surface finish

[neandethal]

Hi, I have tried the search engine and cant find a definitive answer or even strong opinions.
Now i know everybody tries to achieve a smooth mirror finish to their streamliners, (Damn time consuming) but just how much effect does a less than perfect surface finish have 1%, 5% or more
Any thought s welcomed, cheers Ian

[wobblywalrus]

My bike is covered by a bunch of riveted aluminum plates.  "What plates should I remove and replace with a single smooth piece?" I asked the wind tunnel operator.  He said to not worry and to keep the finish I have.  He said it would be a waste of time to smooth it out.  His father operates a wind tunnel next door.  He has a lot more experience.  I asked him why this is.  He gave me an explanation that verified that the plates are no problem.  What he said completely went over my head.  It was very technical.

[WOODY@DDLLC]

WW: Hoerner: Fluid-Dynamic Drag, 5-8 fig. 14
                         Cd
Hex-head bolt - .800
Round Rivet    - .320 [This might be why you don't see them on most aero contraptions! From 30 mph solar to 450 mph airplanes!]
Flush rivet      - .002 [I milled lots of these flush on DC-9s in the late 60s!  ]

Other factors involved in the numbers but this is close to a 1:1 comparison.

[neandethal]

Great info Woody and just what i was after. If recessed rivets which are in reality far from smooth only have a drag coefficient of 0.002, then the resultant roughness after a course sanding down of glass fibre/resin should be even less, and the hateful hours of sanding to achieve a babies bottom level of smoothness unnecessary!
Or am i reading the results to suit my laziness

[Lemming Motors]

If you start putting babies in a wind tunnel to assess bottom smoothness and aero drag you will have other problems. They are really hard to tie down to the wind tunnel floor and they tend to wriggle.

[Mike Borders]

Also, with babies, there is always the "SHTF" issue in a wind tunnel   

[Stainless1]

Johnboy is our regular painter.... we usually are as smooth as an orange..... 
 

[ronnieroadster]

Johnboy is our regular painter.... we usually are as smooth as an orange..... 
 

   I recall a  number of years ago a segment on the Mythbusters TV program where the surface of a golf ball was used as an example of improving gas mileage. The test car was a Ford Torus the MPG was first tested with the stock painted body surface and recorded.  Next the entire body surface was covered in clay and then half round indentations were added to mirror the golf ball surface. The now modified body surface test showed an improvement in MPG over the same test coarse used for the stock body surface testing.   Seems from that test a smooth surface may not be the best way to do this.      Who knows for sure but interesting nun the less.
 Ronnieroadster

[wobblywalrus]

The first things were working on shape and frontal area in my build.  It was a ten year process of learning and trying different things.  Finish was not emphasized.  The objective of the tunnel work was to finalize the shape.  Then the finish would be improved.

The yarn tufts and smoke showed the flow was well attached considering this is a bike with a guy sitting on it.  The roughness would make it attached turbulent rather than laminar flow.  Attached turbulent flow has less drag than attached laminar.

  

[Lemming Motors]

Was it America's Cup yachts where they banned specialised surface finishes (someting akin to a sharks skin at the micro level as used - probably by the Kiwi's 'cause they are smart like that) - I think it created a more coherent bounday layer and then the water (or in our case air) is slipping against water rather than hull.

Re: the golf ball surface experiment - a case of 'improving the driving range' - exactly why golf balls are dimpled - improved driving range.

[Rex Schimmer]

If you look through the Target 550 build you will see some pics of Jim Hume using a long vixen file to smooth the surface of the car. They purposely set all of their counter sunk rivets, and they had thousands of them, a few thou above the surface so that they could get the perfect surface by filing them down to be exactly flush with the surface. They also happen to use 14 ga aluminum (.071 in. thick) to provide extra material for the finish file job.

Rex

[wobblywalrus]

Countersink pop rivets are available with the tops flush with the surface.

[Stan Back]

. . . but probably not as "flush" as Hume would want them.

[Captthundarr]

Look up "laminar flow" and the next time you fly check out how bumpy an airplane wing really is...At high speed air will stick to a smooth surface and actually induce drag.