I'm designing a rocket that will have an avionics bay just forward of the fins. Due to some packaging considerations, the static port will be in line with a fin, or just slightly offset from the fin. How far forward of the fin's leading edge should the static port be placed so as to avoid turbulence issues created by the fin?
Thanks, Bob Chmara
Jerry, could you explain why? Or point us toward why? Nothing too technical. Laymans terms. I'm not real smart, but I hang around some pretty smart people.
da rmr plumber Gary R Goldenbaum NAR #73669 L1 Northwest Florida remove 'nichspam' to reply
Pardon in advance the tech post.
You are taking a reading of an aerodynamic flow from the "leading edge" of a flowfield (ala interference generator).
You want that reading to be in freeflow.
If I am wrong so far, ignore this post.
The points of turbulance aqround a fin or wing or fin/body joint or wing/body joint varies with shape and velocity. I am assuming basic and somewhat aerodynamic shapes.
I am further assuming you have a body/fin type arrangement generally.
The 1/3 of the chord figure comes from wing studies that show that field turbulance (perturbances) happen well in front of the wing in the form of wake or a shock wave. A shock wave is thinner but more powerful and an aero-flow turbulance or wake is thicker and less energetic.
Therefore if you want to take readings from say zero velocity to M0.5 then you should assume turbulant thick flowfields and stick your sensor pretty far out in the wind. If you note pitot tubes on research aircraft, they are 4-8 FEET out in front.
One of the disadvantages to a 1/3 chord distance on a model rocket is it constitutes a major portion of the overall aerodynamics and a similar shape object on the other side as well would be adviseable.
Just Jerry
We now return you to endless politics and senseless attacks. Control of your set is returned to you.
No Don't! THIS is the kind of stuff I like. Rockets and science. Building, flying, and yes, even the occasional glue thread. (Although I showed you guys that they are no longer necessary;-)) You guys know I have been here for a few years but I don't post much. But I do get sick of the political rhetoric and the arguments. [OT can be a welcome diversion;-)]
Thanks Jerry
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The point is that the static port(s) need to be placed where the local streamlines are straight, not curved. Curved streamlines occur due to pressure gradients, which corrupt the static pressure measurement. That means that static ports can't be anywhere the flow is turning or curling, like near a fin leading edge, or near the body/fin root vortex, or near a flow separation bubble caused by a conical body transition, or on a surface curved in the direction of flight (e.g. on an ogive or parabolic nosecone). You may need a wind tunnel to really see where the flow is nice and straight over your rocket. Computational Fluid Dynamics (CFD) can work too, but it's alot of effort.
Brad Hitch
see below
THE BOTTOM LINE IS for a model rocket traveling below M0.5 that is about
3/4" from any surface face. So if you have a pitot tube sticking out of a fin leading edge, it has to be 3/4" or more from the body wall and also 3/4" or more protruding from the fin edge.To keep the rocket from flying crooked a similar size and placed wanker has to be on the other side as well.
I suggested 1/3 chord since if anything this would make the figure typically be more than the needed distances.
Jerry
Jerry, Nice post. Since I'm not an aeronautical engineer, I sent your reply to one that does it for a living. He pretty much agreed with you. I'm impressed. Here's his reply:
"Turbulence is not the real issue unless you are in the wake of something else, for example if the rocket is yawed and the port is on the downstream side of the body, it will be in the wake of the body. However, assuming that is not the case and the flow is subsonic, then the real issue is not turbulence ahead of the fin, but the flow field disturbance associated with the flow curving to go over the fin at some angle of attack. For the subsonic case this is probably more of an issue than supersonic, since at M>1 the air ahead of the fin does not know it is coming until it gets there. My initial guess is a supersonic probe could be closer to the fin than subsonic, and that
1/3 chord is probably a good enough distance for subsonic. You also need to be a reasonable distance from the missile body, which is probably as much an issue as the wing."Mark Simpson NAR 71503 Level II God Bless our peacekeepers
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