Cd deletion

[Varn]

I don't know if he said this on the web site but he mentions that waves are worse than surface fineness. I remember a part of the tape where he was measuring and plotting sagitta along a line on the airplane. He used 3 points about an inch apart. He then graphed it and compared it to other commercial aircraft.

Originally Posted by 3-Wheeler

Bob,

I was thinking of you when I went to the AR-5 website and was reading about just how *smooth* the aircraft was shaped.

Flashed back to our earlier discussion of how laminar flow can be easily *tripped* into turbulence by any roughness in the surface finish.

Jim.

[aerohead]

Originally Posted by Bicycle Bob

"If it's a 'laminar' aircraft you can increase the Cd significantly,as it is not possible to have laminar flow above critical Reynolds number in ground-effect.( 20-mph up )."

Do you have a reference, or a rationale for that? Thanks.

Bob,just now catching your post.Am away from my references here at the copy center so relying on memory ( a scary proposition!),here's what I recall.
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A laminar structure is designed to operate under 'flight conditions' which for aircraft is at altitude,in free-flow environment,with turbulent-free air.
A laminar structure is only 'laminar' up to the first point of minimum pressure ( center of lift ) after which the flow is necessarily turbulent due to the un-favorable pressure gradients present.
Also,in aircraft applications,drag is customarily governed by skin friction and embody zero flow separation,whereas in automotive applications,skin friction plays an insignificant role,with flow separation responsible for the largest portion of drag.
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For road vehicles above 20-mph,and for the surface roughness conditions obtainable in automotive finishes ( or even polished surfaces ) the given surface grain size versus vehicle length/velocity relationship mandates that the Reynolds number go 'critical' and boundary layer transition to ' turbulent' with a ground/air velocity of 20-mph.Which is all good for road vehicles,as the turbulent boundary layer allows the separation point to be moved rearward,allowing pressure regain and reduction of pressure drag,the largest component of profile drag.
All air in ground proximity is considered turbulent and cannot support the environment necessary for laminar boundary layer,unless the velocity is held below 20-mph.( i.e. college teams pulling off 7,000 mpg as they creep along a closed-course ).
From memory,I will credit references I cited in my stickys: Fluid Mechanics by Dauherty and Franzini,Boundary Layer Theory by Schlichting,Aerodynamic Drag by S. Hoerner, Theoretical Aerodynamics ( can't remember author ),I think that Abbott and Von Doenhoff get into it also in their Theory of Wing Sections.
In my Fluids text there were some sample problems for road vehicles.Critical Rn occurred around 500,000 which was triggered at very low ground velocity.
That's all I can do off the top of my head.
At Battle Mountain I grilled the Cal Poly team( I think it was).They were using CFD for engineering their bike and believed that Cd 0.11 was probably the 'range' for bikes running,like Varna Diablo III.The day I was there,Sam went 81.6-MPH in it.
Matt Llewelyn's (sp?) Sylph was designed around something like a Cd 0.0066 laminar wing section and also scored Cd 0.11,in ground proximity in the Guggenheim tunnel at Cal Tech.
It appears that,on the ground, the 'laminar' forms cannot exploit the efficiencies they enjoy in free flight.

[Otto]

Regarding surface texture, about 20 years ago Dan Somers at NASA Langley did wind tunnel tests of factory painted, optically smooth and shiny, and sanded (in direction of flow) wing panels.

Guess what?

Draggiest was the shiny surface, next most draggy was factory painted surface, and least draggy was sanded-in-direction-of-flow surface. This is also essentially the reason 3M skins were used on America's Cup sailboats, to good effect if aligned with flow or no more than ~15 degrees off. Has about the same texture as a phonograph record.

For our purposed, probably best to forego attempts at laminar flow, but try for turbulent but attached flow, which is much less draggy than turbulent and detached flow.

[Patrick]

GM (Aerovironment) claims that they were able to achieve laminar flow with their Sunraycer:

"In the GM wind tunnel, we were able to put a stethoscope against the body and hear whether the flow was laminar or turbulent. If it was laminar, the sound was smooth, a whooshing; turbulence burbled. We were able to confirm that the flow over the front third of the car was laminar everywhere except for two thin wedges just above the side edges starting back about the middle of the front wheels. It was not clear why the flow was not laminar there, but as it represented less than one percent of the drag of the vehicle, we were not overly concerned."

Bart Hibbs, Lecture 2-2: Sunraycer Aerodynamic Development, p 28, in GM Sunraycer Case History, 1990.

PS: Speed was 40 mph.

[Varn]

I would agree that Georgi's bikes are anything but laminar flow bodies. Yet they are quite practical vehicles and can be ridden in 20+mph gusty conditions. I have two hanging on my wall (Varna 2, the grandfather of the diablo3 and a Mephisto clone (unfinished)) and have personally witnessed the Varna2 being ridden on a closed course in dust devils at 50+ mph for an endurance race.

Of course Georgi doesn't use any engineering or computers, just what looks good and experience. Georgi has told me that as long as the surface is not wavy then the finish doesn't make that much difference at the speeds that human powered vehicles run. I believe him. He goes out at night with a flashlight to refine the shape of his plugs using tangential lighting looking for waves.

Back to the subject.

Originally Posted by aerohead

...
At Battle Mountain I grilled the Cal Poly team( I think it was).They were using CFD for engineering their bike and believed that Cd 0.11 was probably the 'range' for bikes running,like Varna Diablo III.The day I was there,Sam went 81.6-MPH in it......
....For road vehicles above 20-mph,and for the surface roughness conditions obtainable in automotive finishes ( or even polished surfaces ) the given surface grain size versus vehicle length/velocity relationship mandates that the Reynolds number go 'critical' and boundary layer transition to ' turbulent' with a ground/air velocity of 20-mph.Which is all good for road vehicles,as the turbulent boundary layer allows the separation point to be moved rearward,allowing pressure regain and reduction of pressure drag,the largest component of profile drag....

[Bicycle Bob]

Aerohead writes "All air in ground proximity is considered turbulent and cannot support the environment necessary for laminar boundary layer,unless the velocity is held below 20-mph."
I have always been skeptical of claims of laminar flow on vehicles, and this is another common consideration, but it seems that when the wind is legal for a speed record, the air might be still enough for some laminar flow.

Anybody trying for it will want to look at longboards for sanding - flexible, two-handed sanding blocks approaching a meter long.

[3-Wheeler]

Originally Posted by Otto

.....Has about the same texture as a phonograph record.....

Hi Otto,

Did the article mention anything about what grit sand paper was used?

Thanks, Jim.

[3-Wheeler]

Originally Posted by Bicycle Bob

....Anybody trying for it will want to look at longboards for sanding - flexible, two-handed sanding blocks approaching a meter long.....

Hi Bob,

Do have any recommendations for this type of sanding board? Maybe some pictures of one's you may have?

Thanks, Jim.

[Varn]

We used sanding belts and slid a section of formica or masonite a bit longer than half the length of the belt. Making in effect a bow. Sanding is done fore and aft using the open side. It works good with two people. Start with 36 grit and work toward finer. Belts can be gotten from Mcmaster for a good price.

Originally Posted by Bicycle Bob

Aerohead writes "All air in ground proximity is considered turbulent and cannot support the environment necessary for laminar boundary layer,unless the velocity is held below 20-mph."
I have always been skeptical of claims of laminar flow on vehicles, and this is another common consideration, but it seems that when the wind is legal for a speed record, the air might be still enough for some laminar flow.

Anybody trying for it will want to look at longboards for sanding - flexible, two-handed sanding blocks approaching a meter long.

[Varn]

Here is a hand written page describing Georgiev's sanding and some construction techniques: I used the last one described.