Sure there is a pressure difference and sure enough Bernoulli's equations describe it and yes some airfoils (all actually except symmetrical ones) will create some lift at at zero or slightly negative angles-of-attack but will any of them provide enough in this state to hold up a typical aircraft in level flight? You might be able to do it with a Piper Cub or a Cessna 172 if you could push them to 500mph but in any remotely practical sense you can't. Most low/medium speed airfoils in use on actual planes won't generate enough lift to stay in level flight with their airfoils at zero AOA never mind negative. My pet peeve concerned mainly the high-school level of explanation of how a wing works whereby Mr. Bournoulli is bandied freely about but rarely heard is any discussion of angle-of-attack. A simple flat-bottomed airfoil, a Clark Y for example, will not support any real aircraft in level cruise flight with a zero angle-of-attack. Discussions of this sort have a chicken-or-egg quality to them sometimes with the chicken being the lower air pressure on top of an airfoil and the positive air pressure on the bottom being the egg. In the real world lift is created by banging air up against the bottom of an airfoil and the shape of the airfoil is what manages the airflow to minimize drag, suppress tubulence, extend the usable speed envelope of the aircraft, and provide the desired handling characteristics. Bernoulli's equations and their modern refinements describe the behavior of airflow in the same way that Newton's equations describe the strength and effects of gravity. They mathematically formalize the study of the phenomenon so that we might make intellectual sense and practical use of it but they aren't the actual phenomenon itself. A blackboard full of formulae describing nuclear fusion is at a far remove from feeling the heat of the sun on our faces. At least the fusion calculations enable you to build a weapon or a powerplant. All the Bernoulli equation filled blackboards in the world are inadequate to the task of convincing the typical aircraft to fly level at a negative angle-of-attack. A jet fighter may in fact have an airfoil that sits a slight negative angle at Mach 2 because that ferocious airspeed will generate enough pressure differential to do it but slow that puppy down to a couple hundred miles per hour and the AOA goes way positive. Aircraft usually don't appear to be tilted up in level flight but that's only because the wing is attached to the fuselage at an angle (decalage it's called) that permits it to actually be close to level in level cruising flight even if the wing isn't. Can't have the drinks cart rolling down the aisle by it's lonesome. With high-speed race cars as much attention is paid to managing airflow under the body as it is over the top. The lower the chassis can ride the less air will get under it and contribute to unwanted lift thereby easing the design constraints of the top of the body. An LSR car that could run a hundreth of an inch off the ground would never generate enough lift to be dangerous. That's not practical of course so killing lift without causing too much drag becomes the source of endless head-scratching and wind tunnel experimentation. Bernoulli's equations even describe the behavior of a vacuum cleaner but you simply can't have suction without a real ocean of air eager to push real dust-bunnies into that bag of man-made nothin'. That's why they call it lift and not suck.
Now you look like you know your aerodynaics, I just can't understand how come you don't know that the majority of the lift comes from the air accelarating on the upper surface, not air banging on the lower surface. The angle of attack only adds to the bernouilli effect. Just study how a stall happens. When air becomes turbulent on top of the wing and stops generating the lift, the plane goes down, no matter how much air is "banging under the surface" High school explanation of an airplane wing might be simplyfied, but it's dead right! Trough 3 years of studying aircraft maintenance and four years mec. engineering, I've never heard anyone pretend the opposite. You did mentionned that it was bunk though???
Is the teardrop really moving through space, or could the drop be at rest and the space around it moves? Pretty heavy Man.
Well, I'm not sure about that. The "Z" crashed in spectacular fashon...taking Lowell Bayles with it. The "R" models all met similar fates. Then again, the modern replica has been flying for some time now and no mishaps. Maybe they were just a little too far ahead of their time. Surely a handful in any case. Neat planes though...the Hot Rod of the skies. Sorry to get O/T...back to the aero discussion...
We're talking around each other here. My contention is that the accelerated air over the top of a wing does not per se cause lift but rather allows it and increases it's efficiency. The two are inseperable. Take the example of a simple flat bottomed airfoil meeting the air at exactly zero angle of attack--that is the flat bottom is parallel to the airflow. Air will accelerate over the top of the wing not the bottom and produce some lift. But it's very rare that in normal flight regimes this would produce enough lift to keep an aircraft at a constant altitude. As I pointed out a jet fighter might go fast enough to accomplish this although they are so heavy that I'm skeptical even then. A wing that is merely a flat plate as is found on some small simple models will produce useful lift even if it's only across a very narrow range of (positive) angles-of-attack. The air flowing over the top is almost entirely turbulent due to the sharp leading edge so this makes it very inefficient but it will in fact work for a very light weight airframe. The same can be said for a wing in a full sized aircraft that is completely stalled. The energy sucking turbulence is severe enough to destroy the wing's efficiency and down you go unless you have enough power to literally hover the plane on the prop. In this high-drag low-speed "stalled" condition the air is no longer banging on the bottom of the wing enthusiastically enough to do any good. Another factor is that lift and thrust are vector quantities. At a high angle of attack the air pressure on the bottom may be pushing the aircraft as much backward as upward. This might be mitigated if you could rotate the engine down (or rotate the wings) and drag the aircraft more forcefully forward. Most propellor engines are fixed, as are wings, and thrust is roughly parallel to the fuselage while ignoring the direction of actual flight. As the elevator of an airplane rotates the wing to greater and greater angles-of-attack the air rushes faster and faster over the top of the wing trying to fill the vacuum created by the passage of the tilted wing through the air. The main job of the upper wing curvature is to manage this airflow, to keep it moving along nicely, and to keep it attached to the surface preventing energy robbing turbulence. When a wing is tilted relative to the flow air is moving towards the bottom and attempting to move away from the top due to the vacuum created by the movement. Keeping this airflow smooth and attached greatly increases the efficiency of the wing. This decreased air pressure is a response to the situation and not directly a cause of it. Imagine you're dragging a sheet of plywood face on to the airflow along at 100mph. In this situation the plywood is in a state of 100% drag with no lift available and complete turbulence behind it. It'd be absurd to argue that the pressure drop behind the plywood is what is causing the air to slam into the front of it. Now put a big rounded back on the plywood. The airflow behind becomes less turbulent. Keep putting bigger and more tapered backs on there and at some point drag will do down to some irreduceable minimum but obviously not zero. Now tilt the plywood down to say 45 degrees. Now the air whacking the bottom produces a vector force that is half backwards and half upwards. Now we got some lift but we also still have a bunch of drag. Now reduce the size of the shroud on the back and taper it away from the direction of flight. Presto, less drag and less power needed to hustle the assemblage along. Now keep the iterations going and tilt it forward until this object ends up looking like a real airfoil. Now you have greatly reduced drag, greatly increased efficiency, and greatly reduced need for the power required to maintain the 100mph speed. Tilt this airfooler to a couple of degrees positive angle of attack and the resultant vector forces abetted by much reduced drag are now able to hold a heavy mechanical object up in the air very nicely at that 100mph speed. So the difference between that upended plywood and a real airfoil is really a matter of degree. An extremely large degree to be sure but the principles do not change. A deeply stalled wing is that metaphorically upended sheet of plywood and a wing in level flight is an appropriately rounded slab of plywood working in it's most efficient range. Tilt the plywood/wing down just a bit though and unless you radically increase the speed of it's passage through the air down you will also go. Sure Bernoulli describes this behavior exactly but be careful not to confuse even perfectly descriptive equations and pressure distribution graphs with real cause and effect. A gut level appreciation requires more than the statement that air goes faster over the top of a wing therefore it flies. Thus endeth the sermon.
Would it be mean of me to point out that a multi-ton aircraft isn't a sheet of paper or a beachball? The "wingloading" of a piece of paper or beachball is in the range of a few grams per sq.ft. With this much area and this little weight simply blowing air across one surface generates a pressure differential large enough to demonstrate the effect of "pure" Bernoulli lift in the science exhibits. Now try to duplicate the exhibit using a 20 ton F16 (around a hundred pounds of weight per sq.ft. of lifting surface) parked on the ramp. Use a large horizontal nozzle to direct air across the top of the aircraft (not just the wings because the fuselage of most fighters generates lift as well). Does anyone seriously think that regardless of how much high speed air is blasted over the top of the plane that it will simply rise off the ground in the same manner as a sheet of paper? In theory since sea-level atmospheric pressure is in the range of 135lbs. per sq.ft. the trick might be done if the over wing pressure could be reduced to essentially zero but in practice this is an impossibility. Expose the entire aircraft to a 500mph airstream and it's a different story. That wall of air molecules roaring past the airframe has a pressure wildly greater than static atmospheric and so the plane carries it's weight just fine with only a very small pressure difference between the top and bottom. Now you may indeed think, "But isn't the principle the same? Aren't we still talking about a pressure differential?". Yes we are but the manner, order, and the relative scale of a given situation in which a principle is applied is all-important. Say you are mad enough to crack some goober over the head and do him damage. Which is going to be more effective (and brutally satisfying)---bopping him on the noggin with a feather a million times or using a baseball bat once? The total force applied may be identical in both cases but in the first case long before any real effects are felt by the goober he's either going to have flounced off to the next county or he will be standing over your bloodied prostrate form whereas in the second case he is the one who is going to be on the ground moaning and leaking red stuff. So much for the "principle" being the same. That metaphorical baseball bat is a 20 ton F16 in flight at 500mph and the feather is that fluttering few grams of paper. A real aircraft needs considerable air pressure on the bottom of the wing to fly and the mere fact of its being dragged along by a prop/turbine produces the vacuum on top of the wing into which air rushes at an increased speed. Again I submit that the decreased air pressure on the top of a wing is a response to the situation and not the direct cause. I'm not saying that the response is not important (it is critical to the efficiency of the system) but what I am saying is that it's the wagging tail and not the dog. If the response were as important as the cause most wings would hold an aircraft up in level flight at a zero angle-of-attack and except in the most exotically extreme cases they simply won't do it. If you could contrive to reduce the weight of an F16 to a mere ton or so then the nozzle experiment might work but back here in the real world F16s weigh as much as an armored personnel carrier, fly Mach 2 at 40,000 ft., and can be wrenched into 6 gee turns. That's one hell of a piece of paper. It is not mere metaphorical quibbling to state that Mr. Bournoulli generously enables Mr. Newton to efficiently get on with the job. They are partners but not quite equal ones.
Frank Costin once said that "one square foot of frontal area is worth yards of streamlining" ...or something along that line. btw: don't go turning people onto the tudors for lsr cars, that is my idea...
Looks like Dave pretty much answered your question. Funny thing about aero-dynamics is that although there can be a close to perfect shape for flying though the air with little resistance it normally doesn't apply much to real life. for instance a raindrop has very little wind drag, but would quite probably be very hard to control in the real world. Give you an example, the E Type Jag is bullet shaped in the rear, this causes it to pull less draft, but at speed they also get a little light in the ass. So the Jag designers had to shape the body accordingly to compensate for the problem. They still suffer if someone pulls up on the rear bumper at speed.
I really liked this one: It's cheaper than most, is applicable to cars, fairly easy to read, but covers the material extensively enough. Jano was indeed a genius, and so was Columbo, and Pininfarina's boys, and everyone else Enzo surrounded around him. --Matt
The man responsible for some of the most beautifull automotive body shapes ever... My personal favorite, the Lotus Eleven. Edit. Actually... He drew some really homely looking cars too....
If the pressure on top of the wing were reduced by 100 psf, then 100psf/2117 psf = .047, or say ~5.0% then my physics says the fighter just levitated! Is there a mathematician in the house? I don't see a problem w/creating the necessary lift if the above is correct.[/QUOTE] My humble apologies for my dumb math mistake but my inference is actually strengthened by correcting the error. In theory you might be able to levitate the fighter in question but could you in a real experiment reduce the pressure that far and just what would that prove in terms of what makes an F16 fly and handle the way it does? An aircraft in flight sees pressures on both sides of the wing not just one and air moving over the curved top surface sees reduced pressure right enough but it's nowhere near zero. In virtually all cases it's not enough pressure reduction to fly a real plane at zero AOA. Now you do have a situation somewhat comparable to the experiment in a high speed automobile where airflow underneath the vehicle is much reduced (but impossible to eliminate entirely in practice). In most cases you don't wany ANY lifting of the vehicle including what results from pure increased airflow over the top. Three basic ways to fight this lift. Get the vehicle as low as you possibly can and still make it practical to use on a race course. Use wings and spoilers etc. to create negative lift. Make the top of the vehicle as flat as practical. Streamliners use the first and third tactics primarily because any wings or other protuberant whatnot would cause too much drag. Racers talk of the bad things that happen when air "gets under" the car and they are right to do so. When it does the lift potential skyrockets and up and over you may go. I've seen plenty of Nascar entries on flights longer than the Wright brother's first one all because unwanted air got under the car. That selfsame air is not only wanted underneath an airplane's wing it's vital to it's operation. To repeat my main assertion there are few if any real aircraft that will maintain level flight with their wings at a zero angle-of-attack.
One of my favorite enzo stories, was that in his office he had a cabinent, and like most cabinents things were kept in it. but this particular cabinent held a collection of various mangled, twisted, and broken car parts. When "il comendatore" would have a conversation with his designers and engineers he would sometimes brandish one of these failed items, thus making a very poignant yet subtle point...as only one can in italian.
Let me throw my hat in the ring here. First off...and before I forget to do it, let me paste this link in for you fellas. I thought it was informative - even if it is event specific. http://www.aerospaceweb.org/question/aerodynamics/q0151.shtml Now with that out of the way let me say that the "ideal" shape depends on the object and the (relative) speed. I say relative because in terms of speed - what is fast? For a street car - Id' say 200 MPH is fucking rocking (yea, there goes my etiquette). Comparatively, for a LSR, 200 MPH is merely a good start, and for something with wings and the intention to fly 200 MPH puts you in the VW Beetle class of airplanes...(maybe a little exaggeration). ...So, in order to answer the original question posted, questions need answered. Questions like what exactly are you needing the shape for? Even the difference between a motorcycle and a car are completely different (though the principles are the same). And let's once and for all kill the raindrop theory. A raindrop shape is ideal for a raindrop. Factoring in that it's not traveling under any power and that it's natural shape - in zero g environment is spherical, and the fact that the shape of a falling raindrop is affected by the forces placed onto it by wind "drag" and gravity at terminal velocity. A raindrop is effectively "sucked' into said shape and whether it could travel faster if it were shaped differently can never be tested as it cannot hold a shape when force is applied to it. Done. Almost... Now let me contradict myself. A so-called raindrop shape is a damn fine shape to aspire to when building for speed. Not perfect but not bad at all. I believe Darwin and the others have been doing a fine job of going after the basic principles of the matters at hand. Obviously - use some basic common sense and you can understand the objectives of overcoming wind resistance. Fat and flat = bad long and lean = good and so on.... I am not going to go on and on about the technical details involved in creating the ideal shape, mostly because I don't have those answers - not many people do. Instead I will state that you should plan the shape according to what you want to do. Find the goal and build a plan to achieve the desired goal.....yea I copped out - so fuckin' what. Now if I HAD built a car to a perfect raindrop shape - no wings no airfoils and I had an exorbitant amount of power in the machine in question, I am going to assume that the raindrop shape has some deficiencies that make it not so perfect after all. I imagine that - if pushed to some undetermined imaginary limit - that the pure raindrop shape would be subject to several forces that could cause my machine to wobble or wag at these limits. Fine for a raindrop - not so fine for a dude in a firesuit with his wife watching from the pits. So now I have many other factors besides shape alone to think about when I try to overcome these issues. As mentioned I could use foils and other things to help overcome the tendancies to wag or wobble but I also have to consider the fulcrum point of weight bias and which wheels are driving my hypothetic car. Anyone see "Worlds Fastest Indian"? -> kick-fucking-ass. (Trying to fullfill my "F" word quota.) It's kinda like moving all of the weight of a common bar dart to the rear of the body. The shape is right, but the fulcrum point is too far back and will cause all sorts of bad things to happen. But let's move that weight back to the front...now we're talking....bullseye! So with an airfoil (or "flight") on the rear and the right placement of weight, we are setting ourselves up for some speed. Now we just need the power to the ground... Alex Xydias was a genius. I'll take this time to ask you respect the effort Ryan put into setting up this connection below. We are truely blessed. Go find your hero and maybe someone new to worship while you're there (Just when you think you know it all - you look and see someone beat you to it long before you were even born): http://www.ahrf.com/pioneer_landing.php Thanks Ryan, I still can't believe we had the opportunity to converse with GODS.
It's been too many years since I studied this stuff, but going from memory I have to say that when I was studying lifting surfaces, I had similar thoughts to Darwin. However, some details I haven't seen mentioned here are the effects of the location of the stagnation point of the flow on the wing, the seperation point, and the movement of the boundry layer. As I remember it, the boundry layer rotates from back to front over the top of the wing/front to back on the bottom, which moves the stagnation point under the wing, and also moves the seperation point to the trailing edge. The location of the stagnation point of the flow adds to the lift, just as Darwin is saying. So no, it isn't simply that the air on top moves faster than the air underneath... This is for subsonic flow by the way. I don't think there is a whole lot of info that can be transferred from paper sheets in a tray to supersonic flight...
I am not trying to stir the pot or anything - just wondering if all of this banter about lift is falling on deaf ears. I mean how many car guys want thier car coming off the ground? Point is - at least what I thought the point we were seeking was aerodynamic autos. Lift science is fascinating, but I don't thing that's what the origional poster was heading at. If I were building a hot rod for the salt I'd start with the front end...
For those that are interested, this months issue of Racecar Engineering (www.racecar-engineering.com) has an entire article dedicated to dimples and how they are related to automobile aerodynamics (Rough around the edges). I haven't read the article yet, so I can't report on what it actually says about the subject. Borders usually carries this magazine. It's very expensive but worth every peny if your interested in racecar technology.
If you want to kill or control lift you sure as heck need to know what causes it in the first place. The optimum vehicle shape for drag reduction is not by any means the optimum shape for one that will stay firmly planted on the ground at 200-400mph. All the forces need to be juggled to get the vehicle to perform in the manner desired. In the case of very high speed situations knowing about how to control unwanted lift is scarcely any less important than any other consideration.
Didn't Tatra's also come with an air cooled V-8 Hemi?? There was one on eGay a few months back and you wouldn't of have expected to see an american muscle motor in those things back in the day. Even in 67 they were pretty aerodynamic..... Ahh yes here it is... Rare Czech-built 1967 Tatra T-603-2 suffered a mishap at the show, caving in the front slightly and causing the hood not to close completely. The car has a rear-mounted air-cooled hemi V-8 and belongs to Alex Veronac of London, Ont.
This aerodynamic device could be of use to people building LSR cars. This is the bottom of the University of Texas at Arlington Formula SAE car. Those tunnels that flare up at the back are call a diffuser. The flare on the tunnels makes a nice low pressure zone right under the car providing downforce, with very little drag. This shape was designed for the 30-60 mph range, so a LSR would need a much shallower angle. But the principle is the same. I see a diffuser as a way more logical approach to traction/downforce issues than putting weight in the trunk. The flat section you see on the outside of the tunnels is the fence, and its purpose is to keep the air outside the tunnels from sucking into the tunnels, reducing its effectiveness. The wider the fence, and the closer it is to the ground, the more effective it is.
The Jalopy Journal
