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Propellers – Aero

Preamble

Are you confused by the numbers used by propeller vendors to describe their products, such as 5040? So was I, so here is a brief attempt at explaining them.

Links

See Propeller size nomenclature?

Propeller description

There are multiple brands out there (i.e. GWS, JP, Graupner, Emax, APC, EMP and Falcon, among others), and each brand has its own properties. Let’s consider just two brands APC and GWS. From GWS prop vs. APC prop:

  • APC draw more current than GWS.
  • APC props are stiffer than GWS
  • GWS can flex at higher speeds.
  • GWS work better at lower speeds.
  • Which type provides more thrust seems to depend on who you ask.

Diameter and Pitch

The prop numbers are usually a set of 2 numbers (9×7 (APC) or 9070 (GWS)). The first one is the total prop diameter. The second is the pitch of the prop blades themselves.

Larger propellers/less pitch versus smaller diameter propellers/larger pitch

Generally, a larger diameter, slower turning prop of a relatively low pitch is more efficient aerodynamically and will produce more usable thrust than a smaller, high pitched one turning at high rpm. The larger prop disc moves a larger volume of air per revolution, but the lower pitch results in a lower inflight maximum airspeed (aka pitch speed). Smallish, high pitch props are more intended for fast, sleek airframes, and in extreme circumstances, the blades can actually be stalled when rotating, at low or zero airspeeds of the plane. The pitch on them is such that the AOA of the blades is too steep to produce much lift (thrust) until the plane is moving forward.

Larger propellers are used with motors of lower KV (850-1050), and conversely, smaller props are used with higher KV motors (2200 – 3500, 5600

Useful tools

Here is a good tool to find the best propeller, called Optimal Propeller Calculator:
http://personal.osi.hu/fuzesisz/opc_eng/

See also

I’ve almost given up trying to nail down the exact constants of these props. It’s almost like the constant is actually a function itself! The problem is that the GWS and APC seem to act quite differently when you pick up air-speed. The following are strictly personal observations during my time of experimenting with different props on my High Desert. When the good doctor will certainly issue the best “static” results, though.

GWS RS: Frankly, I’m not a big fan of this prop anymore. It’s a good prop for hovering if you are running around 6000RPM or less, but the prop just doesn’t seem to “grab” the air when you decide to move forward. Slightly less current draw than an APC SF, but not near the performance in forward flight. This prop is good for slow-flying indoor planes that require minimal weight and never pick up speed.

GWS HD: This is a surprisingly good prop. Much to my infuriation, my LHS insists on stocking the RS series. The GWS HD has a much smaller prop constant than the RS series, so you can go up one inch on both diameter and pitch to get about the same current draw. This prop is good for those motors that spin at least 5000RPM or more. Seems to grab the air rather well, too. Perfect prop for the higher kV applications.

APC SF: This is currently my favorite series for my low kV motors, like the AXI 2212/34. It has a slightly higher current draw than the GWS during hover, but it really “grabs” the air when you move forward. With the GWS RS, by the time I had enough pitch to get the forward speed I wanted, the hover current draw was too high. The APC SF has a very good range of speed. It’s a little heavier than the GWS props, and it is almost too strong. You run the risk of bending a shaft, so I suggest using a “prop-saver” on this (should be called a “shaft-saver”).

APC TE: This is the APC-version of the GWS Hyperdrive. The performance is pretty comparable, but the weight is horrible. <shrug> Frankly, I can’t really see a substantial performance difference between the GWS HD and the APC TE, and the GWS is lighter, cheaper, and less likely to bend your shaft on landing. I don’t use the APC TE props.

So, I use APC SF on my low kV motors (AXI 2212/34), and GWS HD on my high-kV motors (17T CD-ROM). Sorry, no technical numbers or wind-tunnel tests. I’ve given up on relying on static-thrust data since it only tells you half the story with un-stalled props. If you spend most of your flight pretending to be a helicopter, than the static-thrust measurements will be fine.

and

Here are the weights of various 11-13″ props that I have, measured with a pretty accurate set of triple-beams:

APC 11x4.7SF	13.5g
APC 11x5.5E	21.2g
APC 11x7E	21.4g
GWS EP1260	10.5g
APC 12x3.8SF	18.0g
APC 12x6E	23.1g
APC 13x4.7SF	21.6g
APC 13x6.5E	25.7g
APC 13x4E	25.9g

From Low kv and high kv motor specs comparison

Calculator:

Drive Calculator allows you to compare the graphs of 2 different motors: http://www.drivecalc.de/

and

Here is a very crude comparison.

RPM = Kv * voltage
torque = Kt * current
Kt * Kv = 1 (IN THE MKS SYSTEM) or some crazy constant in other systems.
power = torque * RPM = volts * current (with some constants to fix units of measure)

Give two identical motors but motor A has good magnets and motor B has cheap magnets such that its Kv is twice motor A. At a give voltage and current motor B runs twice as fast and has half the torque. However with a 2 to 1 gear box on motor B it will be identical to motor A. The only difference is the efficiency of the motors.

Given a black box with a motor and gear box inside it is impossible to tell what the motor is.

From motor KV higher or lower?

If you’re going for speed runs, higher kv is better. However, the higher the kv, the fewer LiPo cells you can run. This is because BL motors have a redline. For instance, a Neu 1515 motor can only spin to 60,000 rpm. The rotors are balanced for a certain rpm and beyond that, you risk damaging them.

Back to lower vs. higher kv. The lower the kv, the more torque that motor will produce. Higher kv is less torque, but higher top speed.

and

What batteries will you be running?
If I was running 3s Lipo i’d go for the 3500kv
If I was running 2s Lipo or 7 cell NiMH packs i’d go for the 5700kv

 

 

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