Karma Stock Propeller Pitch vs MAS Propeller Pitch

[bfarnam]

For those looking at propeller upgrades, it's good to know the stock propeller dimensions and pitch and how they compare to upgrades like the MAS.

Here are are some averages of stock propellers that I measured:

(Numbers have been rounded for ease of math)

255 mm long (10.0 in)
26.5 mm wide (at 75% from hub) (1.0 in)
27.5 mm wide at widest point
6 mm blade height (at 75% from hub) (0.24 in)

pitch = 2.36 diameter height/width

pitch = 2.36 * 10 * .24 / 1 = 5.664"

OR

pitch = 2.36 * 255 * 6 / 26.5 = 136.25 mm or 5.4"

So it appears that the stock GoPro Karma Propellers are somewhere around a 10 x 5.5 prop.

The MAS upgrades are a 10 x 4.5 prop.

Decreasing the pitch by a full inch should decrease the power usage at any given RPM by about 14% giving you longer batter life as well as being a quieter blade.

Performance at any given RPM would be lost which could be corrected by increasing the RPM and thereby negating some of the efficiency gain. However, since these drones are used in mostly "static" flight modes as a stable camera platform, you should have an overall longer battery life.

power = k rpm^3 diameter^4 pitch

For power in Watts, and diameter and pitch in inches, k is about 5.3×10-15 for an average model airplane propeller.

k = 5.3x10-15 = 0.0000000000000053

Stock Blade at 10000 RPM
power = 0.0000000000000053 * 10000^3 * 10^4 * 5.5 = 291.5 watts

MAS Blade at 10000 RPM
power = 0.0000000000000053 * 10000^3 * 10^4 * 4.5 = 238.5 watts

238.5/291.5 = ~ 18%

When using the Static Thrust Calculator, chose the propeller type of APC.

Cheers!

Brett

References

Propeller Basics

The majority of powered model airplanes use a propeller as part of their power system, and electric models are no exception. Some models use a ducted fan to simulate jet flight, and some even use propane or kerosene powered turbines (real jet engines). There are also a very few models that use...

www.stefanv.com

Static Thrust Calculator - STRC

 

[SurfDog]

OK, for those of us who are not rocket scientists; why did GP use the ones they did if there was a better design? What is being "lost" by using an "upgrade". Automotive I understand, swap out a camshaft and ad gears and you can pick up higher RPM HP but you'll lose the lower end torque, etc. What would you lose on the prop upgrade and where?

 

[bfarnam]

This is a complicated question.

Lets define some terms to make this easier to understand:

Pitch: This is a measurement of how far in inches the propeller will move through the air for every 1 revolution as if it was a screw turning into a semi solid like ballistic gelatin.
Thrust: This is the maximum amount of "push" that that propeller can "move" through the air. This is usually determined by the diameter of the propeller and the speed in RPM but can be affected by the width and style of the propeller and for multi-rotor aircraft, this is also affected by the pitch.

I would imagine that GP was faced with the dilemma of drone stability and maneuverability vs. battery life. By decreasing the pitch on the upgraded propeller, you do not have to do as much work to produce the same amount of thrust at any given propeller RPM. This means that you will use less amperage and increase your battery life. The trade off? This would be a decrease in the overall max prop air-speed (not rotational RPM). In a multi-rotor aircraft this translates to the ability to overcome windy conditions and turbulence while stationary and the ability to move into a headwind.

In short: Decreasing pitch increases battery life but also decreases potential maneuverability and top speed.

To put this in perspective, you can increase the MPG of a car by going to stiffer thinner tires (less rolling resistance) but you lose traction in the trade off.

For a drone, by decreasing the pitch you will get an increase in battery life, but you will lose some "traction" in the form of the amount of air the propeller is able to move through the air. For most of us this trade off is barely noticeable during regular flying days. I would imagine in very windy and turbulent conditions, the flying platform will not be so stable. However, GP has a great stability platform in the camera stabilizer so this should overcome most of this. However, the ability to overcome gusts of air and high winds will be effected. How much? I don't know.

Will I buy a set of MAS props? Sure. Will I use them all the time? Absolutely not. Only on calm days. This is why all modern prop aircraft and helicopters have adjustable pitch blades. This allows the pilot (or computer systems) to adjust the prop pitch for maximum efficiency at any given moment.

I hope this helps.

Cheers!

Brett

 

[Shon]

I've been doing quite a bit of research lately on MAS. They are based out of California, and have been in the prop making business for many, many years, longer than drones have been around. They have a loyal following among RC Aircraft hobbyists.

Be careful with cheap Chinese knockoffs...loose a prop...lose your drone...

 

[bfarnam]

Shon,

I used to fly RC Aircraft and am very familiar with MAS and trust them. However, the dynamics involved in drones is very different than RC Aircraft. It is more like RC Helis in the aspect that the propeller directly creates lift vs. aircraft where the wings create lift and the propeller drives the aircraft with sufficient speed to create lift. Unfortunately, the blades in drones are more similar to aircraft props. This is why it is so hard to compare apples to apples.

And I agree 100% with you . . . stay away from cheap Chinese knockoffs!