Mechanical Engineering Department
BS in Mechanical Engineering
The following report encompasses the Cal Poly Human Powered Helicopter team’s efforts during the 2010-2011 academic year. The intention of this project is to further the knowledge of human powered helicopter design and to validate an ideal configuration through experimental tests and analysis.
The Sikorsky Prize offered by the American Helicopter Society has been the catalyst for many attempts at Human Powered Helicopter (HPH) flight. The requirement to win the prize is a continuous, human powered flight of more than 60 seconds that stays within a 10 meter square box and reaches an altitude greater than 3 meters at some point during the flight. As of 2011, there have been over thirty different attempts. Most of these attempts have not flown or produced anything significant due to serious design issues, fabrication and execution problems, failure to record and publish information, or lack of funding, among other problems. In addition, a large number of the projects are no longer underway because of failure to fly. In fact, only three HPHs have ever achieved flight: the Da Vinci III constructed by students at California Polytechnic State University (Cal Poly), the Yuri I, constructed by students at Nihon University in Japan, and most recently the Gamera, constructed by students at the University of Maryland. All of these successes came after many years of experimentation and the combined efforts of numerous individuals.
Some of the main design challenges for human powered flight include maintaining a very low vehicle weight while achieving the highest power output from the limited energy that a human can produce. From previous research and experiments performed at Cal Poly, it was concluded that the limiting element in using human power is the body’s respiratory system -- the series of organs involved in re-oxygenating blood as it circulates the body. As with any engine, an athlete’s body can only perform within the limits of its respiratory system before reaching exhaustion. For example, simultaneous use of arms and legs to provide power to a helicopter is of little advantage if the body is already processing oxygen at a maximum rate. However, the phenomenon of ground effect is known to reduce the power required for hover during flight at very low altitudes. Unfortunately typical helicopter theory, especially related to low altitude hover, does not entirely apply to the HPH because rotor shape and operational RPM are so drastically different. The HPH relies on rotors that resemble rotating conventional aircraft wings with longer chord, greater camber and twist, and operate at much slower RPM than conventional helicopter rotors. Because the majority of publicly available data relates to traditional helicopter analysis, there is a considerable absence of published experimental data needed to make critical design decisions for an HPH. Therefore, the experiments performed by the Cal Poly HPH Team were done to simulate possible HPH rotor configurations and situations in order to better understand the dynamics of HPH flight.
C. Ground Effect
In studying the past HPH designs, our team realized that the most obvious problem is overcoming the limitations of human power. The best attempt at fighting this was Naito’s Yuri I, a craft that utilized ground effect by running rotors very close to the ground. The success of the Yuri inspired us to study Dr. Naito’s research, and try to understand what made his helicopter so successful. After reviewing Dr. Naito’s research, we identified our areas of work. We came to understand ground effect (GE) as an increase in blade efficiency for rotors operated at less than one radius above the ground. The improved efficiency, or lift to drag ratio, is due to two things:
1. Reduced downward velocity of induced airflow, leading to:
a. Less induced drag and a more vertical lift vector
b. Decreased pitch angle and less power needed for hover
2. Reduced tip vortices, resulting in:
a. Improved efficiency of outboard portion of blade
b. Reduced system turbulence from ingestion of recirculating vortex swirls
As a result of low altitude hover, the downwash from the wing or rotor blade is deflected by the ground. This deflection reduces the vortices on the tips of the wings and rotors. Because these vortices cause a slight downward and backward drag force on the lifting surface, their depletion allows increases in lift. In a helicopter, this lift is being achieved by power applied to a rotor. Therefore, ground effect should theoretically allow for increased thrust (or lift) from the rotor without a corresponding increase in power input. Ground effect is especially noticeable in the HPHs that have actually flown. The Da Vinci III, Yuri I and Gamera were only able to fly in deep ground effect; in fact the Yuri and the Gamera rotors were only inches above the ground. The Da Vinci rotors were over four feet high, but the rotor length was very large. Ground effect’s influence on HPHs is undeniable, and better designs for HPHs can be achieved by understanding this phenomenon. In Figure 1 below, the in-ground-effect rotor on the left has smaller tip vortices and decreased pitch angle than the rotor out of ground effect seen on the right. The benefits of GE begin around one rotor radius above ground, and then increase exponentially the lower the rotors fly. To reap the greatest benefits, our HPH should fly as close to the ground as possible.
D. Intermeshing Rotors
The Yuri I had four rotors, and was the most stable and successful flying HPH. However, the large structure needed to keep the four rotors from colliding with each other had to be made very, very light. This meant the structure was not very robust and collapsed after a minor impact between rotors. If the structure size requirements could be reduced, the remaining structure could be made much more robust and could withstand forces from a control system, without an increase in craft weight (compared to previously flown helicopters). In addition, if the rotors could be brought together, it would also decrease the amount of material in the drivetrain as well as reduce complexity and weight. To accomplish this, the rotor blades would have to intermesh or spin inside of each other like gears that do not touch, otherwise obvious disaster would occur. The question is then whether a drive train could be designed to ensure that the rotor blades don’t hit and if intermeshing rotors come with any thrust or instability penalties that would have to be considered before using them on a full size helicopter.
Manufacturing of Carbon Fiber Tubing for Use on the Human Powered Helicopter
CLTTubeDesign.m (8 kB)
Composite Tubing Design Program
Appendix A-Composite Spars (W11).pdf (445 kB)
Appendix A- Composite Tubing Winter 11
Appendix B-Natural Frequency and Ultimate Loading of Composite Spars.pdf (1808 kB)
Appendix B- Composite Tubing Testing and Design