The 152 can be flown at airspeeds as low as 57 Knots, and as high as 149 Knots. High speeds give excellent controllability, but at low airspeeds you need careful and gentle handling.
To land an aeroplane on a runway gracefully you will need to slow down while raising the nose so that only the main wheels touch down. As you slow down just above the ground, raising the nose increases the angle of attack of the wings (this is called Holding Off), until the stalling angle is reached, and with the consequent reduction of lift the aeroplane descents gently onto its wheels To do this youre going to need lots of practice!
So imagine you are holding off when a vehicle or perhaps and animal crosses your landing path You are going to have to Go Around, so you apply full power but your airspeed is low and so this requires careful handling of the controls until sufficient speed has been built up to climb away. This situation is precisely why we practice Slow Flight.
To practice slow flight we will start from the cruise and slowly reduce the airspeed by reducing the power and raising the nose to maintain our altitude by increasing the angle of attack.
As the angle of attack increases, so will the Drag. You can feel this drag increase if you put your hand in the slipstream as your car travels along the road; with your hand flat it is easy to hold it there, but if you rotate your hand 90° to the wind, it will be blown backwards, this is a definite increase in drag! Likewise, as the angle of attack of the wing is increased so is its drag.
There are other factors that increase the drag, and these are called Vortices. To see vortices, fill the kitchen sink with water and then take a knife and move it through the water flat side in the direction of the movement. You will create swirls of water, eddies, where the water spins around to fill the gap, vortices are similar to these. Theres a lower pressure on the top of the wing than the bottom, and so the air spills around the tip and swirls like a miniature tornado. The pressure is very low in this vortex and so it pulls back on the wing like someones holding the wingtip! Vortices get bigger as the angle of attack increases.
Slow Flight in the Cessna 152
In this exercise we reduce the power gently and note the Indicated Airspeed shown for each 100 rpm of power reduction.
These are typical figures: 2000 rpm / 82 KIAS (Knots Indicated Airspeed), 1900 rpm / 73 KIAS, 1800/67, 1700/57, 1800/50, 1900/44!
We went down to 1700 rpm and found that we could maintain height at 57 KIAS, this is the minimum power we needed to fly level, it would give us the least amount of fuel use, and so we could stay in the air for the most amount of time. We call this speed the Best Endurance Speed.
Reducing the power further to fly at 50 KIAS caused us to descend and so the power had to be increased again to maintain our height. This was because the drag had increased so much that we needed more power to be able to fly slower! When you have to add power to fly slower while maintaining your height, you have entered the Slow Flight regime.
The slipstream from the propeller will try to yaw the aeroplane to the left so we have to apply sufficient right rudder to keep the ball in the middle.
We are very close to the stall, and the ailerons are quite ineffective at these low speeds so the rudder is now the primary control for roll, well learn why in the next lesson on stalling
Recovery from slow flight is simple; apply full power, keep straight with the rudder, and lower the nose.
© Michael Peare 2001
Next: Lesson 7.