Ice and Rain Protection — Page 261, Lesson 331

Let’s pick up where we left off with the propeller de-icing system, but now we’re moving into the electrical heating side of it. I want to walk you through how the heating elements are built, how they get their power, and how the system is controlled — because this is where the real engineering detail lives. First, the basis of effective de-icing in electrical systems is resistance wire heating elements. These are bonded to the leading edges of the propeller blades. Now, for turbine engine propellers — that is, turboprop installations — the elements are either woven or sprayed onto the front shell of the spinner. The spinner is the aerodynamic cone at the very front of the propeller hub. So you have heating on the blades themselves, and on the spinner shell for turbine engines. The power for these elements can be either direct current or alternating current, depending on the aircraft type. And here’s the key: the power is not applied continuously. It’s applied in a controlled sequence by a cyclic timer unit. That timer is what orchestrates the on-off cycling we’ll talk about in a moment. In turbopropeller engine installations, the propeller heating circuit is part of a larger power unit de-icing and anti-icing system. And the cyclic control for the propeller is integrated with the engine air intake heating circuit. So the same timer that cycles the propeller heat also cycles the intake heat — they’re tied together. Now let’s look at the construction of the elements themselves. They’re sometimes called overshoes, and the construction varies between propeller types. In one commonly used propeller, the heating element wires are interwoven with glass threads, and that forms a glass cloth base. This glass cloth base is then cemented between sheets of rubber. So you have a sandwich: rubber, glass cloth with the wires, rubber. On top of that, a protective guard of wire gauze is cemented beneath the outer rubber covering. So the gauze sits under the outer rubber layer, protecting the element from damage. The whole overshoe is shaped to fit around the blade leading edge and is cemented to it. In some cases, the overshoe is cemented into a rebate — that’s a machined recess — in the leading edge, so it lies flush with the blade surfaces. That keeps the aerodynamic profile clean. Now, power supplies. The power is conveyed to the elements via cables, slip rings, and brushes. The brushes are contained within a brush block housing. The slip rings are normally mounted at the rear of the propeller hub, or on a starter ring gear. The brush housing is normally on the engine front casing. But in some systems, the mounting is the reverse way round — so don’t assume a fixed arrangement. The cables are made of sufficient length and positioned so as to allow for movement of the blades throughout their designed pitch range. That’s important — the blades pitch, they rotate, and the cables have to accommodate that motion without breaking or pulling loose. Now, heating control. This is the clever part. Efficient operation of these systems requires a relatively high consumption of electrical power. That’s a problem, because you don’t want to draw that much power continuously. So the system uses a cyclic de-icing technique. Here’s how it works. A short unheated period allows a thin film of ice to build up on the leading edges of the propeller blades. You deliberately let a little ice form. Before that film builds up enough to interfere appreciably with the aerodynamic characteristics of the blades, the cyclic control applies heating power. Now here’s the physics: the ice that has already deposited acts as thermal insulation. As the ice in contact with the blade surfaces melts, the main ice catch — the bulk of the ice — is carried away under the action of centrifugal and aerodynamic forces. So the heat melts only the thin layer right at the blade surface, and the spinning propeller flings the rest off. That’s why you let a little ice build up first — it makes the shedding more effective and saves power. Let me show you the schematic circuit for this. Now, beyond the propeller and spinner, there are other items that may need heating. These are the miscellaneous items. Let me list them for you, because you need to know each one: - Pitot heads or probes — those measure airspeed. - Alpha probes — those measure angle of attack. - Q feel probes — those provide aerodynamic feel forces to the controls. - P1 probes — those measure total pressure, the ram air pressure. - Waste water drain horn — that’s the drain for the lavatory waste system. - Total air temp heads — those measure the outside air temperature, corrected for ram rise. - Aerials — the antennas. - And water pipes for “in line” heaters — those keep water lines from freezing. So the same principle of electrical heating applies to all of these, but each has its own specific installation and purpose. The propeller system is the major one, but these miscellaneous items are all part of the ice and rain protection picture. That’s the electrical de-icing system for propellers — the construction, the power supply, the cyclic control, and the other heated components.