<center><img src="http://www.edmunds.com/edweb/brauer/photos/00BMW.3.Coupe5.jpg"></center><p>
Here's a pic of the a mirror on the new BMW 3 series coupe. Notice the little ribs on the top edge. I guess these work the same way as that pop-up piece on the A4 sunroof.

Question is, how exactly does a rib-like structure work to reduce wind noise?


-K. '99 A4 1.8tqMS<ul><li><a href="http://www.edmunds.com/edweb/brauer/2000BMW.3Series.Coupe.rt.html">Edmunds' first peek at the 3-series coupe for my2000</a></li></ul>

I visited a helicopter rotor factory once, and this was explained in very good detail. Similar ribs appear near the edges of helicopter rotors for a slightly different reason, but might contribute to BMW's logic in adding this feature to mirrors.

Helicopter rotors are not, as you might first think, simply opposed wings made to rotate. Instead, they are a hollow composite structure designed first to create lift, second to decrease turbulence, and only third to act with conventional airfoil characteristics.

Let's back up for a bit of interesting information. Helicopter rotors are hollow and evacuated with a vacuum sensor inside the rotor. If the rotor develops a stress crack, the cavity in the rotor is reoccupied and the pilot is notified. I've had this warning go off in two helicopters I've been in. One was a false alarm from a bad sensor. One resulted in a near-to-ground crash in Latin America. The false alarm was in a Robinson, the ground impact in Central America was in an older Bell. Just a side-note.

You see ribs on high performance helicopter rotors because, as you can easily visualize, the outside portion of the rotor has to cover far more distance per rotation (augmented radius) than the inside portion of the rotor. In some cases, mostly military (Huey II, RMLF, etc.), part of the rotor is traveling at supersonic speeds (or barely subsonic speeds) while the other portions of the airfoil are acting at lower speeds. The whack whack whack you hear in Vietnam footage is partly resonance inside the helicopter from which the film was taken, but partly the rotor breaking the sound barrier or decellerating (note that leaving >1.05mas speeds also generates enough disturbance to cause a distinguishable sound). The ribs on these rotors are "cleaning spoilers" that supposedly clean up the air as it passes off the trailing edge of the rotor.

Would they work on car mirrors? Who knows. My guess is that it's more a styling cue to repeat the patterns in the headlight assembly than an aerodynamic feature designed solely to reduce wind noise.

Cameron

These eddies produce less drag than the larger flow patterns that would result if there were just a smooth surface.

Ever wonder why golf balls with dimples go farther than those without?

dimples on a golfball help create a lifting force due to rotation of the ball. This is why it flies farther. A smooth ball would produce almost no lift. It's the same thing with the threads on a baseball. If you put heavy back spin on a ball you get a slight lift (normal throwing). Topspin pushes the ball down (slider). Left or right spin causes an opposing sideways force (curve ball). No spin results in a less predictable (wobbly) flight characteristics (knuckle ball).

Laminar flows ALWAYS produces less drag, but having mild turbulence (eddies) can help to break up vibration due to harmonics since you don't get uniform flow characteristics. This could help reduce any whistling or rushing noises. On the other hand, heavy turbulence can lead to very negative harmonic vibration. Ever seen a road sign flutter and twist from left to right in a hurricane or high wind? This is an extreme example of turbulent flow separation which follows a sinusoidal pattern. It has been known to rip the wings right off of some aircraft.

Just my $0.02.

(BTW...I was an aerospace enginerring student in college so I'm not just BS'ing you guys)

Chris

That we were notified when we lost blade integrity! The UH-60 Blackhawk has a nitrogen filled titanium spar that runs dow the blade about 2/3 of the way... There is a indicator that turns from yellow to red when the pressure drops below a certain level. We inspect it on pre-flight.

Cam's right about all the rotorhead talk, although our rotors don't need no steenking 'ribs' on them... but we're too busy picking up 9,000lb. loads to worry about noise.. :)

P.S. A ROBINSON had a warning system for the blades in the cockpit? Man, that thing's lucky to have a VHF radio!!! Musta been a SOF (Safetly of flight) message requiring the mod.

Rich<ul><li><a href="Quinland">http://home.att.net/~richquinlan</a></li></ul>

if that's your .02, i'd hate to see a buck or 2 from ya....

seriously, thanks!

nt