Technology

How We Put So Much Performance in Such a Small Package

The Rocketail Wing reduces aerodynamic drag using an extremely compact, internally braced design that requires no involvement by the driver. The Wing was designed to have the best performance-to-size ratio ever achieved in a rear drag reduction device, making it maximally effective while minimizing the causes of collisions and damage. To achieve this ratio, Rocketail has two unique patented design features which allow us to smooth and redirect aerodynamic wake while producing forward lift.

First, Rocketail is placed one inch away from the trailer wall, avoiding the boundary air layer to interact with higher volume, more uniform airflow. Second, we utilize an advanced multi-element airfoil similar those on jet aircraft. These two features enable us to “close the wake” while using a profile that’s 50% to 80% smaller than competitive designs.

The primary cause of drag is separation of airflow. When a solid object travels through a fluid, the area of fluid close to the solid surface is known as the boundary layer. Boundary layer separation occurs when the fluid flow becomes detached from the surface of the object, and instead takes the forms of eddies and vortices.


The Challenge: Aerodynamic Drag

The current semi-truck design is a result of the industry seeking to maximize freight efficiency while adhering to DOT restrictions. Consequently, semi-trucks are square and bulky to allow for maximum storage capacity, which results in poor aerodynamic properties. The majority of the power generated by a Class 8 tractor (50% to 70%) is utilized to overcome drag forces.
(http://www.freightwing.com/on-aerodynamics.php).

As a result, in the United States, on-road freight is the fastest growing source of transportation related fuel consumption—$150 billion in 2012—and greenhouse gas emissions1. Aerodynamic drag accounts for 65% of the total drag on a Class 8 truck with the remaining 35% attributed to the tires2.

As the trailer moves through the air, an area of low pressure forms directly behind the trailer creating separation and generating a turbulent airflow (i.e. back pressure)3. This turbulent airflow directly behind the trailer represents 16.25% of the energy consumed by the truck when it is traveling at a constant velocity on a level surface at highway speeds. The objective is to minimize separation and generate a laminar airflow to reduce tail-end drag (i.e. back pressure).

In order to minimize the wake and create a laminar flow to reduce drag on the tail-end of the trailer, airflow separation must be reduced, which is achieved by neutralizing the negative pressure (i.e reduce back pressure).

Reduce Separation
Turbulence occurs when a fluid boundary layer3 travels far enough against an adverse pressure gradient and separates from the surface. In the case of the back of a semi-truck, the trailer surface makes a 90° turn and consequently, separation from the trailer is inevitable at higher speeds. The best technique for reducing drag is to reduce the back pressure behind the trailer by reducing the size of the aerodynamic wake.

Neutralize the Negative Pressure
Without a “boat-tail” to encase the haphazardly growing air pressure behind the trailer, the increased pressure will continually eject itself. This repeating pattern of swirling vortices is known as a Karman Vortex Street and it causes considerable drag on the truck. It is critical to choke off this repeating pattern of vortices. By doing so, back pressure behind the trailer is minimized, effectively reducing the size of the aerodynamic wake.


Aerodynamic Principles

Aerodynamic drag refers to forces acting opposite to the relative motion of any object moving with respect to a surrounding fluid. In the case of a semi-truck, there are two relevant forms of aerodynamic drag to consider.

Parasitic Drag
The combination of form drag and skin friction drag. The majority of the aerodynamic impact is a result of parasitic drag.

Induced Drag
The drag created from redirecting airflow to obtain a desired outcome. Rocketail generates induced drag to redirect airflow however, the desired outcome significantly outweighs the induced drag.

Objects that allow for a laminar flow have minimal drag, while objects that generate a turbulent flow have the opposite effect.

Laminar Flow
An object that allows for a laminar flow will have minimal drag, and therefore, is the most efficient shape.

Turbulent Flow
An object that creates turbulence in its wake is inefficient and has increased drag. In the most fundamental sense, the primary cause of drag is separation of the boundary layer, which translates to turbulent airflow. When a solid object travels through a fluid, the area of fluid close to the solid surface is known as the boundary layer. Boundary layer separation occurs when the fluid flow becomes detached from the surface of the object, and instead takes the forms of eddies and vortices.

1 Trailer technologies for increased heavy-duty vehicle efficiency.
2 At highway speeds on level road.
3 Separation explained in Aerodynamic principles section