Drone Design #1 – Selecting an Airfoil

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Drone types
Rotary wings, quadcopters, for example, use the vertical thrust of the propellers to keep the drone in the air. A fixed-wing drone, however, relies on conventional wings to generate the required lift, just like an aeroplane as it travels through the air. In most cases, this setup eliminates any hovering capabilities, but it greatly increases efficiency, giving you much longer flight times.
Fixed wing drones come in many shapes & designs. Some look just like minified aeroplanes, with a propeller at the front, a fuselage in the middle with a long slender wing at both sides and a tail with vertical and horizontal flaps. Blended wings, on the other hand, look very futuristic, with fuselage and wings morphed into a single piece, without any tail at all.

Airfoil basics
Whichever design you go for, you’ll need to choose some kind of wing section, called an airfoil, to generate lift. In more advanced designs, the size or even the shape of this airfoil can change along the width of the wing, but it’s always a good starting point to do some basic hand calculations first.
Let’s start with naming the basic parts of an airfoil. At the front, you have the leading edge, at the back you have the trailing edge. They are connected via the upper surface, also called the suction surface, and the lower surface also called the pressure surface.
The chord is the straight line connecting the leading & trailing edge. The camber line, on the other hand, runs nicely in between the upper and lower surface, showing the centre line of the wing.
The angle of attack is the angle between the chord and the relative wind direction. The relative wind is not only composed of the wind vector but also the velocity of the drone itself.

Lift and drag
Essential to airfoils is how much lift & drag they generate. Lift is the vertical force perpendicular to the relative wind direction. Drag is the horizontal force along the wind direction. These vary in function of the angle of attack. There is a great website called http://www.airfoiltools.com/ that provides you with tons of data on different airfoils. To understand drag & lift curves, let’s illustrate this using a symmetric airfoil, where upper and lower surface are identical. An example is the NACA0012.
At zero angle of attack, the lift is zero as well. There is only drag. As soon as the airfoil rotates its nose into the air, creating a positive angle of attack, it starts generating lift. The bigger the angle of attack, the larger the lift. Beyond a certain critical angle of attack though, the lift will start to decrease again. This operating region beyond the critical angle of attack is called aerodynamic stall and is caused by a separating flow at the suction surface of the airfoil. Trying to pull up too fast during take-off, for example, is a typical scenario in which planes can go into the stall, lose lift and risk crashing.

Another effect of increasing the angle of attack is the increase in aerodynamic drag, which could cancel out the positive effect of lift. To find the sweet spot, we can use the lift-over-drag curve, which plots the ratio of lift over drag in function of the angle of attack. The NACA0012, in this case, reaches its maximum efficiency at an angle of attack of around 8°. At this point, the lift generated by the wing is 80 times bigger than the aerodynamic drag!
This is not the best you can get through. In contrast to symmetric wings, asymmetric wings sacrifice performance at negative angles of attack to generate more lift and less drag at positive angles of attack or even at zero degrees. With airfoil tools, you can easily compare two different airfoils, like the symmetric NACA0012 versus the asymmetric NACA6412. You’ll see that the NACA6412 peaks at a lift over drag ratio of over 140!
As you may have noticed in these curves, lift and drag are expressed as coefficients Cl and Cd, rather than the real lift and drag force. This makes it easier to compare different airfoils, irrespective of their size. The coefficients are calculated by dividing the lift or drag per width of the airfoil by the product of stagnation pressure and the chord length.

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27 Comments

  1. Ansh Nagpal on December 31, 2020 at 10:45 pm

    Really good Explanation!

  2. Saber Dokmak on December 31, 2020 at 10:48 pm

    Also I subscribed, you legend

  3. zhiwan omed on December 31, 2020 at 10:50 pm

    Please i need help how can i contact with you

  4. AC Drone Video on December 31, 2020 at 10:51 pm

    Hi Wouter, my name is Angelo Conte and I’m an aerospace engineer who has working on the designing of propeller for a multirotor. I would like to ask you a question: suppose that I want to design a propeller (10 inches of diameter and 4 for pitch). What approach do you can suggest me to analyze the blade of propeller near the hub in hovering condition, where the Reynolds number is very very low and the beta angle is about 30 degrees? I thought to use xfoil with viscous analysis but in that condition the convergence of solution is never reached. Thank you in advance and my compliments for your videos. Great job. Ciao

  5. Chris Drake on December 31, 2020 at 10:54 pm

    A new easy way is to grab the free Airfoil Tools add-in for Fusion360 – it asks you how big/fast/high/etc, and inserts the CFD-optimised shape you need directly. https://apps.autodesk.com/FUSION/en/Detail/Index?id=5447707798035545266&appLang=en&os=Win64

  6. Mike Ellertson on December 31, 2020 at 10:57 pm

    Excellent information. I found it very easy to understand. I appreciate that you provided the math. It really helped to understand the underlying formula so I can tinker with different hypothetical designs before deciding on a few prototypes. Thanks very much!

  7. Bilal Khan on December 31, 2020 at 10:58 pm

    hi, im an engineering student and i m making a drone whose function is to carry an attachment that will throw a ball to a particular place at some height , i searched about drones , and i got to know that copters that lift more weight are difficult and too costlier to develop, so my q is can i opt for fixed wing to perform the same operation, whats your opinion?

  8. Savio Christopher on December 31, 2020 at 11:00 pm

    i didn’t understand the statement you said about the shape of the sections of rudders, flaps, elevators. symmetrical or an asymmetrical aerofoil which is favorable.

  9. Imran Mani on December 31, 2020 at 11:00 pm

    Hello Buddy, You explained it very well.
    I’m an undergrad student and I’m supposed to design a fixed wing vtol with 2 hours flight time and 2 kg payload carrying capacity, so which airfoil you would suggest to achieve these milestones?

  10. Pilot 76 on December 31, 2020 at 11:00 pm

    Is it possible to create a drone for general purposes, while not having a special education, but having good knowledge in the field of aviation?

  11. none None on December 31, 2020 at 11:04 pm

    Do you know any hooks to teach designing airfoil? I need to learn to design airfoil wings but I have no knowledge. If you let me know what are the books I need to buy I thank you

  12. Mohit Joshi on December 31, 2020 at 11:08 pm

    Sir make more vedios

  13. Reeshad Muntasir on December 31, 2020 at 11:11 pm

    Why don’t Airlines’ use vortex generators to prevent drag, just like race cars & high performance cars use vortex generators to create downforce? Ok I just searched on google, that they do, after I actually typed the comment. I know i can delete it but I won’t.

  14. sphericalsphere on December 31, 2020 at 11:23 pm

    Really good video on the topic. Little tip: If you mention a website in the video, you should also link it somewhere!

  15. سبايسي مان on December 31, 2020 at 11:23 pm

    Iam frome sudan iwant learn about air but idont know english

  16. Stefan Raghavan on December 31, 2020 at 11:23 pm

    1/2*rho*v^2 is not the stagnation pressure. It’s the dynamic pressure.

  17. Vicky Singh on December 31, 2020 at 11:25 pm

    I study in 7th standard but I interested in mechanism of vehicles

  18. James Oluwadare on December 31, 2020 at 11:27 pm

    this is a good video. though I’m currently having a hard time selecting an airfoil for my fixed-wing drone. can you help me?

  19. robert brander on December 31, 2020 at 11:29 pm

    Great explanation , but use more Diagrams !

  20. Saber Dokmak on December 31, 2020 at 11:32 pm

    I just stumbled onto this video and I’m stunned how perfect it is. Good job 🙂

  21. Hurtless status on December 31, 2020 at 11:32 pm

    Hi sir
    I am an Aeronautical student doing my undergraduates .I am interested in designing drones but I don’t have any proper guidance of doing it done .Can you say me what to do exactly to learn to design and what are the things I need to mainly look after for getting it done .

    Waiting for your reply

  22. Bello Aliyu on December 31, 2020 at 11:37 pm

    This is amazing. Thanks for uploading this video.

  23. rai saro on December 31, 2020 at 11:39 pm

    I have to search in english because there isnt anything in spanish.

  24. Soufiane Boumkar on December 31, 2020 at 11:39 pm

    Thank you sir

  25. Madhu Kumar on December 31, 2020 at 11:43 pm

    Sir I am an aeronautical engineer. Which designing software do you prefer for me for today aeronautical field. I really learned a lot of things by your videos.

  26. Balaji Gunasekar on December 31, 2020 at 11:43 pm

    Hi Wouter

    Am a Mechanical Engineering Student planning to do my Undergraduate thesis On Unmanned Aerial Vehicle could you suggest some books and website where i can start from scratch to build a UAV and all the basic concepts in aerodynamics to understand in a much detailed manner.

    waiting for your reply

  27. Djsolstice on December 31, 2020 at 11:44 pm

    No axis labels on your charts. I have no idea what your charts mean. Please explain them in future.

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