# Correcting OpenRocket's CP calculation for fin stall

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#### finspin

##### Member
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.

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3 fins or 4?

Sorry I did not read carefully through everything above. I skimmed, and read just enough to see there seem to be a number of incorrect assumptions being made that may result in seriously incorrect conclusions being drawn.

This problem is a lot more complex than it appears at first glance.

The stall angle depends on the aspect ratio of the fin, its shape, and Reynolds numbers, as well as the airfoil (and even mach numbers for that matter though that is NOT likely to apply for our rockets leaving our launch rails!). It even depends on the rate of change of the angle of attack. For these stubby low aspect ratio wings we call fins, the flow is highly three dimensional when the angle of attack deviates from zero. You can't use the 2D flow predictions to determine stall angle. The actual stall angle will be much greater. This is particularly accentuated due to the sweep of the leading edge on most rocket fins. The predominant mechanism for generating lift at very high angles of attack may well be the high velocity vortex sheet off the swept leading edge, rather than anything even vaguely resembling classical 2D flow.

Even oversimplifying to the 2D case, steady-state stall angle is a function of Reynolds number. Typically the higher the Reynolds number, the greater the stall angle of attack. Most data you will see on 2D stall angles is going to be for Reynolds numbers up around a million or so. At our speed off the launch rail for our scale rockets, the Reynolds number will be a lot lower; therefore the 2D steady-state stall angle will be lower.

To add even more to the fun, flow separation takes more than zero time. A low aspect ratio wing can potentially briefly generate very lift coefficients at high angles of attack way beyond its normal "stall" angle of attack, in those transient conditions. Low aspect ratio wings such as our rocket fins can easily exceed a Cl of 2, well beyond their maximum steady-state lift coefficient, though only briefly.

Even in a classic stall with a high aspect ratio unswept wing, the lift still doesn't go to zero as the wing's stall angle of attack is exceeded. It just reduces. The drag goes up a lot. Drag located behind the center of gravity can assist in stabilizing the rocket. The collection of fins generates a force vector... that is no longer near normal to the surface of any of the fins. You have to see what that resultant force vector tries to do to the orientation of the rocket.

Additionally, there is hysterisis in the behavior so the results depend on whether you are starting unstalled and transitioning into a stall, or starting stalled and transitioning into unstalled flow.

The flow may separate over one or more of the fins, but possibly not all. The flow may separate over part of a fin, but possibly not all of the fin (tip stall, root stall). The resultant composite force vector generated by the fins at any instant of time may not be directed towards or away from the axis of the rocket but may - and probably will - also contribute torque. Torque + yaw -> coning effects. Obviously the moments of inertia come into play.

The problem as a whole is not a simple one. Even CFD analysis is notoriously bad for most such analysis, even in much simpler cases, when the Reynolds numbers are not large (more error on the drag side than the lift side, from what I've seen). The Reynolds number won't be large for smaller rockets leaving a launch rail.

Hopefully this bit of rambling can give a bit of a direction to what affects to research.

Just a couple pictures:

Gerald

PS - Forgot to mention that acceleration alters the airfoil flow characteristics even in the 2D flow case. A patch of air passing the leading edge of a fin "sees" faster airspeed than did the patch which passed earlier and is now towards the trailing edge of that fin.

The stall angle depends on the aspect ratio of the fin, its shape, and Reynolds numbers, as well as the airfoil (and even mach numbers for that matter though that is NOT likely to apply for our rockets leaving our launch rails!). It even depends on the rate of change of the angle of attack. For these stubby low aspect ratio wings we call fins, the flow is highly three dimensional when the angle of attack deviates from zero. You can't use the 2D flow predictions to determine stall angle. The actual stall angle will be much greater. This is particularly accentuated due to the sweep of the leading edge on most rocket fins. The predominant mechanism for generating lift at very high angles of attack may well be the high velocity vortex sheet off the swept leading edge, rather than anything even vaguely resembling classical 2D flow.

Thanks for the reply, this is very useful to know. Are there any standard methods to estimate the true stall angle other than sticking it in a wind tunnel? Any textbooks you would recommend that go through an analytical treatment?

To add even more to the fun, flow separation takes more than zero time. A low aspect ratio wing can potentially briefly generate very lift coefficients at high angles of attack way beyond its normal "stall" angle of attack, in those transient conditions. Low aspect ratio wings such as our rocket fins can easily exceed a Cl of 2, well beyond their maximum steady-state lift coefficient, though only briefly.

Additionally, there is hysterisis in the behavior so the results depend on whether you are starting unstalled and transitioning into a stall, or starting stalled and transitioning into unstalled flow.

Correct me if I'm wrong, but I don't think this will help in our case, because our fins start out in stalled conditions. The rocket starts stationary with 20 mph crosswind, which gives us a 90 degree AOA, which decreases as the rocket accelerates, down to 17 degrees when the rocket leaves the launch rail.

Even oversimplifying to the 2D case, steady-state stall angle is a function of Reynolds number. Typically the higher the Reynolds number, the greater the stall angle of attack. Most data you will see on 2D stall angles is going to be for Reynolds numbers up around a million or so. At our speed off the launch rail for our scale rockets, the Reynolds number will be a lot lower; therefore the 2D steady-state stall angle will be lower.

We're taking this into account; with ~7 in chord length and 100 ft/s velocity, we have, after converting to SI units, DV/nu = (0.1778 m)(30.48 m/s)/(1.562x10^-5 m^2/s) ~= 3.5 x 10^5, which is the Reynolds number our 2D lift coefficient graph was measured for.

Even in a classic stall with a high aspect ratio unswept wing, the lift still doesn't go to zero as the wing's stall angle of attack is exceeded. It just reduces. The drag goes up a lot. Drag located behind the center of gravity can assist in stabilizing the rocket.

We're accounting for this part of it. I ran this calculation in the 2D case using the drag data for the same airfoil and found the effect to be negligible:
C_N = C_L cos(alpha) + C_D sin(alpha) = 0.7 cos(17) + 0.244 sin(17) = 0.74 (a 6% difference).
At the moment we're not banking on eliminating stall entirely, just getting that stalled lift coefficient to be enough.

The collection of fins generates a force vector... that is no longer near normal to the surface of any of the fins. You have to see what that resultant force vector tries to do to the orientation of the rocket.

The flow may separate over one or more of the fins, but possibly not all. The flow may separate over part of a fin, but possibly not all of the fin (tip stall, root stall). The resultant composite force vector generated by the fins at any instant of time may not be directed towards or away from the axis of the rocket but may - and probably will - also contribute torque. Torque + yaw -> coning effects. Obviously the moments of inertia come into play.

This sounds tough - are there any examples of people who have tried these kinds of calculations, some kind of example to follow?

PS - Forgot to mention that acceleration alters the airfoil flow characteristics even in the 2D flow case. A patch of air passing the leading edge of a fin "sees" faster airspeed than did the patch which passed earlier and is now towards the trailing edge of that fin.

Do you reckon this will help or hurt us, and are there ways to quantify this? Is it negligible?

The problem as a whole is not a simple one. Even CFD analysis is notoriously bad for most such analysis, even in much simpler cases, when the Reynolds numbers are not large (more error on the drag side than the lift side, from what I've seen). The Reynolds number won't be large for smaller rockets leaving a launch rail.

Hopefully this bit of rambling can give a bit of a direction to what affects to research.

What would you recommend we do to figure this problem out? We're currently planning on putting one fin or maybe the whole boattail assembly into a wind tunnel and testing it with threads on it to identify when stall happens.

One question I have is - should we even be worrying about this? It seems as if most people put it in OpenRocket, trust what it says at face value (even though it completely ignores the concept of fin stalls and assumes a linear lift slope), and then fly fine (or not?). But we'd want to have an idea of what wind speeds are safe.

So you're not using a longer rail to get you to a safe launch speed...why? That seems to be the simplest solution.

There are a couple of different but related things going on.

The fin stalling will reduce the corrective force it generates. This will slow the response of the rocket and that may or may not be a problem. (A fancy airfoil should generate more force than the flat plate usually assumed so it might be a wash.)

But the fin stall will have little impact on the moment arm of the fin so the CP/CG relationship will change very little. So the rocket is still stable.

In other words, the effects will be in the dynamic response. This could be part of the reason why I have never been able to duplicate the odd (but not unstable) flight dynamics of a couple of my rockets in any sim.

So you're not using a longer rail to get you to a safe launch speed...why? That seems to be the simplest solution.

Our launch rail is already 34 feet long

(A fancy airfoil should generate more force than the flat plate usually assumed so it might be a wash.)
The slope of the lift curves for our airfoil from experimental data matches OpenRocket's potential flow theory predictions reasonably well, so I don't think this is a major issue

But the fin stall will have little impact on the moment arm of the fin so the CP/CG relationship will change very little. So the rocket is still stable.

I ran calculations on this and it had a massive effect on the CP (I described in the OP but can show more detail on these if you are interested). The fins on a rocket are doing most of the work of shifting the CP back, so if they drop to 40% of their effectiveness the CP shifts forward a lot and we lose 2.6 calibers of stability. This is assuming the lift curve is based on the 2D airfoil results, which as another poster pointed out tends to underestimate the stall angle and maximum lift coefficient, however.

Part of this large effect on CP is that we're dealing with a very long rocket, so we have a lot of body lift at these angles of attack (see the article "What Barrowman Left Out" by Galejs: https://www.apogeerockets.com/education/downloads/Newsletter470_Large.pdf )

Use a wire and pully and bungee to counterbalance most of your weight. This will add effectively 1 g of thrust If you use a weight it can only fall at 1g acceleration rather than being a constant force. You'd need to persuade the LCO and RSO to agree to let you do this.

"A rule of thumb I like to use is that the Angle of Attack just off the Launch Rail should not exceed 12 deg. Most airfoils will go into a leveling off of Lift Coefficient (CL) at 12-15 deg Angle of Attack, and then the CL really falls off and the airfoil stalls. But it is pretty much a straight CL (and a straight CNalpha) with Angle of Attack up to 12-15 deg, with 12 deg the low (conservative) value to use as an Angle of Attack limit.

Note that the Fins can temporarily stall and Jet Damping (included in RASAero II) will keep the rocket pointy-end forward, or at least greatly slow the rotation of the rocket if it is unstable or marginally stable, and during that time period more propellant is being burned off and the CG moves forward. Again, all included in RASAero II.

I've proposed in the past an Angle of Attack limit when leaving the Launch Rail of 12 deg.

As others have noted, you can lower the angle of attack by flying in a lower wind, or by having a higher initial thrust to weight ratio to exit the Launch Rail with a higher velocity."

As was noted, the Angle of Attack for the Fin to stall will be a function of Reynolds number.

Basically, you go through a Reference like Abbott and Von Doenhoff (Fin Airfoil Data) and look for the Angle of Attack for stall for similar airfoils at different Reynolds numbers. Of course, this Fin Airfoil data is 2-D, as was also noted Fin Aspect Ratio and Fin Taper Ratio will also affect stall.

So basically you set an Angle of Attack limit. I've proposed an Angle of Attack limit leaving the Launch Rail of 12 deg.

Charles E. (Chuck) Rogers
Rogers Aeroscience

A bigger motor is always the solution!

"A rule of thumb I like to use is that the Angle of Attack just off the Launch Rail should not exceed 12 deg. Most airfoils will go into a leveling off of Lift Coefficient (CL) at 12-15 deg Angle of Attack, and then the CL really falls off and the airfoil stalls. But it is pretty much a straight CL (and a straight CNalpha) with Angle of Attack up to 12-15 deg, with 12 deg the low (conservative) value to use as an Angle of Attack limit.

Note that the Fins can temporarily stall and Jet Damping (included in RASAero II) will keep the rocket pointy-end forward, or at least greatly slow the rotation of the rocket if it is unstable or marginally stable, and during that time period more propellant is being burned off and the CG moves forward. Again, all included in RASAero II.

I've proposed in the past an Angle of Attack limit when leaving the Launch Rail of 12 deg.

As others have noted, you can lower the angle of attack by flying in a lower wind, or by having a higher initial thrust to weight ratio to exit the Launch Rail with a higher velocity."

As was noted, the Angle of Attack for the Fin to stall will be a function of Reynolds number.

Basically, you go through a Reference like Abbott and Von Doenhoff (Fin Airfoil Data) and look for the Angle of Attack for stall for similar airfoils at different Reynolds numbers. Of course, this Fin Airfoil data is 2-D, as was also noted Fin Aspect Ratio and Fin Taper Ratio will also affect stall.

So basically you set an Angle of Attack limit. I've proposed an Angle of Attack limit leaving the Launch Rail of 12 deg.

Charles E. (Chuck) Rogers
Rogers Aeroscience
The thing most people do not know is that at 50 ft off the ground, the windspeed is double the speed at normal person height. So if you have a 20 mph ground speed wind it's 40mph 50 ft up. Know what you are launching in to.......This is similar to the flow of fluid in a pipe where the ground is the pipe wall. ish........

"A rule of thumb I like to use is that the Angle of Attack just off the Launch Rail should not exceed 12 deg. Most airfoils will go into a leveling off of Lift Coefficient (CL) at 12-15 deg Angle of Attack, and then the CL really falls off and the airfoil stalls. But it is pretty much a straight CL (and a straight CNalpha) with Angle of Attack up to 12-15 deg, with 12 deg the low (conservative) value to use as an Angle of Attack limit.

Note that the Fins can temporarily stall and Jet Damping (included in RASAero II) will keep the rocket pointy-end forward, or at least greatly slow the rotation of the rocket if it is unstable or marginally stable, and during that time period more propellant is being burned off and the CG moves forward. Again, all included in RASAero II.

I've proposed in the past an Angle of Attack limit when leaving the Launch Rail of 12 deg.

As others have noted, you can lower the angle of attack by flying in a lower wind, or by having a higher initial thrust to weight ratio to exit the Launch Rail with a higher velocity."

As was noted, the Angle of Attack for the Fin to stall will be a function of Reynolds number.

Basically, you go through a Reference like Abbott and Von Doenhoff (Fin Airfoil Data) and look for the Angle of Attack for stall for similar airfoils at different Reynolds numbers. Of course, this Fin Airfoil data is 2-D, as was also noted Fin Aspect Ratio and Fin Taper Ratio will also affect stall.

So basically you set an Angle of Attack limit. I've proposed an Angle of Attack limit leaving the Launch Rail of 12 deg.

Charles E. (Chuck) Rogers
Rogers Aeroscience

Based on @G_T 's post above I'm starting to think this limitation isn't necessary. I found a paper ( https://www.scielo.br/j/jatm/a/JdnMCtH6R3PhTBZ69YNqNfd/?lang=en ) that used a wind tunnel to measure the C_L vs alpha for a NACA 0012 section with an aspect ratio of 1 and a Reynolds number of 3*10^5 (attached are the test specimen and the graph). They got a stall angle of 32 degrees, and our aspect ratio is even lower than this. I don't know how reputable the journal is though. Thoughts?

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If it were me, and of course it's not.... I'd take Chucks advice and then figure out how you will achieve it. You're going to have to decide on whose advice you trust rather than looking for someone to produce the advice you want to hear. The reason for wanting this seems to be so you can launch in higher winds. Which is a bad idea.

Norm

Based on @G_T 's post above I'm starting to think this limitation isn't necessary. I found a paper ( https://www.scielo.br/j/jatm/a/JdnMCtH6R3PhTBZ69YNqNfd/?lang=en ) that used a wind tunnel to measure the C_L vs alpha for a NACA 0012 section with an aspect ratio of 1 and a Reynolds number of 3*10^5 (attached are the test specimen and the graph). They got a stall angle of 32 degrees, and our aspect ratio is even lower than this. I don't know how reputable the journal is though. Thoughts?

Check the Fin Reynolds Number for when you exit the Launch Rail. Take the ratio of the Rocket Length to the Fin Average Chord, let's say it's 20:1. Then divide the Rocket Reynolds number (available in the RASAero II Aero Plots Output in the Table on the far right, or available in the RASAero II Run Test Output) by that number, and you have the Fin Reynolds Number. (The Fin Reynolds Number is calculated internal to the RASAero II software, only the Rocket Reynolds Number is printed out in the various RASAero II outputs.)

Check the Fin Reynolds Number, and then compare the Stall Angle of Attack for your airfoil at that Fin Reynolds number.

Just a note; "my rocket velocity exiting the Launch Rail is low, so my Fin Reynolds number is low, so I'm below the Fin Stall Angle of Attack, so I'm fine, because my velocity leaving the Launch Rail is low". Those with experience will immediately identify that "my velocity leaving the Launch Rail is low" is not a good thing, for many reasons.

Charles E. (Chuck) Rogers
Rogers Aeroscience

Check the Fin Reynolds Number for when you exit the Launch Rail. Take the ratio of the Rocket Length to the Fin Average Chord, let's say it's 20:1. Then divide the Rocket Reynolds number (available in the RASAero II Aero Plots Output in the Table on the far right, or available in the RASAero II Run Test Output) by that number, and you have the Fin Reynolds Number. (The Fin Reynolds Number is calculated internal to the RASAero II software, only the Rocket Reynolds Number is printed out in the various RASAero II outputs.)

Check the Fin Reynolds Number, and then compare the Stall Angle of Attack for your airfoil at that Fin Reynolds number.

This gives 8585035/23.23 ~= 3.7*10^5, which is the Reynolds number I calculated for the fin and for which these graphs have been for.

Just a note; "my rocket velocity exiting the Launch Rail is low, so my Fin Reynolds number is low, so I'm below the Fin Stall Angle of Attack, so I'm fine, because my velocity leaving the Launch Rail is low". Those with experience will immediately identify that "my velocity leaving the Launch Rail is low" is not a good thing, for many reasons.
I was under the impression from my research that a low Reynolds number lowers the stall angle rather than increasing it; am I mistaken?

We are quantifying fin stall right now and seeming to find it is not the concern we thought it would be, because our fin is not an infinitely extended 2D airfoil. We have taken into account wind-induced instability due to body lift effects. As for weathercocking, our rocket's moment of inertia is relatively high due to its length, however I'm checking the dynamic stability constants to try to quantify this (we're also going to check it via the Cambridge Rocketry Simulator). What are some of the other reasons?

I was under the impression from my research that a low Reynolds number lowers the stall angle rather than increasing it; am I mistaken?

For the Fin Reynolds Number range you are looking at, it's not that simple.

From the following paper from the AIAA Journal of Aircraft (Volume 55, Number 3, May 2018):

https://arc.aiaa.org/doi/10.2514/1.C034415
It has the following Figure (Titled Illustration highlighting conventional airfoil separation characteristics at different Reynolds number regimes below 10^6 [ My Edit; 1.0 x 10^6, it covers Reynolds Numbers from 10,000 to 1.0 x 10^6 ] )

Note how the Separation is different for each of the Reynolds Number ranges.

The paper has the following data (and predictions from a software/program, what is of interest in this discussion is the experimental data) for a NACA 0012 Airfoil:

Note that from the Experimental Data Points the lower Reynolds number (3 x 10^5, similar to your Fin Reynolds Number) has a Lower Angle of Attack for the drop-off in CL than for the higher Reynolds Number (1.0 x 10^6).

Note that both drop-offs in CL are around Angles of Attacks of 12 deg to 14 deg.

Most Dynamic Simulations use a linear CN versus Angle of Attack for the Fins (the slope CNalpha), equivalent to a Linear CL with Angle of Attack (the left-hand portion of the left-hand curves above below the point of the drop-off in CL). Then a Non-Linear (with Angle of Attack) Viscous CL for the Body is added.

Your Fin Lift hasn't died off at 12 deg to 14 deg Angle of Attack, but you will not be able to use a linear Fin CNalpha in your Dynamic Simulation, you will need to include the entire left-hand curve above (linear to the peak, drop-off, then another linear portion at a lower value with a lower slope). Obviously there could be some interesting dynamics during the Fin CL drop-off.

Check the Cambridge Rocket Simulator, I believe it uses a linear Fin CNalpha. Your dynamic simulation won't be valid once the Fin CL drop-off occurs.

Also, if you are thinking of having an Angle of Attack up to 30 deg, you'll need to look at your Body CP at a 30 deg Angle of Attack.

Looking at all of the above, seems like a lot of work. In fact your rocket can become a low Fin Reynolds Number, Fin Post CL-drop off research rocket.

Or ........

You can plan on a Fin Stall Angle of Attack of 12 deg to 15 deg. And have the Cambridge Rocket Simulator, and all of the other simulators (including RASAero II), create an accurate simulation because your Fin Lift is still linear with Angle of Attack, and you haven't had the Fin CL drop-off yet.

And have a Wind Limit based on an Angle of Attack of 12 deg to 15 deg. My conservative recommendation is the lower value, a WInd Limit based on an Angle of Attack of 12 deg.

<< We are quantifying fin stall right now and seeming to find it is not the concern we thought it would be, because our fin is not an infinitely extended 2D airfoil. We have taken into account wind-induced instability due to body lift effects. As for weathercocking, our rocket's moment of inertia is relatively high due to its length, however I'm checking the dynamic stability constants to try to quantify this (we're also going to check it via the Cambridge Rocketry Simulator). What are some of the other reasons? >>

See the above. Unless you model the Fin CL drop-off if you exceed an Angle of Attack of 12-15 deg, then you won't have an accurate Dynamic Simulation.

Charles E. (Chuck) Rogers
Rogers Aeroscience

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This is the question that I think is most pertinent. Fortunately, We have enough info to calculate an answer. At the end of the 34' rail, they're at 100 ft/s. Let's say they're losing 4 feet of rail length to button spacing, so the effective rail length is 30 feet (probably a little short but maybe not terrible). d = 0.5 a * t^2 = 30 ft and v = a * t = 100 ft/s gives us a = 167 ft/s^2 or T:W ~ 6.2. That's not unreasonable at least.

It sounds like planning for a 15 mph crosswind would really make this easier unless there's a really good reason to need to fly in 20 mph wind.

It sounds like @finspin is part of a group working on a liquid rocket. Liquids tend to be both very long, and relatively low thrust. I don't know where exactly they're planning on flying, but FAR and RRS are two sites that commonly host flights of amateur liquids. The wind in that part of the desert can exceed 20 mph on many days, so designing the rocket to be capable of safely flying in high winds would be a prudent measure to take to reduce the chance that they'd need to cancel a launch for wind.

All that said, I doubt that their group is planning on flying at FAR, since the site has a 60 foot long launch rail intended for liquids, which would be a very simple solution to this problem.

It sounds like @finspin is part of a group working on a liquid rocket. Liquids tend to be both very long, and relatively low thrust. I don't know where exactly they're planning on flying, but FAR and RRS are two sites that commonly host flights of amateur liquids. The wind in that part of the desert can exceed 20 mph on many days, so designing the rocket to be capable of safely flying in high winds would be a prudent measure to take to reduce the chance that they'd need to cancel a launch for wind.

All that said, I doubt that their group is planning on flying at FAR, since the site has a 60 foot long launch rail intended for liquids, which would be a very simple solution to this problem.
And I'd hope their anemometer is at the top of the tower to give a truer indication of what wind speed they are launching in to.

For the Fin Reynolds Number range you are looking at, it's not that simple.

From the following paper from the AIAA Journal of Aircraft (Volume 55, Number 3, May 2018):

https://arc.aiaa.org/doi/10.2514/1.C034415
It has the following Figure (Titled Illustration highlighting conventional airfoil separation characteristics at different Reynolds number regimes below 10^6 [ My Edit; 1.0 x 10^6, it covers Reynolds Numbers from 10,000 to 1.0 x 10^6 ] )

View attachment 518051

Note how the Separation is different for each of the Reynolds Number ranges.

The paper has the following data (and predictions from a software/program, what is of interest in this discussion is the experimental data) for a NACA 0012 Airfoil:

View attachment 518052

Note that from the Experimental Data Points the lower Reynolds number (3 x 10^5, similar to your Fin Reynolds Number) has a Lower Angle of Attack for the drop-off in CL than for the higher Reynolds Number (1.0 x 10^6).

Note that both drop-offs in CL are around Angles of Attacks of 12 deg to 14 deg.

Most Dynamic Simulations use a linear CN versus Angle of Attack for the Fins (the slope CNalpha), equivalent to a Linear CL with Angle of Attack (the left-hand portion of the left-hand curves above below the point of the drop-off in CL). Then a Non-Linear (with Angle of Attack) Viscous CL for the Body is added.

Your Fin Lift hasn't died off at 12 deg to 14 deg Angle of Attack, but you will not be able to use a linear Fin CNalpha in your Dynamic Simulation, you will need to include the entire left-hand curve above (linear to the peak, drop-off, then another linear portion at a lower value with a lower slope). Obviously there could be some interesting dynamics during the Fin CL drop-off.

Check the Cambridge Rocket Simulator, I believe it uses a linear Fin CNalpha. Your dynamic simulation won't be valid once the Fin CL drop-off occurs.

Also, if you are thinking of having an Angle of Attack up to 30 deg, you'll need to look at your Body CP at a 30 deg Angle of Attack.

Looking at all of the above, seems like a lot of work. In fact your rocket can become a low Fin Reynolds Number, Fin Post CL-drop off research rocket.

Or ........

You can plan on a Fin Stall Angle of Attack of 12 deg to 15 deg. And have the Cambridge Rocket Simulator, and all of the other simulators (including RASAero II), create an accurate simulation because your Fin Lift is still linear with Angle of Attack, and you haven't had the Fin CL drop-off yet.

And have a Wind Limit based on an Angle of Attack of 12 deg to 15 deg. My conservative recommendation is the lower value, a WInd Limit based on an Angle of Attack of 12 deg.

<< We are quantifying fin stall right now and seeming to find it is not the concern we thought it would be, because our fin is not an infinitely extended 2D airfoil. We have taken into account wind-induced instability due to body lift effects. As for weathercocking, our rocket's moment of inertia is relatively high due to its length, however I'm checking the dynamic stability constants to try to quantify this (we're also going to check it via the Cambridge Rocketry Simulator). What are some of the other reasons? >>

See the above. Unless you model the Fin CL drop-off if you exceed an Angle of Attack of 12-15 deg, then you won't have an accurate Dynamic Simulation.

Charles E. (Chuck) Rogers
Rogers Aeroscience
Hey Chuck, thanks for the reply. Deadlines are tight right now so I won't be able to give a proper response until next week when things calm down, but I wanted your thoughts on something specific; I don't think I've been clear enough. So in OP I was using your reasoning regarding the stall angle of attack of 12-15 degrees based on the 2D lift vs alpha graph (based in part on posts I've read from you and others on here such as a bobkrech); your posts sparked my concern about this when I noticed our angle of attack was 17 degrees.

For these stubby low aspect ratio wings we call fins, the flow is highly three dimensional when the angle of attack deviates from zero. You can't use the 2D flow predictions to determine stall angle. The actual stall angle will be much greater. This is particularly accentuated due to the sweep of the leading edge on most rocket fins. The predominant mechanism for generating lift at very high angles of attack may well be the high velocity vortex sheet off the swept leading edge, rather than anything even vaguely resembling classical 2D flow.
After reading this and searching online, I found experimental data from a 3D wind tunnel test on a low aspect fin that appears to validate G_T's point, posted here in this message (which also has the attached lift vs alpha graph):
Based on @G_T 's post above I'm starting to think this limitation isn't necessary. I found a paper ( https://www.scielo.br/j/jatm/a/JdnMCtH6R3PhTBZ69YNqNfd/?lang=en ) that used a wind tunnel to measure the C_L vs alpha for a NACA 0012 section with an aspect ratio of 1 and a Reynolds number of 3*10^5 (attached are the test specimen and the graph). They got a stall angle of 32 degrees, and our aspect ratio is even lower than this. I don't know how reputable the journal is though. Thoughts?
But what you are saying is based on 2D airfoils. G_T's basic point is that unlike in aircraft which tend to have high aspect ratio wings, we cannot use the 2D data you are presenting to predict low aspect ratio rocket fins. So 12-15 degrees is not the stall angle. Instead, it is closer to 32 degrees as the 3D wind tunnel tests show. Do you disagree with this reasoning, and if so why?

As for designing to lower winds, at this point that is what we are going to do since the competition rules limit the maximum stability to 6 calibers and making fins big enough to maintain stability off the rail results in exceeding this limit at high altitudes once the AOA goes to 0. That is, until they respond to our clarification on these rules. If we are allowed to instead demonstrate acceptable resistance to weathercocking, then we will design for stability up to 20 mph and if low-aspect-ratio fin stall truly is a problem (and we will do wind tunnel testing if there is uncertainty), we will implement it into the dynamic simulator as you are saying.

Hey Chuck, thanks for the reply. Deadlines are tight right now so I won't be able to give a proper response until next week when things calm down, but I wanted your thoughts on something specific; I don't think I've been clear enough. So in OP I was using your reasoning regarding the stall angle of attack of 12-15 degrees based on the 2D lift vs alpha graph (based in part on posts I've read from you and others on here such as a bobkrech); your posts sparked my concern about this when I noticed our angle of attack was 17 degrees.

After reading this and searching online, I found experimental data from a 3D wind tunnel test on a low aspect fin that appears to validate G_T's point, posted here in this message (which also has the attached lift vs alpha graph):

But what you are saying is based on 2D airfoils. G_T's basic point is that unlike in aircraft which tend to have high aspect ratio wings, we cannot use the 2D data you are presenting to predict low aspect ratio rocket fins. So 12-15 degrees is not the stall angle. Instead, it is closer to 32 degrees as the 3D wind tunnel tests show. Do you disagree with this reasoning, and if so why?

As for designing to lower winds, at this point that is what we are going to do since the competition rules limit the maximum stability to 6 calibers and making fins big enough to maintain stability off the rail results in exceeding this limit at high altitudes once the AOA goes to 0. That is, until they respond to our clarification on these rules. If we are allowed to instead demonstrate acceptable resistance to weathercocking, then we will design for stability up to 20 mph and if low-aspect-ratio fin stall truly is a problem (and we will do wind tunnel testing if there is uncertainty), we will implement it into the dynamic simulator as you are saying.
post the rules and their source. 6 Calibers for a hybrid is tight. The CG significantly moves due to the oxidiser tank position. Have you done the calculations I suggested???????????!
Mine goes from 15 calibers to 7 stability and frankly using calibers as a stability measurement on a long rocket is not neccessarily optimum.

Hey Chuck, thanks for the reply. Deadlines are tight right now so I won't be able to give a proper response until next week when things calm down, but I wanted your thoughts on something specific; I don't think I've been clear enough. So in OP I was using your reasoning regarding the stall angle of attack of 12-15 degrees based on the 2D lift vs alpha graph (based in part on posts I've read from you and others on here such as a bobkrech); your posts sparked my concern about this when I noticed our angle of attack was 17 degrees.

After reading this and searching online, I found experimental data from a 3D wind tunnel test on a low aspect fin that appears to validate G_T's point, posted here in this message (which also has the attached lift vs alpha graph):

But what you are saying is based on 2D airfoils. G_T's basic point is that unlike in aircraft which tend to have high aspect ratio wings, we cannot use the 2D data you are presenting to predict low aspect ratio rocket fins. So 12-15 degrees is not the stall angle. Instead, it is closer to 32 degrees as the 3D wind tunnel tests show. Do you disagree with this reasoning, and if so why?

As for designing to lower winds, at this point that is what we are going to do since the competition rules limit the maximum stability to 6 calibers and making fins big enough to maintain stability off the rail results in exceeding this limit at high altitudes once the AOA goes to 0. That is, until they respond to our clarification on these rules. If we are allowed to instead demonstrate acceptable resistance to weathercocking, then we will design for stability up to 20 mph and if low-aspect-ratio fin stall truly is a problem (and we will do wind tunnel testing if there is uncertainty), we will implement it into the dynamic simulator as you are saying.

Finspin:

The following document (NASA Contractor Report 4745) provides information on the effect of Wing (Fin) Planform, Wing (Fin) Sweep, Aspect Ratio, and Taper Ratio on Wing (Fin) Stall Angle of Attack. So this gives the effects of the 3-D Fin Planform relative to the 2-D Fin Airfoil.

As a side note, Polhamus, the author, is a famous aerodynamicist. The Polhamus Equation is used at Subsonic Mach Numbers to convert the Infinite Span 2-D Airfoil Lift Coefficient Slope with Angle of Attack to the Lift Coefficient Slope for Low Aspect Ratio Wings (Fins). The Barrowman Method (derivation of the Method here):

uses a different equation, an equation from Diederich (Equations 45-50 in the Barrowman Derivation above). The Polhamus Equation is much more widely used. Although interestingly, when you put the same variable values into the Diederich Equation and the Polhamus Equation you get exactly the same answer for the 2-D Infinite Span Airfoil CL converted to the 3-D Wing CL for a Low Aspect Ratio Wing. The Diederich Equation/Polhamus Equation is key to making the Barrowman Method work, as our rocket Fins are typically Low Aspect Ratio Fins.

When you scroll through the NASA Contractor Report 4745 document above for Straight Fins, for Swept Fins, for Delta Wing Fins; for some of the Low Aspect Ratios you do see some Fin Stall Angles of Attack of up to 25 deg to 30 deg due to Vortex Flow. But many of the 3-D Fin shapes have Stall Angles of Attack in the 10 deg to 15 deg range. Hence my simple rule applicable to all rockets of a Fin Stall Angle of Attack of 12 deg.

I'd go through the NASA Contractor Report 4745 document above and look for data for a Wing (Fin) shape similar to yours, and see if your Low Aspect Ratio Fin is one of the Low Aspect Ratio Fin Shapes where there is a substantial Vortex Flow effect and a higher Fin Stall Angle of Attack.

As Neutron95 noted:

<< All that said, I doubt that their group is planning on flying at FAR, since the site has a 60 foot long launch rail intended for liquids, which would be a very simple solution to this problem. >>

There is a reason that for liquids and low thrust hybrids that FAR uses a 60 ft long launch rail. If you can't up the thrust, you can at least lengthen the rail, to achieve the same goal of a higher velocity when exiting the launch rail.

Charles E. (Chuck) Rogers
Rogers Aeroscience

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post the rules and their source. 6 Calibers for a hybrid is tight. The CG significantly moves due to the oxidiser tank position. Have you done the calculations I suggested???????????!
Mine goes from 15 calibers to 7 stability and frankly using calibers as a stability measurement on a long rocket is not neccessarily optimum.
https://www.soundingrocket.org/uplo...est___evaluation_guide-2022_v1.2_20211001.pdf
I think it's wacky too. It's surprising they set the minimum off-the-rail speed to 100 ft/s, and the max wind to 20mph, because this gives 16.3 degrees (15.8 if you angle 6 degrees into the wind).

And yes we did those CG-shift calculations. If you use a .rse file instead of a .eng you can put a custom CG that OpenRocket will use. The CG shifts helps combat the over-6-caliber-stability issue; the shift is not nearly as extreme as yours.

https://www.soundingrocket.org/uplo...est___evaluation_guide-2022_v1.2_20211001.pdf
I think it's wacky too. It's surprising they set the minimum off-the-rail speed to 100 ft/s, and the max wind to 20mph, because this gives 16.3 degrees (15.8 if you angle 6 degrees into the wind).

And yes we did those CG-shift calculations. If you use a .rse file instead of a .eng you can put a custom CG that OpenRocket will use. The CG shifts helps combat the over-6-caliber-stability issue; the shift is not nearly as extreme as yours.
You need to read the rules better.

10.4. Over-Stability - Launch vehicles will not be "over-stable" during their ascent.
● A launch vehicle may be considered over-stable when it has a static margin significantly greater
than 2 body calibers (e.g., greater than 6 body calibers at liftoff.
● Over-stable rockets are particularly vulnerable to crosswind or wind shear effects, which often
occur in New Mexico.

It says MAY be overstable. Not IS overstable. For High L/D ratio rockets you can use a length ratio. ie CG to CP divided by the total length. You'd need to demonstrate what that is, but that gives you more wriggle room than 6. Mine had 15 at takeoff and was stable. 3.5m long 64mm dia.

MAY= might be, but we're not saying definitively
SHOULD=should do this
MUST/SHALL= compulsory to do this

Understand your legal definitions. But make sure you get it approved BEFORE turning up to meet the RSO......
Norm

You need to read the rules better.

10.4. Over-Stability - Launch vehicles will not be "over-stable" during their ascent.
● A launch vehicle may be considered over-stable when it has a static margin significantly greater
than 2 body calibers (e.g., greater than 6 body calibers at liftoff.
● Over-stable rockets are particularly vulnerable to crosswind or wind shear effects, which often
occur in New Mexico.

It says MAY be overstable. Not IS overstable. For High L/D ratio rockets you can use a length ratio. ie CG to CP divided by the total length. You'd need to demonstrate what that is, but that gives you more wriggle room than 6. Mine had 15 at takeoff and was stable. 3.5m long 64mm dia.

MAY= might be, but we're not saying definitively
SHOULD=should do this
MUST/SHALL= compulsory to do this

Understand your legal definitions. But make sure you get it approved BEFORE turning up to meet the RSO......
Norm
This is why we asked for clarification and are still awaiting a response. I'm aware of the length ratio alternative (quoted in this forum before as 10% of the length for adequate stability), but they do not mention it nor provide any rules for what values are acceptable wrt overstability.

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You need to present a case for your stability calculation based on the length and ask them to accept that. It says MAY in the rules so now you're in a grey area.
The stability is effectively the moment arm of an equal force applied to 2 dissimilar areas. The calibers of stability is just an easy way to represent that.
Make a case for it rather than asking for some partially worded clarification like this whole thread. Otherwise you'll still be playing email ping pong on the day. You need to know now who is making the decision.
Anyone know who to contact?

Simple solution . . . Add a Booster to make it faster on the rail . . . The IRIS Sounding Rocket is a good example . . .It uses a clustered Booster.

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