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Flight and spaceflght > Second Year Semester 2 > Flashcards

Second Year Semester 2 Flashcards

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1
Q

What is wrong with water cooled engines?

A

Require large (drag producing) radiators to dissipate the heat.

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2
Q

What is the difference between a rotary and radial engine?

A

In a radial engine the crankshaft moves only with the propeller (cylinders dont move) and in a rotary engine the cylinders rotate around a cam.

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3
Q

What are the pros and cons of a rotary engine?

A

Good air cooling as it rotates, but creates a gyroscopic effect and looses oil as it rotates.

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4
Q

Whats the equation for power?

A

Power = Thrust*Velocity = 1/2 * Density * V^3 * S * Cd

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5
Q

What is the pitch angle of a propeller wing?

A

The angle between the Zero lift line of the propeller and the plane of rotation.

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6
Q

What is solidity?

A

B*c(r)/2*Pi*r B= number of blades c(r) = chord length as function of propeller radius r Needs to be larger if more engine power

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7
Q

What is thrust?

A

Thrust T = mass flow rate of air * Delta Velocity of air

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8
Q

Whats the problem with increasing speed?

A

doubling the speed requires 8 times the power!

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9
Q

What is the pitch?

A

H is the distance travelled by a blade in one revolution. H= 2 * Pi * r * tan(angle) Normally quoted 0.7r

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10
Q

Is less or more blades better?

A

Less blades means less interference and higher performance, however more blades reduces the loading on each blade.

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11
Q

Whats good about contra rotating blades?

A

Recover energy lost in swirling of slipstream gyroscopic effects are neglected and torques are cancelled. Fatigue may be a problem in the second prop.

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12
Q

Why does a propeller require variable pitch?

A

The outer parts of the prop have a larger tangential velocity than the innards and so have a different relative angle of attack.

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13
Q

What is local supersonic flow?

A

In a propeller, if the blade relatively thick, then the tips may go supersonic whereas the roots may not be. This increased torque resistance, drag and noise.

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14
Q

How is the relative velocity of the tips kept as low as possible?

A

High Sweep back, thin section blades. Using more blades.

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15
Q

How were piston engines performances at altitude increased?

A

By using turbo-superchargers The turbocharger uses exhaust gases to drive a turbine which drives a centrifugal compressor which acts as the supercharger.

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16
Q

What are the 2 main compressor types?

A

Axial - many turbines which compress the gas into smaller and smaller areas. Radial - takes the air in and pushes it out along the radius.

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17
Q

What is a bypass engine?

A

Allows some of the air to go around the engine to cool and produce a thrust simply from the shape of the housing.

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18
Q

What are cannular combustion chambers?

A

When they are arranged in a circle.

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19
Q

Explain the joule Cycle

A

Idea thermodynamic cycle for combustion. Constant Pressure during exhaust and combustion: 1-2: Isentropic compression (compressor stage) 2-3: Constant pressure heat addition (fuel burning Qin) 3-4: Isentropic expansion (turbine stage) 4-1: Constant pressure heat rejection (exhaust Qout)

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20
Q

Define the thermal efficiency of a gas turbine:

A

Net work / Heat in or 1 - (T4-T1)/(T3-T2) or 1 - 1/rp^y-1/y

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21
Q

What is the pressure ratio (rp)

A

P2/P1

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22
Q

What is the work ratio?

A

rw = net work / positive work or rw = 1 - T1/T3 * rp^y-1/y

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23
Q

How do you et the maximum work ratio?

A

Largest T3 possible

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24
Q

What doe the intake diffuser do?

A

Increases pressure

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25
How should the intake operate?
With a minimum loss in stagnation pressure over a wide range of operating conditions. From static to supersonic..
26
How do we expect the pressure to act in the intake under cruising conditions?
An Increase of static pressure.
27
Why is there a problem with intake during lift off and how is it overcome?
Separation around the lip of the intake creates higher drag and supplies less air. Combated using doors in the intake walls. The same problem can occur on the outside of the intake during fall.
28
What are the main difficulties with supersonic intakes?
The smooth compression from super to sub sonic. Creates large shock waves.
29
Why is there a gradual decrease in the area between the rotor shaft and the stator casing in an axial compressor?
To maintain an a constant axial velocity of the flow as the density increases throughout the length of the compressor.
30
What are the stator blades in an axil compressor for?
To slow the air down and keep it aligned so that the fans have best efficiency.
31
Will angle of attack change within an axial compressor?
Yes it will change with the axial velocity through the engine so that they dont stall and reduce efficiency.
32
Why is fuel evaporated in engines and how is it done?
To obtain efficient combustion. It is done by producing a fine spray of droplets of fuel.
33
What is an atomiser?
A nozzle which produces a fine conical spray for fuel injection.
34
How does the change of injector pressure look on the sprays?
35
What are the problems with injection droplets which are too large or small?
Too small dont penatrate enough Too large dont evaporise quickly enough
36
What happens to the fuel vapour before combustion?
It is mixed with oxygen which is done in stages:
37
How must the airflow be for sufficient mixing?
There must be enough TURBUENCE The air is moving much faster than the flames in combustion so recirculation is required (particularly in the primary region)
38
What is the worst thing that can happen to a pilot whilst flying?
A randy Gavin appears in the cockpit
39
What are the two different combustion chamber layouts?
Annular and Cannular Cannular is many different combstion "canisters" in a circlar array Annular is more of a singular ring around the engine in which combustion occurs.
40
What are the pros/cons of using Hydrogen as a fuel?
No harmful emmisions, and Heat released per KG is 2.8 times greater that normal fuel BUT requires 3.8 times the amount of volume for the equivalent energy. Large fuel tanks requires which is possible.
41
When is Ignition used? And what are the problems with it?
It is only used forstarting or re-ignition if the engine stops during flight. It is more difficult at lower pressures so igniting at high altitudes can be problematic.
42
What is the purpous of the turbine? How does it do it?
To extract energy from the high pressure exhaust gasses, it reduces the pressure of the gasses and converts the energy to shaft power which drives the compressor or is available as excess shaft power in the case of turboprop or turboshaft engines
43
How does the turbine provide the driving torque?
Several stages, each with one row of stationary guide vanes and one row of moving blades. Higher compression ratios has led to an increase in the number of turbine stages.
44
What are high comression ratio engines?
Two shafts driving high and low pressure compressors
45
What are High bypass ratio fan engines?
Three shafts with an intermediate pressure turbine driving the fan
46
What are turboprops?
Shaft power is provided by a freepower turbine which is independent of other shafts and turbines which may be driving the compressor
47
What are the fixed part of the turbines called and what are some characteristics?
Nozzle guide vanes, they have an aerofoil profile with the passage between adjacent vanes forming a convergent duct to accelerate the flow. The mounting of the vanes must allow for expansion when the engine is running at high temperature. They are usually hollow and may be cooled by passing air from the compressor through them.
48
What are some characterisitic of turbine blades?
Aerofoil profile, Attached to turbine disk by "fir tree" fixing. This gives equal load distributions and alows them to be removed when stationary but not when spinning. The bade tips may be joined by a shroud to minimise tip leakage.
49
How are turbine blades cooled?
They may be hollow and actively cooled. They may have compressor air blown over them, they may be made from porous material which allows transpiration cooling.
50
What techniques allow the turbine blades to preform at high temps and pressures? (manufacturing tecniques)
Directional solidification, Single crystal blades, Ceramic blades
51
What do support struts do?
Act as flow straighteners to remove risidual flow.
52
What does the jet pipe do?
Converts the hot gas from the engine into a cylindrical pipe. It may mix the hot core stream with the cold bypass stream. In other bypass engines the bypass and core streams emerge separately.
53
What are afterburners and what do they do?
It burns any excess fuel air mixture after it has passed throught the engine. The ultimate limit is due to oxygen starvation. Stable combustion difficult as lower pressures, and the pipe must be much longer to allow full ombustion before the nozzle. The heated fuel does not go into another turbine but is simply expeled through the nozzle. It only operates for a short time.
54
Wha is propulsive efficiency?
It is much more effcient to take a small mass of air and give it a small acceleration other than vice versa. Different engines are more efficient at different speeds.
55
What is a Ramjet?
An engine with no moving parts. It compresses intake air by the forward speed of the vehicle. Supersonic air is slowed by a diffuser to speed similar to a turbojet. Used in high speed missiles with rocket motor used to boost missile to ramjet operating speed
56
What is a Scramjet?
Similar concept to ramjet, but combustion is supersonic, allowing flight at high mach numbers. Difficult combustion physics and issues with integration to airframe.
57
What is the minimum fuel trajectory between two planets?
An elipse which just touches the initial and final orbit.
58
What are they key points in spacecraft power?
Photovoltaic cells common in earth orbit but need batteries to store energu in eclipse moments. Radioiscope sources used for outer solar system.
59
Are solar arrays used in orbital bodies?
Yes the SIlicon solar arrays are low cost, but have poor efficiency. Need designed for end life. They need deployment mechanisms unless bodily monted.
60
How do radioisotope thermal generators work?
Converts heat from isotope decay to electrical power from thermoelectric effect. It has poor efficiency and significant waste heat needs to be disposed (cooling required). Long life but toxic radiation.
61
What is thermal controll?
It is required for batteries/electronics and payloads. Passive controll is paints, coatings and insulation etc. Active controll is heaters and coolers.
62
What problems are associated with satelite structures and mechanisms?
Need to transmit launch loads throught the launch adapter. Must avoiud possible resonances. Composite materials used to minimise mass.
63
What problems are associated with deployment mechanics?
Required due to launcher volume constraints. Fails if something cant be deployed. Lubrication in a vacume (space tribology) is an issue.
64
What are problems associated with onboard data handeling?
Large amount of memory storage required. Telecom needed with up/downlink radio frequencies and Low gain (omni-directional)/high gain (boresight) antennae (deep space).
65
How is attitude and orbits controlled?
Sun sensors and star cameras determine attitude and GPS/Ground determins orbit. Data processed onboard by computer software. Actuators control attitude (reaction wheels) and orbit by thrusters.
66
What are propulsion sub systems reuired for?
both orbit control and for large orbit transfer manoeuvres. High thrust (chemical) and low thrust (eg. electric) propulsion available.
67
How does the gas turbine opperation look?
68
How do you increase performance of engines at higher altitude?
Use a turbo supercharger to get more air into the engine! It uses exhaust gasses to drive the compressor.
69
What law are propulsion system relying on?
Newtons third: #actionequalsreaction Ejecting mass gives thrust!
70
How is thrust given?
By momentum: Thrust = mass\*Velocity + Pressuredifference\*Area or simply mV
71
How do you calculate rocket power?
The rate of energy production: P=0.5\*m\*V^2
72
What does a pre-burner do?
Generates hot gas to drive turbo pumps for hydrogen and oxygen (liquid forms). Exhaust fed into combustion chamber to raise pressure.
73
What are the four main components of a rocket engine?
Fuel storage, pumps, combustion chamber, Exhaust nozzle.
74
What shape is the nozzle in rockets?
convergent divergent. This is because of the high pressure in the combustion chamber.
75
What does the injector plate do?
ensures a stable combustion by mixing the oxidiser/fluel mixture
76
What are the pros and cons of a solid fuel rocket?
Much simpler, lighter and reliable. Difficult to control performance during flight. Specific impulse is lower than equivalent liquid fuel engine
77
What does the rate of generation of combustion depend on?
Propellant density, Burning rate, Surface area which is free to burn. Variation in the thrust profile may be achieved by varying the shape of the solid fuel charge
78
What is the equation for a change in velocity of the rocket with time?
79
How do you find the final mass of the rocket in terms of initial mass?
80
What is the specific impulse?
The exhaust velocity divided by g. "The time for which one kg of fuel will produce one N of thrust" Low Isp develops high thrus and vice versa
81
How do you calculate the graity loss during lift off?
g \* burntime \* sin(feefahh)
82
How does g change with altitude?
Decreases by the relation: (R is distance from centre of earth and h is altitude)
83
What is the need in multi stage rockets? ...and how is the final velocity of the rocket given?
AS the burnout velocity is dependant on the initial and final mass. Illimination of the mass of fuel tanks improves vehicle performance. The final elocity is the sum of the delta V's
84
What are the main uses of multi stage rockets and what are the problems associated with multi staging?
Many modern vehicles use paralel staging to provide good thrus to weight ratios at liftoff. These are solid rockets strappe to the side of a main liquid core stage. The problem is it is unreusable and cost ineffective. The world needs fully reusable single stage vehicles. OR we will all die probs.

Flight and spaceflght (2 decks)

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