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Aviation History
1948
1948 - 0212.PDF
184 FLIGHT FEBRUARY 12TH, 1945 Flight-Testing of Helicopters When the pilot displaces the stick from its trimmed position(starting from straight and level flight) and then releases it, the resultant behaviour of the aircraft gives an indication of itsstick-free stability. With the R-4B, if the stick is displaced forward and released, the helicopter dives and the stick movesfarther forward as the dive steepens. The converse is also true, and recovery action must be taken to prevent the developmentof a tail slide. Thus the stick-free stability is a periodically divergent. , . ... ,When the pilot displaces the stick and returns it to its initial trimmed position, the resultant behaviour of the helicopter indicates the stick-fixed stability. Under these conditions the Sikorsky R-4B develops an oscillation, the period and damping factor depending on the initial conditions of airspeed, power, etc. The series of lateral control tests is of a similar nature to the longitudinal control tests, but the rolling moment of inertia is only i/'ioth of the pitching moment of inertia, and the tendency to over-control the helicopter laterally is accentuated. On the R-4B, if the control stick is displaced laterally and released, the initial roll is damped out but the control is out of balance to the left and a turn to port begins which soon develops into a steep spiral dive. With the control displaced lateially and fixed in its initial position, a combined lateral and directional oscillation ensues. The period is of the order of two to three seconds and the motion is rapidly damped in forward flight. Loss of Control The most undesirable characteristic experienced in the flighttests of the Sikorsky R-4B was a loss of control during vertical descent. This occurs frequently in zero forward air speeddescents at values of collective blade pitch greater than 7 deg. and is always obtained under these conditions if the pitchis reduced. The loss of control takes the form of a nose-down pitching of the helicopter, and full backward movement of thecontrol stick cannot prevent the speed increasing rapidly to about 40 m.p.h. with consequent loss of height in regainingcontrol. The explanation lies in the generally unstable nature of theflow in the vortex ring state. The rate-of-descent curve against translational speed takes the form of a cusp at zerospeed, and small changes in translation give considerable re- distribution of the flow under the rotor. These vertical changesin the flow have a considerable effect on the fuselage pitching moment. Flight tests with smoke streamers and wool tuftshave shown that reversals of flow over the rear fuselage take place as the pitching motion is set up. The development ofbackward velocities appears to have a particularly serious influence on the nose-down pitching of the helicopter. This loss of control due to the fuselage pitching moments inthe unstable flow condition should be borne in mind especially when considering;the fitting of a tail- plane to the heli-copter. The recent flightmeasurements o f the longitudinaldynamic stability of the SikorskyR-4B constitute one of the mostinteresting items of research in thepresent phase of helicopter develop-ment. It is par- ticularly importantin view of the com- plete lack of accu-rate measurements in this section ofhelicopter work. For the dynamicstability tests an automatic observerwas essential. De- synn transmitterswere fitted to the two rods operatingthe swash plate. Each of thesetransmitters oper- ated two receivers, INITIAL AIRSPEED 6OM.RH. -RECOVERY ACTION TAKEN. "MA 0 20 40 INITIAL AIRSPEED 7VT ilNITIAL AIRSPEEDi 25 MRH INITIAL AIRSPEED 40 M.P. H| INITIAL AIRSPEED O M.P.H. RECOVERY ACTION TAKEN . 20 SEC 2OSEC. Fig. 7. Typical attitude'time records taken during power-on dynamic stability trials. , one in the .automatic observer and the other in front of thepilot. Desynn transmission was also used for the collective pitch of the main rotor blades and the tail rotor blades. As the aircraft air speed system does not give accurate speedmeasurements, a special trailing pitot-static system was used to obtain pitot and static pressures free from' interference.The pitot-static was mounted in a gimbal and ball race, allowing complete freedom to swivel in any direction. Thedifferential pressure gauge (used as a more accurate type of low-reading air speed indicator), an altimeter and a rate-of-climb meter were connected to this trailing system. These instruments were used, primarily to give the initial conditionsand it will be appreciated that in the phugoid motion, the in- dications may lag behind the actual helicopter conditions. An electrically-driven free gyro was installed to indicatechange in attitude. The gyro could be re-set from the observer's position at the beginning of each run. Normalacceleration, engine manifold pressure, engine and rotor r.p.ni. -0-20J- .20 10 20 3O 4OAIRSPEED -M.P.H. DAMPING FACTCSK, X 2O 3O AIRSPEED-MP.H. PERIODIC TIME OF OSCILLATION OC.G.O-5IN AFT OF SHAFT FOR* DISPI + CG30IN. AFT .. XCG.O-5IN BACK? " AQG3'OIN.FOR?.. DCC-3-OIN. FOR? - »CG3'0IN •• BACK? DISPI FOR? •• BACK? - Fig. 8, a and b. Dynamic stability characteristics (power-on) given in terms of damping factor (a) and period of osci//otion (b). and a timing clock were also included in the automaticobserver. The essential feature in flight-testing technique was to makecertain that the control was returned to and maintained in, its initial position after displacement. It was not consi^sJjWadvisable to fit a rigid clamp to the controls because of the vibration. During the tests the pilot obtained the initialconditions required and noted the Desynn reading of the stick position. The control was displaced, returned to its normalposition and firmly held by the pilot. The firmness of the pilot's grip was the main feature in maintaining the trimmedposition, and the use of the Desynn indicators to ensure that the stick had not wandered from its initial position was asecondary consideration. Attempts were made to maintain constant control position by holding the stick loosely andtrying to keep the Desynn readings constant, but this led to continuous over-correction. Simultaneous time histories of the flight conditions and >.control positions were obtained during the longitudinal phugoid motion. On analysis of the films the only measurements usedwere those in which the control position variations were very small. The power-on tests were made from .straight and levelflight at constant height, except at very low speed where there is insufficient power available to maintain height. Tests inauto-rotation were made mainly at i\ deg pitch but a few measurements were made at 4 deg (the rotor speed beingcorrespondingly lower). Measurements were made for three centre of gravity positions covering a range of 6in. In each condition, measurements of the phugoid motionwere made for forward and for backward initial displacement of the stick. Typical attitude/time records during the power-
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