# Balance and Trim ATC Chapter 3.

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Balance and Trim ATC Chapter 3

Aim To review principals of aerodynamics in balance and trim

Objectives Define balance State the factors effecting balance
State the different pitching moments and their effects on flight Discuss the principals of trim

1. Define Balance Balance Definition:
Balance is achieved when no residual force is attempting to alter the position, direction or speed of an aeroplane

1. Define Balance Balance Balance 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

2. Factors Effecting 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
When 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
Move the fulcrum (pivot point) to where the CoG is located:

2. Factors Effecting Balance
Move the heavier weight closer to the fulcrum:

2. Factors Effecting Balance
Add weight to the lighter side to equalise the weight:

2. Factors Effecting Balance
An 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 balanced During flight it is impractical to move the balance point or weight Therefore 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
On an aeroplane there are two types of forces: Static forces Dynamic Forces Static forces are acting all the time eg, weight Dynamic forces are created when moving through the air – aerodynamic forces eg, Lift

2. Factors Effecting Balance
For an aircraft to be in a state of equilibrium: Lift must equal Weight Thrust must equal Drag LIFT THRUST DRAG WEIGHT

3. Pitching Moments Pitching Moments
However, these forces do not act from the same point 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 LIFT Thrust - Is provided by the engine through the propeller CoG DRAG THRUST CoP Drag - Is the resistance to motion felt by all bodies within the atmosphere WEIGHT

3. Pitching Moments Pitching Moments
Because the forces are not acting from the same point they create a couple A couple is defined as two equal and opposite forces acting about a pivot point creating a torque or turning moment The two couple’s generate opposing pitching moments LIFT L / W Couple = Nose DOWN moment T / D Couple = Nose UP moment DRAG THRUST WEIGHT

3. Pitching Moments Pitching Moments LIFT = WEIGHT THRUST = DRAG
We said that the forces must be in equilibrium, therefore: LIFT = WEIGHT (L / W Couple) THRUST = DRAG (T / D Couple) For the aircraft to be in balance, the nose down moment must equal the nose up moment

3. Pitching Moments Pitching Moments
WEIGHT LIFT DRAG THRUST If 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 Force The forces are now in equilibrium and the aircraft is now in balance

3. Pitching Moments Pitching Moments
When loading the aircraft it is essential that we keep the CoG within a certain range of locations This 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 couple This is important to avoid an undesired high nose attitude in the event of an engine failure If 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
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 LIFT DRAG THRUST Force WEIGHT

4. Trim Trim An aircraft is said to be in trim if the aircraft will maintain a constant attitude without the pilot needing to apply any control input If 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

4. Trim Trim Tabs Trim tabs work by creating a small amount of lift opposite to the control surfaces direction of travel Due to the long moment arm created this small force acts to offset the force created by the control surface

4. Trim Trim Tabs Trim tabs can be either fixed or adjustable and fitted to any control surface In light single engine aircraft fixed tabs are usually fitted to the rudder and possibly ailerons These fixed tabs are adjustable on the ground Adjustable tabs are most commonly installed on the elevator

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