Skelatal fins =advanced=

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Chuck Rudy

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With large fins succumbing to weathercock (which is not a terrible thing when the parachute comes out and the rocket drifts back and and lands near where it took off) my question is.......If a skeletal fins are made which do it's job in vertical flight but are almost immune to weather cocking is this a feasible solution to an age old problem? My guess is that at high speed in vertical flight the outline is the deterimining factor concerning straight line stability, but horizontal winds would have less affect.

Am I out to lunch on this one?

Chuck
 
That solution doesn't make a lot of sense to me.

A stable rocket will always attempt to achieve a zero angle of attack with respect to the apparent wind. That's a good thing. With a skeletal fin you probably will not have enough restoring force and the rocket is likely to be unstable after it leaves the rod. (I've actually seen this happen.)

The apparent wind is the vector addition of rocket's forward velocity and the wind's horizontal velocity. The rocket's fins develop lift which increases as the angle of attack increases, and this lift acts about the rocket's CG to push the rocket into the apparent wind to obtain zero degree angle of attack. This effect is most pronounced at the time when the rocket leaves the launch rod, and the slower the rockets velocity is at this point, the greater the turn into the wind.

Stable rockets have the CG ahead of the CP by a caliber or more. For a given size fin, the correcting forces exerted about the CG become greater as the stability factor increases, so one might be tempted to decrease the fin area if the rocket is predicted to be overstable. This is not necessarily a smart thing to do, because the simple stability criteria such as that found in rocksim does not account for body lift at non-zero angles of attack. and will predict a greater stability that actually exists at large angles of attack.

The best way to counter this effect is to use motors with a higher average thrust so that the angle of attack leaving the rod is minimized due to a higher initial rocket velocity. the faster the rocket accelerates, the less it will turn into the wind.

Bob Krech
 
Originally posted by Chuck Rudy
With large fins succumbing to weathercock (which is not a terrible thing when the parachute comes out and the rocket drifts back and and lands near where it took off) my question is.......If a skeletal fins are made which do it's job in vertical flight but are almost immune to weather cocking is this a feasible solution to an age old problem? My guess is that at high speed in vertical flight the outline is the deterimining factor concerning straight line stability, but horizontal winds would have less affect.

Am I out to lunch on this one?

Chuck

At high speed weathercocking is not much of a problem. It's worse at lower speeds because the ratio of horizontal vector to vertical vector is greater.

At lower speeds it's more of a problem. But so is stability. You want more stability when, unfortunately, you're more susceptible to weathercocking.

However, that makes me think of a sort of variable geometry. If you could drop some fin when accelerating you could lose some weight and drag. Easily done in two stages. See attached. Large fins on a booster that fit into the fins on a sustainer and make up enough area for them since they'd be deficient at lower speeds.
 
For a two stager, Dynasoar's solution is a good one, particularly if the booster has a high average thrust motor.
 
If you allow the fins to pivot at the root corner of their leading edges you get fins that don't affect the CP much! If you do this you can even put larger fins forward of the CG and get stable flight. The pivot makes the fins become non-steering except for the extension of the fin width through the leading edge, sort of like a dowel rod fin. If you want to avoid weathercocking use tube fins or ring tail fins. These types of fins don't weathercock as much since most of the fin area is located near the main airframe.

Bruce S. Levison, NAR #69055
 
Originally posted by Chuck Rudy
With large fins succumbing to weathercock (which is not a terrible thing when the parachute comes out and the rocket drifts back and and lands near where it took off) my question is.......If a skeletal fins are made which do it's job in vertical flight but are almost immune to weather cocking is this a feasible solution to an age old problem? My guess is that at high speed in vertical flight the outline is the deterimining factor concerning straight line stability, but horizontal winds would have less affect.

Am I out to lunch on this one?

Chuck

Are you talking about these?
 
Originally posted by rocketsonly
Are you talking about these?

Those fins, while having a wild look to them, would still weather cock with 20 mph winds because of their square area. I'm looking at shaping an outline of a fin which would allow launch in more severe winds. Stability in normal flight is not at issue, but stability in severe wind.
 
It still seems to me that if you get them 'skeletal' enough to help in wind, they'd be ineffective in general. Can you post a .jpg showing what you mean?
 
By skeletal, do you mean an outline of a fin? Say a 1/4" border on an exsisting fin? By doing this, are you hoping that the turbelence (which results ito drag) from the fin will allow the rocket to be stable?
 
Originally posted by rocketsonly
By skeletal, do you mean an outline of a fin? Say a 1/4" border on an exsisting fin? By doing this, are you hoping that the turbelence (which results ito drag) from the fin will allow the rocket to be stable?

Yes. Exactly. But obviously the top portion would have to be designed in a way in which the air moves past it at speed creating a wall of air which is picked up by the downward edge of the wing. I'm guessing (and just guessing) the if the top of the fin is rounded or ovaled and with almost no drag, then the bottom is far more draggy the wall created on vertical flight can overcome air which may be able to pass through at the same time. Or it's possible the wall of air will not permit the lower speed wind to pass through and force a weather cock, but without trying it's a debate in a vacuum.

Chuck
 
Originally posted by Chuck Rudy
I'm looking at shaping an outline of a fin which would allow launch in more severe winds. Stability in normal flight is not at issue, but stability in severe wind.

Speed is your best friend. Bob's post at the beginning of this thread explains it all. If you have a long rail, and a fast rocket, the relative air-flow vector will start out vertical. The only time you need to worry about this is when the rocket is coasting toward apogee. It will turn into the wind as it slows.

The fins do most of their work trying to keep the rocket from swapping ends during motor burn. After the motor burns out, they don't do as much work. You might be able to get away with dropping some fin area after motor burn out.

I don't see how the skeletal fin idea would work all that well.

urbanek
 
Wow interesting idea. I'd be interested to see how it performed on slower rockets or during the slowest phase off the top of the rail. At what kind of speeds do you think it would become effective for making a rocket stable?
 
Just to clarify, I'm thinking Chuck means fins something like those in the link.

Enjoy my artwork, my Jr. High art teacher called me Vincent Van Gak!

Greg
 
it seems to me that the area of the fin affects how well it will preform. so, in order for your rocket to fly straight up, it would have to have horozontal forces imposed on the fins, these would be to stabalize the rocket, however, if the air pressure is allowed to go through as it pleases, it would seem to me that it would be just as effective as if your skeletal fin didnt have the voids, and was one solid small piece of fin, totaling the entire surface area of the original skeletal fin.

anyone object?
 
Originally posted by r1dermon
it seems to me that the area of the fin affects how well it will preform. so, in order for your rocket to fly straight up, it would have to have horozontal forces imposed on the fins, these would be to stabalize the rocket, however, if the air pressure is allowed to go through as it pleases, it would seem to me that it would be just as effective as if your skeletal fin didnt have the voids, and was one solid small piece of fin, totaling the entire surface area of the original skeletal fin.

anyone object?

Close, but there are 2 other things to factor in:
1: The distance from the centerline of the rocket to the centroid of the wing shape will determine the magnitude of the moment arm, forcing the rocket to rotate around its CG.
2: Each trailing edge and leading edge in that skeleton is going to have its own drag effects depending on its shape, as opposed to a single leading/trailing edge.

I'm going off some old, dusty brain cells here, but that's what I remember from my aero classes.

WW
 
Finally found a picture in the mess which is close to the skeletal fins I was thinking of in case I've confused anyone.
 
Take a look at Jim Flis's photograph of John Haarland's Roto-Ruter flying at the CMASS November 20 launch which is posted at https://fliskits.com/photo_album/cmass_1120_2004/index.htm

The first flight of this rocket did not have the plastic film in the fin and the airframe was a few inchs shorter. RocSim had predicted stability, but it's maiden flight was an unstable lawn dart. With the addition of the plastic film shown in the photograph, it flies straight and pretty.

Bob Krech
 
Originally posted by bobkrech


The first flight of this rocket did not have the plastic film in the fin and the airframe was a few inchs shorter. RocSim had predicted stability, but it's maiden flight was an unstable lawn dart. With the addition of the plastic film shown in the photograph, it flies straight and pretty.

Bob Krech

Bob

In looking at this design I can't help but wonder how much vortex generation is caused by the several skeleton chords. What I envision is an outline of a fin, with an aero curved surface to encourage air flow still the slipstream to the bottom fin, not to interrupt it. The design of all the upper chordlines would be a very thin elipse, the bottom chord would be much more disruptive to allow for drag to accelerate the air between the vacant area between the top and bottom chords.......

Guess it'll take some design testing.

Chuck
 
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