### Propeller and Motor Selection

NOTE: The propellers considered in this section are not the ideal props for quadcopters. Fast propeller response time is necessary for good stability and performance which means that a low mass, low rotational inertia prop should be used. **APC Slow Flier props should be used instead of APC E props.** New values of propeller constants and power factors for APC Slow Flier props can be found at the link given below. The process for finding the ideal propeller and motor for your system (mathematics and physics) is still the same. Just plug the correct values into the downloadable spreadsheet.

https://www.aircraft-world.com/Datasheet/en/hp/emeter/hp-propconstants.htm

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Now that a method for calculating static thrust, an understanding of DC motor power, and an estimated aircraft weight has been established, the proper propellers and motors can be determined. As mentioned earlier, all components are to be bought from Hobby King due to selection and cost. Table 2 is a condensed list of motors from Hobby King along with all the necessary motor parameters. A more extensive list of motors from Hobby King is located in the appendix.

As discussed earlier, the ideal rpm of a motor is 1/4 its maximum rpm. The ideal rpm of each motor is located in Table 2.

The next step is to determine the ideal rpm of the propeller. Ideal rpm for a propeller is found by combining Equations 1 and 6 and solving for rpm. Equation 9 shows the result of this mathematical manipulation.

The mass that is entered into Equation 9 is the estimated mass of 809g divided by 4 because there are four motor/propeller sets that contribute to lift. Table 3 shows the results of Equation 9 for a range of different APC E propellers.

From Tables 2 and 3, it is clear that the APC E 8×6 propeller is the best match to the given set of motors. To be more specific, the 8×6 propeller has an ideal rpm that would support hovering for the given estimated mass of the aircraft, and the ideal rpm of the propeller closely matches the ideal rpm of the motors listed in Table 2.

In addition to the analysis performed above, there are other considerations in choosing a motor, like price and availability. Ultimate the Turnigy 2730 Brushless Motors with a 10 A maximum current and APC E 8×6 propellers where chosen for this project.

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## Relevant Files

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I tried to use your ideal rpm eq. I used all of the information provided to double check my own problem. Using table 3, row 1 and eq 9, I get an ideal rpm of 4.33.

Any ideas?

nevermind i found your file and figured it out

Thanks for checking out my blog. It’s good to hear that you found the info you needed. Please note that my blog will change to suggest a more efficient propeller. The process for selecting a prop will be the same (mathematics and physics), but APC Slow Flyer props are much more suitable for quadrotors.

How can I use the ideal RPM equation in case of a 3-bladed propeller?

Good question. My Ideal RPM equation involves constants which are found at Aircraft-world (link on this page). The values of Power Factor and Power Coefficient given by Aircraft-world are found empirically. Equation 1 on the Static Thrust Calculation page of this blog shows the power equation which is used in subsequent calculations. If you are able to find any equation of power with respect to RPM for your 3 blade propeller, you could follow my mathematical manipulation to achieve a new Ideal RPM equation. Note that there are online calculators that you could use, but these simple plug and play calculators should not be used for writing a report or for understanding the process of propeller selection.

Is it possible to estimate the PC and PF for 3″ and 4″ propellers? I Need the ideal RPM for a quadcopter with estimated weight between 100 and 150g and 3″ Propellers……(max 4″ prop). Thank’s for the help

Andy

Hi Andy, sorry for the delayed response. Check out my April 21st comment on the static trust calculations page for a process to use other power equations. Hopefully Duc works out the equation for use and I’ll post it. You might be able to find resources on the props that you are using.

Well, in your motor parameters excel file (where you have calculated the ideal rpm), you have two sets of calculation for the ideal rpm: for the first one, you use the diameter of the prop. measured in meters and in the second set of calculation, you have used the diameter of the prop. measured in inch!

If I use a motor form hobbyking: D2830-11 (1000kv), a quad which has a 1.35kg, then the second table (where the diameter of the prop. is measured in inch) is more suitable…

So, how should I proceeded? Which sets of calculation is the right one? The one where the diam. of the prop. is measured in meters or the one where the diameter of the prop. is measured in inches?

Thanks,

George

Hi George,

Thanks for including your name and checking out my Blog. The set of calculations using inches was accidentally left in the spreadsheets. I was using it to verify my meters equation. So, do not use the calculations with inches. It will be very wrong, leading to a disappointing build. I’ve updated the files. Thanks for catching this, and good luck on your design. Let me know when you’re finished and I’ll add a link to your videos.

Hi,

I have to questions about the motors. First I saw that you are multiplying kv times 11.1 in order to find the max rpm. Why are you using that number?

On the other hand, how can I choose the motor and propellers if my mass is not constant? Lets say if I want my quadcopter to deliver things. In that scenario, the mass of the aircraft will be constant but the mass of the package being delivered will always be different. How can I achieve hovering?

Thanks!

Hi Jorge,

You are the first person to ask this question (Well Done!!!). I multiplied KV by 11.1 because I used a 3S (3 cell), 11.1V Lipo battery. Because KV is given in rpm/volt, multiplying KV by 11.1 will give me the maximum, zero torque revolutions per min.

As far as your question about mass, this is where your engineering judgment comes in. The method that I’ve developed will lead you to a system with peak hovering efficiency given a constant mass and 50% thrust. This does not mean that the aircraft will not work if weight is added or removed. Keep in mind that I use the word “efficiency” instead of “performance” for a reason. For example, decreasing the weight of a craft will usually increase the craft’s performance (more maneuverability) while increasing the crafts energy consumption (less efficiency). Check out the DC Motor section of this blog to get a better understanding of this concept. Here is a better question: How much weight can be added (or removed) and still have a quadrotor with acceptable performance and efficiency? Another way to tackle this problem is to calculate the maximum expected weight then design to that weight. Flight using a lighter payload will result in a less efficient system, but can you live with that?

I’m always interested in my reader’s progress, so please keep me posted on your project and feel free to ask for clarification on my response. Thanks!

Hi,

I’m still working on this project and I’m having a hard time working with batteries. You said that you multiplied the Kv times 11.1 because you were using a 11.1V Lipo battery. But, how did you know which battery to use before even choosing a motor? On the other hand, where can I find a list of batteries with the specific information I need? Thanks!

Hi Jorge,

I suggest you follow my process. First I just picked a battery by estimating flight time, designed a frame that could hold the battery, estimated total weight, then picked some props and motors for that weight. This is an iterative process, so if you find that your system is too large (heavy and large) and you wanted a small, quick machine, then reiterate the process, choosing a smaller battery. Most of your weight comes from the battery.

I suggest looking at Hobby King because they have a large selection to choose from. Use the Battery and Flight Time page of this blog to help you decipher battery specs.

http://www.hobbyking.com/hobbyking/store/__86__85__Batteries_Accessories-Li_Poly_All_brands_.html

For your design, I would definitely check out the systems from readyquadcopters.com. They have some beautiful designs there.

http://www.readyquadcopters.com/

I hope this has helped. Good luck and send me some links to your project.