Passes Swing Test, but Unstable in Flight

[sr205347d]

For all oddroc scum:

@lakeroadster discovered a phenomenon with an oddroc where it passes a swing test, yet will not fly stable. https://www.rocketryforum.com/threads/lakeroadsters-cygnus-probe-ship.170421/post-2300588.

I hypothesized that the instability could be due to vortex formation on the sides of the cylindrical legs that produce lift on that side: https://www.rocketryforum.com/threads/lakeroadsters-cygnus-probe-ship.170421/post-2438883

To test the hypothesis, I built a rocket with body tubes for fins:



I named it Jack, as it resembles (sorta) this:



The theory is that at high speed (faster than a typical swing test) a vortex will form on one side, and only one side, of each cylindrical fin. If the vortices are symmetrical (all on the same side of each fin) the resultant lift forces will cause the rocket to spin about its long axis. If the vortices are asymmetric, the resultant forces will cause a pitch/yaw moment and the rocket will tumble.

After adding some nose weight to get it to pass a swing test, I flew it three times this morning before one of the "fins" broke off.

The first flight on an A8-3 showed straight flight initially, then a tumble, indicating asymmetric vortices:



On the second flight on an A8-3, it began a spin and remained stable as it did a lawn-dart landing, indicating symmetric vortices:



The third and last flight used a C6, and did some skywriting:



So, that seems to validate my theory of vortex lift on cylinders. This also explains why all of my Bellyfloppers do a corkscrew motion during boost.

 

[lakeroadster]

For all oddroc scum:

@lakeroadster discovered a phenomenon with an oddroc where it passes a swing test, yet will not fly stable. https://www.rocketryforum.com/threads/lakeroadsters-cygnus-probe-ship.170421/post-2300588.

I hypothesized that the instability could be due to vortex formation on the sides of the cylindrical legs that produce lift on that side: https://www.rocketryforum.com/threads/lakeroadsters-cygnus-probe-ship.170421/post-2438883

To test the hypothesis, I built a rocket with body tubes for fins:

View attachment 583992

I named it Jack, as it resembles (sorta) this:

View attachment 583995

The theory is that at high speed (faster than a typical swing test) a vortex will form on one side, and only one side, of each cylindrical fin. If the vortices are symmetrical (all on the same side of each fin) the resultant lift forces will cause the rocket to spin about its long axis. If the vortices are asymmetric, the resultant forces will cause a pitch/yaw moment and the rocket will tumble.

After adding some nose weight to get it to pass a swing test, I flew it three times this morning before one of the "fins" broke off.

The first flight on an A8-3 showed straight flight initially, then a tumble, indicating asymmetric vortices:



On the second flight on an A8-3, it began a spin and remained stable as it did a lawn-dart landing, indicating symmetric vortices:



The third and last flight used a C6, and did some skywriting:



So, that seems to validate my theory of vortex lift on cylinders. This also explains why all of my Bellyfloppers do a corkscrew motion during boost.

Interesting... but I'm not sure how that validates anything?

 

[sr205347d]

Interesting... but I'm not sure how that validates anything?

You wanted to know why.

As promised, I did another swing test. The outriggers appear to make the rocket more stable. The swing test is rock solid.

So why does the rocket tumble AoA at speed? Go figure?

This would seem to validate the reason why.

And, given the reason, I suspect that adding more nose weight wouldn't help.

 

[GlenP]

You are on the right track. Basically vortex shedding is an unsteady phenomenon, and speed is one of the things that determines that frequency and the randomness. You have demonstrated that effect with the different motors, but repeated launches even with the same motor may demonstrate significant differences as well. The vortex is a low-pressure region that acts as a suction when close to the surface, but when it sheds the pressure recovers and the suction is lost. So, you have these shedding asymmetric suction regions that help create unsteady side loads on the tubes. The difference in speed from a swing test to flight with a motor is probably significant enough to change the behavior of those unsteady vortices, as you surmised.

 

[sr205347d]

You are on the right track. Basically vortex shedding is an unsteady phenomenon, and speed is one of the things that determines that frequency and the randomness. You have demonstrated that effect with the different motors, but repeated launches even with the same motor may demonstrate significant differences as well. The vortex is a low-pressure region that acts as a suction when close to the surface, but when it sheds the pressure recovers and the suction is lost. So, you have these shedding asymmetric suction regions that help create unsteady side loads on the tubes. The difference in speed from a swing test to flight with a motor is probably significant enough to change the behavior of those unsteady vortices, as you surmised.

The first two launches both used A8 motors, and one tumbled while the other spun, due to the randomness of the vortex formation.

My guess is that in a swing test, the Reynolds number is too low for a vortex to form at all. The flow remains attached.

At higher speed, an initial vortex will form on one side of the cylinder. It seems, though, that the vortex will not shed if the cylinder is free to move towards the side with the vortex. Or maybe, since the flow is not perpendicular to the cylinder, it does not shed? Anyway, it seems like a steady phenomenon.

 

[Buckeye]

I think you are on the right track, here. Not sure what you mean by "one side of the cylinder", though. Depending on Re, and assuming the fin behaves like a right circular cylinder, the wake will be behind the entire cylinder or alternately oscillating/shedding from side to side.

 

[sr205347d]

As a pilot, I am familiar with unsteady flow, as many aircraft will experience buffeting as the AOA approaches stall.

And the textbook descriptions of flow over a cylinder all depict unsteady flow at higher Re. But they assume the flow is perpendicular to the cylinder.

In this case, and in the case of my Bellyfloppers, the flow angle is acute, and the observed effect is not unsteady.

Not all vortical flow is unsteady:

 

[sr205347d]

What I am saying is that with cylinders at an angle to the air flow, there is a side force generated due to vortex formation, and it is not unsteady.

 

[GlenP]

Not all vortical flow is unsteady:

View attachment 584014

That particular plane has chines up the sides to control the vortices. Without those vortex controls, at a high AoA condition like that the unsteady and asymmetric side forces on the nose can exceed the control forces available to the pilot from the tail (since the tail becomes less effective in the wake) and initiate a flat spin. When a vortex sheds on one side a new one forms on the other side, hence an unsteady side force.

 

[sr205347d]

What I am saying is that with cylinders at an angle to the air flow, there is a side force generated due to vortex formation, and it is not unsteady.

And it is enough force to destabilize a rocket that, by the swing test, should be stable.

 

[techrat]

What if you put "nosecones" on the ends of your tubular "fins" to reduce the vortices and smooth out the airflow? Those open ends are causing the disturbed airflow, making the rocket unstable. Balsa nosecones or SOMETHING in those tips will reduce the turbulance, and act like winglets do on aircraft.

 

[sr205347d]

What if you put "nosecones" on the ends of your tubular "fins" to reduce the vortices and smooth out the airflow? Those open ends are causing the disturbed airflow, making the rocket unstable. Balsa nosecones or SOMETHING in those tips will reduce the turbulance, and act like winglets do on aircraft.

The purpose of this was to validate my theory of why @lakeroadster’s oddroc was not stable.
Post in thread 'Lakeroadster's Cygnus Probe Ship'
https://www.rocketryforum.com/threads/lakeroadsters-cygnus-probe-ship.170421/post-2313620

I doubt that any device on the tip would make any difference.

 

[lakeroadster]

You wanted to know why.



This would seem to validate the reason why.

And, given the reason, I suspect that adding more nose weight wouldn't help.

I guess we'll have to see what happens at the next launch with the longer body tube. Based on what you've proposed, since the vortices will still be there, you feel the rocket will still be unstable, correct?

I'm of the opinion, as are others, that the boat tail makes the base drag hack not applicable for this rocket. Therefore the stretch makes the rocket stable and it should fly stable.

​

 

[sr205347d]

Based on what you've proposed, since the vortices will still be there, you feel the rocket will still be unstable, correct?

Yes, that would seem right.

But we are plowing new ground in model rocket science.

And, if by random motion, all vortices are on the same side of each leg, it will spin and be stable.

 

[BABAR]

Reviewing vids, not sure any of them were stable even under boost.

rationale: the rod or rail gets rocket both Up to speed where aerodynamic forces work, and gives it an initial velocity and directional vector, with generally zero angle of attack. inertia itself will likely keep the rocket “straight and true” for a short time/distance, say anywhere from 5 to 20 feet. At this point most rocket will experience a slight random off angle (maybe only 1 or 2 degrees.). Stable rockets will correct, unstable rockets will start to skywrite at this point. Thing is, with an A motor the thrust burns out during that 5-20 feet.



regarding ”vortexes” as the cause of the problem, my suspicion is that the base drag hack applicability requires near laminar flow. @lakeroadster ’s Cygnus Probe is an aerodynamic nightmare. Your tubular fins also likely disrupt the laminar flow signficantly (not sure I’d call them vortices, but maybe any deviation from laminar could be called a small vortex.). Yours also is not that “fat bottomed” a rocket.

interesting thought on swing tests, the velocities achieved on swing tests aren’t nearly those achieved by launched rockets. For well aligned fins, FASTER is usually better (one of the reasons rockets are more likely to have stable flights using longer rods or rails, the “exit velocity” where the rocket is “on its own” for directional control is usually higher as it has more distance (thus time) to accelerate.

for ”screwy” unconventional rocket fins, airflow is likely to rapidly change with different velocities, so perhaps the slower velocities such as a swing test on such rocket produce more well defined base drag, higher velocities produce more random or eccentric flow.

your personal cylindrical fins would be challenging to calculate In any sim program. Although I believe there are some good studies On airflow over cylinders, yours are attached at an angle which makes it “complicated squared” (maybe cubed?)

then again, the Estes Mars Lander and Outlander also had pretty crazy tail feathers, but at least they were sorta fin like.

 

[lakeroadster]

@lakeroadster ’s Cygnus Probe is is an aerodynamic nightmare.

This statement makes me so proud... ​

interesting thought on swing tests, the velocities achieved on swing tests aren’t nearly those achieved by launched rockets. For well aligned fins, FASTER is usually better (one of the reasons rockets are more likely to have stable flights using longer rods or rails, the “exit velocity” where the rocket is “on its own” for directional control is usually higher as it has more distance (thus time) to accelerate.

My last swing test was of the fuselage for my Estes Logo Rocket. I video taped the test and measured how long the string was. Running the numbers the rocket was traveling very close to 30 mph. That correlates well with the 30 mph "off the rod" speed I always shoot for in Open Rocket.​

​

If the rocket is fairly robust in design it can indeed be swing tested at the speed it leaves the launch rod. Another reason to build Hell-For-Stout. But I'm odd-roc scum and not your conventional lightweight rocket builder.​

​

... the Estes Mars Lander and Outlander also had pretty crazy tail feathers, but at least they were sorta fin like.

Good point, both the Estes Mars Lander and Outlander... have similar leg designs as the Cygnus Probe, and they are stable flyers. The folks at Estes sure have designed some truly awesome rockets.​

​

How about a scratch build Estes Mars Lander'ish odd-roc where the motor spool separates from the lander and comes down via parachute... but the lander itself utilizes a small drone chute for orientation and lands on it's feet.​

​

Or better yet... make it look like an Apollo Lunar Excursion Module.. hmmm... now that would be cool. But talk about "an aerodynamic nightmare".​

Something like this.... a 3-1/4" body


​

 

[Art Upton]

then again, the Estes Mars Lander and Outlander also had pretty crazy tail feathers, but at least they were sorta fin like.

They have covering on each side to make them a fin; not a cylinder

 

[lakeroadster]

They have covering on each side to make them a fin; not a cylinder

But they have round pads on the bottom, I believe that is what the "cylinder" comment is in reference to.

 

[Art Upton]

But they have round pads on the bottom, I believe that is what the "cylinder" comment is in reference to.

But the main flow over the legs is on a flat planar fin like item, not round.

 

[lakeroadster]

But the main flow over the legs is on a flat planar fin like item, not round.

Gotcha Art. This thread is pondering if there are funky aero effects imposed upon the rocket, due to pod style cylinders / flat discs... that's why they were mentioned.

 

[jqavins]

So, that seems to validate my theory of vortex lift on cylinders. This also explains why all of my Bellyfloppers do a corkscrew motion during boost.

Interesting test, and worthwhile evidence, but I think you're getting way ahead of yourself to say anything is validated. The fact that you got some trials unstable and one that spun seems to support your hypothesis, but is a long way from confirming it.

The next steps would be to run several (many) more trials with similar designs and to devise other experiments that may provide evidence either supporting or refuting the hypothesis. Of course, the best thing would be to put such an object into a wind tunnel with smoke trails to show what happens around the "fins".

Or better yet... make it look like an Apollo Lunar Excursion Module.. hmmm... now that would be cool. But talk about "an aerodynamic nightmare".​

View attachment 584938

I've seen a couple three LEMs launched at club launches. Never seen one that didn't tumble violently the moment it cleared the rod. I wouldn't even call them skywriters, as they didn't gain enough altitude to qualify.

 

[BABAR]

. Of course, the best thing would be to put such an object into a wind tunnel with smoke trails to show what happens around the "fins".

Would be interesting, I think would prove what ISN’T stable, but not so sure about what IS stable under real conditions. Part of my doubt stems from the rapidity of velocity changes a rocket experiences from pad to end of rod to coast phase to apogee. I wonder if even if each phase is individually stable (at least from as it leaves the rod) , the airflow between transitions might be ….. interesting, and something that would be hard to simulate even with the best wind tunnel. @lakeroadster put me in my place, I didn’t dream a swing test actually approached rod end spead.

 

[lakeroadster]

... @lakeroadster put me in my place, I didn’t dream a swing test actually approached rod end spead.

Naah... I didn't put you in your place. Merely blinded you.. with science... I didn't know until I did the video, and ran the numbers. It's all good.

 

[sr205347d]

But they have round pads on the bottom, I believe that is what the "cylinder" comment is in reference to.

I wasn’t talking about the feet.

 

[lakeroadster]

I wasn’t talking about the feet.

On a Mars Lander.... In your Opinion.... Why do they fly stable?

They have all the outrigger parts that you have been saying are creating "vortex formation on the sides of the cylindrical legs that produce lift".

Contrast and compare. I'm curious as to your thoughts on this.

 

[sr205347d]

On a Mars Lander.... In your Opinion.... Why do they fly stable?

They have all the outrigger parts that you have been saying are creating "vortex formation on the sides of the cylindrical legs that produce lift".

Contrast and compare. I'm curious as to your thoughts on this.

The Mars Lander doesn’t have cylindrical legs.

 

[lakeroadster]

The Mars Lander doesn’t have cylindrical legs.

Allow me to introduce you to The Red Columbine...

 

[BABAR]

I kinda think the stability from rockets like the Columbine, Sputnik, and @jadebox Akavish (or whatever that thing is) are kind of like sky rockets,


long “tails” that produce drag behind what is essentially a tractor motor.

I’m not sure there is much fancy aerodynamics going on, so shape doesn’t matter. Kind of @Daddyisabar ‘s playground .

 

[jqavins]

Allow me to introduce you to The Red Columbine...

View attachment 585524View attachment 585525View attachment 585526

Red Columbine's legs have flt sides, so they're more fin-like than actual tubes are. I'm not convinced on the cylinder-induced vortices hypothesis, but Red Columbine doesn't provide a truly apples-to-apples comparison.

 

[lakeroadster]

Red Columbine's legs have flt sides, so they're more fin-like than actual tubes are. I'm not convinced on the cylinder-induced vortices hypothesis, but Red Columbine doesn't provide a truly apples-to-apples comparison.

How did I know I that comment was coming .

I guess I can see having this discussion on the rocket that the OP did his tests on.

But that rocket is a far cry from the aero nightmare that is The Cygnus Probe.