Compressor Inlet Ducts in Aerospace Engineering

Compressor Inlet Ducts

• The air intake must provide air to the compressor with minimal loss of energy and uniform pressure distribution across the inlet area.
• The air entrance or flight inlet duct is considered part of the airframe, not the engine, but is identified as Engine Station 1.
• The inlet duct's function is crucial to engine performance, and even small discontinuities can cause significant efficiency loss and engine performance problems.
• The inlet duct must be maintained in close to new condition to prevent drag, and repairs should be done with expertly installed flush patches.
• The use of an inlet cover is recommended to promote cleanliness and prevent corrosion and abrasion.

Movable Inlet Ducts

• A movable spike or plug can be used to alter the shape of the inlet as aircraft speed changes, creating a movable C-D inlet.
• During transonic flight, the movable spike is extended to produce a normal shock wave, and as airspeed increases, the spike is repositioned to shift the C-D duct for an optimum inlet shape.
• At supersonic speeds, the bow wave changes to multiple oblique shock waves, and a normal shock wave develops at the lip of the inlet.

Bellmouth Inlets

• Bellmouth compressor inlets are convergent in shape and are commonly found on helicopters and engine test cells.
• They present a mouth wider in circumference than the engine compressor inlet, smoothly converging to funnel air down to the compressor inlet circumference.
• Bellmouth inlets eliminate the 'necking down' effect of an airstream passing through a plain orifice, allowing the engine to draw all the air it can use.
• They are often used during ground testing and calibration, fitted with mesh screens to protect technicians from ingestion hazards.

Inlet Screens

• The use of compressor inlet screens is usually limited to rotorcraft, turboprops, and ground turbine installations.
• Screens have been tried in high subsonic flight engines, but icing and screen fatigue failure caused maintenance problems.
• Screens may be located internally or externally at either the inlet duct or compressor inlet.

Secondary Air Inlet Doors

• Some aircraft use a system of doors that allow extra air into the inlet duct, reacting to excess negative pressure within the inlet.
• The suck-in doors are pushed open by high external air pressure, allowing extra airflow to the compressor.
• The pressure required to open the door can be altered by adjusting the tension of the door spring.

Inlet Duct Losses

• Inlet duct losses can occur if the aircraft or conditions exceed expected flight attitudes, such as high AOAs or sideslipping, disrupting the smooth inlet airflow into the inlet duct.
• This can lead to compressor stall, and some aircraft are more prone to this phenomenon than others.
• During ground running, crosswinds can disrupt the inlet airflow and increase the AOA of the air into the compressor, causing compressor stall.

Inlet Anti-ice

• Anti-icing systems are used to prevent ice formation on the inlet area, which can cause compressor stall or engine failure.
• During flight, the anti-icing system is turned on before entering the icing condition.
• Some smaller turboprop and turboshaft engines use electric heat strip systems, which are cycled on and off as required by ambient conditions.
• Other systems use hot engine or reduction gearbox oil to anti-ice the engine and inlet.