Stability with fixed vs freely pivoting forward mounted canards

[BABAR]

“Relatively” straight is always a fun rocketry description to read.

The top soil on the tip of the nose cone needs some “splaining” too!

 

[Wallace]

Just a typical Wisconsin flood season swamp landing. Keeping in mind Open Rocket claimed stability margin @ .764 along with a steady 10ish mph Southeast wind, I call "relatively" a good day...

 

[Wallace]

Here Ya go..

 

[tim cubbedge]

Here Ya go..

Dead link

 

[Wallace]

Gotcha. Workin' on it. How 'bout this'n? And don't forget to comment on the UFO in the lower right corner towards the end

 

[Wallace]

1st one should be good now?

 

[tim cubbedge]

1st still dead. 2nd ok.

 

[Wallace]

Ok; Looks like I need to go to plan C...

 

[Wallace]

This "should" do it...

 

[Wallace]

Only appreciable difference between 'em is pivot point. Looks like those whom predicted such behavior were absolutely correct. Or psychic

 

[tim cubbedge]

Was that an optical illusion or was the one on left (in video) really bending?

 

[Wallace]

Classic two'fer as far as concept testing is concerned. Got an immediate side by side behavior comparison. I made 'em silver and black thinking it would be easy to differentiate between the two but glare defeated me. Next time they'll be flat/non-glossy colors? I think my next step may be an NC with fixed canards for stability comparison. I really need to find/fix and or build a logging altimeter for better data also. I actually do have 2 Quantums but neither currently function. I have a built Proton but since it has yet to fly I don't care to use it. What would be my best/cheapest/smallish/lightweight option as far as a logging alt?

 

[Wallace]

Was that an optical illusion or was the one on left (in video) really bending?

Wish I could answer that myself but I'm not sure. Suspect it was simply fluttering and the camera couldn't deal with it properly? My guess is that they're really way too small to be acting like an overcooked noodle. At least without self destructing in the process.

 

[tim cubbedge]

Cheapest off the shelf is probably PerfectFlite SLCF but appears to be out of stock.
https://www.perfectflitedirect.com/stratologgercf-altimeter

 

[Wallace]

Also have a video file from flight #2 but it's corrupted. I've never had good luck with this cheap U8 style camera. Seems to be hit or miss. I do like the form factor and relatively low mass but otherwise...I've attempted several times now to recover/repair the file but no love. Anyone have an idea in that regard?

 

[Wallace]

Cheapest off the shelf is probably PerfectFlite SLCF but appears to be out of stock.
https://www.perfectflitedirect.com/stratologgercf-altimeter

I thought those were no longer being produced?

 

[tim cubbedge]

Still made but they sell pretty fast, as they are cheapest on the market.

 

[Wallace]

Gotcha. Guess I have some searching in my future.

 

[Nytrunner]

Well, I like the righthand config. It looked like it was a better representation of the rocket AoA

The ground video didnt look bad

 

[Wallace]

The second flight was quite a bit uglier off the rail. It actually turned and flew 90 degrees across the prevailing wind. Suspect it was canard induced since I rotated the pad 180 from the first launch just to see what'd change. I'll check for ground based vid...

 

[Wallace]

Had a G 80 ready to go, but after watching onboard video I decided it would probably just self destruct. Would've added another 100 or so fps.

 

[boatgeek]

What are the relative chord points of the silver and black fins? From the pics, looks like silver is maybe at 25% chord and black is at 33% from the leading edge.

If you just want recording on the altimeter, a Perfectflite PNut will do the job. If you need deployment as well it's a more complicated solution.

 

[Wallace]

What are the relative chord points of the silver and black fins? From the pics, looks like silver is maybe at 25% chord and black is at 33% from the leading edge.

If you just want recording on the altimeter, a Perfectflite PNut will do the job. If you need deployment as well it's a more complicated solution.

You pretty much nailed it on the pivot points. The only real reason they're offset is it was easier on the small scale version to just run axles all the way through. I'm gonna do some more testing before I commit to the full scale, but surprisingly enough it actually looks like it has at least some merit. And thanks for the peanut reference, pretty sure it'd be perfect for this...

 

[mbeels]

Was that an optical illusion or was the one on left (in video) really bending?

I think that the appearance of bending is a camera artifact as a result of a rolling CMOS shutter. Basically the camera is scanning pixel rows, one after another, instead of complete images. So if things are moving quickly, they appear bent.

 

[burkefj]

.764 in 10mph is a bit low on margin, then add in a fluttering forward drag component not surprised you got one flight that was wacky... I'd move cg forward to at least 1, fly in lower wind then tape the fins fixed and compare how it flies in similar conditions with identical motor. How much does the cp shift in openrocket with fins fixed versus pivoting? If you have 1 margin pivoting, what do you have with them fixed? .5, ..7 ?

 

[Wallace]

I think that the appearance of bending is a camera artifact as a result of a rolling CMOS shutter. Basically the camera is scanning pixel rows, one after another, instead of complete images. So if things are moving quickly, they appear bent.

I think that the appearance of bending is a camera artifact as a result of a rolling CMOS shutter. Basically the camera is scanning pixel rows, one after another, instead of complete images. So if things are moving quickly, they appear bent.

'
I agree. Just like the "flexible" pencil trick..

 

[Wallace]

.764 in 10mph is a bit low on margin, then add in a fluttering forward drag component not surprised you got one flight that was wacky... I'd move cg forward to at least 1, fly in lower wind then tape the fins fixed and compare how it flies in similar conditions with identical motor. How much does the cp shift in Open Rocket with fins fixed versus pivoting? If you have 1 margin pivoting, what do you have with them fixed? .5, ..7 ?

That's a very good question. As far as I can tell from my Open Rocket sims the pivoting fins had enough of an affect to warrant further testing. If there is a way of attaching fins to a transition in O.R. I'm not aware of it? I have heard it is possible in Rocksim but since I don't have....Anyone care to do a sim for it?

 

[Wallace]

Shafts and bearings have arrived..

 

[Wallace]

The neutral point on a symmetric airfoiled fin is at 25%MAC. The suggestion to have a quarter of the area in front and 3/4 behind is not a good suggestion. That is right around the neutral point. Plus, the neutral point shifts forwards as angle of attack increases. At 25% the results are unstable. You want more area to be behind than that just so the fin weathercocks to the freestream. Just to note, it isn't area in front or behind the pivot, it is pivot location compared to percentage of the MAC (mean aerodynamic chord) and you might want to bias away from some of the area right next to the rocket body as the airflow is different there.

https://aviation.stackexchange.com/...tre-of-pressure-aerodynamic-centre-and-neutra

As you get into the transonic region the neutral point will shift, and continue shifting as the rocket goes faster.

Another issue which I think wasn't mentioned is everyone seems to be applying a statics analysis to a dynamics problem. Think about materials flex, time lag on response due to inertia and non-steady flow, and potential resonances and potential instabilities. You might find those pivoting forward fins at certain speeds can force the rocket into coning or other bad behavior.

Gerald

Since this point does NOT change with angle of attack, what did we see in the onboard concerning the 2 differing pivot points?
"Source: www.theairlinepilots.com

For symmetric airfoils in subsonic flight the aerodynamic center is located approximately 25% of the chord from the leading edge of the airfoil. This point is described as the quarter-chord point.

Thus the aerodynamic center does not change with variation in angle of attack. Due to this, the aerodynamic center, rather than the center of pressure is used in the analysis of longitudinal stability."
"

 

[Wallace]

From what I am seeing twenty five percent is the sweet spot. I obviously have some more testing to do, yet again, can it be be that simple or I am overthinking it?