Props absorb power proportional to the cube of the RPM. If you size your prop correctly so that it allows your engine to turn at 3600 RPM at wide open throttle at maximum level flight speed (Vh) then throttling back to 2700 RPM is only 3/4 of the RPM: you'd be throttled back to 0.75^3=.422 or 42% power to achieve that 2700 RPM and you'd only be making 7.6 hp. Can your airplane fly on 7.6 shaft hp? Remember, your thrust horsepower will be reduced even further due to propeller efficiency. A 60% to 70% efficient prop would not be an unreasonable estimate in these circumstances; can your plane fly on 4.5 to 5.3 Thp? Can it do so at your desired 65 mph?Your ”18hp” engine will be a 14hp engine at the reduced rpm. You will need to spin the engine at 3,600rpm to get 18hp. If you want 2,700rpm AND 18hp, the engine needs to be around 550cc, then throw some extra in for DA. B&S 38 (24hp at 3,600rpm) would be about right.
We have 59" 3-blade propeller, as well as 51" both ground adjustable, so you can find the right pitch for your engine. My estimation - 51" is more optimal than 59" for your RPM. You can cut the blades 1"-2" if necessary.I forgot to note that I'm building a 2/3 scale BF-109E-4 so to stay scale I want to use a 3 blade 60" prop...
Exactly. That is why I advise to start from 51" Our blades are not large, I think 3-blade will be OK for 18 HP. If not, we have 2-blade hubs for experiments.The short answer is, “Describe your application to the prop maker and ask what they recommend.” But even then it will be a guess for a new design.
Classic propeller design starts with setting the diameter so the tip speed will be 0.8 Mach or less at max power rpm. At 3600 rpm that would be 57”. Next, the pitch is chosen so the blade will be operating at its best L/D AOA at the design airspeed (usually full power climb or cruise). Finally, the blade chord and/or number of blades are set to absorb all of the engine’s shaft horsepower.
That said, I suspect an 18HP engine will have trouble turning a 57” prop at 3600 rpm, you’ll almost certainly have to go smaller and/or use two, not three, blades. If you can swing that large a prop the blades will probably be toothpicks.
As others have said, it will probably require testing several pitches and/or diameters. Starting with an adjustable prop can be a good way to go. Non adjustable wood props can have the pitch reduced to some extent (I did it twice on my Hatz prop) but not increased.
I estimate this prop for an electric aircraft (weight shift trike) right now. Almost the same numbers. If I have 18-20 HP for takeoff, 7-8 HP is more than enough for horizontal flight. We all know BF-109E has good aerodynamics. It will probably need even less HP for horizontal flight.Bottom line is that a 18hp engine rated at 3,600rpm will not deliver 18hp at 2,700rpm regardless of what prop you fit. If you want to cruise at 2,700rpm, then you need a bigger engine if you also want 18hp. You can’t have both.