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Building a Tricopter - build thread

#1
I've been tinkering with a scratch built tri for the past month or so, experimenting with various set ups, power systems, control boards etc.

This is my chronicle on my endeavor.

Why
I wanted to get into doing aerial photography/videography and eventually FPV. A multi rotor is a great platform for both. Why not a quad or a hex, or even a Y6? I liked the general simplicity of the tri and its portability and its initial setup cost. I wasn't sure I would be into it for the long haul and didn't want a big upfront investment. I've learned a lot along the way and thought I would share it here.

Version 1
My thought process was to utilize smaller props, thus generating less vibrations, less rotating mass. I decided to use a 1400kv motor (HobbyKing 2826-10) with the assumption that I could go with an 8" prop and a 3s battery. After much testing with various prop sizes, 8x3.8sf is the best with this power, or a 9x6sf works pretty well with a 2s battery. There are other limiting factors that makes this set up less than ideal

photo.JPG

So here is my setup for V1
Frame: 1/2" Aluminum channel / 3mm Lexan
Motor: HK 2826-10 1400kv (http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=12920)
ESC: HK 30A RedBrick (http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18004)
Controller: HK KK v3 (http://www.hobbyking.com/hobbyking/...i_Rotor_Control_Board_V3_0_Atmega328_PA_.html)
Tail Servo: HK HKSCM12-5 digital 10g servo (http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=16261)
Battery: Zippy 25c 2200mah 3s (http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=7636)
Final model weight - without battery : 724g (1.6lbs)

I will go into detail about the pros and cons of such a set up and what worked and didn't work for me. I will say that none of the initial parts are being used on the final or V3 copter.

Pictures and video to come...
 
#2
A note about motor/prop selection

My initial idea about running smaller props for less vibration did prove to work. The vibrations were kept at a minimum, at a price.

I hadn't considered the limitations of the propellers. In order to generate enough lift, I would have to go with a slow-fly prop, the e-props are much more efficient, but don't produce enough lift. I was at 3/4 throttle with some e-props just to get it to hover.

Using RCExplorer's tricopter setup as a baseline, he uses 10" or 11" props, I wanted to use 8" or 7" props. I quickly found that my RPMS were exceeding the propeller's ratings.

In general for SF props max RPM calculation is:

65000/d=limit

Where d is the prop diameter, so for my 8" prop

65000/8=8125 Max

My motors are 1400kv thats 1400 rpms/volt, so with a 3 cell battery we have:

1400x11.1=15540*

*This is nominal with no load and not completely accurate, but its a good baseline. I'm using the voltage rating for the battery, but remember a fully charged batter can be around 12.4v. So what does this mean?

Everybody's favorite: In flight prop failure. I found that APC props can withstand more but its not by any means a guarantee that they won't take a crap in the air, and they draw more amps. 9x4SF were at the upper limit of my ESC's.

Lesson learned. Pick a motor/prop combo that is right for your model weight and intended use. Or just use proven setups like that of RCExplorer's.

I've since learned that this rig will fly pretty well with the EP style GWS propellers which are rated at 10000/d RPM, and I found that a 9x5DD or EP prop (The ones with the squarish tips and straight shape have a thicker base at the hub) will do a good job of producing lift, but it is still underrated for the RPMs I'm running.
 
#4
My experience is to use a beefier tail servo as they tend to break, at least they did for me. And also use metal gears. But if that one works, fine, they're cheaper.
 
#5
My experience is to use a beefier tail servo as they tend to break, at least they did for me. And also use metal gears. But if that one works, fine, they're cheaper.
Agreed. I was going to address that in later posts. But I used that servo initially because I had it laying around. And it worked (at least for a few flights and a couple light crashes). But it started falling apart, literally, the screws had come out at some point and the bottom plate came loose and then the gears eventually stripped.

I'm on my third iteration for this tricopter, still waiting on parts for the final build. I'll go into detail about building the frame in the next post.
 
#6
Setting up your airframe: Angles man! Finding the proper arm angle

A tricopter has a couple different configurations, a "T" primarily used when carrying a larger payload up front, and the more common "Y' configuration.

I'll discuss the Y configuration. The arm angles should be (for optimal balance/stability) at 120[SUP]o[/SUP]. Basically 360/3.

So here is my method for finding the angles without any fancy tools, just stuff everybody has in a drawer somewhere.

1 pencil
1 string
1 ruler
1+ Beer/wine (optional ;) )

Note: You should probably have your arms cut out, or at least an idea of the size of your machine so you can make a proportional platform to mount your arms and electronics on.

You can use some simple geometry to find the arm angles using circles, first figure out how big the fuselage should be. Then draw a line at that length.



Next make your circle drawing apparatus (a protractor/compass also works well for this step). Put a pin on the center of your line then using the string tie a loop on one end anchored by the pin. Then lay it out to one end of the line and tie another loop such that taught string is exactly the radius of the line, then with a pencil on the inside of the outer loop you can draw a circle using the string as a guide anchored to the center of the line.

string1.JPG

You should end up with a circle that looks like this.

Line_Circle.png

Next using the same string, pin, pencil. move the pin to one end of the circle where the straight line and circle edge meet, and draw another circle.

string2.JPG

Your circle should now look like this.

bisecting_circle.png

Now draw a line from the center to each point where both circles meet.

120_deg_lines.png

Now you can see the Y shape taking form, and you've found the anges for your arms, each leg of the Y is 120[SUP]o[/SUP]

From here you can draw out your fuselage shape and cut it out of your chosen material. Remember to mark the center point and the end points, so you can properly arrange and drill for your arms.

frame.jpg