Weight and Aircraft Momentum Flashcards
What limits and aircraft’s structural weight?
As weight is opposed by lift the weight of the aircraft is dependant on the amount of lift that the aerofoil can produce. Lift on the other hand is dependant on the speed that the aircraft can achieve, therefore the power and thrust that the engines can produce, the size of the aerofoil and the air density. (L=Cl x 1/2& x V2 x S)
What are the effects of excessive aircraft weight?
- Increased take-off and landing distance
- Reduced rate of climb
- Reduced range and endurance
- Reduced maximum speed
- Increased stalling speed
- Reduced manoeuvrability
- Increased wear and tear on tyres, brakes, undercarriage
- Reduced structural safety margins
Describe centre of gravity (CoG)!
The CoG of an object is the point through which the total weight of a body will act. In other words it is the point through which all of the objects gravitational forces are combined.
Describe a component arm!
A component arm is the distance from a datum to the point at which the weight of a component acts. For a constant weight, the longer the arm the greater the moment. (Weight x Arm = Moment)
How is the pitching moment of the lift-weight couple balanced?
If the lift force is forward of the weight vector, this will result in an aircraft nose up pitching moment. To stabilise this turning moment the elevator has to produce a force in the opposite direction by lifting the tailplane.
Describe the CoG range!
The CoG range is the furthest forward and rearward position on the aircraft’s longitudinal axis where the CoG may position itself. Inside this range the elevator has enough authority to stabilise the aircraft. Outside of this range un-commanded nose pitch up or down moments and stalls with negative stall and spin recovery may occur.
What are the reasons/effects of keeping a CoG inside its limits?
The forward limit of the CoG ensures that full authority of the tailplane during flight and that the aircraft is not too nose heavy, which could lead to negative controlling characteristics during rotation and flare. It also maintains the elevator in a streamlined position reducing drag.
The rearward limit of the CoG ensures full authority of the tailplane during flight, and ensures the aircraft is not too tail heavy which could lead to stalls, spins and reduced recovery effectiveness. Further, the streamlined elevator will again prevent extra drag from forming increasing performance.
What are the effects of a CoG outside its limits? (FWD CoG)
CoG FWD:
- Increased longitudinal stability
- Reduced pitch control (flare and rotation)
- Increased elevator pitch down force, opposing lift, increasing the AoA of the main wing, increasing drag, reducing, performance
- Increased stalling speed, due increase effective weight
- Min in-flight speed restricted due to low elevator authority in that speed range
- Greater forces required from the elevator controls
What are the effects of a CoG outside its limits? (AFT CoG)
CoG AFT:
- Decreased longitudinal stability
- Increased effectiveness of elevator which could cause over stressing
- Increased elevator pitch up force, decreasing AoA on main wing, reducing drag, increasing performance
- Decreased stall speed, less effective weight
- Reduced recovery effectiveness during stall
- Max in-flight speed restricted due to lower elevator authority in that speed range.
CoG Position: FWD: AFT:
Stability:
Stick Force:
Drag:
CoG Position: FWD: AFT:
Stability: More Less
Stick Force: More Less
Drag: More Less
If you were loading and aircraft to obtain maximum range, would you load it with a forward r aft CoG?
Aft. An aft CoG requires the elevator to produce and upwards force to counter the lift-weight turning moment. This upwards force acts in the same direction as lift reducing the overall requirement of lift therefore reducing drag and increasing performance. Further, the elevator is in a more streamlined position (in most jet aircraft) reducing drag further.
How does a forward CoG affect the stall speed of the aircraft and why?
It increased the stall speed of the aircraft. This is due nose down pitching moment created by the fwd CoG. To counter this the elevator needs to produce a nose up pithing moment by pushing down on the tailplane. This force acts in the same direction and the weight vector increasing the effective weight of the aircraft. This in turn requires a higher speed, at a given AoA, to produce more lift and counter the increased effective weight of the aircraft. So, for the same AoA (this could be the stalling AoA) we now have a higher speed.
Why does a jet aircraft have a large CoG range?
A jet has a large CoG range because of the large changes in the aircraft weights and the position of the CoG during flight. (Fuel in swept wings)
What causes CoG movement?
- Fuel Burn (swept wing aircraft)
- Passenger movement
- Icing on the airframe
- Loading and unloading of cargo (parachutists, water, cargo,…)
- Speed changes (changes in the forces on the tail pane, either increasing or decreasing the effective weight of the aircraft and moving the CoG fwd and aft.)
How does Vmcg and Vmca change with CoG position?
An aft CoG creates a small vertical tail plane moment reducing its effectiveness at a given airspeed. Therefore to counter the reduced moment a higher Vmcg/ Vmca is required at a given CoG position. This is true vice versa with a fwd CoG.
