[Dinger325xi]

I was listening to the NPR show, Car Talk, today and a woman called in who was complaining about that helicopter type noise you get when the rear windows are down. She had a jeep liberty. Then Tom or Ray (the hosts of the show) said that at the moment they are driving a new 3 series BMW and that he can't put any combination of windows or sunroof down so that the noise does not occur. My 325xi is on the car carrier Freedom and is scheduled to dock in New York tomorrow so I was wondering if any of you guys who already have there E90's have experienced the helicopter noise with the windows down and how bad it is. Thanks

[LEDZEP]

I don't think it's any worse than any other car - and not nearly as bad as some. If anyone expects silence when they have an open window and are driving their car, I think the expectation is not in line with reality. Opening the sunroof alone will create a buffetting effect (most similar the the "hellicopter" noice described) in any car at significant speed, because air is rushing past a hole in the roof and some air is being pushed in while other air is being drawn out. Putting the rear windows down about 2" greatly reduces this noise because it balances out the pressure within the car better.

Bottom line is, if you want quiet - run the AC and keep the windows up. If you want the breeze, feeling, and enjoyment of open windows and sunroof then be prepared to tolerate some noise. Generally, when the weather is Spring/Fall like and I prefer fresh air, I will put the sunroof into the vent position and let fresh air in through the vents. I find this to be less noisy, less windy, and doesn't fill the car full of pollen and other crap. In the pop-up or 'vent' position the sunroof is aspirating air from the car to the outside. This creates a negative pressure condition inside the car and pulls air nicely in through the filtered vent system.

My $.02

[e90wraith]

The “helicopter noise” you are describing is the Helmholtz resonator effect. Essentially, the air inside the car (or any volume) is excited acoustically by the shear layer created by the air rushing past an opening of the volume such as a car window or sunroof. It is analogous to blowing over the opening of a coke bottle. The bottle “whistles.” In the case of cars, it’s a little more complicated due to their geometry, but it’s the same physical phenomenon.

As noted previously, the sunroof opening creates the same effect. In order to reduce this effect, engineers use some sort of protuberances that stick out of the opening in order to “mix” the air quickly to reduce the excitation by the shear layer. I believe the E90 has them as well – when you open the sun roof, there’s an array of rectangular flat plates which rises over the roof, i.e., a sunroof wind deflector. The drawback is that the protuberances must be big or tall enough to stick out past the boundary layer – hence they create a lot of drag.

The protuberances will work for windows as well, although I have never seen them factory installed. The protuberance would have to retract when the window is closed to reduce drag. I have seen some aftermarket protuberances- They are sold as wind and rain deflectors, and are made from plastic and look like little sunroof wind deflectors that you place in front of the window frame. They can indeed work to reduce the “helicopter noise,” if designed properly. However, you will have higher drag, i.e, lower fuel economy, and even worse, I think they are aesthetically atrocious.

[parafrog]

check out the big brain on e90wraith! help me with this...if what ledzep says is true, wouldn't the air be trying to come in the sunroof since air moves from an area of high pressure to low pressure. higher pressure being outside the car. it's some aerodynamic principle. some aviator help me out........ just like the air moving slower (relatively) over the top of the car than underneath it. the slower air (relative to that under the car) creates downforce to help hold the car to the road.

[e90wraith]

parafrog,

Actually, the air pressure at the top of the car (near the sunroof) is lower than the ambient pressure. So in fact, the net effect is to produce a vacuum which will tend to pull air out of the car, as correctly stated by LEDZEP. The term you are looking for is the Bernoulli’s effect where high speed air has lower dynamic pressure than low speed air. The situation is opposite of what you describe. The air over the top of the car is moving faster than the air underneath the car. Contrary to what most people believe, virtually all production cars generate lift, not downforce. Besides perhaps the new Bugatti Veyron, even exotic cars such as Ferrari at most generate tiny amount of downforce. Of course it’s a different situation for racing cars. Cars such as F1 and Indy cars can routinely generate 3g+ of downforce. They generate these enormous downforce by combination of wings (essentially inverted air plane wings) and careful sculpting of the underbody and sides. If you look at the underbody and sides of these cars, you’ll see that they curve downward from front to rear. They are trying to exploit the “venturi effect” (essentially Bernoulli’s effect – I’m referring to too many 'effects' here! ) to create a vacuum at the bottom of the car to generate downforce.

By the way, I want to clear up some misconception about drag on cars. The shape of the upper body of modern cars IS NOT the biggest contributor of aerodynamic drag. The two biggest sources are the underbody and engine cooling. That is the reason why the E90 has plastic shields underneath the car. It’s also the reason why the old Lexus LS used to have the lowest drag coefficient for production cars even though it was shaped like a brick. The Lexus engineers took the time to clean up the air underneath the cars, and that was the reason why it had lower drag coefficient than Ferraris, Porsche, etc. of that era - cars you’d think would have far lower drag because of their slippery looking bodies.

[jubol]

E90,
You are so correct!!
That is why race cars have spoilers!!
What they are, is an upside down airfoil, that creates a downward ( lift) force on the rear of the car!!
Fred

[exAudiGuy]

e90wraith-I smell another Aero Engineer in the house besides me Pasadena...JPL or CalTech? Inquiring minds want to know!

[ver.e90]

Dinger325xi, when I had my '03 325xi and drove highway speeds with just the rear window(s) down, I did experience the "helicopter" noise. However, the minute you open the front window on the same side as the rear window that's open, the noise goes away, and you just hear regular wind noise. I don't know about the new 3 series, though.

[Chesmu]

If you crack the rear windows a little the nosie will disappear. While my car was in the shop (for routine stuff) I was lent a new X5 with panoramic sunroof. Holy cow! The nosie when that thing was open was horrific! I coule feel my eardrums popping! I cracked thye rear windows just a tisch and the noise disappeared.

[e90wraith]

Originally Posted by AudiGuy

e90wraith-I smell another Aero Engineer in the house besides me Pasadena...JPL or CalTech? Inquiring minds want to know!

You got me - I'm actually alumni of both Caltech and JPL. In the past, I was an aerodynamics engineer. These days, I'm working on artificial intelligence for air/spacecraft.

[puck724]

wow. learn something new everyday. bravo.

[ON325i]

Originally Posted by e90wraith

The two biggest sources are the underbody and engine cooling.

Thanks for the lesson but do you mind telling us why engine cooling affect drag coefficent? Thanks so much!

[parafrog]

thanks e90wraith

[LEDZEP]

Originally Posted by e90wraith

You got me - I'm actually alumni of both Caltech and JPL. In the past, I was an aerodynamics engineer. These days, I'm working on artificial intelligence for air/spacecraft.

Can't beat a man at his profession! Nice explanation - thanks!

[e90wraith]

Originally Posted by LEDZEP

Can't beat a man at his profession! Nice explanation - thanks!

Thanks, LEDZEP. To clarify, I wasn't a car aerodynamics engineer. My speciality was in turbulent flow over bluff bodies (bodies that are not streamlined - cars fall in somewhere in the middle. They are not true streamlined bodies like airplanes - in fact, they are closer to bluff bodies than streamlined bodies...). I worked on different models and theories, not really applied to anything "useful." Car aerodynamics is just my hobby.

One other tidbit - did you know the "ribbed" tail lights on the old Mercedes were functional? If you look at some automotive society journals from the 1960's, Mercedes engineers wrote some papers on the optimal surface characteristics for keeping the plastic tail lights clean. They found that the "ribbed" surface (see attached picture) reduced dust accumulation over a smooth flat surface. The reason is that little eddies (or vortices) are created inside the depressions and keep the dust from going inside. The Mercedes engineers thought of everything in those days! I think they dropped the "ribbed" tail lights on the new models - the stylists probably thought they were too old fashioned without realizing their history and functionality...

[e90wraith]

Originally Posted by coreslogic

Thanks for the lesson but do you mind telling us why engine cooling affect drag coefficent? Thanks so much!

Sorry I was a little vague. What I mean by engine cooling is that air has to be diverted from the front of the car into the engine compartment, passed over the radiators and the engine, and released back into the airstream. The radiators are semi-porus and create a large resistance for the air to pass through them. This generates huge drag. The engine also isn't exactly aerodynamic at best, and this also creates drag. Finally, the air inside the engine compartment must escape somehow. The air is usually allowed to escape to the underside of the car. However, this creates a lot of headaches for aero engineers because the air coming out of the engine compartment tends to disrupt airflow on the carefully sculpted underside (or lack thereof). All of these factors contribute to drag.

If you look at NASCAR (probably no one here does! ), the crew chief tapes up the front of the car to reduce the air going into the engine compartment and thereby drag. If he puts too much tape, the car overheats. If he puts too little, the car is a little slow. Obviously for production cars, there's a huge safety factor in the amount of air entering the engine compartment such that they don't overheat (especially at low speed). So for production cars, drag reduction is a really secondary consideration for engineers. If you want better mileage, go ahead and tape up your grills. Just make sure you don't overdo it and overheat!

[wchew]

Originally Posted by e90wraith

You got me - I'm actually alumni of both Caltech and JPL. In the past, I was an aerodynamics engineer. These days, I'm working on artificial intelligence for air/spacecraft.

Shouldn't you be at least driving a Mercedez SL or something ? (if you love cars)

[LEDZEP]

Originally Posted by e90wraith

Thanks, LEDZEP. To clarify, I wasn't a car aerodynamics engineer. My speciality was in turbulent flow over bluff bodies (bodies that are not streamlined - cars fall in somewhere in the middle. They are not true streamlined bodies like airplanes - in fact, they are closer to bluff bodies than streamlined bodies...). I worked on different models and theories, not really applied to anything "useful." Car aerodynamics is just my hobby.

One other tidbit - did you know the "ribbed" tail lights on the old Mercedes were functional? If you look at some automotive society journals from the 1960's, Mercedes engineers wrote some papers on the optimal surface characteristics for keeping the plastic tail lights clean. They found that the "ribbed" surface (see attached picture) reduced dust accumulation over a smooth flat surface. The reason is that little eddies (or vortices) are created inside the depressions and keep the dust from going inside. The Mercedes engineers thought of everything in those days! I think they dropped the "ribbed" tail lights on the new models - the stylists probably thought they were too old fashioned without realizing their history and functionality...

I actually DID know that! That's attention to detail... or shall we say "attention to de tail". Damn, I just crack myself up.

Most of what I know about aerodynamics (all of which could barely fill the inside of a matchbook) is either common sense or conversations with aero engineers. Love the discussions though!!

[FeeBear]

MIT Flash back. I thought I would never hear another AeroDynamic conversation again! I should have never stop the Shock Therapy..

[exAudiGuy]

Originally Posted by FeeBear

MIT Flash back. I thought I would never hear another AeroDynamic conversation again! I should have never stop the Shock Therapy..

Would that be oblique shock therapy? (This is for you wraith!)

[noflash]

Heavy.

Let me just add that this "wub-wub" effect has never shown itself in my e46 or e90 sedan.

It does happen in our 323it (e46 wagon). Three things will solve it:

1. Crack the rear windows.
2. Open the sunroof only halfway.
3. Extend one finger just out the sunroof towards the front. I swear this works. It must diffuse the air. Makes me want another Bee-stinger antennae on the front of the roof.

[e90wraith]

Originally Posted by AudiGuy

Would that be oblique shock therapy? (This is for you wraith!)

Thanks! It's sure brings back a lot of memories. It looks like the solution for a weak shock with constant gamma (Cp/Cv). Don't get me started on the derivation for a non-constant gamma using the fundamental gasdynamic derivative!

I'm now recalling the pain of the Chapman-Jouguet point... (Only Caltech students will understand)