Lesson One - Introduction To Race Car Aerodynamics
Lesson One – Introduction
Aerodynamics is a science and an art.We cannot calculate everything that goes on with aerodynamics perfectly.The shapes are too complex for any group of people or any Computational Fluid Dynamics computer model, to prove exactly.Proof of this statement is, in aircraft design, we still need test pilots to find out what really happens.
We design race cars to produce the fastest lap time over an entire race.In order to achieve this goal, the weight and balance, the chassis geometry, the setup, the aerodynamic package, and the driver all must blend together correctly.If any of these are off, the performance will be less than desired.
Some people can see what is happening with air flow better than others, and some think they have to rely on a computer.We aren’t trying to make you a computer programmer / engineer.We are trying to give you the understanding and foundation so that you can be creative to find a way to be successful in your racing, or work.
All forms of racing should find at least some bit of information here to improve what they are doing.That spans from F1 to Cup to a late model, or to sprint cars, drag racing or engines.
The goal with this course is to give you a good understanding of what is going on with air flow around a vehicle, how to factually find that air flow, and to make you think.
Nobody knows it all.I won’t tell you how I do all this, because then you will focus on my thinking.Be creative, do testing, and maybe you will come up with the next new great technology.That is what I am trying to spark you to do.
Racing is about engineering and making something better than your competitors.You won’t get that from a computer or a book.You need the understanding of air flow to be creative in exploring new ideas, new methods, new formulas, to make it better than all those that came before you.This course is about getting you to think about how to make it better.
I have been very fortunate, and blessed, in that very high caliber people have stepped in to my life at times.I listened.I questioned.I thought about how to do it better.I have been driven my entire life to make all things better.I tested on the race track and with aircraft.
I was lucky that some of those people were top level Aerodynamicists at NASA.They would always listen to every idea that I had, and then we would discuss it.A few test flights were very scary.But most were learning experiences you cannot get from a school, book or computer.
I have doubled the lift, and halved the drag of most airfoils used on aircraft today.I have dramatically improved propeller and airfoil technology on aircraft.I have won many races.My engines have won championships, rookie of the year, many races, quick time, and a few world records for the time.
My aircraft have won engineering awards, and have been featured on the History Channel and Discovery Channel, and many magazines.I am not a do it like the book Aerodynamicist.I am a “How can I make it better” person.
What can you do?Here is a funny story that hopefully makes you think about what you could do to make it better.I had met several top level PhD friends at NASA Ames in Calif.All are retired now.I was developing a new aircraft, and went down there to have a meeting about the airfoil I wanted for the wing.I gave them the parameters I wanted, which were against what is normally taught in an Aerodynamics school.
This is one of the many wings I developed and fabricated.This is a Sprint Car wing, that would have better aero properties, and create some ram air for the engine induction, if the hood were designed right.
I explained why I wanted those characteristics, and they agreed that my reasoning was sound.So, they gave me a book on computer generated airfoils.This was more data than I had ever seen for any airfoil.I thought this was great.I told them I would go back to my office and digest all of this, and then get back to them.
About a month later I called these Aerodynamicists back and set up another meeting.So, I arrived in their office, and started explaining what was good and what was bad about their airfoil.It mostly met the parameters I wanted.Mostly.
So I got up at their board and started drawing and explaining how I was going to “FIX“ their airfoil.One of them stopped me and said “We don’t like people bastardizing our airfoils “.We all laughed out loud.Then I said, just hold on here, and listen to what I want to do to fix this.
After drawing and discussing for a bit, they all agreed I was onto something, and that it sounded reasonable.Then they said I have their blessing to go try it.I said thanks, but I was going to try it anyway.We all had another belly laugh.
I went back to my office and started building this new aircraft.After the second flight, I went back for another meeting at NASA about all this.I explained to the Engineers what the plane did, and that in fact I increased CL (Coefficient of Lift) max, dramatically decreased CD (Coefficient of Drag) and that the stall characteristics were unreal fantastic.
A few months later one of them came out to fly the plane to verify data. Days after that flight I went back to NASA for another meeting.We determined on the NASA Cray Computer that the CL max was just over 4 (the highest that NASA had ever tested).The CD was amazingly low, even with the flaps down, and was less than half what NASA expected.They also said it had the best stall characteristics of any aircraft NASA had ever tested.
So, I hope that this series sparks ideas to make you succeed in racing or any development involving aerodynamics.Always strive to make things better.We don’t care about doing it the old way.Even the F1 people can make it better.But the more they get locked-in to their computers, the fewer creative solutions will come.
Choose your racing series with care.Read the rule book before choosing a racing series, and make sure it all meets your interests and goals. The information presented here should help you improve the aero performance of whatever car you choose.
Definition Of Terms - Below is a list of cursory definitions of the terms associated with aerodynamics.Later on, in the lessons, we will get in to more detail of these terms.There will be charts further explaining these terms within the lessons.
AOA Means angle of attack.The angle is determined by the direction of travel, relative to a straight line drawn from the center of the leading edge, through the center of the trailing edge, or chord line.
Chord Line is the line through the center of the leading edge and the center of the trailing edge.This is the longest straight line through the chord.When we say the chord of an airfoil, we are talking about the length of the chord line.
Direction of travel, for a car is always horizontal, even if you are going up or down a hill.Direction of travel is horizontal to the road surface.
Understand that in aerodynamic terms, the airfoil on a car is upside down.That means the upper surface of the airfoil, would be the lower surface on your car.
When speaking of a race car, an airfoil produces lift in the opposite direction, which is downforce.
Camber Line is a line drawn from the leading edge to the trailing edge of an airfoil that is half way between the upper and lower surfaces of that airfoil.This line, relative to the chord line determines lift and where that lift is between the leading and trailing edge.
Center of Lift, or Center of Pressure is the point on the airfoil where all the lift is centered.In the speed range of race cars, this is normally around the 25% chord point.That means it is normally centered about 25% of the length of the chord, back from the leading edge.
Lift or Downforce is the force created aerodynamically 90 degrees to the road surface.
Pitching Moment.When the center of lift is not on the 25% chord, we talk about the distance between the center of lift relative to the 25% chord position as Pitching Moment.This causes a pitching moment, and or rotation of the airfoil about the 25% chord position.
Leading Edge can be the actual leading edge radius of the airfoil or the leading edge area of the airfoil.The leading edge radius is determined by a percentage of the chord.
Trailing Edge of the airfoil can be the exact end or the area around the exact end.
Thickness Ratio of an airfoil is determined by dividing the maximum thickness, by the chord.
If the thickest part of the airfoil is around 25% back from the leading edge, we call that a forward cambered airfoil.
Upper Camber is the upper curvature of the airfoil as on an airplane.
Lower Camber is the lower curvature of the airfoil as on an airplane.
CL means Coefficient of Lift.This is a number created for each airfoil shape, relative to its angle of attack, for mathematically determining the lift of an airfoil.
CD means coefficient of drag.This is a number created for each airfoil for mathematically determining the drag of an airfoil or object.Drag is the force opposing the direction of travel.
Drag Bucket.If we were to plot the drag of an airfoil relative to its angle of attack, we would see an area of relatively low drag, at low angles of attach.As we increase angle of attack beyond this range, the drag will rise quicker with AOA.The steeper rise in this plot would determine the outer boundary of the drag bucket.Sometimes this is not a definite spot on the plot.
Drag Count. A drag count is a dimensionless unit used by aerospace engineers where 1 drag count is equal to a of 0.0001.If the CD was .0335, and you made a drag reduction where it is now .0334, you reduced the drag by one count.Don’t worry much about this number.You always want to do everything you can to reduce drag.
Side Force is the force parallel to the ground 90 degrees to the direction of travel.Side force is generated by turning the car, banked tracks, wind, or sliding the car.
Station.An airfoil is derived from mathematical tables, where the airfoil is divided up in to many points along the chord.Each point is called a station.
Flap.A flap is a moveable section of an airfoil near the trailing edge that adds lift and drag.The flap is an airfoil in itself, but not normally the same shape as a stand alone airfoil.
Gurney Flap or Wicker Bill.This is a device that Dan Gurney invented that is installed on the trailing edge of an airfoil, to modify the lift characteristics.The device adds lift and drag.It is a relatively small lip extending 90 degrees to the direction of travel, off the trailing edge of the airfoil.This device should not angle forward of the airstream.
Multi Element Airfoils are made up of a series of airfoils, that modify the air flow between each other.These can at times create a greater force than a single element, but not always so.
Downwash is the flow stream created by an airfoil aft of the trailing edge.The airfoil changes the direction of the airstream as a result of the force it created.
Bow Wave.An object or airfoil traveling through the air, pushes a certain amount of that air in front of it.That air being pushed along in front of it is called a Bow Wave.
Air flow is the actual air surrounding a vehicle or object as it passes through relatively stationary air.It is not the same as the air traveling through a wind tunnel, nor are they necessarily relative to each other.
Wing Span is the dimension from one wing tip to the other.
Square Area is the chord multiplied by the wing span.
Aspect Ratio is wing span squared, divided by the square area.Higher aspect ratio’s are considered to be relatively more efficient than low Aspect Ratio’s.Higher Aspect Ratio’s though can cause wing deflection to where this general rule is broken.
Stall occurs when the air on the upper surface becomes detached and turbulent.This is caused by too much angle of attack for the speed the airfoil is traveling.At stall, the lift does not go to zero.With aircraft, at stall, the lift reduces to something less than the weight of the aircraft.
Wind Tunnel.A device that has a floor, walls, and ceiling that channels high velocity air in a controlled manner, so that objects can be tested within it.
Momentum.All things have momentum.Momentum is the tendency of an object or substance to continue in its motion, until some other force changes that motion.Air has momentum, just as a piece of lead has momentum.
Bernoulli Effect.Bernoulli discovered that when he moved air through a tube at high velocity, and then constricted the inside diameter of the tube, that the velocity through the constriction went up, and the pressure went down.The venturi in a carburetor is built this way, and the low pressure within the carburetor is metered to pull fuel out of a float bowl.This effect has nothing to do with how an airfoil creates lift.
Slugs is a mathematical unit of air relative to altitude or density altitude.
Density Altitude is the corrected altitude, corrected for temperature and pressure.The actual density altitude is what we need to know to make all of our aerodynamic calculations.
Dynamic Pressure, or “ Q “ is( Slugs / 2 ) X ( feet per second, squared ).Feet per second is the velocity in the direction your vehicle is traveling.
Velocity is in feet per second.Take ( miles per hour x 5,280 / 3,600 ) = feet per second, or fps.
Kinematic Viscosity, is the viscosity of air, used in calculating Reynolds Number.
Reynolds Number is ( fps x ( distance from the leading edge in feet)) / Kinematic Viscosity.Reynolds Number was a mathematical equation created to categorize airfoils for certain speeds and altitudes, in an attempt to correlate wind tunnel data to actual flight data.Do not assume that choosing an airfoil by Reynolds Number will give you the correct airfoil, nor the correct data.Something is missing from this formula, along with what is missing from higher aspect ratio’s give greater airfoil efficiency.As for Reynolds Number, the shape of the airfoil may be more important than the number itself.There is no real single number that can determine what airfoil to use at a certain altitude or speed.
Vortice or Wing Tip Vortice is a horizontal tornado created by an airfoil at and behind the wing tip, as a result of lift.The tornado will always rotate in the direction from high pressure to low pressure.The left and right wing tip will spin the air in opposite directions of each other.If we are behind a race car, looking forward, the right wing tip vortice will spin clockwise and the left will spin counter clockwise.In addition, both vortices will descend to the ground and converge towards the center of the car, well behind the car as it travels.It is generally thought that the larger your vortice is, the greater the drag is.This is not always so.If you obtained greater attached flow, and a greater vortice, the overall drag could go down.
End Plates on wings.End plates primarily provide side force when the car is sliding, and advertising area.They also diminish in an inefficient manner, the wing tip vortice.End plates as used today typically reduce downforce when the car is sliding, due to their poor design or placement.End plates do not trap the air making more force.They simply move the pressure leak to a different place, and that does not cause more force.
Airfoil Efficiency.You can invent your own airfoil, or pick one from a catalog, or modify an existing one.Understand though, that there are many factors that can be very detrimental to performing as your calculations showed.The paint, dirt, rain, lap joints on the airfoil skin, rivet heads, dents, pin stripes, inaccurate forming, wing skin deforming from air pressure, all affect the airfoil efficiency.You must consider this when developing airfoils.Accuracy is very important.
Yarn Tufting is a method of taping small lengths of yarn to anything traveling through the air, and recognizing how those yarns act.If you want real data on what the airflow really is doing on your car, you must drive it at the speeds and with the conditions the car will be used at.A wind tunnel will distort what is really happening.
Ram Air is term used for the high pressure air we try to capture for some benefit.We can calculate the Dynamic Pressure as described above, but we cannot seem to capture much of this and put it to good use.The reason is, as soon as we capture some of that pressure, the inside pressure builds up a small amount and then just spills out the opening.Always try to capture ram air, but don’t make the opening too large, or your drag penalty will be greater than the goal you achieved.Know how much air your engine needs to make that power at that speed.
Ground Effects are any part of the car that modifies the air flow under the car in order to obtain a force.If done right, this can cause a great force.Again if done right, that force will be in the proper place.Done really well, wings may not be needed.
Area Ruling is a method of attaching aerodynamic components that minimizes the drag created by the intersection of those components. Air can flow freely around a vehicle until it is restricted.Any devise attached to the car, exposed to the airstream, restricts that flow somewhat, increasing the drag.We can reduce that restriction by area ruling, which in turn will diminish the drag.Area Ruling blends smoother, the actual displaced volume of the car from the front to the rear as it passes through the air.
Intersection Drag is the drag caused by any devise attached to the car, at theintersection of the attachment.
Choking Drag is the drag caused by two parallel or nearly parallel components compressing the air between them.The two surfaces can be parallel to each other and still cause this negative affect, because they have surface drag.The new GT 40 has a huge amount of this drag.
Surface Drag is simply the drag caused on the surface of a vehicle traveling through the air.All the surface exposed is causing some drag, so the surface area of your vehicle should be kept to a minimum.
Coanda Effect uses exhaust velocity to accelerate the local air around the exhaust exit for some aerodynamic benefit.this can typically be used for extracting air out from underneath the car, which in turn creates downforce.This have been used on aircraft since the 1940’s or 1950’s, but only recently (maybe the last 20 years) has it the method been employed with race cars.
Attached Flow.When air flows over a surface where it is considered attached, that will create the least amount of drag.Attached flow is when the air flows over an object in an orderly and controlled fashion. When you yarn tuft a car and see that the yarn tuft is against the surface and not moving around, you are seeing good attached flow.If it is against the surface, but moving around a bit on that surface, you have a weaker attached flow.The yarn should point back in the direction of the airstream with attached flow.
Detached Flow.This flow causes drag.If yarn tufting were depicting this condition, the yarn would be up off the surface of the vehicle a bit and waving around a limited amount.This condition causes more drag than Attached Flow.
Turbulent Flow is where the air flows over a surface in a totally uncontrolled manner, creating the most drag.If yarn tufting were depicting this, they yarns would be way off the vehicle surface, possibly standing up on end, or even pointing backwards, and waving around frantically.If you have this condition on an airfoil, the airfoil is stalled in the worst way.
Finally, understand that much of the science has been derived around the use of wind tunnels.It has been a constant battle to get wind tunnel data to correlate with the real world.You are essentially trying to modify the wrong data, to something you can use.But, you must remember it is wrong data.The correction factors change when the wind tunnel velocity changes, the shape of the model changes, or the size of the model changes relative to the size of the wind tunnel.
Most of you taking this course can’t afford wind tunnel time, nor have the access to the math wizards that can modify this data. But you do have access to the real data, for about $5.00.
Every racing series out there today is missing at least some part of these lessons, and that includes Formula One.Some of these series limit too much of what you could do.Even with those series, gaining a greater understanding should help spark some ideas of how to improve the car.