FLIGHT TESTING METHODOLOGY AND PROCEDURE OF SPIN CHARACTERISTIC ON BASIC TRAINING AIRCRAFT SLAĐAN PEKMEZOVIĆ MIROSLAV JOVANOVIĆ ZORAN ILIĆ Abstract: This article describes methodology and procedures of measuring and calculating aeroplane characteristics during flight tests. Measuring ning characteristics was simultaneously done by equipment build in plane and opto-theodolites from the ground. Spinning characteristics is calculated from date obtain from both systems, with aim to satisfy certification request for basic training aircraft Key words: aircraft, flight test, measure,. 1. INTRODUCTION There is probably no other type of flight-testing which requires a more comprehensive and logical build-up program than initial testing. This is due to frequently unpredictable and disorienting post-stall aircraft characteristics, sometimes-unanticipated modes and the requirement for a rapid pilot analysis of the situation and initiation of immediate corrective action. All tactical and training aircraft require thorough testing and although a has no value as a combat tactic, the military pilot must have confidence that the aircraft can be successfully recovered from any uncontrolled flight maneuver, which may be reasonably encountered. An airplane that cannot be consistently recovered after an accelerated stall or a probably will not be flown to its utmost tactical capability. For these reasons, test programs must be conducted to demonstrate that the test aircraft can be satisfactorily recovered from post-stall gyrations, as well as incipient and steady state s with reasonable and consistent piloting techniques under all conditions in which such motions are likely to be encountered during the aircraft s service operations. Spin is a sustained auto rotation motion of an airplane at high positive (or negative) angles of attack (AOA) above the stall angle of attack (α s ); caused by interaction of aerodynamic and inertia forces and moments. The fullydeveloped (steady) is attained when the spiral trajectory has become vertical and the characteristics are approximately repeatable from turn to turn. Classification of modes is made depend on different characteristics: 1. Angle of attack: o Erect for α> o Inverted for α< 2. Direction of airplane rotating: o Left - airplane rotating to the left-hand wing panel. o Right - airplane rotating to the right-hand wing panel 3. Mean angle of attack (attitude): o extremely steep : α<4 ; o steep : 4 <α<6 ; o flat : α>6. 4. Variation characteristics (mean values parameters of airplane motion) from turn to turn: o equilibrium (steady) : mean values angle rate p, q, r, pitch angle θ, roll angle φ, etc. repeatable from turn to turn; o no steady : mean values angle rate p, q, r, pitch angle θ, roll angle φ, do not repeatable from turn to turn. 5. Airplane parameter variation relative to mean values during a turn: o smooth : instant values p, q, r, pitch angle θ, roll angle φ of are close to mean values; o oscillatory : instant values of p, q, r, pitch angle θ, roll angle ø differ notably from mean values. 6. Ability of the airplane to maintain equilibrium at fixed settings of control devices for a certain time interval: 128
o stable : the airplane maintains parameters; o unstable : the airplane naturally declines from initial parameters. In picture 1 is present position of aircraft in in relation of main axis of rotation. 2.2 Measuring Equipment Measuring equipment which was used for this testing consist of: Telemetry system, Optotheodolite system and Data acquisition system. The nature of tests makes it mandatory that certain known critical items be monitored continually by engineering personnel on the ground; therefore, telemetry (from the safety viewpoint alone) is a firm requirement for testing. For this testing was used PCM/FM telemetry system LORAL for real time data processing, test control, and a data processing system for post flight analysis. This system includes both airborne data acquisition part and ground telemetry system Picture 1: Position of aircraft in in relation of main axis of rotation 2. EXPERIMENTAL DETAIL 2.1 Research Aeroplane Flight testing methodology and procedure was applied to prototype I of airplane LASTA-95 with purpose to investigate their characteristics. The Lasta is new generation primary and basic trainer, developed to provide high teaching effectiveness and easy transition to any advanced training aircraft. Its purpose is for: basic flying, figure flying, navigation flying, basic elements of night flying, category II instrumental flight, basic elements of gunning, rocketing and bombing (GRB) and light close air support of counterinsurgency operations and area patrol/light attack missions. The LASTA is powered by the Lycoming AEIO-54- L1B5D piston engine with maximum power of 22 kw. Also, LASTA is equiped with propeller HARTZELL HC- C2YR-4CF/FC 8475-6, which diameter is 1.98 m and maximum RPM 27. The structure of LASTA is all metal. The construction of fuselage is semi-monocoque with integral vertical stabilizer, ventral and truss type engine support. The wings have two spars. Picture 2: Prototype I of airplane LASTA-95 Picture 3: Ground telemetry station Purpose of using optotheodolite system in this kind of testing is for tracking tested aeroplane in the air, computation of object coordinates (x, y, z) in function of time and recording measured data. Used optotheodolite system was produced by Swiss factory CONTRAVES. Configuration of this system is two theodolites with theodolite stations positioned at a relative distance from 2 km This system use two measuring metod: laser method (better accuracy at larger distance, only one theodolite is needed) triangulation method (completely passive, better accuracy at smaller distance, more then one point can be measured) For this testing was used triangulation method because it was necesary to measure more than one point. 129
4. CARRYING OUT SPIN FLIGHT TESTS Picture 4: CONTRAVES optotheodolite system For acquisition data from sensor and probe during flight testing we used multi-function airborne acquisition system MFAS-AE1. Multi-function airborne acquisition system MFAS-AE1 was developed in Military Technical Institute in Žarkovo. Picture 5: Multi-function airborne acquisition system MFAS-AE1 The flight tests are expected: to identify all possible modes, to work out procedure for airplane recovery from a all possible modes, to evaluate dependence of ning motion and recovery on airplane configuration, weight distribution (including center-of-gravity position and moments of inertia). The test should cover development and features of when entered at various altitudes and flight modes, at speeds from the minimum to the maximum, at or without sideslip, lateral control stick position in Pro- and Anti- and recovery. Because of everything we said above, we must define conditions for flight tests. The initial altitude for entering during flight tests must be such that the pilot be capable of trying at least twice to recover the airplane from a without resorting to anti- devices, in our case H i =3m. The should be entered at speeds exceeding V min by 1-2 km/h, in our case V i =12, 13, 14 km/h. Also, the testing was done for clean and landing configuration, and both for forward and backward center of gravity. To improve safety of the flight testing, in first time the airplane was equipped with anti- parachutes (ASP). The anti- parachutes is engaged by the pilot who has a special control panel in the crew deck. For this kind of testing it was necessery to measure following parameters of flight: Control surface positions (δ l, δ m, δ n ), Roll, Pitch, Yaw rate (p, q, r), Euler's angles (θ, ψ, φ) and Normal acceleration in direction of main axis (a x, a y, a z ). 3. CALCULATION THE PARAMETERS OF SPIN On the basis of recording parameters of flight, we can calculate main parameters of with next algorithm: Number of turn A turn is defined as a 36º change in heading. One of methods which is used to resolve the count is integration of the resultant rate Ω. Angle of attack Ω 2 =p 2 +q 2 +r 2 (1) α=π/2+θ (2) Radius of rotation c.g. of airplane around axis a x = - cosψ * (9,81*sin θ + Ω 2 R cos θ) (3) a z = 9,81*cos θ - Ω 2 R sin θ (4) Picture 6: Anti- parachutes Testing was realize through three phase of development : entry, incipient, steady state. 4. PROCESSING AND ANALIZING DATA To initiate, we used a few procedure. We established that the best way to initiate was to fully pulled pitch control stick, and after approximately 1 sec. moved pedals simultaneously to the side of the motion planned. The task at the first stage of testing was to perform no more than one turn of left-hand and right-hand while 13
keeping the roll control stick at neutral. When the process and recovery was successful, then the experiment was extended to perform six turns of left-hand and right-hand, with increment of half turn. δ l, δ m, δ n [ o ] 4 3 2 Right-hand - developed with 6 turn δm δn δl A turn is defined as a 36º change in heading. With method of integration resultant rate Ω, we calculate how many turn is nessesery from initiation of recovery control application to stopping rotation. Also, we calculate altitude loss in the course of recovery from a and altitude loss after recovery from a in the course of recovery from a post-stall diving, to the time instant of setting the airplane for level flight. The result are presented on pictures 9, 1, 11. We can see that requirements are satisfied. 1 3195 32 325 321 3215 322 3225 late to recovery to stoping rotation -1-2 Δφ (turn) 2-3 1,5-4 t [s] 1 Picture 7:Control surface positions δ l, δ m, δ n in fullydeveloped,5 1 2 3 4 5 6 7 Right-hand - developed with 6 turn p, q, r [ o /s] 3 25 p q r Picture 9: Number of neccesery turn to stop rotation 2 15 1 ΔH (m) 2 late to recovery to stoping rotation 5 3195 32 325 321 3215 322 3225 15-5 -1 t [s] 1 Picture 8: Roll, pitch and yaw rate in fully-developed 5 1 2 3 4 5 6 7 The first characteristic which we analyzed was development of roll, pitch and yaw rate in in function of time. In picture 8, on first look we can see that is very oscillatory, and after three turn became fullydeveloped (steady). Picture 1: Altitude loss in the course of recovery from a Next very important characteristic which we analyzed was recovery technique.a satisfactory recovery is usually defined for this class of airplane as one that is accomplished within one and half turns from initiation of recovery control application. The procedure required for recovery should not involve unusual or difficult pilot techniques and the recovery must be consistent and repeatable. It is preferable that only the primary flight controls be used for recovery. 5 ΔH (m) 4 3 2 1 late to recovery to return to level flight We concluded that best recovery technique is to move pedals fully against rotation Anti-, while keeping the roll control stick at the neutral position; after some time of approximately 1s, set the pitch control stick to the neutral position or the position that corresponds to straight level flight at angle of attack. With rotation terminated, set the pedals to neutral. 1 2 3 4 5 6 7 Picture 11: Altitude loss after recovery from a to the time of setting the airplane for level flight 131
Picture 12: Radius of rotation calculate from a x and a z For calculation radius of rotation c.g. of airplane around axis we used two method. The first method is analytical and the radius of rotation is calculated from normal acceleration in direction of x and z axis. Algorithm is presented in equation (3) and (4). The results calculate from equation (3) and (4) are presented on picture 12. From picture 12 we can see that results calculate from equation (3) and (4) are very different. The second metod for calculation radius of rotation around c.g. of airplane is from data registered with optotheodolite system. The results are presented on picture 13. Picture 13: Trajectory projection of characteristics point in horizontal plane The results calculate from equation (4) are in accordance with results from optotheodolite system. Because of that the radius of rotation calculate from equation (4) is reliable and its value is R 2,75m. 5. CONCLUSION We concluded that primary flight controls are enought and very effective for satisfactory recovery from any mode. A satisfactory recovery for any mode is accomplished within.8 turns from initiation of recovery control application. For this class of airplane requirement is one and half turns. The best recovery technique is to move pedals fully against rotation Anti-, while keeping the roll control stick at the neutral position; after some time of approximately 1s, set the pitch control stick to the neutral position or the position that corresponds to straight level flight at angle of attack. With rotation terminated, set the pedals to neutral. Airplane LASTA has very oscillatory, because its parameters roll rate and pitch rate varied relative to mean values during a turns. After three turns became fullydeveloped (steady) because the characteristics became approximately repeatable from turn to turn. Radius of rotation center of gravity around axis is R 2,75m. Reliable method for calculation radius of rotation is analytical way from normal acceleration in direction of z axis (equation 4). REFERENCES [1] Charles A. Sewell, Raymond D. Whipple, "Initial tests", Grumman Aerospace Corporation, Bethpage Long Island, 1972. [2] H. Paul Stough III, Daniel J. DiCarlo, James M. Patton, "Flight Investigation of Stall, Spin and Recovery Characteristics of a Low-Wing, Single- Engine, T-Ta [3] il Light Airplane", NASA Technical Paper 2427, May 1985. [4] "Stability and control flight test techniques", USAF test pilot school, 1977. 132