Aerodynamic lift- A real problem

[Cd]

I'm seeing a pattern here.

[Christ]

Nerys isn't Hermie. He's also on another forum that I go to. They may have the same ideas, but they're most certainly not the same person.

Incidentally, I only say "he" as a matter of assumption. (I don't know if Nerys is a guy or not.) But the name is as follows:

The girl's name Nerys \n(e)-rys, ner-ys\ is of Welsh origin, and its meaning is "noblewoman". Modern coinage. The given meaning is based on the name's similarity to the Welsh word "ner", meaning "lord" and the endings of the names Gladys and Dilys.

[Nerys]

It also has origins in another nation as a surname. I heard it once on a History channel show and liked the way it sounded (this was well before Deep Space nine) definitely a HE :-)

Thats interesting Christ that we both have metro's and both have Caravans (ok mines a voyager but same thing)

any chance you got an old strange volkswagon lying around somewhere? (I have a 74 Thing)

[Frank Lee]

If lusting after an old VW counts, he matches!

[Christ]

Still haven't gotten my Metro yet.. Still trying to find a way to move them, and it's a crap shoot whether the one is even salvageable.

No old VW's here. Tried getting a Type 2 (I believe it was) fastback, but they wanted like $4000 for it, and didn't even have any paperwork on it, other than the title.

Frank

[TOOSTUBBORN2FAIL]

wait, so let me get this straight, im removing 170 pounds from in front of the firewall on my car, does that mean its going to turn into an airplane from lift? JK

ive taken my completely stock neon to some pretty stupid speeds when i has a life or death situation before, and at 120 mph the car may not have handled awesome, but unless i did something stupid, it wasnt going to fly off the road on its own.

with the fact that basjoos has taken his car to 100 mph, id say that 99% of people on this site dont need to worry about lift

[Christ]

Throughout my 5,000+ posts on this site, and the numerous posts on other sites of differing nature, one could probably derive that I didn't always drive the way I do now. In fact, the day after I put the turbo on my Escort, I took it to redline in 4th gear, which is an unknown speed (never bothered to look it up). I did this on a public highway, which I'm not proud of, but nonetheless, I still had steering enough to switch lanes without losing it altogether, and though it felt a little lighter in the loafers, I never had the thought that I was putting myself in danger of losing control due to front-end lift.

Same for my Civic. I've gotten pulled over in that car in excess of 100 MPH, been ticketed at 97 MPH. Again, never had a problem.

At ~120MPH, my Uncle's old T-bird (the one w/ the boat nose, not the newer bodies) had issues with steering, but was not uncontrollable. He says that at some point beyond 120 MPH, you could actually move the wheel for some distance without ever steering, because the front tires had no real contact w/ the road.

Assuming that car weighed about 4,000 lbs, and was equally distributed, that would be something to the tune of 2,000 lbs of lift at over 130MPH... someone scale that back down and figure out what it would be at 65-70...

Since aero concern increases with the square (or is it cube?) of speed, it's going to be a pretty insignificant number at highway speeds, especially considering that NYS's highway speed was something like 55 or 60 MPH back then, as opposed to 65 or 70 now.

So, you've doubled the speed, you've either increased the aero factor by 4 times or 8 times, depending on which answer above was correct.

If it's square of speed, it's 2*2=4 times more (which means about 500lbs of gross lift at highway speed, not considering whether he was even exaggerating)

If it's cube of speed, it's 2*2*2=8, which means that 2,000/8 = 250 lbs of gross lift on the front end, which is more believable, actually, given the shape.

Note that most of the lift on that car would have been due to a high stagnation point and the venturi effect compressing air under the nose, which looked like the hull of a flat-bottom boat.

[winkosmosis]

I found this thread today on Miata race aerodynamics don't believe in aero on a Miata ? - MX-5 Miata Forum

I think this post is relevant because it has to do with free downforce

Originally Posted by TrackDayHookey

The simple answer is that it's complicated

A more complicated, airplane-geek answer:

Other than sealing the wheel wells or other serious body work, the splitter/undertray/diffuser route is probably going to reduce drag the most.

Air reaching the front of the car has to choose between "up and over", "out and around" and "down and under". On the stock car that decision point (called the stagnation point) is pretty high, so a lot of air takes the "under" route. The under route is fairly rough so the velocity is slow and the pressure and drag are high. The over the hood route is long and relatively smooth, so high velocity and low pressure, leading to lift.

The splitter causes the air to decide to split at the height of the front of the splitter rather than bumper center. This leads to much less flow under the car and lower pressure, so lower drag and less lift. The area on top of the splitter is subject to the stagnation pressure of the essentially "stopped" air in front of the bumper. At 125 mph you could see 1/4 psi on top which adds up. At 62 mph it's only 1/16th psi which is unlikely to generate much force on all but the craziest of aero packages.

Undertray stuff keeps the velocity high, so even lower pressure and less drag.

Air likes to speed up. As it accelerates the pressure is reduced - the air flows from higher pressure to lower pressure and it likes that, which means it resists separation from whatever surface it is running along. Air is much pickier about slowing down - the unfavorable pressure gradient (progressively higher along the flow path) causes the air to separate with almost no provocation. It needs to be carefully herded along, so that the energy in the high velocity, low pressure air can be recovered into high pressure, low velocity aft of the car.

The diffuser helps with that. It provides a progressive increase in cross section between the skinny slot of fast air under the car and the big open volume aft of the rear bumper.

Other aero doodads (wings, canards, ricey stickers) are probably going to just trade downforce for drag. Not an improvement for a stock power car.

Tire drag is a tough nut. Bonneville-style bodywork is impractical. Most efforts I have seen are big, flat plates used to deflect air from something that act like... a big, flat plate. It is probably more productive to try and prevent the static, high pressure air in the wheel wells from getting sucked into the carefully arranged stream created by the splitter/undertray/diffuser. LMP1/2 cars vent the wheel wells to low pressure air on top. Who will be the first to go louver happy?

Mirror drag is probably 1-2% of total car drag. You can probably reduce it by half, so look somewhere else unless you like carbon and hate spare cash.

[MadisonMPG]

Originally Posted by Christ

Since aero concern increases with the square (or is it cube?) of speed, it's going to be a pretty insignificant number at highway speeds, especially considering that NYS's highway speed was something like 55 or 60 MPH back then, as opposed to 65 or 70 now.

square

[Christ]

winkosmosis -

Read the last sentence of the first part you bolded in that quote.

" At 62 mph it's only 1/16th psi which is unlikely to generate much force on all but the craziest of aero packages. "

That's 1 lb per 16 sq inches of added contact surface (in plan view) of added downforce. The only real advantage is pushing less air under the car, more out and around/up and over it, into less turbulent areas, so less energy is being used to compress air between the road and the car, and there is less of a venturi effect.

That's cool and all, but you could lower your stagnation point without a lip/chin spoiler or splitter, just by changing the shape of the bumper.