New full scale unmanned winged tiltrotor VTOL

ArduPlane VTOL Plane
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Hi,
We have been trying to improve the stability of the pitch PID for several weeks day and really have not made much progress.

Fundamentally the problem is that our pitch fundamental mode of oscillation about its CG is very low, on the order of 1 Hz, and the PID loop is not able to correct for this. We think this is because the PID loop has a filter at its input that filters out the low frequency input stick frequencies so the PID controllers does not respond to them. Unfortunately our vehicle fundamental pitch frequency is so low that the system thinks the oscillation is a stick command and is not getting passed onto the PID controller to correct for it.
Is there any way to change the frequency of this filter. So that the PiD controller can try to correct for this oscillation? Our vehicle has so much rotational inertia that it does not need fast stick corrections. So this filter can be reduced to help pass the oscillation signal to the PID controller.
Thanks William

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Summary of progress so far and lessons learned:

  1. We have tried 3 different PID values with varying levels of success: P much lager than I, I much larger than P, and large P=I. The last P=I setting seems the most promising and the signals seem to track the setpoint signals well until the motors rail, which then causes the PID loop to fail and the motor signals start to turn off an on. With low I parameter it was observed that the vehicle pitched down a lot (~5deg) with rapid reduction in throttle, whereas increasing throttle had no effect on pitch. Increasing I seemed to reduce the problem a lot.

  2. The vehicle is under powered and hovers at 70% throttle, which should be ok for a vehicle that just takes off vertically and transitions immediately to forward flight. But the vehicle has a vacuum problem in back caused by the back motor thrust going under the back end of the vehicle’s side panels. As a result the vehicle will not takeoff unless the back end is popped up immediately on takeoff, which causes the vehicle to tilt forward a lot during take off. Correction of this pitch as the vehicle started to hover has caused the motors to rail, which then causes the PID loop to lose lock and start to turn off and on, crashing the vehicle. We are considering trying using PX4 to see if its PID loops behave better.

  3. Adding a thrust blocking skirt to the inside of the vehicle’s side panels significantly reduces the vacuum problem in back and enables the vehicle to take off more easily, and the vehicle has hovered once well with this modification.

  4. Flying the vehicle with the batteries not fully charged has caused the vehicle to hover at 80% throttle, and this reduced the headroom needed for maneuvering and caused the motors to rail more easily. So it is important to fly with the batteries fully charged.

  5. Because of the problems with the PID loop instability when the motors rail, we are considering testing the vehicle without a flight controller, using only transmitter receiver mixing. The flight controller will be used only to collect flight data for log analysis. Realflight simulations have shown that the vehicle can be flown easily without a flight controller due to the vehicle’s large rotational inertia, which slows down the vehicles effects and gives time for pitch and roll corrections. A slight forward movement of the vehicle improves the stability a lot as the air starts to move over the vehicle and produce stabilizing aerodynamic forces. If successful, this will allow us to study the vehicle’s flight dynamics directly. Afterwards we will understand the vehicle better and can then try to integrate a flight controller to automate the vehicle and make the transitions more smoothly.

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  1. 1 deg pitch and roll fluctuations at ~1Hz are observed, which do not grow with increases in PID values. This is attributed to setpoint drift due to fluctuations of the motor thrust, which is observed in all drones and keeps the drones from hovering perfectly still.
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Here are the motor signals, along with associated pitch and roll desired signals from the log files of our flights:
1_PID Good Flight Motor study.pptx (624.3 KB)
2_PID Motor study.pptx (654.5 KB)
3_ IPD Motor study.pptx (595.1 KB)
4_ I=P Motor study.pptx (599.9 KB)

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Hi all, here is an update on the Sky Chaser project. We think we have found the problem causing the vehicle’s motors to wildly oscillate, causing numerous crashes. Here is an example:

Link

Motor tests without a prop revealed that the back motor ESC was sticking at high throttle and releasing only when completely throttled down. Because it was in feedback loop in the flight controller, this caused the motors to wildly oscillate from full off to full on. It is amazing we did not destroy the prototype. A simple ESC software update solved the problem.

A CAD model of the 90kg full scale unmanned Sky Chaser flown in RealFlight 9.5S revealed that because of the vehicle’s large rotational inertia, the vehicle can be flown very easily with out a flight controller. The vehicle appears to be a great flying aircraft and is incredible maneuverable.

Land based demo of 90kg unmanned prototype:
Link

Water based demo of 90kg unmanned prototype:
Link

A flight test of the real 90kg full scale prototype without a flight controller showed this was true, and the vehicle flew in a straight line for about 100 m. The test showed that the vehicle is very stable, but has about 1s time delays, between issuing a command and the vehicle responding to the command. A turn was initiated at the end, caused the vehicle to bank, but I decided to abort when I ran out of field. The back motor also stuck at high throttle, causing the vehicle to pitch down when it landed. Now we know the cause of the problem was the back motor ESC.

Link

Because the goal of the project is to develop a fully autonomous vehicle we decided to continue trying to get the flight controller to work properly in the vehicle. Bill Geyer, a member of the community kindly helped me develop a full scale SITL model of the prototype, to develop better flight controller parameters for the vehicle, since they were based on the much smaller Convergence RC model. After some difficulty getting SITL to work reliably on my Win10 laptop, it is now working well, and the vehicle is flying well in SITL. This also helped me practice as a pilot for the test flight. I am very grateful to Bill Geyer and others in the community that helped me troubleshoot the many problems throughout the development process of this unmanned full scale prototype.

Link

As soon as the weather gets better, will be ready for another VTOL only test of the real 90kg full scale unmanned prototype.

Here is presentation of the Sky Chaser project:
Link

Here are pictures of the 90 kg full scale unmanned prototype:
Link

Here is a video of our first successful test hover of the 90kg unmanned prototype:
Link

As you can imagine developing something like this is very expensive. It has taken 8 years and about 100k USD investment to get to this point. And it will require much more to develop the manned version. It is looking like I will soon be going to Abu Dhabi, UAE to work with a few partners there, and the UAE gov to develop a Racing Park for race cars and flying cars. It will have grandstands, several racing tracks, hotels, shopping, a school, and R&D development facility to develop the manned Sky Chaser flying car and other flying vehicles we will be developing for ourselves and others. We will showcase the Sky Chaser there as well as race it:
Link

Sky Chaser is the 1st REAL Blade Runner style roadable flying car that looks and drives like a car, and flies both vertically like a drone and horizontally like a plane, in addition it is amphibian. It has no exposed rotors, uses it body as a wing so there are no unfolding wings, and will be both manual and fully autonomous. It will be powered by a hydrogen fuel cell giving hours of range. Here is an artistic picture of what the final product may look like:
Link

William Walker