Printed high speed AVC copter and crash

Hi all,

I thought I would share the copter I hoped/hope to fly at ACV this year. I wanted to try to see what speeds I could get from a quad. I also thought it would be interesting to see what I could to with a 3d printer. I have access to a Makerbot 2x and I was also adding a heated bed to a friends printer. So based on the maximum dimensions of the Makerbot I designed the quad you see above and rendered below.

The quad consists of 4 arms with a semi aerodynamic cross section that is angled forward by 60 degrees. The round nose holds a tennis ball and the bottom swings open using a servo to drop the ball. The battery is held in the center of the frame and the 4 esc's are recessed into the side of the body to provide them with cooling without adding too much drag. The copter is held together using screws and nylock nuts that are pressed into the plastic.

I did some basic calculations to try to get an idea of what angle I was going to be able to sustain. My calculations suggested that I should be able to maintain 60 degrees if the air frame drag stops the copter from moving fast enough to reduce the lift of the props. I found I had to make some guesses about the air frame drag (I used coefficient of drag of a tennis ball as a starting point) and the area of the copter (I think this may have been a little low). In the end I concluded that without wind tunnel testing and a much better understanding of propeller aerodynamics I wasn't going to get too much out of the calculations.

What I did get was a reasonable estimate of the power I was going to need from each motor, 700 W total or 175 W per motor. This was well below the 285 W the NTM 28-26A 1200 kv motors are rated at. It also looks like it may be easier to get higher speeds as the weight of the copter increases. This is because the increased weight means the copter must hold a lower pitch angle at a given speed because of the increased lift required to stop the copter from loosing altitude. I limited my pitch angle to -60 degrees to ensure the copter didn't get too close to gimbal lock. The calculation of the airflow angle over the arms as speed increases and the copter pitches forward. What I found is the airflow becomes dominated y the forward airflow very quickly. In my case the airflow is only 3 degrees off the 60 degree angle of the arms at maximum speed.

So I did an autotune and took the copter out to the local model club to see what I could do. I found the copter would accelerate very quickly and the frame was amazingly strong and ridged. While the flame wheel frame I tested the power train on would bend under full acceleration, the 3d printed frame was a rock. With the 45 degree maximum pids I couldn't take the throttle past about half way without climbing. So I checked over the logs for any obvious problems (other than the one I had) and increased the maximum angle to 60 degrees. I also found that I had to adjust the yaw pids to control the frame a high speeds because it wanted to turn the frame sideways to the airflow. This is a well known effect. Just shoot an arrow without feathers or fire a rocket without fins. I was able to adjust the yaw pids to keep the copter right where I wanted it in the following flight.

The following flight I was able to really wind it out. I was very happy with the performance of the copter. It wasn't getting up to the speeds I was hoping for but things were looking good. After about 3 minutes of high speed passes I saw a clear line of white smoke coming from the copter followed by...... unhappiness.

The front left motor was burning hot and the smoke was the insulation on the winding's burning off. The logs showed that the front left motor was pegged during each high speed run and there was obviously a problem with it. The logs also showed maximum velocity of 107 km/h but it was happy sitting at 90 km/h even into a small head wind. The props / motor / battery combination would max out at 126 km/h on a normal plane.

So overall I was pretty happy with the performance. And I can say that a quad can be very effectively 3d printed provided the arms are designed to take advantage of the strengths of 3d printing. In this case each arm and body section attached to it was 70 g. The strength was high enough such that I couldn't break a arm by trying to bend it in my hands.

So now I need to work out if I can print a replacement before I fly to AVC. I need to print 6 parts, each taking approximately 12 hours. So each is an overnight print and I have 6 nights before I need to leave.... I may need to fly the Y6 :(

Things to remember. (this is a "note to self" but I thought I would share)

Always check the position of the bell on the motor shaft and adjust it to get the optimal location. This problem sounds like a bearing problem in flight. This is also a good time to ensure the grub screw is tightened properly.

After the first flight check the motor output of the autopilot at maximum throttle to see if any motor is consistently maxing out before others. This is the tell tail sign of a motor or esc problem and would have saved me here. I didn't look at this because all the motors were brand new.... bugger :)

Always check every screw and prop before the first flight. This is where little things can go wrong.

Yaw stability in a copter will take a big load of the yaw controller in a high speed copter.

Views: 7264


Moderator
Comment by Gary Mortimer on June 8, 2014 at 2:40am

Oh my hat that's cool you are going to be inundated with requests for the files. If I were you I would concentrate on the competition first!

Comment by Greg Dronsky on June 8, 2014 at 3:01am

Really nice design, i will you post stl's? NTM motors are poor quality, i ve had few, and i heard negative opinions form others, as well.

Comment by Stefan Gofferje on June 8, 2014 at 3:13am

Does anybody make glass-filled nylon filament? :) I was thinking of getting a 3D printer myself for UAV-stuff and the stability of the material was kinda concern for me.

Comment by Julien Dubois on June 8, 2014 at 3:28am

Very nice design Leonard!

I would have opened the bottom swing laterally (from left to right for example) because here, if you open it at a decent speed, I fear that will pull the swing or nosedive your drone.

For maintaining altitude at high pitch angle (60°), maybe some little ailerons both sides of the copter would help.

Good luck for AVC!

Julien

Comment by Tipu on June 8, 2014 at 3:53am

Bravo.

Very Cool.

Are you going to share files ?


Developer
Comment by leonardthall on June 8, 2014 at 3:56am

Thanks Gary, yeh, with getting the code ready for ACV and making sure I have something to fly it on means I need to keep my eye on the ball :)

Hi Greg, I have had 12 of these NTM's and not had a problem so I can't claim quality. Although I will say that they do get a bit slack when placing the bell on the shaft. In general though I am very happy with the value for money of these motors and even after this incident I am not overly shaken.

Hi Stefan, No glass-filled nylon yet 3d printed abs is very strong if used correctly. The designer needs to understand how to create and use that strength though.

Hi Julian, yeh I looked at that option but the mechanical drive to do it was to big and complex so I went for the simple option. I figured I can always drop the ball while stationary instead of on the move if it turned out to be a problem. It wasn't the pitch that it was having trouble with, it was maintaining the yaw. It would suddenly "spin out" when going fast. A simple rudder like tail on the back would make the copter behave much better when going fast.

Thanks for the comments all :)

Anybody have a makerbot 2x and live in Adelaide :)


Developer
Comment by Max Levine on June 8, 2014 at 4:21am

Love that frame :)

@ Stefan Gofferje
Carbon Fiber Reinforced PLA 

Comment by Julien Dubois on June 8, 2014 at 4:36am

Yeah, I'd got your yaw issue. I suggested the ailerons also to reach higher velocity by having enough lift at high pitch angle.

As you said, a kind of plane tail would help for both cape satbility and sustaining altitude on high pitch angle.

Comment by Adrian on June 8, 2014 at 6:15am

It was definitely awesome to watch fly... It is good to see someone who has taken some time and effort to design a purpose built quad frame.

I too have used the NTM motors on my quad frames and I never had any issues. One of my FPV frames is near the 2kg mark and the NTM motors work well..

Hopefully soon the frame can be rebuilt (or in this case, re-printed) and get some video of its performance.

Good luck at AVC2014!

Comment by Greg Dronsky on June 8, 2014 at 7:08am

Leo, the main downside of NTM's is poor engine balance quality, my friend buys them, and first that he does is rebalancing. One of my engines just cought fire and died 5 minutes in maiden flight...

Are you going to post .stl files? I really like your design, and would love to print it my self :)

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