Measurement of In-Flight Rotor Blade Loads of an Autogyro Helmut Rapp, Peter Wedemeyer Institut für Aerospace-Technologie, Hochschule Bremen, Bremen, Germany Christian Teuber STN Atlas Elektronik GmbH, Bremen, Germany Abstract go ahead in gyroplane development until there was Autogyros, or gyroplanes, are rotary wing aircraft extensive pressure due to military requirements. with no driven main rotor. The rotor keeps rotating In later stages their gyroplane was able to take only by the airflow resulting from the plane’s forward off vertically, to proceed the so-called "Direct Take- speed. Since WW2 there has been only a few investi- Off" over a 10 m obstacle and a vertical landing, if gations concerning the flying characteristics and per- required. Several gyroplanes were obtained by the formance of autogyros including blade loading. US-Military and a thorough research programme was This work covers both the theoretical and exper- undertaken at NACA-Laboratories compared to the imental investigations of rotor blade loading. The small research programmes done by the British, Ger- main parameters for the flapping moment are rotor man and French military. However, due to different speed and mass distribution of the rotor blades. For design the later developed Gyrocopter does not reflect the experimental investigations, a small telemetric the NACA results. In some short term research this system was developed. Up to four strains in the ro- type of aircraft was covered, even as a solution for a tor blades can be measured by using strain gauges. Mars-Landing-Vehicle and Pilot-Recovery-Systems. Wireless transmission of the strain data from the ro- Major design effort was put into gyroplanes for the tating rotor to a computer inside the fuselage is ac- civil market, reflected in the McCulloch J-2 and Um- complished by 433 MHz transceivers. A simple data baugh 18A, which is still in production as a fully cer- protocol to detect and correct faulty data was imple- tified aircraft. mented. In addition to the strain measurements, video In Europe, development was pushed forward, find- sequences of the rotor blade motion are recorded by a ing its success in the Fairey Rotordyne, as the prob- small rotor hub mounted video camera. ably most advanced aircarrier with 68 seats. Any further development targets were too ambitious and 1. Introduction therefore were unsuccessful, mostly due to fact that If someone talks about the technology of autogyros the designers’ and project leader’s requirements were this technology seems to be well known because of expecting to combine helicopter and gyroplane char- the commonly-known rotary wing aircraft i.e. heli- acteristics. copters. Since the 50’s or 60’s there scarcely has been Since these short studies it is not known about any further research in this particular kind of aircraft, any further research until during the last 6 years two especially with small gyroplanes. United States based Companies started to develop a The autogyro, a type of aircraft originally devel- competitor to the helicopter. They are called "Hawk" oped in Spain in 1920, made its first successful flight and "CarterCopter", both developments are sponsored on 9th January 1923 [1]. After a short period of work through indirect governmental sources. in Spain, major development was moved to the United In 1993 first steps were made to establish a gyro- Kingdom, where the military found much interest in plane aircraft class in the United Kingdom. Due to this particular type of aircraft. Very soon other coun- the number of complaints these regulations didn’t be- tries obtained the rights to use the patented designs for come effective. Meanwhile, several fatal accidents their particular developments. At this time the major with "AirCommand" gyroplanes lead the CAA to take research was done in the United Kingdom by Cierva direct action and started a research program at Glas- Autogiro Company of Juan de la Cierva and his team gow University [4] [5]. and also in the United States by the American Au- In 1993, a German programme to establish the gy- togiro Company of Harold F. Pitcairn and his team. roplane within the Ultralight-Aircraft class began and Both teams in 1937 were able to develop, build and was pushed until end of 1999. Despite notification fly prototypes of helicopters. of the requirements through the EC there has been no However, both companies were always aware that German progress, but France and Italy have made sev- the helicopter would be the more expensive to oper- eral decisions and pushed their regulations. ate solution for minor operations. This lead them to Since no progress has been in Germany, it was presented at the 26th European Rotorcraft Forum, 26 - 29 decided in 1997 to start a gyroplane research pro- September 2000, The Hague, Netherlands gramme at Hochschule Bremen, a university of ap- 101.1 plied sciences, supported by the director of ARROW Introducing λ into (1), together with the solidity ratio : Engines (UK), Ltd., who at this time was fully in- (c0:7: blade chord at 0 7R) volved in the German activities. This at least gives σ c0:7b = : (5) a little bit of industrial support, and the Hochschule 0 7 πR Bremen need not obtain, maintain and handle a gyro- the rotor speed Ω of the autogyro with thrust is equal plane and a pilot just for the reason of research and to weight T = W follows from education. Ú Ù Due to European connections of ARROW Engines 1 Ù W : Ω = Ù σ θ λ (6) (UK), Ltd., these research results will find their way Ø R 0:7a π 2ρ · R directly into safer designs and better performance of 2 3 2 light gyroplanes. In the present report, a thorough investigation, us- At least, the important vertical sink speed vvert of the ing both theory and experiment, has been carried out, autogyro can be determined by Ú especially where blade loading is concerned. The ba- Ù Ù W = ; sic theory for calculating blade bending moments is vvert Ù (7) Ù 2 CT π ρ shown. Telemetric equipment is developed to mea- Ø 2 R 2 CT 4λ · 1 sure in-flight blade loadings. 2λ2 CT denotes a thrust coefficient 2. Basic autogyro theory σ θ λ Basic theory of pure autorotation is well known by he- 0:7a : · CT = (8) licopter specialists [2], [3]. In the following the basic 2 3 2 equations are given. The theoretical results of these Table 1 shows basic data as well as the calculated ro- equations are to be verified later by flight measure- tor speed and vertical sink speed of the VPM-M16 ments. autogyro. 2.1 Steady autorotation Steady autorotation means that there is no forward Table 1: Data of the VPM-M16 autogyro speed with respect to the aircraft. The airflow is MTOW 450 kg strictly perpendicular through the rotor plane and the engine power 120 HP loading of the blade is independent of its angular po- maximum speed 78 kts sition. According to [2], total rotor thrust T follows minimum speed 22 kts from integration of the local air loads along the rotor: radius of rotor 4115 mm weight of one blade 18.2 kg θ λ 1ρ Ω2 3 number of blades 2 · ; T = abc R (1) 2 3 2 precone angle β 2.0 Æ pitch angle θ 2.5 Æ where ρ denotes the air density, a the slope of the lift curve, b the number of blades, c the blade chord and lift curve slope a 5.73 R the rotor disc radius. The blade section pitch angle profile drag coeff. CD0 0.012 θ is assumed to be constant over the rotor radius. For blade chord c0:7 250 mm 9 2 ¡ a given 1g flight condition, thrust must be equal to the flapping stiffness position 1 15:25 10 N/mm 9 2 ¡ known weight of the autogyro. flapping stiffness position 2 2:867 10 N/mm To determine the unkown rotor speed Ω and inflow inflow coefficient λ 0.0209 coefficient λ, rotor torque MT has to be considered solidity ratio σ 0.0339 (CD0: profile drag coefficient). rotor speed Ω 388 1/min vertical sink speed v 9.6 m/s θ λ vert 1ρ Ω2 4 CD0 λ · : MT = bc R a (2) 2 4 3 2 2.2 Theory of blade loading The rotor of an autogyro is not driven so the overall In steady autorotation, the blade loading is indepen- torque has to be zero: dent of the angular position of the rotorblade. There is no change of rotor loading with time. Figure 1 shows : MT = 0 (3) the principal out-of-plane loading of a rotor blade. For this investigation, in-plane drag is not considered. With this, equation (2) can be solved for the inflow The aerodynamic lift results from local airspeed coefficient λ µ ρ v´r and the local lift coefficient CL ( air density, A × reference area): θ θ 2 λ 1 a CD0 ρ · · : = a (4) 2 µ= ´ µ : f ´r v r C A (9) 3 a 3 2 z 2 L 101.2 Static loading of rotor blade (blade weight) Aerodynamic lift fz (r’) 200 Deformation in mm Bending Moment in Nm Q(r) M(r) 0 fr (r’) Centrifugal Force β = 3.0 deg. N(r) β = 0.0 deg. w(r’) β = 1.0 r −200 deg. r’ β = 2.0 deg. ra Deformation, Shear Force, Bending Moment Figure 1: Loading of rotor blade −400 0 1000 2000 3000 4000 Rotor Radius in mm µ The centrifugal force fz ´r can be obtained by Figure 2: Flapping moment and deformation of the ¼ 2 µ= Ω ; fr ´r m r (10) non-rotating blade where m ¼ describes the local mass.