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.
Comments
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 :)
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.
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.
What would be a great option, is using those new folding propellers from DJI, but build a teetering hub to mount them on. You'd get flap, and some lead/lag freedom from the folding hinge.
Small hovering quads seem to be able to ignore this stuff. But once you start going faster, or making bigger machines, it merits consideration. You basically have a bunch of helicopters chained together, so all the same factors apply. Not that Leonard would solve all these problems before AVC. But this is becoming an interesting blog thread.
It may be worth experimenting with the folding (therefore flapping) props. They dont do any fine pitch enough for normal multicopter but for speed they my be perfect. Would have to find a way to limit the folding while on the ground.
Leonard and I actually talked about propellers for a while yesterday. I also proposed he use something like an 8x6 prop instead of the 10x4.7. But the theoretical model he has developed, and I eventually agreed to, shows that it won't actually make the quad go faster, all else being equal. For example, if you limit the pitch range to something, like 45°, and you define the flight condition such that you have to maintain level flight, not climbing, then that means that your horizontal thrust is equal to your aircraft weight. So that defines your maximum forward speed, regardless of propeller or motor selection.
Where different props and more powerful motors come in, is if you allow the pitch to go much greater than 45°. Say, 80°. If you still have enough power to fly at that angle, then you can go very fast. But in this case, Leonard is limiting pitch to 60°. So his speed still won't go faster even with different props, because the angle causes the limit, not the power or pitch speed.
At least, this is what we come up with. Anybody have any other ideas?
Leonard, yeah, the APC SF's are definitely not the way to go for this. Even the APM MR's would be better. They are designed for higher RPM, and you can get them with higher pitch (at least 5.5). But other guys doing high speed quads use Graupners a lot.
One thing to consider about this stuff... I have more questions than answers but...
Multirotors in high speed flight have to be concerned about disymmetry of lift in fast forward flight. The advancing blade creates more lift than the retreating blade. This happens on helicopters too, and the effect is stronger because our rotor RPM is lower. But our swashplate and head dampers allows us to "flap to equality". The entire rotor disk tilts. And the swash plate tips over to cause the advancing blade to have less pitch, and more pitch on the retreating side. This is actually required to prevent the single rotor heli from simply rolling over, in the direction away from the advancing blade.
Multirotors don't have those features. You still get the disymmetry of lift, but the roll torque is countered by another propeller spinning the opposite direction. So you don't see a roll. But each blade is still experiencing a rolling torque.
Light blades like the APC SF's will flex easily. To some degree this allows them to flap to equality. Stiffer blades, like the APC MR's, or Graupners, will resist the flapping force. This leads to vibration. You should be able to see this in the logs. If you plot the accel noise vs. speed, you should see the correlation. Believe the SF's will have less increase in vibration.
So just something to think about. SF's will probably have less vibration. But the props are weaker, and may fail like Vince experienced.
I'm using graupner eprops.12" *6, on 500w, 1300 kv motors on 4s. THey have very high pitch in the centre, and narrow tips.
I'd love to know what current it pulls on full throttle but I don't dare use a current sensor, it's a major point of failure, and given that I did see 110amps on the beast when it was just a quad doing 90mph, I've taken the precaution of hard wiring everything.
And yes, this is probably one of the least efficient setups around, but boy is it fast :) And also, yes, with APC's i can't get anywhere near the performance, too much flat surface and not enough pitch.
you defiantly want props with much more pitch and smaller diameter than the normal recommended ones. ecalc will probably show very poor efficiency for the multicopter section but try the prop calc section and check the pitch speed.
Is there a Brit team in AVC this year?
I was talking to Chad from 3dr about my copter and I think I know why I am getting so much more drag than I was expecting. I think the slow fly APC's are creating a bunch of extra drag that I didn't think about in my calculations. Dave C swears by the Graupner e-prop and I have some carbon versions that I was going to try once I got it all run in.
I am printing up a second one so I hope to have some more results this weekend.
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