PART I

Why eSTOL?

For a long time, sci-fi movies have been imagining aircraft navigating through skyscrapers in a bustling modern city. Unfortunately, there are many obstacles to materializing these imaginations. The first aircraft that you can think of in this context is likely a helicopter, relying entirely on the lift created by its fast-rotating propellers. They require a powerful engine, which can be a significant source of cost, noise, and air pollution, severely limiting their usability in an urban environment. Another option, of course, could be a conventional airplane that utilizes the lift generated by its wings. These are much more efficient, but they need a runway, which is impractical in a city.

We can think of combining the properties of these two aircraft and imagine a vehicle that can take off and land like a helicopter, but also fly like an airplane. The tiltrotor “Bell-Boeing V-22 Osprey” is an example of this category. It has a rotor system that can transition from a vertical position (for takeoff and landing) to a horizontal position (for cruise). Unfortunately, the complex engineering, the additional weight involved, and the compromised wing and rotor areas degrade the endurance and range in these aircraft. These factors render them economically unsuitable for private enterprises.

Through intense research, battery technology has been steadily progressing over the years, and we are reaching an interesting period when the battery energy density is large enough to sustain practical driving ranges in mass-produced electric cars. The cost of manufacturing such batteries, the energy stored per pound, and their power output are of primary consideration. Fortunately, the battery technology has matured so that we can develop electric aircraft for short-haul travel. Such airplanes are in service for pilot training.

Electric aircraft benefit from the ease of integration and size of the motors powering the propellers, which can be much smaller than a conventional fossil-fuel-powered engine, without compromising performance. This gives us the ability to use several motors on each wing. Such a “distributed electric propulsion” makes it practical to design aircraft that take advantage of widespread aero-propulsive interactions. One example of these types of vehicles is an electric Short TakeOff and Landing (eSTOL) aircraft. It uses the deflection of the propeller’s air flow stream over the wing and trailing edge flaps to increase the lifting capability of the aircraft dramatically. This enables relatively low flight speeds and short takeoff and landing distances. Both these properties are extremely important for envisioning air travel in an urban environment. Some of the recent aircraft utilizing these ideas can operate on a football-field-sized runway and can accommodate about 20 seats.

Parameter Study

The aircraft's performance will vary depending on how the flow stream of the multiple electric rotors interacts with each other, with the wing, and with the flaps. Therefore, the design of eSTOL presents a very large configuration space to explore and optimize. For example, propeller motor count, size, position, flap type, size, and angle will strongly impact the aircraft's performance. As we mentioned earlier, eSTOL vehicles are only beginning to become practical, and the design parameters that can best optimize a vehicle’s performance, given external and practical constraints, remain largely unexplored.

The scientists and engineers at Electra collaborated with Flexcompute to shed more light on this interesting problem. Our primary objective is to study the air flow properties of a model eSTOL aircraft using computational fluid dynamics (CFD). Compared to the actual windtunnel experiments, the CFD approach is much more flexible and allows rapid iterations of the relevant design elements. However, it must be noted that a CFD approach is still theoretical and achieving sufficient agreement with a set of wind tunnel tests will be an essential requirement for a successful design pipeline.

Compartmentalizing the Problem

Our modeled aircraft comprises a fuselage, a high aspect ratio wing, and four electric rotors per wing (see Figure 1). However, instead of directly examining the fluid dynamics of the entire aircraft, we begin by studying the essential elements that define the blown-wing system.

Figure 1: Our modeled eSTOL aircraft. Note that we only model the left side since we can assume symmetric flow for this configuration and operating conditions simulated.

The Centerbody houses the electric motor, the Main Element represents the primary wing, the propellers are attached to the Centerbody, the pylon connects the Main Element to the Centerbody, and the flap is attached to the trailing edge of the Main Element; these are the important sub-components (see Figure 2). Studying these separately would be fruitful, as these simplified setups are essentially quasi-2D approximations of the full 3D model aircraft.

Figure 2: The important sub-components of our modeled eSTOL aircraft.

Out of these subcomponents, modeling the fluid dynamics of propellers can be a significant challenge, followed closely by the flow impingement on the wing and flaps. The rotation speed of the propellers is extremely high—typically in the thousands of RPM—so as to generate enough thrust for the aircraft. This poses a substantial challenge when simulating the physics involved in this system. When we track each and every propeller of the rotors and the associated flow, we can refer to it as a “fully resolved” (FR) approach. Its high computational cost creates a strong impetus for developing simpler models, at least for preliminary design work. This concludes PART I of the series. In the next article (PART II), we will discuss various propulsion approximations for modeling the propeller physics and provide details on how our solver, Flow360, employs the finite volume method to conduct such simulations.

If you’d like to learn more about CFD simulations, how to optimize them, or how to reduce your simulation time from weeks or days to hours or minutes, stop by our website at Flexcompute.com or follow us on LinkedIn.

For the expanded version of the paper this content is derived from, see Impact of the Propulsion Modeling Approach on High-Lift Force Predictions of Propeller-Blown Wings.