The SDP is a theoretical level at which the pressure of the atmosphere is 29.92 “Hg and the weight of air is 14.7 psi. As atmospheric pressure changes, the SDP may be below, at, or above sea level. Pressure altitude is important as a basis for determining aircraft performance, as well as for assigning flight levels to aircraft operating at above 18,000 feet.

The pressure altitude can be determined by any of the three following methods:

1. By setting the barometric scale of the altimeter to 29.92 “Hg and reading the indicated altitude,
2. By applying a correction factor to the indicated altitude according to the reported “altimeter setting,” see figure below.
3. By using a CX-3 Flight Computer.

Let’s try a sample problem using the above chart.

1. Determine the pressure altitude at an airport that is 1,386 feet MSL with an altimeter setting of 29.55.
A—1,631 feet MSL.
B—1,731 feet MSL.
C—1,778 feet MSL.

Looking at the above chart you will see that 29.65 is not actually shown so we will need to interpolate between 29.50 and 29.60. Subtract 298 from 392 to get 94 and then divide by 2 since 29.55 is directly in the middle. You get 47 which you can now add to 298 to come up with 345 altitude correction. Add 345 onto you airport elevation to find the pressure altitude, 1,386 + 345 = 1,731 PAlt.

The correct answer is B, 1,731 feet MSL.

Here is another problem which can be solved by using your CX-3 Flight Computer.

2. Determine the pressure altitude at an airport that is 3,563 feet MSL with an altimeter setting of 29.96.
A—3,527 feet MSL.
B—3,556 feet MSL.
C—3,639 feet MSL.

Using you CX-3 open the FLT menu and select Altitude from the list. Enter your IAlt (indicated altitude) or airport elevation of 3,563 feet. Scroll down to the Baro field and enter your altimeter setting of 29.96. The CX-3 will then give you a PAlt of 3,527 feet MSL. The correct answer is A.

The post CFI Brief: Pressure Altitude Conversions first appeared on Learn To Fly.

Runway conditions affect takeoff and landing performance. Typically, performance chart information assumes paved, level, smooth, and dry runway surfaces. Since no two runways are alike, the runway surface differs from one runway to another, as does the runway gradient or slope.

Runway surfaces vary widely from one airport to another. The runway surface encountered may be concrete, asphalt, gravel, dirt, or grass. The runway surface for a specific airport is noted in the Chart Supplement U.S. (formerly Airport/Facility Directory). Any surface that is not hard and smooth increases the ground roll during takeoff. This is due to the inability of the tires to roll smoothly along the runway. Tires can sink into soft, grassy, or muddy runways. Potholes or other ruts in the pavement can be the cause of poor tire movement along the runway. Obstructions such as mud, snow, or standing water reduce the airplane’s acceleration down the runway. Although muddy and wet surface conditions can reduce friction between the runway and the tires, they can also act as obstructions and reduce the landing distance. Braking effectiveness is another consideration when dealing with various runway types. The condition of the surface affects the braking ability of the aircraft.

The amount of power that is applied to the brakes without skidding the tires is referred to as braking effectiveness. Ensure that runways are adequate in length for takeoff acceleration and landing deceleration when less than ideal surface conditions are being reported.

The gradient or slope of the runway is the amount of change in runway height over the length of the runway. The gradient is expressed as a percentage, such as a 3 percent gradient. This means that for every 100 feet of runway length, the runway height changes by 3 feet. A positive gradient indicates the runway height increases, and a negative gradient indicates the runway decreases in height. An upsloping runway impedes acceleration and results in a longer ground run during takeoff. However, landing on an upsloping runway typically reduces the landing roll. A downsloping runway aids in acceleration on takeoff resulting in shorter takeoff distances. The opposite is true when landing, as landing on a downsloping runway increases landing distances. Runway slope information is contained in the Chart Supplement U.S. (formerly Airport/ Facility Directory).

Water on the Runway and Dynamic Hydroplaning
Water on the runways reduces the friction between the tires and the ground and can reduce braking effectiveness. The ability to brake can be completely lost when the tires are hydroplaning because a layer of water separates the tires from the runway surface. This is also true of braking effectiveness when runways are covered in ice.

When the runway is wet, the pilot may be confronted with dynamic hydroplaning. Dynamic hydroplaning is a condition in which the aircraft tires ride on a thin sheet of water rather than on the runway’s surface. Because hydroplaning wheels are not touching the runway, braking and directional control are almost nil. To help minimize dynamic hydroplaning, some runways are grooved to help drain off water; most runways are not.

Tire pressure is a factor in dynamic hydroplaning. Using the simple formula in the figure below, a pilot can calculate the minimum speed, in knots, at which hydroplaning begins. In plain language, the minimum hydroplaning speed is determined by multiplying the square root of the main gear tire pressure in psi by nine. For example, if the main gear tire pressure is at 36 psi, the aircraft would begin hydroplaning at 54 knots.

Landing at higher than recommended touchdown speeds exposes the aircraft to a greater potential for hydroplaning. And once hydroplaning starts, it can continue well below the minimum initial hydroplaning speed.

On wet runways, directional control can be maximized by landing into the wind. Abrupt control inputs should be avoided. When the runway is wet, anticipate braking problems well before landing and be prepared for hydroplaning. Opt for a suitable runway most aligned with the wind. Mechanical braking may be ineffective, so aerodynamic braking should be used to its fullest advantage.

The post Aircraft Performance: Runway Surface and Gradient 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.

This requires a different pitch attitude and a different power setting. To slow the airplane, the pilot reduces power and gradually raises the pitch attitude to maintain altitude; to increase airspeed, the pilot increases power, and gradually lowers the pitch attitude to maintain altitude.

Once the desired airspeed is achieved, the pilot adjusts the power to maintain it. The precise power required for steady flight will depend upon the amount of total drag, which, in level flight, varies with angle of attack and airspeed. Higher power will be required for:

• high speed cruise (when total drag is high mainly due to parasite drag); and
• low speed cruise (when total drag is high mainly due to induced drag).

Medium power is required for normal cruise. The ASI confirms whether or not correct power is set. The ASI is the primary performance guide to power requirements during level flight if you fly a particular airspeed.

Practicing airspeed changes in cruise is excellent instrument flying practice since pitch, bank (and balance) and power changes must all be coordinated to maintain constant altitude and heading. When the pilot changes power, a single-engined propeller- driven airplane will tend to move around all three axes of movement. If the propeller rotates clockwise as seen from the cockpit (the usual case), adding power will cause the nose to pitch up and yaw left, with a tendency for the airplane to roll left.

The pilot can counteract this by applying forward elevator pressure to prevent the nose pitching up, with right rudder and right aileron pressure to overcome the tendency to yaw and roll left. The converse applies when reducing power, hold the nose up and apply left rudder pressure. Refer to the AI to keep the wings level and hold the pitch attitude, and keep the ball centered.

Some hints on changing cruising speed follow:

• The attitude indicator gives a direct picture of pitch and bank attitudes.
• The ball gives a direct indication of coordination.
• Useful performance instruments are the altimeter and VSI—they ensure altitude is being maintained, and the heading indicator to ensure heading is being maintained.
• The airspeed indicator indicates the power requirements. If too slow, add more power; if too fast, reduce power.

The pilot’s scan rate of the flight instruments during any power change needs to be reasonably fast to counteract the pitch/yaw effects smoothly and accurately. For this reason, it is good to develop the skill of judging power changes by throttle movement and engine sound, rather than only by observation of the power indicator. This allows the pilot to concentrate on the flight instruments until after the power change has been made, at which time a quick glance at the power indicator for fine adjustment suffices.

When you memorize the approximate power settings necessary to maintain the various cruise speeds, then power handling and airspeed changes become simpler to manage.

Small airspeed changes (say five knots either way) can generally be handled by a single small power change, then allowing the airplane to gradually slow down or accelerate to the desired speed. Large airspeed changes, however, are most efficiently achieved within a few seconds by underpowering on the initial power change for a speed decrease, or overpowering on the initial power change for a speed increase. This allows more rapid deceleration or acceleration to the desired speed, at which time the necessary power to maintain that airspeed is set.

Once the desired airspeed is achieved and suitable power is set, the ASI will indicate if further fine adjustment of power to maintain airspeed is required. In level flight, the ASI is the primary guide to power requirements.

The post Aircraft Performance: Changing Airspeed in Straight-and-Level Flight first appeared on Learn To Fly.

Preparing the Airplane
The information manual for your airplane will contain a list of items that must be checked during:

• the preflight inspection (external and internal);
• the preflight cockpit checks;
• the engine power check; and
• the before-takeoff check.

At first, these checks may seem long and complicated, but as you repeat them thoroughly prior to each flight, a pattern will soon form. It is vital that the checks are carried out thoroughly, systematically and strictly in accordance with the manufacturer’s recommended procedure. Use of written checklists, if performed correctly, will ensure that no vital item has been missed, but some pilots prefer to memorize checks. The comments that follow are only general comments that will apply to most airplanes.

The External Inspection
Always perform a thorough external inspection. This can begin as you walk up to the airplane and should include:

• the position of the airplane being safe for start-up and taxi (note also the wind direction and the likely path to the takeoff point); and
• the availability of fire extinguishers and emergency equipment in case of fire on start-up (a rare event, but it does happen).

Some of the vital items are:

• all switches off (master switch for electronics, magneto switch for engine) as a protection against the engine inadvertently starting when the propeller is moved;
• fuel check for quantity and quality (drain into a clear cup);
• oil check; and
• structural check.

A list of typical walkaround items is shown below. Each item must be inspected individually, but do not neglect a general overview of the airplane. Be vigilant for things such as buckling of the fuselage skin or popped rivets since these could indicate internal structural damage from a previous flight. Leaking oil, fuel forming puddles on the ground, or hydraulic fluid leaks from around the brake lines also deserve further investigation. With experience, you will develop a feel for what looks right and what does not. The walkaround inspection starts at the cockpit door and follows the pattern specified in the checklist provided by the aircraft manufacturer.

Cabin

• Parking brake on.
• Magneto switches off.
• Landing gear lever (if retractable) locked down.
• Control locks removed.
• Master switch on (to supply electrical power).
• Fuel quantity gauges checked for sufficient fuel for the planned flight.
• Fuel selector valves on.
• Flaps checked for operation; leave them extended for external inspection.
• Stall warning (if electrical) checked for proper operation.
• Rotating beacon (and other lights) checked, then off.
• Master switch off.
• Primary flight controls checked for proper operation.
• Required documents on board: MAROW plus airman certificate and medical certificate for the pilot. (Note: under some circumstances a medical certificate may not be required.)
• Cabin door securely attached, and latches working correctly.
• Windshield clean (use correct cloth and cleanser).

Wing

• All surfaces, the wing tip, leading and trailing edge checked for no damage or contamination; remove any frost, snow, ice or insects (on upper leading edge especially, since contamination here can significantly reduce lift, even to the point where the airplane may not become airborne).
• Wing tip position light checked for no damage.
• Flaps firmly in position and actuating mechanism firmly connected and safety-wired.
• Aileron locks removed, hinges checked, correct movement (one up, the other down) and linkages safety-wired, mass balance weight secure.
• Pitot tube cover removed and no damage or obstructions to tube (otherwise airspeed indicator will not respond).
• Fuel contents checked in tanks and matching fuel quantity gauge indications; fuel caps replaced firmly and with a good seal (to avoid fuel siphoning away in flight into the low-pressure area above the wing).
• Fuel sample drained from wing tanks and from fuel strainer into a clear container. Check for correct color (blue for 100LL, green for 100-octane), correct fuel grade, correct smell (aviation gasoline and not jet fuel or kerosene), no water (being denser, water sinks to bottom), sediment, dirt or other contaminant (condensation may occur in the tanks overnight causing water to collect in the bottom of the tanks, or the fuel taken on board may be contaminated).
• Fuel port, or fuel vent (which may be separate or incorporated into the fuel cap) clear (to allow pressure equalization inside and outside the tanks when fuel is used or altitude is changed, otherwise the fuel tanks could collapse or fuel supply to the engine could stop as fuel is used).
• Stall warning checked (if possible).
• Inspection plates in place.
• Wing strut checked secure at both ends.

Fuselage

• All surfaces, including underneath checked for skin damage, corrosion, buckling or other damage (corrosion appears as surface pitting and etching, often with a gray powdery deposit); advise a mechanic if you suspect any of these.
• No fuel, oil or hydraulic fluid leaking onto the ground beneath the aircraft.
• Inspection plates in place.
• Static ports (also called static vents)—no obstructions (needed for correct operation of airspeed indicator, altimeter and vertical speed indicator).
• Antennas checked for security and no loose wires.
• Baggage lockers—check baggage, cargo and equipment secure, and baggage compartments locked.

Main Landing Gear

• Tires checked for wear, cuts, condition of tread, proper inflation, and security of wheel and brake disk.
• Wheel oleo strut checked for damage, proper inflation, and cleanliness.
• Hydraulic lines to brakes checked for damage, leaks and attachment.
• Gear attachment to the fuselage—check attachment, and be sure there is no damage to the fuselage (buckling of skin, popped rivets).

Nose Section

• Fuselage checked for skin buckling or popped rivets.
• Windshield clean.
• Propeller checked for damage, especially nicks along its leading edge, cracks and security (and for leaks in the hub area if it is a constantspeed propeller).
• Propeller spinner checked for damage, cracks and security.
• Engine air intake and filter checked for damage and cleanliness (no bird nests or oily rags).
• Nose wheel tire checked for wear, cuts, condition of tread, proper inflation, and security of nose wheel.
• Nose wheel oleo strut checked for damage, proper inflation (four to six inches is typical), security of shimmy damper and other mechanisms.
• Open engine inspection panel; check engine mounts, engine, and exhaust manifold for cracks and security (to ensure that no lethal carbon monoxide in the exhaust gases can enter the cockpit—exhaust leaks may be indicated by white stains near the cylinder head, the exhaust shroud or exhaust pipes).
• Check battery, wiring and electrical cables for security (firmly attached at both ends).
• Check the oil level; top up if necessary (know the correct type and grade of oil to order); ensure that the dipstick is replaced properly and the oil cap is firmly closed to avoid loss of oil in flight.
• Close the inspection panel and check its security.

Other Side of Airplane
Repeat as appropriate.

Empennage

• Remove control locks if fitted.
• All surfaces checked for skin damage (vertical stabilizer and rudder, horizontal stabilizer, elevator and trim tab); remove any contamination such as ice, frost or snow.
• Control surface hinges checked for cracks, firmness of attachment, safety-wiring and correct movement.

Chocks and Tiedown Ropes
Chocks and tiedowns removed and stowed (after checking the parking brake is on).

Overall View
Stand back and check the overall appearance of the airplane. It cannot be emphasized too greatly just how important this preflight inspection by the pilot is. Even if you have no experience in mechanical things, you must train yourself to look at the airplane and notice things that do not seem right. Bring any items that you are unsure of to the attention of your flight instructor or a mechanic. At this stage, you are now ready to seat yourself in the airplane and begin the internal cockpit inspection.

The Cockpit Inspection
Always perform a thorough cockpit inspection. The cockpit inspection involves preparing the cockpit and your personal equipment for flight. It should include:

• Parking brake set (on).
• Required documents on board (MAROW items).
• Flight equipment organized and arranged in an efficient manner so they are readily available in flight (flight bag, charts prefolded to show your route, computer, pencils, flashlight, and so on).
• Fuel on.
• Seat position and harness comfortable and secure, with the seat definitely locked in position and rudder pedals (if adjustable) adjusted and locked into position so that full movement is possible.
• Ignition switch (magnetos) off (so that the engine is not live).
• Master switch on (for electrical services such as fuel gauges).
• Flight controls checked for full and free movement (elevator, ailerons, rudder and trim wheel or handle). Trim set to takeoff position.
• Engine controls checked for full and free movement (throttle, mixture control and carburetor heat).
• Scan the instruments systematically from one side of the panel to the other for serviceability and correct readings.
• No circuit breakers should be popped nor fuses blown (for electrical services to operate).
• Microphone and/or headsets plugged in (if you are to use the radio) and test intercom if used.
• Safety equipment (fire extinguisher, first aid kit, supplemental oxygen if planning to fly high, flotation equipment for overwater flights) on board and securely stowed.
• Loose articles stowed.
• Checklists on board and available.
• Read the preflight checklist, if appropriate.

Checklists
Normal checklists are found in Section 4 of the typical pilot’s operating handbook, and emergency checklists are found in Section 3. Written checklists are used to confirm that appropriate procedures have been carried out, for example, the before-takeoff checklist or the engine fire checklist. In earlier days, when airplanes were simpler, checks were usually memorized. Nowadays, in more complex airplanes and in a much busier operating environment, many checks are performed with the use of standard written checklists for that airplane. Checklists are usually compiled in a concise and abbreviated form as item and condition (for example, fuel—on), where the item to be checked is listed, followed by a statement of its desired condition. Explanations for actions are usually not included in the concise checklist, but may generally be found in the pilot’s operating handbook if required.

Vital checklists are best committed to memory so that they may be done quickly and efficiently, followed by confirmation using the printed checklist if required. Emergency checklists, such as the engine fire checklist, often have some items that should be memorized, since they may have to be actioned immediately, before there is time to locate the appropriate checklist and read it. These items are often referred to as memory items or phase-one items, and are often distinguished on checklists by bold type or by being surrounded with a box. The method of using checklists may be one of:

• carrying out the items as the checklist is read; or
• carrying out the items in full, followed by confirmation using the checklist.

Be sure to check back Thursday for more on preflight from our CFI as well as something interesting from SunState Aviation!

The post Ground School: Preflight Inspection 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.

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.

Frost is described as ice deposits formed on a surface when the temperature of the collecting surface is at or below the dew point of the adjacent air, and the dew point is below freezing. Frost causes early airflow separation on an airfoil resulting in a loss of lift. Therefore, all frost should be removed from the lifting surfaces of an airplane before flight or it could prevent the airplane from becoming airborne.

Test data indicates that ice, snow, and frost formations having a thickness and surface roughness similar to medium or coarse sandpaper on the leading edge and upper surface of a wing can reduce wing lift by as much as 30%, and increase drag by 40%.

Prior to flight, it is extremely important that the aircraft is completely cleaned and clear of any frost or ice. This is often accomplished through deicing.

Deicing is a procedure in which frost, ice, or snow is removed from the aircraft in order to provide clean surfaces. You may often see ground deicing accomplished by applying a fluid solution containing glycol and various other chemical agents to the aircraft. The image below depicts a typical commercial airliner going through a deicing process using a glycol solution. If you do not have deicing fluid on hand, simply moving your aircraft into a heated hanger for a short period of time will solve the problem. However, it’s important the water resulting from the melting is wiped off or you could encounter a re-freeze as you taxi to the runway. Other times all it takes is a quick brushing of the aircraft to remove frost from the surfaces.

Take a look at some of these sample Private Pilot test question dealing with frost and ice.

1. Which conditions result in the formation of frost?
A—The temperature of the collecting surface is at or below freezing when small droplets of moisture fall on the surface.
B—The temperature of the collecting surface is at or below the dewpoint of the adjacent air and the dewpoint is below freezing.
C—The temperature of the surrounding air is at or below freezing when small drops of moisture fall on the collecting surface.

2. Why is frost considered hazardous to flight?
A—Frost changes the basic aerodynamic shape of the airfoils, thereby decreasing lift.
B—Frost slows the airflow over the airfoils, thereby increasing control effectiveness.
C—Frost spoils the smooth flow of air over the wings, thereby decreasing lifting capability.

3. How does frost affect the lifting surfaces of an airplane on takeoff?
A—Frost may prevent the airplane from becoming airborne at normal takeoff speed.
B—Frost will change the camber of the wing, increasing lift during takeoff.
C—Frost may cause the airplane to become airborne with a lower angle of attack at a lower indicated airspeed.

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

Takeoffs involve much more than smooth piloting skills; they involve careful planning and preparation. A very smooth takeoff is of little value if the airplane, once airborne, is faced with obstacles impossible to avoid. The takeoff performance of the airplane needs to be matched to the runway and the surrounding obstacles prior to actually taking off. Today, we’ll take a look at one of the factors affecting takeoff performance: air density. This post is excerpted from the new fourth edition of The Pilot’s Manual: Ground School (PM-2).

One cause of an increase in density altitude is a decrease in air density. This results in a longer ground run and takeoff distance to clear a 50-foot obstacle. A decrease in air density can be caused by a number of factors.

A lower air pressure will decrease the density and this can occur as a result of a different ground-level ambient pressure or as a result of a higher airport elevation. This effect is covered by pressure altitude, which relates the actual pressure experienced by the airplane to a level in the standard atmosphere that has an identical pressure. High elevation airports lead to longer takeoff distances.

A higher air temperature will also decrease the air density, reducing airplane and engine performance.

If the air density decreases, the engine–propeller combination will not produce as much power and so the takeoff distance will increase. In addition to the power-producing performance of the engine–propeller decreasing, the aerodynamic performance of the airplane will also decrease as air density becomes less.

To produce the required lift force (L = Lifting ability ½ρV² × S), a decrease in air density (ρ) means that for the same required indicated airspeed, an increase in the velocity (true airspeed, V) is required and a longer takeoff distance will result. Not only does a lower air density affect the aerodynamic performance of the airframe (controlled by ½ρV²), it also decreases the weight of the fuel/air mixture in the engine cylinders, causing a decrease in engine power.

The post Aircraft Performance: Air Density first appeared on Learn To Fly.