Type Half Prop-Blade
For additional information on this topic see VW Type 1 & 2 Prop-Blade.
An ideal prop for a 1/2 VW will have a maximum WOT rpm of no more than 3400rpm. It is recomended that static rpm on the ground not be less than 3000 rpm. There is a big difference in thrust between 2800 and 3000 rpm perhaps as much as 20-30%. A propeller is not just chosen for an engines horspower and rpm, it must also match the aircraft it is being used with. Selecting the proper propeller is half art & skill, and the other half is trial and error.
It is usually best to work from the advice of others that own the same type aircraft and make fine tuning guesses from there.
Mosler Prop Sizeing & Performance
W hat performance can be expected from a Mosler 40 (1083cc?) The Mosler manual lists 3600rpm as "redline" and states it will make 40HP at 3200 rpm (though some say 38HP is more realistic).
On one Fisher 404, with a prop labeled 56x27 a pilot reported 3000 rpm static, and 3200 rpm at 60 knots. Frank at Performance Propeller said it should be at least 3200 static and 3600 WOT. The builder of the Fisher 404 tried other props and noted: The best performance was with a 56x24 or 54x27 prop. two other props were tried; a 55-24 & a 58-22. Both props where a little weak. In testing he got 150-200 more rpm with the 58-22 but less performance. Be aware these small 2 cyclinder VW engines do not like heavy props. They hurt performance & in some cases the motor.
Lightweight wood with a hi aspect ratio is best. The 58-22/24 Peery prop is a good example of what is needed. Mosler recommends a 54-24 prop. It may be OK for an ultralight, but not for a heaver Fisher 404. Using it caused a loss of 1/2 ROC & 10 MPH cruise.
Other 1/2 VW
My 1/2 vw at 38 hp came with a 54x24 prop installed on it. I would see 3100 rpm's in cruise flight at WOT.
Scott Casler from Hummel engines provided one builder with the biggest 1/2 vw he makes - about 45 hp. Scott told me the engines that he see's comeing back a lot are ran at or over 25 on the MP gauge. He recomends running at or under 24 and the engine will last a long time.
Tennessee props makes props in a narrow tip and one as a regular tip. Scott told me to stick with the narrow tipped blade or prop. I tested both the narrow and regular tipped Tennessee prop. The regular tipped prop produced the rpm's I wanted but not much else. I was seeing at WOT in cruise around 3200 RPM's.
Next, a order for a narrow tip prop from Tennessee prop was made. Now in Cruise at WOT, the airplane was getting around 3650 -3700 rpm's. This engine was not happy running this fast, nor was it good for it. Using the narrow tipped Tennessee prop with manifold pressure of 32 gave over 3600 rpm's. At around 23 on the MP gauge I was seeing around 3300. At 21 on the MP gauge I was seeing 3100. The engine was free spinning to much. I had been dealing with oil pressure fluctuation and now its even worse. I knew this engine was under loaded. I was lucky to burn 1.2 gallons an hr. which showed I was not getting all available hp out of the engine.
Using the regular tipped Tennessee prop. At 32 on the MP gauge at WOT in cruise I was seeing 3200 rpms. At around 23 on the MP gauge using this prop I was seeing around 2800 RPM's. But my speeds sucked or was lousy. I knew this engine could do better.
Steve at Tennessee propeller was amazed my little engine is seeing these numbers. We come to the agreement I need more bite in my blade. Neither one of us wanted to see 3500 in cruise at WOT. We both agree that more on the lines of 3200 is what I want. So he makes me a 54x28 prop. What a difference this prop makes. Now at WOT in cruise I am seeing around 3100 rpm's. At 23 on the MP gauge I am seeing around 2700 rpm's. WOT cruise speed has gone up by almost 10 MPH over any other prop I tried.But I still was not happy.
In cruise I want to see around 3200 RPM's and in climb out I want to see around 3000 RPM's. At 54x28 I was seeing around 2900 on climb out. So I remove one inch off each blade to make this prop a 52x28. I re-balance the prop and install it and go for a flight. Now in climb out I am seeing 3000 rpm's. In cruise flight at 23 on the MP gauge I am seeing around 2800 RPM's. At WOT in cruise I am now seeing 3200 rpm's. Just what I was looking for. My speed at all throttle settings went up by about 7-10 MPH. Fuel consumption went up to what a 45 hp engine should be, around 1 3/4 gallon an hr. This is telling me I am using almost all the power from this engine. My oil fluctuation went away. Oil temps are perfect as is CHT's.
The Culver 54x24 was by far the smoothest prop. My engine never ran so smooth as it did with a culver. I mean silk smooth. But the 54x24 Culver prop did not have much thrust. My speeds went down by 10 mph across the board. Meaning at WOT in cruise and at partial cruise settings.
If you have a 38 HP 1/2 vw I would stick with the 54x24. But if you have the 45 HP version of this engine, 54x24 just is not enough prop to make this engine happy. IMHO 3500 RPM's is just to many revs for this little engine. Steve at Tennessee propeller agree's with this. As does my little engine.