Aerodynamic drag effects of static port holes?

I'm working on a B-altitude contest bird for this weekend, and it's time to cut the static port holes for the altimeter. But it seems like a hole in the side of the BT (much less 3 of them) would have significant drag effects. Do they? And if so, is there anything that can be done about it? For example, would placing little fairings above them help, or just create more drag and add weight that would totally negate the benefits? I'm not planning on anything like this, but I'd like to minimize drag as much as possible. And, from the drag standpoint, what is the best shape for the holes, or does it not matter?
Thanks in advance.

In order for an altimiter to function correctly, it must sense the static pressure from the outside air, and if the static porthole interferes with that outside air, then the altimiter will return a faulty reading. That's why eliminating burrs on pitot-static tubes is important. So, if portholes don't interfere with the outside airstream, then they don't interact with the outside airstream, and, therefore, produce no drag. I say this based on the little I know about fluid mechanics, so if someone can speak from real-world experience, please correct me.

JMO-NO fairings! The turbulence from these fairings is a bad thing and create more surface area for drag-like a launch lug, and will create a negative pressure zone behind them-giving false readings. That is why you drill the holes in 'clean air'. The holes need to be clean with no 'fuzz' or lip on them. Drill a small hole, put a drop of CA on it and then drill again. Another drop and then sand till smooth. Inside too! On a "B" motor, you'll only be in laminar flow for a short time and the boundary layer won't separate due to the short period of time. Make sure your nose cone/BT interface is seamless and you have a a high polish. Wax but don't paint-that is a weight penalty. Foil your fins and if you can make a lite (cardstock) tailcone, go for it! Clear skies, buddy-hope you win!

Not only will fairings make the readings incorrect, they'll also add more drag. If your holes aren't excessively large, they shouldn't cause significant drag regardless, so I wouldn't worry about it too much.

Thanks everyone. Like I said, I wasn't planning on adding fairings or anything; that was just abstract speculation. GA, I don't know why I never thought of that. :bangpan:
Fyrwrx, how important is it to have a seamless joint between the NC and BT? That may be somewhat difficult, but I'll try. The fins are 1/32" balsa, and I have foiled them, as much as I can, anyway. A cardstock boattail is out, wit the model being minimum diameter, but the tube, fins, and NC are all pretty smooth. What do you mean by "wax"? And don't worry, I wash;t going to paint it, either.
I figure I stand a pretty good chance of at least placing, if not winning; the contest is only open to those aged 10-16, so I don't have as fierce competition as I might if that were not the case. I just hope I don't lose the club's Alt1...

Boundary layer turbulence is an issue related to the nose cone/body tube joint; a smooth transition from nose cone to body tube will keep the air flowing smoothly over the rocket. Rough joints cause turbulence and drag.

GrossApproximator said:

Boundary layer separation is an issue related to the nose cone/body tube joint; a smooth transition from nose cone to body tube will keep the air flowing smoothly over the rocket. Rough joints break the air away from the body tube causing turbulence and drag.

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Not exactly. At the nose cone to body tube joint, you won't get boundary layer separation unless you have a very, very large irregularity (and even if you do, it's likely to be a small separation bubble, not a large separated region). Tailcones might have some separation, but only if they're fairly steep. The bigger issue at nose cone to body joints is that they will tend to trip the boundary layer from laminar to turbulent, which will slightly increase drag. It's pretty hard to avoid though. Of course, the smoother the better from an aerodynamic point of view, but you'd need a rocket with pretty horrible irregularities to start seeing flow separation problems.

Thanks cjl. Post corrected.

Just keep your Ports as small as possible and debur. my normal practice is to add a drop of CA at the port site then sand back to completely smooth.
Not sure what Altimeter your using, the Adrel Alt-LED NAR approved Czech altimeter needs only .3mm to .4mm vent holes. (.0118"-.0157"). I went with 3- 1/64" (.0156") vents in my 3x1/2A 13mm cluster altitude models recently. This Altimeter and Pico's now uncertified P1 both performed perfectly with these tiny ports.

I'm hoping to get the ports done today, but I might not because of the homework load.
Micromeister, the altimeter is a Jolly Logic Altimeter One. I guess I still have a little research to do, since I realized I don't even know how big the holes have to be in a BT-20 rocket. :bangpan:

I'll see if I can save you a little time...link to Altimeter Port Sizing-->Here

It would be nice for someone to come up with a chart for smaller than 1.14" (27.94mm) though. Anyone?


My head would pop if I even tried to figure out the math...

I just pasted this from the Adept site...

The general guideline for choosing port size is to use one 1/4 inch diameter vent hole (or equivalent area, if multiple holes are used) per 100 cubic inches of volume in the altimeter chamber. For instance, An eight-inch long four-inch diameter tube has a volume of about 100 cubic inches. Use one 1/4 inch port, or three or four 1/8 inch ports evenly spaced around the tube. An altimeter chamber two inches in diameter and eight inches long (25 cubic inches) needs one 1/8 inch vent hole or three or four 1/16 inch vent holes. Try to keep hole sizes within -50% or +100% of the general guideline. Do not make the holes too small, and especially do not make them too large. Obviously, a vent or vents in a small body tube (18 mm BT-20 or 24 mm BT-50) will be quite small. However, in general, the vent holes need never be smaller than 1/32 inch. Also, the vent hole diameter need never be less than the thickness of the body tube.

Adept Rocketry completed the research on static port sizes in 1990. The information provided here has remained the industry standard ever since those early years.

When possible, vent holes should be a minimum of four body diameters below the junction of the nosecone with the rocket body. This is necessary with high performance (high speed) rockets. The tremendous pressure on the nosecone leeches down the rocket body as much as three or four body diameters before it dissipates. However, with lower speed rockets, the "minimum of four body diameters" rule may be reduced to one or two.

I did the highlights in red...


Kinda funny this came up, I'm trying to figure it out for an AltimeterOne in a Wildchild. I actually made 3 holes, but wondered if they were too small....

Thanks, that helps a lot! I really need to finish this thing....
What?! It's Thursday already?! :y: Off to go cut some static vent holes! :horse: (Not really. )

I like the adept website. There is a lot of good info on it. I especially like the quick and dirty vent hole calculations.

While you haven't said what your building your model out of rocketbuilder. Most Current Estes, BMS & Totally Tubular type cardboard BT-5 to Bt-50 tubes have a .013" wall. Quest tubes are a bit thicker, if that helps.

me said:

Kinda funny this came up, I'm trying to figure it out for an AltimeterOne in a Wildchild. I actually made 3 holes, but wondered if they were too small....

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I flew my WildChild today, and tried to test the AltOne. I'm guessing my holes are too small, because the reading of 808' just does not seem right. It sim'd at over 1200'. I wonder if it took the reading at deployment, because the holes couldn't 'breathe' fast enough at apogee? It was on an F24.

That's ok though, it's much easier to make a small hole bigger, than a big hole smaller....

It seems that alot of people just poke a few 1/16" holes and call it done. I'd like to use the smallest hole possible. I may have to try to figure out that math after all, if I want to get all technical about it, which I do. (I over-complicate everything. :blush: )

<hijack off>


Sooo, Rocketbuilder, how did your flight go this weekend?

You said it was windy today, right Jeff? That alt reading is probably right considering the conditions and typically rather optimistic sim estimates.

McKailas Dad said:

Sooo, Rocketbuilder, how did your flight go this weekend?

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It didn't. I was planning for my contest entry to be the second flight of the day, but because of crapperhead igniter difficulties with my (attempted) first flight, I didn't get a single rocket launched before the whole thing was rained out.

Jeff,
Could flight trajectory be the cause of flying lower than sim? I've seen swings of 40% between a straight up flight versus an arcing one...

John, thanks for chiming in! While, yes, it was windy, the altitude did seem pretty respectable. Considerably higher, visually, than it did with an E18. It did arch over some though.

On a previous launch, the reading I got on an E18, with no vent holes, was 767' (so the reading was at deployment) I drilled the delay, and it was shortly after apogee.

In contrast, the F24 delay was quite late.

To give you an idea, the holes I made will admittedly tiny. If I were to estimate, I would say maybe 1/32, or maybe the diameter of a thumb-tack needle. It was more of a pinhole, than a vent hole. I have since made the holes larger (1/16"), but have not yet tested it.

Totally Agree with John.
Sim's are notorous for being between 10-25% high depending on how accurate the input data was on the model. Couple that with windy day non-vertical flight path, Motor 10% thurst/duration variences & Altimeter altitiude reading varience, it's easy to see that kind of difference between Simulated flight over actual achived altitiude. You'd also have to include at least two station visual altitude tracking along with your altimeter to get a really accurate feel for what your model really did.

Barring any actual manufacture instruction on the size of the port for your particular altimeter the original 1/32" holes are were likely more then sufficent though i'm sure 1/16" ports won't bother or alter future flight readings.

Did you update your sim with the real weights when you had finished building the rocket? You'd be surprised how much weight paint and glue adds!

Nope, never bothered updating the file. The components themselves were so light that my scale wasn't sensitive enough to measure the weight, at least for the fins. The whole rocket did weigh about what the sim said. As for paint, nope, none of that! Glue, maybe, but I used very little and most of it was CA. To avoid heavy fillets, I used very thin tissue paper with just a little white glue to reinforce the joint between the fins and the BT. It's actually a fiberglassing technique I picked up over in the HPR forum, just on a very small scale and with tissue paper instead of FG.