Teardrop aerodynamics... in reverse!

[Frank Lee]

Well said!

[Bicycle Bob]

Originally Posted by hypermiler01

Roll bars' purpose isn't to be aerodynamic. If the tubing had a teardrop shape, it would have very poor resistance to lateral bending loads.

Who proposed re-shaping the tubing? I mentioned Adding cardboard and tape.
I'm going to ignore this thread starting now - that "beating a dead horse" graphic should have warned me off.

[MazdaMatt]

Any reference to a race car to back up the pointy nose theory is totally moot. There are very few cars on the road that have a higher .Cd than an F1 car. They are not designed to cut through the air - they are designed to scoop under it and generate 5g's of downforce. Simply look at the list of the lowest .Cd cars in the world and you'll see that the pointer nose is not used. Period.

[Piwoslaw]

How about A-B-A testing:
Two identical teardrops (same size, weight, etc.).
One teardrop has pointy nose added.
Drop from tall tower.
Tower height small to ensure subsonic speed at bottom.
Wife at bottom with stopwatch.
Pad and pencil (or spreadsheet).

[winkosmosis]

Originally Posted by Bicycle Bob

Who proposed re-shaping the tubing? I mentioned Adding cardboard and tape.
I'm going to ignore this thread starting now - that "beating a dead horse" graphic should have warned me off.

I think you're right about roll bars, but if those racecars really wanted to be aerodynamic they'd have a canopy.

I don't think land speed record cars are made by engine guys. They must have aerodynamic experts.

[winkosmosis]

BTW, how can this Lexus possibly be 0.25cD?

It has a big flat front, not even a dome like a teardrop, and a notched rear rather than gently sloped rear window.

[hypermiler01]

Yes, the implication that speed record guys don't carefully plan the aerodynamics is ridiculous and insulting. Might have been true about a few of them who had no real competition.

Another good one to watch is the HPV (human powered) races. They show how important good aero is at relatively low speeds.



And yes, any racing series where top speed is important doesn't have exposed roll bars, or exposed cockpits.

I have noticed the last few years that the auto designers seem to be pointing the sides and headlight areas to direct a lot more air to the sides. The overhead view is also very important. That Mercedes fish car has a quite V shaped nose.

[hypermiler01]

Originally Posted by Piwoslaw

How about A-B-A testing:
Two identical teardrops (same size, weight, etc.).
One teardrop has pointy nose added.
Drop from tall tower.
Tower height small to ensure subsonic speed at bottom.
Wife at bottom with stopwatch.
Pad and pencil (or spreadsheet).

Those are what bullets are. It is well known that pointed bullets are faster, even the subsonic airgun pellets, and they penetrate any fluid with less resistance, including water and air.

The problem is that if the point is too long, it causes directional instability. Pointed nosed tear drops dropped from a tower wouldn't fall straight without tail fins like a dart to make them spiral.

And depending what speed they are operated at, there are diminishing returns where the extra surface area causes more friction.



Looking back at that chart, you can see that the highest value of pressure, either positive or negative, is directly in front of the traveling sphere, where the surface is at a 90 degree angle to the oncoming air. The lowest pressure is not behind the ball, but at the point in front where the airflow is about half way between perpendicular and parallel with the surface.

By making the nose of bullets and airplanes parabolic rather than hemispherical, both the pressure and the volume of the pressurized air in front of the object are greatly reduced.

[hypermiler01]

The reason that the noses and wing edges of airplanes are not razor sharp is not because it wouldn't be more efficient, but because they are frequently subjected to gusts and angles where the leading edges are not in line with the oncoming air, and the air would shear at the edge and stall, rather than following the curvature around to the top wing surface.

But the radius of the leading edge is kept as small as possible to avoid pushing air while being large enough to do the job of maintaining attached flow at the necessary range of pitch and yaw.

[Bicycle Bob]

[QUOTE=hypermiler01;92004]Ye
And yes, any racing series where top speed is important doesn't have exposed roll bars, or exposed cockpits.

Drag races are won by ET, but they still keep records of top speed. With open-cockpits, and using round tubes for wing supports and two big roll bars, they are hitting 300 MPH.