I think I have decided on the engine choice now, but some additional "brainwork" regarding engine and propellers makes the engine choice almost a "no brainer". The reason for this is that finding the right (fixed pitch) propeller for an arbitrary engine must be very difficult. Theoretically you must know the full performance graph of the engine, that is torque vs rpm for all power settings, at least at WOT and max continuous. Then you must know the performance of the aircraft (speed vs thrust) and the performance of the propeller (efficiency vs advance ratio). I don't know any of this, so if I should chose a "non standard" engine, I would have to get at least a ground adjustable propeller, or a CS propeller to have any chance of hitting it somewhere right. For a fixed pitch I would probably have to try several different ones, 10-20 maybe.
For instance. The power needed to drive a propeller is (for one single arbitrary air speed):
P = k * rpm³ * D⁴ * pitch
P is power, k is a constant depending on units and D is diameter.
Great Plains has in the catalog at page 39 a nice note about this. Their 1835 cc engine has a known horsepower of 60 at 3400 rpm. The propeller used for this engine has D = 52" and pitch = 42". I can use this as a base and plug in the numbers for some other engines and propellers. There are more to a propeller than pitch and diameter, but without advance ratio and efficiency this will have to do. The actual numbers here may be off, but the principle is right.
The Aerovee with the Sensenich propeller will output 73 HP at 3400 rpm according to this. This is probably about right, because the larger GP 2276 cc shall output 76 HP, and will do so by slightly more pitch. The Revmaster is a different setup because it is designed to output 80 HP at 3000 rpm. With the recommended Prince propeller, it will only output 52 HP at 3000 rpm, so obviously Prince is wrong. A more "correct" propeller would be 55" diameter and 65" pitch. With the Sauer and ULPower, a usable propeller would be difficult to get right without also extending the landing gear/larger wheels due to their much lower rpm which requires larger D, or by adding more blades.
Regarding "propeller difficulties" the AeroVee is no problem. Sonex has done all the testing. The larger capacity GP and Hummel should also be relatively easy I guess. It is only a matter of adding some pitch. The Revmaster could be much more difficult. It is designed to run on lower rpm. The Sauer and ULPower would be even more difficult, and probably would need new landing gear to get a larger diameter propeller.
For instance. The power needed to drive a propeller is (for one single arbitrary air speed):
P = k * rpm³ * D⁴ * pitch
P is power, k is a constant depending on units and D is diameter.
Great Plains has in the catalog at page 39 a nice note about this. Their 1835 cc engine has a known horsepower of 60 at 3400 rpm. The propeller used for this engine has D = 52" and pitch = 42". I can use this as a base and plug in the numbers for some other engines and propellers. There are more to a propeller than pitch and diameter, but without advance ratio and efficiency this will have to do. The actual numbers here may be off, but the principle is right.
| Engine (at max continous rpm/HP) | HP | rpm | D | pitch |
| GP 1835 | 60.0 | 3400 | 52 | 42 |
| GP 2180 | 70.0 | 3400 | 52 | 49 |
| AeroVee 2.1 | 73.1 | 3400 | 54 | 44 |
| GP 2276 | 76.4 | 3400 | 54 | 46 |
| Hummel 2387 | 79.7 | 3400 | 54 | 48 |
| Revmaster 2300 with recommended Prince propeller | 52.5 | 3000 | 54 | 46 |
| Revmaster 2300 with "correct" propeller | 79.2 | 3000 | 56 | 60 |
| Sauer 2400 UL | 79.7 | 2700 | 58 | 72 |
| ULPower 2600i | 81.8 | 2800 | 57 | 71 |
The Aerovee with the Sensenich propeller will output 73 HP at 3400 rpm according to this. This is probably about right, because the larger GP 2276 cc shall output 76 HP, and will do so by slightly more pitch. The Revmaster is a different setup because it is designed to output 80 HP at 3000 rpm. With the recommended Prince propeller, it will only output 52 HP at 3000 rpm, so obviously Prince is wrong. A more "correct" propeller would be 55" diameter and 65" pitch. With the Sauer and ULPower, a usable propeller would be difficult to get right without also extending the landing gear/larger wheels due to their much lower rpm which requires larger D, or by adding more blades.
Regarding "propeller difficulties" the AeroVee is no problem. Sonex has done all the testing. The larger capacity GP and Hummel should also be relatively easy I guess. It is only a matter of adding some pitch. The Revmaster could be much more difficult. It is designed to run on lower rpm. The Sauer and ULPower would be even more difficult, and probably would need new landing gear to get a larger diameter propeller.
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