The Aerodynamics of Airfoils
Last time, I talked about the designs of airfoils and how they generate lift. In today’s blog, let’s explore the angle of attack and aerodynamics!
The Angle of Attack (AoA) is the angle between the chord line of the wing and the relative direction of the wind parallel to the flight path. Refer to my expertly drawn diagram.
The angle matters a lot. A low AoA has air flowing smoothly over the airfoil and creates slight pressure differences, resulting in minor lift or drag. A moderate AoA increases the deflection of air downwards to create more pressure differences between the upper and lower surfaces (creating lift). However, at high AoAs, the airflow separates from the upper surface, causing turbulence and reduces lift. When the AoA exceeds a critical angle of attack (15 degrees for my Piper Archer pilots), we start to stall our airfoil.
A stall is a loss of lift to the point where the aircraft cannot maintain its altitude and loses its aerodynamics``. We may see a nose drop or a roll. Typically, this is a good thing because a downwards pitch lowers our AoA and restores lift. There are different type of stalls but I will cover that later on. To recover from a stall, we want to create more below-airfoil pressure so we reduce the AoA and add power to recover lost airspeed.
In extreme cases, a spin might occur when an aircraft starts to spin in a yawing motion due to one airfoil (wing) stalling more intensely. To recover, we utilize the PARE method:
P - Power to idle
A - Ailerons neutral
R - Rudders opposite direction
E - Elevator forward
Notice how we add power in a stall, but idle the power in a spin. This is because in a standard stall, the aircraft is not yawing so we can safely utilize power and a reduced AoA to gain speed and lift. Conversely, power can actually aggravate the spin because of gyroscopic precession and asymmetric thrust (due to the wings stalling at different levels). We can recover from a stall without adding power, but it is much safer to do so with the power because it gets us out of the danger zone faster. In the case of a spin, after we idle the power and neutralize the ailerons, we apply rudder to the opposite direction of the spin to counteract the yaw. Once it has stopped, we can move the elevator forward (pushing the nose down) just like we would in a normal stall. This is important to study and memorize because it might save you from starring in an NTSB page one day!
The angle of incidence is the angle between the chord line and a “longitudinal line” representing the axis of the airplane. A higher angle of incidence generates more lift at lower airspeeds but can cause more drag at higher airspeeds, whereas a lower angle of incidence may reduce drag at high speeds but require higher nose-up attitudes at lower speeds to generate equivalent lift. Attitudes are the relationships between the pitch and bank angle to the horizon. Each aircraft is designed with a compromise to match their intended use-case. Some Boeing planes utilize a zero-angle-of-incidence so the fuselage pitches up during cruise.
Throwing down more terminology, the center of pressure (aka center of lift) is the point where lift is perpendicular to the direction of flight relative to the airfoil. This can be affected by the dihedral - the angle of the wing from the root to the wingtip. The wing’s planform is the shape of the airfoil from a bird’s eye view. The leading and trailing edges can be tapered or straight, creating four combinations of planforms. In addition, we have sweptback wings that are slanted backwards and delta wings (F-22).
Wow that was a lot of stuff to learn! In the next section, I will go over the aircraft’s structure in more detail.