1. Forces and stability in aircraft
Fly straight
Forces and stability in aircraft

2. Fly Straight: Forces and stability in aircraft
This presentation covers the following:
Forces involved in flying
What is stability
Side-to-side stability
Nose-to-tail stability
At the end you should understand:
How aircraft are designed to be stable
How this applies to your glider

3. There are 4 main forces acting on aircraft:

4. This is Newton’s 1st law of motion.
The aircraft will continue at the same speed and direction unless acted upon by an unbalanced force.
This is Newton’s 1st law of motion.
If Lift > Gravity
Gain altitude
If Thrust > Drag
Speeds increases
If Drag > Thrust
Speeds decreases
Teacher notes:
Newton’s first law of motion is often stated as “an object will remain at rest or in uniform motion in a straight line unless acted upon by an unbalanced force”.
The net force experienced by an object is a sum of all the individual forces acting upon that object, taking into account that these forces have direction (they are vectors).
If the forces on the flying aircraft are unbalanced then the aircraft will change speed or altitude:
If the Lift > Gravity: The aircraft will gain altitude
If the Gravity > Lift: The aircraft will lose altitude
If Thrust > Drag: The aircraft will increase its speed
If Drag > Thrust: The aircraft will decrease its speed
In cruise condition, maintaining speed and altitude: Lift = Gravity and Thrust = Drag
What this means for our glider:
Because the flight of the glider is not powered, the lift generated is not enough to counteract gravity and our glider gradually loses altitude.
If Gravity > Lift
Lose altitude
In cruise conditions: Lift = Gravity and Thrust = Drag

5. We also want the aircraft to be stable…
But what happens if there is a sudden disturbance?
We also want the aircraft to be stable…

6. What is stability?
A stable object will come back to its original position after being disturbed.
Teacher notes:
If you poke this cone it will likely return to its original resting position: it is stable. However, even a stable object can become ‘unstable’ if the disturbance is large enough - here the cone would fall over on its side if pushed too far.

7. An unstable object will not come back to its original position after being disturbed.

8. An object may have neutral stability
Teacher notes:
If you poke this cone it can either come back to its original position, or roll away from it.

9. Side-to-side stability
horizontal length left side = horizontal length right side
lift on the left side = lift on the right side
Teacher notes:
This slide shows the situation when the aircraft is level side-to-side (lateral direction).
The lift generated is dependent on the horizontal (projected) length of the wing, labelled ‘wing length’ on the diagram. As the length of the wing is equal on both sides, the lift generated is equal on both sides.
What this means for our glider:
It is important to position the wing centrally so that the wing length is the same on both sides. As well as altering the lift, having the wing off centre would also unbalance the weight of the glider causing additional problems.
The forces are balanced and the aircraft continues level.

10. A gust of wind now tilts the plane. What happens?
horizontal length left side > horizontal length right side
lift on the left side > lift on the right side
Teacher notes:
As the aircraft tilts, the horizontal (projected) length of the wings on the left side is now greater than the length on the right side. This in turn generates more lift on the left compared to the right.
What this means for our glider:
The optimal angle for the winglets is between 20 and 45 degrees from horizontal. This provides the best balance between increasing stability whilst maintaining maximum lift from the wing overall. Decrease the angle and you decrease the difference in wing length as the glider tilts, reducing stability. Increase the angle too much and you make the wing length shorter, decreasing the lift.
This imbalance in lift causes the aircraft to return back to its starting position. The aircraft is stable side-to-side.

11. If the plane tilts left with the winglets point down:
horizontal length left side < horizontal length right side
lift on the left side < lift on the right side
Teacher notes:
Commercial passenger aircraft have the winglets in the upwards position to increase their stability. However, military aircraft may actually have the winglets pointing downwards in this less stable configuration. Having a very stable aircraft makes it less manoeuvrable, as you need to make large disturbances to overcome the tendency for the plane to self-correct its position. As military aircraft need to be very manoeuvrable, making them less stable makes it easier to change direction quickly.
What this means for our glider:
Try attaching the wings onto the glider upside down and see what difference it makes.
The imbalance in lift causes the aircraft to continue rotating. The aircraft is unstable.

12. The forces are balanced and the aircraft continues level.
Nose-to-tail stability
The centre of gravity acts as a balance point, the lift from the wing and tail create rotation around this point.
Teacher notes:
Both side-to-side (lateral) and nose-to-tail (longitudinal) stability involve the actions of unbalanced forces. However, there are two additional concepts that come into play when it comes to the nose-to-tail stability:
1. Lift acts at right angles to the direction of air flow.
The horizontal stabilizer is angled down compared to the direction of air flow. This means the flow of air is directed up over the top surface so the lift generated by the horizontal stabilizer actually points down in this instance. In comparison, the air flow is directed underneath the wing, resulting in upwards lift. For commercial aircraft, the wing will be positioned at a slight angle, called the angle of attack, in order to generate lift. For our model glider, the wing is horizontal compared to the fuselage. However, the glider is falling as well as travelling horizontally when it flies, and this gives enough of an angle of attack to allow the wing to generate lift.
2. The forces on the aircraft follow the same law as levers
The balance point (fulcrum) of the aircraft is located at the centre of gravity. The lift generated by the wing and horizontal stabiliser causes the aircraft to tip or rotate around the balance point (it creates torque). The amount of rotation is equal to the force (lift in this case) multiplied by its distance to the balance point.
When the aircraft is level
The centre of gravity acts as the balance point: the lift from the wing acts to tip the nose down, lifting the tail section up. The downwards lift from the horizontal stabilizer counteracts this, tipping the nose up. Because the lift generated by the horizontal stabilizer is in the opposite direction to the lift generated by the wing, the rotational motions cancel each other out. The forces are balanced and the aircraft continues to be level.
The horizontal stabilizer is further away from the balance point than the wing. Remember, the law of levers means that the rotation is equal to the force multiplied by distance. As the horizontal stabilizer is more distant it does not need to produce as much lift to generate the same amount of rotation as the wing. This means that the horizontal stabilizer can be much smaller than the wing and still generate enough rotation to keep the aircraft level. This difference in size between the wing and horizontal stabilizer is important for our next point.
Lift versus rotation
The longitudinal stability is a result of cancelling out the rotational forces generated as described above. This is different to the overall lift experienced by the aircraft. The total lift is equal to the lift from the wings plus the (negative) lift from the horizontal stabiliser. This means the horizontal stabilizer does decrease the overall lift but, because the wing is so much bigger, the overall lift is sufficient to keep the aircraft flying.
What this means for our glider:
The position of the wings needs to be adjusted correctly so that rotational forces generated by the wing and horizontal stabilizers cancel each other out. As the horizontal stabilizer is smaller than the wing, the distance between the wing and stabilizer needs to be sufficient to compensate for this through the law of levers.
The forces are balanced and the aircraft continues level.

13. The aircraft is returned to its starting position. It is stable.
A gust of wind tips the nose of the aircraft down.
The horizontal stabilizer is angled down. This creates even more lift downwards, tipping the nose of the aircraft up.
Teacher notes:
When the aircraft pitches down, the horizontal stabiliser is now angled even further down compared to the direction of air flow. This generates even more lift downwards, which in turn increases the amount of rotation generated around the centre of gravity. This rotation acts to tip the nose of the aircraft back up.
The aircraft is returned to its starting position. It is stable.
The aircraft is returned to its starting position. It is stable.

14. The aircraft is returned to its starting position. It is stable.
Now another gust of wind tips the nose of the aircraft up.
The horizontal stabilizer is angled up. This creates lift up, tipping the nose of the aircraft back down.
Teacher notes:
When the aircraft pitches up, the horizontal stabilizer becomes angled up compared to the direction of air flow. The lift generated by horizontal stabilizer and wing now both point up. With the centre of gravity acting as the balance point, this lift acts to tip the nose of the aircraft back down.
The aircraft is returned to its starting position. It is stable.
The aircraft is returned to its starting position. It is stable.

15. The aircraft will tip upwards. It is not stable.
If the centre of gravity is moved behind the wing:
The lift from the wing will tip the nose up, and the lift from the horizontal stabilizer will add to this rotation.
Teacher notes:
The nose weight plays an important role on the position of the centre of gravity. If the centre of gravity is positioned behind the wing, then the lift from the wing generates a rotation tipping the nose up. This then adds to the rotation generated by the horizontal stabilizer. The aircraft will tend to tip upwards.
What this means for our glider:
The nose needs to be sufficiently weighted to bring the centre of gravity of the glider to just in front of the wing. To far forward and the whole glider will become unbalanced and plunge nose first to the ground. Conversely, too far back and the glider will be unstable in flight.
The aircraft will tip upwards. It is not stable.

16. Acknowledgements
This presentation was developed by Paul Lancelot and Juliet Jopson as part the AMEDEO Marie Curie Initial Training Network project, funded from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, please attribute to AMEDEO ITN, EU FP7 Grant no