We conducted another flight test over the weekend and it did not go well, but we did learn a lot from it.

The airplane was stable when under power, as it had been in previous flights. It even flew relatively well in a very stalled scenario. (It is not necessarily visible from the video, but it was very stalled throughout even the beginning of the flight.) Our problems resulted from pilot error/misinterpretation as well as from a poor configuration of the airplane prior to the flight.

Prior to take off we noticed that the rear wing of the airplane was slightly pitched down with respect to the fuselage and that the canard was slightly pitched up when the joystick was neutral. We decided that it was not extremely important for both canard and main wing to be parallel with the fuselage for flight, which we still believe is true, (I will discuss this separately) but we did not comprehend the full impact and consequences of this configuration until too late.

We had not realized that this configuration would mean that "normal" fuselage attitude during climb would actually put the canard and main wing at high angles of attack. As a result of this we took off in a stall and maintained a stall throughout without comprehending what had happened and why the airplane wasn't performing as it had in our previous flights. With the fuselage in an appropriate climb attitude it was not immediately apparent what the problem was.

Flight Analysis
The airplane took off well from a hand launch in zero wind conditions. It began to climb, but as it climbed it became apparent that it was mushing through the air rather than establishing a proper climb. The pilot then controlled down slightly, but it was not enough and the airplane continued to mush through the air. As the airplane came to a complete stall it slid right and entered a sort of flat spin. It recovered from this and began to gather speed ending the stall, but at this point we decided to autorotate in an effort to gather what we could from a failing test.


Unfortunately, prior to the flight we had decided on an autorotation sequence that was improper and could never have worked given our greater understanding of the problem at hand. This sequence was: 

1. Kill engine
2. Change remote control to high rate setting (this allows the pilot to rotate the wings over large angles for autorotation rather than the low angle limits used during conventional flight).
3. Begin wing rotation (~80 degrees)
4. Approach ground and increase angle to ~100 degrees so as to generate sufficient lift to arrest descent.

The problem with this scenario is that as soon as we killed the power we lost yaw stability and the airplane went into a bit of a flat spin. As you may recall the airplane has no vertical stabilizer. It is therefore very unstable in yaw, and cannot be maintained in coordinated, controlled flight without thrust vectoring. The ability to use thrust vectoring is lost if the motor is turned off. Thus our flight plan was doomed to fail because we had planned to cut power before entering the autorotation. The airplane was already out of control before the autorotation sequence was initiated control was never regained.

The damage to the aircraft was not as serious as it may look on video. The part of the fuselage made of aluminum bent and I will need to machine a new one. Other than that, the airplane is in good condition for future flight tests and we will be conducting these as soon as possible.

We conducted one flight test today and the results are pleasing and will certainly lead to more testing of the current airframe. The most incredible part of the flight test was that the airplane actually flew in coordinate, controlled, straight, and climbing flight. Due to various design constraints, the airplane has no vertical stabilizer and it is therefore unstable about the yaw axis. A gyroscope, and gyroscope-controlled thrust vectoring were incorporated into the design making coordinated flight possible. That said, it would be impossible for a human to control the airplane without the gyroscope. Additionally it is interesting to note, although not entirely surprising that the airplane was controllable in roll. As previously explained, there are no ailerons on the airplane. Instead, the whole main wings rotate in opposite directions to achieve roll control. I know of no other airplane that controls roll in this manner and we were all pleased to see the airplane fly as well as it did.

The rotoplane was hand launched into the wind from a moving vehicle. It climbed in coordinated, controlled flight to an altitude high enough for an autorotation. At this point the motor was cut and the airplane was set into a roll. This roll merged into a situation where the airplane was spinning about its longitudinal axis, and parallel to the gravity vector as designed.

The pilot then changed the pitch of the wings to a “positive” angle of attack. It is unclear why, but the airplane then reversed its autorotation such that the what was supposed to be the top part was now the bottom. It continued in this nose upward attitude for a brief period without rotating significantly until the controls were adjusted to accomodate an autorotation in this orientation. At this time the airplane began to rotate about an axis through the center of its fuselage although pointed upward.

Without enough altitude left to rectify the situation, the test pilot let the airplane spin up and prepared for the flare at the end of the autorotations. At an altitude of about ten feet he flared. By this point, the airplane was below the tree-line and isn’t as clearly visible as I would have liked. However, it did arrest its descent as is noted by the brief pause only a couple feet off the ground.

This test was a great success for the rotoplane even though it wasn’t a perfect demonstration of the technology being developed. It did demonstrated that an airplane of the rotoplane’s configuration could fly, and that not only could it fly; it can autorotate. Future tests will be conducted with the same airframe in a couple weeks once I have time again. 

Future work includes gathering data from the autorotation such as rotational rates, wing angle and descent rate. In order to accomplish this I would like to install electronic sensors. I am in the process of learning to program microcontrollers. I eventually hope to install some custom made data loggers in the airplane to gather data during autorotations.

I would like to thank William Grossman (UC Berkeley) who flew the airplane for all test flights from the very beginning of the project. I would also like to thank Cedric Jeanty (Caltech) for all the useful ideas and advice during the design and construction of the airplane. I would also like to thank professor Culick for his help and guidance along the way and Caltech for having a senior thesis program. Finally I would like to thank my cousin Beth for filming.

On February 2nd, 2008 we did high speed taxi and preliminary flight tests for the rotoplane. The results are positive, but the airplane design leaves some to be desired. The most important result of the test is that the airplane has enough yaw stability to maintain straight and level flight due to the gyroscope-controlled thrust vectoring. Many high speed taxis without take offs were conducted and then we graduated to low altitude flights (1-3 feet above ground). 

All flights unfolded in approximately the same manner. The takeoff runs were smooth and transition into flight was easily accomplished with pitch control from the canard. We were able to maintain level pitch attitude and the wings parallel to the runway as well as maintain heading. The problems were always with the landings. The airplane can maintain heading when it’s motor is powered because it can vector its thrust. Upon reducing the thrust to land, the ability to maintain heading is lost and one wing slips ahead of the other. During this slip one wing generates more lift  than the other sending the plane into an awkward sort of cartwheel/roll landing as you’ll see in the videos. Fortunately, the airplane isn’t designed to land in a conventional manner.

Because the landing gear was an afterthought, designed solely for high speed taxis and initial testing, it interferes with large rotations of the main wings and thus we cannot attempt an autorotation landing from a ground roll take off. We would have liked to fly again and take off without landing gear so that we can attempt an autorotation, but it began to rain and we will have to wait for better weather.

I believe, based on my experience with the mockup as can be see in the droptest video, that we could accomplish an autorotation. Although the current airplane is heavier than that used in the droptest, the wings are much stronger and hopefully will not break as one did in the droptest. In fact, autorotation simulations in Matlab and wing loading tests confirm that the wings are strong enough and we look forward to a chance to autorotate.

See video below.