Simulator Study on Gyrocopter Pilot Performance in Different Takeoff Procedure Scenarios
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Simulator Study on Gyrocopter Pilot Performance in Different Takeoff Procedure Scenarios F. Sachs, I. Pruter, H. Walter, M. Friedrich, O. Mielke German Aerospace Center (DLR), Lilienthalplatz 7, 38108 Braunschweig Abstract This paper deals with different takeoff procedures of gyrocopter. Most common takeoff procedures for cur- rent gyrocopter are very similar to those of airplanes. Nevertheless, there are distinct differences in perform- ing a takeoff with an airplane in contrast to a conventional gyrocopter. A further takeoff procedure highlighted in this work is the vertical takeoff of the gyrocopter, the so called jump takeoff procedure. To compare the two takeoff procedures with respect to pilot’s behavior and performance the unique gyrocop- ter simulator at the Institute of Flight Systems is used for an extensive pilot-in-the-loop simulator study. Three groups of participants were invited to the experiment: fixed wing pilots, gyrocopter pilots and unexpe- rienced persons without any pilot license. Each participant got the task to perform a series of takeoffs at dif- ferent wind conditions using the conventional takeoff procedure and the jump takeoff procedure. During the experiment the participant’s interaction with the simulator as well as their eye movements were logged. Dur- ing each experiment the participants had to fill in a questionnaire on subjective pilot ratings. The paper presents a short theoretical introduction on gyrocopter technology and explains the two takeoff procedures as well as their particularities, advantages and challenges. Furthermore the experiment on the two takeoff procedures and the results of the comparison of the both takeoff procedures are shown. 1. INTRODUCTION Since gyrocopters were certified in the Ultra-Light The rotating wing of a gyrocopter creates lift inde- class in 2004 the number of licensed gyrocopters in pendent from airspeed as long as a sufficiently Germany increases continuously even if the boom of strong upward airstream through the rotor plain is the first years seems to be broken. assured. Therefore stall is not a threatening issue for They are popular especially with private pilots. Their gyrocopter pilots. Due to the high maximum angle of main advantages are low acquisition costs, low op- attack of the rotor disk very low forward speeds are erating costs and affordable spare parts. Further- achievable. more gyrocopters offer unique flight characteristics Ultralight gyrocopters can perform level flights with which are attractive for a vast range of customers. flight speeds of 18 to 20 kts. Decelerating the autogi- ro to zero forward flight speed evokes the transition The German Aerospace Center investigates the to vertical autorotation. At zero forward speed the flight physics of gyrocopters for many years. For gyrocopter sinks with a stabilized sink rate after a several test campaigns and projects two flying de- short transition phase. monstrator are operated: an MTOsport and Cavalon. The gyrocopter is the only flying vehicle that is oper- Both types are produced by the manufacturer Auto- ated in an autorotative state within the complete gyro Ltd. flight regime. The transition from forward flight to vertical autorotation is often performed by pilots as a normal flight maneuver. Low disk loadings and a favorable thrust to weight ratio of current ultra-light gyrocopter allow short takeoff roll distances (~120 m) and thanks to low landing speeds very short landing roll distances (~20 m). Moreover, due to comparatively small moments of inertia, those vehicles show a high agility. Especially during landing the pilot can use the rotor’s Figure 1: Modern Gyrocopter Configurations characteristics perfectly to flare and to stop the air- One main research goal is to look beyond the bor- craft within a short distance. To slow down the vehi- ders of current vehicle designs and respective regu- cle on the ground the rotor is tilted backwards and lations. The possibility of taking off vertically, chang- the rotor force is used to decelerate. es the gyrocopter’s flight envelope radically. For the conventional takeoff procedure the rotor has 1 to spool up during the takeoff run to reach a suffi- 2. THE TAKEOFF PROCEDURE ciently high rotor RPM for flight. At the same time the gyrocopter has to accelerate to reach a velocity high enough to fly. 2.1. The Conventional Takeoff Since the ground run ought to be as short as possi- ble those two purposes are contradictory, because The conventional takeoff is the current standard the rotor and the resulting rotor force are highly in- takeoff procedure for gyrocopter with an acceleration phase on the ground, similar to fixed-wing takeoffs. clined to enable a significant airstream through the rotor itself. Due to this inclination the rotor generates 2.1.1. Conventional Takeoff Procedure a braking force, which counteracts the thrust of the propeller and decreases the acceleration. The takeoff procedure comprises 6 distinct phases. The gyrocopter leaks one main advantage of the The first step is the preparation phase (PP). In that helicopter. This is the ability to takeoff and climb phase the pilot starts the engine and releases the vertically. rotor brake. This phase is followed by the pre- However this gap can be filled by the realization of rotation (PR) of the rotor. While the gyrocopter the so-called jump takeoff, which would make the stands on the ground the pilot couples the rotor with gyrocopter more competitive for rotorcraft applica- the engine and the rotor rotational rate is increased tions. to a takeoff value (approx. 200 rpm). There have been several activities in the field of In the third phase (G-AC) the rotor is tilted back- jump takeoffs already in the 1930th. wards and after a pilot throttle command the gy- The first successful demonstration on a C30 gy- rocopter accelerates rapidly. In that phase the rotor rocopter took place in 1935. rotational rate increases after a short decay at initial- ly low forward speeds. The fourth phase (RT) starts with a pitch-up move- ment of the gyrocopter which has to be counteracted by pilot inputs at a moderate pitch angle. The mo- ment that rotates the gyrocopter about the main landing gear axis is caused by a strongly tilted rotor thrust vector in combination with an increasing rotor thrust. The pilot has to balance the gyrocopter in a pitch-up attitude to further accelerate the rotor to the maximum rate possible without pitching further up before reaching sufficient lift to take off safely. (Figure 6) Figure 2: Cierva C40 in jump takeoff 1938 [1] As soon as a sufficient amount of lift is provided by the rotor, the gyrocopter lifts off. This indicates the Some of the current autogyro designs are already beginning of the fifth phase, the in-air-acceleration equipped for jump takeoffs. Examples are the Carter phase (A-AC). Copter PAV [2], the Carter GyroDemonstrator or the Groen Brothers Hawk 4. While the technical realization has already been demonstrated, the particular demands on pilot’s abilities during such a takeoff have not been fully investigated. This simulator study on jump takeoff is one first step to investigate in how far the jump take- off could be an alternative to current takeoff proce- dures with respect to the necessary pilot actions. The following research questions were addressed by this study: How do pilots accept such a jump takeoff system? Are they capable to act with a modified gy- Figure 3: Gyrocopter at a conventional takeoff in the moment rocopter properly and safely? of rotation How do they evaluate such a takeoff proce- Depending on its weight the gyrocopter lifts off with dure? different speeds. To enhance safety margins the gyrocopter is accelerated close to the runway to a speed that gives sufficient time to react in case of engine breakdown or any other major failure. More- 2 over a speed of best climb has to be reached to perform an efficient and safe climb. As soon as that speed is reached the gyrocopter starts to climb to the desired altitude, this is the last phase of the takeoff (CL). Figure 4 shows key pa- rameters for the conventional start. Figure 5: Drag curve of a reference gyrocopter Other typical hazards in takeoff might be the follow- ing pilot errors: Pulling the Stick to late at already high for- ward speed or not to far enough blade flapping on the ground Inclining the rotor with the wind possible rollover due to wind passing the rotor from below No reaction to strong pitch up movement in takeoff run tail strike Figure 4: Simulated conventional takeoff data [3] Contact of front wheel with runway while rudder deflected at speeds above taxi speed strong yaw moment and tipping danger 2.1.2. Hazards In general the gyrocopter shows very good flight 2.2. The Jump Takeoff characteristics. Nevertheless some flight maneuvers should be avoided. One of such conditions is the The jump takeoff differs strongly from the conven- slow flight close to the ground or to obstacles. Figure tional takeoff. The gyrocopter lifts off vertically and 5 shows the drag curve of a reference gyrocopter. the acceleration phase is done in the air. Minimum drag is reached at approximately 90 km/h. At slower velocities the drag increases. This region 2.2.1. Jump Takeoff Procedure is often designated as the backside of the curve. Similar to the conventional takeoff the first phase of Due to the increase of drag slowing further down the jump takeoff is the preparation phase (PP) too. It requires more thrust. At very low speeds the gy- is also followed by the pre-rotation phase (PR). The rocopter has to sink to accelerate and to decrease differences to the conventional takeoff are that the drag because the engine thrust is insufficient.