if a fin is askew???

[skybuster]

Hey everyone,

So, getting fins on straight is important for a proper flight, however with 3 and 4 fin designs, they have to be spaced out at an even 120 or 90 degrees, with most rockets. Now, we're not perfect and not everyone uses the fin alignment guides, so what happens with a rockets performance if the fins aren't exactly spaced apart at the perfect angle?

Thanks, Happy holidays!

 

[dlazarus6660]

So what's it askin'...askew? Bless you!

 

[bjphoenix]

Some of them don't fly straight. They may wiggle a little bit, or they might turn in circles like a small corkscrew.

 

[dlazarus6660]

Wobble, weebulls wobble but they don't fall down. As long as the CP and CG are correct, they rocket should fly fine.

 

[sodmeister]

Some time ago (10 years),I built an Estes Bullpup and the fins were less than 90° apart and not the straightest (beer may have been involved)

You know ,the darn thing flies pretty darn arrow straight.It finally went into the bushes due to a bad parachute ,and is now probably compost.


I`m a lot better at it now


Paul t

 

[[POW]Eagle159]

For smaller rockets ( like estes) there will be probably very little that you will notice. Mine fly good/straight even though the fins arn't perfect with each other.

 

[foamy]

Some time ago (10 years),I built an Estes Bullpup and the fins were less than 90° apart and not the straightest (beer may have been involved)... Paul t

Hmm. I believe I had that same experience last week. One bloody little fin (out of eight). I think I'm just going to cut it off and make a new one. That and I've gotta put the rockets down after cocktails.

 

[spacecadet]

Hmm. I believe I had that same experience last week. One bloody little fin (out of eight). I think I'm just going to cut it off and make a new one. That and I've gotta put the rockets down after cocktails.

Eight is far too many!
I went down to three for a lot of rockets to save work and weight.
Why not fly it? You'll then have some data about the importance of straight fins.

 

[jflis]

Assuming that the only "issue" is that the fins are located in a position other than a specific angle from the others, but are otherwise attached straight, there really shouldn't be much of any problem.

The CP will be affected (though, to be honest I have never really done the math, it would be easy enough...).

If you assume a 4-fin model and 2 opposing fins are in the correct spot but the other 2 are rotated, say, 1 degree clockwise there would be virtually no affect. If the other 2 are rotated, say 89 degree there would be a massive affect to the CP and the model would likely be unstable.

fwiw...

 

[rocketguy101]

Eight is far too many!
I went down to three for a lot of rockets to save work and weight.
Why not fly it? You'll then have some data about the importance of straight fins.

uh, because a Bull Pup would look funny w/ three fins?

Just kidding!!!!!

I agree w/ Jim. I think the "axial alignment" is more important than the "radial alignment" (see my drawing) for spinning, wobble, etc.

 

[[POW]Eagle159]

uh, because a Bull Pup would look funny w/ three fins?

Just kidding!!!!!

I agree w/ Jim. I think the "axial alignment" is more important than the "radial alignment" (see my drawing) for spinning, wobble, etc.

yes.

 

[David987Fox]

If the fin is too askew, the rocket will noodle. If the airframe isn't strong enough, that noodling could cause separation. That separation could cause your parachute to come out while the rocket is ascending and cause all sorts of chaos.

 

[Fe Dude]

I helped someone build a Big Daddy and they managed to get 3 of the 4 fins looking like the wonky one in the first example of rocketguy's post. Don't you know, that rocket went up without a hitch.

[YOUTUBE]Wzqk6pDUR04[/YOUTUBE]

 

[rocketguy101]

I helped someone build a Big Daddy and they managed to get 3 of the 4 fins looking like the wonky one in the first example of rocketguy's post. Don't you know, that rocket went up without a hitch.

[YOUTUBE]Wzqk6pDUR04[/YOUTUBE]

wonky...I like that...when I think of wonky fins I think of these bad boys and they fly pretty straight...

 

[JPVegh]

When I started in rockets all of my fins were off by a few degrees. It ain't no biggy. They all flew fine.

Even now I only get precise with fin placement on competition rockets.

 

[JStarStar]

The highly-technical, precisely-calculated answer is ...... uhhh, it depends. :confused2:


The effects on the flight can range from negligible (if the fin root angle is slightly off) to moderate (a slight wobble or slow spin) to major (if, for instance, you have two opposing fins on a 4-fin rocket both canted in the same direction so they effectively act as flaps/elevators). :y:

Bottom line is it's better to get the fins on razor-straight and perfectly aligned and distributed. But we all knew that already.

Old rocketeer's trick: if a fin angle is just slightly off, hold it an inch or so away from a high-wattage incandescent light bulb (no, a CFC curly bulb will NOT work; an infrared bulb if you have one is best) for a minute or so (expose both sides to the warming light), that can soften up the glue and maybe allow you to straighten it out. However that method usually works less well in proportion to the quality of glue used in the first place. And usually, it only softens the glue to the extent you can slightly straighten out a crooked mounting angle, not move the fin's actual location on the body tube.

Old-fashioned drop lights, if you are still using old-style 100W light-bulbs, can get pretty hot. That's not good for a lot of other reasons but it is good for this particular trick. A hair dryer (on high setting) can also work.

Using double-glue joints helps in getting fins on straight: if each piece already has a thin surface coating of dried glue, the glue joint usually "grabs" fairly quickly, allowing you a brief time for minor adjustments before it sets firmly in place. (Not as fast as CA but still pretty fast.)

Single-glue joints usually take much longer to "grab," necessitating you hold it in place with your quivery shaky hands (in my experience the usual reason for slightly screwy fin placement). With double-glue joints, they usually grab hold and then you can quit messin' with 'em.

 

[UrbanKnight]

This is exactly what my students learned at their first launch of 10 identical kits (Estes Star Trooper). The ones with the straightest and most carefully airfoiled fins flew the highest, whether they were perfectly perpendicular or not. The more canted the fins were, the more erratic the flights were, especially if each fin was canted to a different degree. A few sloppy builds lost fins on lift-off.