major effect of the Ducktail is to reduce drag!.

[RSBob]

I know with my duck on the freeway at a sustained 70 - I get 19.5 to 20 MPG on a '73S with MFI. This is about the same MPG as a T (20-22)

[nostatic]

my first roommate in grad school was in ChemE, and I remember when he took fluids. He was not a happy camper. Kept mumbling something about low Reynolds numbers and contemplating suicide.

[Mike Kast]

I am tring to post in the spirit of Aerodynamics for Dummies

And that was quite wise of you Island, you have assessed this situation well

Which would reduce the drag more? A ducktail coated with Swepco or one coated with M1? I assume if you coated your ducktail, you'd be foolish not to coat your front spoiler. Failure to do so could cause serious instability and possible barrel roles.

[tbitz]

I'm not a ME, but isn't induced drag simply the component of the aerodynamic force that is opposite to the direction of travel? The perpendicular component is lift. Here is what I mean:



One could see that if the total aerodynamic force is reduced (let's say by adding a duck tail) then then both the lift and drag components would also be reduced. Please no flames I'm only an EE

[nostatic]

Mark, you're close, but you forgot the critical step: tasting.

My guess is that Swepco has it all over M1 in that department. Case closed...and delicious!

Oops, my bad......

[Randy Webb]

to the EE -- Yup. And I hope you get that other E someday....

to Zeke -- Sorry if my brief reply made you unhappy. Brevity may be the soul of wit, but Email isnt the most snesitive of interactions.

to Isl: I just always use fluid dyn. not aero as the principles do not differ among (conventional) fluids -- gas, liq. or air. Where ya wrong? Well, if I interpreted your post correctly it was a general stmt. and that's just not the case. But for sticking things on cars, you could be right -- but that's a limited universe of bodies in flow regimes. All the sime-empirical eqns. that you see in Eng. texts are great but they don't explain the whole of lfow regimes. The NS eqns. do -- BUT they have never been solved(!) Great grad. thesis for somebody.

There are lots of counter-intuivie things in flow dynamics -- the golf ball one is agood example. Some guy wrote a physics for poets type book called "the physics of everday objects" or some such. It's a fun read.

Also, I can top the insanity/mumbling story:
Georg Cantor went insane from thinking about the real line.
- Now that's something to think about....

[island911]

I'm not following what you are saying.
"There is no _necessary_ relation between lift and drag." (?)

do you mean "There is no _single_ relation between lift and drag." (?) . . .that I could buy; kinda. . . .if you mean "total drag" when you say "drag."
But, I can not imagine aerodynamic lift, w/o induced drag.

Furthermore, as general results go, (read: a wing from the massless-rope, & frictionless-pulley closet) induced drag *is* proportional to the the lift . ..or proportional to the sqr of the lift divided by (pi*A*R)

And, to show where the induced drag fits in the scheme of drag contributors:


So when I say 'Less lift = Less Drag' I do so, fully aware of other drag components being in the mix.

My comments were also in context, and response to the first reply in this thread:

Originally posted by Ho Hum 74
If my memory serves me correctly. There is a fundamental law of aerodynamics that says: Whenever you produce lift you produce drag as well. This is called "lift induced drag".
.. .

[Drago]

Originally posted by nostatic
Mark, you're close, but you forgot the critical step: tasting.

My guess is that Swepco has it all over M1 in that department. Case closed...and delicious!

I dunno nostatic...have you ever smelled Swepco???

[nostatic]

well, ever smelled Stilton cheese?

Swepco is an acquired taste...and it's gentle on the hands. In fact, you're soaking in it!

[Randy Webb]

Yes, total drag. And I mean that lift and drag need not covary when I say no necessary relation. The lift induced drag you are thinking of is indeed a factor for aircraft. I don't think it's going to be of any significance for cars (could be wrong, esp. for 200 mph cars). Regardless, I wouldn't call it fundamental in any sense. It's just an empirical generalization, IM(NS?)HO.

I dug out a fun source for flight & aerodynamics stuff: Georg Ruppell, Bird Flight. 1977.

Now let me cross-link this thread to the PNW / rain thread -- How much does rain increase the drag on a car?

[island911]

cross-link ? . . . you mean about the nekid women in Eugene? . . .so if they're topless, is there still a necessary relation between they're white socks with their Birkenstocks?

[Randy Webb]

Interesting questions, all... but by cross-link I meant the thread on how the rainy weather is making our cars the fastest.

[FranksPlace2]

I have been think about this for several days and want to add to the confusion.

The back of the 911 looks a little like an airplane wing. The paricles of air coming over the top need to speed up to meet the particles of air going underneath.

This produces a slighly lower pressure region on the back through the same "Bernoulli effect" that makes planes fly and sailboats sail.

The lower pressure acting on the surface produces a force perpendicular to the surface. See tbitz's diagram. That force has a horizontal componet which slows down the car (drag, but not frictional type drag) and a vertical component which raises the car (lift).

To reduce this effect, we add a device that messes up the flow field. (Let's call it a spoiler.) With the ducktail, the flow coming over the back is turned about ninety degrees.

Turning the flow results in a force on the ducktail. This is the same effect as when you blow on a pinwheel or in a turbine.

That direction of that force will be roughly in the middle of the angle of the turn. Looking at RSBob's car, that makes ir very close to perpendicular or straight down!

Therefore, ideally speaking, the internal part of the ducktail provides a vertical force and no horiziontal force.

Now engineers know there is no free lunch. So there will be lots of form and friction effects as well as the pressure distribution on the back of the duck tail that also result in increased drag.

But the ducktail is very well designed for its purpose.

Frank

[Don Plumley]

Cars in cross section look and act like wings - not just the back half of a 911. The entire 911 looks quite a bit like a wing. The flat bottom and rounded top of all cars create lift. It is through the Bernoulli principle, but not exactly as you have said. The particles of air do not "speed up" over the top of the curved surface to meet the particles underneath. This is a common misperception about the Bernoulli principle and it is often described wrong, even in text books. This is called the "longer path" theory or "equal transit time" theory which are incorrect. It's a minor point, but thanks to my kid's science fair project a few years back, we (I) learned a lot about the Bernoulli principle!

However, the air moving along the curved surface is moving faster. It's simply moving faster because of the curved surface, not trying to catch up to anything. Faster moving air has less pressure. The net of slower air below the car (or wing) having higher pressure and faster air above the car (or wing) having less pressure is lift. As you said.

If you watch an airplane land, as the tires make contact spoilers are raised along the top surface of the wing. These spoilers, which look a lot like a ducktail, spoil the airflow moving over the top surface of the wing, and hence, spoil the lift.

As for the original ducktail/drag argument, I'll let the engineers continue to debate.

Don