ultimate fin discutions and recomendations

Discussions about rockets, construction materials, adhesives, nozzles, nosecones and fin design.
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Jelo
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ultimate fin discutions and recomendations

Post by Jelo »

Hi
Im new on this forum bu had quite an experience with water rockets before.

I tried getting information here about the key part in a rocket to keep it stable and efficient but found almost nothing! So i

desided to creat this kinda chat room to keep all the information in a single topic :mrgreen: PLEASE POST

ONLY INFORMATION OR QUESTION REGARDING FINS!!!!


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Brian
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Re: ultimate fin discutions and recomendations

Post by Brian »

id say that circular fins would be the best because they go behined the rocket and move the CP backwards. i think that the best thing to do is to put extra weight in the nosecone to move the CG forwads. extra weight also con make the rocket go further.
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Jelo
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Re: ultimate fin discutions and recomendations

Post by Jelo »

first the basics, there are two key points to be considered in making a rocket stable,

-its center of gravity (its CG) -the point where an object is in nutral equilibrium, which means it is in perfect balance. This can easily be found by tying a piece of string around the rocket and moving it along its body until it is horisontal wen hangin in the air.

- the second point to find is the center of pressure of the rocket. this is found by holding your rocket loosly in between your two indexes and placing...
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Re: ultimate fin discutions and recomendations

Post by Jelo »

continuation of part above

...it in a windy area, now move it up and down until you find the spot were it will stay upright, that will be the center of presure of your rocket. An easier way of doing it with big heavy rockets is to make a cardboard copy of it, however this method will be less acurate.

Now how to use these points: the CG must be higher by at least two diametre of the rocket (twice its whith) than the CP for a nice straight flight.
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Re: ultimate fin discutions and recomendations

Post by Jelo »

okay now i am going to show you how to make the most basic type of fins, flat fins. there are usualy made out of solid plastic or balsa, a type of wood, altough some do put cardboard, which i strongly recomend you do not try for a number of reasons: first, cardboard gets destroyed by water, but this can be solved by painting it or putting tape or clink film over it. Secondly, cardboard is not very strong and will actualy wave around uselesly ouround the rocket durring flight.

i make my fins out of ice cream box covers, which is light, slim and strong. I usualy put 3 fins only for the only reason its always easier to make 3 rather than 4, although 4 fins provide a bit more stability.

i drew out a diagram of my fin design, D stands for diameter, the whith of the bottle.

just draw out your shape on the material your using and cut it out using a craft nife, sicors, or a saw in the case of balsa. also, when using balsa, make sure to sand the fins with fine sand paper.
happy flying!
jelo :WRA2:
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my fin design, simple, easy to make and effectiff
my fin design, simple, easy to make and effectiff
Untitled.jpg (18.04 KiB) Viewed 235 times
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Jelo
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Re: ultimate fin discutions and recomendations

Post by Jelo »

the next type of fins are my favourite as they are very easy to make and put on and are the ones that i find most stable for the moment. the only problem is that when they crash at high speeds, these types of fins get destroyed very easily, bu fortunately, there easy to repare.

Circular Fins:
this type of stabiliser is a round piece of plastic taken from the flat, round bit of a bottle. it is attached to the main body of the rocket by rods, mine are bamboo skewers, they are easy to get (100 for around 2 euros) and are quite resistant, athough the best ones for bigger rockets are longer, bigger carbon fiber sticks, like the ones used to make the modern arrows. the further away you put the round bit from the main rocket, the more stable the rocket will be, but, this might call for a laucher re-design as the nozzle is further away from the ground with long fins... you can stick the rods with glue, but my favourit way is to use strong tape, but make sure if you are using tape to avoid gaps between the tape and the plastic as quite a lot of water goes there and will make the tape useless.
This system makes the center of pressure lower than with flat fins and also makes your rocket look very stylish :mrgreen:
These kind of fins however, are not suitable for the top stages of a multi-stage rocket.
here are some pictures and diagrams(sorry bou the diagrams there made on paint in two sec :mrgreen: )
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side view
side view
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my rocket
my rocket
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RaZias
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Re: ultimate fin discutions and recomendations

Post by RaZias »

Just try to put inverted wings, like they did in the SU-47 russian jetfigher.

I don´t know what would happen...
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Re: ultimate fin discutions and recomendations

Post by Jelo »

hum...good question, i think it wouldnt make much of a difference appart from the fact the tips of the fins would actualy be made higher, therefore making it less stable, but weither the fins are put one way or another, as long as they have the same shape, they provide the same drag, just at a different point.
ill try it out when i can :wink: might get interestiong results.

as for flat fins, i made some out of very flexible plastic yesterday, as i suspected, the rocket did not have any stability and just zig-zaged through the sky, but, then, i tryed putting just a bamboo skewer on the leadign edge of the fins and re-inforcing them with normal tape and they worked fabulous, all this to say if people live in an area with little choise of materials, soft ones can be re-inforced by everyday items :D :D
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Re: ultimate fin discutions and recomendations

Post by RaZias »

Here are some photos of the X-29, they needed a 40 corrections per second by a computer to make it stable. The advantage is the higher manuverbility.
(I don´t see a advantage for a rocket)
x-29-EC87-0182.jpg
x-29-EC87-0182.jpg (106.13 KiB) Viewed 200 times
x29.jpg
x29.jpg (72.65 KiB) Viewed 200 times
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Jelo
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Re: ultimate fin discutions and recomendations

Post by Jelo »

yep i know this model, i am very much into jet fighters again :mrgreen: my favourite been the french rafale!

yup ill try it, ill tell you the results as soon as i can, one thing i taught about, the very tips of the fins woudl have to me extremely stiff in order for them not to bend...
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Re: ultimate fin discutions and recomendations

Post by RebelRockets »

Hi everybody,

Jelo had requested me to tell him about the type of fins that I use, here in this discussion. There is now full instructions on how to make the style of ring fin that I regularly use on my web site. http://rebelrockets.wordpress.com/fins/
Since it turned out rather lengthy it was better to post it there for those who are interested than to repeat it here and take up lots of space.
I hope it will be beneficial to someone.

Dennis
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Jelo
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Re: ultimate fin discutions and recomendations

Post by Jelo »

hi dennis
nice fins, they look very strong :mrgreen: ! i prefer using flat fins with my rockets, its easier to access to nozzle, and they turn out more resistant than circular fins, i made two rocket lauches yesterday, one with flat and one with circular fins, the circular fin got ripped off at lauch... while the flat fins stayed on even when the rocket smashed trough the branches of a tree...
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Re: ultimate fin discutions and recomendations

Post by Jelo »

i tested the inverted fin tecnique, without very positif results, the very tip of the fins were not supported enough and caused the rocket to fly in a very zig-zaggy fashion... tell you if iv more results 8)
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Re: ultimate fin discutions and recomendations

Post by Jelo »

by the way, RaZias, are you from france?
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Re: ultimate fin discutions and recomendations

Post by RaZias »

Jelo wrote:by the way, RaZias, are you from france?
I am a Portuguese living in the North of France (Department 14, Calvados).
I live near the main city of Calvados, named Caen.
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