Finish versus height.

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OccamMD

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At the last HERC event I saw some beautiful smooth as glass paint jobs. This is great for looks, but my question is, wouldn't you want your surface roughed up a bit if you are going for height? Obviously not every model is built for this, but if you where going to build for sport in the height category I would think that you would want a slightly roughed surface to decrease the drag on the rocket during flight.

I was thinking of building 3 identical kits with differing roughness to test this out, tough correlation given all of the factors, but maybe possible.

Any comments?

Thanks,
Ray
 

OccamMD

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Just a note, I would think that this would be dependant upon length 'cause we are talking about the region where we develop laminar flow after turbulence created by the nose cone.
 

Micromeister

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Did the same experiment a while back, the Smooth as a babies butt model, waxed reached 22% greater tracked altitudes with motors weights within .2g and model Ewts adjusted to be identical.
one model had a granite granular type finish the other a Finesse-it polished, smooth as glass, waxed finish.
Heres a composite pic of the 2 models

Ps: when the two were flown on C6 matched motors the Smooth javelin was lost out of sight while the Rock-it finished model never left the field of view.

I'm not sure, but I believe the golf ball dmple effect to reduce drag has to have a specific pattern and depth. I'm sure one of the engineers here can give a more scientific explanation.
 

OccamMD

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Thanks John,

Your rough finish is more than I was thinking of. I was thinking really smooth, fine sandpaper smooth, and medium sandpaper smooth. Just enough to break up the laminar flow.

?
 

Silverleaf

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To be honest, I think John's test is a perfect example of drag/aerodynamics in action. I'd love to see this test for myself, so I think I'm going to buy two kits and do the same.

John, did you do this just for you, or to teach students about drag/aerodynamic forces ?

How did you finish the rough rocket ?

Thanks for the idea,
 

OccamMD

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I would have thought that all you would want to do is break up the laminar flow against the tube/fins. NO? It looks like his rough rocket is pretty rough. John, how high are the rough spots?
 

OccamMD

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I am going to see if I can get some flow analysis done at a place I work, just for fun. Any interest?
 

eugenefl

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Originally posted by OccamMD
I am going to see if I can get some flow analysis done at a place I work, just for fun. Any interest?
Ray, this type question might do better, in terms of responses pertinent to the question, under the "Techniques" forum. This is pretty ambitious I tell ya! Personally, I'll take looks over performance any day. All we really care about is smoke and fire right? :D
 

powderburner

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Originally posted by OccamMD
I was thinking really smooth, fine sandpaper smooth, and medium sandpaper smooth. Just enough to break up the laminar flow.
For a given length/wetted area, laminar flow is the condition with the lowest drag. You WANT laminar flow. You LIKE laminar flow. You NEED laminar flow.

For competition models, you may even want to permanently attach the NC, fill and polish the joint between the NC and forward BT, and use mid- or rear-ejection. This gives you the best chance to generate smooth airflow over the nose and preserve it as far as possible along the length of the rocket.

Same reasoning goes into shaping the cross-section of the fin leading edges as rounded, or ideally, parabolic. Fin surfaces should smoothly transition to maximum fin thickness, and then smoothly taper to a knive edge trailing edge. That is, for absolute minimum drag. Such a fin configuration is just begging for dings and dents if you put it onto a sport flier.
 

Micromeister

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Laminar flows our friend...Fire is good....Fire is our friend...Oops i'm back:D

The "Rock-it" was painted with a Granite look 2 part kits I picked up at Micheal's, it's a base coat and a "splatter coat" type over paint. The peaks and valleys are very irregular. Best I can measure the highest peak to valley is only .003" most are .002" or less. I think the model actually looks rougher then is really is:)
Yes the experiment was part of a Boy Scout Mod-Roc camporee/ launch/seminar weekend. This particular segment was "It's a DRAG! aerodynamic forces friend or foe"... or something dumb like that. I'm looking for the handouts and/or results data sheets but haven't been able to put my hands on them.
We added tiny amounts of clay to match the prepared weight of both models less the motors. matching the motors was the biggest problem, i seem to recall having to purchase 3 or 4 25 motor bulk packs of B6-6 motors from Estes to get 24 that were close enough to be considered "matched". of those 24, 12sets were flown and tracked over two weekends. the first 6 at the camporee the last 6 at a MR club launch the following month. Tracking at both events was done by the same team of experienced adult mod-roc competition trackers. as I mentioned only models that closed within 2% were used in the study. relying on the note in the photos only 8 dual flights closed for use in this experiment. I'll do some digging in the shed over next weekend if I can find the data i'll scan it and post it latet.
 

OccamMD

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Thanks for the discussion, this is great. My understanding is that laminar flow is actually a high drag condition, at least when I was researching the development of a solar powered autonomous sub we found that the most efficient design was a teardrop body shape with juggling of frontal surface area vs required volume. The actual torpedo body shape was not very efficient. The reason is that the laminar boundary could not develop on the trailing edge of the sub in the teardrop. The thrust portion was strung behind on a small diameter shaft.

This started me thinking about the boundary layer on the rocket. Now there is no doubt that it is speed dependant, and we may be going fast enough that the laminar flow never re-establishes itself. In that case I would agree that the smoothbody would be best for less interaction of the body with turbulent region.

One example of this is a draft meter we have a patent on. It measures the pressure wave found behind a moving vehicle, like a tractor trailer, and allows you to optimize your drafting distance to optimize fuel economy by showing the difference of ground speed vs wind speed. You end up in the turbulent region, its periodic for some distance, and help yourself out by as much as 20% in fuel economy. I did this a number of times across the Howard Frankland bridge here in Tampa. This isn;t exactly a laminar flow example I know.

Thanks for the info, thoughts? :rolleyes:
 
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