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The Delta Wing - Nose to Tail

Part 4 of 7 by Scott Morris
Tuesday, March 02, 2010

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Current style of IndyCar design, in a trailing-car wind tunnel setup


The originating intent of the Delta Wing design was to create better racing that allows the drivers to really race, especially at the superspeedways, and be able to trail another car and draw close enough to have the exit speed to pass a car coming out of the turn and heading down the straight.

Being able to really race in traffic, which is a big appeal of the NASCAR variety of racing, is key to the quality of the competition and entertainment value.

In this fourth installment of our 7 part series, we talk about how this car will bring all of that to IndyCar racing, restoring a style of racing that is truly American style and as challenging for the drivers as it is exciting for the fans to watch.

The key to this car design is that nearly all of it's downforce comes from underneath the car. Of course, a lot of the downforce of most open wheel cars comes from the underside of the car. However with the traditional design, the wings also develop a sizable downforce and the two parts (wings and underbody) work together.

The problem with this is that the turbulent air that results from a winged open wheel car greatly reduces the downforce produced by the wings, and the underwing. So it makes it impossible to maintain momentum when drawing up on a car into a turn. The driver has to come out of the throttle because the car loses downforce, and hence traction.

Indy 500 throttle and downforce data from Dario Franchitti's Car, in comparative open track and traffic conditions.
Current design style downforce vs. distance behind leading car
Note the huge drop at about 2 seconds behind another car

Check out this graph that shows Dario Franchitti's throttle position, speed and effective downforce throughout the 2009 Indy 500 . You can see when he is on clear laps, even for an extended period of time, the throttle stays pegged flat at 100%. When he hits traffic, you can see sharp and frequent dips to no pedal at all, which on a track like Indy, should be rarely the case. These throttle dips were because Dario had to get out of the throttle simply because there was no grip behind another car, and you can see the downforce number dip as well as the speed.

The next graph shows the data from the 2009 Indy 500 for reduction in downforce and its relationship to the distance behind a leading car. First of all, note the significantly lower downforce on the left side of the graph, at the shorter time intervals between cars. Amazingly, look at the spot where it take a huge downward spike at about 2 seconds. It's almost as if someone threw a car cover over the car.

The recurring remarks on the radio and in the post-race briefing had the drivers saying that they had a very tough time even passing cars that were much slower. Well, that is clearly a problem that needed to be solved.

Telemetry data indicated that Dario was experiencing a loss of downforce of as much as 38%. This is a huge amount when considering that a car cornering at the limit only needs the lightest tap to send it into chaos. So it not an issue of the driver just braving  it into the corner.

Jackie Chiles, the lawyer in a number of Seinfeld episodes would say "If the car won't grip, it just won't grip and the speed has to dip."

Much effort and study was dedicated to this problem. Check out the opening image of the two current-style cars in the wind tunnel above. Nobody had ever done this before.

Many other issues arose in trying to quantify this effect, and then produce a solution. Unlike many other aerodynamic problems, this proved to be very difficult to test in a wind tunnel with a moving ground plane, as there were none that could fit two cars.

CFD and other simulation methods also proved to significantly overestimate downforce loss, which is something to keep in mind as we move on...

After all of this, it became obvious that some kind of departure in design would be required. So the new design was born out of seeking the only apparent path to a real solution.

The first thing that had to be addressed was the turbulence that a rotating wheel out in the middle of the airflow creates. Limiting that by covering most the wheels, and sending the air over the majority of that rotating surface, was a huge step forward.

Secondly, eliminating the wings altogether was the next step, followed by creating a huge low pressure zone under the car, which sucks the car to the ground. This ground effect is not dependent on airflow from wings, because there are none. There is some downforce created by the nose and body shape, but is a cleaner airflow.

With all of the downforce being created under the car, and a very slick body shape, the overall drag of the car is dramatically lowered, and the trailing airflow behind the car is cleaned up immensely. Since the trailing car is not depending on airflow to the wings to make downforce, the overall loss is also greatly reduced.

Delta Wings
Nose to tail CFD simulation - top and bottom
Check out the CFD illustrations with one Delta Wings trailing another. The brighter red colors are the higher pressure areas, indicating the downforce generated. There is very little difference between the downforce on the leading car and the trailing vehicle. In fact, Ben tells us that the loss they are seeing in the CFD simulations is about 12.5%, which is a groundbreaking result. The most interesting part, is that the CFD simulations tend to overestimate the downforce loss vs. the real world loss by as much as 20%. So that means the real world expectation could put downforce losses at less than 10% overall for the Delta Wing design.

In fact, Ben Bowlby tells us that they are seeking to actually have no loss of downforce at all when trailing behind another car, and he sounds pretty confident about that goal. He also added that with a slight offset in the line of the trailing car, it is possible that the trailing car could have an increase in downforce.

There is one other interesting factor that ties into one of our previous installments. This is that the body shape with the narrow front and wider rear, presents a very efficient shape to the oncoming airflow. Interestingly, the bull-nosed leading edge tends to create downforce at higher yaw rates as well. So in theory, the driver could hang the car out quite a bit behind another car, and actually get some of that lost downforce back by offsetting the racing line a bit, and allowing that bodywork to create downforce that could exceed that of the leading car. That might be wishful thinking, but is a great goal to work toward.

What does this mean to you the fan, watching the race from the stands or on TV? Well, you will see a trailing driver being able to get right up on the tail into the turn, and moving to a slightly offset line, hanging the tail out and actually gaining grip and picking up speed, and then carrying that through the turn and making a pass in the turn itself, or coming off the corner.

Now, all first impressions aside, isn't that what we need in this sport?

If this is starting to sway your opinion, wait until you read our next installment that will detail the advantages of an active differential. Mario Andretti really loves that part.

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