Features

Vibrating-reed ice detectors from the 1980s are designed for controlling inflatable rubber boots, but today’s UAVs require a more modern approach.

The perverse thing about ice on the tailplane of a general aviation aircraft is that the pilot sits and looks forward, but the tailplane is aft. You can’t see it from the cockpit.

One sign there’s ice on the tailplane of a general aviation (GA) aircraft in flight is when the controls become “mushy”. That’s a tactile clue, but when there’s no human being with hands on the controls, it’s a compound problem for unmanned aerial vehicles.

As for ice on the wings etc., GA pilots are trained to “look for ice”. Again, without a human being in direct control, it’s a double problem for UAVs. But even so, how do you see clear ice? How do you see white ice on a white wing? How do you see ice at night?

Clearly, ice sensors are de rigueur for UAVs.

UAVs can fly at great heights, and for long periods of time. But in order to get up there, they have to transit through the hazardous ice-formation zone below 20,000 feet; that’s where ice forms. Above that altitude, it’s generally too cold for ice to form on the airframe. Because the temperature lapse rate of our atmosphere is -3.5 deg F per 1000 feet, H2O molecules in the air up there may already be in their solid phase, and they just bounce off.

It’s when liquid water molecules (clouds) impinge on the airframe and freeze in place – that’s the danger.

Ice forms on thicker members of an airframe later (windshields and wings). That’s because thicker cross-sectional members compress more H2O molecules, and their cumulative ram-air heating effect is greater than it is on thinner cross-sectional members (horizontal stabilizers and rudders of the tailplane empennage). Less compressional heating means the tailplane remains colder, and ices up earlier, before the wings do.

Figure 1. This commercial, off-the-shelf, in-flight ice sensor monitors the optical characteristics of whatever substance is in contact with the optical surfaces of the probe, either air (NO ICE) or water ice (ICE ALERT). Ambient wind blows standing water away, but ice sticks. Made of nonconductive delrin and acrylic plastics, probe is electromagnetically compatible with its host aircraft radio environment, and can be installed in close proximity to radio antennas.

So, the perversity of ice on an aircraft tailplane is that ice is more likely to form there than on the wings; the tailplane can’t be seen, and the deleterious effects of ice on the horizontal stabilizer can be greater than ice on the wings, especially on landing.

Wing ice on the leading edges and upper surfaces is bad enough. It destroys lift, and the weight of it (90% that of water) not only burdens the powerplant, but is mostly forward of the center of gravity, inducing the nose to point down, toward the ground.

On landing, the effect of ice on the horizontal stabilizer is definitely worse than on the wing. An aircraft’s horizontal stabilizer is actually a small wing, mounted upside-down. It creates downward “lift”, forcing the tail down, and the nose up. If the horizontal stabilizer stalls on landing, the nose can pitch down very abruptly and violently. Ice-induced tailplane stall on landing is really bad, because there may not be enough altitude remaining in which to recover.

In-flight situational awareness is even more critical in a UAV than in a manned aircraft because of the absence of tactile feedback to a human pilot.

Bristling With GPS and Other Gear

Most large air transport and military aircraft are normally equipped with pneumatic de-icing boots made of rubber. These boots have been around since the 1940s and earlier; they can be activated manually by the pilot, or automatically, by an on-board ice sensor.

Figure 2. Test program conducted at NASA Glenn’s Icing Research Tunnel demonstrates sensor conformity with defacto standard SAE AS 5498 ¶5.2.1.1.1 minimum operational performance for in-flight icing detection systems. Also listed in SAE AIR 4367 ¶ 4.11. (Note NASA Pitot tube in left foreground, rotating deck for adjusting wind tunnel icing angle-of-attack).

Since the advent of rubber boots, more recent developments for aircraft de-icing include hot bleed air from the engine’s exhaust, electrical heaters, ethylene glycol weeping wings, capacitive discharge loops that literally blast ice off the leading edges, and others. Historically by default, most automatic deicing schemes use ice-sensing technology based on a vibrating reed, from the 1980s.

Vibrating-reed ice sensors protrude from the fuselage of an aircraft into the ambient airstream and resonate in free air at 40 KHz. When ice forms on the probe, its mass reduces the frequency of vibration. The sensor housing contains a circuit board that translates that change in frequency to an equivalent mass on the probe. Then another circuit board translates that equivalent mass to a theoretical thickness of ice, which, if it exceeds a threshold of typically 0.020 inch, the unit reports ice alert. This signal then activates pneumatic boots which expand and crack the ice away from the wing’s leading edge.

Even for aircraft that do not employ pneumatic de-icing boots, vibrating ice detectors have been the default ice sensors for 30 years, providing advisory ice alerts to pilots, advising them to take some corrective action – climb up out of the clouds to clean air, descend below the clouds to warmer air, or turn around and go back. All this is well and good, but when it comes to modern unmanned aerial vehicles, mechanically vibrating ice sensors are less than an optimum solution. In a major breakthrough in ice sensor technology development, today’s newest and most up-to-date ice sensors are available on the open market for subsonic aircraft.

Figure 3. Simple field test procedure sprays tetrafluoroethane component cooler on air gap optical surfaces, freezes ice out of ambient moisture, and tests rate-of- accumulation: ICE ALERT, MORE ICE, SATURATION.

Optical ice sensors are especially suitable to UAVs because they are small, lightweight, sensitive, and their probes can be made entirely of nonconductive plastic.

Modern UAVs are used as airborne platforms for surveillance, mapping, communications, fire fighting, agriculture, search & rescue, and other radio-intense applications. Compared with traditional general aviation aircraft, modern UAVs tend to be smaller and lighter, with less powerful engines, a lower energy budget, and bristling with GPS and other radio gear.

H2O Phase Change

Just as any other radio antennas, transmitting and receiving antennas on UAVs require a sphere of elbow room in which to propagate and receive electromagnetic signals correctly, without interference from nearby electrically-conductive structures that might distort and corrupt their frail, low-level satellite and terrestrial radio signals.