Drag and shape

[260DET]

You've got to remember, not everybody here is en engineer or physicist. Some of us are just amateurs and beginners that need more information/help here and there.

 

Very good point, if commentators are not sure of what you are asking they should seek clarification. First

[Guest Rolling Parts]

Again, it was a general question and whether the flow was compressible or not

 

I did not think he was asking as a physicists/aerodynamicist and wanted to ponder "compressible flow". I too see non-teardrop shapes being used on cars and on 100-250mph airplanes and that's an interesting thing. The cool thing is that an oblong shape can be just as efficient at 100-200mph as a so-called raindrop (because they are being used).

 

The fact is that every time you look at a "winglet" on a fixed wing plane or "turbulators" on a horizontal stabilizer that you should realize that there is a LOT more to aerodynamics than simple profile shapes as far as efficiency (and that's the fun part!). 3D.

[bearcat]

I'll weigh in. Let's move away from raindrops and airfoils for a minute and talk about cars. Because I have a '68 Corvette which I've owned for many years, I'm most familiar with that. When the Vette evolved, the C2 (1963-1967) used to experience severe front end lift at about 152 mph, to the point where you couldn't steer it anymore. The C3 was designed with a much sharper front nose and longer, smoother body. It's beautiful to look at, but actually wasn't that aerodynamic either. The C3 Vette, (1968-1982) had almost the same CD as the 240Z in fact. But it did reduce lift significantly over the C2, for a higher top speed, approaching 170 mph, so it was an improvement. The Kamm tail of the early C3 was actually more aerodynamic than the smooth taper tail of the later C3. So they brought it back starting with the C4. The wedge shape of the C4 (1984-1994) was a big improvement. The C5 was the best, aerodynamically, but it's not nearly as attractive.

 

So my point is, the C5 and C6 have more of a rounded nose, not unlike a teardrop. But unlike a teardrop, the abrupt end to the tail was found to be far better. The Corvette was designed as an early attempt at aerodynamics, but on paper only. Through trial and error, they finally learned what to do. Just like Nissan has also learned what to do, and has improved their Z aerodynamics through the years. The new 350Z is far more rounded than the much more beautiful 240/260/280Z.

 

Thing is, what works at very high speeds is different than what works at lower subsonic speeds. That is why supersonic jets have sharp noses. The behavior of the slipstream becomes much more fluid as speed increases, and the rules change. Simple as that.

 

I really don't worry about the coefficient of drag on my '68 Corvette. It has a 427 race motor in it. I figure if I need more power to hit 170 than a later generation Vette, I just hit it with a much bigger mallet. What is much more important than maximizing your CD is maximizing your ground-sticking ability. The C3 Vette had almost a 20 mph higher top speed than the C2, because they may not have fully understood how to build a lower CD, but they very much did know what worked on the race track. You can always install a bigger hammer, guys. Then it doesn't matter how low your CD is.

 

Cheers.

 

[hoov100]

I'll weigh in. Let's move away from raindrops and airfoils for a minute and talk about cars. Because I have a '68 Corvette which I've owned for many years, I'm most familiar with that. When the Vette evolved, the C2 (1963-1967) used to experience severe front end lift at about 152 mph, to the point where you couldn't steer it anymore. The C3 was designed with a much sharper front nose and longer, smoother body. It's beautiful to look at, but actually wasn't that aerodynamic either. The C3 Vette, (1968-1982) had almost the same CD as the 240Z in fact. But it did reduce lift significantly over the C2, for a higher top speed, approaching 170 mph, so it was an improvement. The Kamm tail of the early C3 was actually more aerodynamic than the smooth taper tail of the later C3. So they brought it back starting with the C4. The wedge shape of the C4 (1984-1994) was a big improvement. The C5 was the best, aerodynamically, but it's not nearly as attractive.

 

So my point is, the C5 and C6 have more of a rounded nose, not unlike a teardrop. But unlike a teardrop, the abrupt end to the tail was found to be far better. The Corvette was designed as an early attempt at aerodynamics, but on paper only. Through trial and error, they finally learned what to do. Just like Nissan has also learned what to do, and has improved their Z aerodynamics through the years. The new 350Z is far more rounded than the much more beautiful 240/260/280Z.

 

Thing is, what works at very high speeds is different than what works at lower subsonic speeds. That is why supersonic jets have sharp noses. The behavior of the slipstream becomes much more fluid as speed increases, and the rules change. Simple as that.

 

I really don't worry about the coefficient of drag on my '68 Corvette. It has a 427 race motor in it. I figure if I need more power to hit 170 than a later generation Vette, I just hit it with a much bigger mallet. What is much more important than maximizing your CD is maximizing your ground-sticking ability. The C3 Vette had almost a 20 mph higher top speed than the C2, because they may not have fully understood how to build a lower CD, but they very much did know what worked on the race track. You can always install a bigger hammer, guys. Then it doesn't matter how low your CD is.

 

Cheers.

 

 

Personally, I don't think you fully grasp what it takes to hit speeds above 160mph, if you think all you need is a bigger motor and stickier tires.

 

(as a side note, stickier/wider tires will slow you down, from the increased rolling resistance, extra rotational mass and extra weight.)

 

shape isn't as important as where the air is traveling around and through the shape of the vehicle.

[bearcat]

Personally, I don't think you fully grasp what it takes to hit speeds above 160mph, if you think all you need is a bigger motor and stickier tires.

 

(as a side note, stickier/wider tires will slow you down, from the increased rolling resistance, extra rotational mass and extra weight.)

 

shape isn't as important as where the air is traveling around and through the shape of the vehicle.

 

And I don't think you grasp how much power 631 hp really is. L88's built to similar but lesser levels (due to technology) hit that speed routinely. The '68 L88 427 Corvette won its class in the 24 hours of Lemans many times throughout the 70's, hitting speeds in the high 160's and into the 170's.

 

All you need is a bigger hammer. Do the research, it is historically recorded. I'm not makin' this up.

 

Maybe I should start by asking, what are your goals here? You have a car which has roughly the same coeff of drag as my Corvette. At the legal speeds encountered in the US, this isn't an issue at all. If you're racing your Z, and you have limited horsepower, then yes, CDA is more important. But honestly, dudes, if you've got a big block Chevy that can hit 7000 rpm, the point is moot. If you want to hit 160 in your Z, install a bigger "mallet", because there's only so much you'll be able to do to improve the CDA of your car.

 

Absolutely no offense intended. Just trying to help, Cheers.

[roger280zx]

Take that 427 out of the vette, slap it in a sleek body, and watch 170mph look like just another tic on the speedo. I think that is the point the guys are making. Just look at TonyD. He's going that fast with around 120 cubic inches. Just my .02

[bearcat]

Take that 427 out of the vette, slap it in a sleek body, and watch 170mph look like just another tic on the speedo. I think that is the point the guys are making. Just look at TonyD. He's going that fast with around 120 cubic inches. Just my .02

Yes I understand. With a 2.4 liter engine, the coefficient of drag is much more critical. With a 7 liter engine, a brick wall could punch a hole in the atmosphere at over 100 mph. That's all my point was.

 

I was suggesting more power. For example, a turbocharger would give a lot more top end and still keep it at 2.4 to 2.8 liters.

 

My point was also that it is a lot more important to keep the car from flying off the ground than it is to be more aerodynamic.

[hoov100]

Yes I understand. With a 2.4 liter engine, the coefficient of drag is much more critical. With a 7 liter engine, a brick wall could punch a hole in the atmosphere at over 100 mph. That's all my point was.

 

I was suggesting more power. For example, a turbocharger would give a lot more top end and still keep it at 2.4 to 2.8 liters.

 

My point was also that it is a lot more important to keep the car from flying off the ground than it is to be more aerodynamic.

 

Your shooting yourself in the foot here. power will only get you so far, when it comes to speed. once you reach a certain threshold, aero and weight becomes more important then power. these are things every major automaker who has ever whent to Bonneville has learned. Say your vette made 1300WHP and had some ridiculously tall gearing, with all of the supporting chassis mods. you would still be lucky to break 170mph. your seriously neglecting the laws of fluid dynamics and physics, if your thinking all you need is more power.

 

 

 

(Tony where are you!?!?!?!?!?)

[Zmanco]

IIRC, the power required to overcome drag goes up by the cube of speed. So for example, the incremental power to go from 120 mph to 130 is ~27%. To go from 130 to 140 requires an additional 32%. Or put another way, to go from 120 mph to 150 requires twice as much power! And none of this takes into account losses due to rolling resistance, etc.

 

So in theory, it is possible to "throw horsepower at it", but in practice getting the CD down to a reasonable level matters a lot too.

[Globerunner513]

I'm going to chime in quickly, and visit something that was said earlier in the thread. I'd like to just point out that I think the tapered end to airliners actually doesn't have much to do with aerodynamics. Although it's helps for sure, the bigger reason is to allow pilots to take off at a higher angle of attack.

 

Would be difficult to pull the nose up if the tail was as wide as the rest of the fuselage.

 

[JMortensen]

I'll throw a little more in too. Most purpose built race cars have horrible drag coefficients, because you can go faster around a circuit by making downforce than you can by reducing drag. I like the bigger hammer and more downforce way of doing things, but if you guys are going for land speed records I can't begrudge you that lower CD is going to be faster there.

 

EDIT--Here is a formula from Competition Car Aerodynamics:

BHP absorbed by drag = cd x Area (sq ft) x v^3 (mph) / 146600.

 

I'll leave it to someone else to figure out the frontal area of a lowered Z car and figure out how fast you can go and how big a hammer you need.

[JMortensen]

Here's a related link on another site: http://www.atlanticz.ca/forums/index.php?topic=651.0

 

That link says frontal area of 22 sq ft for a 240z. Sorry bearcat, I'm not going to look that one up for your car, but we'll play with some numbers here for fun. We'll use drag of .45. I think that is pretty close for a Z car.

 

According to the formula, you need 540 bhp to do 200 mph. Keep in mind this tool is for estimating and isn't exact. 1300 bhp gets ~267 mph according to the equation. I can't remember Tony's exact speed, but I worked it out for 172mph and it comes up with 343 bhp, which sounds at least ball parkish. I think a stock 240Z ran out of legs at about 122 mph as reported in that other thread. According to the formula, that requires 122bhp. So it's sounding like a reasonably accurate tool to me, at least to get a general sense of what it takes to go xxx mph.

 

Looks like I should be able to hit about 180mph in my car if I get the 400 hp I'm looking for (not that I expect to find a place where I can reach that velocity on a road course). My take on this? Bigger hammer works.

[bearcat]

Hoov100, I'm in Taos, New Mexico. But I might be moving further south...hopefully! If the job I want there opens up.

 

You are both right. Hoov, the bigger hammer, downforce and stickier tires only goes so far. You usually can't have both downforce AND great coeff of drag, because downforce USUALLY means greater drag. But this doesn't stop NASCAR racers from going 200 mph. They use the bigger hammer approach too. A bigger hammer DOES work to a point. And this is where JMortensen is correct. He and I agree on that.

 

The Corvette body can go faster, because a 427 has enough balls to allow it, if the gearing is there. And that's the problem. I have a close ratio Muncie 4 speed with a 1:1 fourth gear and a 3.36. Although the 3.36 is not optimized for drag racing, it will allow a 165 to 170 mph top speed. But I'll be gear-limited, and I'll probably be using about 400 hp to travel at that speed. I have higher power than that, but not the gearing. I did deep research on the available 5-speed overdrive trannies out there and most of them will actually make me less quick while improving my top speed. I finally came to the conclusion that because I live in America and we don't have autobahns, what do I need an overdrive for?

 

Guys, I actually built my car to race in the Silver State Classic. It has the gonads to do it, but I can't race the unlimited class for two reasons. #1, I don't have the gearing to keep up with those 230 mph monsters, and #2, I don't want to basically destroy a valuable classic Vette by permanently altering the car for all the safety requirements the sanctioning body has for that class. In all likelihood, I'd probably realistically race in the 120 mph class, which won't require me to dedicate my car to a race-only configuration.

 

I have always admired the first gen Z-cars and would love to own one, but at the present time, I don't. Maybe someday. But when I do, I'd like to keep it stock-appearing, but have a nice big turbocharger on the straight six. I do miss turbo cars.

[bearcat]

Just thought I'd add, I'm the original owner of this particular Shelby Charger. That's my son standing in front of it. He knows I'm giving it to him, so he's all smiles. This car is in need of a restoration but it's extremely straight.

 

Anyway, the purpose for this pic is that this exact car went 140 mph on a turbocharged 2.2 liter 4-banger. This was right after I intercooled it and got it up to probably between 180 and 210 hp. I don't know for certain. But originally, they were 147 hp and they weigh 2450 pounds. So power-wise and weight-wise, they are near twins to the Datsun 240Z.

 

When I did my 140 mph run, it was at 5000 ft ASL, in Fort Collins, Colorado, on I-25. I had a taller geared transaxle in it while my original was being built (3.07:1) and decided to see what it could do with the right gearing. I hit 138 very quickly, but those last 2 mph took about 3 miles to reach, and then that was about it.

As you can see, not all that aerodynamic, with headlight pockets and hatchback not much different than a 240Z either. But with a turbocharger, 15 pounds of boost, and an intercooler off of a Mitsu Eclipse, it acheived an honest to goodness 140 mph, and to this day, I'm proud of that. Please note the ground effects and rear spoiler. They WORK too. At that speed, the car just sucks down to the ground and is very smooth.

 

That's why I think simply turbocharging a 2.4 Z engine would vastly help.

 

[djwarner]

I don't think the OP ever had his question answered, namely why use a relatively blunt nose on an airfoil.

 

Comparing two airfoils, one with a sharp nose and a similar one with a blunt nose, in a windtunnel at varying angles of attack, you will find the slope of the CL is much steeper in the sharp nosed airfoil. So as you decrease the angle of attack, the sharper nosed foil will suddenly go from more than enough lift to just enough to very little to negative lift very rapidly.

 

This is an unattractive trait in a general purpose aircraft - more useful in a fighter jet though. For general aviation a blunter nose does develop a stagnation zone near the leading edge. As the angle of attack varies, the stagnation zone can shift up or down the gently curving leading edge helping to maintain smoother flow and better lift over a wide range of angles of attack. So the blunt nose is preferred for lift purposes - not drag.

 

The other half of his question asked why not a needle shape to reduce drag. You need to consider the what is takes to maintain laminar flow over the surface. The flow immediately above a solid surface is actually traveling at almost the same speed as the surface. This is caused by friction with the surface. As you move away from the surface, the air is not slowed down as much, Go farther and the air is hardly slowed at all. The change in velocity as distance from the surface increases is a shearing stress. The longer the shearing action occurs, the more likely the laminar flow will turn turbulent with resulting increas in drag. So long straight body lines are apt to induce turbulent flow and higher drags. The longer the shape is exposed to turbulent flow, the higher drag will be. So as general design guideline, delay the transistion from laminar to turbulent and after that minimize the surface area after that. 

 

You can visualize this by watching the smoke rising from a candle after you blow it out. Its smoke rises on the warmed air fairly straight for some time and then suddenly burbles into turbulent flow. This is caused by the warm currents rubbing against the cooler still air next to it.

 

This is also a phenomina found when a wings stalls. The air moving over the surface transisions from laminar to turbulent causing at loss of lift, not all, just most. At the same time the drag increases dramatically and the power required to keep the airplane flying goes up dramatically - usually beyond that available from the engine (again, some modern fighter jets excepted).

 

If you actually closely examine modern airliners would would find most fuselage/wing cross-sections do vary smoothly throughout its length. Fifty years ago, some airliners were stretched by adding uniform sections of fuselage as a midlife design kicker (ie DC-8). However these aircraft were always less efficient than a clean sheet design.

 

Hope this helps.