Downforce
12-18-2005, 01:20 PM
#31
Lift and downforce are the same force applied in different directions.
With an airfoil, lift is the result of the pressure difference which results from the air moving faster over the top of the wing that the air travelling under the wing. With the increased velocity, the pressure acting on the upper surface of the wing is less than the pressure under the wing causing lift.
The same holds true for cars only applied in the opposite direction.
With an airfoil, lift is the result of the pressure difference which results from the air moving faster over the top of the wing that the air travelling under the wing. With the increased velocity, the pressure acting on the upper surface of the wing is less than the pressure under the wing causing lift.
The same holds true for cars only applied in the opposite direction.
12-18-2005, 02:01 PM
#32
See my last post. What would make the air move faster over the top of a wing? What requires air to meet back up again at the rear of the wing? I was taught this in school also, but when you think about it, it doesn't make sense.
Hold a book out the window of your moving car. The book has 2 sides that are of equal length and shape, yet gets pushed up or down when you angle it. The deflection of air causes the lift/downforce, not the speed differential of the air traveling over the surfaces.
This is quite interesting, because it isn't what has been commonly taught.
Hold a book out the window of your moving car. The book has 2 sides that are of equal length and shape, yet gets pushed up or down when you angle it. The deflection of air causes the lift/downforce, not the speed differential of the air traveling over the surfaces.
This is quite interesting, because it isn't what has been commonly taught.
12-18-2005, 02:10 PM
#33
Originally Posted by shiners780
Hold a book out the window of your moving car. The book has 2 sides that are of equal length and shape, yet gets pushed up or down when you angle it. The deflection of air causes the lift/downforce, not the speed differential of the air traveling over the surfaces.
This is quite interesting, because it isn't what has been commonly taught.
The elevator is completely different from the wing.
In reality, the air molecules seperated at the leading edge do not meet again at the trailing edge. Those that travel under the wing do pass from under it first. What really causes lift is the pressure difference exerted on the wing. Even though the air passing under the wing exits first, the air passing over the top is moving faster, therefore exerting less force.
12-18-2005, 02:15 PM
#34
Race Director
Originally Posted by shiners780
Lift has nothing to do with the higher rate of air passing over the top of a wing (car):
In air (or comparably in any fluid), lift is created as an airstream passes by an airfoil and is deflected downward. The force created by this deflection of the air creates an equal and opposite force upward on an airfoil according to Newton's third law of motion. The deflection of airflow downward during the creation of lift is known as downwash.
Common misconceptions
Many readers new to this topic may be looking for the explanation that is commonly put forward in many mainstream books, and even in scientific exhibitions which touch on flight and aerodynamic principles. Known as the "equal transit-time" explanation, it states that the parcels of air which are divided by an airfoil must rejoin again, and due to the greater curvature (and hence longer path) of the upper surface of an aerofoil, the air going over the top must go faster in order to "catch up" with the air flowing around the bottom, therefor due to its faster speed, pressure of the air above the airfoil is lower, etc. Despite the fact that this "explanation" is probably the most common of all, it must be made clear that it is utterly false.
In air (or comparably in any fluid), lift is created as an airstream passes by an airfoil and is deflected downward. The force created by this deflection of the air creates an equal and opposite force upward on an airfoil according to Newton's third law of motion. The deflection of airflow downward during the creation of lift is known as downwash.
Common misconceptions
Many readers new to this topic may be looking for the explanation that is commonly put forward in many mainstream books, and even in scientific exhibitions which touch on flight and aerodynamic principles. Known as the "equal transit-time" explanation, it states that the parcels of air which are divided by an airfoil must rejoin again, and due to the greater curvature (and hence longer path) of the upper surface of an aerofoil, the air going over the top must go faster in order to "catch up" with the air flowing around the bottom, therefor due to its faster speed, pressure of the air above the airfoil is lower, etc. Despite the fact that this "explanation" is probably the most common of all, it must be made clear that it is utterly false.
I don't see (and have never seen) any explantion regarding the deflection of air flow.
Ground effects (true ground effects) are caused by air speeding up in the venturi tunnels on the underside of the car, thus creating a low pressure area and since the air pressure above the car is greater you get downforcec.
I won't say that I know for a fact the air speeding up is the god's honest scientific answer, but I've seen it written by some extremely credible sources and nothing to the contrary. However, I'm open to new information if you can provide a link to credible information to the contrary.
12-18-2005, 02:16 PM
#35
Race Director
Originally Posted by streckfu's951
Lift and downforce are the same force applied in different directions.
12-18-2005, 02:18 PM
#36
Race Director
Originally Posted by streckfu's951
What you are describing here is what an elevator does (the flight control surface that is part of the horizontal stabilator.
The elevator is completely different from the wing.
In reality, the air molecules seperated at the leading edge do not meet again at the trailing edge. Those that travel under the wing do pass from under it first. What really causes lift is the pressure difference exerted on the wing. Even though the air passing under the wing exits first, the air passing over the top is moving faster, therefore exerting less force.
The elevator is completely different from the wing.
In reality, the air molecules seperated at the leading edge do not meet again at the trailing edge. Those that travel under the wing do pass from under it first. What really causes lift is the pressure difference exerted on the wing. Even though the air passing under the wing exits first, the air passing over the top is moving faster, therefore exerting less force.
12-18-2005, 02:22 PM
#37
Race Director
Originally Posted by shiners780
See my last post. What would make the air move faster over the top of a wing? What requires air to meet back up again at the rear of the wing? I was taught this in school also, but when you think about it, it doesn't make sense.
Hold a book out the window of your moving car. The book has 2 sides that are of equal length and shape, yet gets pushed up or down when you angle it. The deflection of air causes the lift/downforce, not the speed differential of the air traveling over the surfaces.
This is quite interesting, because it isn't what has been commonly taught.
Hold a book out the window of your moving car. The book has 2 sides that are of equal length and shape, yet gets pushed up or down when you angle it. The deflection of air causes the lift/downforce, not the speed differential of the air traveling over the surfaces.
This is quite interesting, because it isn't what has been commonly taught.
While I have never actually looked into the science of who an aircraft flies inverted, I have always assumed it was done by a combination of configuring the control surfaces and angle of attack so they ultimately reconfigure the airfoil to work in the opposite fashion (making the air travel faster on the inverted underside.
12-18-2005, 02:30 PM
#38
In the same respect that 5 is less than 7.
Think of Lift and Downforce as a force applied over a number line.
-5 -4 -3 -2 -1 0 1 2 3 4 5
What we call downforce is applied to the left while lift is applied to the right. 0 represent a symetrical surface that produces neither lift nor downforce.
If you are producing lift, your force is represented to the right of 0. To ask "is the reduction of lift the same as downforce", the answer is no as it is only less lift. Say you start with a value of 5 Lift. By adding a splitter and spoiler, you reduce that force by 3. Now you have a value of 2 Lift.
This is all a numbers example. In reality, I agree with you Geo, that almost all cars generate downforce. Splitters reduce the air traveling under the car which becomes turbulent and really messes with the cars stability.
Take a look at the shape of F1 cars. The wings are all shaped to push down on the car. They are all curved upward to use the same deflective force Shiners describe (which is not the same principles that describe aircraft wings). Also, the underside of F1 cars is convex to act like an inverted wing.
Aircraft manufacurers do build their aircraft with the wings angled slightly upward to take advantage of the deflective force. It's just not adjustable like Shiners used in his example. It's a matter of the aircraft speed generating enough force to overcome the gravitational pull of the aircraft.
Think of Lift and Downforce as a force applied over a number line.
-5 -4 -3 -2 -1 0 1 2 3 4 5
What we call downforce is applied to the left while lift is applied to the right. 0 represent a symetrical surface that produces neither lift nor downforce.
If you are producing lift, your force is represented to the right of 0. To ask "is the reduction of lift the same as downforce", the answer is no as it is only less lift. Say you start with a value of 5 Lift. By adding a splitter and spoiler, you reduce that force by 3. Now you have a value of 2 Lift.
This is all a numbers example. In reality, I agree with you Geo, that almost all cars generate downforce. Splitters reduce the air traveling under the car which becomes turbulent and really messes with the cars stability.
Take a look at the shape of F1 cars. The wings are all shaped to push down on the car. They are all curved upward to use the same deflective force Shiners describe (which is not the same principles that describe aircraft wings). Also, the underside of F1 cars is convex to act like an inverted wing.
Aircraft manufacurers do build their aircraft with the wings angled slightly upward to take advantage of the deflective force. It's just not adjustable like Shiners used in his example. It's a matter of the aircraft speed generating enough force to overcome the gravitational pull of the aircraft.
12-18-2005, 02:57 PM
#39
Three Wheelin'
FWIW, to relate back to the original question (not that anyone reads my posts) I recall and article in Flying magazine, talking about aerodynamics of cars, and it stated that a 911 a 80mph created about 500lbs of LIFT. it didnt state what version 911 so id assume it wasnt a turbo, or had any kind of spoiler
12-18-2005, 03:08 PM
#40
Nordschleife Master
ahh the good old downwash vs pressure differential arguement. Well I'm not touching that one with a 10 foot pole.
but I can tell you this:
I've driven my cavalier at 115mph and it gets floaty. there is very definately a lot of LIFT generated by it's body at that speed.
I've driven my 951 at 165 mph, and it's the exact opposite. It sucks itself down. This is the exact opposite of the cavalier.. there is definately some downforce being generated.
Certain cars, like the 959 for example, have zero lift and zero downforce. You might notice some 959's actually have a little plastic strip installed on the bottom of the rear spoiler.. I presume to help add a bit of downforce since the car doesn't have any in stock form.
but I can tell you this:
I've driven my cavalier at 115mph and it gets floaty. there is very definately a lot of LIFT generated by it's body at that speed.
I've driven my 951 at 165 mph, and it's the exact opposite. It sucks itself down. This is the exact opposite of the cavalier.. there is definately some downforce being generated.
Certain cars, like the 959 for example, have zero lift and zero downforce. You might notice some 959's actually have a little plastic strip installed on the bottom of the rear spoiler.. I presume to help add a bit of downforce since the car doesn't have any in stock form.
12-18-2005, 03:20 PM
#42
Nordschleife Master
another thing to remember, you can't base your argument on slow speed experiences. If you've never driven your car at go-to-jail speeds, then you don't have any real first hand experience of what it feels like when a car's body generates lift or downforce.
an airplane is generally very light.. a 2000lbs cessna 172 with those huge wings starts to take off at 75mph... a car weighing almost twice as much with a much less efficient wing-shaped body, will not feel the effects of downwash until about 120mph at least...
I've been fortunate enough to drive a an open wheel car capable of generate 2 or 3 times the weight of the car in downforce... however, the suspension is so stiff, that you never feel the race car squatting. Same with most closed-wheel race cars with 400+ lbs springs. On a regualr street car, the suspension is so soft that you can feel the car squating or lifting at 120mph+ speeds.
an airplane is generally very light.. a 2000lbs cessna 172 with those huge wings starts to take off at 75mph... a car weighing almost twice as much with a much less efficient wing-shaped body, will not feel the effects of downwash until about 120mph at least...
I've been fortunate enough to drive a an open wheel car capable of generate 2 or 3 times the weight of the car in downforce... however, the suspension is so stiff, that you never feel the race car squatting. Same with most closed-wheel race cars with 400+ lbs springs. On a regualr street car, the suspension is so soft that you can feel the car squating or lifting at 120mph+ speeds.
12-18-2005, 03:24 PM
#43
Nordschleife Master
and one other thing... one of the reasons that the 959 was designed with zero lift and zero downforce, was to achieve a higher top speed.
Everyone knows that if you increase downforce, you also increase drag. I think Porsche purposely made the 959 a zero lift / zero DF car so that it could have less drag and reach a higher top speed. (knowing full well that those tests would be done on a straight line road..)
Everyone knows that if you increase downforce, you also increase drag. I think Porsche purposely made the 959 a zero lift / zero DF car so that it could have less drag and reach a higher top speed. (knowing full well that those tests would be done on a straight line road..)
12-18-2005, 04:36 PM
#44
Geo,
Admittedly, I know squat about airfoils, and couldn't design a plane if my life depended on it.
I have found this information at www.en.wikipedia.org/wiki/downforce, which is really interesting. The source of the info is unknown, but makes reasonable conclusions. It shows how Bernoulli's principle has historically been misused in providing an explanation of aero lift. In fact, in studying Bernoulli's equations, you can see that Bernoulli never stated that an increase in velocity creates a decrease in pressure. His equations show that an increase in velocity and a decrease in pressure occur simultaneously. The information on the website uses a venturi to demonstrate Bernoulli's principle, and how an increase in velocity occurs simultaneously with a decrease in pressure. The result would still be lift/downforce, but not in the way commonly expressed.
I guess we're walking the fine line of semantical bullsh!t, cause and effect, etc.
Interesting reading nonetheless.
I can say, without a doubt, that my car with it's 93k miles very soft original suspension squats at speed. That, I would believe, is downforce. (that was the original question...wasn't it???
)
Admittedly, I know squat about airfoils, and couldn't design a plane if my life depended on it.
I have found this information at www.en.wikipedia.org/wiki/downforce, which is really interesting. The source of the info is unknown, but makes reasonable conclusions. It shows how Bernoulli's principle has historically been misused in providing an explanation of aero lift. In fact, in studying Bernoulli's equations, you can see that Bernoulli never stated that an increase in velocity creates a decrease in pressure. His equations show that an increase in velocity and a decrease in pressure occur simultaneously. The information on the website uses a venturi to demonstrate Bernoulli's principle, and how an increase in velocity occurs simultaneously with a decrease in pressure. The result would still be lift/downforce, but not in the way commonly expressed.
I guess we're walking the fine line of semantical bullsh!t, cause and effect, etc.
Interesting reading nonetheless.
I can say, without a doubt, that my car with it's 93k miles very soft original suspension squats at speed. That, I would believe, is downforce. (that was the original question...wasn't it???
)
12-18-2005, 04:38 PM
#45
Drifting
Newton actually develloped the particle deflection idea for boats, and while the priciple still applies regardless of the density of the fluid, we all know only the bottom of the boat is interacting with the fluid.
Give this page a read for a better explanation than I can give:
http://travel.howstuffworks.com/airplane7.htm
Give this page a read for a better explanation than I can give:
http://travel.howstuffworks.com/airplane7.htm