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: 7341

Comment by Rob_Lefebvre on June 10, 2014 at 7:35am

I've actually thought about using the DJI 28" props, sawn in half, on a 450 heli.  Should be more efficient than the standard heli blades.  But those props are so stupidly expensive...  $10 for a set of 450 blades, vs. $200 for the DJI's.  Huh?  Seems a bit much.

Comment by Vince Hogg on June 11, 2014 at 5:17am

My curiosity got the better of me so I decided to do a little experiment tilting my motors on the carbon quad.

I think if I was not limited with the space below the trees it would have got quite a shift on. One of the support bands snapped half way through, hence the rattle.


Developer
Comment by leonardthall on June 11, 2014 at 5:34am

Thanks Dave :)

Nice job Vince, I would be interested to see it fully wound up.

I have been thinking about things a little further as well Rob. Based on my calculations, the drag I am getting is somewhere between 5 and 10 time greater than what I expect. Assuming the idea that it is actually the propellers that are primarily responsible for this then it may make sense to use smaller props at higher rpm (Rob suggested that). The idea is that the smaller the prop the lower the drag, purely a guess but it sort of makes sense.

The smallest Graupner e-prop style prop I can find is 8x5 but it would be interesting to go even smaller to 6x5's or something like that. This wouldn't be an efficient copter but it would be small and fast :)

Comment by Rob_Lefebvre on June 11, 2014 at 7:12am

Vince, pretty cool.  I definitely thing this is the way to go for a quad designed primarily for FFF and not just hovering.  It may not work perfectly for an out-and-out speed run where you're using 45° tilt, as you will get some issues, like the rear props riding in the flow of the front props.  And issues with prop clearance on take-off and landing.  But for a quad who's design intent is primarily ~10 m/s flight for mapping or something, 10-15° tilt makes a lot of sense.

Leonard, you could always take it the next step... ducted fans. :)

I was actually wondering if the propeller themselves actually can be modelled as semi-solid disks, and therefore the larger props cause more drag.  It's hard to say, as they are powered, so should be creating thrust, not drag.  I could see that the larger disk might waste power from the motors, but not create drag?

I do remember from my RC Airplane days, I used to like building my models with a 4-stroke, and with as large of a prop as I could fit, with low pitch.  This gave them good climb rates, but with limited speed which is actually what I wanted.  And I do remember that on landing, they slowed down really nicely.  The large prop definitely acted like an air brake when the motor power was cut and the airspeed was higher than the pitch speed of the prop.  But that's just not the case here, because your props are powered.

Comment by Vince Hogg on June 11, 2014 at 7:51am

Actually if you dont care about efficiency I suspect ducted fan could be very fast and lend itself to an in flight tilt system Avatar style.

Comment by Oz on June 11, 2014 at 3:08pm

A year ago a friend experimented with several ducted fans, his conclusion, 80% of the power went in to making noise, LOL, I am sure that was partly sarcastic and improvements in design has happened since. I would think they still are not better than good props. Good Luck


Developer
Comment by leonardthall on June 15, 2014 at 7:41am

Talk about egg on face!!!!

I realized why that motor failed. That one had an 11" prop on it while the others all had 10". I knew I was working the motors hard with 10"s on them and that static load was going to exceed the rating of the copter but 11" at sea level torched it!

I also did a static test on the motors with a 10" prop and I could push the current up to 36A with a fully charged 4s battery. This is WAY over the rating of my motors of 17A. However my motors should only be pulling about 3A at hover and would unwind considerably at maximum speed making it hard for me to push the motor over it's maximum rating. In fact at maximum speed all motors were sitting at 80% and in static conditions the motors reached maximum current rating at 75% so I think my motor choice is on the limit but ok for the design I have. I shouldn't be doing high throttle punch outs though :)

Mental note: 11" props look a lot like 10" when on a frame.

I was rushing to get this done and tested in time and had been up very late the night before finishing things off. I also showed the copter off to about a dozen experienced modelers and no body picked it out. Looking at the 3 photos above can you see which one has the 11". I can now :)

Comment by Oz on June 15, 2014 at 9:03am

I have many of the NTM 2826-1200kv motors (~16-20), great little motor for little money. I have tried many different props (even low cost CF), APC MR 8x4.5 is the best with 4S on all my quads ranging from F330 (700g) to my 600mm (1200g). This setup pulls just under 250 watt per motor WOT, the real rating of the motor is ~180, but ~5 seconds WOT here and there is fine. 8x5 was too much, sluggish response for acro and pulled to much power, I did try GF 9x4.7 and again did not have the snap the 8x4.5 gave, 10x4.5 was way over prop'ed. Maybe on 3S, but I love the smell of 4S in the morning :)  Good Luck.


Developer
Comment by leonardthall on June 15, 2014 at 9:27am

Hi Oz,

I agree. I started with them on 3s and they work really well at 10x4.7. When I started experimenting with my high power frame I have been nervous about them but they have held up well because they don't get full throttle until they are going very fast and have unloaded the prop. I suspect I might not be able to get the power out of a smaller prop at high speed.

However, I would feel much better if I was using these right now!

http://www.hobbyking.com/hobbyking/store/__25077__NTM_Prop_Drive_28...

I will see what I can do with what I have and baby them as much as I can.

Comment by Oz on June 16, 2014 at 10:14am
If you need bearings for the NTM 2826 motor, I get them from here, $1 each, mostly I replace the top (smaller) bearing, as it has more side loading during flips. I oil them about every 10 packs.
bearings


Comment

You need to be a member of DIY Drones to add comments!

Join DIY Drones

© 2019   Created by Chris Anderson.   Powered by

Badges  |  Report an Issue  |  Terms of Service