Design process of a Human Powered Helicopter

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1 Design process of a Human Powere Helicopter Wm. B. Patterson Assoc. Prof. Cal Poly State. University., San Luis Obispo, CA Abstract The paper outlines the 9 year esign an evelopment process that le to the first ocumente human powere helicopter flight. Nomenclature P R c n wt C 0 W I xx power rotor raius rotor chor number of blaes weight air ensity rag coefficient angular velocity moment of inertia about the x axis rotation about the 'x' axis will be rotation about the 'y' axis will be rotation about the 'z' axis will be an the rate is /t an the rate is /t an the rate is /t Design Process A esign team has two istinct an very ifferent tasks. The first an most ifficult is to etermine the layout an configuration of the artifact being constructe. In the case of a totally new object, no human powere helicopter was currently flying, the task is inee formiable. The conceptual esign, or invention, was carrie out in an has been the basis for all further evelopment. One of the jobs of the manager/avisor is to keep new members of the team focuse on the original concept an not have the team fall victim to the "not invente here synrome". The secon task is to esign the components that will be use to allow the artifact to achieve its goals. This task is, component esign or simply engineering. It is the kin of esign taught in engineering schools. The only problem occurs when the neee components are not like anything seen before. Then we are back to step one on a smaller scale. We are then asking our esign engineer to be an inventor. Conceptual Design The Human Powere Helicopter project at Cal Poly began uring the winter quarter of AHS offere a prize to be given for the first hover uner human power. How was such a

2 machine to be configure? Torqueless rive is the obvious system of choice, the power plant is limite an the losses generate by a tail rotor coul not be tolerate. Later, it became obvious that the sie war thrust of the tail woul also eman a sophisticate control system. Three systems were consiere: - single rotor, tip riven - ual rotor (either tanem or coaxial) - multiple rotor The current configuration of the successful Japanese entry was not consiere. We all felt that multiple rotor heas woul ecrease efficiency. The number of rotor blaes was always a consieration. A large number of rotor blaes woul impose a weight penalty an woul increase profile rag. The minimum number of blaes was consiere to be beneficial. Again, this may not be as important as we ha anticipate. Our successful competitors have 8 blaes an have suffere no loss in performance. The tip riven configuration ha several isavantages. The riving propellers woul be some istance from the power source, so the rive system woul be more complicate an woul suffer aitional losses. The system woul also require new propellers to be esigne an constructe for any change in tip spee or power requirements as the Da Vinci evolve. The ual rotor coaxial system woul have a more compact rive mechanism but woul nee a minimum of four rotor blaes, two for each rotor hea. The tanem rotor system woul use four rotor blaes an woul also require an extensive support structure. The ecision was to try to fly with a tip riven system because it coul be flown with only two rotor blaes. See Figure 1. Power Consierations Inuce power y z x Inuce power is the power neee to generate lift. It is the preominate power requirement in a hover. Happily, inuce power is reuce significantly near the groun. (in groun effect, IGE). Cursory analysis of the inuce flow inicates that a human powere craft with a gross weight of 200 lbs. will nee a very large rotor to hover out of groun effect (OGE), see ref. l.

3 P wt 3 /2 1/2 / R forp 0 8 hp 440 ft lbs/ s wt 230 lb R 65 ft Profile power The above oes not inclue Profile power, which is the friction an form rag neee to move the rotor through the air. The pilot must also provie the aitional power for this purpose. profile power C 0 n R 4 W 3 c /8 0 1 hp Groun effect The first effort was to etermine the effects of ultra low in groun effect (IGE) on inuce power. Much effort was spent with electrically riven moel rotors showing that as the rotor approaches the groun plane the inuce flow an the inuce power is inee sharply reuce. The test ata are ifficult to scale up to a Vinci size. Current expectation is that inuce power necessary to hover is inee less that 50 per cent that for OGE. The Leonaro a Vinci project can be clearly unerstoo by looking at the five major components separately. These components are: the rotors; the propellers; the centre section; the rive system; an the control system. Rotors Da Vinci I The purpose of the rotors is to provie the lift necessary for flight. Originally a tip spee of 35 fps was impose for safety reasons. This fiat put success back by a perio of 3 to 4 years. Using the limitations impose on them, the stuents esigne a tapere an twiste rotor blae system that was very similar to the successful craft that flew 8 years later. The main structural member of the Da Vinci I was a 4 in. iameter carbon graphite beam with an 8 egree wrap to withstan the 6000 ft-lb. bening moment. The main spar faile its static test. The rotor was then raise to 4 ft. above the groun. This allowe a guy wire to be use to relieve bening moment stresses. The airfoil was a Liebeck esign. The chor tapere from 6 ft. at the tip to 8 ft. at the root. The twist was linear -6 egrees. Da Vinci II The next generation rotor blaes were esigne using numerical analysis. The ecision was to conquer the performance problem with size. The new blaes were built with no taper an no twist. The raius was increase to 70 ft. possibly the largest rotor system ever built. The problem was that the new machine was not stable an crashe upon liftoff.

4 Da Vinci IIB The rotor system was a 120 ft rotor with a tapere chor. it was teste a Vanenberg AFB uring the spring of Da Vinci III The rotor system was reuce to 100 ft. with smaller chors to use a higher tip spee. DaVinci IV The rotor system will be similar to successful craft but with lighter structures. Propellers Da Vinci I The propellers function is to provie the rive to turn the rotors. The initial props were built by han an were just mae to look correct. No analysis or test was attempte. The first props were never use. During the hiatus of 1983 while the rive system was being sorte out, a proper set of props were esigne an built using the Larrabee erivations for a minimum inuce loss propellers. The props were prouce in a mol using expaning foam. They were then covere by fiberglass or kevlar. The esign criteria were to use minimum power at 35 fps at 8 lbs. of thrust turning 350 rpm. This allowe the props to absorb approximately 1 HP. Da Vinci II-IV The new craft ha a much higher tip spee an neee less thrust. Flight attempts with the ol props inicate that they were inaequate to operate in the ifferent of a flight regime an were subject to quality control problems. New props, optimize for a higher spee an lower thrust were neee. The ecision was mae to reesign the props an to test them thoroughly before use. Each new prop is now built accoring to the same set of equations but is carve by han out of balsa an covere by mylar. The props were then teste in a NASA Ames win tunnel. The successful test gave the crew a great eal of confience in their performance. This propeller esign is currently in use an will be applicable for all future machines that fly at a tip spee of approximately 55 fps. A severe stability problem was encountere uring actual flight. Very small ifferences of propeller thrust create a pronounce swirl of the center section. Present plans for future stabilizing systems will not be able to overcome propeller thrust problems. The Da Vinci IV props must be ajuste precisely for a successful recor attempt. Center Section Da Vinci I The center section supports the pilot an the main rotor blaes. Originally the pilot was above the blaes an the rotor hea was very near the groun. The rotor blaes were cantilevere in orer to keep the blaes as eep in groun effect as possible. Failure of the main spar uring tests emane the use of guy wire system so the rotor hea was elevate 3.5 feet. Later the

5 sprocket an chain to be ae. The slight increase in weight was offset by the ease of changing gears on the new craft. Da Vinci IV Current esign is uner review, see ref. 2. Control System The system as flown uring 1989 was unstable. The control system will consist of a weight shift of the pilot in the rotating system. Only instabilities ue to rotations will be aresse. A rotating axis system must be efine. 'x' axis is perpenicular to the rotor axis in the horizontal plane. 'y' axis out the rotor axis. 'z' axis points own to form a right hane system. The rotors are contemplate to weigh 65 lbs. or to have a mass of two slugs. They can be moele as a slener ro 100 ft. long. Common practice inclues the mass of air containe in the wing cyliner (1 slug) when fining I xx I xx = 1/12 x 3 x I xx = 2500 slug ft 2 I zz = 1/12 x 2 x I zz = 1700 slug ft 2 The moment if inertia about the wing axis is very low. We assume the pilot is a point mass of 4 slugs at 3.5 ft. The system is chosen for convenience not at the mass center, an oes not inclue coning of the rotor system. I yy = 50 slug ft 2 The major instability seems to be prouce by a rotation about the wing axis ' '. This rotation changes the lift on the rotors an prouces a very large moment along the negative 'x' axis. Each rotary wing prouces 150 lb. of lift at a pitch of approximately.15 raian above zero lift. So the force generate on the right wing of a pitch 'theta' is an the lift on the left wing is The lift vector occurs at 3/4 raius which prouces a moment about the 'x' axis of -125,000 ft. lbs. Remember that we are in a rotating frame, so the moment equations must inclue coriolus accelerations. Hz is because t equals -1 ra/sec. The moment equation about the 'x' axis is as follows. M x I xx 2 t 2 I zz I yy t t

6 t t The weight of the pilot provies a small restoring moment about the 'y' axis. M y I yy 2 t 2 I xx I zz t -500 = t t t We can use a step function movement of the pilot as a control, or a aeroynamic surfaces to stabilize the system. Figure 2 is a plot of roll versus time Time secs Roll angle raians Figure 2 We will attempt to fly with an open loop controller. The rules are somewhat vague on the subject. For a more complete analysis see reference #3. Wish us luck! Reference 1. Helicopter Performance. Stability an Control, Raymon Prouty, PWS, ISBN Human Powere Helicopter: The Next Sam, Wm. B. Patterson, Fourth International Human Powere Vehicle Scientific Symposium Control of a Human Powere Helicopter in Hover, Joseph Totah an William Patterson, NASA Tech Memo

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