at
NASA Glenn Research Center The wing will ordinarily
stall at a lower
angle of attack, and thus a higher
airspeed, when contaminated with
ice because of the significantly lowered
lift coefficient and increased
aerodynamic drag. Even small amounts of ice will have an effect, and if the ice is rough, it can be a large effect nonetheless. Thus an increase in approach speed is advisable if ice remains on the wings. How much of an increase depends on both the
aircraft type and amount of ice. Stall characteristics of an aircraft with ice-contaminated wings will be degraded, and serious roll control problems are not unusual. The ice accretion may be asymmetric between the two wings which requires calibrating. Also, the outer part of a wing, which is ordinarily thinner and thus a better collector of ice, is likely to stall first.
Effect on unmanned aircraft Unmanned aircraft are an emerging technology with a large variety of commercial and military applications. In-flight icing occurs during flight in supercooled clouds or freezing precipitation and is a potential hazard to all aircraft. In-flight icing on UAVs imposes a major limitation on the operational envelope. Unmanned aircraft are more sensitive and susceptible to icing compared to manned aircraft. The main differences between UAVs and manned aircraft when it comes to icing are: •
Size and weight: Small aircraft accumulate ice faster, and more ice per unit area, compared to large aircraft. UAVs are typically smaller than manned aircraft and therefore more sensitive to icing. Furthermore, the added mass from ice accretions can have quick negative effects on UAVs with stringent weight restrictions. •
Flight velocity: High airspeeds lead to heating on the wings or propellers of the aircraft, which can counteract icing to some degree. Current UAVs generally fly at lower velocities than manned aircraft and will not benefit from the same heating effect. Therefore, icing on UAVs can occur at a broader range of temperatures than on manned aircraft. •
Laminar flow: The
Reynolds number for UAVs is approximately an order of magnitude lower than that for manned aircraft. This leads to UAVs operating in flow regimes where laminar flow effects are more prevalent than turbulent flow effects. Because laminar flow is more easily disturbed than turbulent flow, the negative effects of icing are bigger. •
Type: Rotary-wing UAVs are typically more sensitive to icing than fixed-wing UAVs. The parts of the UAV most exposed to icing are the airspeed sensor, the leading edge of aerodynamic surfaces, rotors, and propellers. Icing on UAVs is a global phenomenon, and icing conditions at the operational altitude can occur year-round around the world. However, icing risks are particularly big in the sub arctics, Arctic and Antarctic. In large parts of the Nordics, for example, icing conditions are present from 35% to more than 80% of the time from September through May. ==Prevention and removal== Several methods exist to reduce the dangers of icing. The first, and simplest, is to avoid icing conditions altogether, but for many flights this is not practical.
Pre-flight protection If ice (or other contaminants) are present on an aircraft prior to takeoff, they must be removed from critical surfaces. Removal can take many forms: • Mechanical means, which may be as simple as using a broom or brush to remove snow • Application of
deicing fluid or even hot water to remove ice, snow, etc. • Use of infrared heating to melt and remove contaminants • Putting the aircraft into a heated hangar until snow and ice have melted • Positioning aircraft towards the Sun to maximize heating up of snow and ice covered surfaces. In practice this method is limited to thin contamination, by the time and weather conditions. All of these methods remove existing contamination, but provide no practical protection in icing conditions. If icing conditions exist, or are expected before takeoff, then anti-icing fluids are used. These are thicker than deicing fluids and resist the effects of snow and rain for some time. They are intended to shear off the aircraft during takeoff and provide no inflight protection.
In-flight protection systems on the wing of a
Dash 8 aircraft. The ridges are the result of the boot being inflated with air to crack and remove accumulated ice. To protect an aircraft against icing in-flight, various
forms of anti-icing or deicing are used: • A common approach is to route engine "bleed air" into ducting along the leading edges of wings and tailplanes. The air heats the leading edge of the surface and this melts or evaporates ice on contact. On a turbine powered aircraft, air is extracted from the compressor section of the engine. If the aircraft is turbocharged piston powered, bleed air can be scavenged from the turbocharger. • Some aircraft are equipped with pneumatic
deicing boots that disperse ice build-up on the surface. These systems require less engine bleed air but are usually less effective than a heated surface. • A few aircraft use a
weeping wing system, which has hundreds of small holes in the leading edges and releases anti-icing fluid on demand to prevent the buildup of ice. • Electrical heating is also used to protect aircraft and components (including propellers) against icing. The heating may be applied continuously (usually on small, critical, components, such as
pitot static sensors and angle of attack vanes) or intermittently, giving an effect similar to the use of
deicing boots. In all these cases, usually only critical aircraft surfaces and components are protected. In particular, only the leading edge of a wing is usually protected.
Carburetor heat is applied to carbureted engines to prevent and clear icing.
Fuel-injected engines are not susceptible to carburetor icing, but can suffer from blocked inlets. In these engines, an alternate air source is often available. There is a difference between deicing and anti-icing. Deicing refers to the removal of ice from the airframe; anti-icing refers to the prevention of ice accumulating on the airframe. ==Related accidents and incidents ==