- Have an understanding of what a stall is
- Be able to recognise and react in a stall situation
- Be able to avoid a stall
Understanding the Stall
Look at the pictures above. In the first image the wing has a smooth flow of air over its upper surface. This smooth flow of air around the wing is what creates lift and keeps the aircraft in the air.
In the second picture we have started to pull the nose of the aircraft up. This tilts the wing against the oncoming flow of air. This is called changing the angle of attack of the wing. As this happens the air flowing over the upper surface of the wing begins to lose its grip on the surface and does not follow the shape of the wing any more. You may think this is what happens when we climb, but in actual fact in the climb we also increase thrust so the aircraft still penetrates the air as if it was travelling straight and level – as in the first picture. A stall occurs when the nose is raised but we have insufficient power to climb. In this situation the plane continues to fly straight and level only at a slower speed and with the nose raised.
In the third picture the wing is completely stalled. The angle of attack is so great that air can no longer stick to the top surface of the wing and it breaks away, swirling around in small ‘eddies’. When this happens the wing is no longer generating any lift and can no longer counteract the force of gravity, so the aircraft falls from the sky!
Reacting to a Stall
To learn to react to a stall your instructor will ask you to fly the following steps:
- Starting from a normal circuit, you will create a stall during the upwind leg
- At a safe height, close the throttle and start to pull back gently on the elevator to raise the nose a little
- Continue to raise the nose as the aircraft slows down
- Once a stall occurs the nose of the aircraft will drop sharply, keep the aircraft straight using the ailerons and centre the elevator
- Allow the nose to drop and apply full throttle
- As the aircraft gains speed, use the elevator to bring the nose up and regain straight and level flight
- Climb back to circuit altitude and continue to fly the circuit
This whole process will take no more than a few seconds and the aim is to keep the aircraft flying straight without dropping a wing and losing as little height as possible.
Avoiding the Stall
This lesson becomes important when you are flying slowly and close to the ground, i.e. just after take-off and just before landing. Practice stalling at altitude so that you can recognise when you model is going to stall and how slowly you can fly before a stall occurs.
If you think a stall is going to happen, the methods to avoid it are simple:
- Let the nose drop a little back towards a level attitude
- Increase power
Practice flying close to a stall and then pulling out of it before the nose drops. If you can recognise the signs and react before the stall occurs you should never be in danger of nose diving into the runway!
A tip stall occurs when just one half of the wing stalls, causing the aircraft to tip violently to one side and enter a spiral-dive. This occurs when an aircraft turns too tightly at too slow a speed, which most often when you are making the final turn towards the runway for landing. For this reason you must always make sure that your slow turns are made as wide as possible with as little bank in the wing as possible – this will be covered in more detail when you learn to land.