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Help! So many questions about propellers

#1
I am new to the this forum, and new to RC flying in general. I have a few planes that were purchased, but recently received a used Airfield RC 800mm F4U Corsair from my uncle. I don’t have any specs for the plane other than what I can find online.
Here’s is what I could find about the plane and parts: https://www.nitroplanes.com/95a302-800-f4u-blue-rtf-24g.html
My question is: what size prop should I use? I know the plane originally came with a four blade prop, but don’t know the numbers. The motor is a 3128 out-runner brushless KV1300 so can someone tell me or perhaps explain how I can determine what size prop to use? What diameter/pitch/number of blades?
This might be a complicated question, but any help will be greatly appreciated!
 
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Merv

Well-known member
#3
My question is: what size prop should I use? ....The motor is a 3128 out-runner brushless KV1300 so can someone tell me or perhaps explain how I can determine what size prop to use? What diameter/pitch/number of blades?
I would start by looking up the motor. See if you can find any recommend props from a seller. Sometimes sellers will give a range, something like this motor is good for 2-4S and the props 10x6 to 14x5. The 12 inch prop is for 2s & the 10 inch prop is for 4S. Something in between, say 12 inch for 3S. In addition, look for the maximum amps the motor is built for.

You prop size will change by the cell count of the battery you use. Each motor has a Kv rating, that is how many revolutions per volt. The more volts the faster it will spin. Spinning faster makes the prop pull harder & draw more amps. Which means you need a smaller prop will a higher cell count battery so you don’t exceed the maximum amp draw of the motor.

Let’s say the motor can spin a 12 inch prop but the plane sits too low to the ground. There is only enough ground clearance to swing a 10 in prop. That’s when you would look at a 3 or 4 blade prop. You need more thrust but can’t have a larger diameter prop.
 
#4
I would start by looking up the motor. See if you can find any recommend props from a seller. Sometimes sellers will give a range, something like this motor is good for 2-4S
Like I said, the plane was given to me used, so no manual. I found the plane on NitroPlanes but it seems like the plane is out of stock/discontinued so I can't get the replacement propellers.
https://www.nitroplanes.com/95a302-800-f4u-blue-rtf-24g.html

https://p11.secure.hostingprod.com/@hobbypartz.com/ssl/ibuyrc/manual/95A302.pdf
I found a pdf of the manual, but it only shows the parts for assembly, not the specs of propellers. The motor on the plane is unbranded and the manual doesn't give a brand either but I know it has a 3128 out-runner brushless KV1300. I also know that the plane has a Hobbywing Skywalker 20A ESC with T-plug and I was given a couple of 2S 7.4V 35C 1500mAh batteries with T-plug to go with the plane.

I've tried to comp other KV1300 motors to see what propellers are suggested, but the sizes vary. I know the plane originally had a 4 blade prop, but I would prefer to switch to a 2 blade prop as they are easier to find, but if the clearance doesn't allow, I could also use a 3 blade. Do you have any suggestions?
 

FastCrash45

Well-known member
#5
Like I said, the plane was given to me used, so no manual. I found the plane on NitroPlanes but it seems like the plane is out of stock/discontinued so I can't get the replacement propellers.
https://www.nitroplanes.com/95a302-800-f4u-blue-rtf-24g.html

https://p11.secure.hostingprod.com/@hobbypartz.com/ssl/ibuyrc/manual/95A302.pdf
I found a pdf of the manual, but it only shows the parts for assembly, not the specs of propellers. The motor on the plane is unbranded and the manual doesn't give a brand either but I know it has a 3128 out-runner brushless KV1300. I also know that the plane has a Hobbywing Skywalker 20A ESC with T-plug and I was given a couple of 2S 7.4V 35C 1500mAh batteries with T-plug to go with the plane.

I've tried to comp other KV1300 motors to see what propellers are suggested, but the sizes vary. I know the plane originally had a 4 blade prop, but I would prefer to switch to a 2 blade prop as they are easier to find, but if the clearance doesn't allow, I could also use a 3 blade. Do you have any suggestions?
Like I said, the plane was given to me used, so no manual. I found the plane on NitroPlanes but it seems like the plane is out of stock/discontinued so I can't get the replacement propellers.
https://www.nitroplanes.com/95a302-800-f4u-blue-rtf-24g.html

https://p11.secure.hostingprod.com/@hobbypartz.com/ssl/ibuyrc/manual/95A302.pdf
I found a pdf of the manual, but it only shows the parts for assembly, not the specs of propellers. The motor on the plane is unbranded and the manual doesn't give a brand either but I know it has a 3128 out-runner brushless KV1300. I also know that the plane has a Hobbywing Skywalker 20A ESC with T-plug and I was given a couple of 2S 7.4V 35C 1500mAh batteries with T-plug to go with the plane.

I've tried to comp other KV1300 motors to see what propellers are suggested, but the sizes vary. I know the plane originally had a 4 blade prop, but I would prefer to switch to a 2 blade prop as they are easier to find, but if the clearance doesn't allow, I could also use a 3 blade. Do you have any suggestions?
By looking at the prop compared to wing size it looks around a 5.5 in or 6 in prop for a 4 blade. So maybe a 9in prop for a 2 blade prop
 

Merv

Well-known member
#8
This is the motor that comes in the plane. No branding, no info, makes it really hard to know what to use.
That look pretty close to the HK motor. The HK motor is a bit higher Kv & uses 7x7.5 prop on 2s.

I think I would start with a 8x4 or 8x5. A watt meter would be helpful but not necessary. Install the prop and run it at full speed for 10 seconds or so. Stop and grab hold of the motor & esc to check the temperature, warm is ok but if you don’t want to hang on to them, it’s too hot. If it passes the 10 second test, try a 30 second run, then 60 seconds, then 2 minutes. If it passes the 2 minute test, you could try a larger prop if you wanted. If anything starts getting so hot you don’t want to hold it, stop and try a smaller prop.

@FastCrash45 may be right, a 9 inch prop could work.
 
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#11
I’d try the 7 inch prop first, use the test procedure above. If it passes, you could test the 8 inch prop.
Thanks, I will try your suggestions. From what I'm gathering there's no single propeller that must be used as several different sizes and styles can be viable.

I know there can be harm if I use a propeller that's too big, but is there anything wrong with using a prop that's too small other than the plane feeling underpowered?
 

Merv

Well-known member
#12
Thanks, I will try your suggestions. From what I'm gathering there's no single propeller that must be used as several different sizes and styles can be viable.
Correct, for each motor, there is a range of suitable props. There is an interplay between diameter, pitch and blade count that you can change to accomplish what you want. Larger diameter lower pitch props have more thrust at low air speed. Smaller diameter higher pitch props have more top end speed. Think of a transmission, low gear vs a higher gear.

I know there can be harm if I use a propeller that's too big, but is there anything wrong with using a prop that's too small other than the plane feeling underpowered?
No, you may be waisting potential, but you will not burn anything out like is possible with a prop that is too large.
 
#13
Larger diameter lower pitch props have more thrust at low air speed. Smaller diameter higher pitch props have more top end speed. Think of a transmission, low gear vs a higher gear.
Wow, that's a good way to explain it. Thank you so much for the info, explanations, and all the help!