In the last lesson we learned about how the controls are used to create movements that allow us to manoeuvre in the air. In this lesson we will apply what we have learned.
First of all I need to give you a couple of definitions!
The angle of attack is the angle at which the wing meets the air. A wing has a curved shape that is called an Airfoil and this is designed to create Lift in the most efficient way for the particular aeroplane. To create lift the airfoil has to be at an angle to the airflow. To experiment for yourself, just stick your hand into the Slipstream that passes your car as you are driven along a road; if your hand is directly in line with the slipstream theres no lift, but if you angle your hand to the airflow it will lift up, or down. This angle of your hand to the airflow is called its Angle of Attack. A wing needs a positive angle of attack to develop lift to keep the aeroplane in the air.
In the "three point", landing attitude this Cessna
170's wing reaches a high angle of attack to
provide lift at low airspeed for comfortable
The same aeroplane in cruising flight where the wing
is able to develop the same amount of lift with a
smaller angle of attack due to to its higher
The only true measure of distance on the Earths surface is called the Nautical Mile. It is easily measured on a map because it equals 1 Minute of Latitude. There are 60 minutes in one degree, and there are 180 degrees of latitude from the North Pole to the South Pole. Knots are Nautical Miles Per Hour and the Airspeed Indicator is calibrated to read airspeed in Knots. 1 Knot = 1.85 Kilometres Per Hour, so an airspeed of 100 Knots = 185 Kph.
Here we see a section of a map.
From 41°30' to 42° is 30 Nautical Miles
We can measure distances directly off the map if we
use nautical miles as minutes of latitude using a line of
longitude (Meridian) as a scale.
Minutes of longitude cannot be used for this purpose
since these get smaller as we approach the poles.
Now let's get down to the nitty gritty of this lesson
First of all there is one basic rule to be learned, that is Attitude + Power = Performance!
A Cessna 152 will cruise happily at 80 Knots with a power setting that gives 2,000 rpm, and with the nose just below the horizon. To an extent, Attitude can be said to control the airspeed. If we do nothing else but lower the nose, then the speed will increase, just like rolling your car down a hill, and if the nose is raised above the horizon, just like rolling the car up a hill, the speed will decrease. This idea will be very important later when we have to land the aeroplane!
Supposing we want to increase our cruise speed to 90 Knots. We will need to lower the nose to gain the speed, but, if we do, we will lose height! So first we add more Power, maybe 2,200 rpm will do the trick, and then lower the nose (Attitude) slightly as the aeroplane accelerates to achieve the new cruising speed. At this higher speed the wing needs to meet the airflow at a smaller angle of attack to generate the required lift Do you remember the Trim? Yes! we have to trim the aeroplane to fly at the new attitude and speed. So to increase our cruise speed, we need to apply the correct Power, lower the nose Attitude by moving the control column forward as the aeroplane accelerates, and then Trim to remove the control forces so that the aeroplane can fly "Hands Off".
Conversely, to fly at a reduced cruise speed, we reduce the Power, raise the nose to a higher nose Attitude as the aeroplane decellerates, and then Trim it once again.
In a car you have to put your foot down on
the accelerator to go up a hill, likewise an aeroplane needs more
Power to climb higher. If the aeroplane is trimmed
to fly level at 80 Knots and we add power, the nose will rise
automatically into the climb Attitude and the aeroplane
will continue at the same airspeed. Even though the nose is
now above the horizon, the Angle of Attack is the
same! The attitude of the aeroplane is not necessarily an
indicator of the angle of attack of the wings. In this case
the Power and Attitude have been altered to give
Conversely, if we reduce the power to give 1800 rpm while trimmed to fly at 80 Knots in level flight at 2,000 rpm, the nose will automatically lower itself to maintain the trimmed speed, and the aeroplane will descend.
In the next lesson we will carry out turns.
© Michael Peare 2015