Put some canted motors up front!
Winged and forward winged designs unstable for rockets?
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Poor boy sim for the Lacrosse CP...
So this method is supposedly more conservative, so CG was *definitely* behind the CP (CP is actually about where my mind sim said).
So this method is supposedly more conservative, so CG was *definitely* behind the CP (CP is actually about where my mind sim said).
Kelly
Usually remembers to get the pointy end up
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.
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?
Some images of the real Lacrosse missile, for comparison to this "Lacrosse" design.
Dave F.
Dave F.
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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.
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!
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Great looking Tiger!
Kelly
Usually remembers to get the pointy end up
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:
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.
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!
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
Usually remembers to get the pointy end up
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.
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.
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Plane? Oh yes..... the plane......
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.
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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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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.
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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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.
Excellent explanation!
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.
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.
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.
Kelly
Usually remembers to get the pointy end up
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...
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...
Noris makes a V1 kit. Hostile Projectiles made a winged V2 kit.
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!
Pic of another I've built waiting to launch:
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