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Help! Which type of 10x4.7 propeller to use with Pack C Emax Motor?

#1
So I'm at the point where I've broken both the 10x4.7 props that come with the FT power Pack C Emax Edition through crashing my Bushwacker. I'm looking to buy a new 10x4.7 prop but can't decide between APC slow fly vs APC Electric. Has anyone tried these? What are the differences and which would be better? My Bushwacker's weight with battery is about 820 grams.

P.S. I also had a dilemma about APC vs GWS propellers but after reading a lot of forums on this matter, I think APC would be more suited as the Pack C motor is well above 200W and has a moderately high RPM. Any suggestions on this would be amazing as well! Thank you.
 
#3
The APC 12x3.8 slow fly work very well on my FT3D. I believe both props will work.
I take it that both would work fine, but what is the difference between the slow fly and electric props? I read that some APC props have a very large and thick hub, but I'm not sure which ones.

Also, I have this nagging question for a while now. As the pitch increases, the distance travelled per revolution increases right? But the thrust is actually decreasing (from motor manufacturer test), so how can the plane be travelling faster?
Does the decrease in thrust just mean a lower acceleration (F = ma)? And the top speed (or effective terminal velocity of the plane) is higher?
 

tamuct01

Active member
#4
Assuming an incompressible fluid (of which air is not), the higher-pitched prop of the same diameter and surface area would travel farther for each revolution. The increased pitch also requires more torque from the motor to move past the air assuming equal drag on the airframe. I don't pretend to know all the fluid dynamics of prop design, but in general, a high pitched prop = more speed, but a lower-pitched prop allows more torque and faster throttle response. That's why the 3D guys like to swing big props with lower pitch to get that instant torque response.

One drawback to higher pitch props and that torque requirement is that it can induce torque rolls on the airframe. I built a Duster once and happily used the 9x6 APC I'd used on the power pack C and it was uncontrollable due to the induced torque roll (short wingspan). I switched to a 10x4.5 and it went away. It's all about tradeoffs and I recommend Propcalc (https://www.ecalc.ch/motorcalc.php) to help you pick the best combination.
 

Merv

Well-known member
#5
As the pitch increases, the distance travelled per revolution increases right?
Correct

But the thrust is actually decreasing (from motor manufacturer test), so how can the plane be travelling faster?
All motors have a maximum wattage limit, that is the maximum power they can output. It takes the same power to move a move a lot of air slowly or a small amount of air quickly. That is a large diameter low pitch prop, big area, slow exit velocity. Vs a small diameter high pitch prop, small area, high exit velocity. As exit velocity increases, you must reduce the area (diameter) to not over load the motor, burn it out.

All props will produce less thrust, as the air speed reaches the exit velocity. They will eventually reach an equilibrium and the air speed can no longer increase. Because the exit velocity from a high pitch prop Is faster, this equilibrium will happen at a higher air speed.

All static thrust testing is done at zero air speed. The larger, lower pitch prop will always produce more thrust at low air speed. A smaller, higher pitch prop will continue to produce thrust at higher air speeds. The question is, at what air speed do you want the thrust.
Does the decrease in thrust just mean a lower acceleration (F = ma)? And the top speed (or effective terminal velocity of the plane) is higher?
If I understand the question, the answer is yes & yes.
 
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#7
Thanks everyone, these prop suggestions really helped.

All props will produce less thrust, as the air speed reaches the exit velocity. They will eventually reach an equilibrium and the air speed can no longer increase. Because the exit velocity from a high pitch prop Is faster, this equilibrium will happen at a higher air speed.

All static thrust testing is done at zero air speed. The larger, lower pitch prop will always produce more thrust at low air speed. A smaller, higher pitch prop will continue to produce thrust at higher air speeds. The question is, at what air speed do you want the thrust.
This made everything really clear, thank you! Just bought a APC 10x4.7 SF propeller. Will try it out soon.