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Aerodynamics

Introduction To be fully in control of your aircraft, pilots need to understand how and why an aircraft flies, as well as the basic physics and mechanics of flight. We call this area of aviation science “Aerodynamics.” This video will go over the basic aspects of aerodynamics that a student pilot needs to understand.

This lesson will cover:

• The four forces of flight • And the four fundamental maneuvers

The Four Forces The study of aerodynamics begins with the understanding of the four forces of flight.

While an airplane is in flight, there are four forces acting upon it. They are: lift, weight, thrust and drag. Lift is the upward force, created by the wings as air flows around them; and keeps the airplane in the air. Weight is the downward force toward the center of the Earth, opposite lift, which exists due to gravity. Next, we have thrust. This is the forward force, generally created by the aircraft’s propellers or turbine engines, which pulls or pushes the aircraft through the air. Finally, there is drag. Drag is the force, acting in the direction opposite of thrust, which fundamentally limits the performance of the airplane.

When an aircraft is maintaining its heading, altitude, and airspeed, it is said to be in straight-and-level, un-accelerated flight. In un-accelerated flight, lift equals weight and thrust equals drag.

Let’s look at these four forces in a little more detail.

Lift The key to an aircraft being able to fly is lift.

Looking at a cross-section of a wing, we can better understand how the lift gets generated. A wing is a type of airfoil. Airfoils, in general, are just any surface that generates an aerodynamic force as a fluid, in our case AIR, moves around it. Don’t confuse a fluid with a liquid. Fluids are any substance that deform under an applied stress. Liquids, gases, and plasmas are all considered fluids. In addition to the wings, all the flight control surfaces, as well as the propeller are considered airfoils. The aircraft’s fuselage is even an airfoil, but it is not very good at producing lift.

Before we get too in depth, let’s introduce a few new terms.

The forward-most point of the wing is called the Leading Edge. The aft-most point is called the Trailing Edge. If we connect these two edges together with an imaginary line, this line is called the Chord Line. 2

As an airplane flies through the air, the path that the plane travels along is known as its Flight Path. The airflow that flows around the airplane as it travels through the air is known as the Relative Wind. The Relative Wind is parallel to, but opposite the aircraft’s flight path.

The angle between the wing’s chord line, and the aircraft’s relative wind, is called the aircraft’s Angle of Attack. The angle of attack is a major factor as to how much lift the wings generate.

So, now that we’ve got those terms out of the way, how does a wing actually create lift? Well, there are two major theories, working in unison, that explain the creation of lift. These are Newton’s Three Laws of Motion, and Bernoulli’s Principle.

While all three laws of motion apply to flight, the third law has the most significance to lift production. Newton’s third law states that, for every action, there is an equal and opposite reaction. If you stick your hand out of a moving car’s window, with your palm flat and thumb forward, and then rotate your hand, thumb-side up, you will notice that your hand will want to lift up. By rotating your hand, you are deflecting the air that comes in contact with your hand downward. And, as a result, the air will push your hand up. This is similar to how a wing works. In normal flight, as air flows around the wing the air gets deflected downward, as it flows smoothly around it, and as a result, the wind will lift the wing up.

The other main theory of lift is Bernoulli’s Principle. This principle states, “As the velocity of a fluid (in this case, air) increases, its internal pressure decreases.” We can visualize this by having air flow through a tube with a narrower middle section, which we call a venturi. As air enters the tube, it is traveling at a known velocity and pressure. When it arrives at the narrower portion, the velocity increases to allow all the air through. As the air’s velocity increases, the air’s pressure decreases. Then, as the air exits the narrower portion, it returns back to its original velocity and pressure. Now, let’s flatten the top part of the tube. Granted, the effect will not be as pronounced, but there still will be a change in velocity and pressure as the air moves through. Now, how does this relate to a wing?

Well, if we replace the bottom protrusion of the tube with a wing, in essence we have the same thing as the venturi. As the air passes over the wing, each layer of air gets deflected less and less, until finally, we reach a layer where the air is not disturbed at all from the wing. This can be thought of as the top of the venturi.

An airplane’s wing is shaped similar to that of a venturi. The top is rounded, while the bottom is relatively flat. Because of this, the air traveling over the wing will increase in speed, and as a result, will have a lower pressure than the air below the wing. This imbalance in pressure is called a pressure gradient. Wings are designed to create this kind of pressure gradient because air always moves from areas of high pressure to areas of low pressure. Since the wing is stuck in-between the two areas of unequal pressure, it is “lifted” towards the area of low pressure by the force of the higher pressure air trying to move to the low pressure side of the wing.

Now that we’ve covered the two theories behind lift, let’s discuss all the factors that determine how much lift is produced. The best way to discuss this is through the Lift Equation. Don’t worry though; this isn’t a math lesson. 3

Lift equals one half times the air density times the surface area of the wing times the airplane’s velocity squared times the coefficient of lift.

For the most part, this should be fairly straight forward. The only one that might confuse you is the coefficient of lift. The coefficient of lift is simply just a number that is associated with a particular shape of an airfoil, as well as the airfoil’s angle of attack.

Generally speaking, there are really only two ways a pilot can control the amount of lift the wings can generate: airspeed and angle of attack.

The faster the airplane travels, the more lift the wings will generate. Similarly, the higher the airplane’s angle of attack, the more lift the wings will generate. However, there is a limit to this angle. Let’s look at this using a chart. This chart is plotting the coefficient of lift of a particular wing as its angle of attack increases. Lift will continue to increase until a certain angle of attack, called the critical angle of attack. After this point, the wings still create lift, but the amount of lift created is decreased. This is called a stall. When some people think of the word “stall”, they think about the engine stalling. However, we are talking about an aerodynamic stall, which has nothing to do with the engine. Stalls happen when the normally-smooth airflow over the wing separates from the wing’s upper surface, resulting in a turbulent airflow. The parts of the wing that have turbulent airflow passing around them are not producing any lift. For a given airplane, a stall will always occur at the same angle of attack, the critical angle of attack, regardless of its airspeed, attitude, or weight.

Here is a wing at a normal cruise angle of attack, something small like 4 or 5 degrees. Increasing the angle of attack will progressively cause more and more disruption of the airflow on the upper surface of the wing. Early on, the airflow will separate at the trailing edge. As the angle of attack increases, the separation of the airflow moves from the trailing edge of the wing to the leading edge. As this separation occurs, the airflow will become more and more turbulent, and less and less lift gets generated.

Different airplanes will exhibit different stall characteristics, but generally coincide with mushy, sluggish flight control responses, a stall warning horn, a buffeting or shaking feeling of the airplane and pilots’ controls; and once the stall occurs the aircraft’s nose will pitch down, and the aircraft will begin losing altitude.

Since the stall is a result of an excessive angle of attack, the only way to recover from a stall is to reduce the angle of attack below the critical angle. Power should be added to minimize the amount of altitude lost in the recovery and increase the airplane’s speed as quickly as possible.

In addition to the aircraft’s airspeed and angle of attack, there are other factors that affect the amount of lift created by the wings. However, the pilot does not have control over these factors. These factors have to do with the design of the wing, itself, and consist of the wing’s planform, camber, aspect radio, and wing area.

The wing’s planform refers to the shape of the wing when viewed from above. 4

• The elliptical wing is ideal for flight at slow speeds, but does not have favorable stall characteristics. They are also very expensive to build. • The rectangular wing is less efficient than the elliptical, but has better stall characteristics. • Tapered wings result in less drag, and more lift, especially at high speeds. o Low-speed aircraft, like the Cessna 172, will use a combination of rectangular and tapered to get the best of both. • Sweptback, and delta wings are most efficient at high speeds.

The camber is the curvature of the wing. A wing with zero camber would be considered symmetrical about the chord line. Camber is usually designed into an airfoil to increase the maximum coefficient of lift, and thereby minimizing the stalling speed of the aircraft.

The wing’s aspect ratio is the relationship between the length and width of the wing. Generally, the higher the aspect ratio, the more efficient the generation of lift is. For example, gliders have really long, skinny wings, giving them a higher aspect ratio compared to a Cessna 172.

Finally, there is the wing area, which is simply the total surface area of the wings. The larger the wing area, the more lift the wing can produce.

Looking back at the lift equation, we can see that the wing area is incorporated into the equation. The rest of the wing shape factors are merged into the coefficient-of-lift variable.

We saw how pilots can control the lift generated by the wings by changing the aircraft’s airspeed and angle of attack. However, most airplanes come equipped with one or more additional ways for the pilot to manipulate the wings, and in essence, change the shape of the wings. These are called high-lift devices, the most common of which are trailing-edge flaps, or just “flaps”, for short.

High lift devices, such as flaps, are designed to increase the lift and drag generated by the wings at low airspeeds. Flaps are particularly important for the approach and landing phases of flight. Use of flaps during a landing allows the pilot to fly at a fairly steep descent angle without gaining airspeed, and allows the airplane to touchdown at a much slower airspeed.

There are four popular types of flaps, each with their own set of characteristics that designers use to best suit their airplane.

• The plain flap is attached to the wing by a hinge at the top of the flap. When deflected downward, it changes the chord line, and increases the camber, which both increases the wing’s ability to create lift. • The split flap is hinged on the bottom of the wing. When deployed it looks like the wing “splits”, which is where the split flap gets its name. This leaves the top edge of the wing unaffected and significantly increases the amount of drag created compared to the plain flap. • Next, is the slotted flap. This acts similar to the plain flap, but, when extended, the flap slides slightly backward, creating an opening, or slot, between the two surfaces. This slot allows the air below the wing to flow through the slot and on to the top side of the flap. This air flow 5

energizes the air on top of the flap and delays the air from separating away from the wing, which will allow the wing to generate more lift. • Finally, there are fowler flaps. Fowler flaps ride along a track and slide outward from the back of the wing before rotating down. When extended, they increase the size and surface area of the wing which if you remember from the lift equation will increase the total amount of lift created by the wing. This type of flap is often found on larger jets and on some aircraft there can even be slotted fowler flaps which allow the airplane to use the advantages of both a slotted flap and a fowler flap when creating lift.

Flaps can generally be lowered in steps, or more precisely, in set degree amounts. Initially, the input of flaps will increase lift by a larger amount, with only a small increase in drag. As the flaps are extended further, usually around the halfway point, lift increases only slightly and the amount of drag created increases rapidly.

Weight Now that we have an idea on how lift is generated, let’s discuss the three remaining forces, starting with weight.

Weight is the force of gravity, pulling the aircraft back down to the ground. This force always acts vertically downward to the center of the Earth, no matter what the aircraft’s attitude. The weight force always extends and pivots from the center of gravity, also known as its CG. Keep in mind that the weight of an aircraft is not constant. It will vary with the equipment that is installed, as well as the passengers, cargo, and fuel. Throughout the flight, the weight will slowly be decreasing as fuel is burned to power the engine.

Thrust Next is thrust. Thrust is the forward-acting force, opposing drag, which propels the airplane through the air. In most general aviation airplanes, thrust is generated from the propeller. Larger jets get their thrust from their turbine engines.

Similar to lift, thrust is generated from the same principles as lift but in a horizontal direction. A propeller is an airfoil. As such, as it rotates, its blades accelerate the surrounding air towards the aft end of the aircraft. And, as illustrated with Newton’s Third Law, the equal and opposite reaction results in the aircraft moving forward.

Drag And finally, we reach our last force, drag. Drag is the force opposing thrust, which limits the forward speed of an aircraft.

There are two types of drag, Parasite and Induced drag.

Parasite drag is a direct result of the air resistance as the airplane flies through the air. There are three types of parasite drag: form drag, interference drag, and skin friction drag. 6

• Form drag results from the turbulence created as the air tries to flow around the aircraft. Aircraft with larger cross-sections will have higher drag than thinner, more streamlined designs. Other items like the landing gear and the antennas on the aircraft will also create form drag. • Interference drag occurs in locations over the aircraft where different surfaces meet, for example, where the wings attach to the fuselage. Placing two objects close together will create up to 200% more drag, than if each object were separate. To minimize this, manufactures will place smaller angled pieces at these locations. • Skin friction drag is caused by the rough imperfections of the airplane’s surface. A good example of this are the rivets located on the airplane’s skin. These bumps disrupt the air from otherwise flowing smoothly along the surface. Keeping the surfaces clean and waxed, while also utilizing flush-mounted rivets will minimize the production of skin friction.

The amount of parasite drag varies with the speed of the aircraft. Air the airplane’s speed increases, the amount of parasite drag will increase. In fact, the amount of parasite drag you experience is directly proportional to the square of the airspeed. For example, an aircraft traveling at 120 knots will experience four times as much parasite drag as the same plane going 60 knots, at the same altitude.

The other kind of drag is lift-induced drag, more commonly called induced drag. While the wing is creating lift, behind the wing is a downwash of air. At the same time, the airflow around the wing tips are creating vortices that spiral from below the wing to above the wing. As these vortices wrap around the wing, they actually change the downwash angle of the air flowing over the wing. This, in effect, tilts the direction of the lift created backwards. This shift from completely-vertical lift to slightly-aft is due to induced drag. Induced drag is higher at slower airspeeds and decreases as we increase speed. This is because induced drag is worse when the airplane is flying at a high angle of attack, like when we are flying slowly.

One way that a pilot can experience reduced induced drag is by flying in ground effect. When flying within a wingspan of the ground, the ground itself changes the downwash of the air flowing over the wings. This shifts the lift vector forward and reduced the amount of induced drag. Pilots can take advantage of ground effect when performing a soft-field takeoff. This lets the airplane lift off the ground before the regular liftoff speed. However, they’ll need to hover over the ground for a few seconds to increase their speed before they can continue to climb out.

If we take both induced and parasite drag, plot them on a graph, and add them together, we get a new curve representing total drag. The lowest point on the total drag curve shows the airspeed at which we make the most amount of lift and the least amount of drag. This value is called “L-over-D-max,” or as pilots know it, our best glide speed. Pilots should be familiar with this number, because in the unlikely event of an engine failure, this is the speed at which they’ll want to glide down to the ground. In a no- wind condition, this speed will give the pilot the best glide ratio, meaning that they’ll be able to stay aloft the longest, to maneuver to their intended field for emergency landing.

Note on the left side of the total-drag curve. The slower you fly, the more drag you create. In this region of airspeeds, sometimes called the “backside of the power curve,” the pilot will actually have to add 7 more and more thrust to counter the high amounts of drag being created. In fact, if they want to accelerate out of this range of airspeed, they will have to add an excessive amount of power, maybe even full power.

One other thing to keep in mind at slow airspeeds is that there is much less airflow traveling over the flight control surfaces. As such, any input you make on the flight controls will not have the fast response one would be used to. The flight controls will feel “mushy”, and may require large inputs before any real response is felt.

The Four Fundamentals Now that we have a basic understanding of the four forces, let’s take a look at how we use these four forces to control the airplane through flight. To do this, we use four fundamental maneuvers: straight- and-level flight, turns, climbs, and descents. Every flight maneuver, whether basic or advanced, is based off a combination of these four maneuvers.

Straight-and-level Flight The first and most basic maneuver is straight-and-level flight. Straight and level flight is flight in which a constant heading and altitude are maintained. However, this is far from a hands-off maneuver. Pilot will have to make small corrections from time to time for deviations in heading and altitude caused by bumpy air, wind changes, and unintentional turns, climbs, and descents.

It is important to remember that when flying these maneuvers, the pilot’s primary focus should be looking outside on the horizon, with only a quick glance inside to check the instrument panel. In fact, the FAA recommends that pilots should keep their focus outside 90% of the time, and inside only 10% of the time.

The pitch attitude for level flight is obtained by referencing the aircraft’s nose with the horizon. If the plane starts to climb, then the pitch attitude is too high, and should be lowered. If you are losing altitude, the plane’s pitch is too low, and should be raised.

To maintain a constant heading, simply keep the plane in level flight, laterally. To do this you can just look out the side windows and compare both of the airplane’s wingtips with the horizon. Both wingtips should be the same distance above or below the horizon. If one wing is higher than the other, the airplane is turning.

Although both the airspeed and angle of attack control the aircraft’s lift, in straight-and-level flight, the throttle is primarily used to maintain a desired airspeed, and the elevator is used to control the altitude. Keep in mind, that the pitch necessary to maintain level flight is not always the same. Since lift increases with airspeed, the slower you are flying, the more you have to pitch up to maintain your altitude.

In normal cruise flight, when an aircraft is maintaining a constant airspeed, thrust and drag are equal. If the pilot increases their engine output via the throttle, the engine and propeller will spin faster, and generate more thrust. This will accelerate the aircraft, as there will then be more thrust than drag. As 8 the aircraft accelerates, more drag will be created, and eventually, the amount of drag will equalize with the amount of thrust, and the aircraft will stabilize and maintain its new cruise airspeed. Again, remember that as you increase your airspeed, you will need to lower the aircraft’s pitch, or you will start climbing.

When needing to slow down, the pilot will usually decrease the throttle, resulting in less thrust being generated. There will then be more drag than thrust, and this is cause the aircraft to slow down. As the aircraft slows down, the pilot will need to smoothly and continuously pitch up more to maintain their altitude. As the aircraft slows down, the amount of drag created will also decrease, and similar to before, the amount of drag will eventually equal the amount of thrust, and the aircraft will, once again, maintain its airspeed.

Climbs To get a plane to climb, we need to create more lift than weight. Pilots typically do this by pitching up and adding power. As an airplane enters a climb, it changes its flight path from level to a climb attitude. In a climb, weight still acts straight down, no longer perpendicular to the flight path, and because it’s aligned in a rearward direction, this causes an increase in total drag. Additional thrust is required to balance out the forces. There are typically three different types of climbs, each with different power settings and pitch attitudes, which a pilot can use during different phases of flight.

• The best rate of climb, also known as VY, is the climb profile that allows a plane to gain the most altitude in the least amount of time. In other words, this climb will give the maximum rate of climb in feet per minute.

• The best angle of climb, also known as VX, is the climb profile that allows a plane to gain the most altitude in the least amount of horizontal distance. This climb speed will result in the steepest climb path, but the plane will take longer to get there, compared to the best-rate airspeed. This type of climb is most suitable to be used when trying to climb over an obstacle. • Finally, a cruise climb is the type of climb used most often, especially when a pilot is in no rush to get to a higher altitude. The airspeed that is used for a normal climb is generally higher than the airplane’s best rate of climb, which allows for better engine cooling, easier control, and better visibility over the nose.

To return to straight-and-level flight from a climb, it is necessary to initiate the level-off at approximately 10 percent of the rate of climb. For example, if the airplane is climbing at 500 feet per minute, leveling off should start 50 feet before the desired altitude. After the airplane is established in level flight at the desired altitude, climb power should be retained temporarily to allow the airplane to accelerate to cruise airspeed. Once the aircraft has reached the desired cruise speed, the power can be reduced to an appropriate cruise setting. Keep in mind, that as the aircraft accelerates in level flight, it will want to continue to climb. Because of this, the pilot must continually keep decreasing the aircraft’s pitch to maintain that altitude…until the aircraft has finally stabilized. 9

Descents Descents occur when the amount of lift produced is less than the weight of the aircraft. Pilots can pitch down to reduce their angle of attack, or reduce their airspeed, by reducing the output of the engine, or both. Unlike climbs, the forward component of weight in a descent will add additional thrust to the aircraft. Different techniques will be used for different situations. Similar to climbs, there are three types of descents.

• Partial Power Descents, also known as cruise descents, or en-route descents, are the normal method used to descend. The target descent rate for this should be about 500 feet per minute. Ideally the pilot should set their pitch to maintain their desired airspeed, and use the throttle to control their descent rate. • Descents at Minimum Safe Airspeeds are a very different approach then a partial power descent, and are primarily used during an approach to landing on a short runway. These types of descents are much steeper than a partial power descent, but must be flown with caution. The aircraft will be flying at a very low airspeed, and as such, the margin of error between flying and stalling is significantly reduced. If it feels like the onset of a stall is happening, large amounts of power may be required to accelerate out of the situation. • Finally, a glide is a basic maneuver in which the airplane loses altitude in a controlled descent with little or no engine power. Forward motion is maintained by the forward component of weight. Aside from gliders, who have no engine, other aircraft can use this technique for normal landing procedures, as well as a controlled descent to landing during an engine failure. Pilots will usually want to glide down at the minimum drag airspeed, to maximize their gliding distance, giving them more potential landing locations.

Just like with climbs, when leveling off from a descent, the pilot must lead the altitude by 10% of the vertical speed, or they will end up overshooting their target altitude.

Turns Turns are the last fundamental maneuver that all pilots must master. A turn is made by simply banking the wings in the direction of the desired turn. Although, it may seem that only the ailerons are used to control a turn, there is much more to it. Let’s look at an airplane that’s about to make a left turn.

Like we saw in the previous lesson, a bank is made through the use of the ailerons. The aileron on the right wing rotates down and the aileron on the left wing rotates up. This makes the right wing create more lift than the left wing, which will roll the airplane to the left.

When an airplane is in a straight-and-level attitude, 100% of the lift produced is used to counteract weight. However, when an airplane is in a bank, the lift produced follows the direction of the bank, and no longer is directed straight up. With this new attitude, we still have our vertical lift, which opposes weight, but we also now have a horizontal component of lift. This horizontal component is actually what causes a plane to turn. Now, because some of the lift is used to turn the plane, that means that there is less vertical lift holding the plane up in the air. If nothing was done about this, every time a plane turned, it would lose altitude. Therefore, the total lift needs to be increased to get the vertical 10 component to still equalize the weight. This is done by slightly pitching up a few degrees…just enough to maintain our altitude.

But we aren’t done yet. Since the right wing of the airplane is producing more lift, it is also producing more drag. As we are trying to make our left turn, the added right drag will actually pull the nose to the right. This is something called Adverse Yaw. In order to counteract this, the pilot should also press the rudder pedal in the direction of the turn, in this case left, to help force the nose in the correct direction.

So as we turn to the left, we use left aileron, left rudder, and a little bit of back pressure on the elevator. That means we are using all three primary flight controls at the same time. Different bank angles will require different amounts of control input, but with some practice it’ll become second nature.

When it’s time to roll back out to straight and level flight from a turn, the pilot must lead the roll out before arriving at the target heading. Because a plane will continue turning as long as the wings are banked, the plane will continue turning while the roll out is happening. So, the general rule is to lead the roll out by one half of the amount of bank. For example, in a 30 degree bank, the roll-out should begin 15 degrees before the desired heading.

Also, remember that as the roll out is occurring, the vertical component of lift is returning, so the additional elevator pressure that was added for the turn should be smoothly and simultaneously reduced.

Conclusion While the finer details of aerodynamics can seem a bit overwhelming at first, gaining a basic knowledge of how the airplanes flies provides the pilot full understanding of all the forces at work and the best methods and technique for controlling their aircraft. A good pilot does not “drive” their plane from Point A to B, but, instead, understands the art and science of how their plane flies.