Passes Swing Test, but Unstable in Flight

[Daddyisabar]

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".




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.

Back in the day, Bruce (Mariviski), the former President of CRASH in Denver Colorado, flew a LEM to win a NARAM competition. I briefly saw the model when I helped the club move his estate donations. I was an idiot newbie so I didn't examine the model for stability secrets. It was very light an flew on a D12. I saw the trophy and a cover photo on a Sport Rocketry Magazine. Maybe an old timer who knows more can chime in.

 

[jqavins]

How did I know I that comment was coming .

And from whom, I expect.

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.

Quite so. The OP experiment lends dubious support to the hypothesis that tubular legs for fins create vortices that remain stable on one side or the other and can cause instability or spin. And it's really very little or no support to the hypothesis that that's is responsible for Cygnus Probe's failure because, as you say, there are so many other aerodynamic atrocities in that model.

And Red Columbine's success provides only weak counterevidence regarding the vortex hypothesis* because the legs are not tubular. It's apples to oranges. And the OP experiment regarding Cygnus Probe is apples to outhouses.

* "Vortex Hypothesis" sounds like a great name for a bad sci-fi movie. Or a rocket.

 

[aerostadt]

Theoretical Fluid mechanics is complicated.

 

[BABAR]

Theoretical Fluid mechanics is complicated.

Not as complicated as Real Fluid Dynamics!

 

[aerostadt]

Not as complicated as Real Fluid Dynamics!

If all terms are included, it is just as bad.

 

[jqavins]

If all terms are included, it is just as bad.

But that's only possible if all terms are known. Which tey never are, if you look close enough.

 

[aerostadt]

But that's only possible if all terms are known. Which tey never are, if you look close enough.

Well, let's say use CFD with turbulence terms included. Admittedly, turbulent models are not perfect, but they are checked out to be reasonably valid. Some aerospace companies have gotten to the point where they are confident enough that they have dropped their experimental labs and rely solely on CFD.

 

[jqavins]

My comment was made with tongue part way in cheek. Knowing "all the terms"when you look arbitrarily close means knowing things like the wake that a bird left while flying near, or the rate of change of ambient temperature down to a hundredth of a degree per minute, and stuff like that. Which one never knows, and can't account for in the models or the lab tests, but which do affect real world results to some miniscule (and irrelevant) degree. Thus, BABAR's statement that, as monstrously complicated as theoretical fluid dynamics is, it's not as complicated as the real world results.

 

[Grog6]

The vortex shedding force is calculable , and oscillatory around the restoring force;you'd see the rocket yaw about the direction of travel, until it randomly changes direction after falling sideways to the d.o.t. Remember the suspension bridge?(tacoma narrows) That was vortex shedding.

 

[sr205347d]

The OP experiment lends dubious support to the hypothesis that tubular legs for fins create vortices that remain stable on one side or the other and can cause instability or spin.

I formed a hypothesis to answer @lakeroadster's question. My test supports it.

What is your hypothesis?

 

[jqavins]

I'm referring to your hypothesis. I don't believe that one test with a similar but not near identical configuration provides, at best, enough support of your hypothesis to make it worth a more rigorous experiments. Without such experiments it is less than a theory, but merely speculation with some weak but real support, which is more or less the definition of a hypothesis.

 

[aerostadt]

Vortex phenomena has a lot of aspects. Post #6 summarizes vortex shedding flow in a visual manner for a cylinder normal to the direction of flow. Alternate vortex shedding (similar to aeolian tones that can be heard from wires) occurs according to Schlicting "Boundary-Layer Theory" from a Reynolds number of about 100 to 10,000. (Vortex shedding frequency can be characterized by the non-dimensional Strouhal number versus Reynolds number). Note that the spin of the vortices (von Karmon vortex street) is opposite from the top as compared to the bottom. The frequency of shedding can be fairly high and the force on the structure will oscillate in opposite directions fast, too. I taught an engineering class in experimental methods using hot-wire aneometry years ago and we concentrated on this subject. In any case model rockets move pretty fast and without doing any calculations, I would suspect that after the rocket is a few feet up the Reynolds number is way beyond the 10,000 range. Getting to the high Reynolds number range the turbulence in the wake washes out the vortex shedding and it is really no longer there (as depicted in Post#6).

Now the cylinder-fin model has the fins at angle to the direction of the flow, so off the top of my head I have no idea what the vortex structure is (If any and if it is there what direction is at and as mentioned before the correpsonding force will be opposite.). In the case of an airplane wing some people talk about a starting vortex that roughly stays at the airport and the circulation on the wing is opposite in direction and stays with the plane creating lift. Note that in this case the vortex is steady and the corresponding lift force is steady, too.

 

[sr205347d]

I'm referring to your hypothesis. I don't believe that one test with a similar but not near identical configuration provides, at best, enough support of your hypothesis to make it worth a more rigorous experiments. Without such experiments it is less than a theory, but merely speculation with some weak but real support, which is more or less the definition of a hypothesis.

Here is another test:

If my hypothesis is correct, then strakes on both sides of each leg (kind of like the strakes on both sides of the F-18) would create a pair of counter-rotating vortices. There would be no net lateral force on a leg. The rocket should then fly straight and not tumble or spin.





Looks like this test further supports my hypothesis.

So, if @lakeroadster wanted his Cygnus Probe to fly straight, he could add strakes to the legs. I don't know how large they would need to be. Mine are likely overkill.

 

[BABAR]

Hard to test. I still think big hunking cylindrical fins or Cygnus Probe like lattice networks, while the flow disturbances may be correctly called vortices, have the main effect of distorting smooth flow (as opposed to standard fins or even absent fins), and this “disturbance of the Force” is what nullifies the base drag hack.

 

[lakeroadster]

Here is another test:

If my hypothesis is correct, then strakes on both sides of each leg (kind of like the strakes on both sides of the F-18) would create a pair of counter-rotating vortices. There would be no net lateral force on a leg. The rocket should then fly straight and not tumble or spin.

View attachment 587124View attachment 587125



Looks like this test further supports my hypothesis.

So, if @lakeroadster wanted his Cygnus Probe to fly straight, he could add strakes to the legs. I don't know how large they would need to be. Mine are likely overkill.

Sweet.

The thing about doing that on the Cygnus Probe is it's a semi-scale model, adding the strakes may mess that up... plus its got outriggers too.

I'll try it with the stretched body tube... and we'll go from there.

Did you do multiple flights with the strakes or did the lawn dart render the test bird inoperable?

 

[sr205347d]

Did you do multiple flights with the strakes or did the lawn dart render the test bird inoperable?

Three flights with the same outcome.

 

[sr205347d]

The thing about doing that on the Cygnus Probe is it's a semi-scale model, adding the strakes may mess that up... plus its got outriggers too.

Another potential fix would be to use tabs somewhere to induce a spin before the vortices form. Then the vortices should be forced to be symmetrical, and it should fly stable like my second attempt.