Leroy R. Grumman Cadet Squadron

Civil Air Patrol - The official auxiliary of the United States Air Force

The Four Forces of Flight - Drag

Drag opposes thrust. Imagine sticking your hand out the window of a moving car. The force that pushes your hand back is called "drag". As your hand pushes on the wind, the wind also pushes against your hand. Drag is unwanted because it makes the plane inefficient by requiring more thrust to fly successfully. To reduce drag and increase efficiency, planes are streamlined by using cowlings, nacelles and fairings.

Types of Drag - Parasitic and Induced

Parasitic Drag - Types

 

 

Methods of Reducing Turbulence/Drag - Diagram

 

Form Drag
Generated by the aircraft due to its shape and airflow around it. Examples include engine cowlings, nacelles, antennas, and the aerodynamic shape of other components.

 

Interference Drag
Comes from the intersection of air streams that creates eddy currents, turbulence, or restricts smooth airflow. For example, the intersection of the wing and the fuselage at the wing root has significant interference drag. It is also highest when two surfaces meet at perpendicular angles.

 

Boundary Layer Airflow interaction with Wing Surface - Diagram

 

Skin Friction Drag
The aerodynamic resistance due to the contact of moving air with the surface of the aircraft.  Even though a surface appears extremely smooth to the naked eye, there are still textures and imperfections existing that can be viewed under a microscope.  These textures and imperfections interact with air molecules within the boundary layer closest to the aircraft's surface, slowing the movement of the boundary layer as the aircraft moves through the air.


Induced Drag
In level flight the aerodynamic properties of a wing or rotor produce a required lift, but at the expense of a certain penalty. Induced drag is the name of the penalty. Induced drag is inherent whenever an airfoil is producing lift, and this type of drag is inseparable from the production of lift. It is always present of lift is produced.

 

 

Me-109G - Percentages of Drag

Cowlings and Nacelles

 

Turbofan Engine Nacelle

 

Turbofan Engine Nacelle - Diagram

 

Nacelles - Aerodynamic Design Considerations

 

 

Turboprop Engine Cowling

 

 

Nacelles and Cowlings reduce parasitic drag by reducing the surface area, having a smooth surface and thus leading to laminar flow, and having a nose cone shape, which prevents early flow separation. When used to surround an Internal Combustion (IC) engine, a cowling's inlet shaping and airflow design in combination lead to an isotropic speed reduction around the cooling fins and, due to the speed-squared law, to a reduction in cooling drag.

 

 

What you don't want to see during flight!

 


A Townend Ring is a narrow-chord cowling ring fitted around the cylinders of an aircraft radial engine to reduce drag and improve cooling.

 

 

The Townend ring was the invention of Dr. Hubert Townend of the British National Physical Laboratory in 1929. Patents were supported by Boulton & Paul Ltd in 1929. In the United States it was often called a "drag ring". It caused a reduction in the drag of radial engines and was widely used in high-speed designs of 1930-1935 before the long-chord NACA (National Advisory Committee for Aeronautics) cowling came into general use. It was also said to generate forward thrust from the expansion of the air as it passed over the engine, adding 10 to 15 mph to the aircraft's top speed.

 

Early claims portrayed it as a superior design to the NACA cowling (see below), but later comparisons proved aircraft performance worse when using a Townend ring at airspeeds above 250 mph.


 

The NACA cowling is a type of aerodynamic fairing used to streamline radial engines for use on airplanes and developed by NACA in 1927 from the ‘Anello Magni’, patented by the Italian engineer Pietro Magni. It was a major advancement in drag reduction, and paid for its development and installation costs many times over due to the gains in fuel efficiency that it enabled.

 

 

 

Most propeller aircraft in the 1920s had radial engines. In this configuration, the engine cylinders were mounted in a circular pattern. Most of these engines were air cooled by air flowing back through the propeller and over the cylinder heads (the tops of the cylinders sticking out from the engine). But this presented a problem, for the cylinder heads stuck out from the aircraft's skin and created drag. In the later 1920s, NACA developed a covering, or cowling, for the engine that not only reduced drag but also improved cooling of the engine. It was a major technological breakthrough achieved through careful, systematic research and testing.

 

NACA researchers at Langley started an effort to explore engine cowlings under the direction of Fred Weick, a young Chicago-born engineer. Weick designed ten different cowlings, from a partial covering to a complete covering of the engine. These were fitted to a Wright Apache biplane with a J-5 Whirlwind air-cooled engine. The goal was to produce a cowling that reduced drag but still cooled the engine as much as an uncovered engine.

 

The ten cowlings were tested on the engine in Langley's PRT. Engineers measured drag, propulsive efficiency of the propeller (in other words, how efficient was the propeller at pushing air past the cowlings), and the engine temperature. The best versions of the cowlings were then modified, with the addition of vents and with changes in their shape. This was all done in a methodical manner. Eventually, the engineers decided upon a cowling design they designated "No. 10," which completely covered the engine and its protruding cylinder heads, letting in air at the front. The air was then directed over the hottest parts of the engine and out the sides along the fuselage. What they also learned was that the shape of the airplane behind the cowling was important to understanding how to design the cowling in the first place—the cowling had to smoothly connect to the fuselage so that the air flowing over it was not disrupted.

 

The No. 10 cowling reduced drag by a factor of almost three. This was such an impressive improvement in performance that Weick chose to make the results public immediately so that industry could take advantage of them. In November 1928, Weick wrote Technical Note 301, directed at airplane manufacturers, which described the cowling. Weick stated that using a cowling that completely covered the engine was practical, but warned that "it must be carefully designed to cool properly."  NACA's Washington office announced that a cowling could be installed for about $25 per airplane and that the possible overall savings from the industry's use of the cowling was at least $5 million (which was more than had been spent on NACA since its establishment in 1915).

 

Langley engineers then mounted cowling No. 10 onto a borrowed Curtiss Hawk AT-5A biplane that used the same Wright Whirlwind J-5 engine that had been tested in the PRT. The tests showed that the airplane's maximum speed increased from 118 to 137 miles per hour (190 to 220 kilometers per hour) with the cowling. On February 4-5, 1929, Frank Hawks, a barnstorming pilot, flew a Lockheed Air Express equipped with a NACA low-drag cowling from Los Angeles to New York nonstop and established a new record. Hawks flew this distance in 18 hours and 13 minutes in an airplane whose top speed had been increased from 157 to 177 miles per hour (253 to 285 kilometers per hour).

 

The NACA engine cowling was an important technological development. But its importance was not so much developing a thing as it was improving understanding of aeronautical design techniques. Airplane designers could not simply stick an "engine cowling"—a piece of tin—on any aircraft and have it work. They had to specially design a cowling for each aircraft. But once they understood the principles behind this device, and once they understood the method for developing it, they could design engine cowlings for existing and new aircraft and improve their efficiency. This then led to further research on the proper placement and cooling of propeller engines, particularly on large multi-engine craft, such as bombers. For instance, most multi-engine craft before the mid-1930s, such as the Ford Tri-motor, had the engines mounted below the wings in pods (or "nacelles"). But further research at NACA, using the same methods developed for the low-drag cowling development, demonstrated that the best place to mount the engines was directly in front of the wing, blended into it. Thereafter, planes like the B-17 bomber had engines that were fitted into the wing itself and achieved greater efficiency.

Fairings

A fairing is a structure whose primary function is to produce a smooth outline and reduce drag.  These structures are covers for gaps and spaces between parts of an aircraft to reduce form drag and interference drag, and to improve appearance.

On aircraft, fairings are commonly found on:

Fixed Landing Gear Junction Fairings

 

 

Landing gear fairings reduce drag at these junctions.


Flap Track Fairings

 

 

 

Most jet airliners have a cruising speed between Mach 0.8 and 0.85. For aircraft operating in the transonic regime (about Mach 0.8–1.2), wave drag can be minimized by having a cross-sectional area which changes smoothly along the length of the aircraft. This is known as the area rule.

 

On subsonic aircraft such as jet airliners, this can be achieved by the addition of smooth pods on the trailing edges of the wings. These pods are known as anti-shock bodies, Küchemann Carrots, or flap track fairings, as they enclose the mechanisms for deploying the wing flaps.


Strut-to-Wing and Strut-to-Fuselage Junctions

 

 

 

Strut end fairings reduce drag at these junctions.


Tail Cone

 

 

Tail cones reduce the form drag of the fuselage, by recovering the pressure behind it. For the design speed they add no friction drag.

 

 


Wing Root Fairing

 

 

Wing roots are often faired to reduce interference drag between the wing and the fuselage. On top and below the wing it consists of small rounded edge to reduce the surface and such friction drag. At the leading and trailing edge it consists of much larger taper and smooths out the pressure differences: High pressure at the leading and trailing edge, low pressure on top of the wing and around the fuselage.


Wing Tip Fairings

 

 

Wing tips are often formed as complex shapes to reduce vortex generation and so also drag, especially at low speed.

 

 


Wheel Pant or Wheel Spat Fairings on fixed gear aircraft

 

 

Wheel fairings are often called "wheel pants", "speed fairings" or in the UK, "wheel spats". These fairings are a trade-off in advantages, as they increase the frontal and surface area, but also provide a smooth surface, a faired nose and tail for laminar flow, in an attempt to reduce the turbulence created by the round wheel and its associated gear legs and brakes. They also have the important function of preventing mud and stones from being thrown upwards against the wings or fuselage, or into the propeller on a pusher craft.

The NACA Duct

A NACA duct or NACA scoop is a common form of low-drag intake design, originally developed by the National Advisory Committee for Aeronautics (NACA), the precursor to NASA, in 1945. When properly implemented, it allows air to be drawn into an internal duct, often for cooling purposes, with a minimal disturbance to the flow. The design was originally called a "submerged inlet," since it consists of a shallow ramp with curved walls recessed into the exposed surface of a streamlined body, such as an aircraft.

 

 

 

Prior submerged inlet experiments showed poor pressure recovery due to the slow-moving boundary layer entering the intake. This design is believed to work because the combination of the gentle ramp angle and the curvature profile of the walls creates counter-rotating vortices which deflect the boundary layer away from the intake and draws in the faster moving air, while avoiding the form drag and flow separation that can occur with protruding inlet designs. This type of flush inlet generally cannot achieve the larger ram pressures and flow volumes of an external design, and so is rarely used for the jet engine intake application for which it was originally designed.  It is, however, common for engine and ventilation intakes.

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