Up, Away and Rudderless..

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Prandtl D in flight (photo: courtesy http://www.nasa.gov website)

On October 28, 2015, at the NASA’s Armstrong Flight Research Center, a boomerang shaped aircraft model( flying wing), remotely piloted, made a graceful flight after a bungee like launch. The 25 foot wingspan model was in the air for just about 1 minute and 33 seconds and made a soft landing on the dry lake bed.

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Prandtl -D with streamers showing the vortices(photo: courtesy http://www.nasa.gov website)

However, the short flight could forever change the way airplanes have been flying so far. Spearheading this effort, NASA Chief Scientist Al Bowers, used German Aerodynamicist’s 1933 theory of minimum induced drag and bending moment to come up with a design that could

  1. overcome adverse yaw without using a vertical fin or rudder.
  2. result in 11% reduction in total aircraft drag due to span load.
  3. result in 20-30% efficiency gain due to elimination of the rudder.
  4. resultant improvement of of 15.4% in propulsive efficiency.

Let us take a closer look at the component that matters most in flying – wings. Wings help birds and airplanes fly. How? By generating lift. And what factor decides the lift generated? Wing Loading!

In simple language, wing loading is the effort required by the wings to support the aircraft weight in flight. It is denoted by a ratio of the total aircraft weight(W) and the total area of the wings(S). Units used are N/sq.mtr  or  lbs/sq.ft.

wing loading

Higher wing loading means smaller wings, greater take off and landing speeds, higher stall speed, poor maneuvering, reduced drag etc.

Lower wing loading means larger wings, lower takeoff and landing speeds, lower stall speed, better maneuvering, increased drag etc.

The shape and size of the wing decides the wing loading. As of now, till today, almost all the aircraft wings are designed using the elliptical span load. The problem with elliptical span loaded wings is that they generate an “adverse yaw” while turning.

Adverse yaw

For example, if the airplane is turning right, it would increase the lift on the outer wing(left) and reduce the lift on the inner wing(right) resulting in a bank or partial roll. Now the wing that generates more lift, in this case left wing, also generates more induced drag at the wing tip resulting in a net force against the direction of the turn which is called adverse yaw. The vertical fin or the rudder helps counter this adverse yaw.

However, the rudder extracts its price by way of weight and drag.

Bell Spanload

Bell Shaped Span Load wing

Elliptical Spanload

Elliptical Span Load wing

 

Birds on the other hand, do not have a rudder but still turn effortlessly and even sharply. How do they do that? Bell span load wings! In a bell span load wing, the resultant force at the wingtip is twisted forward which results in an induced thrust instead of drag. Applying that to our air plane turning right, this induced thrust at the wing tip results in a net force in the direction of the turn creating a proverse yaw. So if you have to turn right, just increase the lift at the wing tip of the left wing and voila, the airplane would turn right without the need of a rudder!

That’s how birds fly and turn and like Al Bowers said “They never told us about it!”

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Albatross in flight

It is still a puzzle as to why aviation experts and airplane designers took almost 80 years to take cognizance of and put Ludwig Prandtl’s Bell span load theory into practice. However, the best part is yet to come. Al Bowers and Dave Berger, a NASA Armstrong aeronautical engineer have come up with an idea for Prandtl-M which is planned for flight on Mars in 2020.

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Prandtl-M (photo: courtesy http://www.nasa.gov website)

 

The plan for Prandtl M is to glide for about 10 minutes in the Martian atmosphere at about 2000 feet, overfly some of the proposed future landing sites for a manned mission to Mars and send back high resolution photographs of the landing sites.

Turns out that the bell shaped span load is the best solution for the adverse yaw problem without a rudder, reducing drag and getting much better efficiencies and explaining the mysteries of bird flight.

Contrails

 

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Aviation never ceases to amaze! One of the wonders that aviation offers is contrails(condensation trails). Yes, you must have seen them or continue to see them. Beautiful lines drawn in the sky when an airplane passes high above. Sometimes the lines remain for hours together or disappear after few minutes. What are these trails and how are they formed? Well, they are something similar to vapour formed when we exhale through the mouth on a cold winter morning. Contrary to the popular belief, contrails that we see are not the hot jet exhausts. Commercial jets typically fly at altitudes in excess of 25000 feet. Jet exhaust basically contains carbon di-oxide, water vapour, soot and unburned fuel. All these components are released at high temperature and pressure into the atmosphere. The ambient temperature at those altitudes is typically between -41 to -44 degrees C and the pressure is around 0.37atm.

contrails

The water vapour and some impurities in the exhaust like soot in extremely fine form act like condensation nuclei facilitating formation of a linear cloud. However, this isn’t always the case. The formation of contrails depends on a few factors. When the air at the altitude at which the aircraft is flying is wet and cold, the chances of contrail formation are more and they can stay around for a longer period of time. Sometimes the trails spread and stay around for longer time assuming different shapes resulting in what is known as “chemtrails”. Now chemtrails is a conspiracy theory but for now let us not venture in there. Not all aircraft flying in the same airspace create contrails. The factor that is in play here is also the type of engines. The aviation industry has gradually moved away from the turbojet engine to the turbofan engine. Turbofan engines are more efficient. They have  a higher bypass ratio which releases cool air around the hot exhaust gases through the core. A study by Ulrich Schumann has shown that such engines operating at higher propulsion efficiency reach conditions which result in higher relative humidity for the same ambient temperature resulting in formation of contrails.

AirCrossroads

 

To test this theory, two large commercial jets, a A340-300 and a Boeing 707 were flown together, wing to wing, at designated altitudes during ascent and descent. The A340 is powered by CFM56-5C4 high bypass ratio(6.8 – more efficient) engine whereas the Boeing 707 is powered by JT3D – 3B low bypass(1.4 – less efficient) engine. The ambient conditions and the contrail formation was observed from a research aircraft. It was found that the A340 created contrails in many more instances than the B707. Since early days of aviation, military planners were interested in the formation of contrails. If an aircraft is forming contrails and if it flies over enemy territory, it can easily give away itself and would thus help the enemy take counter measures. A scientist named H.Appleman devised a chart that could predict whether a jet plane would or would not produce a contrail. This chart named as the Appleman chart is still used widely across the world to predict the formation of condensation trails.

appleman chart

Another factor that affects the contrail formation is the distribution of wet and dry air in and around the clouds. Inside the cloud, the concentration of wet air is much higher than outside. However, this is not just limited to the clouds. The variation in wet and dry air can be found throughout the clear air. So a jet flying through wet air to is lot more likely to produce contrails than the one flying through dry air. This is the reason why a jet flying at a particular altitude leaves behind contrails while another one in the same vicinity does not. Sometimes the wet and cold air can co-exist within few feet of each other. Sometimes, rockets flying vertically up have been known to demonstrate this. as soon as it hits wet air, it can be seen tracing a contrail. As it enters dry air, the contrails disappear.

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Contrail left by the Solar Dynamics Observatory Rocket

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So next time you see a jet streaking by with a beautiful white trail, just take a few minutes out to savour the sight.

Arial Super Highways

Jetstream

Recently, British Airways passengers flying from New York to London were pleasantly surprised to arrive at their destination, one and half hours earlier than the scheduled time. No, the aircraft did not have more powerful engines neither did it take a shortcut. It was just lucky enough to ride a powerful jet stream! In spite of all the abuse heaped upon her, mother nature is generous enough to offer this gift to the aviation industry. Jet streams are fast flowing currents or streams of air usually found anywhere between 23000 ft to 52000 ft above sea level. A jet stream is usually thousands of kilometers long, few hundred kilometers wide and few kilometers thick. The most natural question would be how and why they occur. Well, they are formed when two masses of air with substantial temperature contrast meet. Now the air does not flow from the high temperature mass to the low temperature mass. Instead, it flows along the boundary of the air masses due to the Coriolis effect.

jetstream-110512 (1)There are two types of jet streams. The polar jets are the stronger ones found between altitudes of 23000 ft and 39000 ft. The subtropical jets are the weaker ones found between altitudes of 33000 ft and 52000 ft. The northern and southern hemispheres have their own polar and subtropical jets. The speed of the jet stream depends on the temperature gradient. Greater the temperature difference, greater is the speed. While the average speed is around 110mph, speeds up to 250mph have been recorded. The effects of the jet stream were noticed as early as 1883 when the Krakatoa erupted. The smoke and dust that rose up high into the atmosphere was carried a long way and was tracked over several years. In 1920s, a Japanese meteorologist, Wasaburo Oishi, used balloons to track these winds. The first man to fly solo around the world in 1933, American pilot, Wiley Post, noticed that sometimes his ground speed greatly exceeded his air speed. However, the real understanding of the jet streams is credited to WW II pilots who flew repeated sorties and found tail winds in excess of 100mph.

There is another interesting story about Sir Richard Branson when he and Per Lindstrand successfully attempted flying across the globe in a balloon. When flying across the Pacific, as luck would have it, they had lost two-thirds of the fuel but had another 7000 miles to cover. They had two options, resign to fate and wait to get drowned in the Pacific or try to get into a jet stream and cross the Pacific. Per Lindstrand was an experienced balloonist and they somehow managed to catch a jet stream. However as soon as they got into the Jetstream, the balloon which was more than 6 storeys high rocketed ahead horizontally with the capsule being pulled behind!! The airspeed shot up from 70mph to 230mph! They missed their target landing area, Los Angeles, by 4000 miles and landed up in the Arctic but survived. Also, mountaineers, trying to scale peaks like the Mt. Everest routinely encounter jet streams clocking more than 100mph. They have no option but to stay put in the tents.

The jet stream does affect the climate. In some regions it is known to have caused hurricanes while the Hawaiian Islands have mostly managed to avoid those due to jet streams. While the aviation sector has been using these to cut down on fuel and time, scientists are finding out ways to harness the potential wind energy to generate power. It is estimated that only 1 percent of the wind energy in the jet streams is sufficient to meet the world’s energy needs! However, the technology might take another 15-20 years to develop. The bleak side of the story is, the jet streams are  expected to get weaker as a result of global warming. Hope we learn from our past mistakes and stop messing around with mother nature so that we are able to take advantage of this wonderful phenomenon. Long live the jet stream!

Flight

Flight is freedom, flight is fancy, flight is fantasy, flight is grace, flight is poetry. The mere mention of the word “flight” brings up a torrent of emotions. Just observe an eagle soaring high up with wings spread out or an airliner taking off and beauty of flight sinks in. We have always harboured an unbridled fascination for flying. For us earth crawling creatures, flight denotes breaking free, letting yourself go, savouring the freedom and exploring the vertical dimension. Haven’t we all made countless paper planes in our childhood, some of us even do it now, and played with them hours together?a-soaring-bald-eagle-john-stoj

I am lucky enough to have a vast open farm field at the back of my house, rich in flora and fauna. It attracts birds of all kinds and I do get a sneak peek in their flying styles and habits. The best and the easiest way to understand flight is to observe birds. After all that was how the airplane was built. A kite(bird family) or an eagle soaring up above with its wings spread wide open not only exudes grace but also style and is in perfect harmony with the wind. Once it catches a thermal, it can stay aloft for hours together without bothering to flap its wings. It is completely serene and peaceful. Watching an eagle glide has a calming effect on me Blue-Heron-Fly-4272014-1024x682just like watching fish in a fish tank. When it dives to catch its prey, wings folded, beak out and eyes locked on its target, you cannot help but get awestruck by the speed, grace and power. Its nature’s ultimate ground attack machine. Flight of smaller birds is swift, agile and random. They can change direction, speed and altitude anytime. At all stages, their flight is effortless whereas larger birds need a significant effort to get airborne but once in the air, they are equally at ease. However, they have one thing in common, they are in complete sync with gravity and air. Another interesting aspect of a bird’s flight is the ability to stay still in the air when faced with strong headwind. Smaller birds usually flap their wings and manage to kind of hover while the larger birds just spread their wings wide open and stay still  as if they are jet takeoff2hanging from a string. Watching a bird soar or glide has a soothing effect because it instantly connects with our soul’s inherent desire to break free and fly. Maybe that is the reason why we spread out our arms, hold our head up and close our eyes in ecstasy when we are standing on a high windy cliff.

afterburner3Similarly, watching birds like cranes, swans or large passenger/military aircraft take off is absolute treat. The birds run a little distance on land or water to generate air speed and then slowly start gaining altitude. It feels like gravity is gradually letting them go. Similarly large jets build air speed over the runway, their wings arching up as the speed increases and finally lift off.  While the larger jets have grace in their flight, the smaller combat aircraft radiate power, swiftness and arrogance. Not only do they get effortlessly airborne within few hundred meters or runway, they also cock a snook at gravity by climbing 30000 feet in few seconds! A sure shot way of getting that adrenaline rush is to watch a fighter jet take off with full afterburners. With advanced flight controls and thrust vectoring, these aircraft have turned the conventional laws of flight upside down. Rockets and missiles jet1present another enjoyable aspect of flight. If you happen to watch a rocket lift off live or even on television, it doesn’t get more dramatic. The humongous thrust it builds up behind the smoke plumes, the gradual lift off and then the overarching trajectory on its way to space is nothing but heavenly.

But you don’t always need to look at birds and airplanes to appreciate flight. Simple objects like shuttle takeoffa boomerang that curves back to you or even a flying disc or a frisbee skimming through the air is pure joy to watch and play. The bottom line is, the phenomenon of flight continues to fascinate us and activities like hot air ballooning, parasailing, paragliding, gliding, sky diving and bungee jumping reinforce this fact.

I can’t help but sign off with lyrics from the song “Plane” by the band Jefferson Airplane:

I’m getting ready for a great leap forward, ready for a leap in the pool
Ready to touch the stars again, ready to go back to school
Puttin’ the pieces together, put on your wings and come with me and
Fly, fly, fly to the centre of the sky, let’s go flying….”