Study of Air Intake in aircraft report

[Study of air intake in aircrafts] [2009]

A TECHNICAL SEMINAR REPORT ON Study “ Study

of air intake configuration in aircraft ” ”

Submitted in partial fulfillment of requirements for the 1st semester MASTER OF TECHNOLOGY IN AERONAUTICAL ENGINEERING Submitted by

CHIRAG.D.SONI M.Tech, 1st Semester Dept of Aeronautical Engineering MVJ College of Engineering Bangalore-560067

[MVJCE, Department of aero]

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DEPARTMENT OF AERONAUTICAL ENGINEERING

This is to certify that MR. CHIRAG.D.SONI has satisfactorily completed the seminar of 1st semester Master of technology in aeronautical engineering prescribed by VTU, Belgaum during the academic year 2009-2010. The seminar has been approved and satisfies

the

academic

requirements

in

respect

to

the

work

prescribed for 1st semester Master of technology.

Name of examiners

1.

……………………….

Signature of HOD

………………………..

2. ……………………….


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Abstract Topic: Study of air intake configurations in aircraft. The air intake is that part of an aircraft structure by means of which the aircraft engine is supplied with air taken from the outside atmosphere. The air flow enters the intake and is required to reach the engine face with optimum levels of total pressure and flow uniformity. These properties are vital to the performance and stability of engine operation. Depending on the type of installation, this stream of air may pass over the aircraft body before entering the intake properly. Selection of the correct type of intake and the associated inlet geometry has important consequences to any airplane design. For that reason, intake design receives considerable attention in the design phase of an airplane.


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[Study of air intake in aircrafts] [2009] Contents Chapter1

Page no:

1.1

Introduction to air intake

1

1.2

Need of air intake system

1.3

Air intake Design requirements

3

1.4

Intake configurations

4

2

Chapter 2 2.1

Jet engine intake (subsonic)

11

2.2

Determination of size of the stream tube

15

2.3

Deceleration of airflow

16

2.4

Air intake characteristics of Lockheed C-141

19

Chapter 3

3.1 Jet Engine Intakes: Supersonic

22

3.2 Flow conditions over wedge and cone

26

3.3 Intake configuration and operation

30

3.4 Examples of oblique shock diffusers

34

3.5 Supersonic air intake case studies

36

References [MVJCE, Department of aero]

41 Page 4


List of figures Chapter 1

page no:

1.1 Air intake in aircraft 1.2 Turboprop engine air intake

2 4

1.3 Plenum Inlet

5

1.4 Subsonic Bifurcated Inlet

6

1.5 Subsonic Podded Nacelle Inlet

7

1.6 Pitot type intake

8

1.7NACA Submerged Inlet in a Euro Fighter

9

Chapter 2 3.1 Intake flow field

12

3.2 Intake flow field at high speed

17

3.3 Air intake in Lockheed C-141

19

Chapter 3 3.10 Supersonic flow over wedge and cone

26

3.12 Comparison of supersonic flow

28

Over wedge and cone


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[Study of air intake in aircrafts] [2009] 3.11Total pressure loss and static pressure

29

Increase due to shockwave. 3.13 Operation of normal shock diffuser

31

3.15 Characteristics of oblique shock diffuser

33

3.16 Examples of oblique shock diffusers

34

3.17 F-16 intake characteristics

38

3.25 F-14 intake characteristics

41


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Introduction


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[Study of air intake in aircrafts] [2009] Chapter 1

INTRODUCTION 1.1What is air intake? In any application subsonic transport or supersonic fighter the air intake is essentially a fluid flow duct whose task is to process the airflow in a way that ensures the engines functions properly to generate thrust.

Fig 1.1[air intake system] 1.2 Need of air intake in an aircraft. A widely used method to increase the thrust generated by the aircraft engine is to increase the air flow rate in the air intake by using auxiliary air intake systems. The air flow enters the intake and is required to reach the engine face with optimum levels of total pressure and flow uniformity hence need of an air intake system. Deceleration of airflow at high flight mach numbers or aerodynamic compression with help of air intake.


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[Study of air intake in aircrafts] [2009] 1.3 Air intake design requirements The airflow first passes through the air intake when approaching the engine, therefore the intake must be designed to meet certain requirements of aircraft engines such as: The air intake requires enormous effort properly to control airflow to the engine. The intake must be designed to provide the appropriate amount of airflow required by the engine. Furthermore this flow when leaving the intake section to enter the compressor should be uniform stable and of high quality. Good air intake design is therefore a prerequisite if installed engine performance is to come close to performance figures obtained at the static test bench. The engine intake must be a low drag, light weight construction that is carefully and exactly manufactured. These above conditions conditions must be met not only during all phases of flight but also on the ground with the aircraft at rest and the engine demand maximum, thrust prior to take off


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[Study of air intake in aircrafts] [2009] 1.4 INTAKE CONFIGURATIONS Broadly the intake configurations may be classified as 1. Piston engine intakes 2. Turbo propeller intakes 3. Jet Engine Intakes: Subsonic 4. Jet Engine Intakes: Supersonic

Jet Engine Intakes: Subsonic These are of the following types: 1. Plenum Intake 2. Bifurcated Intake 3. Podded nacelle Inlet 4. Pitot Inlet 5. NACA Submerged Inlet

Turboprop engine air intake as seen below fig [1.2]


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[Study of air intake in aircrafts] [2009] Subsonic intakes Plenum Intakes These are used mainly in combination with double-sided centrifugal flow compressors. In this case the engine is installed in a region of large volume, the

‘plenum chamber’, in order that front and rear compressor intakes can receive equal air supplies. The aircraft intake feeds directly into the plenum chamber. Fig 1.3 shows a sectional view of plenum intake.

Fig 1.3 Plenum Inlet


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[Study of air intake in aircrafts] [2009] Bifurcated intakes are used primarily in single engine installations with side intakes Fig 1.4 shows a bifurcated intake.

Fig. 1.4 Subsonic Bifurcated Inlet


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Fig 1.5 Subsonic Podded Nacelle Inlet .


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[Study of air intake in aircrafts] [2009] Pitot type intakes have been applied to many fighter airplanes. They are not influenced by the flow field of other airplane components. However, they require very long ducts which cause extra weight and loss in pressure recovery.

Fig 1.6 shows a pitot type intake


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[Study of air intake in aircrafts] [2009] The NACA submerged type intake is not very efficient for use with propulsion installations. However, they are frequently used as intakes of auxiliary systems (auxiliary power unit, heating and avionics bay cooling) as seen in Fig 1.7

Fig 1.7NACA Submerged Inlet in a Euro Fighter

Except for the Pitot and the Podded nacelle type intakes, all jet engines intakes are equipped with boundary layer diverters (or B.L. Splitters). If such boundary layer diverters are not used, large pressure recovery losses (thus losses in thrust) are incurred.


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[Study of air intake in aircrafts] [2009] A major consideration in jet fighter intake design is the behavior of the intake at very high angles of attack and sideslip. Compressor stall and engine surging are easily induced in such conditions. In subsonic installations, the intake is kept as short as possible. Long ducts translate into weight and pressure recovery losses. In jet fighters and in jet trainers long ducts cannot always be avoided.


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Jet engine intake (subsonic)


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[Study of air intake in aircrafts] [2009] 2.1Subsonic air intakes The standard air intake has found widespread application with high subsonic civil and transport aircraft. Being of quasi circular cross section, the air intake forms the forward part of the engine nacelle. Subsonic air intakes are also applied to some combat aircrafts and virtually all jet training aircrafts that operate near the speed of sound. Here we find intake shapes of elliptical ,half circular ,or even irregular cross section ,with intake mounted on the fuselage sided or under the fuselage .


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[Study of air intake in aircrafts] [2009] Observed, the bounding streamlines of which will terminate in stagnation points on the cowl. With aircraft velocity increasing, stagnation points continue to move forward of the cowl.

2.2 Determination of size of stream tube Cross section A 0, of the stream tube well ahead of the intake is determined by the engine mass flow rate, the size of the stream tube may simply be determined by applying the continuity considerations. Continuity requires mass .

flow rate m at any cross section within the stream tube to be the same, which is hence a constant. constant. Mass flow rate at cross-section cross-section A 0, in particular ,exactly equals mass flow rate at the compressor face A=2=,which itself reflects engine mass flow .hence: .

.

m 0=m 2 Mass conservation may be expressed for the a particular flow path station (upstream infinity) and 2(compressor face) as follows Station 0(upstream infinity) .

m 0=p0v0A0 Station 2 (compressor face): .

m 2=p2v2A2 Therefore cross section of the stream tube at upstream infinity will result as simple expression: A0= (p2 /p0)*(v2 /v0)*A2 If air density is assumed not to change within the stream stream tube between the stations 0 and 2 ,then stream tube cross-section A 0 depends only on aircraft flight speed v0 , because air stream velocity at compressor face is determined


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[Study of air intake in aircrafts] [2009] by the compressor ,with compressor entrance cross-section A 2 a constant by design.

2.3 Deceleration of airflow at high flight mach numbers or aerodynamic compression with help of air intake. We know that for an air breathing engine to function correctly compression of air is a prerequisite. Aerodynamic compression occurs in flow ducts whose cross-sectional area gradually increases in stream wise direction. A duct with the ability to retard the flow and convert energy into pressure energy is termed as diffuser. diffuser. At sufficiently high mach numbers, for instance at cruising flight, airflow approaching the engine will be faster then would be tolerable for the compressor. Due to the diffuser action of air intake which is deceleration of the air flow and a buildup of pressure, airstream velocity will be adapted to the need of the compressor as seen in fig 3.2a. Additionally, due to the rise in pressure, a considerable benefit to the engine cycles results so that less mechanical energy is required for compression.

Pressure recovery and nose suction In order to prevent the flow from separating along the walls , the interior surface of the diffuser must be carefully shaped , and be smooth and unobstructed by steps or kinks , otherwise the sensitive

boundary layer

(between main stream and diffuser wall ) may separate. This would result in partial losss of kinetic energy and its conversion into unusable heat, a process termed friction which always results in a degradation of total pressure. If it were possible for the deceleration flow to convert all its of kinetic energy into pressure , then total pressure of the flow would remain constant and so-called pressure recovery would be 100%


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[Study of air intake in aircrafts] [2009] Nose suction formation Today’s high-subsonic cruise flight mach numbers which are in range of mach 0.78 to 0.85 call for an air intake design which features a relatively ‘thin’ intake i.e. where external dimension of intake is not much greater than the internal diameter. This will result in a small radius, leading to a relatively thin lipped air intake.

If the external flow is made to pass the intake lip ‘ correctly’, additional drag resulting from ram effect ahead of the intake may effectively be reduced. Such a reduction is accomplished solely by the air stream flowing around the nose. As the flow follows the contour of the nose, excessive velocities can develop which may even attains (low) supersonic speeds. This will cause a zone of low pressure around the intake‘s circumference , leading to the exertion of an aerodynamic force with a component acting in the direction of engine thrust and termed as nose as nose suction [3.2b].


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[Study of air intake in aircrafts] [2009] Air intake characteristics of Lockheed C-141 strlifter military transport 2.4



The intake is particularly noteworthy because of its short duct, denotes

as ‘zero-length inlet’ by Lockheed, which enabled a light weight constructions of high aerodynamic performance (fig ( fig 3.3). 

Due to its small radius, the intake lip is relatively sharp-edged which made necessary a secondary intake system that comes into effect at high airflow rates with aircraft static , or at low speed.



The slotted inlet embodies 12 sets of outer doors pivoted at the cowl.



The door opens against a spring force if a [pressure drop exists between the low static pressure on the engine side of doors relative to that of the external side of the doors.


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Jet engine intake (supersonic)


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Chapter 3 3.1 Jet Engine Intakes: Supersonic They are of the following types: 1.

Pitot Intake

2.

External compression Intake

3.

Mixed (or external/internal) compression Intake

A Pitot Intake has a number of attractive features, notably low drag and a stable flow characteristic with good flow distribution. Its disadvantage lies in the level of pressure recovery achieved. As shown in Fig 1.6, this type of intake has been used in aircrafts like the Mig 21.

Fig 1.6 Mig-21 Air Intake


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[Study of air intake in aircrafts] [2009] Proper inlet design is extremely critical to supersonic aircrafts. A long inlet duct is often needed to assure smooth flow deceleration (to around M=0.4 at the compressor face) and to assure full use of the favorable pressure distribution in the inlet duct. A typical intake for a twin engine aircraft is shown Fig 1.7. Different types of supersonic intakes are given in Fig 1.8 and some examples of supersonic intakes are shown in Fig 1.9.

Fig 1.7 Supersonic Twin Engine Inlet


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Fig 1.8 Supersonic Inlets


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[Study of air intake in aircrafts] [2009] Pressure waves in air


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[Study of air intake in aircrafts] [2009] 3.2 Flow conditions over wedge and cone In the design of supersonic air intakes flow conditions over wedge and cone are of the greatest importance as these are simple geometric bodies and relatively easy to manufacture.

First let us consider supersonic flow over a wedge. Such a device is installed in the air intake of the majority of modern supersonic combat aircraft such as F15 F-14, MiG-29, Su-27, but also in the airliner Concorde. We assume a wedge of unlimited length to be latterly immersed in a supersonic gas stream ((fig fig 3-10a). Flow conditions here are similar to the


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[Study of air intake in aircrafts] [2009] previously discussed corner flow where streamlines, after passing the shock front, are everywhere tangent to the wedge cross-section. Due to the compressive effect of the shock, the stream line pattern downstream of the shock is more compact hen it is upstream. If the wedge angle exceeds the maximum value permissible for that particular Mach number, the oblique shock will no longer remain attached but will jump abruptly upstream to form a (detached) bow shock. Part of the bow shock immediately ahead of the wedge apex acts like a normal shock causing the region between shock and wedge to be sub sonic, i.e. M<1 ( fig 3-10b) adjacent regions of the shock surface bounding the center normal shock region, increasingly bend in a downstream direction to form an oblique shock with, finally, degenerates into a (weak) mach line (not shown) . In order to design aircraft of low wave drag, the angle of the shock front must be small. This implies, apart from the supersonic Mach number flown, that nose sections of intake and wing must be given a knife-edge shape. We now understand why subsonic intakes with their well rounded nose sections are of less use in supersonic flow: the detached bow shock creates high drag which

will absorb much of the engine’s thrust, so that supersonic flight is virtually unattainable.

Comparison of supersonic flow over cone and wedge The major advantage of a (supersonic) conical flow is a smaller total pressure loss (when compared to a wedge of the same half-angle), together with the fact that a conical shock sustains lower mach numbers until it becomes detached to form a high loss bow shock. A major disadvantage of conical flows is that it is less tolerant to asymmetric flow conditions which cause distortion to the intake flow. As combat aircraft are frequently required to maneuver at higher angles of attack, the flow inevitably gets asymmetric- hence a performance for the (horizontally arranged) wedge in all modern combat aircraft, despite its reduced efficiency.


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[Study of air intake in aircrafts] [2009] Total pressure loss and static pressure increase due to shockwave.


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[Study of air intake in aircrafts] [2009] 3.3 Intake configuration and operation Present-day turbine aero engines require subsonic flow at the entry to the compressor, even if the aircraft is flying at supersonic speed. The task of air intake is therefore to decelerate the supersonic external flow to a subsonic speed acceptable to the compressor. As intake discharge mach number are required to be in range of mach 0.4 to 0.7 great care must be exercised when decelerating the flow in order to keep total pressure losses to a minimum .

Normal shock diffuser For aircraft operating at a maximum speed equivalent to mach 1.5 a normal shock diffuser is generally sufficient to decelerate the supersonic airflow efficiently to the speed needed by the compressor. The action of diffusing i.e. the deceleration of flow and build up of pressure is accomplished in two steps: The supersonic flow is (abruptly) decelerated, through the normal shock , to subsonic velocity with an accompanying abrupt increase in static pressure; In the diverging (subsonic) duct, where the flow is sill faster then would be acceptable to the compressor, deceleration of the flow continues with pressure increasing further. Case 2 Suppose the air flow demand of the engine is reduced. Then static pressure p 2 at the compressor face will rise ,less air is allowed to enter the intake, the excess airflow after being processed through the shock front is forced to flow outside the inlet as a so-called spill-over so-called spill-over flow (fig (fig 3-13b). Case 3 Suppose the air flow demand of the engine to be greater than the intake can so provide. At first, this is equivalent o pressure drop at the compressor inlet, either pressure decreasing upstream, too. This will eventually cause the shock


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[Study of air intake in aircrafts] [2009] to be swallowed, and the airstream to enter the subsonic diffuser at supersonic velocity .The inconsistency of duct geometry and flow velocity results in a complex

shockwave

pattern


within

the

duct

((fig fig

3.13c).

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[Study of air intake in aircrafts] [2009] Oblique shock diffuser intake characteristics The operational characteristics of an oblique shock diffuser may be summarized in three typical conditions.

Case1: Case1: If the normal shock that terminates the supersonic flow regime is exactly at the position of the diffuser throat (i.e. where the cross section is a minimum), the airflow rate is a maximum fig (3.15). (3.15). This condition is denoted as critical . The inclination angle of the first oblique shock wave is then determined both by the free stream mach no and the apex angle of wedge or cone. Such a shock configuration assures acceptable intake efficiency and usually corresponds to the design pint of the diffuser.

Case 2: 2: In case of a pressure drop at the compressor face, the normal shock will be swallowed to adopt a quasi-stable position farther down-stream within the intake duct (fig 3-15c). This condition is denoted as supercritical and, due to greater strength of the (terminating) normal shock, poor flow quality results.

Case 3: Now assume a rise in the pressure at the compressor face such as caused by a reduced airflow demand of the engine. The normal shock will then be expelled from its throat position, air flow is reduced. Intake operation in this case is subcritical (fig 3-15b).such 3-15b).such a shock position is highly unstable, the shock oscillating at high rate between swallowed and expelled positions. This oscillating motion causes high frequency pressure oscillations in the intake; known as diffuser buzz a sound feared by pilot’s as it can indicate one of the most dangerous conditions of the propulsion system.


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[Study of air intake in aircrafts] [2009] 3.4 Examples of oblique shock diffusers

Mirage ||| fighter with side mounted oblique-shock diffuser fig (3.16a)

Axisymmetric oblique-shock diffuser (Lockheed SR-71) fig (3.16b)


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Two dimensional oblique shock diffuser (Northrop F-5 with vertical ramp) Fig (3.16c)


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[Study of air intake in aircrafts] [2009] 3.5 Supersonic air intake case studies An aircraft showing the typical application of a normal shock diffuser is the American F-16, now a product of Lockheed, but developed and built originally by the general dynamics corporation.


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[Study of air intake in aircrafts] [2009] The F-16 intake is of fixed-geometry type, without moveable parts a decision made fairly in design process to save costs. What is remarkable about the inlet is its positioning fairly well under the fuselage a solution resulting from the requirements of aircraft The F-16 was designed to have exceptional maneuverability and this required to operate at high angle of attack. In these considerations the long fuselage fore body performs a shielding function which serves to align the (inclined) axis of intake (fig 3-17a). The intake itself features a short duct which not only contributes to the light weight design of the aircraft, but also minimizes flow distortion ahead of the compressor. Another problem facing the combat aircraft is the hot gas from gun muzzles that may be ingested and cause engine flame out. By placing the gun muzzle above the leading-edge extension or strake, the high temperature gas from the gun will be kept effectively away from the intake before being carried away by the external flow as shown in fig 317 below. The intake cowl features a moderately blunt lower lip that transitions into a sharp leading-edge extension or splitter plate on the upper side (close to the fuselage). The splitter plate extends 25cmsahead of the lower cowl lip to isolate the inlet normal shock from the fuselage boundary layer (fig-17b). A short length of splitter plate keeps boundary layer buildup small, so eliminating the need of boundary layer bleed on the splitter.


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Fig (3.17)


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[Study of air intake in aircrafts] [2009] Intake characteristics of F-14


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References Jet engine fundamentals theory and operations by KLAUS HUNECKE. Cowl - Wikipedia, the free encyclopedia. encyclopedia . Aircraft engine controls - Wikipedia, the free encyclopedia.. encyclopedia Turboprop - Wikipedia, the free encyclopedia. encyclopedia . Janes’ All the World’s Aircrafts ’ , 2000. ‘ Janes’


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Study of Air Intake in aircraft report

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