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?