Cd deletion

[Bicycle Bob]

I usually use a thin wooden board, ripped to the thickness suitable for the contours I'm working on. Spray glue is pretty good for holding on sandpaper until it is time to change it. End handles can be screwed or glued on.
If you are really into making smooth shapes, have a look at foamed glass blocks. They wear into any contour as you work, cut like 40 grit, and finish like 100 grit. They are often called Fartstone, being blown with hydrogen sulphide.
I'm also very fond of a 1" hand-held belt sander with all the junk removed so that I can use the unsupported side of the belt.

[aerohead]

Originally Posted by 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.

Patrick,did Bart say that the boundary layer was laminar,or the flow outside the boundary layer?

[aerohead]

Originally Posted by 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.

I think that the general flow outside the boundary layer ( the 'ideal' or 'inviscid' flow ) is laminar,as with any structure free of 'stall' or 'separation.'

[aerohead]

Originally Posted by 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.

Abbott and Von Doenhoff report of an investigation conducted by NACA in which aircraft surfaces,sanded in the direction of airflow with # 320 sandpaper exhibited no more drag than highly polished surfaces,and that this became their 'standard' for the high speed tunnel work.

[aerohead]

I've compiled some data which concerns 'laminar' aircraft technology as it might pertain to EcoModding and will post as a separate thread 'Laminar' Aircraft and EcoModding.
I thought the topic deserved its own thread and will probably pop up from time to time.
Hope it's okay.

[Patrick]

Originally Posted by aerohead

Patrick,did Bart say that the boundary layer was laminar,or the flow outside the boundary layer?

He didn't say, but since they were listening with a stethoscope pressed against the body, my guess would be that it was the boundary layer.

[rumdog]

Bumping this old thread as i have been watching these excelent videos full of design and construction details... heres the link https://youtu.be/U2PX1p7nbw8

Mike uses sandwich construction with foam and fiberglass/epoxy to build this slippery one seat plane, which would transfer to aero caps or covers nicely.

Look up The Arnold Company on you tube. Many excellent vids like "moldless low drag wheelpants"

[freebeard]

That's quite the hot-wire foam cutter. Here's mine:



And here's the Lionel transformer my folks used to cut foam around the time Burt Rutan was starting out:



I'm only 42 minutes in, now to go back and finish.

edit: The wheel pants thing is great. There are interesting design decisions from offsetting the strut to turning the split laterally.

The whole process in miniature.