NCFMF Video: How to Reduce Drag
 

I found this very interesting video on another forum, but it is very applicable here so I thought I'd share:
http://www.youtube.com/watch?v=ftq8jTQ8ANE

There is a whole collection of these National Committee for Fluid Mechanics Films (NCFMF) videos. More here: National Committee for Fluid Mechanics Films

An excellent contribution darcane, I feel like I learned something today.

Wow, I knew streamlining was important, but I didn't realize how much of an impact the little things can have on drag. When I saw the wire compared to the foil, I couldn't help but think, car antenna. Every little bit can make a big difference.

Thanks for the video. Good find.

Mental note to self: Wing-shape that antenna stalk :)

cool video; thank you for posting!

So I just did!
http://imageshack.us/a/img689/7600/aerotenna.jpg
Cut a sheet from some hard plastic bagel box and glued the opposing edges together, nudging the fold into a rounded edge for that more or less wing shape.
Might do something more permanent later.
http://imageshack.us/a/img153/842/wingtenna.jpg
Too early for A-B-A testing results yet ;)
Honestly, if it would reduce the drag by 1N at highway speeds that would be great, but I'd have to drive for thousands of miles to save just 1 liter of fuel by that. But even at a drop at a time any improvement still is, well, an improvement :snail:...

That was fun to watch & think about.

Three points: 1) the rectangle didn't have the same cross section as the foil and the original rod, so the drag value really can't be compared.
2) the data about round vs airfoil was implicit in the drag value of the rod, even before the wire was tested.
3) I didn't calculate Reynolds number of this test - if I think of it, I'll check with Hoerner to see how close this test data matches.

No, but it certainly is telling - it had a much reduced frontal area, and yet displayed much greater drag.

An equally telling display, I think, would be to build this same rig and test three shapes: a square, add a rounded nose to the square for a bullet shape, then add a proper tail to the bullet to create an airfoil. That would keep the frontal area constant.

telling
 

If you can find a copy of Hoerner's 'Aerodynamic Drag' you'll find all those numbers.

I looked at some of the neighboring videos. Here's some interesting stuff:

http://www.youtube.com/user/yshsuzuka/videos
9 videos. Desktop/office-sized (1/7th-1/24th scale) moving belt wind tunnel. "Reynolds number is no problem." Since 1974.

http://wwwDOTyoutubeDOTcom/watch?v=gHFFZ1ru0Pk
Scale models submerged in water. It works about as well as desktop CFD software. Some obvious areas for improvement: The tunnel is too narrow and should be closed on top. Needs immiscible 'smoke' and a steerable nozzle.

I could see a tank divided in thirds with walls/baffles that have four wind/waterwheels, one at each end of both walls. Do you think it would minimize turbulence that way?
http://ecomodder.com/forum/member-fr...6-12-26-10.png

Wow! A rod has 10 times the drag of an airfoil.

Glad I did that antenna relocation to under the bodywork.

First results, if it can be named so, from the wing shaped antenna sleeve.

Arriving at work after 35 km the rear edge had split open in the middle; apparently the glue failed. The sleeve had slid up slightly and the point of the antenna stuck out of the gap. So I took it off.

Version 2 had internal bracing to give it a better wing shape, all in bagel box plastic, but it meant more glue was needed to keep all in place. Tried to speed up the drying time in an oven, but overcured it so it deformed enough to just bin it.

Version 3 had internal bracing by simply adding extra folds to create a triangular shape inside the wing, providing a large glueing surface while mimicking the ideal wing shape quite well.
I blew against its leading edge with the antenna inside, and against the antenna without the wing, and sure enough the wing did reduce drag noticeably even in this crude handheld windtunnel substitute.
Looked much better on the car too. Stuck quite firmly to the antenna.
5 km on the highway, doodling along between the semis, I heard a buzzing sound, then a nasty pop (even felt that, but that might be imagination) and off went the plastic sleeve, onto the A12.

So even though the stalk is quite short on the Insight, the force on it is substantial so any aero work on that does bring benefit and needs sturdiness.
Version 4 will be secured firmly, that's for sure.

The buzzing sound was vortexes shedding on alternate sides of the winglet. You'll see that in some of the other videos. Think: Tacoma Narrows Bridge.

Make a fin of solid foam so the sides don't flex, and put the antenna at 30% of the chord, instead of clear at the front. That will give any vortexes less leverage.

Thanks for the tips.
It did not buzz initially, it only started just before it came off completely.
It already had the stalk at roughly 30% btw and its internal structure made it quite rigid; the trailing edge was a glued double layer. But I will use a solid wing in the final version.
Maybe I'll give it a slight shark fin shape, expanding the (bulky) base forward. Cant do much on the back of that as the spray nozzle sits there and the hatch needs to open fully.

:thumbup:

So here is 4up. Same design as number 3, but slightly narrower and it now has 3 flaps with holes that fold over the antenna bolt, so by screwing that down both antenna and wing sleeve are fixed sturdily.
http://imageshack.us/a/img32/6826/0t2i.jpg

Again I folded the plastic back end like a double layered flat triangular box to create the wing shape and make the back end rigid.
http://imageshack.us/a/img268/4236/ed.JPG

It looks quite crude from the top, but down from that the shape improves to being just mildly crude.

So will it reduce the antanna's air resistance by 90%?
Who knows. I'd be happy with just 50%.

The force on the (unwinged) antenna is quite big at highway speeds. A 50% reduction in air resistance for the antenna might save 0.1% or 0.2% in total fuel usage.
The wing sleeve weighs almost nothing. The whole bagel box was just a few grams.
As the total cost was maybe just 1 cent for the glue I reckon this mod will have earned itself within the first tank full...!
Going for 100 mpg one tiny step at a time...:snail:

The next experiment would have been nice to see how drag with the airfoil increase with angle of incidence.

I've been thinking of giving it a little bit of play so it could adapt like a vane.
But not too much, so in really strong sidewind it could act like a sail and net some forward thrust ;)

mind blown! definitely going to look at doing something with my antenna and window wiper arms. would also like to see what happens when the pole is at somewhat of an angle as the antenna on my car is at a very inclined angle. however the first part of my antenna is flexible so might change it as per pic.

Attachment 18466

Do you tighten down on the three tabs by gripping the whole assemble at it's base or the tip of the antenna through the hole in the top? (And how would that work with the weathervaning?)

Because it is normal to the antenna, not horizontal. If you could enclose the top, there's a bit of a shark-fin opportunity there.

I made a hole in each tab, they fold over the antenna's bolt end and get locked when I screw that in.

Which was quite fiddly as it rotates all the way until it reaches base tight position, so I had to redo it a few times to get it tight while pointed in the right direction.
Wouldn't help much when the direction is 90° out :)

The wing now has a large crack, I'll put some black duct tape (great stuff for Ecomodders btw!) over it so it should hold until I get time tom do the new winglet.
Just got word that the BMSes for my plugin pack are shipped so it will probably have to wait some :)

The same technique may work on the spokes of a cycle wheel.