158 replies to this topic

[schuy]

In Autocar magazine's 4th May issue Chris Harris joined Noble Automotive in testing their new Noble M400 to find out it's top speed.

As those of you familiar with the Noble M12 GTO will remember, the Noble has a huge rear wing which Noble persisted aids with handling characteristics.

When they removed the wing in order to see how much it hurts top-speed, they found out it's top speed was not affected at all(not even 1mph) by the removal of the wing.

And this raises a question- Does more downforce necessarily mean more drag?
And is it fair to suggest that the wing, which apparently made no effect on the drag co-efficient would also create no downforce?


Liran.

[Double Apex]

Maybe the wing is only there to stabilize the air and thereby stabilize the car? i.e. it's not designed to produce any downforce?

[saudoso]

I've heard of wings creating drag with no lift (stall, or a simmetric foil with no angle). Never heard of lift (or downforce) with out drag.
This suggests that the wing was receiving no air at all (I don't know the car, but it does not sound possible) or some other detail in this test was not consideried, like entering the straight too slow due to low grip, or the wind, or what so ever...

[dosco]

Originally posted by schuy
And this raises a question- Does more downforce necessarily mean more drag?
And is it fair to suggest that the wing, which apparently made no effect on the drag co-efficient would also create no downforce?

Drag is a function of wetted area, profile, and in the case of a wing section - angle-of-attack.

If, by removing the wing, there is no change in top speed, then the wing is not producing any downforce.

Increase downforce => increase drag.

Decrease downforce => decrease drag.

The same general principle applies to a diffuser as well - but diffusers are (generally) more efficient than wings so the drag penalty is usually lower.....

Do you have a pic of the car?

[dosco]

Originally posted by Double Apex
Maybe the wing is only there to stabilize the air and thereby stabilize the car? i.e. it's not designed to produce any downforce?

It can't stabilize the car if it doesn't produce any force.

If it's not designed to produce downforce, it's only a decoration.

[Todd]

The Noble's top speed wouldn't be limited by the car's gearing, would it? The other possibility is a governor. Even decorative wings designed to do nothing on purpose tend to create drag. The only exceptions are wings which reduce drag by compensating for a terrible design failing in the car by cleaning up an area of trememdous turbulence.

[Greg Locock]

When I worked for a well known manufacturer the stylists decided we needed a rear wing, to screw up the looks of the blandified version of the original car. It took the aero guys two weeks testing to establish an optimum setting and position for the wing that did not degrade top speed or yaw stability.

[onepablo]

Originally posted by Todd
The Noble's top speed wouldn't be limited by the car's gearing, would it?

I hesistate to post, as I'm not trained in the area, but that's the first thing I thought of too. Exactly how fast were they running it when they hit top speed? For alot of supercars, there exist only a few tracks (mostly manufacturer test tracks) long enough that their top speed potential can even be gotten close to. Aero/drag effect generally don't come into play until relatively high speeds are reached.
Slightly OT, but does anyone know the drag coefficient for the Ferrari Enzo? I remember reading that despite its lack of wings (replaced by difusser), it had some considerable number.

[zac510]

I found a photo for this thread:

M12 GTO3R


The above photo is the 3R model, which according to the noble cars website has the higher top speed (170mph) through gear ratios only. Here are the stats, including power, weight and gear ratios: http://www.noblecars...site/models.asp

[zac510]

If they took the wing off it would look like an Elise

[PPatrik :: LKF]

Originally posted by zac510
If they took the wing off it would look like an Elise

That's it! They tried the Noble, it ran X mph. Than they chop the wing and started again. In the same moment GOD looked down from above and saw the car. Thouhgt: "It's a Lotus Elise, so it must be as fast as X mph!" So it ran again X mph...


Seriously. Maybe Todd is right: the car's rear end can produce such turbulence to make big drag. When you can clean this turbulence with a wing you can build up even just same drag for producing good downforce.

[Lukin]

Greg, would that be the T3?

[schuy]

Originally posted by Todd
The Noble's top speed wouldn't be limited by the car's gearing, would it?

Apparently not, the gearing would allow a higher top speed...

Originally posted by onepablo
Exactly how fast were they running it when they hit top speed?

173mph/277kp/h.


BTW, the M400's stats have yet to appear in Noble Automotive's website, it's not yet in production.

[Lukin]

I read an article a few years ago (waiting to go into the doctors I think - **** medical system means I coulda read a Russian novel, but I digress) and it did the old cotton balls glued to the rear of the car trick. The first time there was no rear spoiler and the pictures showed how the air was turbulent from the roof/window backwards and then another one with a rear spoiler to 'channel' air and it was a lot more laminar than before and as a result increased adding the wing actually increased the top speed of the car by reducing the overall drag of the car. I guess the drag the spolier created was more than compensated by the overall drop of drag from the car.

In the Noble's zero change to top speed, could the air flow be smoothed out by the rear wing but the addition of the wing itself perfectly cancels out the gain. That said if it makes no difference why is it there? That could be a marketing ploy.

When the new monaro (ie a commonwhore executive with two doors welded shut and a 20K prince increase. damn I digressed again) was released a few years ago the market research showed that customers wanted a rear wing even though the designers hated the idea. That said, the one they put on was bloody fugly, so I guess they got their way in the end.

[dosco]

Originally posted by zac510
I found a photo for this thread:

Nice looking car.

I suspect the wing's effectiveness could be improved by adding vortex generators to the roof right where the rear windshield meets the roof. The resultant vorticity would "clean up" the airflow and allow the wing to receive "cleaner" air.

That said, if they can take the wing off and it doesn't affect the speed/handling of the car, who cares? With the wing, it looks cool. Without the wing you'd have 1 less piece of bodywork....

Hhhhmmmmm.....the other thing that comes to mind is that the designers may have added it as a decoration....therefore the wing is set to it's "zero lift" angle-of-attack at top speed as not to generate additional drag. This implies that it actually creates lift and *lifts* the tail at lower speeds. Probably not too much of a big deal....and with this in mind, a user could increase the downforce (by adjusting wing incedence) as needed to improve cornering at various tracks.....

Better yet, do you think the designers/owners/marketing weenies may have adjusted the wing for zero drag at top speed? Perhaps a marketing gimmick? Like they fiddled with the car on purpose to create the "odd" results?

BTW, does this thing have an undertray? What does the ride height look like?

[Torquer]

Originally posted by saudoso
I've heard of wings creating drag with no lift (stall, or a simmetric foil with no angle). Never heard of lift (or downforce) with out drag.
This suggests that the wing was receiving no air at all (I don't know the car, but it does not sound possible) or some other detail in this test was not consideried, like entering the straight too slow due to low grip, or the wind, or what so ever...

I've heard of a wing that produces negative drag(thrust) plus downforce.;) You just have to change the direction of airflow. If you can channel the air or "bend the air" then you can use a wing to develope thrust.

[schuy]

Following the mentioned test, Mr. Noble of Noble Automotive stated that the wing was put on the car after specific tests which showed that the car had more downforce with the wing on, especially at higher speeds(duh).
He went on to say that the car is stable enough without the wing, but can be driven without it as well...

[Greg Locock]

Lukin: No not the T3. A fast car.

Torquer: it is trivially easy to generate thrust from a wing section. It is impossible for you to do so and generate more useful thrust than the drag it causes at the same time. The Laws of thermodynamics say so. Even Bernie can't legislate those away.

[Torquer]

Originally posted by Greg Locock


Torquer: it is trivially easy to generate thrust from a wing section. It is impossible for you to do so and generate more useful thrust than the drag it causes at the same time. The Laws of thermodynamics say so. Even Bernie can't legislate those away.

The drag the wing would produce would mainly be downforce pushing the car down rather than rear drag pulling the car back. I have tried it on a simple cfd program and it works. You just have to bend the airflow by the use of the coanda effect. If you can redirect/bend the airflow by 45 degrees then you could have a wing that generates thrust plus downforce.

[Greg Locock]

Good oh. So just take this airfoil, and mount it on 4 wheels, then give it a push.

Are you claiming it will accelerate all by itself?

[McGuire]

Originally posted by Torquer



The drag the wing would produce would mainly be downforce pushing the car down rather than rear drag pulling the car back. I have tried it on a simple cfd program and it works. You just have to bend the airflow by the use of the coanda effect. If you can redirect/bend the airflow by 45 degrees then you could have a wing that generates thrust plus downforce.

Dude, forget CFD for now. You are dragging an object through a fluid. The object has volume; the fluid has mass. Molecules of the fluid must be displaced to get the object through the fluid, producing drag which squares with speed. No matter how you manipulate the fluid flow around the object, you are not going to generate net thrust. What you propose is a type of perpetual motion machine.

[desmo]

Put enough of these "wings" on a car or plane and engines will become unnecessary! Woo hoo!

Better keep quiet about this one or the oil company hit men will pay you a visit for sure.

[Torquer]

Originally posted by Greg Locock

Are you claiming it will accelerate all by itself?

No, it still needs an external force to move the car. It is kind of like a ram air intake. It will give a boost at speed.

[Torquer]

Originally posted by McGuire


What you propose is a type of perpetual motion machine.

Not at all. It wouldn't be able to generate enough lift/thrust to overcome the drag of the car itself, not even close. It is just a boost like ram air.

[Dmitriy_Guller]

Originally posted by Torquer


Not at all. It wouldn't be able to generate enough lift/thrust to overcome the drag of the car itself, not even close. It is just a boost like ram air.

So, if you get the car up to speed, keep the throttle floored, and then somehow detach the wing, it will overtake you and wave goodbye?

[Torquer]

Originally posted by Dmitriy_Guller


So, if you get the car up to speed, keep the throttle floored, and then somehow detach the wing, it will overtake you and wave goodbye?

Nope, as soon as the wing enters the "unaltered airstream" it will lose it's ability to function. You have to remember that the airstream that is feeding this wing isn't coming straight across from left to right or right to left. Instead the airflow is being bent by using the coanda effect and by exploiting this bent air a wing can be tilted so it's lift can produce thrust.

[dosco]

Originally posted by Torquer
It is kind of like a ram air intake. It will give a boost at speed.

I'd like to hear your explanation of how a ram air intake gives a "boost" at speed.

Please elaborate.

[Torquer]

Originally posted by dosco


I'd like to hear your explanation of how a ram air intake gives a "boost" at speed.

Please elaborate.

I shouldn't have to, the term ram air is self explanatory, isn't it? When moving at speed air is rammed into the intake giving you a boost in performance. A ram air intake is not funtional if it isn't moving. Cowl induction is similar as well by using the high pressure around the base of a windshield when at speed.

[WPT]

A diffuser intake (airplanes, cars, bikes, etc....) has increasing cross sectional area in the direction of the flow. Pressure increases in the direction of flow in this intake. Hence, a force in the direction of vehicle movement is generated in the intake system. The SR-71's intake provided about 50% (maybe more) of the necessary thrust at M=3. WPT

[dosco]

Originally posted by Torquer


I shouldn't have to, the term ram air is self explanatory, isn't it? When moving at speed air is rammed into the intake giving you a boost in performance. A ram air intake is not funtional if it isn't moving. Cowl induction is similar as well by using the high pressure around the base of a windshield when at speed.

LOL.

No, ram means "stagnation pressure." Stagnation means the air is stopped and the pressure is raised. Which means drag is generated.

Cowl induction uses the "high pressure" generated by the windshield-hood stagnation zone.

FWIW, the use of "ram pressure" for a car intake - the pressure is increased by like 4%. Which for an F1 car is a big deal, but for your road car is jack.

[dosco]

Originally posted by WPT
A diffuser intake (airplanes, cars, bikes, etc....) has increasing cross sectional area in the direction of the flow. Pressure increases in the direction of flow in this intake. Hence, a force in the direction of vehicle movement is generated in the intake system. The SR-71's intake provided about 50% (maybe more) of the necessary thrust at M=3. WPT

1. You're getting supersonic and subsonic flows confused. "Ram air intakes" function differently in the different speed regimes.

2. The "50% increase in thrust" for the SR-71 had only to do with the intakes shocking down the flow with the greatest pressure recovery (lowest drag) for the engines. The intake itself did not generate any thrust. IIRC Ben Rich (the guy who designed the intakes) quoted a figure of 30%.

3. In your post you are describing a negative pressure gradient (the flow is low pressure, the intake is high pressure). Since air tends to seek equilibrium, the air "wants" to flow from high pressure to low pressure. The force that is generated is drag, not thrust.

4. In supersonic flow, diffusing the air will increase its mach number. If you put a normal shock wave in the flow prior to the diffuser, the flow will decelerate.

[WPT]

The shock(s) to reduce supersonic flow to subsonic ocurrs before the diffuser section. You are wrong in your thinking. WPT

[dosco]

Originally posted by WPT
The shock(s) to reduce supersonic flow to subsonic ocurrs before the diffuser section. You are wrong in your thinking. WPT

I posted

"4. In supersonic flow, diffusing the air will increase its mach number. If you put a normal shock wave in the flow prior to the diffuser, the flow will decelerate."

Your post (above) just restated what I already said.

Let me elaborate to clear it up for you.

If the upstream flow is supersonic, and there is no normal shock wave prior to the diffuser, the flow will accelerate. This is how supersonic wind tunnels, rocket engines, and (to a certain extent) afterburners work.

If the upstream flow is supersonic, and there IS a normal shock wave prior to the diffuser, the flow will decelerate. This is how supersonic engine intakes work. This is also how closed-circuit supersonic tunnels work to bring the flow back down from high mach numbers.

[McGuire]

Originally posted by Torquer


Not at all. It wouldn't be able to generate enough lift/thrust to overcome the drag of the car itself, not even close. It is just a boost like ram air.

Here, what applies in first principles applies to all. There is no "boost" as there is no introduction of new energy into the system.

If you pull an object (a wing or shape) through a fluid, the displacement of fluid by the object will produce DRAG.

If this object could be somehow shaped to produce positive THRUST when pulled through the fluid, then by definition less net DRAG is produced than if there were no object in the fluid. Obviously, no such object exists. There is no wing or aerodynamic device which can produce net thrust simply by being dragged through a fluid. Remember -- first, something must propel the object through the fluid.

The power to propel the vehicle through the fluid is provided by its engine. With no speed, no lift or drag are produced. If you can force more air into the engine, it may make more power and allow the vehicle to go faster. But you are not reducing the net drag on the vehicle, only increasing it. Nor are you converting drag into thrust. Any energy in the fluid around the object (here, air pressure around the vehicle) is not free for the taking -- it was produced by the vehicle's engine.

What you are proposing is perpetual motion, a violation of the second law of thermodynamics. No system can produce more energy than it consumes.

[McGuire]

Originally posted by dosco
FWIW, the use of "ram pressure" for a car intake - the pressure is increased by like 4%. Which for an F1 car is a big deal, but for your road car is jack.

You are being generous. Assuming a very efficient intake (90+ %) a 4% increase in airbox pressure would require over 200 mph airspeed.

[Greg Locock]

This is all very amusing, but our cfd novice does have some slight evidence on his side. It is possible to add an aerodynamic device to a car that reduces the overall drag, so to some extent the device does add thrust. eg a Kann tail spoiler (?name) for instance, adds downforce and reduces drag.

But that is a very perverse view, rather like when the government repays you some of the tax you've paid.

[dosco]

Originally posted by McGuire


You are being generous. Assuming a very efficient intake (90+ %) a 4% increase in airbox pressure would require over 200 mph airspeed.

Well, I try to be generous when helping out "those in need."

[dosco]

Originally posted by Greg Locock
This is all very amusing, but our cfd novice does have some slight evidence on his side. It is possible to add an aerodynamic device to a car that reduces the overall drag, so to some extent the device does add thrust. eg a Kann tail spoiler (?name) for instance, adds downforce and reduces drag.

But that is a very perverse view, rather like when the government repays you some of the tax you've paid.

Well...optimizing the lift-to-drag ratio "adds thrust" by reducing drag. I could claim that thrust is generated by gurney flaps and/or serrated features added to wings/flaps.

I do, however, find his theory of ram air intakes to be rather entertaining.....FWIW, I think Torquer should head up Macca's aero department. With his groundbreaking new theories of aerodynamics he could spark Kimi and DC on to victory...

PS: got a link to Kann spoilers? A quick google search didn't bring up anything of note.....

[McGuire]

Originally posted by Greg Locock
This is all very amusing, but our cfd novice does have some slight evidence on his side. It is possible to add an aerodynamic device to a car that reduces the overall drag, so to some extent the device does add thrust. eg a Kann tail spoiler (?name) for instance, adds downforce and reduces drag.

But that is a very perverse view, rather like when the government repays you some of the tax you've paid.

And let us not forget the miraculous aerodynamic device which does indeed generate useful thrust. It's called a sail. All you need is a star to steer her by...and a good stiff breeze, of course.

[dosco]

Originally posted by McGuire


And let us not forget the miraculous aerodynamic device which does indeed generate useful thrust. It's called a sail. All you need is a star to steer her by...and a good stiff breeze, of course.

Careful, now. Someone might think that the sail produces thrust and no drag.....;)

[McGuire]

Originally posted by dosco
PS: got a link to Kann spoilers? A quick google search didn't bring up anything of note.....

Try Kamm, with two m's (first name Wunibald). Interesting dude. Best known for the Kamm effect tail (see Cobra GT by Pete Brock) which surmises that a sharply cut-off shape and the low-pressure area trailing behind it will approximate a long teardrop shape.

[McGuire]

Originally posted by dosco


Careful, now. Someone might think that the sail produces thrust and no drag.....;)

Quite right...and the more drag in the sail, the greater the wind required to overcome it and move the boat. I can also see how messing around with a CFD program long enough could cause one to forget what is making the air move in the first place. Personally, I blame the educational system. Lacking any formal education, I have never gotten any of these ideas.

[dosco]

Originally posted by McGuire


Try Kamm, with two m's (first name Wunibald). Interesting dude. Best known for the Kamm effect tail (see Cobra GT by Pete Brock) which surmises that a sharply cut-off shape and the low-pressure area trailing behind it will approximate a long teardrop shape.

Found it.

Yah, in the low energy side of a body, in many instances it is better to "chop" the end off and leave sharp edges than to have a radiused or rounded surface (sailboats, for example....the racers like to have a really sharp trailing edge so the water separates quickly). If done well, drag numbers can be improved. If done poorly.....well.....take your pick.

I'm not sure the addition of a Kamm spoiler gets you "something for nothing" (although I suppose in some instances the drag is worth it for the increase in rear-end stability).

[dosco]

Originally posted by McGuire
I can also see how messing around with a CFD program long enough could cause one to forget what is making the air move in the first place. Personally, I blame the educational system.

Bah!

Stupid is as stupid does.

[BRIAN GLOVER]

You communist you. A friend in need is a friend to avoid.

Originally posted by dosco


Well, I try to be generous when helping out "those in need."

[McGuire]

Originally posted by dosco


Bah!

Stupid is as stupid does.

I'm sure you're right. On the other hand, there are some theories which are are so incredible that a graduate degree is required in order to believe them. A mind of average intelligence must first be subjected to years of academic abstraction and contextual confusion before such ideas even have a place to take shape.

[dosco]

Originally posted by McGuire
On the other hand, there are some theories which are are so incredible that a graduate degree is required in order to believe them. A mind of average intelligence must first be subjected to years of academic abstraction and contextual confusion before such ideas even have a place to take shape.

I know where you're coming from.

The problem is the shift from practical, hands on kinds of stuff (wind tunnels, tuft tests, etc) to high-altitude math and academia....with no correlation between the two.

What theories, in particular, are you thinking of (the only one that immediately comes to mind for me is Quantum Physics).

What always struck me as funny is the prof saying...."let's take the Euler equations, make this assumption, and derive...." Ugh. They never taught us the "other stuff" like who Euler was, what his equations were all about, etc etc etc. They just took off and you either got it or not.

Problem with CFD is much like calculators.....they teach people how to use the tools and not the fundamentals....then the student assumes the tool (CFD, calculator, whatever) is correct and have no way to do a sanity check.

Ah well.

Sorry to rant for so long.

[WPT]

Well, I was wrong. The subsonic diffuser in the SR-71 provides 70% (not 50%) of the thrust required to sustain Mach 3. In the Concorde at Mach 2.2 the subsonic diffuser provides 75% of the required thrust, the engine only 8%. Of cousre, the engine is necessary for the mass flow through the system. For those interested in this subject may I suggest ' Modern Combat Aircraft Design' by Klaus Huenecke, Naval Istitute Press, 1987. WPT

[WPT]

The inlet where the shocks decelerate the supersonic flow to subsonic produces drag. But from the last shock to the engine is a subsonic diffuser where forward thrust is developed. WPT

[McGuire]

Originally posted by dosco


I know where you're coming from.

The problem is the shift from practical, hands on kinds of stuff (wind tunnels, tuft tests, etc) to high-altitude math and academia....with no correlation between the two.

What theories, in particular, are you thinking of (the only one that immediately comes to mind for me is Quantum Physics).

Well, since you asked, sorta...

Of course the classic example of theory vs. practice in motorsports is poor old Roger Huntington SAE, RIP, who in the 1950's proclaimed a theoretical maximum acceleration attainable by wheel-driven vehicles: one g. Oops.

By necessity, physical theorists must live in a world of their own creation, which is comprised of smooth surfaces, as well as perfect spheres, etc. and a lot of other things which don't exist. (Which is an interesting way to study "reality" when you think about it.) Engineers and mechanics had better not live there. There is no money in mistaking the map for the territory.

Quantum physics is an interesting case. Is a photon a wave or a particle? Neither: it's a photon. Any paradox is solely our own. When some of the implications of the "true nature" of physical reality became known to me, they were troubling for a while. But eventually I realized that the trouble was not with reality but with me. Physical law truly exists, as much as we can say anything truly exists. The problem lies in the epistemological limitations of what we arrogantly call "physical law." There are no laws of nature truly known to us, only rules of science...which are subject to change without notice, every time we humans get a little bit less stupid. So who's right: Einstein or Newton? Both and neither, depending on their ability to describe how stuff really works. There is no higher meaning.

We now know, for example, that there is no such thing as "electricity" in the classic textbook sense. That was entirely our own invention; it made sense at the time. In metaphysical truth, we have no particular reason to believe that the entire universe does not contain only one electron (albeit an extremely busy one). But on the other hand: I can design and build an electrical circuit, graphing its characteristics and properties on a sheet of paper. Then when I hook the circuit up to an oscilloscope and turn it on, I will get a trace that lays perfectly over my sheet of paper. That is all the reassurance you will get of the existence of a natural world which operates according to physical laws which can be grasped from a human perspective, and that is good enough for me.