Basic structural dynamics in multicopters

[afernan]

I´ve performed a test to determine the Young Modulus of the 16 mm carbon fiber tubes that I use in my copters. They comes from following shop:
http://rc-innovations.es/Accesorios/Fib ... tubo-16mm-

Result is E = 54.000 N/mm2.

Probably other CF tubes will have similar E values, but can be also very different! . So the best is to perform the simple test described in the document annexed

Angel

[Trnquill]

Few things are still missing to make the process super simple for rest of us:

1) What software to use for analyzing the vibration file? (Or if someone knows an app which shows the frequency reliably straight up in the phone, please do tell!)
2) You could solve the equation for E so that given all the known variables one could simply type in the values and get the E. Now one has to do the solving part, too. (Although there are great online (and desktop) tools for tasks like this, try http://web2.0calc.com/ for example)

[afernan]

Below a video showing modeshapes of modal frequencies of arms and legs moving corresponding to my HEXA monoblock desing. It worths to see how it moves and frequency values.



Enjoy

[afernan]

@Trnquill:
In the document attached before showing the test performed in a CF tube, I´ve shown two ways of analyzin the accelerometer signal taken from the smartphone:
1.- Very simple by "counting waves". Afortunally we are dealing with low frequencies. Frequency = number of waves per second.
2.- Much more complex using FFT (Fast Fourier Transform (pease read in the web abouth that). I use Matlab.

To compute the "E" you can use an excel file. I attach the excel I use to do that. I guess is auto explicative, if you know what to do.

I offered before to compute "E" for tipical tubes you all use, for instance Flyduino or Viacopter ones. But I need to get a 1m length tube to perform the test (shorter ones are not valid). This is one time task for all. Maybe Paul and Jussi are interested.

For tubes from RC-Innovations I did it and reported the E value already.

Angel

[afernan]

Main guidelines to design your copter free of frequency problems. Summary

1.- To be safe, use soft arms and legs combined with high RPM (small propellers). This is the MK style: small tube section (10x10) and long alu tubes. Then you´ll have the excitation frequency f(motor) high enough from arm ones. Typically we can get f(arm) = 20-30 Hz, f(motors) > 100Hz.

2.- As you increase the propeller size, rpm decrease, so freq.excitation. For instance a typical HEXA (mass = 2.5Kg, propeller = 12x4) f(motor) = 60-70Hz. With arms around 320mm, 16mm we have f(arm) = 35 - 40 Hz that is OK. Is suitable in this case to have longer arms to reduce more the arm frequency. Don´t use large diameter arms (18+mm), or shorter arms since this will increase f(arm). Torsion is not an issue (very high at f>400Hz).

3.- For propellers > 15" a lot of care must be taken, since the f(motor) can be very low (<50Hz). So we need to decide to put f(arm) freq behind or below f(motor). I prefer below (soft and long arms), it´s safe.

4.- Last but not least, the legs: usually we want to be behind f(motor). So use short, light and stiff legs. A typical "T" leg, 200mm long has f(leg) > 100Hz, that´s good.

Hope this will clarify general design ideas.

Angel

[Astudillo]

Hi Angel,

Thank you very much for the excel sheet!

I think you ment to show this video


Cheers,

Ray

[afernan]

[chschmid]

Hi Angel

Thanks so much. I just could verify that my octo is within a good range with 32.9Hz (boom) / 69Hz (Prop).

Cheers
Christof

[afernan]

If someone wants me to make the numbers, you only need to give the data, and I´ll do it for you.