Stability and control characteristics of model helicopters

[摘要] ENGLISH ABSTRACT:A need exists for the development of an unmanned rotorcraft capableof autonomous flight, as would be required for the survey of highvoltage electricity supply lines. A program was initiated at theUniversity of Stellenbosch in December 2002 in order to develop suchan aircraft.The first goal of this thesis was the development of software that couldcalculate the stability and control derivatives of a model helicopter.These derivatives could then be used in the formulation of anappropriate helicopter control strategy. The second goal of the thesiswas an investigation of the stability and control characteristics of modelhelicopters.The trim settings of the helicopter were required in the calculation ofthe stability and control derivatives. A computer program wasdeveloped to determine the trim settings of a helicopter in forwardflight. Another program was developed to calculate the stability andcontrol derivatives, using the results of the trim analysis.The trim analysis was based on the assumption of negligible couplingbetween the longitudinal and lateral modes of motion. The methodproposed by Bramwell (1976) was used to perform the trim analysis.The stability and control derivatives were calculated by obtaining thetrim settings from the trim analysis. These derivatives were then usedto solve the roots of the characteristic equations of the longitudinal andlateral modes of motion. The stability of the helicopters wereinvestigated firstly by examining the stability derivatives and secondlythrough root-loci analyses.The most important results were the following:• The root-loci analyses indicated that a helicopter without ahorizontal stabiliser suffered from instability of the phugoidmode. It was also found that the short-period motion of thesehelicopters was heavily damped. Fitting a horizontal stabiliser tothese helicopters caused the phugoid motion to become stableeven at low speeds. This was achieved at the cost of a reductionin short-period motion damping.• The periods of the lateral and longitudinal motions were smallerthan those found on full-scale helicopters. This was attributed tothe small mass and inertia properties of the model helicopters. An increase in speed is generally accompanied by an increase inthe stability of the helicopters. This could be attributed to theeffective operation of the tail surfaces at higher speeds.• The axial climbing speed of a helicopter is influenced by the rotorspeed. A low rotor speed allows higher climbing velocities at agiven power setting. This was due to lower induced power lossesat low rotor speed, assuming that no blade stall occurs.• The rotor speed does not influence the incremental amount ofpower (M:,) required to achieve a certain climbing velocity, dueto the fact that the profile power losses are constant for a certainrotor speed.• The simplified horseshoe-vortex theory can be used to analysethe downwash angle at the horizontal stabiliser if the helicopteris in high-speed forward flight.

[发布机构] Stellenbosch University