German

Hirth F23 engine

The original Italian KFM engine proved to be not powerful enough for the J-5. Also the exhaust system was too loud, especially with the exhaust pointing directly downwards. So it was clear that I will switch to an state of the art 21st century engine for my J-5.

  

F23 in the box


The dream of  every single seat experimental airplane builder is a 40 hp, 6 cylinder, 4-stroke engine with a weight of less than 20 kg (44 lb) and a TBO of more than 3000 hours with a small price tag. Unfortunately such engines don't exist. This class of engines also is not the focus of 4-stroke engine manufacturers. So we will not see 4-stroke engines which come even close to these parameters. Looking at 2-stroke engines is a different story! Companies like Solo (2489er series), Rotax (447 or 503) or Hirth (F23) offer great engines.

   

looking into the box

In contrast to the 2-cylinder inline Rotax and Solo engines, the Hirth F23 is a boxer. Comparing the mass compensation of these engine configurations we see that the inline engines do have 1st and 2nd order mass moments, while the boxer has almost non. On the other hand the 2-cylinder inline engines do fire every half revolution, while the boxer fires only once per revolution. This means that the Hirth F23 will come with less vibration, but has also a less advantageous moment characteristic per revolution. A damper in the gear is the minimum reaction required.

     

front view

Finally I made my decision in the favor of the Hirth F23. The boxer configuration is a better fit for the J-5, simplifying the cooling design. Also there was an brand new engine offered at a very good. After one short email to Hirth the head of the company called me the same day, asking about the project and ensuring that Hirth is supporting its customers all along the way. Hirth being only 50 min by car from my workshop, all of this looks like a good setup for great support.

   

rear view

Unpacked the first thing which caught my attention was the ignition box. It is in the place where I want to see the intake silencer. So I will have to move the ignition system to the back of the engine.

  

engine position

The next task is to take a mold of the rear fuselage and to build a mockup of the engine installation. But before this, I will become a member of the OUV (the German equivalent of the American EAA). There I'm looking forward to good discussions with experienced builders. During the Aero this year in Friedrichshafen I meet the head of the OUV project committee who has an impressive technical knowledge as well as good contacts into the experimental aircraft scene. The first inspectors report will evaluate the projects chance for success, my workshop and my person as a builder.

 

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