After more than a year of design and development, I've built version 2 of the iQuad custom tilt-rotor quadcopter which is capable of achieving high forward speed by tilting its rotors forward in flight. After several days of flight testing and tuning, I was able to achieve stable hover and incredibly fast forward flight. Near the middle of the video, you can see the iQuad hitting a top speed of 53.3 miles per hour on the third flight of the day! This was achieved with a 40 degree forward tilt of the rotors.
For stability control, I'm using an APM 2.5+ and a modified version of the 3.0.1 arducopter firmware. Big thanks to R_Lefebvre and Gary McCray for convincing me to switch to the 3.0 version and other suggestions. To support tilt-rotors, I created a new body configuration, which adds servo control and does the proper mixing of the pitch, roll and yaw inputs depending on rotor tilt. I also added logging for the tilt input and servo angles, which was very helpful for debugging some initial stability problems. For the final phase of the project, I'm planning to add four wings that will wrap around the metal arms (and maybe some tail-fins), to allow a full transition to forward flight.
Here's another video of me having some fun with the iQuad. Near the middle of the video you can see the iQuad hovering in place while tilting forward/back.
Also, Have you considered raising the rear blades?
I wonder if you can link the forward tilt to the accelerometer such that the tilt is not enabled until after a predetermined amount of forward momentum has been established. The idea being that the quad having first established forward movement and the quad having become tilted forward10 or 15 degrees to help with prop wash.
A few ideas you probably have already gone through. Love the quad! Keep up the great work!
"I was actually trying to decide whether to use a symmetric or an asymmetric airfoil, and one the things I was concerned about with an asymmetric airfoil is that I'd always have to tilt the rotors slightly forward during hover"
Conversely, if you use a symmetrical airfoil you will always have to have some tilt-up in forward flight at a cost in efficiency. I guess it all boils down to where you plan to spend the majority of your flight time: in hover mode or forward flight.
Nice job Ilya!!!
Gary, that's an interesting point... I was actually trying to decide whether to use a symmetric or an asymmetric airfoil, and one the things I was concerned about with an asymmetric airfoil is that I'd always have to tilt the rotors slightly forward during hover, and the amount of tilt necessary would vary depending on thrust.
I wasn't thinking of using the asymmetric airfoil as a way to control tilt in forward flight, but you're absolutely right, it should be possible to control yaw and roll simultaneously by speeding up the rotors on one side or the other. However, I'm not sure whether this would be a good thing -- it may cause a lot of unintended consequences like the differential tilt (which seemed like a good idea at the time).
I need to think about this some more, although my first instinct is to get rid of all the possible complications by going with a symmetric airfoil.
I do like the idea of a rudder/fin for the same reason you mentioned. You can probably tell from the video that the quad wasn't necessarily flying in the direction that I was pointing the nose in, which is kind of scary at the speeds it was going.
Hi Ilya Yes I can understand the yaw thing, probably best kept in sync.
After watching how you were also able to change angle successfully front and back without moving it also occurred to me that you could probably skip the fixed wing control surfaces (except total angle) simply by speeding up one sides motor / prop pair relative to the other.
The turns would skid, but if you put on a top fin (I'd say rudder, but probably better if full length and not too high,) it would probably reduce the skid. And you could probably use total wing angle to control altitude just fine.
In fact speeding up the motors on one side relative to the other would also cause more lift over the wing on that side and would cause it to bank in the appropriate direction, pick the right airfoil and at least at some speed / angle you could make a coordinated turn.
The wings turning with the props will make for some interesting flight characteristics and capabilities at various angles.
Cant wait to see your video with the wings on.
@BayAreaCrasher... yep, that's Baylands Park. Let me know when you'd like to meet up. Would be fun.
fastest agiliety iv ever seen for a quad, I am thoroughly impressed.
Thanks Gary! The most important lesson I learned from my flight testing is that the front and rear tilt servos need to be in sync for yaw control to work. During the first two days of flight testing, I had uncontrollable yaw. As soon as it started to yaw in one direction, it would keep doing it, no matter how much rudder I applied in the other direction. I thought the flight controller was doing something wrong, so I added logging of servo angle, and what I realized is that any time the front servo was tilted forward more than the rear, it would go into uncontrollable yaw.
The reason the servos were moving at different rates was that initially, I implemented differential tilt control, where the pitch output from the PIDs was used to control the relative tilt of the front and rear servos. The idea was that if the quad pitched forward, the front servo would tilt up and the rear would tilt down and vice versa if the quad pitched back to keep the quad level. The unintended consequence of this is that the set of rotors that were tilted further forward would dominate yaw control. So, for example, if I was trying to yaw to the right, but the front set of motors was tilted forwrd, the front-right motor would actually pull the quad to the left. The more right yaw I gave it, the faster it turned to the left.
Adding to this was that during forward flight, the rear motors become less efficient since they're in the propwash of the front rotors. So, the rear motors were almost always tilted up higher than the front.
To fix this, I got rid of the differential servo control and tried to get the two servos to move perfectly in sync. I also moved the CG of the quad slightly forward of center so that it would be more balanced in forward flight. Those two changes did the trick. Besides that, and some code to mix the yaw and roll control, it was surprisingly easy to get stability. I didn't even change the PID constants from the defaults!
Really great job Ilya, major hurdles overcome in firmware to get such smooth transition without tilting frame at all.
Looks like it could be an interesting and useful capability for a camera or video platform too.
Wings should be interesting too.
if you went with separate servos for each side you could use wings for forward flight control too maybe.