Infrared-sensor systems for real-time detection of accretion of ice on helicopter rotor blades are undergoing development. By providing early warnings of icing conditions, these systems would enable pilots to activate deicing equipment or take other corrective action to avoid the severe hazards posed by icing.

The accretion of ice on a helicopter rotor blade begins and is concentrated at the leading edge. Because the freezing process releases latent heat of fusion, the leading edge becomes warmer than the remainder of the blade surface. The present icing-detection technique is based primarily on infrared measurement of the icing-induced variation of temperature between the leading and trailing edges. Secondarily, the technique also involves the use of infrared signatures to determine whether a blade is dry or whether ice has already accumulated.

Figure 1. Infrared Radiation From a Passing Rotor Blade can be measured to detect a leading-edge temperature rise indicative of icing.

A system of the type under development includes an upward-staring infrared sensor mounted on top of a helicopter fuselage (see Figure 1). The sensor measures infrared radiation indicative of the temperature profile across the blade as the blade passes by overhead. Experiments have shown that the optimal sensor for this application is a thermoelectrically cooled PbSe photodetector, which is sensitive to radiation in the wavelength range of 3 to 5 µm. A prototype system that has shown promise in experiments includes such a sensor equipped with a germanium lens to focus on a small spot in the rotor-blade plane, plus electronic circuits for digitizing the sensor readings and processing the digitized readings.

Figure 2 shows infrared-sensor readings taken from three representative helicopter-blade passes during a field test. The curve labeled "Dry" indicates a fairly uniform blade temperature under non-icing conditions. The curve labeled "Icing" was obtained at the commencement of icing caused by the impingement of supercooled water droplets; this curve clearly shows the expected temperature rise at the leading edge. The curve labeled "Static Ice" manifests an apparent cooling of the leading edge after ice had accumulated on the blade but icing conditions were no longer present; this leading-edge-cooling effect is observed consistently under such circumstances and could thus be a basis for detecting ice already present after icing conditions have passed.

Figure 2. Plots of Infrared-Sensor Readings of a passing rotor blade exhibit distinct shapes indicative of the icing and non-icing conditions under which the readings were taken.

The most noteworthy feature of the infrared blade signatures of Figure 2 is that it is apparently possible to distinguish among the three indicated conditions from a single blade pass - an observation time of the order of 2 ms. Even if an ice-detection system employs an algorithm that processes digitized sensor readings from multiple passes to increase the robustness of the decision as to which of the three conditions has been detected, it should be possible to achieve a response time of the order of 1 s.

This work was done by R. J. Hansman of Massachusetts Institute of Technology and R. J. Rieder and S. Krishnaswamy of Visidyne, Inc., for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4-8
21000 Brookpark Road
Ohio 44135

Refer to LEW-16944.

Photonics Tech Briefs Magazine

This article first appeared in the July, 2001 issue of Photonics Tech Briefs Magazine.

Read more articles from the archives here.