Thrust reversers have various types Blocker doors, C’Ducts and Integrated Cold Stream reversers. The clam shell type utilizes two doors at the rear of the engine, which come together and deflect the exhaust gas to flow in the opposing direction to the motion. This is a fairly simple method of thrust reversal and is common on JT8D engines used on Boeing 737 – 100, … 200’s. The C’Ducts are the modern type and these have several functions. The prime function is thrust reversal. There are two halves to this type and in thrust reversal there is a translating sleeve which deploys and moves backwards. This causes “Blocker doors” to come up onto the by-pass air and deflect cascade fairing, which direct the air forward to oppose the motion of the aircraft. The second function of this type is noise suppression and acoustics are an important consideration in these units. The other type works similar to the C duct but rather than being a two-half reverser, it is a full annular section. This is commonly used on the RB 211-524 G/H units. Again it uses a translating sleeve and blocker door system to deflect the air from the by-pass through the cascades to oppose the forward motion of the aircraft. The thrust reverser acts along with the brakes of the aircraft to reduce the speed of the aircraft during the touchdown segment of landing. They are particularly useful in situations where the brakes fail or there is a shortened runway.
Requirements and operating conditions
Blocker doors in thrust reversers are generally subjected to high load conditions: the pressure of the jet stream, the high temperature of exhaust gas and flaring of the jet engine, vibrations and shocks occurring at engine start up. Moreover, components should have the lowest possible weight and feature high strength characteristics; they shall also be reliable and capable of withstanding fatigue.
The function of blocker doors is as illustrated in figure below.
1. Blocker door
2. Shifting system for folding wing
3. (Jet) engine
4. Direction of movement of aircraft
5. Reversed jet stream
8. F – resultant force from the pressure exerted by the reversed jet stream
Blocker doors material
The selected material, carbon fiber reinforced epoxy resin, is suitable for this type of application. Composite blades for helicopter metal blades are now being replaced with blades made of composite materials – S glass fibers in an epoxy matrix. They have high stiffness, strength, resilience and temperature and fatigue resistance, high impact strength also. S-glass features higher fatigue life than E-glass. They are superior to “aramid” or carbon -reinforced composites. But glass or carbon fiber reinforced hybrid plastics are now being developed for high temperature applications with continuous use ranging up to about 300 C. These are used in DC-10 and also Boeings 727 & 777 (9% by total weight- vertical and horizontal tail). Reinforced plastics have reduced fuel consumption by about 2%. Substitution of Al in large commercial aircrafts with graphite-epoxy reinforced plastics could reduce both weight and production cost by 30% with improved fatigue and corrosion resistance. If we examine the diagrams of Tensile strength/% Reinforcement; Impact energy/% Reinforcement; Flexural modulus/% Reinforcement; Flexural strength/% Reinforcement for a carbon fiber-based and glass fiber-based (short and long) composite material, we can see that most carbon fiber (CFRP) characteristics are superior to those of glass-fiber composites with the exception of impact energy.
Design considerations and selection of the suitable method
We know that the RTM (Resin transfer moulding) method features the following characteristics: it is used to manufacture more complex parts than compression moulding and for higher production rates; some scrap is lost; medium tooling cost. In our particular case this method ensures best characteristics of manufactured components to meet the high operational requirements. Components shall be light in weight, they have coupling elements to connect to particular assembly units involved in the overall mechanism (the reverse thrust system) and ribs are required to be provided along with sudden transitions between thin and thick walls and supports. The RTM method ensures uniform strength for different loading directions which is due to the uniform mixing of reinforcement fibers in the overall mass of the matrix – in our case the epoxy resin. Additionally, the method ensures complete filling of the mould without porosity. Despite of the higher cost of the moulds, the RTM method is very suitable for this particular application.