Moment (also known as torque) is the rotational force applied to a body. Think of your airplane like a seesaw. The fulcrum is the wing because, like the fulcrum of a seesaw, the wing is keeping you from touching the ground. Continue reading “Weight and Balance Basics”
One of my favorite maneuvers when conducting a flight review is the steep turn. This innocuous looking maneuver provides a window into a pilot’s stick and rudder skill that allows me to quickly find areas of deficiency where the pilot being reviewed might need more work.
Please remember that the flight review is not a test and my goal is not to fail anybody. Rather, I want to find areas where the pilot is out of practice and try to give them a boost!
The steep turn requires a combination of just about all of the basic flying skills in one maneuver. It requires a pilot to: Continue reading “Steep Turns”
Flaps are very useful for giving an airplane good handling characteristics at low speed. They are vitally important for giving fast planes the ability to go slow for takeoff and landing.
There are 6 types of flaps
A simple hinge at the rear of the wing is used to make plain flaps. They are easy to design but they can’t provide much lift before the drag increases very significantly.
Since increasing drag is one of the purposes of a flap the performance is not too bad for landing. When taking off though, drag is not desired.
The reason this flap has so much drag is because the air from above the wing tends to separate and become turbulent when it has to flow downwards at such a steep angle. Think of it like a car on the highway having to make a sharp turn to stay on the road. This airflow separation is like that car spinning out because it can’t make the turn.
Turbulent airflow separation above the wing reduces lift.
The split flap surface is actually below the wing and pushes down out of the bottom. It does provide some lift but it creates a lot of drag and is almost like a speed brake under the wing.
This is the most common type of flap because it is relatively simple to design and build but provides a huge benefit over the plain flap. The slot created when the flap extends allows air to flow from under the wing.
This airflow provides a cushion for the air from on top of the wing to keep it from separating. It also flows along the top of the flap surface. These two benefits combine to create a lot of extra lift.
Fowler flaps are complex but provide a lot of benefit. They act like slotted flaps opening up a channel for air, but they also slide outwards as depicted below. By sliding out from the wing they greatly increase wing area.
Put simply, they make the wing bigger. A bigger wing means lots of lift!
Most modern airliners use double or triple slotted fowler flaps. These have several flap surfaces that extend out from each other as the flaps are lowered creating a much larger wing with several slots for air to pass through.
This modification to the leading edge of the wing provides a channel of air that is pushed up over the wing and channeled towards the upper surface. At low speeds, this simple fixed device can increase the critical angle of attack (the angle at which the wing stalls). This means that the plane can fly much slower without stalling.
Slats are another type of leading edge flap. They slide down at low speeds and provide a large increase in lift like the fixed slot. The benefit of the slat is that it is retractable and won’t create extra drag at high speeds because it slides up into the wing surface.
Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?
Mach number is a measurement of the aircraft’s speed relative to the speed of sound. Mach 1 would mean the aircraft is flying at the speed of sound and Mach .5 would mean it is flying at half the speed of sound. The mach number itself is generally determined by an air data computer gathering pitot-static and temperature data. Continue reading “Flight Instruments: Mach Indicator”
Indicated airspeed is the speed that the plane “feels”. It might help to think of it as the number of air molecules hitting the plane. This is the speed that matters for the performance of the plane. It can be read directly on the airspeed indicator.
2. True: TAS
As you climb the air gets thinner. As the air gets thinner there are fewer air molecules in a given volume of air. This allows the aircraft to fly faster.
For example, if your plane has enough power to fly at 100 knots and you maintain 100 knots while climbing your true airspeed will increase. True airspeed is your actual speed through the mass of air. As you climb and the air thins out, if you are still at 100 knots then you are still encountering the same amount of air over time, but since that air is spread out over a longer distance you are flying at a faster speed. True airspeed is the same as groundspeed if there is absolutely no wind.
This bonus in speed and better fuel economy are the reasons that planes bother to climb all the way up to high altitudes.
3. Calibrated: CAS
Airspeed indicators aren’t perfect. When flaps are down or the plane is at a high angle of attack the airspeed indicator may be off by several knots. This error is studied and a placard is provided with the correct numbers. So calibrated airspeed is more precise than indicated airspeed but it is not displayed directly on the airspeed indicator.
4. Ground Speed: GS
This is not an airspeed, but it is worth including here. Ground speed is the speed that really matters for getting somewhere, it is very simply your speed over the ground. It is equal to your true airspeed plus or minus a tailwind or headwind.
When there is enough wind it is possible to gain an enormous amount of extra speed. This is why jets love to take advantage of the jet stream where the wind speed can often exceed 100 knots.
It is also possible to make a plane stop or fly backward. See the video below that illustrates this concept.
5. Equivalent Airspeed: EAS
Equivalent airspeed is a further correction of calibrated airspeed that corrects for airspeed indicator errors due to compressibility. It is most prominent at high altitudes and high speeds. Modern planes that can reach these altitudes and speeds generally have an air data computer that handles the calculation of EAS but a simple performance chart can be used as well. For light aircraft, EAS is generally ignored because it is very close to being the same as CAS.
6. Mach Number: M
Aircraft that fly at higher altitudes and speeds, like jets, generally refer to their speed in terms of mach number. This speed is measured as the ratio of the speed of sound. For example, mach 1 means you are flying at the speed of sound, and mach .5 means you are flying at half the speed of sound.
Pilots generally pronounce mach numbers like “mach point seven five”, or “mach point eight” for M.75 and M.8, respectively.
Most planes fly at subsonic speeds, less than the speed of sound.
Faster planes like the Concorde, and some military fighters and bombers can fly at transonic speeds, at the speed of sound. They can then accelerate to supersonic speeds great than the speed of sound.
Anything greater than M5.0 is considered to be a hypersonic speed. Hypersonic aircraft are certainly being studied but as far as I know, there aren’t any flying.
As an aircraft reaches higher altitudes the mach number is used to measure speed instead of IAS. An aircraft is limited in IAS by aerodynamic pressure and in mach number it is limited by aerodynamic shock waves. Since there are two different limits they both need to be considered.
For example, a Boeing 757 has a Vmo (maximum operating airspeed) of 350 knots and an Mmo (maximum mach number) of M0.86.
At a low altitude near sea level M.86 is 568 knots, well beyond the Vmo limit. However, at a high altitude like 40,000 feet M.86 is approximately 262 knots, well below the Vmo limit. This is why pilots will reference IAS at low altitudes and mach number at high altitudes. During climb there is a crossover altitude at which the transition is made from thinking in IAS to thinking in mach.