Oasis TR

Put simply, aerodynamics is to do with the flow of a gas or a liquid over a static object and the forces created due to this. Conversely, it also describes what happens to the gas or liquid when a moving object cuts through it.



Consider a tank full of water. If we were to drop a brick, vertically, into this tank we would observe a big splash with serious disruption to the whole surface of the water. Now, if we were to have a rhombus-shaped wedge, made of brick but of equal mass, and drop this into the tank, point first, what would we observe?










What we would see would be the wedge cutting the surface of the water gradually, causing hardly any splash, and substantially less disruption to the surface. This, in essence, is what happens as a car cuts through the air. Even though air has a much lower density than water, a coach (our brick) causes great disruption to the air around it while moving, whereas a hatchback (our wedge) cuts the air not with a large flat face (our bus), but with a small, gradually expanding surface area. What this means is that the power required from the engine to propel the vehicle through the air can be significantly reduced as we are causing less disruption as we proceed. The main aim of the aerodynamicist is to make the air move as smoothly over the vehicle, to make as little a ‘splash’ in the air as is possible.



However, once the air has moved over the top of the car, what happens then? This is where we encounter drag. From Newton’s third law we know if a body is to maintain a constant velocity that every action has an equal and opposite reaction. Drag is our reaction to the driving force, so in order to accelerate the force driving the car must be greater than the aerodynamic drag.



As air flows over the car, the air particles closest to the surface of the car body form a boundary layer. The shear forces (slipping particles) produced by this layer cause the airflow to slow down and saps the kinetic energy from the airflow. The boundary layer is turbulent, and the thickness of the layer increases as it moves along the body. This is because the particles in the layer are moving in a random motion and the thickness of the layer increases as these particles lose energy due to collisions with the surface of the body. The layer clings to the body contour, and so enables the airflow to follow the body contour as well. As the body falls away so does the boundary layer. This causes large-scale turbulence, or a wake as it is sometimes known. In this wake, we have low pressure, and the car must ‘drag along’ this area of low pressure (in effect a bit like having a vacuum at the back of the car). This is drag, and the energy to overcome this drag must come from the car’s engine. So, to accelerate, the car’s engine must produce a greater driving force than the force generated by the area of low pressure at the back of the car that is effectively trying to pull the car backwards.



Another important aspect that the aerodynamicist must take in to consideration is generating downforce.

Daniel Bernoulli was a Swiss mathematician and physicist who in 1738 came up with the principle that

‘As the speed of a moving fluid (liquid or gas) increases, the pressure within that fluid decreases’

So, as air flows faster, its pressure drops. This principle explains why certain things fly and some don’t. If we consider the shape of a wing:






The air has a greater distance to flow over the top than it does under the bottom. The air, therefore, flows faster over the top than it does under the bottom. So, because of the rise in velocity of the air over the top of the wing, the pressure of the air decreases. This causes a pressure difference therefore generating lift. However, the greater the lift generated, the greater the aerodynamic drag, so a happy medium must be reached. Drag coefficient Cd is a figure used to represent the drag per unit area over downward pressure per unit area:



Cd = (N/m2)/( N/m2)



So, our bus will have a high drag coefficient as the drag is large because of the disruption to the airflow. The sportscar will have a low drag coefficient as it ‘slides’ through the air, causing minimal drag. Therefore the lower the drag coefficient, the better.

Oasis TR

This is to you guys who dont think that a rear wing makes a difference on a Truggy or Buggy..You must remember that it's all about air flow and how the rear wing controls this. For every one who thinks a "small" rear wing wont make a difference on a 10 pound + truggy, why does a rear wing work on an F1 car..(I know that they dont jump but aerodynamic theory does not change)..Weight and wing size..

1. Aspect Ratio:
The amount of down-force produced by a wing is determined by its size. The larger the wing the greater the down-force. The length/width ratio is called the aspect ratio. The higher the aspect ratio, the more efficient the wing. The aspect ratio is the span of the airfoil (the long dimension perpendicular to the airflow) divided by its chord (the dimension parallel to the airflow).
2. Angle of Attack:
The efficiency of a wing is its down-force/drag ratio. The amount of down-force generated is dependent upon the angle or the tilt of the wing. The angle of attack is greater on a road course rear wing setup than on a speedway setup. The greater the angle of attack, the more down-force and drag. The angle of attack and the size of the rear wing will vary from road, short oval and speedway setup.
3. Drag:
While increasing down-force a wing also increases unwanted drag. Drag increases with the angle of attack. The down-force generated by the wing works in a vertical, downward direction, while drag acts in the opposite direction.

And I'm not trying to start a flame war..I'm just trying to put out some information that might help explain why aerodynamics really do have an effect on RC cars..This is a hard subject to discuss because their is not very much information that directly address this subject.

chedster

Hey I am with you that aerodynamics really do have an effect on rc cars, heck anything moving in the outside world feels the effect. I just say the effect is pretty small and that it does not play a huge role in the handling of the truggy. It seems that the typical race truggy cannot reach speeds great enough to feel large effects of down force and drag. There is an equation for generating down force, for us it's on a wing. If you look up the equation for down force you will notice that the velocity (v) is squared. This means that its rate of change is not linear, and grows much faster then a linear relationship. The bigger the numbers the faster they grow. V is going to be the biggest number in the equation thus dominating the magnitude of the down force, measured in Newtons. Often times there is also minimum speed at which down force becomes applicable. So your right while blazing down the back stretch at 40mph, your going to have way more down force then you would cruising thru the whoops section at maybe 10mph, assuming you are minimizing drag. Remember this is only down force created by the wing; I have not even taken in to account the lift or down force created by the truggy. All in all you can see this can become a complicated problem with many calculations, but in the end well worth it if you take racing serious. In no way am I saying I'm am right, heck I myself am an aspiring engineering student. I know there are plenty of bright individuals on here with more knowledge then I on the subject. Just because the truggy has a wing on it does not mean it is automatically going to handle better, it first has to agree with the laws of the universe.

Oasis TR

I raced 1/10 touring cars (mostly carpet but some asphalt) and it may not seem like aerodynamics would play much of a role especially on a small carpet track. But it was HUGE, just running a different style of body would make a big difference in lap times (I practiced 4 times a week anywhere from 6 to 12 hours a day..so I tested a lot..lol). As far as a Truggy goes it's hard to explain but I'll try..when your buggy or truggy is in flight try to think of it as if it was on a pendulum balanced in the center.. it does not take that much down force to move it..it does seem to help keep the truggy level in flight..I race a ST-RR and the rear wing is held on by body clips, on a long main one of the clips came off so the wing was moving all over the place, it made such a huge difference in the way the truggy handled I had to come into the pits and fix it (put another clip on it..lol)..It would not jump straight if it was off to one side it would actually try to barrel roll..just a handful to drive..this is how I know that it makes a bigger difference than what some would think..I'm not all that great in mathematics but I at least understand the basic theory's of what your trying to say..but we forget velocity and the down-force it creates..The acceleration that RC cars generate is basically unseen in the "real" world of auto racing, the closest thing that i can think of is F1 cars..this is why some of the basic automotive theories on aerodynamics have a hard time crossing over to RC cars..the performance of the RC cars/trucks we have nowadays is simply amazing if you put everything in perspective..

chedster

Ya I see what your saying, it seems hard to think the same principles that apply to 1:1 race cars also apply to little RC cars, heck that’s why I got into RC, they are amazing little machines. My neighbor was looking at mine the other day and he was amazed by it. He could not believe it even had little brakes. I then let him drive it...bad idea as he tried to jump a curb with it, luckily just some scratches

Jerzferno

Yes wings work at HIGH speeds. We dont see the speeds where these wings will provide any "significant" down force on 1/8th scale off road racing. Wings work on formula 1 cars because they are going 100 mph plus on most of the track. If you think 2 ounces of downforce is alot, then I am with you. Thats what I saw when I experimented with fans and scales. 2 ounces total. Does that make a difference between winning and not wiinning? I have no idea? Probably not.

Oasis TR

I think that we do reach speeds where aerodynamics work..I'm just going by the basic formulas..1. Aspect Ratio:
The amount of down-force produced by a wing is determined by its size. The larger the wing the greater the down-force. The length/width ratio is called the aspect ratio. The higher the aspect ratio, the more efficient the wing. The aspect ratio is the span of the airfoil (the long dimension perpendicular to the airflow) divided by its chord (the dimension parallel to the airflow). what we forget is that 20 mph on an 1/8 scale is not the same as 20 mph on a full size car..and if you look at the Aspect Ratio of a typical 1/8 rear wing if you put it on a 1 to 1 car it would be huge..lol..

Jerzferno

Being a Mechanical Engineer, Im well versed in Dynamics.

slayerphonics

People are obessed with the wing debate,

Until someone post some real data, besides opinions. I'm not commenting anymore on it.

This makes me want to build a wind tunnel and test it,

Because I think it has very little to no affect on the truck, especially with jumping, I was jumping monster trucks for years with no wings and no problems, jumping my truggy with a wing was no diffrent, you use the gas and brake to govern the jump not the wing,

chedster

I have a question, If you shot a bullet and dropped a bullet off a cliff, both at the same time and same height. Which bullet will hit the ground first? We'll assume wind drag is the same on both bullets?

sloppyG

I think that you meant to say this happens in a vaccuum


wind drag can't be the same on 2 objects travelling such hugely different speeds

in a vaccuum, they'd hit the ground at the same time

slayerphonics

How would that work?

One is propelled.. the other isn't. the propelled one would hit first vaccum or not.

chedster

Ya vacuum whatever just disregard wind drag. One is dropped and the other is shot horizontally