E-Racer Blended Winglet and Interference Drag CFD Analysis

leroy cook once said that every airplane is a compromise though no design is perfect cheryl dickey's eraser offers a unique balance of features making it exceptional this side-by-side seating canard configuration offers impressive cruising speeds at reasonable fuel consumption while having a fair amount of cap and space the fully retractable landing gear makes it the perfect design for my potential use case despite the efficiency i noticed something interesting while studying the plans the vertical stabilizers are attached directly to the wing tips with a sharp 90 degree angle which immediately reminded me of something from an aerodynamics course interference drag a 90 degree angle creates considerable drag on an aircraft so i wondered if the design could be modified to have a curved or blended intersection instead it did not take long to realize i was not the first person to think of this or even try the modification jack morrison built an e-racer and later decided to make the changes to his plane with the help of marcus eatlin mark presented the results at oshkosh in 2007 and found a curve of about 9 inches improved cruising speeds by about 10 knots that presentation is available on mark's website and linked below the improvements were encouraging but it left me wondering how much of a curve is ideal too little might lead to minimal drag reduction while a curve that is too large would be difficult to build and cut into the vertical stabilizer so much that i may not have a sufficiently large rudder making aircraft control difficult this was the start of my research bert routine the now famous air and spacecraft designer unveiled his very easy canard pusher to the world in 1975 also at the oshkosh air show according to the eaa the primary feature of a canard aircraft is moving the horizontal stabilizer from the tail to the nose this can theoretically improve efficiency but it creates several other problems an aircraft designer must address for a more detailed look at the advantages and disadvantages of carnard aircraft please see the bold method article linked in the description one of the drawbacks discussed was figuring out where to put the vertical stabilizer bert routine solved this with the ver easy by giving it two vertical stabilizers and mounting them to the tips of the main wings this was very effective but by attaching them directly with no curve or blending considerable drag was created routine did not originally intend to sell plans for his new plane but he changed his mind after seeing how interested people were in it routine eventually made several improvements to his design and began selling plans for the long easy which became even more popular cheryl dickey based his eraser on routine's long easy as such it has similarly attached vertical stabilizers around the same time bert routine was designing his very easy nasa engineer richard whitcomb was working on a project of his own by attaching a small vertical wing to the tip of a plane's main wing whitcomb found that efficiency and therefore speed and fuel economy could be improved after extensive research a boeing 707 was fitted with winglets and saw a 6.5 improvement in fuel burn during a series of 1979 and 1980 nasa test flights although routine and whitcomb's winglets may seem similar in concept mark zeitlin noted an important distinction where whitcomb's winglets were intended to reduce drag and increase lift to save fuel rutan and by extension dickey's winglets were simply the plane's vertical stabilizers they were placed on the wing tips since that was a good place to put them but that also meant that efficiency improvements were not the primary intention behind the design choice leaving room for improvement there are two main types of dragon aerodynamics induced in parasite real engineering made a video which concisely explained how winglets mitigate induced drag which is strongest at low air speeds by fighting the creation of wingtip vortices that is also linked in the description along with the aerodynamics textbook i originally learned from parasite drag on the other hand increases with airspeed in other words the faster an airplane flies the higher the parasite drag this type further divides into four categories one of them being interference drag before diving into what interference drag is the concept of boundary layers needs to be explained as a wing or almost any object moves through a fluid like air that fluid has to move out of the way for it the fluid closest to the object moves very slowly relative to it due to surface friction as distance from the object increases the air around it moves faster move far enough away and no obvious impact on the fluid can be observed one of the best examples of boundary layers can be seen by looking at rocks in a river the water very close to a rock flows relatively slowly while the water farther from the rock flows faster anywhere the speed of the water is influenced by the rock is part of the boundary layer returning to aircraft wings propellers fairings and other surfaces are designed to make the boundary layer of air behave in a specific way wings for instance accelerate the boundary layer on their upper surface and slow it down on the underside due to bernoulli's principle the pressure of the slower moving air under the wing increases the pressure and the pressure of the faster moving air on the upper surface decreases in this way the air both pushes and pulls the wing up creating a force we call lift by maximizing lift and minimizing drag or the force that tries to slow the plane down the lift to drag ratio is increased an improved lift to drag ratio results in higher aircraft efficiency with these concepts in mind consider the boundary layer on an eraser's wing this can be visualized with animated streamlines in a computational fluid dynamics program called simscale as expected the airstream splits to flow above and below the wing with the closest streams forming a boundary layer the vertical stabilizer also has a boundary layer but this time it forms on the sides of the surface since it is oriented vertically now let's look at the joint between these two components at this sharp 90 degree angle the wing and vertical stabilizer's boundary layers interact and disrupt each other you can see how some of the streams are pushed both to the side and up while others follow a more chaotic path this creates turbulence and high pressure which slows down the air and is the exact opposite of what we need to create lift so the result is a force that steals energy from the plane slowing it down by definition drag planes like the cessna 172 reduce interference drag with fairings such as these by rounding the edge it helps decrease the interactions between the boundary layer from the wing and the strut you also see them down here where the wheel strut attaches as well as here with the strut attaches to the body of the airplane now one place there is a very harsh 90 degree angle is where the wing intersects with the fuselage the reason they can't put a fairing here is because the door has to open and when this door opens it uh yeah it swings right under the wing so not really any way to put a fairing on that so lots of interference drag there theoretically smoothing the transition between an eraser wing and vertical stabilizer should fight this interference drag by giving the boundary layers a shallower angle and more space to interact less dramatically mark zeitlin tested this theory by modifying jack morrison's eraser after it had flown for some time with the original winglet intersection with the new nine inch internal radius blended winglets the plane flew almost exactly the same as before with no noticeable changes in handling the one difference that was recorded was the speed with the same amount of engine power as before the plane could fly faster significantly faster at about 75 normalized power an average line of best fit between multiple tests showed that the plane was originally capable of flying at about 86 knots to airspeed in level flight with the blended winglets this same power allowed for a true airspeed of about 197 knots this is an improvement of 11 knots or almost six percent with no increase in fuel burn after reading zeitlin and morrison's results i wanted to see how different curvatures would affect lift and drag generated by the wing a wind tunnel would be ideal but that would be extremely expensive and time consuming fortunately there is a more modern cheaper alternative computational fluid dynamics or cfd this uses computers to model the flow of air over a wing or any other object the first step was to create computer models of the airplane this was easier said than done but after a few months of learning how to use solidworks then using that knowledge to follow the eraser plans i had a digital version of the airplane the next step was to create several versions of the wings with varying degrees of curvature i created one with an internal radius of 3 inches another with 6 then 9 12 and 15. the 3 inch curve would likely provide little benefit while the 15 inch version cuts significantly into the vertical stabilizer making any further curve unsafe and therefore not worth testing with the original design this was six different wings to test the next step was to find a cfd tool i started with sim scale but the free version had a limited number of simulations which would be problematic since i wanted to test each wing multiple times further sim scale could not handle the entire airplane and each simulation failed as a result i decided another program would be necessary so i tried loading the plane into freecad which connects to open foam and open source free cfd program this too could not handle the whole airplane at this point i'd spent several more months just learning how to use two cfd tools with no simulation results to show for it after considerable thought though i had an idea instead of testing the whole plane i could try running only a single wing through the simulation this would be a significantly less complicated model making it far more likely to be successful simscale was able to handle this simplified test but the results between different wings varied wildly which i knew was not correct freecad on the other hand provided consistent results and had no limit on the number of simulation runs for the first round of simulations i wanted to test all six wings in a configuration very similar to jack morrison's this way i could use the cruise configuration figures that he and mark zetland had found i started by setting a cruising speed that was the same as the plane was able to achieve with the normal stock winglets and that was about 185 knots which after some weird shenanigans with unit conversions came out to about 208 miles per hour in freecad i set the angle of attack to about two degrees this probably was a bit too large but it was good enough for my test these were tested at a resolution of about one inch so the computer was doing calculations at about one inch intervals from there i calculated the lift and drag took pictures of those to document the distribution of the lift and drag on the wing and then recorded those results into a table for some reason peer review the program used for processing simulation results cut out the outer half of the wings so at this point we were just dealing with the outer third of the wing since i had only done the last two thirds of it but even so the results were very helpful because there were marked changes between the different degrees of curvature for example as the curve radius became larger generally the lift would increase the drag would decrease and the lift drag ratio would increase there was some variability but i was able to generate my own lines of best fit to filter out other variables such as my imperfect method of attempting to draw out a curve in solidworks between the wing and the winglet for the next round of tests i increased the resolution to just under an inch in hopes of obtaining more detailed results as expected a slightly finer surface just as it would for a real airplane resulted in reduced drag part of the simulation process is watching residuals which is essentially variability in the flow of air over the wing as the residuals get lower that indicates that the wing is stabilizing so that the results can be analyzed accurately if the residuals are extremely high this would indicate that the wing is still in a state of changing flow uh that being that the air currents are moving around still and haven't quite calmed down to where they would be in a nice stable cruise so the simulation is not successful until those residuals fall below a certain value this is important because on the next test i wanted to see what would happen if i tested the wing at a higher angle of attack perhaps close to landing or very close to a stall so i decided to go with 12 degrees at about 90 miles per hour since the plane was theoretically capable of flying that slow and 12 degrees sounded pretty reasonable even after running the simulation all night over eight hours the residuals never stabilized they kept fluctuating one extremely stress relieving feature of the free cad open foam paraview combination is that even if the results have not stabilized to parameters to indicate a successful test the results can still be viewed so i went ahead and did that and what i saw was surprising there were points where the boundary layer had completely stopped and in some places reversed this would indicate flow separation or where the boundary layer had detached from the wing this means that at 12 degrees angle of attack the wing on an eraser will stall which is very bad for canard aircraft fortunately the plane is designed so that you cannot push it to this point if it's loaded properly the canard will stall first the nose will lower and the angle of attack will not be able to make it to a point where a stall would happen even so it was fascinating to see that about a 12 degree angle of attack would result in a main wing stall after seeing this i reduced the angle of attack to about 8 degrees kept with the speed of around 90 miles per hour and went back to the more granular resolution of about 1 inch these results were also recorded with the numerous data points now in a spreadsheet i was able to generate graphs putting the internal radius of the winglet wing curvature on the horizontal axis and the change of the parameter i was looking to measure on the vertical axis we had viewable results starting with the two degree angle of attack there was some variability which again could have come down to just my imperfect method of doing the blending in solidworks though after calculating a line of best fit it was very clear that small increases in the curvature of the wing resulted in small increases in lift also as expected there was a law of diminishing returns meaning that the three inch curve resulted in a significantly larger change than say the 15 inch curve compared to the 12 inch curve now of course we have to look past the variability just from imperfect modeling methods drag was also reduced slowly now dividing the lift and the drag to get the lift to drag ratio these results were much tighter a three inch curve resulted in a lift drag ratio improvement of about five percent where the 15 inch curve resulted in an improvement of about 17 percent as before there is this law of diminishing returns moving on to the 8 degree angle of attack the lift was all over the place it was so spread out i couldn't even generate a line of best fit that seemed to indicate anything and this does make sense a curve is designed to reduce interference drag which is a part of parasite drag at 90 miles per hour and 8 degrees angle of attack induced drag is far more powerful than parasite meaning that the effects of the interference drag would be quite small drag on the other hand did decrease slightly with an increase in curvature at an 8 degree angle of attack but these changes were also relatively small the lift to drag ratio graph also supported this returning to the 2 degree angle of attack this was approximately where the plane should be flying in cruise flight so using the original cruise speed obtained by jack morrison at 75 normalized power and then comparing that to the lift to drag ratio improvements given by different curvatures i was able to calculate a ratio and a rough line that shows the expected improvement in cruise speed with varying degrees of curvature so these are the final results we could expect an improvement of about five knots on the cruising speed with a three inch curve increasing that to six inches would net about an eight knot improvement nine inches gives 11 knots and this was the benchmark obtained using jack morrison and mark zedlin's results 12 inches would give an expected improvement of about 13 and a half to 14 knots and 15 inches would give an improvement of about 16 knots to summarize cheryl dickey's eraser wing and winglet intersection was not originally intended to reduce drag and improve cruising speeds this was just simply where the vertical stabilizers were mounted even so by rounding or blending this intersection this cfd analysis would suggest that efficiency and cruising speed can be improved the degree of improvement does depend on the degree of curvature with a very small curve resulting in relatively little improvement where a larger curve would result in a greater improvement for aircraft builders looking to use these results please be very careful to not to use a curve that is too large 15 inches or more is going to significantly reduce the amount of rudder area available which could be very dangerous from an aircraft control standpoint nine inches worked very well for jack morrison and marcus eatlin and this curvature should result in a significant improvement in cruising speed 12 inches may also be worth trying but getting it to the 15 inch range is starting to get into territory that might not be worth the risk there is room to improve this study such as testing a wider range of wings like using one-inch intervals or using a more powerful cfd analysis with a more powerful computer but for the resources i did have available i'm very happy with these results i have line of best fit equations that can be used as a helpful aid to pick the degree of curvature that i will want to my own aircraft when i get to that point and it should result in a significant improvement in the plane's fuel economy and capabilities thank you so much for taking the time to watch and listen i hope you found this helpful if you're another aircraft builder and even if you're not building an airplane you found a new appreciation for the work that goes into designing them if you know someone who you think would enjoy this or find the results helpful please do share it and i hope you all have a very nice day i'd also like to thank mark zeitlin for his help answering my questions on his original study with jack morrison's eraser as well as rock larocca for answering many other questions related to the project the depth and detail of the study would not have been possible without their help

Channel: Walker Weathers

Views: 259,065

Keywords: airplane winglets purpose, eracer aircraft, aerodynamics simulation software, freecad openfoam, cfd external flow, cfd analysis, solidworks, paraview, blended winglets, interference drag aviation, homebuilt aircraft, experimental aircraft, aerodynamics, building an airplane

Id: 1REKYJGNJMQ

Length: 17min 52sec (1072 seconds)

Published: Tue Jul 26 2022