Winged and forward winged designs unstable for rockets?

Kelly

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I will eventually, but I have to get Rocksim or similar first and learn it. What are you looking for in the sim?

I woud just like to know where the Cp is, so we can all stop speculating about whether this is a "stable design with the Cp forward of the Cg".
If you could post the critica dimensions - length and diameter of the body tube, nosecone, fins etc. - I could whip a sim up for you. Or, post a side-on photo of the rocket, trying to eliminated perspective as much as possible, and I can guess at the dimensions from the photo.
 

Daddyisabar

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Those big fins up front just chop up the airflow and reduce the effectiveness of those cute little fins in the rear. So my mindsim would put the CP about mid wing. The pitfalls of sports scale. Will it whistle on the way up?
 

KenECoyote

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I woud just like to know where the Cp is, so we can all stop speculating about whether this is a "stable design with the Cp forward of the Cg".
If you could post the critica dimensions - length and diameter of the body tube, nosecone, fins etc. - I could whip a sim up for you. Or, post a side-on photo of the rocket, trying to eliminated perspective as much as possible, and I can guess at the dimensions from the photo.
Idk if you've missed it, but I provided a 2D profile cutout just before your post with it balancing. That cutout is extremely precise...I measured the body tube dimensions and took the actual rear fin (had broken off) or a direct tracing of the front fin to make the cutout.
Other than that, the dimensions:
OAL including 1/2A motor in the back = 11"
Main bt = 7 5/8"
Motor tube extended out back = 5/16"
Nose cone = 3"
Bt-20 = ~3/4"OD
Rear fins = 7/8" L x 3/4" H at root x 1/4" H at end
Mid wings = 2" L perpendicular to bt x 1 1/4" H at root x 1/2" H at tip, Drop from root to tip = 2.5"
Thanks! :)
 

Kelly

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Oh, I did miss that pic; thanks. Working on your measurements instead of the cutout (I'm out of time tonight) this is what I get, first pass:

rocket.jpg
It would help to know the distance from tip of rocket to root of forward fins/wings, and also the sweep of the wings - in degrees, or inches. Ignore the Cg, because I didn't enter accurate data to calculate that.
 

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KenECoyote

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Oh, I did miss that pic; thanks. Working on your measurements instead of the cutout (I'm out of time tonight) this is what I get, first pass:

View attachment 457423
It would help to know the distance from tip of rocket to root of forward fins/wings, and also the sweep of the wings - in degrees, or inches. Ignore the Cg, because I didn't enter accurate data to calculate that.
The tip of rocket to root of forward fins/wings is 5 - 1/16", the wings are swept at a 45 degree angle (or add on 90)... I'd forgotten what sweep I used and was happy to see it match 45 exactly!
Thanks!
 

Kelly

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OK, with those updates, I get this:

rocket.jpg
It shows the Cp as being 6.7" back from the tip, or maybe 1/2" back from the back edge of the root. FWIW.
 

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Ez2cDave

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Idk if you've missed it, but I provided a 2D profile cutout just before your post with it balancing. That cutout is extremely precise...I measured the body tube dimensions and took the actual rear fin (had broken off) or a direct tracing of the front fin to make the cutout.

The "Cardboard Cutout" method of determining CP tends to place the CP somewhat AHEAD of its actual "mathematical location". In my experience, the Cardboard Cutout method provides a SAFE approximation of CP location, but I am curious how much it differs from Barrowman and the various "Sim" programs.

https://www.rocketryforum.com/threads/center-of-pressure-cardboard-cutout-vs-barrowman-vs-computer-sims.163511

https://apogeerockets.com/education/downloads/Newsletter18.pdf


Dave F.
 

KenECoyote

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The "Cardboard Cutout" method of determining CP tends to place the CP somewhat AHEAD of its actual "mathematical location". In my experience, the Cardboard Cutout method provides a SAFE approximation of CP location, but I am curious how much it differs from Barrowman and the various "Sim" programs.

https://www.rocketryforum.com/threads/center-of-pressure-cardboard-cutout-vs-barrowman-vs-computer-sims.163511

https://apogeerockets.com/education/downloads/Newsletter18.pdf

Dave F.
Makes sense that it is conservative...I later realized that means it's more likely farther forward so you can place the CG farther forward for a safer measure.
 

Kelly

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I think, based on where you show the Cg, and where the sim shows the Cp is, and the flight video, that the pseudo-lacrosse is marginally stable. Still, if you have some novel stabilization mechanism, we'd love to see it.
Looking at the Gnikiv, and the flight video, it seems that this is unstable, though the extremely short flight duration keeps it from doing anything too crazy.
 

KenECoyote

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I think, based on where you show the Cg, and where the sim shows the Cp is, and the flight video, that the pseudo-lacrosse is marginally stable. Still, if you have some novel stabilization mechanism, we'd love to see it.
Looking at the Gnikiv, and the flight video, it seems that this is unstable, though the extremely short flight duration keeps it from doing anything too crazy.
Estes should make the Lacrosse! They've done much worse. :p
 

icyclops

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Ok, think of it as a weather vane.....the wind stabilizes the fins in the back so it points into the wind, but if you also add fins too far near the front ...it doesn’t know where to point and swings around crazy....unstable big time. Your video shows this well....a good demo.
 

tab28682

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Covid lockdown lead to alot of building experimenting and tried a couple of WW2 designs of Winged Rockets. Both were miserable failures. But looked cool before flight!

Both of those will boost just fine if you get the CG in front of the CP.

Here is my RC A4b during the boost.
 

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tab28682

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Cardboard cutouts err slightly to the side of conservative due to the fact that it correctly represents wings and fins as flat plates and incorrectly represents the body tube and nose cone as flat plates, which they are not.

When the model has a small deflection from straight flight, the flat fins and wings have a bigger effect on the airflow than the curved surface of the body tube and nose cone has on the airflow. If the deflection from straight flight gets large, the tube and nose cone start having the drag effect of a tube section with the relative wind 90 degrees to it, which starts having a very large effect.

RC model wise, aircraft of fairly normal configuration can be balanced for flight based on the wing planform only (again, CG in front of CP). However, aircraft models of things like an F-104 must take the fuselage planform into account, as it is large compared to the wing area.
 
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tab28682

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I think, based on where you show the Cg, and where the sim shows the Cp is, and the flight video, that the pseudo-lacrosse is marginally stable. Still, if you have some novel stabilization mechanism, we'd love to see it.
Looking at the Gnikiv, and the flight video, it seems that this is unstable, though the extremely short flight duration keeps it from doing anything too crazy.

I agree. Looking forward to finding out more about the new stabilization system.

I have flown a RC EDF jet with a very faintly negative stability margin, with a good rate gyro assisting in pitch. Once it was trimmed and the gyro tuned to best effect, it worked well.
 

KenECoyote

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Cardboard cutouts err slightly to the side of conservative due to the fact that it correctly represents wings and fins as flat plates and incorrectly represents the body tube and nose cone as flat plates, which they are not.

When the model has a small deflection from straight flight, the flat fins and wings have a bigger effect on the airflow than the curved surface of the body tube and nose cone has on the airflow. If the deflection from straight flight gets large, the tube and nose cone start having the drag effect of a tube section with the relative wind 90 degrees to it, which starts having a very large effect.

RC model wise, aircraft of fairly normal configuration can be balanced for flight based on the wing planform only (again, CG in front of CP). However, aircraft models of things like an F-104 must take the fuselage planform into account, as it is large compared to the wing area.
Excellent explanation!
 

KenECoyote

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Looking at the Gnikiv, and the flight video, it seems that this is unstable, though the extremely short flight duration keeps it from doing anything too crazy.
Try to watch the video more... particularly 0:11-0:12. From what I can tell the rocket goes unstable and horizontal into the wind, but then stabilizes and actually shifts vertical and perpendicular to the wind...very odd, but I believe it shows it stabilizing itself.
Screenshot_20210330-183553.png
Screenshot_20210330-183609.png
Screenshot_20210330-183404.png
Unfortunately for me, I've had MAJOR issue at work and now need to deal with lawyers in what seems to be a losing battle. Will still try to update to get away from things for bits.
 

KenECoyote

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So I don't leave everyone in the dark, the system I'm working on is movable fins that have pendulum weights for gravity assist in vertical stabilization. These are mechanical and can be adapted as small as needed (currently BT20 & BT50s). I already have several versions built and a somewhat complex one to test as soon as I find time and weather permits.
IMG_20210307_220620923.jpg
 

Kelly

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Well, it's intriguing. Can you describe how the weighted/pivoting fins are supposed to work? I just can't work out in my mind what this could be doing that is useful. Normally, in order to stabilize a rocket, fins need to move in the opposite direction they would tend to go naturally, which means some sort of active control is necessary.
Does the Gnikiv use this? Unfortunately, 11-12 seconds is about the entire flight, and the rocket loses speed so quickly that it's tough to determine whether it is 'stabilizing' or just 'randomly orienting' itself.
Sorry to hear about the work issues; "I need to deal with some lawyers" ranks right up there with "The government is going to help with that" in terms of phrases you never want to hear...
 

KenECoyote

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Well, it's intriguing. Can you describe how the weighted/pivoting fins are supposed to work? I just can't work out in my mind what this could be doing that is useful. Normally, in order to stabilize a rocket, fins need to move in the opposite direction they would tend to go naturally, which means some sort of active control is necessary.
Does the Gnikiv use this? Unfortunately, 11-12 seconds is about the entire flight, and the rocket loses speed so quickly that it's tough to determine whether it is 'stabilizing' or just 'randomly orienting' itself.
Sorry to hear about the work issues; "I need to deal with some lawyers" ranks right up there with "The government is going to help with that" in terms of phrases you never want to hear...
Thanks! Fins forward past CG work to easily stabilze...My first build is attached to the nosecone and was pretty cool and can be moved to other rockets... making me think "Valkyrie" build. Later design I have to come would have rear fins going opposite, but more as non-flying demo first... should be cool!
IMG_20210315_074609095.jpg
Pic of another I've built waiting to launch:
IMG_20210329_204728351.jpg
 
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