Bald head, it's been done in the U.S. with PA-25 Pawnee. In the ~1990's the Fixed Base Operator in Taos, NM had a succession of glider tugs. With field elevation 7094'/2162 metres, Density Altitude was a real factor, commonly running in excess of 10,000'/3050 metres in the good summer soaring season. One of the decent tugs was a PA-25B with a 290 or 300 hp (I forget which) IO-540 Lycoming. It did a good job with all except the heaviest two-seaters, but was difficult to start hot, and cylinders ran a bit hot too. As I recall, it had been converted in Texas by Juan Batch, who was involved with lots of towing, but I may have that garbled. I don't recall which variant of the IO 540 was used, there are quite a few, with subtle to significant differences. It was licenced "Restricted", and probably with a Field Approval. I don't know how that would work in your jurisdiction. [Its successor the following season was a 450hp AgCat, which was quite a treat to tow behind in a Ka6 with a C of G tow release....]
The Call-Air/Rockwell A-9, U.S. Type Certificate A-758, was available with both 290 and 300 hp IO-540 Lycomings, utilising a 90" version of the Pawnee McCauley fixed pitch prop. It is rumoured that these props sometimes have transported themselves onto the nose of Pawnees, in agricultural operations. The Call-Air is somewhat heavier than an equivalent PA-25, and parts availability may be a problem.
There is a U.S. STC by AirCare to replace the standard restrictive PA-25 exhaust system with dual straight pipes (three into one on each side, no mufflers), which claims a 15 to 20 hp increase. I fly one of these with Howard Kron's 250 hp STC, but don't know if the lack of mufflers would be permissible in your jurisdiction; it's a little louder than the stock system, but honestly the prop noise is most of it. Frankly, they're overpriced for what they do, but it's an option, and the system is virtually maintenance free; no cabin heat though. Here's a link:
http://www.aircareinc.us/aircaresystems/systems.html. Now if Larry Vetterman would come up with a more efficient STC'd system....
There is also a VERY cool PA-25 with an IO-540, Alaska Bushwheels and many other clever improvements, which used to show up at Supercub fly-ins in the western U.S., based in Utah. I believe the owner/developer frequents this site, and images pop up regularly on the rotating pictures of
www.supercub.org . Unfortunately, it's licenced "Experimental" (Racing and Exhibition?), so no glider towing in the U.S.
I hope this is of some help. Please let us know how you get on with this project.
Thanks. cubscout
