So what’s so special about these aerodynamic devices?
(First Published at Globalair in November 2007.)
Recently I attended the Annual Meeting and Convention of the National Business Aviation Association, which this year was held in Atlanta, GA. I had the pleasure of spending the a day at the associations static aircraft display at the Fulton County Airport, with a gentleman that is relatively new to aviation. While admiring the new aircraft being displayed by Dassault Falcon Jet, he asked me what the purpose and function of the new winglet feature on the Falcon 2000. I had to stop and dredge up old and nearly forgotten knowledge to explain why Dassault had decided to install these new features on their proven and venerable wing design. His question along with my long-winded explanation has thus spawned the subject of this month’s article.
Many of you have probably noticed how slowly our aviation world is being populated by more and more aircraft that are sporting winglets, tip-sails, raked tips, non planer tips, etc. Like my friend, you may also have been wondering why this is the case. Please let me try to explain.
A wings aspect ratio is one of the design factors that determines how fast, how far, and how much a wing can support aloft. Aspect ratio is determined by dividing the wingspan by its mean aerodynamic chord (MAC.) High aspect ratio wings are normally long and narrow, the ultimate rendition being those normally sported by high performance sailplanes. Low aspect ratio wings are normally short and wide, think F104 Starfighter.
Everything in aircraft design, as in life, is a trade-off. High aspect-ratio wings normally exhibit poor roll-rate, which can be attributed to both the lever-arm distance between the wing tip and the fuselage, and also due to the fact that a long wing has to overcome more forces to change its position, i.e. it’s inertia (the tendency of a wing to maintain its state of rest or uniform motion in a straight line. Additionally, long wings take up a lot of ramp and hangar space, and they also have much higher bending stresses and therefore must be have greater structural reinforcement than which is required by a shorter wing.
On the plus side, high aspect ratio wings do create considerably less induced drag, which allows an aircraft with this type of wing to reach its self supporting and lifting, flying speed at a much lower required engine power setting or level, i.e. less power required = less fuel burnt, or more load can be supported for a smaller amount of power required. Parasite drag is higher with a high aspect ratio wing and therefore long wings are not normally associated with high speeds.
Low aspect ratio wings are the aerodynamic inverse of the long skinny variety, borne out by the fact that they have much less parasite drag, and greater induced drag, i.e. this type of wing is generally capable of faster speeds at the expense of more power being required to reach and attain flying speed = more fuel burnt. Remember my reference to the F104 Starfighter in association with short, wide wings (stubby is the term that leaps into my mind.)
The concept of the winglet was contemplated and patented all of the way back in 1897 by Mr. Fred Lanchester, a failed heavier than air, flying machine inventor from England. He realized through visualization that a wing end-plate would better control wingtip vortices
A vortex is generated at an aircrafts wingtip due to the high pressure fast moving air that is beneath the wing, mixing and colliding with the slow and low pressure air that is above the wing having been created by the design curvature of the wings aerofoil (remember Mr. Bernouilli’s principle?)
With higher the speeds, the forces creating the vortices also increase. A violently collapsing and turbulent vortex will rob a wing of a large part of its efficiency, unless it is smoothed, reduced and/or channeled. Now this is the area of drag where the winglet device really shines. If designed properly, it will provide a much smoother area of transition for both types of air pressure to collide and mix, thereby reducing the vortex forces, reducing the vortex in size (maybe) and ultimately reduced the amount of drag being created by the vortex.
There are many types and designs of winglet systems that are in existence. Almost all of them achieve the following:
1. The aspect ratio is increased without increasing the overall wingspan of the subject aircraft.
2. Overall drag reduction.
3. Improved available lifting force.
4. Improved take-off performance.
5. Improved climb performance.
6. Range increase/better economy due to the aircrafts improved ability to climb higher, in less time for the same amount of energy.
7. Increased payload capability (sometimes.)
8. Improved stall characteristics.
9. Higher cruising altitudes are also attainable.
10. Lower airport noise.
Even though the concept of the winglet was patented before the Wright Brothers conquered heavier than air, powered flight at Kittyhawk in the Kill Devil Hills of North Carolina, the principle was not properly applied to aircraft design on a large scale until after world war two. The winglet principle was manifested in the production of aircraft that featured Non-Planer wingtips, i.e. the wing was angled up towards the tip into a polyhedral configuration, i.e. local dihedral (think of the Avions Jodel series of aircraft starting in 1946, designed and built by Frenchmen Edouard Joly and his son-in-law, Jean Delemontez.) The ultra-modern Airbus A350XWB also has this type of wing tip configuration.
The non-planer wingtip in conventional aircraft design circles was soon overtaken by the Raked Wingtip, meaning a wing tip that features a higher degree of sweep-back than the rest of the wing. The latest iterations from the Boeing line of commercial aircraft feature this type of performance enhancing, tip design. The blended winglet is probably the most common type of device seen today. This is effectively a wing extension that is attached perpendicular to the existing wing through a large radius and smooth chord transition structural fairing.
Now back to the Dassault Falcon 2000 series that now sports the Blended Winglets from Aviation Partners, Inc. Seattle, WA http://www.aviationpartners.com/ The Falcon 2000 derives the following performance improvements directly attributed to the new winglets:
• Reduced Fuel Consumption (5% at Mach 0.80; 7% at the Long Range Cruise setting.)
• Increased Range (up-to 260 NM.)
• The need for Step Climbing is either reduced or eliminated.
• Optimum Cruising Altitude is elevated by 1,500 feet.
• Higher Speeds attained for the same power settings.
Pretty impressive by any standards. Now I hope that I have managed to answer the question, “What’s so special about winglets?”