The Aerodynamics of a Stall
The wings of your
aircraft are simply an airfoil, which creates lift when air is pushed past
it at the propper angle. The best way to demonstrate this (believe
it or not) is by driving down the road with your hand out the window of
your car! With your fingers held together and your palm facing
towards the ground and your thumb at the leading edge, your hand (the airfoil)
hits the wind with an angle of attack of 0 degrees. This doesn't
create any lift, but if you increase the angle of attack (rotating the
thumb upward), then your hand is pushed upwards (Diagram
1).
The force pushing your hand upwards is called LIFT, and the force pushing
your hand backwards is called DRAG. The Angle between the oncoming
airflow and the airfoil (your hand) is called the ANGLE OF ATTACK (AOA).
Note that if we have a low angle of attack, we produce a small ammount
of lift and a small ammount of drag. If we increase the angle of
attack, we produce more lift, but also produce more drag (Diagram
2).
Diagram
1
Diagram 2
Say you are
travelling at 60 kmph and keep the same angle of attack , but increase
speed to 100 kmph, what happens? Well more airspeed creates more
lift, and your hand gets pushed upwards with even more force, but you also
produce more drag. Similarly if you slow down and maintain
AOA, lift decreases, and so does drag.. If the airspeed is too slow,
the ammount of lift produced is less than the force of gravity pushing
your hand down, and so your hand falls. This is not a stall - it
is a similar situation to when you are on the runway a bit short of the
required speed for takeoff. From this we learn that the more speed
you have, the more lift you can produce.
What happens if we increase the AOA and keep the speed constant?
More AOA creates even more lift, but as you increase the AOA of your hand,
it gets to a point where you cease creating lift, your hand drops down,
and is pushed back by the force of the oncoming airstream. (Diagram
3) At this point you are creating no lift,
but a lot of drag. This situation is what we call a "STALL".
Note that if you maintain this angle and increase speed, you cannot re-establish
lift, because the AOA is too extreme.
A stall situation simply means that the airflow past the airfoil has become
very turbulent (Diagram 4) instead
of streaming past and smoothly re-joining at the trailing edge
. The only way to re-establish the correct airflow and subsequently
lift, is to decrease the AOA- it's really that
simple. When the airflow is right on the edge of stalling for
that particular speed, the airfoil is at it's CRITICAL Angle of attack.
If for example your hand has exceeded this critical AOA as in the top of
diagram 3, doubling your speed will not create any lift, it will just create
extra drag.
It is important to realise that your aircraft can stall at any speed.
At very high speeds you are more likely to black out before you encounter
a stall. (You do have the Blackout/Redout option on, don't
you??!!!)
Diagram
3 Diagram
4
A simple stall can be encountered by flying along slowly (160 kmph or 100
mph), keeping the wings level, and then pulling the nose up 20-30 degrees
and holding it. As the airspeed bleeds off, the lift generated decreases,
and more back stick pressure is required to hold the nose up. It
gets to a point where the wings exceed the critical AOA, and bingo - you
have stalled the airflow. If the wings are level, gravity takes over
and helps recover the stall for you. With no lift, the weight of
the aircraft forces you to lose altitude, the airflow over the tail section
pushes the tail up and the nose down, and you dive, thus regaining airspeed
again. This type of stall is a piece of cake to get out of - you
just point the nose downwards (Decrease AOA), open up the throttle
and build up some speed, then slowly pull out of the dive. Where
you get into trouble, is if the wings aren't quite level, and then it is
more likely that you will enter a Spin.
Click here to go to Aerodynamics
of a Spin.
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