Hey guys, I wanted to share something I've been doing and get your thoughts on it. We're already at the limits in terms of launch rail length, and there isn't much propulsion can do to increase off-the-rail speed (100 ft/s currently). We want to design the rocket to be stable in winds up to 20 mph. These conditions result in a 17 degree angle-of-attack (15.8 degrees if we angle the rod 6 degrees into the wind), which is well past the point of stall for our airfoil, which is about 8 degrees (NACA 0006, though we are probably going to thicken the fins a bunch). I found data online for the coefficient of lift versus angle of attack for a NACA 0009 for a Reynolds number similar to our flow (attached).

OpenRocket assumes the normal force coefficient is linear with angle of attack: C_N = C_Na * alpha

where C_Na is the normal force coefficient derivative, which it assumes to be equal to the coefficient of lift derivative (a good approximation even after stall, see http://www.aerospaceweb.org/question/aerodynamics/q0194.shtml ), which is approximately 2pi for thin airfoils (then it does some stuff to correct for the planform).

So what we can do is find the true C_L at our angle of attack, and divide that by OpenRocket's assumed value, 2pi * alpha, where alpha is in radians. This will give us a correction factor. Using the data from the chart, the true C_L is 0.4 of what OpenRocket assumes, meaning the fins lose 60% of the effectiveness. To correct OpenRocket's CP calculation, we use the component analysis tool to get all the pieces, and then set the angle of attack to 17. It gives us the CP and CNa for each part. The total CP is calculated as a weighted average of each individual CP, weighted by the CNa. After verifying we get the same results as the listed total CP, we now do the same calculation with 0.4 times the fin CNa. This gives us a CP 2.6 calibers higher than without stall, which is sure to be unstable unless the fin size was increased dramatically.

Now onto how we're trying to deal with this. First is increasing our stall angle by searching for a better airfoil; it's challenging to search for symmetric airfoils with a high angle of attack however (if you know any please share). Second is installing vortex generators, and testing it in a wind tunnel similar to this video

to find the stall angle. Even if it's still stalling, it can reduce the instability a lot by virtue of the max lift coefficient being greater, maybe the true C_L being 0.75 times what is expected, at which point simply making the fins bigger could be workable.

But an 8 degree stall angle isn't very much, a lot of rockets seem to reach numbers like 11-13 degrees, so it's very surprising this is a little talked about issue and seemingly not encountered. One thing to consider is that as the rocket speeds up, the angle-of-attack will decrease. If this happens fast enough, stall will be ended before the rocket gets a chance to rotate sideways due to the instability. OpenRocket shows we get below the stall angle ~0.5 seconds off the rail, and even setting it up with it the predicted stalled-CP, is pretty slow to actually rotate to the point where the angle of attack goes up and sideways. This could explain why such rockets don't end up going sideways even though they stall and are briefly unstable, but no one knows it. Regardless, this is quite a risky theory, we intend to test it a lot more and possibly just accept a lower wind speed rating. I'm probably getting some things pretty wrong, so I'm curious on your guys' thoughts on this.

OpenRocket assumes the normal force coefficient is linear with angle of attack: C_N = C_Na * alpha

where C_Na is the normal force coefficient derivative, which it assumes to be equal to the coefficient of lift derivative (a good approximation even after stall, see http://www.aerospaceweb.org/question/aerodynamics/q0194.shtml ), which is approximately 2pi for thin airfoils (then it does some stuff to correct for the planform).

So what we can do is find the true C_L at our angle of attack, and divide that by OpenRocket's assumed value, 2pi * alpha, where alpha is in radians. This will give us a correction factor. Using the data from the chart, the true C_L is 0.4 of what OpenRocket assumes, meaning the fins lose 60% of the effectiveness. To correct OpenRocket's CP calculation, we use the component analysis tool to get all the pieces, and then set the angle of attack to 17. It gives us the CP and CNa for each part. The total CP is calculated as a weighted average of each individual CP, weighted by the CNa. After verifying we get the same results as the listed total CP, we now do the same calculation with 0.4 times the fin CNa. This gives us a CP 2.6 calibers higher than without stall, which is sure to be unstable unless the fin size was increased dramatically.

Now onto how we're trying to deal with this. First is increasing our stall angle by searching for a better airfoil; it's challenging to search for symmetric airfoils with a high angle of attack however (if you know any please share). Second is installing vortex generators, and testing it in a wind tunnel similar to this video

to find the stall angle. Even if it's still stalling, it can reduce the instability a lot by virtue of the max lift coefficient being greater, maybe the true C_L being 0.75 times what is expected, at which point simply making the fins bigger could be workable.

But an 8 degree stall angle isn't very much, a lot of rockets seem to reach numbers like 11-13 degrees, so it's very surprising this is a little talked about issue and seemingly not encountered. One thing to consider is that as the rocket speeds up, the angle-of-attack will decrease. If this happens fast enough, stall will be ended before the rocket gets a chance to rotate sideways due to the instability. OpenRocket shows we get below the stall angle ~0.5 seconds off the rail, and even setting it up with it the predicted stalled-CP, is pretty slow to actually rotate to the point where the angle of attack goes up and sideways. This could explain why such rockets don't end up going sideways even though they stall and are briefly unstable, but no one knows it. Regardless, this is quite a risky theory, we intend to test it a lot more and possibly just accept a lower wind speed rating. I'm probably getting some things pretty wrong, so I'm curious on your guys' thoughts on this.