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Helicopter anemometer for measuring aircraft's speed relative to air, uses a pressure/speed formula including total pressure measured by probe, static atmospheric pressure and a Mach number
Helicopter anemometer for measuring aircraft's speed relative to air, uses a pressure/speed formula including total pressure measured by probe, static atmospheric pressure and a Mach number
A pressure/speed formula is used where: Pt = Ps(1 + 0.2M2)(7/2) where Pt is the total pressure measured by a probe, Ps is the static atmospheric pressure, M is the speed expressed as a Mach number. Method of measuring helicopter speed using a probe (14) mounted a rotor blade (12) in which: (a) N pressure (Pi) samples are taken spread over the complete rotation of the rotor; (b) a first binary signal (F1) is produced and changing state at the passage of the blade situated at 0 and 180 degrees for a blade reference orientation and a second binary signal (F2) in phase quadrature to the first and changing state at 90 and 270 degrees. The following sums (S1,S2) are made during a complete rotation; (c) S1 - sum of samples obtained during a first state of the first binary signal diminished for the sum of samples obtained for the second state of the signal; (d) S2 - sum of samples obtained during a first state of the second binary signal diminished for the sum of samples obtained during the second state of the binary signal; (e) calculate a numerical value representing arctangent(S1/S2). The result determines a phase corresponding to a speed direction searched for. To determine the speed amplitude N pressure samples, used during determination of phase phi are stored as well as the corresponding measuring instants. values =P / 2 and 3 * pi / 2 are added to the phase phi (modulo 2 * pi ) to produce two other phases =Fmax and =F min. The latter values represent the theoretical instants of the max. and min. values of the pressure curve measured during rotation. In the stored samples a search is made for max. and min,. pressure values (Pmax,Pmin) corresponding to the instants. The instantaneous values Vmax and Vmin are calculated from the pressure values as a function of a speed/pressure formula and the amplitude of the helicopter speed is determined from Vmax and Vmin
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