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Old Sep 24, 2014, 08:34 AM
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Portage MI
Joined Aug 2009
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Mini-HowTo
Vortex Generator Experimentation and Design Tips

Vortex Generators (VGs) can be used for various purposes such as changing stall characteristics, lowering takeoff and landing speeds and even reducing drag (typically on large transonic airliners). All of these effects result from the fact that VGs help to control and alter the boundary layer over a surface.

In my case, I wanted to improve the stall characteristics of one of my wings. The wing in question is a scratch build balsa wing that has a tendency to tip stall. To correct this problem, I designed VGs that are placed on the outboard 1/3rd of the wing. In depth details of the VG design is provided below. But first I will show you some results to prove that they work.

Fist I would like to show an image of a “clean” wing with no Vortex Generators. Figure 1 below shows the stalled clean wing fitted with tufts (aka. pieces of string). The tufts give us an idea of what the flow is doing on the surface of the wing. The image shows that the flow is very chaotic over the entire span of the wing. The result while flying is a tip stall that leads to the aircraft rolling over.


Figure 1: Clean Wing Stall

Now we can take a look at a wing stalling with Vortex Generators installed on the outboard 1/3rd span. As you can see from figure 2, the tufts behind the VGs are straight back while the rest of the tufts on the wing are again arranged in a chaotic fashion. The end result is that the wing has a much more controllable and gentle stall with no bad tip stall characteristics.


Figure 2: Wing Stall with Vortex Generators

Overall, the Vortex generators helped to mute bad stall characteristics and make the plane more controllable during a stall. The video below shows 4 stalls (2 with a clean wing and 2 with VGs). I have also provides some tips on designing your own Vortex Generators below.

Vortex Generator Experiment on RC Plane (1 min 48 sec)
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Old Sep 24, 2014, 08:34 AM
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Ryan2010's Avatar
Portage MI
Joined Aug 2009
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Tips for Designing Vortex Generators

In this section, I will provide you with all the equations I used to design the Vortex Generators that I installed on my plane. The math is not two involving and just requires some algebra. These equations do have their limitations, which I have mentioned below, but they will serve as a good estimate on designing your VGs.

1) Determine the shape of your Vortex Generators
There are many types of Vortex Generators being used on aircraft today. For simplicity, I made simple rectangular VGs for my aircraft. Below is an image containing various styles of VGs. Ultimately, you can make them in almost any shape you want.

2) Determine Reynolds Number of flow
Before designing your VGs you need to determine what Reynolds Number you will be operating at. For this you will have to approximate or know the stall speed of your aircraft. Equation 1 below shows how to calculate this value.

3) Determine length of VGs and their location along the chord of your wing
Next you will need to determine the length of the VGs and where along the chord of the wing the VGs will be placed. The length of your VGs should be around 5-8% of the chord length of your wing. This is just a value that I have found to work for the square VGs I made.

Now you can calculate the position of the VGs along the chord. The VGs should be placed just in front of the laminar to turbulent transition of the boundary layer on your wing. This transition point is located at approximately 16% back on the wing chord from the leading edge. Thus you will want to place the leading edge of your VGs at a length equal to (16% of the chord – length of VG) back from the leading edge of your wing. This will place the VGs just in front of the laminar to turbulent boundary layer transition.
4) Determine the height of your VGs
Next you will want to determine the height of your VGs. VGs work to control the boundary layer and thus they are most effective inside the boundary layer. On larger general aviation aircraft and airliners, VGs typically have a height 80% that of the laminar boundary layer right before the laminar to turbulent transition point on the wing. However, on model aircraft that will typically result in a VG with a height well less than 1/64th of an inch (my model has a boundary layer height of 0.00097 inches). This makes them very hard to manufacture and thus I made my VGs have a height of 1/8th of an inch to make them easier to handle. To calculate the height of the laminar boundary layer on your wing before the transition point you can use equation 2 below.

5) Calculate span wise spacing of VGs
The next step is to determine the spacing of the VGs along the span of the wing. To do this I derived an equation which determines the size of the vortex being generated by the VGs. This equation is derived from the lift equation and the equations governing vortex flow. The end result is equation 3 below. After determining the radius of the vortices being produced by the VGs, I recommend that you space the VGs at least two radiuses away from each other. I also recommend that VGs be placed at a 15 degree angle to the flow going over the wing. This will allow the VG to work effectively and produce the largest vortices. If you would like to experiment with different angles, remember to change the Coefficient of Lift (CL) for that angle.

I also recommend that VGs be placed on the outside 1/3rd to ˝ span of the wing. This will make it so that the inboard sections of the wing will stall first while the outboard section continue to have smooth airflow. This will lead to gentle stalls and will help to alleviate tip stalls.

6) Making and installing your VGs
The final step is to make the Vortex Generators and install them on your aircraft. To construct your VGs I recommend using some thin yet sturdy material. I made my VGs using some sturdy poster board. You could also make them from plastic folders or a similar material. If you want your VGs to be removable, I recommend gluing them to some scotch tape and then placing them on your aircraft.

To make installing the VGs easier, I recommend using a simple guide made from a piece of paper to help in aligning your Vortex Generators on the wing. Below I have included some pictures of my installed Vortex Generators to give you an idea of how they could look.





Limitations
These equations and tips have various limitations. The equations do not account for the varying velocity profile in the boundary layer when determining the size of the vortices. Some of the things I mentioned above are just things that I have found to work. Thus, if you decide to try making VGs following the steps I mentioned above and they do not work as expected, I recommend experimenting some to see what works for you.
Hope you enjoyed and happy experimenting!
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Last edited by Ryan2010; Sep 24, 2014 at 08:43 AM.
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Old Sep 24, 2014, 08:48 AM
yank and bank!!
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Nice job!
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Old Sep 24, 2014, 08:49 AM
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Nice work.

Azarr
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Old Sep 24, 2014, 08:56 AM
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Portage MI
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Thanks guys!
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Old Sep 24, 2014, 09:28 AM
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United States, GA, Eastman
Joined Mar 2014
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What did you make your VGs out of?
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Old Sep 24, 2014, 09:46 AM
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Portage MI
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Quote:
What did you make your VGs out of?
I made my VGs using some sturdy poster board. You could also make them from plastic folders or a similar material.
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Old Sep 24, 2014, 10:08 AM
Sokol
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United States, CA, Walnut Creek
Joined Jul 2013
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Ryan,
could you elaborate on the fact that the VGs are not parallel to the main longitudinal axis ? Is there a way to determine the angles ?

you mention 15 degrees.
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Old Sep 24, 2014, 10:13 AM
buyer of the farm
United States, FL, DeLand
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This looks like a fun project for my flying wing!
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Old Sep 24, 2014, 10:13 AM
MGA Aero Club Safety Officer
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United States, GA, Eastman
Joined Mar 2014
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Quote:
Originally Posted by Ryan2010 View Post
I made my VGs using some sturdy poster board. You could also make them from plastic folders or a similar material.
Had I read and not skimmed the post I would have seen that, thanks for replying.

Next question, if you have a symmetrical airfoil and you fly upside down a lot, would it be logical to put VGs on the top and bottom of the wing? I have an Edge 540 with aggressive stall issues, mostly inverted tip stalls. Would you say that, that would be worth playing with.?
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Old Sep 24, 2014, 10:28 AM
I fly 3-C Crash,Crunch,Crumble
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USA, NM, Clovis
Joined Feb 2010
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Looks like a good candidate for My Yak. It has a horrible tip stall. I am also interested in the underside of the wing for inverted flight. Thanks for the info Roy
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Old Sep 24, 2014, 10:29 AM
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Portage MI
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Quote:
Originally Posted by flyingdreamz View Post
Next question, if you have a symmetrical airfoil and you fly upside down a lot, would it be logical to put VGs on the top and bottom of the wing? I have an Edge 540 with aggressive stall issues, mostly inverted tip stalls. Would you say that, that would be worth playing with.?
If you are flying inverted allot, you could put them on the bottom of the wing. If you are doing 3D maneuvers with your Edge 540 it will help to smooth out your inverted harriers and other maneuvers that can stall the wing while inverted.
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Old Sep 24, 2014, 10:34 AM
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Portage MI
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Originally Posted by JureZ View Post
Ryan,
could you elaborate on the fact that the VGs are not parallel to the main longitudinal axis ? Is there a way to determine the angles ?

you mention 15 degrees.
Essentially, the VGs act like little wings protruding perpendicular from your wing. They need to create lift to create vorticity. Thus they must be at an angle and they will create lift (and vorticity) similar to a flat plate airfoil. The maximum angle of attack that a flat plate can be at before it stalls and loses lift (and vorticity decreases) is 15 degrees. Also, when I say "lift" it is lift in their reference frame of the VG (perpendicular to the wing). Thus they are not creating lift that adds to the lift of the wing. Lift and vorticity can be interchanged here because lift is a result of vorticity. You can't have one without the other.

That is the long answer. The short answer is if they were parallel to the flow they would not create a vortex and if they were perpendicular to the flow they would just create drag.
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Old Sep 24, 2014, 10:36 AM
Sokol
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United States, CA, Walnut Creek
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Thanks Ryan !
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Old Sep 24, 2014, 10:40 AM
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Portage MI
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You're welcome JureZ
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