Thrust value and tractor propeller for a Hirth F-33


Well-Known Member
Jun 12, 2022
the rear cockpit
We were playing around with a 313 cc F-33 with a 34mm Bing, slide valve, side draft carb. It is attached to a wire trussed aluminum A frame without wings. Engine RPM readings come from a "Tiny Tach," and the 59" x 10* composite two-blade is spun through a 1:2.5 belt redrive.

To determine thrust, we used the NASA approved method-- no not THAT NASA, I mean the "Northern Assinboine Sport Anglers." We leveled the airframe on its main wheels and tied a rope to the elevated tailwheel. The rope went around a firmly set post trapping a bathroom scale that when compressed gave us a reading of just over 190 "somethings--" ostensibly pounds, probably not accurate, or even linear, but definitely proportional-- as the throttle was opened up.

The Hirth performance on a F-33 chart runs up to 6700 engine rpm, but has a plateau from 5500 to 6500 engine rpm where hp remains pretty constant and torque actually decreases slightly and fuel consumption increases 27%. Thrust isn't measured (and of course is propeller dependent).

We seemed to max out performance wise at just a shade over 5750 (2300 prop rpm). By the Hirth F-33 performance chart, 5800 engine rpm is 2320 prop rpm, 28 hp, 25 ft pounds, and 3.1 gph.

Small carb? Restrictive exhaust? Poor propeller choice? Normal?

Does anyone have better numbers or propeller recommendations?
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