flight and spaceflight

What is multistaging?

What is multistaging?

Is a means to improve the performance of rockets by getting rid of the dead weight of empty tanks and structures.

Centrifugal turboshaft

Variable geometry nozzles:

used to ensure maximum exploitation of the energy available at the end of the turbine;

Why are  Afterburners used?

Are used to increase the amount of thrust by adding extra heat (i.e. fuel)

what are Centrifugal compressors?

Realize pressure increase by means of a combination of channel shape and centrifugal effect.

A radial flow is established

How does a centrifugal compressor work?

The impeller rotates at high speed and air is continually induced into the centre of the impeller.

Centrifugal action then causes it to flow radially outwards along the vanes on the surface of the impeller disc, accelerating the air and causing an increase in P.

The air leaving the impeller passes into the diffuser section where the vanes form divergent passages.

Each single stage of a centrifugal compressor has a higher compression ratio than a single axial stage: r between 4 - 6

What should the combustion chamber be capable of providing?

- Complete combustion of large quantities of fuel

- Small volume and minimum pressure loss

- Deliver uniform temperature

- Exit temperature low enough for turbine blades

- Expected life of the order 10,000 hours

- Produce low Nitrous Oxide (NOx) and hydrocarbon emissions

- Operate over the whole range of engine speed, flight speed and altitude

typical configuration of a combustion chamber include:

1. Injection zone

2. Ingnition zone

3. Primary combustion zone

4. Secondary combustion zone

5. Tertiary combustion zone

What is thrust?

A generation of a propelling force based on Newton's 3 law.

what does a propulsion system consist of?

A source of mechanical power and a propulsor (means of converting this power into propulsive force).

air breathing propulsive systems

steam /piston engine

jet engines

high-speed engines

non air breathing propulsion

chemical rockets

electric-electromagnetic

experimental future systems

types of propulsors

propeller

nozzle-fan

nozzle

fuel/propellent

liquid

solid

nuclear

Straight Level flight forces

Lift = Weight

Thrust = Drag

Drag equation

Power equation in Straight Level Flight

How does a propulsion system generate thrust?

By accelerating a mass of gas is in the direction opposite to the direction of the intended motion.

Difference between air-breathing and non-air breathing system?

Air-breathing: A mass of air enters the propulsive system and is expelled by the system with a higher momentum.

Non-air-breathing:

A mass of gas/plasma is expelled by the system.

why is specific impulse used in case of rockets?

A. determines the thrust of rocket

B. indicates effecient of propulsive systems

C. Easy way to size the engine.

Rotary engine

An air-cooled engine.

Cylinders with the propeller attached , rotates, at high speed, around the shaft.

Movement cools the cylinders.

Disadvantage of rotary:

Gyroscopic effect - hard to manoeuvre,

Total loss oil system - Centrifual force throw lubricating oil out after its first trip through the engine.

Range of aircraft limited by the amount of oil it could carry as well as fuel.

Otto cycle. Describe the four strokes of piston per cycle.

1. Air fuel mixture is drawn into cylinder at constant pressure.

2. Adiabatic (isentropic) compress of the air-fuel mixture. 1-2.

3. Constant- volume heat transfer ( combustion of fuel-air mixture). 2-3.

4. Adiabatic (isentropic) expansion (power or work stroke). 3-4.

5. Constant-volume heat release while the piston is at the bottom dead centre. 4-1.

6. Release of burnt gases at constant pressure.

Otto cycle. P-V Diagram.

blade geometric pitch, θ

Angle between the aerofoil zero lift line and the plane of rotation.

This angle will vary along the length of the

blade span for a twisted blade.

Soliditiy σ

The fraction of the propeller disk area occupied by solid blades.

σ = Nbc(r )/2πr

pitch, H

The distance travelled by a blade element in one revolution.

H = 2πr tanθ. Since θ(r ) also H(r ) depends on the blade span position

The thermodynamic effeciency of otto cycle increases if

the compression ratio increases

A propeller generates Thrust a

as a consequence of the aerodynamic interaction of the blades (and airfoils) with the incoming airflow;

Combination of velocities?

important since it leads to the idea of variables (twisted blades) and explains why there are limitations in trying to fly a propeller-driven aircraft faster and faster.

why do we use contrarotating propellers?

reduce gyroscopic effects

why do we use sweptback blade tips?

reduces local mach number

why do we use high number of blades?

reduces load per blade

Turbojet cycle

Do turbojets have a high or low bypass ratio?

A lower bypass ratio gives a higher exhaust speed, which is needed to sustain higher, usually supersonic, airspeeds. This increases the SFC .

Do turboprops have a low or high by pass ratio?

A high bypass ratio gives a lower (actual) exhaust speed. This reduces SFC, but reduces the top speed .

the turboprop where the piston engine is replaced by a gas turbine.

what is the primary purpose of the air  intake ?

-increase pressure by means of the socalled ram-effect - ensure uniform flow to -the compressor under diverse aircraft incidences

-does not involve heat or work exchange with its surroundings

what thermodynamic cycle does a jet engine work with?

A joule - Brayton cycle

Pressure is constant in cycle.

What is an effective way of improving the performance of a turbojet?

Multispooling

Low pressure/temp stage of compress = less work than high pressure/temp - more efficient to have them rotating at differencr speeds - multiple co-axial shafts,

Primary purpose of a compressor

increase pressure by guiding the flow of air through a series of carefully shaped channels

why does the compressor require a power supply?

Because it does work on the fluid. The power supply is the turbine.

is the flow in a compressor supersonic or subsonic?

subsonic

De halle crtierion

V2/V1 < 0.72

Compress inlet - outlet differencr

Absolute velocity increases accross the rotor and diminishes across the stator.

-Relative velocity decreases across the rotor.

- Pressure always increases.

What are axial compressors?

Machines where air enters and leaves in a direction parallel to the axis of rotation.

The increase in pressure is gradual as the flow passes through the compressor The pressure rise across each stage is typically between 1.1 and 1.6

Alternative types of fuel for jet engines?

Synthetic oil made from coal

-Methanol and Ethanol

-Hydrogen

-Biofuel

Purpose of the turbine?

To extract energy from the high-pressure gas coming out of the combustion chamber.

This is done by reducting the pressure and temp of gas (expanding the gas).

What is the extracted energy from a turbine used for ?

Converted to shaft power and used to drive the compressor and as used as shaft power in the case of turbo-prop engines.

Why can a turbine work more efficiently to a compressor?

Flow is accelerating. The pressure gradient is favourable and the risk of flow separation is minimal.

The higher the BPR the lower

he lower the maximum velocity at which thrust can be obtained;

What is a nozzle?

The propulsor in the case of turbojet and turbofan engines.

For the turbofan we are also considering the ”nozzle” of the turbofan in the case of non-associated jets.

in the case of turboprops, the nozzle :

gives minimal (is some cases zero) contribution to the thrust.

Thrust vectoring :

obtained by having the possibility to orient the direction of the nozzle.

What are ramjets and scramjets?

Are advanced systems used for high-Mach flights where shock waves are exploited in the propulsive system.

In a scramjet the combustion process is supersonic, in a ramjet is subsonic.

What type of system are rockets?

Rockets are non-airbreathing systems and must carry fuel and oxidizer: T = ˙mO+Fvout;

The specific impulse

I = T m˙ O+F = I (M, T0, P0, γ, R, Pout);

Chemical rockets

are the most widely adopted solution: solid, liquid, hybrid;

Solid

Robust, special design of the grain to control thrust, non-reusable;

Liquid :

More complex, multiple ignitions, hypergolic propellants, throtteable.

What are nozzles?

used to generate thrust also for rockets.

The ratio Pout/P0 :

controls the type of flow that can be obtained;

What is convergent/divergent nozzle?

needed to realize supersonic speed at the exit

What are the rocket equations?

are used to compute the ∆v given a specific amount of propellant or the mass of fuel needed to realize a given ∆v.

What is gravity loss?

refers to the reduction in ∆v resulting from the inclusion of gravity in the rocket equations;

centrifgal turbojet

How does a centrifugal compressor increase the pressure in the air passing through a jet engine?

Centrifugal action causes the air to flow radially outwards along the vanes on the surface of the impeller disc, accelerating the air and causing a slight increase in pressure.The air leaving the impeller passes into the diffuser section where the vanes form divergent passages.The airflow is decelerated and the pressure increased.

Why is a turboprop engine more efficient than a turbojet engine?

A larger mass of air is accelerated through a smaller change in speed