly in this size. The approach used by yours truly is a straightforward rectangular tin tank with the pick-up point being taken from the rear left hand bottom corner. This does seem to cause some surprise with some people who think that the pick-up point should go from the right-hand corner. However, if you realise that the model spends a fair amount of its time in a left hand down knife edge attitude you will see I think, that the left hand corner is the correct one. Incidentally, it is not really necessary to have a separate vent for filling the tank, since it is possible to stand the model on its nose and fill the tank, allowing the air to vent out via the fuel pipe and the carburettor. This will have the virtue of automatically priming the fuel system and tending to push any dirt out of the carburettor. If using a pressure system, ensure that the tank is absolutely air tight. Also, it is in fact a good idea to lap all joints, although there again in the size used this does become rather fiddly.


The course originally used was eight laps around two pylons spaced 110 yards apart or in other words I mile. The disadvantage of this course is that with only a single pylon on the bottom end, models coming out of the turn are flying right over the heads of the pilots, helpers, time-keepers, etc. To end this situation, the Cotswold Club have begun experimenting with a triangular course of roughly the same size. This is made up of two base pylons spaced 40 ft. apart with a third pylon 310 ft. upwind of these two base pylons. Eight laps are again flown and this gives a course distance of 1 mile as before. Disadvantage of this course is that times are now corning down to the region of 1 min. 15 seconds which is not really enough racing. It is, therefore, proposed that the course shown in Fig. 2 be adopted, this having a length of 0.15 of a mile. If 10 laps are flown this will give a race length of 1.5 miles and will bring the times back to somewhere around the two minute mark.


Well there you have it, a pylon race class which is simple, cheap and, if the models do not bounce, then they are at least usually repairable. It is also free from the deadly seriousness of FAl pylon racing and everyone thoroughly enjoys themselves, so I hope to see you at some future racing meetings. II"

DD and MG

Author (left) with Dick Ohm Special racer and Mike Gagg with Miss Cosmic Wind.

Dick Ohm Special

Goodyear racer No.15 or N6H was built in 1949 by Richard Ohm and Jamie Krapf with financial assistance from Gordon Stoppelbein. The aircraft thus became known to posterity as the 'Ohm ond Stoppelbein Special'. For some strange reason, this was frequently shortened to 'Dick Ohm Special' or just 'Ohm Special'.

Originally, it was a rather bulbous machine with high aspect wings but like many of its contemporaries, was progressively modified over the years (only four new heads and three new handles). In the form depicted here it was raced during the 1966 season with, sad to say, little success.

Despite its lack of glory, I have always felt it to be one of the prettier Formula I machines and in 1969 1 built a Formula l (then Goodyear) version, which succeeded in making its mark on the Hullavington runway during the Cotswold rally that year.

When 1/2A racing appeared on the scene, it was discovered that scaling the drawings down to 2/3 gave a model which fitted the rules nicely. Note! If Formula I ever makes a comeback, just scale the plans up 1.5 times and you are in business.


Click on plan for a larger image.


This is straightforward as far as the wings and tail are concerned. The wing spars serve as a jig and the wing can be built 'in the hand' or flat on a board to suit your preference. The lack of dihedral and constant thickness ease matters considerably. The original had no washout and this has caused no handling problems.

However, the fuselage is a little more cornplicated and should not be started until the wing has been built and the aileron horns installed.

Having cut out the fuselage sides and added the 1/16 in. sheet doublers, mark the positions of all formers and (most important) the thrust line on the inside of both sides. Next, mark the thrust line on formers F5, 6, 7 and 8. When this has been done, cement the formers to one fuselage side, on the thrust line only. Do not curve the side at this stage.

The fuselage side, complete with formers, should now be threaded on to the wing from the appropriate side and the other fuselage side threaded on to the wing from the opposite side. The second side should now be cemented to the formers at the thrust line as before.

When this, by now complicated, assembly has set, the fuselage sides should be joined at the rear and the sides curved and cemented to the formers. This is best done, by wrapping with Sellotape at each former location.

The remaining formers should then be added, working progressively forward to F1.

After all formers have set and the fuselage structure checked for straightness, it should be aligned at right angles to the wing and the sides glued to the wing sheeting. This is best done with epoxy to avoid deformation.

The top blocks and bottom sheeting should now be added. The tail and fairings should also be added and checked for squareness. The engine is now bolted to the bearers (yes, I know they are not in the fuselage yet!) using blind nuts or whatever system you prefer. The engine bearer unit is then pushed into the front of the fuselage and the rear of the bearers aligned into FI. It will then be possible to align the engine shaft with the thrust line marks on the front of the sides. Trim the top block to suit and, after ensuring that you cannot glue engine bolts or engine in place for eternity, epoxy the whole lot into the model.

When set, remove the engine and fill in around the bearers with scrap sheet, etc, to make the whole structure rigid.

You may find that the hatch is a little cramped so this may be enlarged to suit.

The wheel fairings shown are still in place on the prototype after some 50 flights from all sorts of surfaces, so do please fit them.

Motor and servos

Left: nose close-up showing tightly cowled Cox TD .049 motor. Good cooling is essential. Right: two function radio installation.


With the CG and control movements shown on the plan, no problems should be experienced, but it is recommended that first flights should be hand launched until you get the hang of it.

The tank shown allows you to stop the motor by rolling inverted but will still permit plenty of aerobatics. The best of luck with your model and please, do try not to beat me. Now then, where's that 'Miss Dara' drawing..

BMPRA 1/2A Pylon Racing Rules

GeneralAll SMAE & BMPRA rules shall apply except as noted below.
AircraftModels should be scale-like in appearance and similar to full size propeller driven aircraft which have competed in closed course or cross country air racing.
EngineMaximum displacement shall be .051 Cu. in. (.85 c.c.).
WingThe minimum wing area including the area displaced by the fuselage will be 200 sq. in. No restriction on planform but wing must have a constant minimum thickness of 7/8 in. from root to tip. In the case of aircraft with rounded tips, the thickness may be tapered over the last 2 In. of the half span only.
Landing GearA fixed main U/C with at least two wheels with a minimum diameter of 1.5 In. will be used.
PropellorsNo restnction on material but must be fixed pitch.
WeightFlying weight less fuel to be 20 oz. min. and 32 oz. max.
Fuselage4" x 2" at the cockpit.

Picture of 'Milibar'

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