Knowing Simple Aerodynamics Helps Your Aviation Career
When it comes to teaching someone about aerodynamics, it is possible to teach simple aerodynamics in a way that doesn’t require a physics course as a prerequisite. I am a firm believer that as your flight career progresses, so should your knowledge. However, we need to take a simple to complex, known to unknown approach. If you are a Flight Instructor, you know this is a key fundamental of instruction. Take ‘Lift’ for example; at what point should someone be expected to know the Coefficient of Lift (CL)? Here’s what NASA says regarding the lift coefficient:
“The lift coefficient is a number that aerodynamicists use to model all of the complex dependencies of shape, inclination, and some flow conditions on lift. This equation is simply a rearrangement of the lift equation where we solve for the lift coefficient in terms of the other variables.
The lift coefficient Cl is equal to the lift L divided by the quantity: density r times half the velocity V squared times the wing area A. Cl = L / (A * .5 * r * V^2)
The quantity one half the density times the velocity squared is called the dynamic pressure q. So Cl = L / (q * A)
The lift coefficient then expresses the ratio of the lift force to the force produced by the dynamic pressure times the area.”
Above, NASA states this equation is “simply a rearrangement of the lift equation”. Perhaps if you have a PHD and work for NASA this equation is simple. Why is this complex lift equation shown to brand new student pilots across the Country? If we know that a key fundamental principle of instruction is to go from simple to complex, known to unknown; why would we ever introduce a complex physics equation to a new student? Instead, when introducing lift, ask your student a few simple questions:
“Have you ever been driving down the highway with your hand out the window? Have you noticed that if your raise your hand up slightly, your whole arm wants to shoot up like you’re waving to oncoming cars? That is lift…simple”
When learning simple aerodynamics, where should one start?
The first thing I have my students learn, are the basic definitions of helicopter aerodynamic terms. I focus on their rote memorization of bullet point definitions. Once they have these memorized, I then focus on their understanding and application. I present aerodynamic terms to my students in a question / answer format. Before a Student Pilot is ready to take their Private Pilot exam, they will need to be able to describe aerodynamics much more in depth. However, if you’re new to aerodynamics, I recommend you start by memorizing the key definitions below. Writing these questions / answers down on index cards is a good idea to aid in your memorization of these key aerodynamic terms.
What are the forces of flight?
A. Lift, Weight, Thrust and Drag
How is lift developed?
A. LIft is developed by creating an area of positive pressure beneath the airfoil and negative pressure above the airfoil.
What is Bernoulli’s Principle?
A. Bernoulli’s Principle states that as velocity increases, pressure decreases. This is also known as the Venturi Effect.
What is Newton’s Third Law?
A. For every action, there is an equal and opposite reaction.
What is Angle of Attack?
A. The angle between the chord line and relative wind.
What are the three types of drag?
A. Profile, Parasite and Induced drag.
What is Profile Drag?
A. Drag caused by the frictional resistance of the blades moving through the air. Composed of Form Drag and Skin Friction.
What is Parasite Drag?
A. Drag caused from all Non-Lifting surfaces of the aircraft.
What is Induced Drag?
A. Drag that is a result of developing lift. Also known as Vortex Drag.
What is Coriolis Effect?
A. As the center of mass moves closer to the axis of rotation, the blades have a tendency to accelerate.
What are the two external factors that cause Coriolis Effect?
A. Coning and Blade Flapping.
What is Coning?
A. Coning is the result of two forces acting at the same time; Centrifugal Force and Lift.
Why do helicopter blades Flap?
A. Helicopter Blades are allowed to Flap to compensate for Dissymmetry of Lift.
What is Dissymmetry of Lift?
A. Unequal lift between the advancing and retreating halves of the rotor disc.
What is Retreating Blade Stale?
A. Due to Dissymmetry of Lift, at high forward airspeeds the retreating blade exceeds its critical angle of attack causing the blade to stall.
What is Translating Tendency?
A. The tendency of the helicopter to drift in the direction of tail rotor thrust.
What is Translational Lift?
A. Improved rotor efficiency resulting from directional flight or surface winds.
What is Effective Translational Lift (ETL)?
A. ETL occurs at approximately 16-24 knots when the rotor system completely outruns the recirculation of old vortices.
What is Transverse Flow Effect?
A. Occurs at speeds just below ETL. Induced flow drops to near zero at the forward disc area and increases at the aft disc area.
What is Gyroscopic Precession?
A. Gyroscopic Precession states that when an outside force is applied to a rotating body, the result of the outside force will occur 90 degrees later in the plane of rotation.
Begin introducing the “Why?”
Once you have memorized some of these key definitions, it will be time to start asking “Why?” The short list above is exactly that…short. Notice I did not include aerodynamics of autorotations, conservation of angular momentum or other complex aerodynamic principles. There is a lot more to learn, but building a foundation of key definitions is where we should start. Once you have the definitions memorized, the next step is to gain an understanding of what is actually happening. Helicopter Aerodynamics can be made simple and enjoyable to learn. Start with the basics and develop a good foundation to build on. This is my approach to teaching aerodynamics to a brand new student. There are other good approaches that Instructors use and they are successful in their teaching. With a goodFlight School you will be surrounded by a large amount of Flight Instructors ensuring that your individual learning needs will be met.
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