There remains a need to develop improved VTOL techniques that are cost-effective and with minimum compromise on cruising flight performance for fixed-wing aircraft. This work proposes an elegant VTOL control method known as PTVC-M (pitch-axis thrust vector control with moment arms) for tailsitters. The hallmark of the approach is the complete elimination of control surfaces such as elevators and rudder. Computer simulations with a 1580 mm wing span airplane reveal that the proposed technique results in authoritative control and unique maneuverability such as inverted vertical hover and stall-spin with positive climb rate. Zero-surface requirement of the PTVC-M virtually eliminates performance tradeoffs between VTOL and high-speed flight. In this proof-of-concept study, the VTOL-capable aircraft achieves a VH of 360 km·h -1 at near sea-level. The proposed technique will benefit a broad range of applications including high-performance spinsonde that can directly measure 10-m surface wind, tropical cyclone research, and possibly serving as the cornerstone for the next-generation sport aerobatics.
In the early 1920s, Juan de la Cierva came up with the concept of autorotation in the efforts to solve the wing- stall problems [
Modern aerobatic model airplanes have the abilities to perform an array of impressive post-stall maneuvers known as “3D aerobatics” such as hovering (including VTOL), waterfall, flatspin, blender, tailslide and their derivatives [
To address the issues above in regard to the constraints imposed by the placement of rudder and elevators, this work proposes the concept of pitch-axis thrust vector control with moment arms (PTVC-M) in an attempt to create an ultra-agile 3-axis VTOL control method for tailsitters. The proposed PTVC-M requires no control surfaces to actuate VTOL flight controls so as to gain independence from the aerodynamic rudder and elevators. For an aircraft with twin-engine configuration (i.e., one engine on each wing), the system will involve two pairs of moment arms; one of which is parallel with the longitudinal axis to enable pitch control and the other pair is parallel with the lateral axis to ensure responsive yaw. The flight performance is investigated via computer simulation using a 1580 mm wing span UAV as a test platform. In this article, the definition of a tailsitter is one whereby the roll axis (longitudinal axis) of the aircraft is approximately parallel with the z-axis of the world frame during the VTOL phase though the landing gears can be attached to any part of the airframe including fuselage and wing and are not restricted to the vertical or horizontal stabilizers at the rear of the aircraft.
In principle, the PTVC-M actuates pitch control by creating moments about the lateral axis (pitch axis) similar to conventional elevator effect, except the force is generated directly by the thrust-producing element instead of surface-derived aerodynamic force. In the case of propeller being the thrust-producing element, the moment arms extend from the center of the propeller disks to the lateral axis. In
Therefore, the further away the location of the propeller from the centerline, the greater the maximum yaw moment. However, if the design intent requires the elevators to be partially immersed in the propeller wash to harness its additional advantages, then there should be an optimal placement of the propeller along the lateral axis because if placed too far out then there would be very little amount of propeller wash reaching the elevators at the rear of the fuselage. Apart from propeller, the thrust-producing element can include turbine fans, rotor blades, or possibly even rockets. The thrust-vectoring mechanism responsible for the tilting of the propeller disk can be realized using several well established methods such as swashplate or servo-actuated motor mounts similar to those being used for yaw control in present day tricopters [
The graphical model of the small UAV used in this work was created using the Autodesk 3ds Max® [
The surface areas of the rudder and elevators were approximately 32% of those of the stabilizers, which were significantly smaller than those on the “3D aerobatic” model aircraft [
control surfaces were ±45˚. Optional retractable undercarriage can be affixed to the UAV to enable takeoff and landing on runway. The aircraft was equipped with two 3-bladed propellers and they were counter-rotating to each other. The variable pitch propeller has a diameter of 305 mm. Each brushless motor has a maximum electrical power of 4 kW. The all-up weight (AUW) of the aircraft was 2.48 kg with a wing loading of 60.3 gdm−2. Gyros (electronic flight stabilization system) were added to the roll, pitch and yaw controls for the piloted simulations in this study. However, flight stabilization for a particular principal axis would be disengaged when performing the angular rate test so as to avoid any possible influence from the gyro.
Pitch: tilting both the propellers in the same direction [
Roll: tilting the propellers by the same angular displacement but in opposite direction [
Yaw: differential thrust by varying the rotational speed of the propellers [
The PTVC-M was deactivated and only the control surfaces were used during level flight evaluation, and the roll, pitch and yaw controls were found to be responsive. As shown in
The maximum rates of the control response at near zero airspeed were 180˚s−1, 220˚s−1 and >700˚s−1 for the roll, pitch and yaw, respectively, but the effectiveness of PTVC-M dropped off with increasing airspeed. Strengthening of the PTVC-M yaw moment as the airspeed approached zero is similar to the fact that a multi-engine airplane with an engine failure will experience a larger yaw moment at low airspeed [
A more comprehensive study on the control effectiveness of PTVC-M as a function of airspeed was carried out and the test method involved launching the tailsitter vertically on its own propeller thrusts. When the airspeed has reached the desired constant value, maximum input was applied to the roll. The test was evaluated at airspeeds of 60 and 150 km・h−1 and the process was repeated for the pitch and yaw (
The ailerons continued to work below the wing-stall speed of 39 km・h−1 because they were immersed in the strong propeller wash. The elevators managed to exert an angular rate of 50˚s−1 at zero airspeed due to the widening of the wake flow angle during the static hover (0 km・h−1 airspeed) [
The rolls of the simulated tailsitter were quite axial and have low tendency to cause directional change or promote angular momentum exchange among the principal axes of the aircraft [
Setting the throttle to maximum while the aircraft was in hovering position and applying full deflection of pitch control resulted in a pitch rate of no less than 650˚s−1, which could be used to create a new form of maneuver based on the well-known “waterfall” maneuver [
Video 2 shows the general VTOL and cruise flight performance of the tailsitter such as hovering, transition to forward flight (and vice versa), loop, and rolls. The maximum controlled vertical descent rate was found to be 8 ms−1. The landing sequence from level flight to final touchdown could be performed with ease, an indication that the coupling between the control surfaces and the PTVC-M has resulted in responsive controls that seamlessly spanned across the full flight envelope.
Spinsonde is a concept developed to acquire high-resolution vertical wind speed profile measurements for atmospheric research [
Control mode | Airspeed (km h−1) | Rotations about the principal axes (˚s−1) | ||
---|---|---|---|---|
Roll | Pitch | Yaw | ||
PTVC-M | 0 | 180 | 220 | >700 |
60 | 70 | 110 | 35 | |
150 | 24 | 80 | 25 | |
Control surfaces | 0 | 230 | 50 | 0 |
60 | 497 | 180 | 115 | |
150 | 1030 | 330 | 140 |
The simulated tests were carried out for horizontal wind speeds of 0 km・h−1and approximately 40 km・h−1 (altitude and terrain dependent) and the results for both scenarios are available in Video 3. The arrows in the second part of the video indicate the direction of wind flow. Also, the local wind intensity and updraft are displayed on the NavGuides. The steady-state stall-spin was generally achieved by applying maximum deflection of rudder and ailerons in the same direction and applying up elevator with no throttle input. For the 0 km・h−1 wind, it achieved an equilibrium descent rate of 6.2 ms−1, and the variation between the wind speed and ground speed was ±3 km・h−1, which was taken as the measurement uncertainty of the wind speed following [
In mesoscale meteorology, the parameter of particular interest to forecasters and the public is the maximum sustained 10-m surface wind [
We had proposed and demonstrated via simulations the concept of PTVC-M in an attempt to develop ultra-agile VTOL capabilities without relying on large control surfaces which often degrade high-speed flight performance. The small UAV in this work equipped with PTVC-M had demonstrated the ability to perform inverted vertical hover with full 3-axis control. PTVC-M had also enabled the UAV to climb in a stall-spin and such feature will
lead to the development of high-performance spinsonde that can directly measure the 10-m surface wind intensity as part of the multi-cycle measurement of the vertical wind speed profile. We believe the proposed PTVC-M will be another indispensable tool in mesoscale meteorology as well as being an enabling technology that brings exciting and enthralling “4D” maneuvers to the world of sport aerobatics.
Chung-Kiak Poh,Chung-How Poh, (2016) PTVC-M for Ultra-Agile VTOL and 300+ km·h-1 Cruising. Advances in Aerospace Science and Technology,01,48-57. doi: