r1dermon Posted September 2, 2011 Share Posted September 2, 2011 flight:: when air rushes over a surface a vacuum is formed, high pressure below, low pressure on top you get lift, like you said vacuum on both sides keeps it true ps watch a 747 take off in a crosswind as soon as it leaves the runway it points in the wind,size has nothing to do with it,its a principle of flight size affects lift...for instance, put piper wings on a 747, and then we'll see what happens. im not sure what you're trying to say here? Link to comment Share on other sites More sharing options...
THEONE Posted September 2, 2011 Author Share Posted September 2, 2011 The fins at the body of the rocket must be completely straight and aligned ? cause as i can see from some on board videos the rocket moves cyclically around himself... Link to comment Share on other sites More sharing options...
allrocketspsl Posted September 2, 2011 Share Posted September 2, 2011 size affects lift...for instance, put piper wings on a 747, and then we'll see what happens. im not sure what you're trying to say here? what I meant was,if air flows over a wing of any size the same principle applies,a large airplne of small one will correct itself into the wind if it has a vertical stabilizer,size has nothing to do with it in this only regard.Like others have said a vacuum is created on both sides of the stabilizer making it find true,then a pilot forces the movement of said stabilizer in one direction to correct the direction he knows will keep the plane straight to some point.Gravity is gravity on a bb or basketball the force is constant only the mass is different.To much said! Link to comment Share on other sites More sharing options...
r1dermon Posted September 2, 2011 Share Posted September 2, 2011 The fins at the body of the rocket must be completely straight and aligned ? cause as i can see from some on board videos the rocket moves cyclically around himself... they dont have to be exact, and 99% of the time, they're not. anomalies in the construction of the fin-can will result in oscillation or rotation of the fin-can (and thus, the airframe). this doesn't mean the rocket is unstable, it's just not aerodynamically optimal. you can't eliminate drag. drag is caused by the vacuum placed on the surfaces of the fins. the fin is displacing air (fluid) as it moves through it, the less air it displaces, the less vacuum is created. a very thin sharp fin is more optimal than a thick blunt fin, however, on large rockets going mach+, the forces put on the materials are just too much for conventional design, and exotics must be used (to decrease the volume of the fin, and increase it's rigidity). when your fins are incorrectly aligned, or not strong enough to deal with the forces of flight...this happens. often the result is Link to comment Share on other sites More sharing options...
nater Posted September 2, 2011 Share Posted September 2, 2011 I had no idea the fins flexed that much during flight. Link to comment Share on other sites More sharing options...
THEONE Posted September 2, 2011 Author Share Posted September 2, 2011 So the fins must be a little bit flexible ? Link to comment Share on other sites More sharing options...
dagabu Posted September 3, 2011 Share Posted September 3, 2011 they dont have to be exact, and 99% of the time, they're not. anomalies in the construction of the fin-can will result in oscillation or rotation of the fin-can (and thus, the airframe). this doesn't mean the rocket is unstable, it's just not aerodynamically optimal. you can't eliminate drag. drag is caused by the vacuum placed on the surfaces of the fins. the fin is displacing air (fluid) as it moves through it, the less air it displaces, the less vacuum is created. a very thin sharp fin is more optimal than a thick blunt fin, however, on large rockets going mach+, the forces put on the materials are just too much for conventional design, and exotics must be used (to decrease the volume of the fin, and increase it's rigidity). when your fins are incorrectly aligned, or not strong enough to deal with the forces of flight...this happens. Keep in mind that r1dermon is explaining the effects of flutter on high speed rockets, not the kind we make here. The first video is an anomaly of the shutter as refuted by the posts below the video. The materials they use cannot handle that kind of flutter. -dag So the fins must be a little bit flexible ? No, they do not have to have any flex to them. -guf Link to comment Share on other sites More sharing options...
THEONE Posted September 3, 2011 Author Share Posted September 3, 2011 Ok i have understand, thanks a lot dagabu and you r1dermon Link to comment Share on other sites More sharing options...
r1dermon Posted September 3, 2011 Share Posted September 3, 2011 Ideally nater, the fins do not flex. In very high speed applications, more rigid materials are sought for use as find material. A good material being used is carbon fiber. Very rigid (zero flex). One of the drawbacks however, is that with enough pressure, it would rather shatter than flex, which will terminate the flight in catastrophe. Debatable dag. I like that video because its a more blown out example of what happens. Real or not, i tend to think that rocket would have disassembled itself. Most find flutter is relatively minor. But on ultra high speed applications, a little flutter can rip your carbon fiber airframe to pieces. Like dag said, this is more for rocketry, science of, than fireworks...although an O motor mainframe lifting a nice 16" header to a mile above the ground would be cool as hell. Link to comment Share on other sites More sharing options...
dagabu Posted September 3, 2011 Share Posted September 3, 2011 Like dag said, this is more for rocketry, science of, than fireworks...although an O motor mainframe lifting a nice 16" header to a mile above the ground would be cool as hell. True that!! All I can say is that as an eddy current and magnaflux dye NDI specialist for two of the local military manufacturers up here in lutefisk land 20 some odd years ago, exposed me to thousands of high velocity fins and the specs for flutter were in the -1% range for variation. I would with all confidence say that the more rigid the better. -dag Link to comment Share on other sites More sharing options...
allrocketspsl Posted September 4, 2011 Share Posted September 4, 2011 True that!! All I can say is that as an eddy current and magnaflux dye NDI specialist for two of the local military manufacturers up here in lutefisk land 20 some odd years ago, exposed me to thousands of high velocity fins and the specs for flutter were in the -1% range for variation. I would with all confidence say that the more rigid the better. -dag every stinger I loaded up onto a jet had stiff metal fins!No flex at all! Link to comment Share on other sites More sharing options...
dagabu Posted September 4, 2011 Share Posted September 4, 2011 Exactly! -dag Link to comment Share on other sites More sharing options...
THEONE Posted September 4, 2011 Author Share Posted September 4, 2011 How you can find how much surface area you will going to need to make the rocket fly well and straight ? Link to comment Share on other sites More sharing options...
dagabu Posted September 4, 2011 Share Posted September 4, 2011 You can't. We are making pyrotechnic rockets, the weight changes drastically when it is lit and most of the fuel is dumped in one second. Use 1/4" by 1/4" by 24" sticks on 1/2" ID rockets and you will be fine. -dag Link to comment Share on other sites More sharing options...
Thunderr1 Posted September 6, 2011 Share Posted September 6, 2011 (edited) Here is a video of a finned rocket. Very small rocket, very small fins and still launched with a stick. Report with single test star http://www.youtube.com/watch?v=FhBin4s0PZU Edited September 6, 2011 by Thunderr1 Link to comment Share on other sites More sharing options...
THEONE Posted September 7, 2011 Author Share Posted September 7, 2011 The fins rocket also has got something like a gide at first (platform) this one is necessary or it is for safety ?... Without the platform the rocket will fly straight ? Link to comment Share on other sites More sharing options...
dagabu Posted September 7, 2011 Share Posted September 7, 2011 No glide, they are launched from a 12" bamboo skewer and a short section of tube that is attached to the body of the rocket. -dag Link to comment Share on other sites More sharing options...
THEONE Posted September 7, 2011 Author Share Posted September 7, 2011 So it is necessary... Link to comment Share on other sites More sharing options...
dagabu Posted September 7, 2011 Share Posted September 7, 2011 No, they are not necessary, I use a guide to be 100% sure that they will launch straight. These are missiles and can be launched with no guide. -dag Link to comment Share on other sites More sharing options...
THEONE Posted September 8, 2011 Author Share Posted September 8, 2011 So the goal is to put more forward the CG than the CP correct ?By addind bigger fins you will successfully add your center of pressure upwardand with heavier nosecone you move your CG forward... So it is not the best to put bigger fins that put more weight to your rocket cause the more heavier the more performance it will loose... Link to comment Share on other sites More sharing options...
dagabu Posted September 8, 2011 Share Posted September 8, 2011 So it is not the best to put bigger fins that put more weight to your rocket cause the more heavier the more performance it will loose... Yes and No. They have to be big enough to make sure the rocket goes straight but small enough to not loose performance. -dag Link to comment Share on other sites More sharing options...
THEONE Posted September 8, 2011 Author Share Posted September 8, 2011 But adding more weight it will also loose performance... Link to comment Share on other sites More sharing options...
dagabu Posted September 8, 2011 Share Posted September 8, 2011 But adding fins that are too small will make the rocket go wild!! -dag Link to comment Share on other sites More sharing options...
THEONE Posted September 8, 2011 Author Share Posted September 8, 2011 But adding fins that are too small will make the rocket go wild!! -dag What do you mean i also say that it will be better to add bigger fins cause more weigh = more loosing performance... Link to comment Share on other sites More sharing options...
dan999ification Posted September 8, 2011 Share Posted September 8, 2011 What do you mean i also say that it will be better to add bigger fins cause more weigh = more loosing performance... fins will probably weigh as much as a suitable stick i would not worry about the performance loss if you use a light weight material. if the fins weigh the same as a stick they will out perform it, more aerodynamic and less drag.if you decide to make a recovery system or any headers the weight of the fins is not important since you add more they just have to be big enough and light enough. dan. Link to comment Share on other sites More sharing options...
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