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All airfoils I have seen become narrow towards the trailing edge.

Is it still possible to create a forward vector force if the shape becomes wider again after a narrow middle section?

I read that the air flows around the airfoil and that that is part of the dynamic that creates the upward and forward force. Is this essential to create a forward/upward force and does the shape in the second drawing below prevent this circulation around the airfoil?

Background: I'm building a vehicle (a bicycle with partial fairings) with the goal of minimizing drag and maximizing the forward force that results from the "sailing effect". The second image below (the one that looks like a fish) is a cross sectional view of this vehicle. Imagine that the vehicle has been cut in half and now we are looking from the top onto the lower half of this cut up vehicle.

I'm trying to figure out whether it is possible to get some forward force given the tail you see in my second drawing. I am aware that an upward and a forward force is created as a result of the wind blowing at an airfoil at an angle. But what I care about is the forward vector of that force, not the upward force (like sailors do).

The first drawing shows a "standard" (as good as my drawing ability allows) symmetrical airfoil. The second drawing is the same as the first drawing but with a different tail. The blue arrows at the bottom left corners show the direction the wind is blowing from. Both drawings are cross sections.

enter image description here

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    $\begingroup$ Before answering your question, are you designing a sail or some vehicle that has no engine? Otherwise, if you have an engine or a propulsion system, let the engine gives you power for the motion and focus on minimizing drag. If you share some more details about the project, someone could provide better answers $\endgroup$
    – basics
    Commented Jan 20 at 14:22
  • $\begingroup$ Good idea. I have added the following description to my original question: I'm building a vehicle (a bicycle with partial fairings) with the goal of minimizing drag and maximizing the forward force that results from the "sailing effect". The second image below (the one that looks like a fish) is a cross sectional view of this vehicle. Imagine that the vehicle has been cut in half and now we are looking from the top onto the lower half. $\endgroup$
    – aehhhhmm
    Commented Jan 21 at 14:28

2 Answers 2

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The tapering shape is the most efficient, in terms of drag vs lift. But if you have a strong engine that can overcome a lot of drag then you can “make a barn door fly”.

Similarly, with a strong enough wind you can use a barn door as a sail. A specific shape is not needed, per se, but a poor shape will lead to poor performance. In equating a sail to an airfoil, the “lift” becomes the forward force and the “drag” becomes a sideways force for e.g. a beam-reach configuration.

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  • $\begingroup$ Thx for your reply. Does the upward force that makes the barn door fly also necessarily come along with a forward force? .... because the forward force is what I care about in the end. $\endgroup$
    – aehhhhmm
    Commented Jan 20 at 13:25
  • $\begingroup$ The forward force is called thrust, it comes from the engine. The wing provides an upwards force called lift and a backwards force called drag. $\endgroup$
    – Dale
    Commented Jan 20 at 14:18
  • $\begingroup$ Sails of boats so rely on being hit by wind at an angle which creates a sideways AND A FORWARD force. The sideways force is equivalent to the upward force of the plane (lift). I know that the first drawing in my post/question above creates a forward force if wind hits it at an angle (besides the upward force). But is a forward force also created in the case of the second drawing? Edit: This forward force is tiny compared to the engine thrust but I don't have an engine so it's relevant:) $\endgroup$
    – aehhhhmm
    Commented Jan 20 at 15:46
  • $\begingroup$ @aehhhhmm the force that pushes boats forward (when the wind is not coming from behind a boat) is provided by the water pushing on the keel. The wind pushes the boat in the direction of the wind (drag) and at a right angle to the wind (lift). The water pushes the boat mostly in the direction at right angle to the keel. Adding he lift force and the keel force give you a net force in the direction of the keel. $\endgroup$
    – g s
    Commented Jan 20 at 21:40
  • $\begingroup$ You can do a related experiment with a toy truck. The friction between the truck wheels and the floor works like the keel. You can provide a force with your finger to stand for lift. You'll have to imagine the direction of the sail that the lift force is at right angles to. Observe that if you push gently, the truck rolls forward or backward, not in the direction that you're pushing: friction cancels out the part of your push that's not in the direction that the wheels can roll. $\endgroup$
    – g s
    Commented Jan 20 at 21:45
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Anything that manages to deflect the airflow downward will generate lift. The sharp trailing edge is not necessary, but it helps to promote a smooth flow in the wake of the wing. Wake turbulence is wasted energy, requiring more thrust to keep the wing moving forward.

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  • $\begingroup$ @john-dotty Your answer gives some "hope" but what I want is not the lift component of the force but the forward component of it (like sailors do). I have not made this clear in my initial question and title. I have edited the title and added some specifics about the project under "background" in the question. Is the following conclusion correct? In my second image the airflow is deflected not only to the side (downwards if you think of the cross sectional drawings above as plain wings) but also to the rear. The latter creates a counter force to the front (forward). $\endgroup$
    – aehhhhmm
    Commented Jan 21 at 14:52

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