AHE - Rotor Modulation

Rotor Modulation

Advanced Helicopter Electromagnetics Laboratory | Comments

Most people have experienced the phenomenon of a disruption in their broadcast television or radio reception as they moved about the room. By standing in one location relative to the receiving antenna, the program they were enjoying degrades into a hiss of static; moving a meter to one side of that location restores the clarity of the reception. In an analogous manner, the motion of a helicopter's rotor blades can result in a disruption of communication. Because the effect is periodic, this disruption is known as rotor blade modulation (rbm). Although AHE-related studies concentrate on rbm associated with helicopters, they are applicable to other rotor-wing and propeller driven aircraft as well.

Rotor blade modulation can be quite oppressive. Nulls in the radiation pattern of a helicopter-mounted antenna due to rbm can be 30 dB or more below the nominal signal level. In the worse cases, the warbling effect garbles voice communications and can render digital communications useless. However, the use of digital signal processing may ultimately provide some relief from the effects of rbm.

The study of rbm is difficult due to the large number of variables involved. The level of rbm varies with frequency, polarization, and observation angle about the helicopter. It is a function of the type and element radiation pattern of the helicopter-mounted antenna involved, as well as its mounting location. The geometry of the helicopter fuselage can have an impact on rbm; but the rotor geometry, including the chord, length, shape, composition, and the number of blades, has the greater influence. Furthermore, the highly complex motions of the rotor blades and disc (pitch, tilt, coning, flapping, etc.) can alter the nature of rbm. To fully exacerbate the situation, very little has been published in the open literature on the subject, and the definitions of rbm (particularly with respect to the inclusion of phase) are not entirely clear.

AHE studies of rotor blade modulation include predictions (full-wave and assymptotic) and scale model measurements. In addtion to generating parametric studies, predictions are continually checked, when possible, against scale model and full-scale measurements (provided by AHE Consortium Members) to verify and assess their accuracy. One such comparison is illustrated in the following figures. In the ray-traced image of our BRL-CAD computer model of the Blackhawk helicopter, a whip antenna mounted near the top of the vertical stabilizer is visible. The rotor modulated (with both the main and tail rotors) radiation pattern of this antenna was predicted using ASU's FDTD-based NEWS code, and is compared with the full-scale measurement. Although there are some uncertainties in both the modeling and measurements, the agreement is quite good.