AERO40002 - Introduction to Aerospace Flashcards
Troposphere
From 0 - 11km AMSL
The temperature decreases with increasing altitude.
Warmer near the surface because low altitude water and CO2 absorb heat radiated from the surface.
Air is generally turbulent.
Most weather occurs here.
Stratosphere
From 11km - 50km AMSL.
The boundary between the troposphere and stratosphere is called the tropopause.
Much less water at lower levels so absorptivity from the ground is lower.
Ozone levels increase over this region, which encompases the ozone layer, increasing
absorption from ultraviolet radiation.
Relatively little vertical mixing observed on this region
Almost all flight is in the stratosphere or below.
Mesosphere
50km to 80km AMSL.
The boundary between the stratosphere and mesosphere is the .
The temperature decreases with increasing altitude due to decreasing levels of solar
heating and increasing cooling by CO2 radiating emission.
Thermosphere
Temperature increases rapidly to ⇡ 100km AMSL, then constant from above 100km
upwards. Density and pressure are very low.
Atmospheric gases separate by molecular mass.
The Karman line (at 100 km AMSL) is considered the edge of space.
NB: A typical low earth orbit satellite orbits at ⇡ 300 km or higher.
Key wing parameters
- Area
- Span
- Aspect Ratio
- Taper
- Sweep
- Dihedral
- Twist
- Airfoil section.
What is the chord?
The horizontal length of the wings.
C root - the horizontal length of the middle.
C tip - the width of the tip of the wings.
How is wing area measured? (S ref)
It is a measure of the area is projected on the ground.
What is the wing span? (b)
Distance between one wing tip to the other.
What is the aspect ratio?
b^2/ Sref
Explain the aspect ratio
A high aspect ratio means the plane has long, narrow wings.
A low aspect ratio means the plane has short, wide wings.
A high aspect ratio gives more lift and higher endurance flights, but low aspect ratios are better for swift manoeuvrability.
Generally, a higher aspect ratio tends to limit an aircraft’s responsive ness because of the higher roll inertia and roll subsidence damping.
Which aspect ratio has increased drag acting on it?
Higher aspect ratio planes.
The long, narrow wings have less induced drag. Long narrow wings have less end edges (tips) and more stable wing area than shorter wider wings- so less drag.
This is also means they have less fuel consumption.
What is the taper ratio? (sign is lambda)
Ratio of the c tip and C root.
Explanation of tapered wings.
Generally, the most strongest tapered wings will have the smallest tip deflection ratio. Therefore, they will have a larger span than the others, giving it a potentially lower induced drag.
Define roll subsidence damping.
The damping of rolling motion. There is no direct aerodynamic moment created tending to directly restore the wings back to their original position.
Aspect ratios of typical airliners
Orders of 7-12.
Aspect ratio of a sailplane
20-50.
Aspect ratios of a supersonic fighter jet or transport
Less than 3.
General taper ratios of commercial airliners
Order of 0.2 - 0.3
Define sweep.
Just an angle.
Unswept means it is 0.
Backward swept means it is less than 0.
What is dihedral
The angle formed between the plane of the wing and the horizontal.
What is twist on an airplane?
It is basically changing the the angles of attack along the span of the wing. An increase in the angle of attack is called a wash in. The opposite is called a wash out.
Wash out is primarily used, to avoid flow separation at high wing angles of attack.
NACA 4 Series
- Max camber (1 in 100) - max difference between camber line and chord.
- Location of max camber (tenths of chord) - usually 30 - 50%
- and 4. Max thickness of the airfoil (1/100)
NACA 5 series
- Multiply by 3/2 - design lift coefficient
- Divide by 2 - location of max camber along chord from LE.
- Reflex - either 1 or 0.
4 and 5. Mac thickness (% of chord)
Explain reflex on a plane
The trailing edge is slightly pointed upwards, to oppose the pitching moment and bring it all to equilibrium.
NACA 6 series
- Always 6 to identify the series.
- Location of min pressure (tenths of chord)
- Design lift coefficient (tenths)
- and 5. Max thickness (1/100s of chord)
What is the point of the NACA 6 series?
Generally was to keep the boundary layer for as long as you can across the chord.
The minimum pressure on the top of the airfoil is when transition flow starts.
What are normal values for the thickness for an airfoil (generally for a NACA 6 Series)
12 -15% for normal- has good Coefficient of lift, low weight and low drag.
5 - 6% for fighter jets (is quite thin)
Disadvantages of a turbojet engine
Are really good at high altitudes and high speeds, but inefficient at lower airspeeds.
Explain a piston engine
It relies on pistons attached to the crankshaft compressing a fuel - air mixture which is then ignited.
The high pressure gases created by the mixture pushes the piston out of the cylinder and then rotates the crankshaft. Having many pistons creates an intake- compression-combustion/expansion-exhaust cycle that can continue to regenerate power.
You can also add a compressor that allows the piston engines to remain at constant inlet air pressure.
Explain a gas turbine engine
Rely on compressing air, mixing with fuel, igniting it and extracting some energy to sustain the cycle.
What is a ramjet for?
It is an engine that is used where turbojets can’t be used because they are limited by the turbine stage inlet temperature. Here, air is compressed by the inlet slowing it down through a series of oblique shock waves.
Define static pressure
The atmospheric pressure recorded on a body at rest relative to the vehicle.
Define total pressure
The pressure recorded when airflow over the vehicle comes to a rest.
How is altitude measured using a pitot tube and a barometer?
There is 1 tube for the altitude measurement. It is then compared to a fixed value of another pressure. It measures the static pressure. As the altitude increases, the static pressure decreases
How do pressure values change as you increase altitude (in general, but also specifically for measuring airspeed)
Static pressure decreases as altitude increases.
The dynamics pressure will stay the same.
So the total pressure should decrease.
If the static pressure tube in a pitot tube is blocked by ice, how does this affect the airspeed.
Assume you are climbing at a constant velocity.
Since the static pressure will remain the same, and so will the total pressure. The dynamic pressure must decrease to give the same total pressure. The velocity is measured using the dynamic pressure, which will mean the velocity will actually be lower than expected. It will under-read.
Explain a VSI (Vertical speed indicator)
It measures the difference in static pressure over a small time duration, by using a calibrated leak valve to a standard altimeter. If the pressure inside the bladder coming from the static tube is the same as the pressure in the enclosure around it, then the aircraft is not climbing. If climbing, the pressure measured by the static tapping will decrease, whilst that outside the bladder will take longer to adjust to the new ambient conditions.
How does a jet engine operate?
Inlet - takes in air and slows it down.
Fan and compressor stage - Increases the temperature and pressure.
Combustion chamber - air is mixed with fuel. So more increase in the temperature and pressure.
Turbine stage - the high energy gases to through a turbine, which extracts energy from it. Decreases pressure and temperature. So useful work is used by the engine.
Why does the maximum thrust produced by a turbofan or turbojet engine decrease with altitude?
Due to the reduction of density with altitude. This affects the mass flow rate of air passing through the engine.
Difference between the true airspeed and the equivalent airspeed
True airspeed is calculated using the actual density. But the Equivalent airspeed is calculated using the reference density.
Explain a ramjet engine
Uses oblique shock waves to compress the high speed freestream air. Subsonic, high pressure and temperature air enters the combustion chamber where it is mixed with fuel and ignited. Exhaust gases then expand through a converging-diverging nozzle to supersonic speeds higher than freestream.
Explain a scramjet engine.
Same as a ramjet, but used for hypersonic speeds, where slowing down the air to a subsonic speed is not possible.
Disadvantages of a ramjet and scramjet
Operate efficiently at speeds above Mach 3 and Mach 6 respectively. They cannot be operated at static conditions, so need to be coupled with another propulsive unit to get them up to speed.
What affects the zero angle of attack?
- Camber
- Reflex
- Setting angle of the wings (reference value that the angle of attack is compared to- could be the fusealage)
How is the wing geometry manipulated such that the Cl is increased during takeoff, climb, descent and landing?
- Leading edge flaps - These will extend the CL curve to a higher AoA before stall. Also, the CL max is increases. This generates positive pitching moment to counteract the negative pitching nose down moment. Generally, this tends to increase the total lift, but requires a large amount of AoA to produce it (therefore making taking off and landing more difficult)
- Trailing edge flaps - Reduces stall angle slightly. Increases the CL max. Allows flying slower at higher AoA.
What creates zero lift drag?
- Drag due to skin friction (due to viscosity and boundary layer)
- Pressure drag (due to flaps. Deploying the flaps can increase the drag and the CL as well.)
- Compressibility effects OR Wave Drag (essentially, when flying at really high Mach numbers, you can lose energy due to the formation of shock waves. These occur when trying to slow down supersonic speeds to subsonic. This energy lost can be in the form of drag. This happens when you go past the critical Mach Number. However, if you go slightly past Mcrit, to Mdd (Mach drag divergence). After this point, the drag increases as a fourth power law. Most airliners will fly at Mdd.)
- Interference drag (effect of engine being in the flow field of the fusealage, antenna etc.)
Why do some supersonic vehicles need really large engines?
These fly at supersonic speeds and have really high Mach Numbers. This means that the flow over the top of the wings has to be brought back to subsonic speeds at the trailing edge. However, this produces a lot of shock waves, therefore resulting in a lot of drag being produced, because energy is always lost with shock waves. There is a point called the Mach drag divergence, after which the drag increases as a fourth power and then decreases again to a lower amount of drag. To get to supersonic speeds, you have to overcome this bump, for which really powerful and large engines are required. You may need afterburns too to overcome this point and then be able to travel at supersonic speeds afterwards.
What is a usual value of the critical Mach number?
Around 0.7
Explain lift dependence drag
When an aircraft moves through a flow field, the wing tips create trailing vortices that flow into the plane. This is because there is suddenly a pressure difference in the surrounding air of the wing tips. There is high pressure below the wing, and low pressure above the wing. Since the air molecules will always travel from an area of high pressure to an area of low pressure, the particles will move into the plane. This creates trailing vortices on either side of the plane, at each wingtip. Any particle inboard of the wing tips (so directly below the plane) will be pushed downwards, and any particles outboard of the wing tips will be pushed upwards and into the plane. This creates a downwash.
This downwash changes the angle of attack of the plane, by changing the angle of the air meeting the plane. This angle is called the induced AoA, which is less than it was before. This small changes causes a small proportion of the lift to act as drag.
1 knot in m/s
0.514 m/s = 1.852 km/h
1 Nautical Mile in m
1,852m
1 pound in kg
0.452 kg.
1 ft in m
0.3048m
Rule of thumb for designing a fusealage
Length to diameter ratios are generally more aerodynamically efficient, yet heavier. And the circular cross sections are ideal when cabin pressurisation is used.
Explain a semi-monocoque system
This is a system which is made up of stringers and frames. Many frames are used and they are placed along the fuselage length to ensure that the fuselage’s aerodynamic shape is being maintained. The stringers extend continuously from end to end, taking some of the bending loads and supporting the skin against buckling. This allows the thickness of the skin to be reduced safely and results in lightweight structures.
Stabilisers
A vertical stabiliser must be placed aft of the vehicle’s centre of gravity to ensure weathercock (directional) stability.
A horizontal tailplane contributes to pitch stability.
What is a power producing engine?
They deliver power using a rotating shaft to a propeller. Often has an internal combustion engine or an electric motor to drive the propeller.
What is a thrust producing engine?
They produce a thrust force by increasing the momentum of a fluid, typically the on-coming airflow.
Examples are:
Gas turbine engines (turbojets, turbofans)
- Ramjets
- Scramjets
- Rockets
How can the lift generated by the tailplane be reduced?
NOTE: The elevators are the horizontal control surfaces at the end of the plane.
By deflecting the elevator upwards, you reduce the lift generated by the tailplane. This produces a negative lift, resulting in a nose up aerodynamic pitching moment.
Define static stability
The tendency of an object or a system to return or diverge from its EQM position once perturbed away from it.
On a graph of velocity(i) against drag, explain Vi1
This is the drag that is dominated by the parasitic drag. This is because the velocity here is quite high, therefore in the drag equation, the first term with is AV(i)^2, will be quite high, but the second one will be much lower. The second term will not affect the drag component, as it’ll near to 0 as the velocity increases (it is inversely proportional to the V).
P.S- In the same way, V(i)2 is the term that is dominated by the lift induced drag terms.
On a graph of velocity(i) against drag, why is V(i)1 the speed stable one?
Any increase in the velocity (perhaps due to a gust of wind) will cause the drag to increase. So it will move up the graph. But as the thrust will stay the same, but the drag will increase, the place will decelerate and return to its equilibrium position.
A decrease in the velocity (again, due to a gust of wind in the opposite direction) will cause the drag to decrease. Since the thrust has remained the same, the plane will accelerate, meaning velocity will increase back to the EQM position.
On a graph of velocity(i) against drag, why is V(i)2 the speed un-stable one?
If there is a slight increase in the velocity (a gust of wind for example) will lower the drag. (Know that V(i)2 is the lift dependent dominated one, (n^2 B)/V(i)^4. So any slight increase in the velocity will reduce the drag by a lot). Since the thrust remains the same, the plane will accelerate even further, so further increasing the velocity. This means that it will simply move down the graph.
A slight decrease in the velocity will cause a big increase in the drag. An increase in the drag means that the plan will slow down even further, causing it to stall.
Since we know that travelling in the V(i)2, or the speed-unstable region can be dangerous, how can planes reduce their likeliness of staying in the region for too long? (Know that they must stay in that region for a bit during the take-off and landing because this required lower velocities)
To avoid operating in the speed-unstable region, you should just reduce V iMD. Can be done by:
- Increasing Cd 0 (by deploying the flaps, undercarriage, spoilers and brakes etc.) This increases the drag.
- Reduce the weight (such that the landing weight is less than the take off weight)
Define specific air range
The air distance travelled per unit fuel mass consumed. Is defined using true speed.
What is the most common airframe for aircraft fuselages?
Semi-monocoque system.
It has frames and stringers supporting a stressed skin.
Frames are placed along the length of the fuselage to ensure that theaerodynamic shape is maintained.
The stringers extend from end to end, taking some of bending loads, and supporting the skin against buckling.
These allow the skin thickness to reduce. Gives a stiff but lightweight structure.
What is a supercritical airfoil?
They allow wing sections to operate with significant portions of supersonic flow on the suction side without the adverse impact that the formation of a strong shock wave would have on drag.
How do stabilisers help to control the plane?
The vertical stabiliser (must be placed aft of the vehicle’s centre of gravity ) - to ensure directional stability.
Horizontal tailplane - Contributes to pitch stability.
Having a V-tail, which is a horizontal and vertical tail combined, can reduce their wetted area, and thus the drag as well.
Explain a turbojet engine
It extracts only the energy from the high-pressure gas exiting the combustion chamber to drive the compressor.
So it’ll generate thrust by expanding the high pressure air through the nozzle and expelling it at a high speed.
Explain a scramjet
Is used to get to hypersonic speeds.
Propfan engines
Also need to understand this..?
Turboshaft
The exhaust gases from a compressor prive a power turbine stage, where most of their energy is extracted and outputted in the form of shaft power.
Explain an electric motor engine
Relies on permanent magnets and electromagnets interacting to convert electrical into mechanical energy.
Shaft power is unaffected by altitude and speed changes.
Conditions to find the absolute ceiling?
L = n W, so that the L = W.
T = Dmin = W/ (L/D)max.
Note that the absolute ceiling of an aircraft is simply given by an equation, for the stratosphere.
Using the T/Ti equation, you can get the density ratio. And then use the table to continue.
How does the TAS differ from the EAS?
The true airspeed is the speed at which the aircraft is travelling through the surrounding air. The equivalent airspeed is the speed that would produce a dynamic pressure equal to the TAS, at a given altitude, assuming that the density is constant, and equal to rho0.
How is an aircraft’s range affected if there is a constant tailwind?
The actual range of the aircraft remains the same, but the range over the ground would increase. It increases by the product of the wind speed and the airborne time.
Airborne time = Range/ velocity.
Then, add the range+ (tailwind speed* airborne time)
HOW TO CALCULATE L/D?!
L = W.
D = THE DRAG EQUATION.
AND CALCULATE THIS WAY!!!
L/D = W/DRAG EQUATION!!
What is AR
b^2/S
How does deploying air brakes affect the amount of drag?
Deploying air brakes increases the drag, therefore decreasing the (L/D)max ratio.
Define a tailwind
It’s a wind that pushes the plane forwards from the tail.
What is required for speed stability?
The Vapproach speed should be greater than V imD
What is the first thing to do when you get a force balance/ equation of motion question?
Write out forces and equation of motion, in F = ma form!!!
Explain the centre of pressure in lift
It is the point where the total lift force acts through, resulting in no pitching moment. When the centre of gravity is on the centre of pressure, the aircraft is in equilibrium. (under the assumption of no other contributions to the pitching moment).
Explain the neutral point of an aircraft
It is the point through which any lift generated by a change in the angle of attack will act through. It determines whether or not the aircraft is stable in terms of its pitch.
How does using takeoff flaps affect the takeoff performance?
Flaps increase the lift.
Therefore increasing the Cl max.
This decreases the Vs, and decreases the ground roll distance.
Flaps however increase the Cl, so the Cd0 in increased. Therefore increasing the overall drag, and decreasing the climb gradient.
At what speed will a propeller driven aircraft maximise its cruising range?
At V = Vimd.
Or, V bar = 1.
If they ask you about endurance, there is literally only one equation to use- so use that one!
!!!
Endurance is maximised at V bar = 1.
How to calculate the maximum energy for any SEP curve?
It will be tangent to the specific energy curves.
Define a level balanced turn
No sideslip and no overall lateral aerodynamic forces.
How can the pilot/ aircraft designer maximise the turn rate or minimise the turn radius?
By reducing the value of VimD, by minimising the wing loading.
Therefore, the load factor must be maximised.
Speed must be minimised.
What information can you get from the SEP curves?
Absolute and service ceiling (500ft/min for a service ceiling)
Maximum possible airspeed.
Point performance.
How do pilots fly using the SEP and energy height curves
They fly ALONG the energy heights, trading kinetic energy for potential energy.
Then, they fly in a climb trajectory, which is normal to the constant energy heights.
How does increasing weight affect the takeoff and landing roll?
Takeoff Roll - Increases.
Landing Roll - Increases.
Increasing weight - the T/W decreases - reducing the acceleration.
Increase in stall speed.
How does headwind of tailwind affect the takeoff and landing roll?
Both decrease the roll distance.
The wind will decrease / increase the effective groundspeed at which takeoff/ landing are possible.
How does increasing temperature affect the takeoff and landing roll?
Both increase.
Increasing the temperature reduces the air density and increases the aircraft’s stall speed.
Also, there is a loss of thrust as the temperature increases- which is the major contribution to determining the roll distance.
How does increasing elevation affect the takeoff and landing roll?
Both increase.
Again, increasing elevation increases the air density, increasing the aircraft’s stall speed.
How does increasing upslope of the runway affect the takeoff and landing roll?
Increases the takeoff roll.
Decreases the landing roll.
Increasing the upslope of the runway results in the aircraft’s weight acting to decelerate the aircraft.
How does wet or icy runways affect the takeoff and landing roll?
Marginal decrease in the takeoff roll.
Increase in the landing roll.
It will reduce the tire friction coefficient. Therefore, loss of traction when braking.
