Mechanics Tutorial 4 - Vehicle Stability
Some people like to buy sports utility vehicles (SUVs). They are very fashionable as luxury cars.
They remind me of a computer I used at work more years ago than I care to remember. It was called a Televideo but that was a con. It was neither a telly nor a video. In the same way, an SUV is neither a sports car, nor a utility vehicle. A sports car is designed to have nimble and agile handling characteristics, but many of these lumpish and brutish vehicles have dreadful handling characteristics that make them rather unpleasant to drive.
Utility vehicles are designed to lug large and possibly dirty objects about the country. However a good number of SUV owners would have a fit at having dusty bags of cement in the back of their exceedingly expensive vehicles with their even more expensive personalised number plates. Some may tolerate muddy dogs in the rear luggage compartment, especially if they have spent a satisfying afternoon blasting wood-pigeons from the sky...
The bus has its centre of mass half-way between the wheels. The distance from the centre of mass to the tyre is d metres, and its height above the road is h metres. If the mass of the bus is m kilograms, its weight is mg newtons.
Suppose we tip the bus over by an angle of q to the road:
The angle Q is q. By simple geometry we can say that angle P is q as well.
There is a critical point at which the bus might fall back or tip over. This is where the centre of mass is directly above the point of contact of the tyre with the road. Point P is vertically above point Q.
Now suppose the bus tips further:
Lorries have a higher centre of mass on their trailers, due to the load. If you live in the country and get stuck behind a hay-lorry, you may see it swaying alarmingly. This kind of accident tends to happen with lorries when they drive through strong cross-winds on exposed roads.
Stability in Aeroplanes
When an aeroplane flies, there is a centre of lift, which is an imaginary point through which all the lift from the wings appears to act. Ideally the centre of mass of the aeroplane should be directly underneath this. Since the line of actions of both forces coincide, there is no turning moment, and the aeroplane is stable.
When the pilot uses the elevators (flaps at the tail to make the aeroplane move up and down), a force is applied to the tail, causing a turning moment to act.
When the pilot wants to turn, he uses the ailerons (flaps at the end of each wing). One aileron is raised, and the other is lowered, to make a couple. The aeroplane rolls.
Now suppose the aeroplane takes off with both wing tanks full of petrol. The pilot can take petrol from both tanks, and he selects which tank to use. He needs to keep both tanks balanced, by using 10 litres from one tank, then 10 litres from the other tank. Every few minutes he needs to change over the tanks.
Suppose he forgets to change over, and takes almost all the fuel from the right tank.
How do you think this will affect the handling of the plane? Explain your answer.
The management of the fuel is an important part of the air-work of the pilot of a light aeroplane. However many light aeroplanes can take petrol from both tanks at the same time. Unless there is a problem with one of the tanks, that is what is done normally.
If the aeroplane is not balanced, it is possible for the turning moment to be cancelled out by using the control surfaces. All aeroplanes have trim tabs which are little flaps that can push the nose up or down to balance the plane.
Recently there was a tragic accident involving an aeroplane in Africa. One of the passengers had smuggled on board a young crocodile that was a pet. It got out from its box and all the passengers rushed to the front of the plane. There was a big nose-down turning moment which the pilot could not balance with his controls. The plane nose-dived into the ground.