Performance and Centre of Gravity Position Flashcards
What’s the Centre of Gravity Position?
CG is a point along the longitudinal axis of the aircraft where the sum of the weight moments acting on the aircraft is zero.
How does the force act through it in level flight or in a steady climb or descent?
Parallel to the gravity vector
Through where does the total lift of aircraft act?
Through the centre of pressure
Where must the CG lie on the ground?
Between nose gear and main gear
How does is the weight over the gear affected by the position of the CG?
Further aft of CG: heavier the weight over main gear
Further forward of CG: heavier the weight over the nose gear
How do we need to consider the strength main and nose gear according to the position of the CG?
Strenght of main gear: when calculating the aft limit at the MTOM
Strength of nose gear: when calcultaing the forward limit at the MTOM
Why do we need to consider nose gear adherence during taxy and take-off?
o CG too far aft will make the nosewheel steering ineffective, which is exacerbated with underwing engines.
What are the 4 forces acting on an aircrat in a level flight?
Lift and weight
Thrust and drag
Through where do these forces act?
- Lift: acts through centre of pressure (CP)
- Weight: through CG
- Thrust: through along the engine centre line
- Drag: through the CP, but the total drag, including the fuselage and tail, may act along a different line
How do these forces act in straight and level flight?
Lift = weight
Thrust = drag
Unless the forces are in line, each pair will exert a twisting moment on the aircraft, which may be in the nose up or nose down sense.
What is alpha (α)?
The angle of attack
What is affected by the relative positions of CG and CP?
The lognitudinal stability of the aircraft
Where do transport aircraft have the CG? why?
Forward of the CP, because it stabilizes pitch
What’s the most common situation where equilibrium is achieved?
- A down force is required
- Tail’s α set to produce downward lift to compensate nose up moment, which requires extra upwards lift from wing
How does the CP change?
As speed and α change
How does the CG change?
As load and fuel change
On what depends the static stability of an objetc?
On its tendency to return to its original position after being deflected
What’s negative stability?
when displaced, it continues to diverge from its original position
What’s Neutral static stability?
when displaced, eventually comes to rest in another position. Lift and weight couple act through the same point.
What’s Positive stability?
returns to its original position. This is what we want from our aircraft
How is the stability of a dart achieved?
- Weight is forward
- Long balance arm
- Tail is fixed in a cross, dealing with stability in two axis (analogous to yaw and pitch in an aircraft
- Deflecting forces will turn dart around its CG and the tail will pull back in line again
What decides the degree of stability?
Tail and CG position
Consider this example:
While flying with all forces in balance and trimmed to hold your pitch attitude, a gust lifts the nose up. The aircraft’s inertia carries it forward on its ballistic path. The following happens:
- α increases instantaneously
- lift increases consequently
Is this a stable or unstable condition?
Depends on the relative position of the CP and CG:
If CP is aft of CG, the increased lift vector will tip the aircraft nose down, therefore, the aircraft will have a POSITIVE static stability
what if in the other example the CP was forward of the CG?
Then, as α increases, more lift would develop, and the aircraft would stall. Therefore the aircraft would have negative logitudinal static stability.
What is increased when CG is forward of CP?
-Aircraft weight because the elevator needs to produce a down force in order to balance the tail up pitching effect from the wings
- Wing lift from the wing is increased in order to counter the down force of the elevator
- Drag
- Fuel consumption
- Stalling speed = Giher Vref
- TOR
- TOD
- ASD
- Stick forces
- Landing distance
What is decreased when CG is forward of CP?
Rate and gradient of climb
Landing Mass (MLM)
What becomes difficult when CG is forward of CP?
Rotation
Flare
In summary, what does a forward CG increase?
Stability
Drag
Fuel Consumption
Stalling Speed
By what is the foward CG limit controlled?
Increasing stability
Elevator Authority
Higher Stalling Speeds of the aircraft
Where can we usually find the aft CG?
Just in front of the wing’s centre of pressure
What happens when the CG is at the aft limit?
o Stalling Speed ↓
o Allowing Higher Altitudes
o Range and endurance ↑
Where is the optimum CG?
Near the aft limit
What gives directoinal static stability on an aircraft?
The fin/vertical stabiliser
How is directional static stability as we increase the CG forward?
The stabilizing force will be greater
How are Vmca and Vmcl calculated?
On a fully aft CG position to ensure adquate control with an engine out
Does directionaly stability call for any CG limits inside those already imposed for longitudinal stability?
No
On what does the performance of an aircraft depend?
Difference between the thrust required and thrust available, latter varying with ambient conditions.
What’s the limiting factor to achieve and maintain an adequate level of performance during all phases of the flight on the Certification Specifications?
Mass
When can we exceed maximum masses publish in the Operations and Flight Manual? Can you give an example?
In emergencies
An example of this is an overweight landing after an uncontained engine fire after take-off
What happens to the factors associated with a forward CG?
Most of them will be affected, especially Fuel Consumption and Stalling Speed
What are the consequences of increasing weight/mass?
↑ Weight = ↑ Lift = ↑ Drag = ↑ Fuel Consumption
↑ Mass = ↑ Stall Speed, which is roughly proportional to the square root of the weight.
What happens to the rate of climb and gradient of climb as weight increases?
↑Weight = Rate of Climb and Gradient of Climb ↓
What is the effect of mass on V1?
It increases, although it is more difficult to prove and it is subject to the field length limitations.
What happens to the second gradient requirement of the NTOFP (Net Take-off Flight Path as mass increases?
When does this usually happen and by what is it limited?
can be limited by the CLTOM (Climb Limited Take-Off Mass)
This generally happens when hot and high and is a weight, altitude and temperature limitation.
What happens when the aircraft is overloaded and the CG is in the safe range?
Structural limits May be exceeded = ↑ risk of structural failure, particularly on landing
Vs ↑
Drag ↑
Thrust required ↑
Fuel consumption ↑
Range and Endurance ↑
Take-off and Landing Distances ↑
Rate of climb and Engine-out performance ↓
Climb gradient ↓
Aircraft ceiling ↓
Wear on the brakes and tyres Excessive
What is the effect of gear and flap position on CG?
Main gear = no change
Nose gear = small but negligible
Retracting flaps = no change in CG but CP moves forward
In Transport Aircraft = flaps move forward as they retract and this can cause a small but significant forward movement of CG
Flap extension = opposite effect as retracting flaps
What is automatic fuel management used for?
to ensure optimum position of CG in flight for:
- Range flying
- Best economy
Where is the location of fuel tanks on an aircraft in order to have a better CG management?
- In the wings (outer and inner) and in a central position of the fuselage
- Fin and horizontal stabilizer
Why do we need a correct sequence when feeding from the tanks?
In order to maintain:
o Wing bending relief
o CG position
How does the CG change as fuel is burnt off?
If burnt from a tank aft of aircraft’s CG = CG will move forward
If burnt from a tank forward of aircraft’s CG = CG will move aft
What do we need in order to compensate for crew and passenger movements while the aircraft is on flight?
Small trim changes
What are the two main reasons for having fuel in the tail fin and stabiliser?
o Provide extra fuel capacity and range
o Help maintain aircraft’s CG at the optimum for minimum drag and maximum fuel economy
What device is used to control the fuel in the tail fin and stabiliser? What does this system do?
CGCC (Centre of Gravity Control Computer)
- Automatically drains fuel from fin and stabilizer as fuel is used from the centre and wing tanks
- Fin and Stabiliser tanks are controlled by transfer systems
What are the Disadvantages of using a CGCC (Centre of Gravity Control Computer)?
- Development and installation cost of such complicated systems is high
- Dangers associated to the failure/malfunction of the system
o CG exceeds the aft limit, which can be overcome by:
Make an unscheduled landing
What’s the MAC of a wing? How is it measured?
It’s the mean aerodynamic chord, which is the distance between the leading and trailing edge of the wing, measured parallel to the normal airflow over the wing
It is the chord at the aerodynamic centre of the wing
How is CG expressed in a straight winged aircraft?
As a distance relative to a datum or reference point
How is CG expressed in a swept winged aircraft?
- CG position related to the mean aerodynamic chord of a wing
- CG is expressed as a percentage of its position along the MAC (%MAC), from the leading edge (LEMAC)
What’s an acceptable handling characteristics of a swept wing aircraft?
If CG is between 15% and 25% of the way back along the MAC, described as 15% to 25% MAC
