Drag is the force that opposes the motion of an aircraft through the air. It affects the performance, fuel efficiency, and safety of the aircraft, especially during takeoff and landing. In this article, you will learn some of the factors that influence drag and some of the methods that aerospace engineers use to reduce it.
Drag is the result of air pressure and friction acting on the surface of the aircraft. It depends on the shape, size, speed, and orientation of the aircraft, as well as the density and viscosity of the air. Drag reduces the acceleration and top speed of the aircraft, and increases the amount of thrust and fuel required to maintain a desired speed. Drag also affects the stability and control of the aircraft, especially at low speeds and high angles of attack.
Induced drag can be reduced (for a constant amount of lift) by increasing speed. Everything else are about design solutions which are built into the aircraft by the manufacturers: Flush-mounted rivets Wheel pants Fairings Gap seals Winglets Making the wing narrower Using new materials (smooth surface) Using specific paintings (chemicals known as drag-reducing agents (DRA) In addition, keeping the aircraft clean is a simple but effective solution. Thus, making the aircraft more streamlined to reduce the separation of the fluid flow around surfaces. In conclusion, reducing causes for friction while increasing the fluid flow.
Drag also depends on surface smootheness, contamination, Reynolds and Mach number. It is a resultant force opposing motion of the body, acting in the opposite direction of object motion. Total drag is made up from different types of contributing drag forces and the best resource for finding out more on this topic is Hoerner's 'Fluid dynamic drag'.
During Takeoff adding a notch of flaps will lower your overall angle of attack and increase camber of the wing adding lift and reducing induced drag. During approach to landing adding full flaps increases camber allowing aircraft to fly at slow airspeeds and adds drag, stabilizing airspeeds to allow for increased controllability during approach.
Drag can be classified into two main types: parasite drag and induced drag. Parasite drag is the drag that occurs due to the friction and pressure of the air on the non-lifting parts of the aircraft, such as the fuselage, engine, and landing gear. Induced drag is the drag that occurs due to the generation of lift by the wings and other lifting surfaces. Induced drag is proportional to the square of the lift coefficient and inversely proportional to the aspect ratio of the wing. The total drag of the aircraft is the sum of the parasite drag and the induced drag. The drag coefficient is a dimensionless number that represents the ratio of the drag force to the dynamic pressure and the reference area of the aircraft. It is a useful parameter to compare the aerodynamic efficiency of different aircraft designs.
There are two types of drag: Induced and parasite drag. Induced drag is a byproduct of coefficient of lift. Increasing AoA increases drag thereby inducing it. Parasite drag is a byproduct of design and increases with airspeed. Parasite stage is subdivided into three types: Skin Friction Form Interference.
One of the ways to reduce parasite drag is to minimize the frontal area and the wetted area of the aircraft. This means designing the aircraft with a streamlined shape that reduces the separation and turbulence of the airflow around it. Another way to reduce parasite drag is to use smooth and polished surfaces that reduce the skin friction of the air on the aircraft. A third way to reduce parasite drag is to retract or fair the components that are not needed during flight, such as the landing gear, flaps, and slats. This reduces the form drag and the interference drag caused by these components.
Some years ago I was invited to visit the Airbus factory to see the prototype A380 under construction. The head of engineering said that the weight of the under carriage was, if I remember correctly, 40 tonnes and of course is only used twice, the rest of the time it's just excess weight. He then invited guests to suggest ways landing gear could be improved. I suggested that you could get rid of it all together if aircraft were designed to take off & land on water as they did in the 1930-1950s but of course that would create other problems, not least from the vested interests of airlines and airport operators. However, it would make an interesting research project to see how today's technology could improve flying boat design.
One of the ways to reduce induced drag is to increase the aspect ratio of the wing. This means designing the wing with a longer span and a narrower chord. This reduces the amount of vortex shedding and downwash at the wingtips, which are the main sources of induced drag. Another way to reduce induced drag is to use winglets or other devices that reduce the strength of the wingtip vortices. Winglets are vertical extensions at the end of the wing that improve the lift-to-drag ratio by redirecting some of the airflow from below to above the wing. A third way to reduce induced drag is to use variable geometry wings that can change their shape and size according to the flight conditions. For example, some aircraft can sweep their wings back or forward to optimize their performance at different speeds and altitudes.
Configure flaps and slats to generate lift while reducing drag once sufficient lift is obtained. Keep wings and surfaces clean and free of contaminants disrupting airflow. Maintain proper tire pressure and condition to minimize friction. Engage thrust reversers on landing to decrease the runway required for deceleration. Pilots should make smooth, controlled movements and maintain proper pitch/bank angles to avoid drag-inducing maneuvers. Ensure the right aircraft weight balance and loading to minimize trim adjustments. To streamline the profile, perform aerodynamic wing clean-up by retracting gear and flaps once at altitude. Implementing these strategies reduces drag, fuel efficiency, and operating costs.
Another way to reduce wing vorteces and drag is to reduce lift at the wing tips. An eliptical planform wing produces an eliptical wing loading. Since a planform is not a viable wing shape for most modern non G.A. aircraft, other means of minimizing lift at the wings maybe be used, such as a higher taper ratio or change in airfoil type towards the wing tips.
Takeoff and landing are the phases of flight that require the most amount of thrust and fuel, and also pose the most risk of stall and loss of control. Therefore, it is important to optimize the drag during these phases. During takeoff, the aircraft needs to generate enough lift to overcome its weight and enough thrust to overcome its drag. To achieve this, the aircraft uses high-lift devices, such as flaps and slats, that increase the camber and area of the wing, and thus the lift coefficient. However, these devices also increase the parasite drag and the induced drag of the wing, so they need to be deployed at the right angle and retracted as soon as possible after takeoff. During landing, the aircraft needs to reduce its speed and altitude while maintaining enough lift and control. To achieve this, the aircraft uses drag devices, such as spoilers and airbrakes, that increase the drag and reduce the lift of the wing, and thus the speed and glide ratio. However, these devices also increase the risk of stall and loss of stability, so they need to be deployed at the right time and retracted before touchdown.
The extended landing gear is responsible for about 1/3 of total drag experienced by aircraft during takeoff and landing phases. Improve the landing system (arms, doors, fairings) can be very beneficial. The best way is try to desing more aerodynamic surfaces with fairings, avoid angular edges and try to make everything very smooth. The cavity where the tires are stored can also be improved, by reducing the recirculating flow inside. The Hoerners Drag and semi emphirical methods can be a good tool to give some direction. CFD can be a power tool to get good result for simpler geometries.
Both take-off and landing are low speed phases of flight, meaning the angle of attack and (the coefficient of) lift is high. Induced drag increases with an increase in lift and parasitic drag decreases with a decrease in speed, so the predominant type of drag in these regimes is induced drag. The usual design solutions for induced drag reduction can be used: - High aspect ratio (slender) wings; - Winglets or other wing-tip devices (are most useful in these flight regimes). Additionally, using flaperons instead of flaps, good wing root fairings, optimal lift distribution, clean aircraft design etc. helps immensely.
If you are interested in learning more about drag reduction techniques, you can check out some of the online courses and books that cover the fundamentals and applications of aerodynamics and fluid mechanics. You can also explore some of the research and development projects that aerospace engineers are working on to improve the drag performance of current and future aircraft. Some of the examples are laminar flow control, boundary layer suction, active flow control, adaptive wings, and blended wing body.
The best way is to read Hoerner's 'Fluid-dynamic Drag' and old NACA technical notes and studies found on NASA's webpage.
A part of the drag is due to the engine cooling system. A well dimensionned cooling system can allow you to reach the cooling needs without increasing to much the drag. The use of aerodynamic simulation software (CFD) can help a lot in designing efficient cooling devices. In the general aviation world, for low speed aircraft (incompressible flows) the software "ACE of Aircraft" can help engineers in designing efficient cooling devices, without requiring CFD expertise. Wings, slats, flaps etc... can also benefit from CFD in order to reduce the drag.
Why not consider to choose the best runway, ir order to secure the most possible headwind for landing and take off. Sometimes ATC uses the most confortable runway in order landing and take off sequencies, but other runways could be better taking in account wind factor.