What prop to use for a 18hp 440cc Duromax engine?

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Enesess31380

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What size and pitch prop do I need for 18hp 440cc Duromax engine? I plan on direct drive, trying to acheive 65mph cruise speed at 2700 rpm. I know the le Pelican and the Quickie had 18hp engines and were capable of that speed.
 

Enesess31380

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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. What I really need to know is what pitch should I make it to make work. Let's assume my empty weight is 250lbs (theoretically it should weigh even less) and I only weigh 115lbs. My plane will be scale so I should have the same lift to drag ratio. I have a 18hp 440cc Duromax gas engine that has max rpm of 3600 cruise rpm is 2700. I would like to cruise at 65mph. I know this possible because both the Le Pelican and the Quickie weighed less than 250lbs empty and were powered by 18hp Briggs & Stratton lawnmower engines and were capable of doing over 100mph with a gross weight over 450lbs. So it's certainly possible. I just need to know how.
 

TFF

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Well it’s not so simple that someone else has used 18 hp so you can too. Both examples are effective at using minimal horsepower. With 18 hp Briggs the LeP used a 43” prop and no way it went 100 with that engine. Maybe the 35 hp version.

So 43” is a starting point. You can add up pitch for the speed you want, but the engine might not pull that much pitch. I’m just guessing around 20” is ballpark for your speed. That’s the ballpark guess answer

What you will probably have to do is make your first best guess prop and see. Probably making your own props would be a thing to do. You can try this and see what they say. Culver Props Page
 

TiPi

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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.
 

davidjgall

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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.
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?

Other thoughts: In this realm, you really don't get to choose your propeller based on scale appearance. As noted above, a 43" diameter is typical for this hp and RPM. Also, the prop efficiency of a two-blade prop in these conditions is already poor due to the small size (Reynolds' number); specifying a three-blade prop would be pure folly.
 

Air Trikes

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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...
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.
 

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Dana

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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.
 

TiPi

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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.
 

Air Trikes

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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.
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.

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.
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.
 

Jsample40

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Good luck with your project!!! I may have a viable prop solution for your aircraft. Reasonable price.
I recommend a ground adjustable variable pitch prop... Having just replaced a smaller KFM engine with a Rotax 503, my new 54" IvoProp carbon fiber prop with SS spinner will not fit. The KFM turns clockwise (viewed from the cockpit in tractor configuration), while the new Rotax I have just mounted turns counter clockwise .... thus I have had to replace a brand new prop. If you are interested, contact me at JSample40@gmail.com.
 
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