Discussion in 'Low Power Rocketry (LPR)' started by bikefxr, Feb 17, 2019.
Yes we launched on the same day same angle same weather conditions
The square pointy bit broke the air up before it hit the square flat bit?
I believe if they were all rounded ( OP points out the difficulty with identical rounding ) your instincts would have been pretty close.
Hey, don't get all technical with me.
Do you really think round fins would actually *invert* the standings?
I really am curious to understand these results. Short of a wind tunnel test I suspect we'll be left to speculate.
For #10 that really is my best guess.
And I don't think it would invert them precisely, just reorder them to match what the collective experience of these forums suggest.
In any case, cool experiment and good science!
I have the pattern for the fin if someone wants to try....all the fins had an area of 4.09 square inches
Ah, I was wondering about that too. Cool experiment for sure.
Very cool, and a very unintuitive result. I wonder if some further fluid modeling / testing might give an answer? I'd also be interested in a deeper statistical dive. Extreme spread, standard deviation.
My guess is the pointy part is probably right inside a boundary / laminar flow layer, although I have no idea what that would mean for the turbulent air on the forward-swept area vs "clean" air on the outer, rearward-swept area.
Like Neil, I'm kind of puzzling over the counterintuitive results. Were all of the fin cans the same weight? Did they all fly straight up with no weathercocking?
You could have made some serious money on bets from the crowd on what we thought would be fastest/highest. I would have picked #9.
I would have picked "anything other than #10".
I would have bet the farm on #1 and lost big.
My competition training always told me that eliptical (and airfoiled) fins where always the best shape for altitude.
The rocket weighed 4pounds 3oz and they all flew fairly straight
Did you try to sim the various fin patterns in OpenRocket or RockSim to see how they compared to your actual results?
I did not do that .. just built then flew
How about uncertainities for the experiment? Winds aloft, motor thrust deviation, wadding mass, and launch rod angle from flight to flight? Most experiments even simple ones have uncertain errors associated with them. I suspect of the uncertainties that the motor thrust from flight to flight being the biggest source of error.
1st let me thank you for posting this thread. Very interesting!
if your are willing, post up the dimensional data for the rocket... and the various fin designs. I'd be glad to run the Open Rocket simulations. If you'd rather, PM the data to me.
On a C6-5? That's like a 1:4 thrust-to-weight.
The second-best altitude result, the clipped delta with rounded corners, seems likely. What was the variance in the three trial sets? I wonder if you might have outliers in your data for #1 and #10?
His fins weren’t even in compressible flow at Mach 0.3 which is more troubling to understand at such low velocities. A wind tunnel or CFD would get you the drag coefficients to compute the drag forces something that open rocket can’t do accurately. The damnest thing about a drag coefficient is itself varies usually from object to object by geometry and along Mach numbers as velocity changes the coefficient sometimes also changes. In theory as the sweep angles increase the induced drag decreases. But that change also affects stability. It could be that #10 was the most stable of the other designs. I would be guessing fully developed turbulent flow and near negligible changes in flow characteristics at such low velocities. Then again I’m not an Aero just a mech here.
Interesting results, I was going to ask about holding the area constant, but that's been covered. is the weight between the 10 fin units consistent? also did you weigh the motors?
I'm spitballing here but lets say that motor performance can vary +5% (There is an official figure but I'm not sure what it is off hand). Setting Fin Unit 10 at the +5% end and looking at Altitude only, fin units 3, 6, and 7 fall in the 10% variance range and #5 is just outside of the low end. (409 ft * 0.9 = 368.1 ft)
#3 being a trapezoidal fin plan makes sense
#6 is an elliptical approximation results make sense
#7 rounded trapezoid also falls in line
#5 swept back trapezoid also makes sense for good performance
Total spread is 32% difference in altitude, and if we toss Unit 1 it's 26%. That's pretty good for a school science fair.
As for why 10 came out on top some thoughts, if you remove the spikes it is roughly elliptical, the spikes push the fins moment of inertia outward, and the boundary layer trip thoughts may have some merit.
Standard deviation of each design?
Sometimes the mean is not the message.
Some research on the ThrustCurve's site shows that NFPA 1125, Code for the Manufacture of Model Rocket and High Power Rocket Motors, allows an alarming amount of motor deviation from stated values....
The total impulse must not have a standard deviation greater than 6.7%.
The ejection delay must not vary more than 1.5 second or 20% (whichever is greater, up to 3s) from average.
The average thrust must not vary more than 20% (or 1N for model rocket motors, 10N for high-power motors, whichever is greater) from average.
That's why running all these through a simulator would be great.
Add the impulse deviation and the average thrust deviation and we end up with results that vary so wildly that it's hard to gather any useful data from a test unless some sort of on-board electronics are used to evaluate that actual motor performance during the test.
Ready for the science fair
What a fun science fair project. Well done.
That's the data I couldn't find thanks, definitely lends to motor variability being the most likely explanation.
The display looks great! Well done hope he does well!
Not sure I would have guessed this result, but having the luxury of Monday-morning Quarterbacking, I might suggest that the base drag is impacted by the unique shape of #10, in that it serves as a trip to make the boundary layer turbulent and slightly less base drag than a laminar fin. Very counter intuitive since skin friction drag would be higher, but overall base drag may be a larger quantity than skin friction.
Granted these are thin fins, but a very simple example would be the dimples on a golf ball make it go farther due to less base drag.
Hey, I told you I liked 10!
I have a construction question. I assume you had one or two rockets, and you swapped out the fin section on them. How are the fin sections held on for flight?
We had one rocket and slid the fin sections over the motor mount then used blue painters tape to make sure they didn’t come off
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