Download - Balance and Trim ATC Chapter 3. Aim To review principals of aerodynamics in balance and trim
Balance and Trim
ATC Chapter 3
Aim
To review principals of aerodynamics in balance and trim
Objectives1. Define balance2. State the factors effecting balance3. State the different pitching moments and their effects
on flight4. Discuss the principals of trim
1. Define BalanceBalance
Definition:Balance is achieved when no residual force is attempting to alter the position, direction or speed of an aeroplane
BalanceBalance consists of two elements:• Total forces acting on the aeroplane• The alignment of the total forces
When the forces are balanced the aeroplane is in equilibrium
1. Define Balance
If we consider a see-saw, when the beam is loaded equally the centre of gravity is located in the centre of the wooden beam aligned with the pivot point
If one side of the wooden beam is loaded heavier than the other side, the CoG moves towards the heavier side, thus the pivot point and CoG are no longer aligned, the see-saw becomes unbalanced
2. Factors Effecting Balance
Balance
BalanceWhen the weight is not equal on either side of the see-saw, we can move the CoG in 3 different ways:• Move the fulcrum (pivot point) to where the CoG is located• Move the heavier weight closer to the fulcrum• Add weight to the lighter side to equalise the weight
2. Factors Effecting Balance
BalanceMove the fulcrum (pivot point) to where the CoG is located:
2. Factors Effecting Balance
BalanceMove the heavier weight closer to the fulcrum:
2. Factors Effecting Balance
BalanceAdd weight to the lighter side to equalise the weight:
2. Factors Effecting Balance
BalanceAn aeroplane needs to be balanced in the same way as the see-saw, however balancing an aeroplane is slightly more complicated
In straight and level, unaccelerated flight the CoG must be aligned with the pivot point to be balancedDuring flight it is impractical to move the balance point or weightTherefore to achieve balance we must rely on aerodynamic forces to align the total lift forces with the total weight forces – similar to adding weight to the lighter side of the see-saw
2. Factors Effecting Balance
BalanceOn an aeroplane there are two types of forces:• Static forces• Dynamic Forces
Static forces are acting all the time eg, weightDynamic forces are created when moving through the air – aerodynamic forces eg, Lift
2. Factors Effecting Balance
BalanceFor an aircraft to be in a state of equilibrium:Lift must equal WeightThrust must equal Drag
2. Factors Effecting Balance
LIFT
DRAGTHRUST
WEIGHT
Pitching Moments
3. Pitching Moments
However, these forces do not act from the same point
WEIGHT
CoP
CoG
LIFT
DRAGTHRUS
T
Lift - Is produced by the wings and acts upwards through the centre of pressure
Weight - Acts straight down through the centre of gravity to the centre of the earth
Thrust - Is provided by the engine through the propeller
Drag - Is the resistance to motion felt by all bodies within the atmosphere
Pitching MomentsBecause the forces are not acting from the same point they create a couple
L / W Couple = Nose DOWN moment
WEIGHT
LIFT
DRAG
THRUST
A couple is defined as two equal and opposite forces acting about a pivot point creating a torque or turning momentThe two couple’s generate opposing pitching moments
T / D Couple = Nose UP moment
3. Pitching Moments
Pitching MomentsWe said that the forces must be in equilibrium, therefore:
LIFT = WEIGHT
DRAGTHRUST =
(L / W Couple)
(T / D Couple)
For the aircraft to be in balance, the nose down moment must equal the nose up moment
3. Pitching Moments
Pitching MomentsIf the moments are not equal, the tailplane makes up the difference
In a correctly loaded aircraft the tail plane will create a small force downwards
WEIGHT
LIFT
DRAG
THRUST
Force
The forces are now in equilibrium and the aircraft is now in balance
3. Pitching Moments
Pitching MomentsWhen loading the aircraft it is essential that we keep the CoG within a certain range of locationsThis will ensure that at all times during the flight the CoG will be in front of the CoP causing the L/W couple to have a pitch down coupleThis is important to avoid an undesired high nose attitude in the event of an engine failureIf thrust is lost, the nose down pitching moment will be greater than the nose up moment, allowing the aircraft to adopt a glide attitude
3. Pitching Moments
Pitching Moments
WEIGHT
LIFT
DRAG
THRUST
Force
If thrust is lost, the nose down pitching moment will be greater than the nose up moment, allowing the aircraft to adopt a nose low
3. Pitching Moments
4. Trim
TrimAn aircraft is said to be in trim if the aircraft will maintain a constant attitude without the pilot needing to apply any control inputIf the aircraft is trimmed correctly the only control inputs required will be to manoeuvre the aircraft into a different attitude
Trim can be achieved by either:• Adjustable springs to hold control position• Trim tabs controlled via cockpit control
Trim TabsTrim tabs work by creating a small amount of lift opposite to the control surfaces direction of travelDue to the long moment arm created this small force acts to offset the force created by the control surface
4. Trim
Trim TabsTrim tabs can be either fixed or adjustable and fitted to any control surfaceIn light single engine aircraft fixed tabs are usually fitted to the rudder and possibly aileronsThese fixed tabs are adjustable on the ground
Adjustable tabs are most commonly installed on the elevator
4. Trim
Questions?