Engines for Sale

Monday, January 31, 2011

OS .30 Wankel Rotary Engine

OS .30 Wankel Rotary Engine
Compact, quiet and almost vibration-free!

The Japanese model engine specialist O.S. Max, the German model building company Graupner and the German automaker NSU, jointly developed a model Wankel engine in the 1970s .

How it works:

Personal Observations:-

I am not sure if the Wankel is considered to be a 2 or 4 stroke design or how the displacement is calculated. Considering it has 3 chambers per rotor and each chamber fires once for every revolution of the rotor or once per ever 3 shaft revolutions. This translates into one firing per shaft revolution, the same as a single cylinder 2 stroke engine, but OS describes their Wankel as a 4 stroke engine.  

Mine is one of the older versions of this engine and it has been my experience that it is extremely powerful for it's weight. I usually run my engine without it's muffler as I find that the engine is relatively quiet in operation. In this configuration the engine makes a very compact scale like installation for models with cowls designed for radial engines.

The engine is easy to start with an electric starter however it may be difficult to start by hand propping because of the low perceived compression at low revolutions, much the same as is observed when hand propping an engine equipped with a Dykes Ring. 

OS 30 Wankel Type 1
Click below to view my old engine run again.

Check inside a running Wankel Engine by clicking on the picture


The OS .30 Wankel is still available today, more than 30 years since it was first introduced in the mid 1970's. The modern type 2 engine has changed little in appearance in that time. OS has added a few improvements such as, more and deeper fins to improve cooling and an improved carburetor featuring a venturi stack to increase power.

49-PI Type II .30 Wankel Rotary Engine

The 49-PI is ideal for boosting power in small-scale model airplanes, reducing weight in mid-size craft and offering  dependable, service in any application.

  • Two needle bearings, a ball bearing and a rotary design result in a very quiet and virtually vibration free power source.
  • A new venturi offers more power by increasing airflow into the carb.
  • A Muffler that can be angled in different directions for increased mounting versatility.


Displacement: 0.30 cu in (4.97 cc)
Practical rpm: 2,500-18,000
Output: 1.27 hp @ 17,000 rpm
Weight: 11.8 oz (335 g)
Includes: glow plug, muffler
Recommended props: 9x6, 9x7, 10x4, 10x5, 10x6, 11x4, 11x5

Type 1 User's Manual

Watch this short video clip of my old Fokker DR1 with its OS Wankel engine.

A special thanks to fellow modeller Jose Maria Gonzalez-Calatayud Saiz for saving and sharing the following data with model airplane engine enthusiasts.

Peter chinn takes a close Look at the 0.S -Graupner Wankel
As mentioned in the November issue, a pre-production model, of this engine was received a few months ago from the factory in Japan for evaluation purposes and, having now run-in the engine, we have put in some time experimenting with different types of plugs, the throttle and starting procedures. Starting As previously stated starting the engine by hand is possible, especially when it is cold. Some sort of starting device is useful for starting a warm engine. Some of the types tried where a modified version of the - pull-cord starter, as used by Fred Militky on the Graupner team, and also a Rand electric starter, kindly loaned by Tony Dowdeswell. Both these work very well (we also tried them briefly, on the flying field on a couple of conventional engines) and will usually cope with Wankel's hot-starting characteristics. Really generous priming seems to bé essential at most times, but, on occasions, when the engine was accidentally stop while we we were experimenting with throttle an application of the starter without priming or choking had it running again. The method employed with C/L speed models of keeping the starter engaged while gradually opening the needle valve until the engine fires and runs under its own steam, did not work with Wankel. However, by holding the electric starter steady with one hand, and momentarily choking the intake with the other, while the engine was turning over, we were sometimes able to get a quick start. This is only possible with the electric starter one needs both hands to operate the mechanical starter.) For cold starting, we found it best to prime all three chambers with three squirts of fuel into the intake with a single revolution of the prop between each squirt. This is essential in the case of hand starting. 

The engine seems to be quite sensitive to the type of glow plug used. The common OS plugs are not suitable. Engineer Kazuhiro Mihaia, who is largely responsible for the development tells us that, at present, the most suitable type of plugs are the old porcelain insulation ones.

The Veco plug, or the Fireball “red plug, both worked well in the first phase of the tests which began during a spell of warm weather in August, and they worked satisfactorily. Later tests were carried out when atmospheric temperature dropped to 54 deg. F. and we found out that engine then seemed to be rather critical to needle-valve adjustment and appeared to be developing not quite so much power. We then substituted the Fireball hot plug but sound and the power dropped off when the battery lead was removed. Franz Kavar had, in the meantime, sent us of his new range of plugs for an opinion and started by trying the “hot version of these in the Wankel, which was quite unsuitable. The engine lost about 800 revs and became extremely critical to mixture control – so much so that it was almost impossible to keep it going. We then tried the Kavan ‘Super' plug, which has a much thicker filament and on this plug, there was a vast improvement and the engine ran extremely well. We also tried a new experimental Kavan plug with a special cone shaped clement coil and this worked even better. There was no power loss on removing the battery lead and the needle could be safely adjusted between rich and lean settings without risk of the engine stopping. This sensitivity to plugs is probably due to the necessity for locating the plug, somewhat remotely from the combustion chamber in a Wankel engine. To allow the threaded plug hole to break through the wall of the rotor housing is totally unacceptable as there would be an excessive gas flow from one chamber to the next as the rotor apex seal traverses the plug hole. The smallest possible hole is therefore desirable and, on the O.S. engine, this is only 2 mm. diameter. 

- Throttle Control 
On the question of throttle control, the manufacturer acknowledges that idling speeds are, at present, higher than they are on or the better reciprocating piston type glow engines. Much depends on ignition and on how well the gas seal is maintained and some improvement may be expected with running in, as with an orthodox engine. Mihara states that the best obtained with prototype engines have been 3,000 rpm, but that 3,500 p.m.is a more reasonable expectation. After approximately 2 hours had been logged and using the special Kavan plug, the safe idling speeds of the engine were 3,300–3,500 r.p.m. on a 10 x 8 propeller and 3600-3800 on a 10X4. 
The engine runs very rich at idle speeds and that is in fact very necessary. We found it best to close the air bleed control completely to achieve this richness. The intermediate throttle range then remains quite reliable he and we were able to vary the speed from idle to full throttle without risk of the engine cutting out. Comments from Fred Militky It was hoped that prior to this article being written we would have the chance to try out the Graupner Wankel engine in the air; a suitable model being available in the shape of a Bill Northrop’s Apprentice, with Digitron proportional (an ideal flying test-bed since it would enable a performance comparison to be made with the Max-S30 engine also 5 c.c.—on which the model has had upwards of a hundred flights). Unfortunately, lack of time, combined with inclement weather, put paid to these hopes, but Fred Militky, who had done a lot of flying with models powered by Wankel engines since he first publicly demonstrated the original Schaegg prototype Wankel in 1967, has passed on some comments on his experiences. ་Confirming the the need for a rich idle setting, Fred tells us that, if the needle-valve is adjusted, in the normal manner, to provide maximum power with the model held vertically upwards, this is just about right for reliable operation when the throttle, is closed. On the subject of running-in, he states that Graupner's policy, generally, has been to run in the engines in the air on an extra rich mixture, after only a brief period on the test bench. The amount of running-in considered desirable has not been excessive and it has been found that the engines generally free off nicely within a couple of hours total running time. Props čustomarily used for the test models are 10 x 4 or 10 x 5. The Johannes. Graupner organisation has been using three Wankel, powered models as part of a sleet of R/C models for flying training purposes under the 2-day UHU modelling instruction scheme for members of the German model trade. Two of these Wankel powered models were used as Graupner Taxis and these were in constant use from early morning,

until the evening on each day with, it is reported, complete freedom from trouble. 

Elimination of Side Seals
Fundamentally, the O.S. Graupner engine follows the basic Wankel design. In the interests of its small size, it makes one change for simplicity and that is in the elimination of the full-size engine's rotor side seals. This means, however that the fit between the rotor sides and the front and back housings is very critical indeed. It cąlls for choosing suitably materials with matched coefficients of expansion (hence the use of a rotor housing of heat treated cast-iron in conjunction with a Meehanite c.i. rotor) and extremely accurate manufacture. It calls for some highly precise flat surface grinding which is a good deal more difficult to achieve than the normal cylindrical surface grinding of a lapped piston reciprocating engine. From a production standpoint, in fact, the O.S.-Graupner Wankel is probably the most ticklish operation ever undertaken by a model engine manufacturer. Rotor The heart of the engine, the rotary piston or rotor is machined, as already stated, from meehanite।n the interests of reduced weight and better circulation of lubricant, it is somewhat less solid than a full size rotor design; the interior being extensively machined away to form a six legged spider form and central, boss to take the 12 mm i.d. caged needle bearing and to form an anchorage for the toothed annulus. The latter, of case hardened nickel-chromium steel has 24 teeth and it engages the fixed 16 tooth gear that is mounted in the center of the rear housing. Apex seals of special cast-iron ride in channels in the tips of the rotor, backed up by leaf springs of high carbon steel. The seals have a corner radius of 1 mm. The complete rotor assembly weighs 44 grams. Rotor Housing The rotor housing or trochoide is machined out of cast-iron and is the heavies part of the engine at 68 grams, but considerable effort have obviously been taken to remove unnecessary weight. The housing is extensively finned and drilled for the eleven assembly screws which sandwich it, between the front and rear housings. The assembly screws, incidentally, are not used to align the three components instead, the rotor housing has two steel tubes which locate in suitable holes in the front and rear housings. The rotor housing contains the exhaust port but not the induction port. Like the Japanese Toyo Kogyo rotary engine use in the Mazda sports car, a side (front housing) port is used. This is virtually essential for an engine relying on its fuel mixture to lubricate all working parts. A radial NSU-Wankel Car engine, would not reach the bearings and gears. Front and 

Rear Housing 
These involve a method of manufacture unique in the model aircraft engine field. They are aluminum alloy pressure die castings with steel wearing surfaces applied be a metal spraying process. This coating averages about 1/16 inch in thickness and after grinding, 

of course, the all-important surface against which the rotor sides must seal. The housing also contains the bearings in which the shaft runs. 

Eccentric Shaft, Bearings and Counterbalancing 
The eccentric shaft—the equivalent of a crankshaft in a reciprocating engine — is of case hardened steel. In the pre-production engine it runs in a 7 mm. i.d. ball journal bearing at the front (production models will have two such bearings) and a 6 mm, i.d. caged needle hearing at the rear. The eccentric shaft has a diameter of 12 mm, and it engages the rotor, as mentioned earlier, by means of a 12 mm, i.d. caged needle bearing. To balance the weight of the rotor and eccentric shaft, the O.S.-Graupner engine has counterbalances fore and aft. At the rear, a mild steel semi-circular counterbalance is keyed to the rear end of the shaft, while, at the front, the machined dural prop driver has cast-in internal counterweights. Carburettor, Cooling-ring, Backplate, etc. The carburettor fitted to the O.S.-Graupner engine is basically a stock O.S. Max-S.30 R/C unit. This is a straightforward barrel type and, with air bleed fully closed, obviously suits the engine quite well. A detachable pressure diecast aluminium alloy finned cooling ring is clamped around the rotor chamber and includes an integral exhaust stub to which a silencer can be fitted. At the rear end, the rear counterbalance chamber is enclosed by a cast aluminium backplate secured with six screws and, to this, is attached the three-point radial mounting ring. In pioneering the manufacture of the model Wankel engine, O.S. are well aware that this is only the beginning and that further development can be expected to yield some big improvements in the near future. Nevertheless, the present engine is a well engineered product that really works. It is certainly the most fảscinating model ảircraft engine to have appeared for a very, very long time,


  1. Where can I purchase an O.S. 49-PI Type II .30 Wankel? I cannot seem to find anyone with it actually available for sale.

    1. Check Ebay. They come up for auction on occasions. But they fetch a good price.

  2. Good review of another clasic and rare engine

    1. Thanks very much for your kind comment Jose and a special thanks for the interesting material that you contributed.

  3. how do one set the timing after motor has been through a bearing change

    1. Can anyone help with this question?

    2. See reply below, it posted in wrong spot.

    3. Re posted from below in case you missed T. Witheld's post.
      Wankel engines are port timed, should only go together the way it was designed. The only way to change timing is by grinding the ports size/ shape.

  4. I need a manual for an OS Wankel I-49 like the one on the triplane.
    Anyone know where I can get one?
    Jim in Tucson

    1. You can find the Type 1 manual here:

  5. Wankel engines are port timed, should only go together the way it was designed. The only way to change timing is by grinding the ports size/ shape.

  6. I've got two of these little rotary engines. They came on a huge twin engine plane, with pneumatic landing gear, I'm unsure of the model.. I'd sell them to someone who will love them and use them, I never have started them. Make me an offer. I'm in Washington state. My email is justfourfrp at Gmail.