The document provides a detailed overview of the design and function of air inlet ducts in turbine engines, explaining their roles in air intake and pressure recovery. It discusses factors influencing air flow efficiency, including the shape and placement of the duct, and differentiates between subsonic and supersonic inlets, including their unique design considerations for different flight conditions. Additionally, it highlights the importance of minimizing drag and maximizing air pressure for optimal compressor performance.

1. G.T.E FUNDAS – ENGINE CONSTRUCTION AND DESIGN.
PART 1 – AIR ENTRANCE. - NISARG MISTRY

2. THE INLET DUCT
The Inlet of a turbine engine is typically located at the front of the G.T.E which is formed by
structural parts located forward of the compressor.
It is normally considered as a part of the Airframe and not the PWR plant.
Its purpose is to admit air to the forward end of the compressor .
The length , shape and placement of the duct is determined to a great extent by the location of the
engine in the aircraft.
Not only must the duct be large enough to supply the proper airflow , but it must be shaped correctly
to deliver the air to the front of the compressor with an even pressure distribution.
Poor air pressure and velocity distribution at the front of the compressor may result in compressor
stall and/or compressor blade vibration.
*Another primary task a duct must do during flight operation is to convert kinetic energy of the
rapid moving inlet airstream into a Ram pressure rise inside the duct. To do this it must be
shaped so that the ram velocity is slowly and smoothly decreasing , while the Ram pressure is
slowly and smoothly rising.

3. *The Air inlet is designed to recover as much total pressure of the free stream air as possible and
deliver it to the compressor . This is k/a “RAM RECOVERY” or “PRESSURE RECOVERY”.
In addition to recovering & maintaining the pressure of the free stream air , inlets can be shaped to
increase air pressure above atmospheric pressure , this Ram effect causes air to “PILE UP” (jama
hona) in the inlet. The faster the Aircraft flies the more air piles up and the higher the inlet air
pressure .
The inlet are rated in two ways -
1) - RAM RECOVERY POINT - The Ram Recovery point is that Aircraft speed at which the Ram-
pressure rise is equal to the “Friction pressure Losses” (due to skin friction drag) or that airspeed
at which the compressor inlet total pressure is equal to the outside ambient air pressure.
A good subsonic duct will have a low RAM Recovery point (about 160Mph or 257.4 Km/Hr).
2) – DUCT PRESSURE EFFECIENCY RATIO - It is defined as the ability of the duct to convert
the kinetic energy or dynamic pressure energy at the inlet of the duct at (Pt1 ) into static pressure
energy at the inlet of the compressor (Pt2 ) without a loss in total pressure. It will have a high value
of 98% if the friction loss is low and if the pressure rise is accomplished with small loses.

4. Air Inlet
Subsonic Supersonic

5. 1) – SUBSONIC INLET -
The air inlet duct also provides a uniform supply of air to the compressor for efficient operation.
The inlet duct is designed to cause as little drag as possible . It takes only a small obstruction to
the airflow inside a duct to cause a severe loss of efficiency
It should deliver maximum volume of air with a minimum turbulence.
This inlet is for subsonic range aircrafts which flies below Mach 1.
On a typical subsonic inlet, the surface of the inlet from outside to inside is a continuous smooth
curve with some thickness from inside to outside. The most upstream portion of the inlet is called
the highlight, or the inlet lip. A subsonic aircraft has an inlet with a relatively thick lip.
Inlet lip

6. To prevent the duct from damage or corrosion an “Inlet Cover” should be
installed anytime the engine isn't operating.
Inlet cover.
Some aircrafts such as military aircraft &
other old commercial aircraft had a wing
mounted inlet or a fuselage mounted inlet.
Wing inlet.
Fuselage Mounted inlet.

7. The design of an inlet duct can also vary by the type of engine used.
Also the amount of air passing through the engine is dependent upon three factors :
1) – The compressor speed.
2) – The forward speed of the Aircraft .
3) – The density of ambient Air.
*Inlet ducts can also be used to pre-clean the air before it enters the compressor .
Traditional filters or screens are not used becoz they would offer too much resistance
to air flow. Particle separators for turboshaft/turboprop engines take advantage of
the natural inertial property of matter to continue in straight line.
DGCA question
Traditional Inlet screen

8. -Ref. Aircraft Gas Turbine engine tech. by Irwin E.Treager page -164 3rd edition.

9. 2) – SUPERSONIC INLET -
The design and construction of the inlet duct for high speed aircraft is of critical importance becoz of
its profound effect on both the airframe and engine.
The supersonic inlet is complex in construction becoz not only must the air be delivered to the face of
the compressor but at an acceptable mass , flow rate, velocity , angle and pressure distribution , it
must it must do this under wide extremes of aircraft speed , altitude and attitude and with as little
loss of total pressure as possible.
At supersonic speed the Bernoulli's theorem is reversed.
In this type of duct , the air is slowed down to MACH 1.0 in the convergent portion of the duct and a
shock-wave is formed. When the air flows through the shock wave , it shows to a subsonic speed & is
further slowed in the divergent portion of the duct before it enters the compressor.
In this way the supersonic inlet works. But when this aircraft flies at subsonic range then this fixed
type supersonic inlet won’t work.
Hence, aircraft that flies at supersonic ranges normally have variable inlet ducts that change their
shape as the airspeed increases.

10. So, the variation in the area of this type of duct is done either by lowering or raising a wedge (just
as a flap which is in RAM AIR INLET) or by moving a tapered plug in or out of the duct.
Therefore, when the aircraft is at subsonic speeds the wedge is raised creating a divergence at
the front part of inlet so the pressure increases and velocity decreases , while going towards
throat again the velocity increases a little to continue the flow and there is divergence at the front
of the compressor creating an increase in total pressure at Pt2 .
Compressor.Wedge.
At Subsonic
At supersonic
Throat Wedge

11. 3) – BELL MOUTH INLET -
This type of inlet is essentially a bell-shaped (GHANTA ) ) funnel having
carefully rounded shoulders , which offer practically no air resistance . The
duct loss is so small that it is considered zero , and all engine performance
data can be gathered without any correction of inlet duct loss being
necessary.
Its interesting to note that the engine manufacturers rate their engines
using this type of inlet.
Next Topic compressor stay tuned….