Four Forces Affecting Flight #

An aeroplane in flight has forces affecting the aircraft. These forces dictate how the aeroplane will perform during flight. The forces counteract each other. Pilots must understand how these forces work to operate an aircraft safely. The four forces are lift, weight, drag and thrust. Each pair counteracts the forces. The thrust is counteracted by drag, while the weight counteracts the lift. Every time an aeroplane flies, all these forces are present regardless of the condition of the flight. But not all aircraft are subjected to the four forces, such as balloons and gliders do not experience these four forces since gliders and hot air balloons do not have engines to generate thrust for the aircraft.

Weight #

The first one is weight. Weight is a force on an object present due to gravity. An aircraft is subjected to this force even if it is on the ground. The weight of an aircraft plays a significant role in its performance. Heavy aircraft have reduced performance. It would take more extended landing and take-off rolls, higher stall speed and decreased engine performance. The force affecting weight is always towards the earth, regardless of the aircraft’s orientation during flight. Weight is a critical factor in the aircraft’s centre of gravity. Before a departure, a pilot computes the centre of gravity in an aircraft to check if it is within limits. It is called weight and balance. These aircraft limits are called CG limits which can be seen in a manufacturer’s manual or the pilot’s operating handbook (POH). Sometimes, it is shown in a graph to plot if an aircraft is within the limit. Here is a sample of a CG (Centre of gravity) Limit chart where a pilot must plot the weight, the CG location, the total weight of an aircraft, and if the lines intersect within the limit envelope given by the manufacturer.

To calculate the CG Location in inches, you must Divide the total moment over total weight in a formula form or $C.G.={Total Moment \above{1pt} Total Weight}$ where moment is also computed by Inches of datum multiplied by the weight. The manufacturer gives the inches of datum or arm. For example, the pilot’s weight is 150lbs, and the arm’s location is 37 inches aft of the datum. The moment of the pilot in the aircraft is 5 550lbs-in. The formula is $Moment = (Arm) x (Weight$)

Effects of CG location on the aircraft’s performance #

The CG location is essential to an aircraft because it affects its performance of the aircraft. An aircraft with a forward CG has increased longitudinal stability. Still, it will have a lower cruise speed because the aircraft will fly at a higher angle of attack to compensate for the added downward force forward. An aircraft with an aft CG has decreased longitudinal stability and higher cruise speed because the aircraft will be pitching down to compensate for the downward force from the aft. The aircraft with an aft CG would also have a poor stall and spin recovery due to extra downward force from the tail section.

How does weight affect the take-off and landing performance of an aircraft? #

Having a light or heavy aircraft has some advantages and disadvantages. But take note of the manufacturer’s limits since different aircraft have different limitations. Here are some advantages of a lightly loaded aircraft

Having a light or heavy aircraft has some advantages and disadvantages. But take note of the manufacturer’s limits since different aircraft have different limitations.

Here are some advantages of a lightly loaded aircraft

• Shorter take-off roll - with a lighter weight, an aircraft gains airspeed faster, which means that take-off is easier with a lighter aircraft.
• Shorter landing roll - a lighter aircraft would require a shorter runway because it is easier to slow down a light aircraft than a heavy aircraft.
• Higher Maneuverability - the control columns on a lighter aircraft have less pressure than on a heavily loaded aircraft.
• Better fuel efficiency - With less engine power required to propel the aircraft forward. The aircraft’s fuel consumption is lower.
• Higher stall speed - lighter aircraft tend to have a higher stall speed due to the efficiency of

Having a lighter profile aircraft can also have its disadvantages.

• More Reactive to the wind - lighter aircraft reacts to wind more than heavier aircraft. It is easier for a light aircraft to get blown by the wind compared to a heavy aircraft.
• Flaring and sinking - lighter aircraft tend to take longer to sink during landing due to the ground effect pushing the aircraft upwards.

Here are some advantages of a heavier-loaded aircraft

• More stable in flight - heavier aircraft tend to have more stability during flight. Since it has more weight, the aircraft reacts less to sudden wind changes.
• Easier touchdown - heavier aircraft reacts less to the ground effect on the landing, which makes the sink rate of a heavier aircraft higher than a lighter aircraft. Only less flight control pressure is required during flaring.

Weight definitions from manufacturers #

These are some terms that weight definitions can find in the Pilots Operating Handbook (POH) to determine the weight limits of an aircraft. The aircraft’s manufacturer gives these limits.

• Standard Empty Weight - weight consisting of the airframe, engines, and all required equipment installed on an aircraft, such as fluids, unusable fuel, and engine oil.
• (Note: Unusable fuel is fuel on the fuel lines that is not included in the fuel tanks. Fuel that connects be drained)
• Basic Empty Weight - these include the optional equipment, unusable fuel and operating fluids such as complete engine oil and hydraulic fluids.
• Ramp Weight - aircraft weight before engine starts.
• Maximum Ramp Weight - the maximum weight of an aircraft for ground operations such as taxi and engine run-ups.
• Maximum Take-off Weight - The maximum allowable weight safe for take-off.
• Maximum Landing Weight - maximum safe weight for landing. This is based on the capacity of the landing gear to absorb the aircraft’s weight on landing.
• Minimum Flight Weight - minimum weight for an aircraft flight.
• Zero Fuel Weight - the weight of the aircraft with empty fuel tanks.
• Gross landing weight - take-off weight minus the fuel burned during flight.
• Licensed Empty Weight - like the actual empty weight but without engine oil. Only unusable fuel is included.
• Payload - the weight loaded on the aircraft, not including the fuel. Such as passengers, baggage, and cargo.

Thrust #

Thrust is a force that propels an aircraft forward. The force produced comes from the aircraft’s engine or powerplant. This force counteracts the drag produced by the aircraft. Thrust is used to increase or decrease the altitude of the aircraft. Airspeed control is also affected by the aircraft’s thrust.

Power is one of the gauges of how much thrust is present in an aircraft. Power is directly proportional to the thrust generated in an aircraft.

Losing thrust or engine power in an aircraft is one of the pilot’s worst nightmares. Losing engine power, especially on single-engine aircraft, means the aircraft won’t be able to increase its altitude. But pilots are trained to handle these kinds of situations.

How does thrust apply to aircraft without engines? #

There are categories of aircraft that do not have an engine on their airframe. Some of these aircraft are gliders and balloons. Gliders are aircraft that glide through the air without an engine. The weight of the aircraft provides the forward motion for the aircraft. Gliders fly downwards but at a shallow angle. Airships, lighter-than-air aircraft, still have engines to propel the craft forward, but some do not like hot air balloons. These aircraft do not have engines to provide thrust. Hot air balloons only rely on the wind for directional movement.

Types of engines #

These are common types of engines according to their type of propulsion system.

• Piston - thrust is generated by a propeller. Pistons drive the propeller to produce thrust. A Cessna 172 has this kind of engine.
• Turboprop - thrust is also generated by a propeller with a small percentage of power coming from a jet blast or jet exhaust. ATR Aircraft has this kind of engine installed on their aircraft.
• Turbofan - thrust is generated by a giant fan located at the front of the engine, with a small percentage of its power also coming from jet exhaust. Modern aircraft, such as the Airbus A330, have this engine.
• Turbojet - all thrust generated from this engine comes from the jet exhaust. Modern military aircraft have these for their powerplants.

Drag #

This type of force is the one that counteracts thrust. Drag is a relative force that opposes any object in motion. Any object moving through the air experiences drags even if the object is not airborne or in flight. Regardless of the aerodynamics of an aircraft, it will still produce drags. There are many types of drags.

Common types of drag #

• Parasite Drag - this type of drag is not connected to the by-product of lift. Parasite drag has different categories. Here are the types of parasite drags.
  • Skin Friction Drag - produced by the aircraft’s surface area. For example, a smoothly painted aircraft has less skin friction drag than an aircraft with a rough surface area.
  • Form Drag - this drag is produced depending on the aircraft’s aerodynamic design. The more aerodynamic the aircraft is, the less form drag it has.
  • Interference Drag - is produced where different parts of the aircraft meet. For example, the location where the wings and fuselage meet the interference drag is present.
  • Profile Drag directly relates to your aircraft’s size, design, and configuration. The profile drag is the amount of form drag and interference drag.
• Wave drag - produced by shockwaves when an aircraft flies at transonic and supersonic speeds.
• Induced Drag - this is the by-product of lift. This drag is inevitable as this is produced every time a lift is produced.

Lift #

Lift is one of the most important forces affecting flight. This force is mainly responsible for the aircraft’s capability to fly. Its lift is created by the difference in the air pressure above and below the airfoil. The opposing force of lift is weight. The wings generate most of the lift of an aircraft. The force acting on the lift is perpendicular to the relative airflow. This pressure difference is an example of Bernoulli’s principle. Bernoulli’s principle states that an increase in speed is directly proportional to a decrease in pressure. As the speed of the air on the upper wing increases, it has a lower air speed. While the speed of the air on the lower wing decreases, the pressure increases, and this pressure difference produces lift for an aircraft. While lift works differently on hot-air balloons and airships, airships are loaded with lighter-than-air gasses, such as helium, enveloped within the airship. The helium-loaded airships generate lift by the differences in the weight of the air outside and inside the airship. A hot air balloon generates lift by creating hot air that expands inside the balloon, eventually giving enough lift for the balloon to fly. Unlike other forces affecting flight, like thrust, all aircraft must have lift to maintain their flight. The lift is still present even in hot-air balloons with no thrust acting on the aircraft. At the same time, gliders use their wings to generate differences in air pressure to produce lift.

Factors affecting lift #

There are a lot of factors that affect an aircraft’s production lift. Here are some common factors that can affect the lift of an aircraft. It varies from the angle of attack to the aircraft’s wing area.

• Lift coefficient - the air density influences this. A lift coefficient is a dimensionless number computed and tested in a wind tunnel.
• Dynamic energy - this is influenced by both air density and airspeed. Dynamic energy results from an aircraft moving through the air and disturbing the air stream around the aircraft.
• Angle of attack - is the wing’s angle relevant to the wind. The angle of Attack (AOA) is determined by an imaginary straight line drawn at the beginning of the airfoil or the leading edge that runs up to the end of the trailing edge of the aircraft.
• Wing Area - it is the surface area of the wings without the factor of the wind. The wing area is simply the computation of the total area of the airfoil of an aircraft.

High Lift Devices #

High-lift devices are movable or stationary components to increase the lift of an aircraft. High-lift devices increase wing surface area. Most of these are used for take-off to maximize lift for shorter take-off lengths. At the same time, high lift devices can also use for landing to increase lift and decrease the stall speed for slow flights.

These are a few of the high-lift devices in an aircraft

• Flaps - Consists of hinged panels installed on the wings' trailing edge. They are powered by a lever in which the pilot can increase or decrease the flaps angle of an aircraft. For example, some Cessna 172s have a maximum flap angle of 30°.
• Krueger Flaps - these are installed at the leading edge of the wings. It is similar to slats, but the deployment is different. These are installed at the bottom part of the wing.
• Slats - these are also located at the leading edge of the wings. This is used to increase lift for low-speed phases of flight, such as take-off, initial climb, final approach, and landing. These are either hydraulically or electrically powered.

Parts of an airfoil #

The airfoil is the primary device that generates lift in an aircraft. The airfoil is commonly divided into these parts.

Here are the common parts of an airfoil.

• Leading Edge - the leading edge is the airfoil part that initially gets in contact with the wind.
• Chord - is an imaginary line that goes through the leading edge up to the trailing edge of an aircraft.
• Camber - these are divided into two parts: the upper and lower camber. The camber measures the thickness of the airfoil.
• Trailing-Edge - the trailing edge is the back part of the airfoil where the upper and lower airflow meets. This part of the airfoil directs airflow to create little turbulence as possible.

Wing location to the performance of the aircraft #

Aircraft have three typical wing configurations or wing locations: the high wing, the low wing, and the mid-wing configuration.

High-wing aircraft are aircraft with wings mounted above the fuselage. One typical example of a high-winged aircraft is the Cessna 172.

High-wing aircraft provide more stability at low flight speeds, and turbulence correction at slower speeds is easier.

Low-wing aircraft are aircraft with wings mounted below the fuselage. An example of a low-wing aircraft is the Airbus A380.

Low-wing aircraft provide the most manoeuvrability among the three configurations.

Mid-wing aircraft have wings located precisely at the midline of the aircraft’s fuselage. A typical example of mid-winged aircraft is modern fighter jets.

Mid-wings aircraft are the most balanced because they can have a larger control surface area. These have high manoeuvrability but less stability than high-wing aircraft.

Different wing locations have differences, but the best profile for aviation training is a high-winged aircraft due to its high stability among the other settings.