The first flight of my 3d-printed Ducted Fan VTOL UAV was on the 27th June 2015. Usually you would not expect that a first flight work perfectly, but it did. No tuning of parameters or modifications were needed. I did attach a GoPro on the second flight to get some nice video from it. I didn’t even unpack my notebook since all the data is already stored in much higher data rate on board as it shown on the ground control station (GCS). In manual flight (commanding attitude and power to the motor) looking on the GCS is not a good idea anyway (at least for my skills). I modified the landing structure a bit to dampen a hard landing and to reduce the risk of flipping over. The following video shows the first flight with some parts of the second flight. I intentionally increased the power until I saw some movement and then accelerated fast. This minimizes the time where the vehicle is still in contact with the ground but half flying.
It was not a lucky coincidence that the vehicle did not require tuning. Some people tune and test the vehicle by holding it and see the reactions. I didn’t like that idea for multiple reasons. I would expect that it is not very precise (I have done it with quad copters before) and you need some courage depending on the vehicle to hold it with propeller rotating that close to your body. Second this vehicle makes significantly more noise than a quadcopter even though the propeller is shielded the psychological effect of the noise scared me on the beginning. Call me chicken but in a closed room that screaming noise is quite an impression (there is certainly room for aerodynamic optimisation).
So I built two test stands with some leftover wood and some 3d printed brackets with ball bearings for the suspension. One test stand was to tune the pitch parameters. Because of the symmetry along the vertical axis I just used the same parameters for roll (in hovering mode). For the yaw parameters I built another test stand where the vehicle was suspended along its vertical axis. The tuning of the parameters was relatively straight forward as well as writing the necessary mixer. Additionally I added some wool thread on the the stators, control surfaces and the inlet of the duct to see if there some airflow separation occurs. On the test stands I didn’t see any airflow separations, even though the incoming airflow was very gusty, since I did those tests in a relatively small room. One wool thread showed the back flow of the air between the propeller and the inside wall of the duct very nicely. The gap , 1mm is relatively large and can be reduced if the inside wall of the duct could be stiffened by an additional rib at this area.
I between the different test stands a few times to make sure that parameters aren’t having any unwanted influence onto other axis. That gave me also a feeling how far I can go with the parameter tuning and what performance I could expect. Once I was satisfied with the parameter tuning I modified the landing structure and waited for a day with sunshine and calm winds (which is not that often the case here). The last thing I did before I left for the first flight was to measure the weight. You never know it might be much more difficult to figure out what the weight of the vehicle was after the first flight. It is 1.3kg with battery ready for take-off.
One last picture before the take-off. Yes, I used in that case a peace of duct tape to hold the receiver in place.
The first flight was very pleasant. I could have not expected more. For the second flight I mounted a GoPro camera on the vehicle and tested the reactions on my inputs. I never used full power, since the vehicle accelerated vertically very rapid with estimated ¾ power.
The next steps will be to tune the parameters for position hold and way-point following (in hovering mode). For the transition into horizontal flight I will have to do some add-ons. Most likely I will build another airframe since the costs for the airframe are insignificant and I can in-cooperate all the changes to improve the assembly and the vehicle it self.
At this point I want to thank all the developers of the PX4 project, without their work I would need a few more weeks and would still have just a basic flight controller. I love that they integrated the off-board control mode, which makes it super easy to send commands and receive vehicle information form a small Linux computer. I am using again the BeagleBone Black (this time just for the high-level control and digital video transmission).