Experimental and Theoretical Studies of Autogyro Flight Dynamics Dr Douglas Thomson Dr Stewart Houston Department of Aerospace Engineering University of Glasgow Glasgow, UK G12 8QQ Keywords: autogyro, stability, simulation, flight testing Abstract in the UK in the early 1990’s the Civil Aviation Authority’s attention was drawn to autogyro’s after a series of accidents A comprehensive flight dynamics study of the between 1989 and 1991 which gave statistics of 6 fatalities autogyro is presented in this paper. A state of the art generic per 1000 hours of flying time. Given that there were less simulation of the vehicle type was developed and validated than 100 aircraft of this type registered in the UK this was against flight data. This validation is presented in the paper constituted a serious problem. The latest statistics show and it is shown that the model can be applied to the autogyro some improvement, Figure 1, [2], however it is clear that with some confidence within well defined limitations there is still a problem with this aircraft type. bounds. It is also shown that the general stability characteristics of the autogyro can be considered as a mix of helicopter and fixed wing aircraft modes of flight. Most 6 fatal accidents 6 reportable fatal accidents accidents significantly the autogyro has a lightly damped, high reportable accidents 5 frequency phugoid mode. Further, it is demonstrated that 5 4 the only significant configurational effect is related to the 4 relative vertical position of the centre of gravity with respect 3 to the propeller thrustline, a centre of gravity which lies 3 2 2 222 above the thrustline being more desirable. Finally, results 2 2 222 1 111 1 1 from preliminary handling qualities trials using an autogyro 1 111 1 1 are presented. 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 1. Introduction The emergence of the autogyro aircraft in the 109 109 average rate of fatal accidents 1920's and '30's paved the way for the development of the peraverage million rate hours of fatal flown accidents 1992-2001 helicopter in the 1940's [1]. Many of the technical problems per million hours flown 1992-2001 associated with rotary wing flight had been discovered and rectified by the early autogyro pioneers most notably Juan de la Cierva's solution of installing flap hinges to accommodate 36.1 36.1 non-symmetric lift from the rotor blades. The development of the autogyro receded as the helicopter became more gyroplanes airline aeroplanes airline aeroplanes public transport popular and successful. In recent years however there has gyroplanes airline aeroplanes airline aeroplanes public transport (<5700 kg) (>5700 kg) helicopters been a resurgence of interest in this type of aircraft both as a (<5700 kg) (>5700 kg) helicopters recreational aircraft and as a low cost alternative to the helicopter with companies such as Groen and Cartercopter Figure 1: UK Autogyro Safety Statistics in Comparison both seeking to market autogyro configurations to with other Aircraft Types commercial and military operators. The autogyro has become a very popular vehicle for hobby flying, possibly Investigation of these accidents was hindered by a due to its flying characteristics but also as they are often lack of contemporary published research into this vehicle, purchased in kit form giving the owner the opportunity to particularly in its aerodynamic characteristics and its flight build and fly his own aircraft. dynamics and flying qualities. This led the UK Civil Aviation Authority (CAA) to fund research in these areas to This resurgence in interest by private flyers has also support a major review of the British Civil Airworthiness led to closer scrutiny by regulatory authorities. In particular, Requirements for autogyros (BCAR Section T) [3]. The aim is to improve the design standard of autogyros in the UK and unlikely that the reduced Reynolds number of the tests so improve their safety. The University of Glasgow has been would produce any significant differences between the supporting the CAA in this activity in a number of ways measured force and moment coefficients and those including wind tunnel testing of an autogyro model, flight experienced by the full-scale aircraft. This is primarily testing of 2 aircraft types and development of comprehensive because the basic gyroplane structure is non-streamlined simulation models. The aim of this paper is to review the and, consequently, insensitive to Reynolds number changes. research carried out on autogyros in the area of flight dynamics by Glasgow researchers. One of the most notable outcomes of the research was the first comprehensive study of the aerodynamics of an autogyro configuration [4], and this research is summarised in section 2 of this paper. More significantly much more is now understood about the flight dynamic characteristics of this configuration [5-8], and section 3 of the paper is devoted to this work. Finally, and most recently, work is now underway to develop handling qualities measures for this aircraft type, and the most recent results are discussed in section 4. 2. The Aerodynamic Properties of Autogyros There were two main aims in undertaking wind tunnel tests of an autogyro configuration. Firstly there was no known data for this type of vehicle and it was essential to Figure 2: Wind Tunnel Model have appropriate information to ensure that the flight (Rotor for display purposes only) mechanics simulations were as accurate as possible. Secondly, there was evidence that some of the accidents A full analysis of the test results is given by Coton which had occurred were related to owners modifying their et al in reference 4, here only some of the more pertinent aircraft by changing aerodynamic surfaces, pod or tailplane, conclusions are discussed. The aerodynamic characteristics for example. The question was just how much were the of the gyroplane configurations considered in this study are forces and moments on the aircraft influenced by such generally benign. It is, however, pertinent to note that there adjustments. The wind tunnel testing therefore included are several effects associated with the cowling which are cases with the pod removed, tailplane removed etc to allow detrimental to stability. Although the cowling on the VPM- comparisons to be made. The effect of propeller wash was M14 is particularly large, it is likely that any 'open' cowling also established by conducting tests with power on and design will be subject to similar effects in the longitudinal power off. mode. Additionally, the length of the VPM cowling is substantial; extending from well in front of the pilot up to the The model used in this study was a powered, one- rotor support column. The increased wetted area which this third scale model of a VPM-M14 gyroplane minus rotor, presents to the onset flow in sideslip acts to oppose the Figure 2. It is normal, in rotorcraft testing, to carry out wind stabilising effect of the tail. The tail of this aircraft benefits tunnel tests without the rotor since scaling considerations of from the additional sideforce produced by the endplates on a combined rotor-fuselage configuration would require the the horizontal surfaces. use of a very large test facility and would be prohibitively expensive. Note that a representation of the pilot is included With this data now available it was possible to as it is likely to be significant for a vehicle of this size. The construct a simulation model of the aircraft with which tests were conducted in the 3m Low Speed Wind Tunnel of to determine its dynamic characteristics. Further, as the Aeronautical Research and Test Institute (VZLU) of most autogyros have the same basic shape it is Prague in the Czech Republic. The particular wind tunnel proposed that this set of aerodynamics data (with used in this study is an atmospheric open-section, closed appropriate scaling) will give a useful estimate for a return, Gottigen style tunnel with a maximum velocity of range of aircraft. around 60m/s. Forces and moments were measured on a six component fully-automatic overhead gravitational balance which is accurate to between 0.01% and 0.05% full scale. The tests were conducted at representative advance ratio and propeller thrust coefficients however the Reynolds number (2.5 ×106) is 40% of the full vehicle value at cruise. It is 3. Flight Dynamics of Autogyros X u X w X q X θ X Ω X η s Z Z Z Z Z Z 3.1 The RASCAL Mathematical Model u w q θ Ω ηs A = M u M w M q M θ M Ω ,B = M η s One of the main aims of the research was to 0 0 1 0 0 0 modify an existing generic rotorcraft mathematical model, Q Q Q Q Q Q RASCAL [9] to simulate an autogyro, which could then be u w q θ Ω ηs used to predict the stability of new or modified configurations. It is appropriate here to present brief details T x = [u w q θ Ω] and u = []ηs of the rotorcraft flight mechanics model RASCAL it is described more fully by Houston [8, 9]. It is a generic This constitutes the longitudinal subset of the rotorcraft simulation code, the nonlinear equations of motion conventional 6 degree-of-freedom rigid-body flight taking the form: mechanics model, with the important (and unique) addition of the rotorspeed degree of freedom. This is required due to x& = f (x, u) (1) the aircraft operating in autorotation, the coupling of rotorspeed, Ω, to the body modes being captured by the where the state vector, x, contains the airframe translational derivatives XΩ , ZΩ and MΩ .