Mach number is the ratio of the true airspeed to the speed of sound (TAS ÷ Speed of Sound). For example, an aircraft cruising at Mach .80 is flying at 80% of the speed of sound. The speed of sound is Mach 1.0. When in high-speed flight we refer to our airspeed in mach rather than true airspeeds or indicated airspeeds. At any airspeeds above Mach 1 you would be breaking the sound barrier.

A large increase in drag occurs when the air flow around the aircraft exceeds the speed of sound (Mach 1.0). Because lift is generated by accelerating air across the upper surface of the wing, local air flow velocities will reach sonic speeds while the aircraft Mach number is still considerably below the speed of sound. With respect to Mach cruise control, flight speeds can be divided into three regimes—subsonic, transonic and supersonic. The subsonic regime can be considered to occur at aircraft Mach numbers where all the local air flow is less than the speed of sound. The transonic range is where some but not all the local air flow velocities are Mach 1.0 or above. In supersonic flight, all the air flow around the aircraft exceeds Mach 1.0. The exact Mach numbers will vary with each aircraft type but as a very rough rule of thumb the subsonic regime occurs below Mach .75, the transonic regime between Mach .75 and Mach 1.20, and the supersonic regime over Mach 1.20.

A limiting speed for a subsonic transport aircraft is its critical Mach number (MCRIT). That is the speed at which airflow over the wing first reaches, but does not exceed, the speed of sound. At MCRIT there may be sonic but no supersonic flow.

When an airplane exceeds its critical Mach number, a shock wave forms on the wing surface that can cause a phenomenon known as shock stall. If this shock stall occurs symmetrically at the wing roots, the loss of lift and loss of downwash on the tail will cause the aircraft to pitch down or “tuck under.” This tendency is further aggravated in sweptwing aircraft because the center of pressure moves aft as the wing roots shock stall. If the wing tips of a sweptwing airplane shock stall first, the wing’s center of pressure would move inward and forward causing a pitch up motion. See the Figure below.

The less airflow is accelerated across the wing, the higher the critical Mach number (i.e., the maximum flow velocity is closer to the aircraft’s Mach number). Two ways of increasing MCRIT in jet transport designs are to give the wing a lower camber and increase wing sweep. A thin airfoil section (lower camber) causes less airflow acceleration. The sweptwing design has the effect of creating a thin airfoil section by inducing a spanwise flow, thus increasing the effective chord length. See the Figure below.

Although a sweptwing design gives an airplane a higher critical Mach number (and therefore a higher maximum cruise speed), it results in some undesirable flight characteristics. One of these is a reduced maximum coefficient of lift. This requires that sweptwing airplanes extensively employ high lift devices, such as slats and slotted flaps, to get acceptably low takeoff and landing speeds. The purpose of high lift devices such as flaps, slats and slots is to increase lift at low airspeeds and to delay stall to a higher angle of attack.

Another disadvantage of the sweptwing design is the tendency, at low airspeeds, for the wing tips to stall first. This results in loss of aileron control early in the stall, and in very little aerodynamic buffet on the tail surfaces.

The post CFI Brief: Mach Number first appeared on Learn To Fly.

When an airplane flies at subsonic speeds, the air ahead is “warned” of the airplane’s coming by a pressure change transmitted ahead of the airplane at the speed of sound. Because of this warning, the air begins to move aside before the airplane arrives and is prepared to let it pass easily. When the airplane’s speed reaches the speed of sound, the pressure change can no longer warn the air ahead because the airplane is keeping up with its own pressure waves. Rather, the air particles pile up in front of the airplane causing a sharp decrease in the flow velocity directly in front of the airplane with a corresponding increase in air pressure and density.

As the airplane’s speed increases beyond the speed of sound, the pressure and density of the compressed air ahead of it increase, the area of compression extending some distance ahead of the airplane. At some point in the airstream, the air particles are completely undisturbed, having had no advanced warning of the airplane’s approach, and in the next instant the same air particles are forced to undergo sudden and drastic changes in temperature, pressure, density, and velocity. The boundary between the undisturbed air and the region of compressed air is called a shock or “compression” wave. This same type of wave is formed whenever a supersonic airstream is slowed to subsonic without a change in direction, such as when the airstream is accelerated to sonic speed over the cambered portion of a wing, and then decelerated to subsonic speed as the area of maximum camber is passed. A shock wave forms as a boundary between the supersonic and subsonic ranges.

Whenever a shock wave forms perpendicular to the airflow, it is termed a “normal” shock wave, and the flow immediately behind the wave is subsonic. A supersonic airstream passing through a normal shock wave experiences these changes:

• The airstream is slowed to subsonic.
• The airflow immediately behind the shock wave does not change direction.
• The static pressure and density of the airstream behind the wave is greatly increased.
• The energy of the airstream (indicated by total pressure—dynamic plus static) is greatly reduced.

Shock wave formation causes an increase in drag. One of the principal effects of a shock wave is the formation of a dense high pressure region immediately behind the wave. The instability of the high pressure region, and the fact that part of the velocity energy of the airstream is converted to heat as it flows through the wave, is a contributing factor in the drag increase, but the drag resulting from airflow separation is much greater. If the shock wave is strong, the boundary layer may not have sufficient kinetic energy to withstand airflow separation. The drag incurred in the transonic region due to shock wave formation and airflow separation is known as “wave drag.” When speed exceeds the critical Mach number by about 10 percent, wave drag increases sharply. A considerable increase in thrust (power) is required to increase flight speed beyond this point into the supersonic range where, depending on the airfoil shape and the AOA, the boundary layer may reattach.

Normal shock waves form on the wing’s upper surface and form an additional area of supersonic flow and a normal shock wave on the lower surface. As flight speed approaches the speed of sound, the areas of supersonic flow enlarge and the shock waves move nearer the trailing edge.

Associated with “drag rise” are buffet (known as Mach buffet), trim, and stability changes and a decrease in control force effectiveness. The loss of lift due to airflow separation results in a loss of downwash and a change in the position of the center pressure on the wing. Airflow separation produces a turbulent wake behind the wing, which causes the tail surfaces to buffet (vibrate). The nose-up and nose-down pitch control provided by the horizontal tail is dependent on the downwash behind the wing. Thus, an increase in downwash decreases the horizontal tail’s pitch control effectiveness since it effectively increases the AOA that the tail surface is seeing. Movement of the wing center of pressure (CP) affects the wing pitching moment. If the CP moves aft, a diving moment referred to as “Mach tuck” or “tuck under” is produced, and if it moves forward, a nose-up moment is produced. This is the primary reason for the development of the T-tail configuration on many turbine-powered aircraft, which places the horizontal stabilizer as far as practical from the turbulence of the wings.

The post Aerodynamics: Shock Waves first appeared on Learn To Fly.

Rime Ice

Rime ice is rough, milky, and opaque ice formed by the instantaneous freezing of small, supercooled water droplets after they strike the aircraft. It is the most frequently reported icing type. Rime ice can pose a hazard because its jagged texture can disrupt an aircraft’s aerodynamic integrity.

Rime icing formation favors colder temperatures, lower liquid water content, and small droplets. It grows when droplets rapidly freeze upon striking an aircraft. The rapid freezing traps air and forms a porous, brittle, opaque, and milky-colored ice. Rime ice grows into the air stream from the forward edges of wings and other exposed parts of the airframe.

Clear Ice

Clear ice (or glaze ice) is a glossy, clear, or translucent ice formed by therelatively slow freezing of large, supercooled water droplets. Clear icing conditions exist more often in an environment with warmer temperatures, higher liquid water contents, and larger droplets.

Clear ice forms when only a small portion of the drop freezes immediately while the remaining unfrozen portion flows or smears over the aircraft surface and gradually freezes. Few air bubbles are trapped during this gradual process. Thus, clear ice is less opaque and denser than rime ice. It can appear either as a thin smooth surface, or as rivulets, streaks, or bumps on the aircraft.

Clear icing is a more hazardous ice type for many reasons. It tends to form horns near the top and bottom of the airfoils leading edge, which greatly affects airflow. This results in an area of disrupted and turbulent airflow that is considerably larger than that caused by rime ice. Since it is clear and difficult to see, the pilot may not be able to quickly recognize that it is occurring. It can be difficult to remove since it can spread beyond the deicing or anti-icing equipment, although in most cases it is removed nearly completely by deicing devices.

Mixed Ice

Mixed ice is a mixture of clear ice and rime ice. It forms as an airplane collects both rime and clear ice due to small-scale (tens of kilometers or less) variations in liquid water content, temperature, and droplet sizes. Mixed ice appears as layers of relatively clear and opaque ice when examined from the side.

Mixed icing poses a similar hazard to an aircraft as clear ice. It may form horns or other shapes that disrupt airflow and cause handling and performance problems. It can spread over more of the airframe’s surface and is more difficult to remove than rime ice. It can also spread over a portion of airfoil not protected by anti-icing or deicing equipment. Ice forming farther aft causes flow separation and turbulence over a large area of the airfoil, which decreases the ability of the airfoil to keep the aircraft in flight.

Effects of Icing

Remember when I said a few paragraphs earlier that ice sucks? Well I didn’t really explain myself as to why.

When structural icing forms, it reduces aircraft efficiency by increasing weight, reducing lift, decreasing thrust, and increasing drag. Each effect will either slow the aircraft or force it downward.  As ice accumulates the performance characteristics of the aircraft will continually deteriorate eventually to a point where the aircraft can no longer maintain sustained flight and stalls.  The image below is a good depiction of this.

As ice forms on an airfoil, it will destroy the smooth flow of air over the surface of the wing resulting in drag and diminishing the maximum lift capable of the wing. NASA wind tunnel testing has shown that icing on the leading edge or upper surface of a wing no thicker then coarse sandpaper can reduce lift by 30 percent and increase drag by 40 percent.

In addition icing can also cause instrumentation errors, frozen or unbalanced control surfaces, engine failures and/or structural damage due to chunks of ice breaking off.

Additional Knowledge to Know

The post CFI Brief: Icing first appeared on Learn To Fly.

Descending a light propeller-driven general aviation aircraft is a fairly simple task. Reduce power to a point where there is more power required than power available, and the basic principle of weight takes over. Under normal flight conditions, descending flight is initiated by the pilot creating a decrement of power (more power required than available). Once the aircraft begins descending, the weight vector can be broken up into two parts, just like with the climb. One component acts perpendicular to the flightpath (down), the other acts forward and parallel to the flightpath, helping accelerate the aircraft.

Gliding flight can be self-induced by bringing the power back to idle, but in most piston aircraft, descents are not conducted at idle power, thus they are called a powered descent. This is because of shock cooling and the possible damage it could cause to the engine. A true gliding descent would be used if the engine fails. Gliding flight can be broken down into two parts, minimum sink and maximum range.

The minimum sink glide is used to prolong the time aloft in the event the engine or engines fail. This is a speed that is not published, but could be useful if you are over your current landing site and wish to stay aloft a little longer. Most light single-engine airplanes will be at (or close to) minimum sink with full aft trim. This is slower than best glide speed. A pilot who elects to use this method should accelerate to best glide once a normal pattern altitude is reached. This will provide a larger margin above stall and the aircraft will have more positive maneuverability. It should be noted that the best glide speed should be used unless the pilot has training and experience flying at the minimum sink glide speed.

The maximum glide range occurs at the speed for maximum range: L/D_MAX. This is generally a published speed and is used when the engine stops or fails in flight. Some Airplane Flying Manuals (AFMs) contain glide ratio charts. There are some concerns with these charts:

1. They do not account for wind.
2. They are usually calculated in a minimum drag configuration (gear and flaps up).
3. They are usually calculated with controllable-pitch propellers in the full decrease position (high AOA).
4. They are usually calculated at maximum gross weight.

Wind is a factor in glide distance and angle. A headwind will decrease glide distance, and the angle of descent will increase (steepen). A tailwind will increase the glide distance and flatten the angle of descent. You experience the effects of both a headwind and a tailwind when you do a power-off approach. On downwind the aircraft has a flatter descent and a higher groundspeed. Turning base to final, the angle of descent steepens and the groundspeed slows.

Weight is also a factor in glide distance if L/D_MAX is not maintained. Without an AOA indicator, the only way to maintain a specific AOA at L/D_MAX is to vary the airspeed. As weight increases, the airspeed would need to be increased to maintain L/D_MAX.

Altitude also affects the airplane’s gliding distance. To understand this we need to step back and look at the effects of altitude on true airspeed. As the aircraft climbs, TAS increases about 2% per 1,000 feet. An aircraft gliding at higher altitudes will have a higher TAS, this means that it will be moving down the slope at a faster rate. This is of particular importance when operating an aircraft at high density altitudes.

The post Aerodynamics: Descent and Gliding Flight first appeared on Learn To Fly.

When flying, we generally consider range in two ways:

1. Maximizing the distance we fly for a given fuel load.
2. Traveling a specified distance while burning minimum fuel.

Endurance
It’s important to understand that range and endurance are not the same. Range relates to distance, endurance relates to time. The formula for endurance is:

endurance = hours ÷ fuel

Hours is simply flight time expressed in whatever units you want: hours, minutes or seconds. Fuel can be expressed in gallons or pounds. A pilot who wants to achieve maximum endurance would slow the aircraft to the minimum power required speed. Figure 5-24 shows the minimum power point being the lowest point in the drag curve.

If the aircraft were to slow even further, to point A, drag would increase rapidly, more power would be required, and the engine would burn more fuel. If the aircraft were to accelerate above point B, drag also increases, which increases fuel burn. As you can see, flying at maximum endurance speed is not practical in the real world; you may save fuel but it would take forever to get to the destination. This speed is also not practical for operations such as holding because it is generally close to stall. From a practical standpoint, endurance comes from the selection of a cruise power setting of 55%, 65%, or 75% endurance charts. The point of this type of flying is generally to minimize or eliminate fuel stops (very time consuming) along the route, or to minimize fuel burn for cost purposes—not necessarily to stay aloft for hours on end.

Range
Range can be broken down into two parts: specific range and total range. An easy way to understand the difference is to use a car trip scenario. If I have a car that has a 20 gallon fuel tank and gets 30 miles per gallon, I can travel 600 miles on one tank of gas. The specific range in this example is 30 miles per gallon, and the total range is 600 miles. In an airplane, specific range is how many nautical miles you can travel on one gallon or pound of fuel. The total range is how far the airplane can fly with the remaining fuel load on board the aircraft. The definition for specific range is:

specific range = NM ÷ gallons of fuel (Note: pounds can be inserted for gallons.)

Specific range is affected by three things: (1) aircraft weight, (2) altitude, and (3) configuration. The maximum range of the aircraft can be found at L/D_MAX. Unlike endurance, which is found on the drag curve where minimum power is required, maximum range is found where the ratio of speed to power required is the greatest. This is located on the graph by drawing a tangent line from the origin to the power required curve (Figure 5-24). Another way to think about this is that as you move from the origin point along the tangent line toward L/D_MAX you increase airspeed at a greater rate than fuel burn (think of the ratio). At L/D_MAX, the ratio of fuel to airspeed should be 1. At any speed above L/D_MAX, the fuel burn ratio increases at a greater rate than the airspeed. Therefore, L/D_MAX is the point where the speed-to-power ratio required is the greatest.

Another aspect of range that we need to examine is the effect of weight on range. Because L/D_MAX occurs at a specific angle of attack, and most general aviation airplanes do not have AOA indicators, the airspeed has to be varied as weight changes to maintain a constant AOA. Figure 5-25 illustrates this: as weight increases, the speed must be increased to maintain the AOA. This is because as weight is increased, the AOA must be increased to produce more lift; the only way to lower the AOA is to increase speed. As weight decreases, the speed must decrease. The reasoning is that as the aircraft becomes lighter, the AOA is lowered to compensate for less weight; the only way to increase AOA is to reduce speed (Figure 5-25).

The effect of altitude on range can be seen in Figure 5-26. Flights operating at high altitude require a higher TAS, which will require more power.

Another aspect of cruise flight relating to range and endurance, one that is often not talked about in textbooks, is cruise performance. From a practical standpoint, the pilot will not fly the aircraft at maximum endurance or range—it is just too slow. In reality, pilots often operate propeller-driven airplanes at 55%, 65%, or 75% best power or endurance.

In order to calculate how to get to your destination as fast as possible, find the highest true airspeed for your aircraft. Most fixed-gear single-engine aircraft that cruise in the 110–130 knot range will have their highest TAS in the 6,000 to 7,000-foot range. This is a good place to start; however, the wind, terrain, and the need for fuel stops will dictate the altitude and speed at which the aircraft ultimately flies.

The post Aerodynamics: Cruise Flight first appeared on Learn To Fly.

I’m sure you have heard of the term wake before, especially if you are a boater. A boats wake is very similar in nature to that of an aircraft. You can see from the image below as the boat motors along it displaces the water leaving behind a wake in the form of waves which spread in an outward direction.

An aircraft’s wake is similar but differs in some characteristics and in the fact that typically wake turbulence created by an aircraft is not visible.

All aircraft leave two types of wake turbulence: Prop or jet blast, and wing-tip vortices.

Prop or jet blast is the thrust stream created by the engine. You will encounter this type of wake on the ground and is hazardous to light aircraft behind large aircraft which are either taxiing or running-up their engines. In the air, jet or prop blast dissipates rapidly.

Wing-tip vortices are a by-product of lift. As a wing produces lift, the higher static pressure area beneath the wing causes airflow around the wingtip to the lower pressure area above. To simplify the high pressure below the wing which creates lift wants to equalize with the lower pressure above the wing. The shortest point for the high pressure to move to the lower pressure area above the wing is at the wing tip. This high pressure moves outward, upward and around each wing-tip. However, because the wing and aircraft itself are moving, by the time the high pressure circulates around the tip to the top, the wing is now gone. This in turn creates vortices that trail behind each wing tip as seen in this image.

The strength of a vortex is governed by the weight, speed, and the shape of the wing of the generating aircraft. Maximum vortex strength occurs when the generating aircraft is heavy, clean, and slow.  A heavy, clean, and slow aircraft will require a greater angle of attack (AoA) to great sufficient lift, as the AoA increases so does the pressure differential. The greater the pressure differential the stronger the vortice.

Vortices generated by large aircraft in flight tend to sink below the flight path of the generating aircraft at a rate of about 500 feet per minute. A pilot should fly at or above the larger aircraft’s flight path in order to avoid the wake turbulence created by the wing-tip vortices. Over time the vortices also tend to move apart and will drift downwind of the aircraft flight path. A common rule of thumb is to fly above and upwind of the path of other aircraft.

Close to the ground, vortices tend to move laterally. A crosswind will tend to hold the upwind vortex over the landing runway, while a tailwind may move the vortices of a preceding aircraft forward into the touchdown zone. Research has also shown that as vorticies come in contact striking the gorund that have a tendency to “bounce” back up as much as 250 feet.

To avoid wake turbulence when landing, a pilot should note the point where a preceding large aircraft touched down and then land past that point.

On takeoff, lift off should be accomplished prior to reaching the rotation point of a preceding departing large aircraft; the flight path should then remain upwind and above the preceding aircraft’s flight path. If departing behind a landing large aircraft delay your takeoff point to a spot past where the landing aircraft touched down.

1. When landing behind a large aircraft, the pilot should avoid wake turbulence by staying
A—above the large aircraft’s final approach path and landing beyond the large aircraft’s touchdown point.
B—below the large aircraft’s final approach path and landing before the large aircraft’s touchdown point.
C—above the large aircraft’s final approach path and landing before the large aircraft’s touchdown point.

2. When departing behind a heavy aircraft, the pilot should avoid wake turbulence by maneuvering the aircraft
A—below and downwind from the heavy aircraft.
B—above and upwind from the heavy aircraft.
C—below and upwind from the heavy aircraft.

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1. When landing behind a large aircraft stay at or above the large aircraft’s final approach path. Note its touchdown point and land beyond it.
Answer (B) is incorrect because below the flight path, you will fly into the sinking vortices generated by the large aircraft. Answer (C) is incorrect because by landing before the large aircraft’s touchdown point, you will have to fly below the preceding aircraft’s flight path, and into the vortices.

2. When departing behind a large aircraft, note the large aircraft’s rotation point, rotate prior to it, continue to climb above it, and request permission to deviate upwind of the large aircraft’s climb path until turning clear of the aircraft’s wake.

The post CFI Brief: Caution for the wake turbulence from the departing 757 first appeared on Learn To Fly.

Aircraft aerodynamics involves the interaction of the four forces: lift, weight, thrust, and drag. The first basic issue to understand is the difference between propeller-driven aircraft power and jet engine thrust. Power is what a propeller-driven engine produces; thrust is what a jet engine produces. In a propeller-driven aircraft, the propeller—not the engine—is said to produce thrust. The thrust on a propeller-driven aircraft decreases with an increase in velocity; in a jet aircraft, thrust remains relatively constant with an increase in aircraft velocity.

Therefore, the power required curve versus the power available curve for a propeller driven aircraft and a jet aircraft will look different.

Propeller Efficiency
Propeller efficiency is a measure of how much power is absorbed (transmitted) by the propeller and turned into thrust. In order to understand propeller efficiency, it’s helpful to start with a basic review of propeller principles. Propellers on aircraft consist of two or more blades and a hub. The blades are attached to the hub, and the hub is attached to the crankshaft on a piston-powered aircraft and to a gear reduction box on most turbo-prop aircraft. The propeller is simply a rotating wing that produces lift along the vertical axis. We call this lift force thrust.

Looking at a cross section of the propeller blade, we can see that it is similar to a cross section of an aircraft wing. The top portion of the blade is cambered like the top surface of a wing. The bottom portion is flat like the bottom surface of a wing.

The chord line is an imaginary line drawn from the leading edge of the propeller blade to the trailing edge of the propeller blade. Blade angle, measured in degrees, is the angle between the chord of the blade and the plane of rotation. The pitch of the propeller is usually designated in inches. A “78-52” propeller is 78 inches in length with an effective pitch of 52 inches. The effective pitch is the distance a propeller would move through the air in one revolution if there were no slippage. On a “78-52” propeller this distance would be 52 inches.

There are two types of propellers that can be installed on most general aviation aircraft: a fixed-pitch propeller or a controllable-pitch propeller. The fixed-pitch propeller is at the blade angle that will give it the best overall efficiency for the type of operation being conducted and for which the aircraft was designed. For most aircraft this would be a cruise setting. A controllable-pitch propeller allows the pilot to adjust the blade angle for the different phases of flight. On takeoff and climb out, a low pitch/high RPM setting is used. During cruise flight a high pitch/low RPM setting is generally used.

On the ground with the aircraft in a static condition, the propeller efficiency is very low because each blade is moving through the air at an angle of attack that produces a very low thrust to power ratio. This means that a lot of power is being used to sustain the engine and rotate the propeller while very little thrust is being produced. The propeller, unlike the wing, moves both rotationally and forward (dynamically). The angle at which the relative wind strikes the propeller blade is the AOA. This produces a higher dynamic pressure on the engine side, which in turn is called thrust. Thus thrust is the relationship of propeller AOA and blade angle.

Since an aircraft moves forward through the air, it is important that the pilot understands how forward velocity affects the AOA of the propeller. The figure above shows the propeller in a static condition on the ground. At this point the relative wind is opposing propeller rotation. As forward velocity increases, the relative wind moves closer to the chord line, decreasing the propeller AOA. This can easily be demonstrated in an aircraft with a fixed-pitch propeller by pitching up or down without changing power. When the aircraft is pitched down, RPM will increase as the relative wind moves closer to the chord line and the AOA is decreased. When the aircraft is pitched up, the RPM will decrease as the relative wind moves farther from the chord line and the AOA is increased.

Propeller efficiency is a ratio between thrust horsepower and brake horsepower. Brake horsepower (BHP) is the horsepower actually delivered to the output shaft. Brake horsepower is the actual usable horsepower. Thrust horsepower (THP) is the power that is imparted by the propeller to the air. Propeller efficiency is the relationship between brake horsepower and thrust horsepower. If the BHP of the engine is 200, the THP is less (20–40%). Some power is lost to turn the engine and propeller. Propeller efficiency usually varies between 50 and 80% on light general aviation aircraft.

The measure of efficiency is how much a propeller slips in the air. This is measured by the geometric pitch (theoretical) which shows a propeller with no slippage. Effective pitch is the distance that the propeller actually travels.

The post Aerodynamics: Power first appeared on Learn To Fly.

One of the biggest differences to point out revolves around the way the flight controls are moved. Because of the high air loads, it is very difficult to move the flight control surfaces of jet aircraft with just mechanical and aerodynamic forces. So flight controls are usually moved by hydraulic actuators. Flight controls are divided into primary flight controls and secondary (or auxiliary) flight controls. The primary flight controls are those that maneuver the aircraft in roll, pitch, and yaw. These include the ailerons, elevator, and rudder. Secondary (or auxiliary) flight controls include tabs, trailing-edge flaps, leading-edge flaps, spoilers, and slats.

Roll control of most jet aircraft is accomplished by ailerons and flight spoilers. The exact mix of controls is determined by the aircraft’s flight regime. In low speed flight, all control surfaces operate to provide the desired roll control. As the aircraft moves into higher speed operations, control surface movement is reduced to provide approximately the same roll response to a given input through a wide range of speeds.

Many aircraft have two sets of ailerons—inboard and outboard. The inboard ailerons operate in all flight regimes. The outboard ailerons work only when the wing flaps are extended and are automatically locked out when flaps are retracted. This allows good roll response in low speed flight with the flaps extended and prevents excessive roll and wing bending at high speeds when the flaps are retracted.

Spoilers increase drag and reduce lift on the wing. If raised on only one wing, they aid roll control by causing that wing to drop. If the spoilers rise symmetrically in flight, the aircraft can either be slowed in level flight or can descend rapidly without an increase in airspeed. When the spoilers rise on the ground at high speeds, they destroy the wing’s lift which puts more of the aircraft’s weight on its wheels which in turn makes the brakes more effective.

Often aircraft have both flight and ground spoilers. The flight spoilers are available both in flight and on the ground. However, the ground spoilers can only be raised when the weight of the aircraft is on the landing gear. When the spoilers deploy on the ground, they decrease lift and make the brakes more effective. In flight, a ground-sensing switch on the landing gear prevents deployment of the ground spoilers.

Vortex generators are small (an inch or so high) aerodynamic surfaces located in different places on different airplanes. They prevent undesirable airflow separation from the surface by mixing the boundary airflow with the high energy airflow just above the surface. When located on the upper surface of a wing, the vortex generators prevent shock-induced separation from the wing as the aircraft approaches its critical Mach number. This increases aileron effectiveness at high speeds.

As you progress through the ranks of aviation and begin flying larger aircraft you will start noticing some of these secondary flight controls installed on your aircraft. But many of the training aircraft like the Cessna 172’s, Piper Archers, or piston powered aircraft you might be flying today won’t have secondary controls such as spoilers installed. The majority of the time these aircraft are just not large enough, heavy enough or fast enough that spoilers would be an effective or beneficial flight control. It is however beneficial to gain experience in the knowledge of these flight control systems as it will help you later on in training when you merge your private and professional aerodynamics lessons into practice.

For more advanced information on aerodynamics check out our collegiate level textbook, Aerodynamics for Aviators.

The post CFI Brief: Flight Controls of a typical Commercial Airliner first appeared on Learn To Fly.

In subsonic aerodynamics, the theory of lift is based upon the forces generated on a body and a moving gas (air) in which it is immersed. At speeds of approximately 260 knots or less, air can be considered incompressible in that, at a fixed altitude, its density remains nearly constant while its pressure varies. Under this assumption, air acts the same as water and is classified as a fluid. Subsonic aerodynamic theory also assumes the effects of viscosity (the property of a fluid that tends to prevent motion of one part of the fluid with respect to another) are negligible and classifies air as an ideal fluid conforming to the principles of ideal-fluid aerodynamics such as continuity, Bernoulli’s principle, and circulation.

In reality, air is compressible and viscous. While the effects of these properties are negligible at low speeds, compressibility effects in particular become increasingly important as speed increases. Compressibility (and to a lesser extent viscosity) is of paramount importance at speeds approaching the speed of sound. In these speed ranges, compressibility causes a change in the density of the air around an aircraft.

During flight, a wing produces lift by accelerating the airflow over the upper surface. This accelerated air can, and does, reach sonic speeds even though the aircraft itself may be flying subsonic. At some extreme angles of attack (AOA), in some aircraft, the speed of the air over the top surface of the wing may be double the aircraft’s speed. It is therefore entirely possible to have both supersonic and subsonic airflow on an aircraft at the same time. When flow velocities reach sonic speeds at some location on an aircraft (such as the area of maximum camber on the wing), further acceleration results in the onset of compressibility effects, such as shock wave formation, drag increase, buffeting, stability, and control difficulties. Subsonic flow principles are invalid at all speeds above this point.

The speed of sound varies with temperature. Under standard temperature conditions of 15 °C, the speed of sound at sea level is 661 knots. At 40,000 feet, where the temperature is –55 °C, the speed of sound decreases to 574 knots. In high speed flight and/or high-altitude flight, the measurement of speed is expressed in terms of a “Mach number”—the ratio of the true airspeed of the aircraft to the speed of sound in the same atmospheric conditions. An aircraft traveling at the speed of sound is traveling at Mach 1.0. Aircraft speed regimes are defined approximately as follows:

• Subsonic—Mach numbers below 0.75
• Transonic—Mach numbers from 0.75 to 1.20
• Supersonic—Mach numbers from 1.20 to 5.00
• Hypersonic—Mach numbers above 5.00

While flights in the transonic and supersonic ranges are common occurrences for military aircraft, civilian jet aircraft normally operate in a cruise speed range of Mach 0.7 to Mach 0.90.

The speed of an aircraft in which airflow over any part of the aircraft or structure under consideration first reaches (but does not exceed) Mach 1.0 is termed “critical Mach number” or “Mach Crit.” Thus, critical Mach number is the boundary between subsonic and transonic flight and is largely dependent on the wing and airfoil design. Critical Mach number is an important point in transonic flight. When shock waves form on the aircraft, airflow separation followed by buffet and aircraft control difficulties can occur. Shock waves, buffet, and airflow separation take place above critical Mach number. A jet aircraft typically is most efficient when cruising at or near its critical Mach number. At speeds 5–10 percent above the critical Mach number, compressibility effects begin. Drag begins to rise sharply. Associated with the “drag rise” are buffet, trim, and stability changes and a decrease in control surface effectiveness. This is the point of “drag divergence.”

V_MO/M_MO is defined as the maximum operating limit speed. V_MO is expressed in knots calibrated airspeed (KCAS), while M_MO is expressed in Mach number. The V_MO limit is usually associated with operations at lower altitudes and deals with structural loads and flutter. The M_MO limit is associated with operations at higher altitudes and is usually more concerned with compressibility effects and flutter. At lower altitudes, structural loads and flutter are of concern; at higher altitudes, compressibility effects and flutter are of concern.

Adherence to these speeds prevents structural problems due to dynamic pressure or flutter, degradation in aircraft control response due to compressibility effects (e.g., Mach Tuck, aileron reversal, or buzz), and separated airflow due to shock waves resulting in loss of lift or vibration and buffet. Any of these phenomena could prevent the pilot from being able to adequately control the aircraft.

For example, an early civilian jet aircraft had a V_MO limit of 306 KCAS up to approximately FL 310 (on a standard day). At this altitude (FL 310), an M_MO of 0.82 was approximately equal to 306 KCAS. Above this altitude, an M_MO of 0.82 always equaled a KCAS less than 306 KCAS and, thus, became the operating limit as you could not reach the V_MO limit without first reaching the M_MO limit. For example, at FL 380, an M_MO of 0.82 is equal to 261 KCAS.

The post Aerodynamics: High Speed Flight first appeared on Learn To Fly.

A spin is a condition of stalled flight in which the airplane follows a spiral descent path. As well as the airplane being in a stalled condition, and yawing, one wing is producing more lift than the other, which results in a roll. The dropping wing is more deeply stalled than the other, and the greater drag from this wing results in further yaw, further roll, and autorotation develops. Upward pitching of the nose will also occur. You can induce a spin on purpose by yawing an airplane that is stalled, or just on the point of stalling.

In a spin, the airplane is in motion about all three axes. In other words, lots of things are happening in a spin! The airplane is:

• stalled;
• rolling;
• yawing;
• pitching;
• slipping; and
• rapidly losing altitude at a low airspeed (close to the stall speed).

In a spin the wings will not produce much lift, since they are stalled. The airplane will accelerate downward until it reaches a vertical rate of descent where the greatly increased drag, now acting upward, counteracts the weight. The altitude loss will be rapid as the airplane spins downward around the vertical spin axis but, because of the high angle of attack and the stalled condition, the airspeed in the spin will be quite low and fluctuating.

Characteristics of a developed spin include a low airspeed (which does not increase until recovery action is initiated), and a high rate of descent.

Spin Recovery
To recover from a spin, you must ensure power is off, oppose the yaw, and unstall the wings. First note yaw direction and apply full opposite rudder, and then move the control column forward to unstall the wings by decreasing the angle of attack. Once the airplane has stopped spinning, ease the airplane out of the dive and resume normal flight.

Misuse of Ailerons
Trying to raise a dropped wing with opposite aileron may have the reverse effect when the airplane is near the stall. If, as the aileron goes down, the stall angle of attack is exceeded, the wing may drop quickly instead of rising, resulting in a spin. The application of aileron after a spin has developed may aggravate the spin. Discuss the spin characteristics of your particular airplane with your flight instructor.

The Spiral Dive
A maneuver that must not be confused with a spin is the spiral dive, which can be thought of as a steep turn that has gone wrong. In a spiral dive the nose attitude is low, and the rate of descent is high, but neither wing is stalled and the airspeed is high and rapidly increasing. A spiral dive is really just a steep descending turn. However, because the pilot may be disoriented it is often mistaken for a spin. The high and increasing airspeed indicates that the airplane is in a spiral dive rather than a spin (when the airspeed would fluctuate at a low value).

Recovery from a spiral dive is simple. Roll wings level and pull gently out of the dive. Beware of overstressing the airplane by pulling too quickly out of the dive—remember the controls will be very effective because of the high airspeed.

The post Aerodynamics: The Spin first appeared on Learn To Fly.