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The Aerodynamics of Flight | How does a plane fly?

The Aerodynamics of Flight | How does a plane fly?

Feb 6, 2016


 

Since childhood almost all of us are fascinated by planes and flying objects. Ever wondered what makes those heavy objects fly high in the sky? Here is the brief basic explanation on aerodynamics of flying vehicles like planes, Helicopters, Quadcopters & Gyrocopters.

Forces on Flight:

Lets begin with the Forces acting on a plane in air.

Forces acting on a flightLift: It is an artificial force manipulated by pilot. It is generated through the wings, acts perpendicular to the relative wind and wingspan.  The theoretical concept that summarizes the direction and force of lift is the center of pressure.  Lift opposes weight—during level cruise, lift equals weight; during climb, lift is greater than weight; and during descent, weight is greater that lift.


Weight:
It is a natural (uncontrollable) force generated by gravity (g force) that acts perpendicular to earth’s surface; theoretically, weight is exerted through the center of gravity, and opposes lift.

Thrust: It is an artificial force manipulated by pilot and generated through engine that acts horizontally, parallel to flight path; thrust opposes drag—when airspeed constant, thrust equals drag; when airspeed accelerating, thrust is greater than drag; and when decelerating, drag is greater than thrust.

Drag: It is the natural resistance of an airplane while it is moving through air; it is partially controlled by pilot.  Drag is a horizontal force acting parallel to flight path, and is opposed to thrust.

Misconception of Lift:

Everyone knows that wing generate lift due to its characteristic shape.  Since air travels farther over the top of the wing so it goes faster than the air underneath. So that the both streams meet simultaneously at the trailing edge, and according to bernoulli’s principle, Faster flowing air exerts less pressure than the slower air beneath the wing. This pressure difference creates upward force which we call Lift. From those assumptions, we infer this way:

  1. Air on the upper side must travel faster than on the lower side because of the difference of distance to be traveled.
  2. By Bernoulli’s principle accelerated airflow has lower pressure.
  3. Pressure is then lower on top of the wing, and higher on bottom.
  4. Hence the wing receives a force which has a vertical component upward. This component balances the weight of the aircraft, and allows to stay aloft.

Such explanation, a one that is very often used, including in aviation books- is a misconception. If lift is computed from fluid laws, based on airspeed on both sides of the airfoil, the result will not be in line with what is observed in real life. Moreover, this theory cannot explain why a symmetrical airfoil works, or how an aircraft can fly upside down. Even the Right brothers’s first succesfull aeroplane flight had wings which had no airfoil shape, and they were flat.

Experiments show that air streams on air-foil don’t meetup at the trailing edge of the wing. Air over the top goes significantly faster reaching the trailing edge first.

How does a wing generates lift?

Lift is achieved by either asymmetric or cambered airfoil or increasing the angle of attack. Air under the wing is deflected down and by coanda effect the air is guided along the upper part of the wing down as well. Since air is slowed and deflected down by the wing. It pushes the wing up(Lift) and back(drag). Thus now since we have understanding of air flow of the wing. Now we apply burnoulli’s principle and we understand the flight in the right way.

Newton third law applied to aerodynamics

ARelative wind angle of attackngle of Attack:

It is the most critical phenomena for the Lift. It is an angle between body’s reference line and the oncoming flow. Perfect example of understanding the angle of attack is when we try to put our hands out of window from a fast moving car. Its easier to keep our hand with finger tips pointed in the direction of motion of the car until we try to move with an angle.

Coefficient of lift and angle of attack relation

Relation between angle of attack and lift: Angle of attack is associated with increasing of the lift co-efficient to maximum limit after then the lift co-efficient decreases.

Lift Equation:

Lift depends on the density of the air, the square of the velocity, the air’s viscosity and compressibility, the surface area over which the air flows, the shape of the body, and the body’s inclination to the flow.

One way to deal with complex dependencies is to characterize the dependence by a single variable. For lift, this variable is called the lift coefficient, designated “CL”. This allows us to collect all the effects, simple and complex, into a single equation. The lift equation becomes:

Graphic13

For given air conditions, shape, and inclination of the object, we have to determine a value for Cl to determine the lift. For some simple flow conditions and geometries and low inclinations, aerodynamicists can determine the value of Cl mathematically. But, in general, this parameter is determined experimentally.

 

Lift of a Helicopter

Ground effect of helicopterWhile a helicopter is a far more complex machine than an airplane, the fundamental principles of flight are the same.  The rotor blades of a helicopter are identical to the wings of an airplane –when air is blown over them, lift is produced.  The crucial difference is that the flow of air is produced by rotating the wings or rotor blades rather than by moving the whole aircraft.  When the rotor blades start to spin, the air flowing over them produces lift, and this can cause the helicopter to rise into the air.  So, the engine is used to turn the blades, and the turning blades produce the required lift. Very simple!

images (1)As you can see in CFD analysis of Helicopter’s airflow. The red color is showing the High pressure that is generated by the wings rotating in helicopter. This high pressure gives helicopter a Lift.

Movement motions of a Flight:

Pitch roll yaw

Helicopter Torque:
Torque reaction on helicopter
Using Isac newtons third law of motion. Every action has equal and opposite reaction. Since the helicopter has rotor that spins in one direction CW or CCW. Now there is a reaction in motion of Helicopter due to the direction of blade travel, and it is called as torque reaction.

Now to make Helicopter to stay in stable direction we need to counter this turning effect on it by counter it by anti-torque system. Where we have a tail rotor that generates a thrust just like in any conventional aeroplanes head. This tail rotor which produces thrust opposes the direction of the torque generated by the main rotor

This tail rotor also allows helicopter to turn. When we want our helicopter to turn in “yaw” mode. We slow down or fasten up the tail rotor depending upon CW or CCW turn required.

Swash Plate Mechanism:

download (1)A swashplate is a device that translates input via the helicopter flight controls into motion of the main rotor blades. Because the main rotor blades are spinning, the swashplate is used to transmit three of the pilot’s commands from the non-rotating fuselage to the rotating rotor hub and mainblades.

It is responsible for entire flight control and movements of helicopter. It also helps to control the lift produce by increasing angle of attach on the main blades.Helicopter aerodynamics

 

Quadcopters

quadcopter_x_flight_dynamics_yaw_pitch_rollQuadcopters are four-rotor helicopters that fly with two pairs of blades spinning in opposite directions. Unlike a traditional helicopter, all four blades on a quadcopter work together to produce upward thrust. The quadcopter’s movement is controlled by varying the relative thrusts of each rotor. This design creates a more stable platform than traditional helicopters, making quadcopters ideal for applications such as surveillance and aerial photography.

From a technical perspective, quadcopters create an interesting challenge, since, in order to be balanced, the quadcopter must continuously make minute adjustments to the speed of each rotor to keep the entire craft level. Since performing these adjustments manually in real time would be extremely difficult, a flyable quadcopter must be able to make these adjustments autonomously. This requires that the quadcopter be nearly perfectly balanced and have a sophisticated control system that is continually making adjustments.

While traditional helicopters generate most of their lift with a single blade, the weight of a quadcopter is split between four separate rotors, two of which rotate opposite the other two. Because each rotor generates a portion of the total lift, the flight of a quadcopter is controlled by varying the the relative lift and torque of each rotor.

imagesWhen a quadcopter is hovering in the air, each rotor is generating the same amount of lift and the torques of two sets of rotors cancel out, keeping the quadcopter from spinning. In reality, due to the many variables affecting a quadcopters flight, hovering is not a fixed setting, but rather the result of continuously adjusting several flight parameters based on sensor input.

A quadcopter can be thought of as having four controllable degrees of freedom: roll, pitch, yaw, and altitude. Motion along each degree of freedom can be controlled by adjusting the thrusts of each motor.

For example, to roll or pitch, one rotor’s thrust is decreased and the opposite rotors thrust is increased by the same amount. This causes the quadcopter to tilt. When the quadcopter tilts, the force vector is split into a horizontal component and a vertical component. This causes two things to happen: First, the quadcopter will begin to travel opposite the direction of the newly created horizontal component. Second, because the force vector has been split, the vertical component will be smaller, causing the quadcopter to begin to fall. In order to keep the quadcopter from falling, the thrust of each rotor must then be increased to compensate.

 

Gyrocopters

Also called as Gyroplane or Autogiro. It looks like a small two seated helicopter but works totally different, its partly an aeroplane and a helicopter. What makes this type the most unique is it has a unpowered rotor in autorotation to develop lift and an engine powered propeller to produce thrust. The main auto rotating rotor is all dependent on the wind. The blades of the center main rotor are like an airfoil which enables the blades to turn into the airflow rather than be pushed round by it. It was first invented by a Spanish engineer called Juan de la Cierva. First successful autogiro flight was in 1923.

Basic components of GyroCopterToday, many gyrocopters are built from kits by hobbyists. If they’re light enough, they are governed by the FAA’s rules on ultralight and amateur-built aircraft. Modern autogyros are also remarkably slow for planes, with an FAA(Federal Aviation Administration) mandated top speed of no more than 63 mph. Of course, the same law governing ultralight aircraft notes that “No person may operate an ultralight vehicle over any congested area of a city, town, or settlement, or over any open air assembly of persons.”

Ultralights also typically operate in the low sky, above 500 feet everywhere and 1000 feet in congested areas. Drones typically operate below 400, though not always. And thanks to their low cost and basic construction, ultralights predate drones as cheap border-hopping vehicles for drug smuggling. While they’re limited in carrying capacity and speed, ultralights are tricky enough for radar to spot that in 2012 the Department of Homeland Security gave a $100 million contract to a company developing radar specifically to look for ultralights.

Powering a Gyrocopter:

To provide the needed thrust to our gyrocopter to fly smoothly. We have two options:

  1. Rotor blade powered by an Engine: To produce thrust for our gyrocopter we use 4 stroke or 2 stroke engines to power the electric motor to spin the rotor blade. The Rotax is a famous brand name for internal combustion engines which are used in wide variety of small land, sea and airborne vehicles. Rotax 913, Rotax 914 & Rotax 915 are the popular engines that are used by Ultra-light auto gyrocopters.

Gyrocopter needs to roll faster and faster on the field before the rotor spins fast enough in the wind for the autogyro to lift off. 100 meters or so of runway is needed for take-off. Many autogyros have a link shaft from the propeller engine and a cogwheel drive so the rotor can be pre-rotated before take-off. The take-off distance required is then shorter.

 

  1. Rocket powered GyrocopterRocket powered: The rocket powered tips of rotor blade of gyrocopter could help reduce the operational costs and also let us make almost vertical take offs, almost like a helicopter. By attaching small hydrogen peroxide fuel rockets on the top of main rotor blade. So far, very less information available online for this sort of Gyrocopter. There have been problems faced for the transfer lines from fuel storage to the tips of fast turning rotor blades.

The best known project is Fairey Rotodyne. Fairey Rotodyne could carry about 50 passangers and the first flight took place in 1957. Vertical helicopter style take-offs and landings were made, using the tip rockets. Rotodyne was cruising at 310 km/h with help of the thrust from propellers and with the rotor spinning free. Despite the project was a tremendous technical success it was still finally cancelled, because the noise was said to be too high when starting and landing in heavily populated city centers.

Fairy Rotodyne Gyrocopter

Applications of Gyrocopters:

Initially Autogyros were developed for leisure or sports flying. But current uses are progressively developing for serious areas of service like fire surveillance, fumigation, border and traffic patrol. Simplicity, easy handling and low operational costs make it very interesting for public services military or police.

Touristic flights: Flying package tours and adventure tours organized by tourist offices, sight-seeing flights, flights over monumental sites, natural attractions, etc.

Aerial photography and film: Having a tandem arrangement with a semi-open cockpit allows simple capture of photography and film images.

Fumigation: The ELA 07 Agro autogyro is currently the only autogyro specifically designed for fumigation work. Key sugar cane enterprises in Central America use the 07 Agro to fumigate their fields.

Surveillance: Without doubt, the gyroplan is the ideal machine to perform surveillance work due to various reasons: Pleasant and easy to fly, superb visibility due to the tandem arrangement in an open-air cockpit, vast flexibility in airspeeds, large stability even in strong winds or flight through thermals.

  • Control of activities in urban areas
  • Surveillance of roads, power lines, pipelines and train tracks
  • Border and coast patrol
  • Fire spotting and fire-fighting coordination
  • Communications relay platform
  • Support of drug-traffic and smuggling control operations

How safe are Gyrocopters?

If you take care of the measures seriously have a proper training and consider all safety measurements right, flying gyroplanes is considerably safer than driving cars, and all of us do that. There are Federal Aviation Administraion certifications for flying Gyrocopters.

gyro     220px-La_Cierva_C-6

In the above images, the left one is “Cierva C.6” replica in Cuatro Vientos Air Museum, Madrid, Spain. The right image is of ELA 07. It is a latest design of conventional autogiros.