Light Canard Research Aircraft

Light canard research aircraft (LCRA) is an all-composite aircraft with canard and is based on the Rutan long-EZ design. It is a two-seater aircraft with pusher propeller and has a tricycle landing gear with retractable nose wheel. The wings have moderate sweeps and the canard has trailing edge flaps that function as the elevators. The LCRA has two rudders. The ailerons are situated in the wings. SP and DR maneuvers were carried out at altitude = 1.53 and 2.74 km and at three speeds 65, 85, and 105 knots at each altitude. Table 9.6 shows the comparison of the analytical (values that were almost constant) and the average values of FDDs of LCRA for longitudinal and LD axes. The scatter and the CRBs for most of the estimated derivatives were reasonably low [21]. Also, SP natural frequency and damping ratio were fairly constant over the AOA range and hence average values are given in Table 9.6. The asterisk (*) indicates that there is a slight trend of the damping-in-roll derivative (C) with AOA and signifies the reduction in the damping with increasing AOA, whereas the analytical value is constant. This shows that the flight data provides additional information and hence enhanced confidence in the dynamic characteristics of the vehicle. This exercise shows that for this light and small aircraft, the aerody­namics are fairly linear with respect to AOA. It also shows that though the FDDs are not far away from the analytical values, the pitch and directional static stability are reduced in flight. However, the dihedral static stability is increased.

9.4.5 Helicopter

The flight tests on a helicopter consisted of 14 sets of recorded data runs at the uniform sampling rate of 8 samples/s. The longitudinal modes were excited by

applying a doublet input command and a 3-2-1-1 like input for the longitudinal cyclic control, and lateral maneuver was excited. The longitudinal derivatives are shown in Table 9.7. Since the reference values were not available, no comparison could be made with the FDDs. Hence, the approach used was to analyze the individual data sets and also to use the corresponding data in a multiple maneuver analysis mode. The latter involves the concatenation of the time histories of the repeat runs (obtained at the same flight conditions) and use in the OEM such that average results are obtained. Since the results from both these approaches were almost identical, except for a few cases, and the estimates had low CRBs, the FDDs were believed to be of acceptable values. In most aircraft parameter-estimation applications, the forces and moments acting on the aircraft are approximated by terms in Taylor’s series expansion, resulting in a model that is linear in parameters. However, when the aerodynamic characteristics are rapidly changing, mathematical that have more num­ber of parameters and cross derivatives may be required. Table 9.8 gives the lateral FDDs of the helicopter estimated with and without cross derivatives. The improve­ment in the lateral velocity component and yaw rate response match is observed when cross-coupled derivatives Nu, Nw, and Nq are included in the yawing moment equation

during estimation. With cross-coupled derivatives included, estimated values of other yaw derivatives, particularly Nr, register a change resulting in improved match. The time-history match with flight data improved when the model included some cross­coupled derivatives [22]. This aspect is important for the flight data analysis of a helicopter. The search for obtaining adequate aerodynamic models that can satisfac­torily represent the helicopter characteristics can be further pursued using SMLR and the model error estimation (MEE) method.