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How wingtips use vortices to create lift.

Jeremy A. Davis  http://oas.uco.edu/96/T96/Jdavis.htm
Hefner Middle School • Oklahoma City, Okla.

Abstract

The problem investigated in this project is how to reduce the vortices around the wingtip in order to increase lift. The goal was to design a wingtip that totally eliminated the negative effects the vortex caused. The new design and wingtips that are used today were compared to the constant, a wing without wingtips.

The airplane was mounted inside the wind-tunnel. The computer was set up to run ten trials. After the tests the airplane was taken out, then the Monokote peeled off, the wingtips replaced, and the airplane remounted in the wind-tunnel. After the trials were done, the lift was averaged out for each of the wingtips.

The wingtip with the most lift was the Corkscrew wingtip with .136 lbs. per square inch (psi), followed by the Drooping wingtip with .128 psi. After that was the Leaning and the Hanging wingtip, which had the same lift at .126 psi. Next to last was the Standing wingtip with .119 psi. Last was the Straight wingtip with .109 psi.

This experiment showed that if the vortex is used correctly, it will produce more lift. Interestingly the most common tip which is added to a wing, the Standing, had the least lift. The others which are hardly used had more lift, and the Corkscrew disign had the most lift.

For further studies in this matter one might test the wingtips with the wings at different angles of attack. One might test drag, or test them at different velocities. One could also test the wingtips on different wing shapes.

Introduction

The problem being solved is how to reduce the vortices around the wingtip in order to create more lift. The reason for trying to find a wingtip design that has the most lift is because that would be the most efficient, energy saving design. A vortex occurs when the high pressure on the bottom of the wing curls around the wingtip into the low pressure on top of the wingtip. The vortex creates a problem in lift because it pushes down on the top of the wing when it curls around it causing it to lose lift. Another type of vortex is caused by angles. When air blows past a corner (for instance where the wing intersects with the fuselage) it spirals. This is because the air hits one side and bounces to the other and it just follows that pattern in a spiral. This causes a problem because when the air hits the top surface of the wing it pushes it down. Adding a wingtip blocks the vortex from pushing down on the wing. A problem with the wingtips used today is that they have an angle which causes another vortex. The goal of this experiment is to design a wingtip that will totally eliminate the negative effects on the lift the vortex causes. The Corkscrew wingtip was invented to solve that. It is named the Corkscrew because it is shaped like one. Four others were tested that are used today and were compared to a constant, a wing without wingtips.

Coming up with the Idea

On summer break the researcher bought a balsa wood P-51 Mustang model kit and built it in five days. At the time he was trying to come up with a science experiment to do. He went back to last year's results and found that the cause for some wings having less lift than others was because of the vortices they created. The researcher wanted, then, to find a way to eliminate or greatly reduce the effects of the vortex on lift. Wingtips would have to be added to a balsa wood airplane to accomplish this. The engineering department at Oklahoma Christian University of Science and Art (O.C.U.S.andA.) was contacted and Dr. James Cutbirth (the head of the engineering department) arranged that their wind-tunnel could be used. He was very helpful in giving advice and in showing how to operate the wind-tunnel and its computer.

Designing the Wingtips

The purpose was to create a wingtip that would eliminate the negative effects of the vortex. The author researched the causes of the vortex extensively. Then he got the idea to make a wingtip almost the same shape as the vortex itself, only larger. He then looked for existing wingtips that are used today so he could compare his wingtip s lift to existing ones.

Building the Airplane

Dr. Cutbirth said the wingspan on the model had to be thirteen inches or less to fit in the wind-tunnel. This meant the model that was built in the summer could not be used. So the reseacher ordered a Peanut-Scale P-51. First, the researcher built the wings, modifying the ends so just the wingtips could be changed while using the same fuselage and wings. That way, different results would be from the wingtips and not from using several different models. On the outside one-third of the wings the leading and trailing edges were moved closer to each other. On the wingtips there would be two bars that would become the leading and trailing edges to replace the ones on the wing that had been moved inward. The bars slid over the edges that were moved in. This is shown in Figure 1.

Next the reasearcher built the body. He modified the body by adding a block in the center of the fuselage that was strong enough to attach a metal bracket to, so he could mount the airplane in the wind-tunnel. He also left off the propeller, since it would block some of the airflow. The reasearcher glued everything together and covered it in Monokote. Monokote is a plastic film that shrinks and glues itself to the balsa wood when a heat-sealing tool is rubbed over it. He then attached the wingtips by sliding them on and wrapping the Monokote around them and sealing it on. It was very tight and held the wingtips firmly. After each test the author planned to peel the Monokote off, replace the wingtip, and recover it with new Monokote.

Trial Test

The researcher tested his airplane in the wind-tunnel to see if it would hold together. When he turned the wind-tunnel on, the wings dihedral (the angle between the fuselage and the wing) increased by three inches. The balsa wood in the wings was too flexible. Also, after changing three wingtips, the wings were totally destroyed.

Rebuild the Wings

The researcher made new wings, only this time he reinforced them with metal. In front and in back of the mounting block he inserted two .5cm metal rods to keep the wings from bending. He used flower arranging wire along the front and back edges of the wing so that when he took off the Monokote to put on the wingtips the wing would not get ripped up. The researcher also replaced the thin wood that made the airfoil shape of the wing with thicker wood.

Final Tests

The author went back to O.C.U.S.&A. after rebuilding the wing and tested the wingtips. Reinforcing the wing really helped. On his trial test the velocity was .0123 (about 10 feet per second) and the dihedral raised three inches. This time the velocity was set at .0220 (about 20 feet per second) and the dihedral did not change.

Hypotheses

The reseacher is testing six different wingtip designs. They are the Corkscrew, Drooping, Hanging, Standing, Leaning, and Straight. It is necessary to understand Bernoulli s airfoil principle as a background for these hypotheses. All around us is atmospheric pressure pushing equally in all directions. The faster a fluid (such as air) moves, the lower its pressure. The slower it moves, higher its pressure. An airfoil has a rounded top and a flat bottom. The air going over the top must go faster than that on the bottom because the air on top has a longer distance to travel. Therefore the air on the top has a lower pressure than the air on the bottom. This difference in pressure creates lift.

Hypothesis 1

 

The Corkscrew wingtip should have the most lift. It would have the most lift for three reasons. First, it directs Vortex A around and off the outside of the curve. (See Figure 2.) This keeps the vortex from pushing down on the wingtip and reducing lift. Second, it creates a vortex in the center of the Corkscrew. (See Figure 3.) As the air curves up, centrifugal force pushes the wingtip up. Third, the inner and outer vortices actually cause more lift. Vortex B has a shorter distance to travel, so Vortex A must go faster. This makes a higher atmospheric pressure inside the wingtip. Because the pressure is reduced on the top of the corkscrew, the high pressure on the inside pushes the wingtip up.

 

Hypothesis 2

The Drooping wingtip should have the second most lift. Vortex A causes the Drooping wingtip to have less lift than the Corkscrew. (See Figure 4.) As it curves around the end, it pushes down some on the wingtip. But Vortex A has less of an effect on the lift than it would on a straight wingtip. This is because, when it pushes on the wingtip, it is mostly pushing the wingtip in toward the fuselage due to the angle at which the wingtip is drooping. Also, because of the angle of the wingtip the vortex will roll off the end. Also the wingtip has a triangular shape, so it only has a small area on the very tip to push against.

The other vortex, Vortex B, is created by Angle 1. This vortex is a kind of turbulence, which slows the air down, thus causing a higher pressure under the wing. This creates more lift because the high pressure is pushing the wing up as the low pressure above the wing is pulling it up. This is like the effect of the two vortices on the Corkscrew. The Corkscrew has more lift, though, because it has more area that can create lift.

Hypothesis 3

After the Drooping wingtip will be the Leaning. The Leaning will have less lift than the Drooping because Vortex B that was pushing up on the bottom of the Drooping is now on the top of the Leaning. ( See Figure 5.) This slows the air on top of the wing and reduces lift.

There is another vortex which is created by Angle 1. It is made by the high pressure under the wing wanting to go up around the wing at Angle 1. It runs into the leaning vertical wingtip and pushes it up. But this upward pressure is canceled by the downward force created by Vortex B. The vortex then follows the leaning wingtip up and curves around the end.

Vortex A does the same thing in the Leaning as on the Drooping wingtip. The only difference is the air will slide down toward the wing instead of off the wingtip. This will cause the wing to have even less lift because the vortex is pushing down on a larger area.

Hypothesis 4

 

Next will be the Hanging wingtip. Although the wingtip stops the high pressure underneath the wing from curling around to the top, there is still a vortex (see A in Figure 6) created by the low pressure on top of the wing. The higher pressure above and to the outside of the low pressure forces the airflow into a circle. This slows down the airflow and reduces lift.

There is a vortex pushing the wing up from the bottom. (See B in Figure 6). This is caused by Angle 1 making the air swirl. It does not have any affect on the lift because it is canceled out by the downward force of Vortex A.

Hypothesis 5

 

The second to last wingtip will be the Standing. The Standing wingtip blocks the Vortex A from hitting the wing. (See Figure 7.) Vortex B pushes the wing down. Vortex B is caused by Angle 1 which breaks up the air flow and causes high pressure pushing down on the wing. In this case there is no force opposing the downward force so it decreases the lift more than on the Hanging wingtip.

Hypothesis 6

 

The last and the least lift should be the Straight wingtip. It does not have anything to block the vortex so Vortex A pushes down with no resistance to any upward force except lift. (See Figure 8.) The Straight has less lift than the Standing because the Vortex A on the Straight is more powerful than Vortex B on the Standing wingtip.

Experiment

  • Equipment

    • Balsa Wood
    • .5 cm Aluminum Rods
    • Flower Arranging Wire
    • Top Flight Monokote
    • Top Flight Heat Sealing Tool
    • Testors Extra Fast Drying Cement for Wood Models
    • Conair Supreme 1500 Hair-dryer
    • Comet P-51 Models
    • X-acto Knife
    • Subsonic Wind-tunnel with air pressure sensors, computer and analysis program
    • Mount for Airplane in Wind-tunnel

Procedure

For the actual tests the researcher went back to O.C.U.S.&A. First he attached the Corkscrew wingtip. He then took off the back panel of the wind-tunnel and mounted the airplane inside. The researcher then replaced the panel and secured it. After that he had to zero out the lift sensor so that the readings would have the same starting point each time. He set up the computer program to run ten trials. Then turned on the wind-tunnel s fan and set the velocity at 0.022 (approximately 20 feet per second). In each trial the computer took 1,000 samples per second for ten seconds. This was 10,000 data points per trial, which the computer averaged to give a score for each trial. The researcher took a record of each trial in his journal. He then removed the back panel again and took out the airplane. Then the researcher peeled off the Monokote, replaced the Corkscrew with the Drooping wingtip, recovered the wing, and remounted the airplane in the wind-tunnel. Then he repeated the process of zeroing out the sensors, setting up the computer, setting the wind-tunnel velocity at 0.022, and recording the data in the journal. He repeated this process exactly for each of the wingtips. After the trials were done, the reseacher averaged out the lift for each of the wingtips.

Discussion

When the project was first started, the reseacher was trying to find a way to eliminate the vortex, not use it to create more lift. After he designed the Corkscrew, he reasoned that it would create lift because it had a fast-moving vortex on top and a slower vortex inside. This made him realize, as the researcher worked on his hypotheses, that the placement and speed of the vortex would affect lift.

Results

The results came out exactly as the reseacher had expected. (See Table 1) The Straight wing had the least lift (.109 psi) because it did not have a wingtip to block the vortex. Next was the Standing wingtip (.119 psi). It had more lift than the Straight because the small vortex created by the Standing wingtip has less downward force than the large vortex created by the Straight wing. (See Figures 7 and 8 above.)

Table 1

Lift Generated (lbs. per square inch)
Trial Number 1 2 3 4 5 6 7 8 9 10 Avg.
Cork Screw .135 .136 .136 .136 .136 .136 .136 .136 .136 .136 .136
Drooping .127 .127 .127 .127 .128 .128 .128 .128 .128 .128 .128
Hanging .126 .126 .126 .127 .126 .126 .126 .126 .126 .126 .127
Leaning .126 .126 .126 .127 .126 .126 .126 .126 .127 .126 .126
Standing .118 .118 .118 .119 .118 .119 .118 .119 .119 .119 .119
Straight .109 .109 .109 .109 .110 .109 .109 .109 .110 .110 .109

The next wingtips, the Leaning and the Hanging, had the same amount of lift (.126 psi). On the Hanging, the high pressure from the vortex on top and the high pressure from the vortex on the bottom cancel each other out. (See Figure 6 above). On the Leaning, the upward force from the high pressure of the vortex on the bottom is reduced by the slant of the wingtip and is canceled by the high pressure from the small vortices on top. (See Figure 5 above.)

Though the Drooping wingtip created slightly more lift (.128 psi) than the Leaning and Hanging, the difference was not a major one. (See Table 1) It allows a larger vortex to form than the Hanging wingtip, but still keeps it under the wing. The droop of the wingtip reduces the size of the vortex at the end of the wingtip and reduces its downward force. (See Figure 4 above.)

The Corkscrew had the most lift (.136 psi). Its design acts as an airfoil, creating extra lift because of the fast-moving vortex on the outside and the slower moving vortex on the inside. (See Figure 3 above.) It also stops the downward force on the wing because the vortex is guided off the wing s back edge. (See Figure 2 above.)

Reducing Error

In the experiment the reseacher tried to eliminate possible errors. An error that could have occurred is that the pressure sensors in the wind-tunnel could have varied between tests. The reseacher avoided this problem by zeroing out the sensors. This was hard to do, and the closest he could get was + or - .002. If he did the tests again, he would want a system that could zero out the sensors perfectly.

Another possible way an error could have affected the results is if the velocity for each test was different. When the velocity is higher, the wing will have more lift because the air is going faster and reducing the pressure even more. Like with the sensors, the best the reseacher could get in controlling the velocity was + or - .002. If he did the tests again, he would run the velocity at a much higher speed. This would mean that changes of + or - .002 would not be as important.

A major problem the reseacher faced was getting the Monokote smooth on the wingtips so the airfoil would remain constant. This would make a large impact on the amount of lift generated. If the Monokote is not smooth, it disrupts the airflow and reduces lift. A way the reseacher could have stopped this problem is by practicing putting Monokote on the wingtips many times before the experiment. If he tested this again, he would allow more time to cover the wingtips carefully. The reseacher had little time to cover the wingtips when he ran the tests because he had so many wingtips to test. If he reran the tests, he would come with the first wingtip already attached to save time. He would also test fewer wingtips per session.

Further Study

For further studies in this matter the reseacher may test the wingtips with the wings at different angles of attack. He would do this to see what their reactions are in climbs, dives, and glides. He might also test drag to find the best compromise between their lift capabilities and speed characteristics. He could test them at different velocities to see which are the best for low speed planes and which are best for high speed ones. The reseacher could also test the wingtips on different wing shapes, such as high speed swept wings versus low speed straight wings. He may also test them in a bank to find heir stall speeds in a turn. If they made the stall speed higher in a turn they could make an uncontrollable spin and cause a crash.

Conclusion

All the hypotheses were correct. The wingtip with the most lift was the Corkscrew, followed by the Drooping. After that was the Leaning and the Hanging, which had the same lift. Next to last was the Standing and last was the Straight.

In doing this experiment the reseacher learned that the vortices have a definite effect on the lift of an airplane. If the vortex is used correctly, it will produce more lift. If not, it will reduce lift. In some ways the results were a little strange because the most common tip which is added to a wing, the Standing, had the least lift besides the Straight. The others which are hardly used had more lift and the Corkscrew, which is not used at all had the most lift. If you rank the results from most to the least lift, it would also be ranked least to most used.

One reason why this could be is due to the amount of drag a wingtip produces. If it produces too much drag, it is not very thrust efficient. If you lose in drag what you gain in lift, there is no point in adding the wingtip. Another reason may be the cost of manufacturing the wingtip. The more complex an airplane s parts are, the more the plane costs. If it is not cost efficient, you would not want to purchase it. A third possible explanation is that at different speeds different wingtips may produce different lift characteristics. For instance, the Standing wingtip is most commonly used on high speed aircraft. If the author tested these again at a higher velocity, the results might be different.

Acknowledgments

Three people that deserve thanks for helping out with on the science project. First, Linda Davis, who provided financial support and supplies. Woody Davis who also provided financial support and he gave advice on different aspects of the report. A very special thank you goes to Dr. James Cutbirth who lent his time and wind-tunnel so the project could be completed. He explained many important facts needed to be understood to form the hypotheses and conclusions.

Bibliography

  • Lopshire, Robert. A Beginner's Guide to Building and Flying Model Airplanes Harper and Row. 1967.
  • Smith, H. C. The Illustrated Guide to Aerodynamics. TAB 1992
  • Walker, Ormiston H. Experimenting with Air and Flight. Venture, 1989.
  • Corel, Corel Draw 5. Corel Corporation, 1994.
  • WordPerfect. WordPerfect, v. 6.1. Novell, Inc., 1994

 

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