April 6, 2022 | Into The Wind Tunnel, Part Two

We are the architects of tomorrow, determined to reimagine what travel can be and designing the way to get us there. To that end, we recently completed an extensive wind tunnel testing campaign. Over the course of a month, our team carefully studied the aerodynamic characteristics of Maker’s motors and propellers. Conducting the tests were Giovanni Droandi (Aerodynamics Manager), Mike Kerho (Senior Aerodynamics Test Engineer), and Kalem Dinkel (Integration and Test Engineer). The team segmented the tests into two distinct phases, each focused on unique scenarios and goals.

PHASE ONE

Our initial phase was dedicated to testing a full-scale motor and propeller system for both the forward tilting propellers and the aft lifter propellers. In fact, both the motors and propellers were actual flight hardware taken right from Maker, Archer’s full-scale demonstrator eVTOL aircraft. You can view these motors and propellers in action here. These tests were done with the motor/propeller in isolation, meaning the aerodynamic performance was investigated without the influence of other aircraft components, such as the boom and wing.

In this phase our team measured the isolated propeller performance across the full operating envelope of the propeller, which is defined by a large matrix of conditions with varying airspeed, propeller blade pitch, motor RPM, and (for the tilter) propeller disk tilt angle. Examples of the key measurements taken include thrust and torque forces, pitching and yawing moments, hub vibration levels and aerodynamic efficiency metrics. Of particular interest to our team was exploring those conditions where airflows are unsteady and thus difficult to predict analytically with high accuracy. These are often referred to as “corner of the envelope cases” of which high angle of attack propeller blade stall conditions are a good example.

PHASE TWO

The second phase of the wind tunnel campaign continued with testing of the same tilter and lifter motors and propeller blades while mounted to a full-scale boom. The boom was then attached to a representative full-scale wing section. This setup provided our team with a wealth of data in a controlled environment.

The focus of this phase was to investigate and characterize the complicated aerodynamic flow patterns and forces induced by the forward tilter propeller onto the wing and boom during cruise conditions. As you may imagine, a propeller spinning at high rate introduces swirl into the airflow that travels downstream and interacts with the wing and boom aerodynamics in a complicated and variable manner, depending on motor RPM, propeller blade pitch, and wing angle of attack. Measurements taken for this configuration included forces and moments on the boom and both static and dynamic surface pressures on both the wing and boom. Special attention was also given to characterizing the drag created by the aft-mounted lifter propeller, which is stopped during cruise conditions in such a way that the two-rotor blades are aligned fore-aft with the streamlines of the local airflow.

This portion of the test also included several exciting days of Natural Laminar Flow (NLF) testing. A sophisticated method known as infrared thermography (IRT) was used to visualize the transition point of laminar-to-turbulent flow within the wing boundary layer. Thermography refers to the graphical visualization of temperature fields on the model surface. IRT works by detecting the heat radiation emitted from the wind tunnel model surface and correlating that to the local temperature within the boundary layer. It is then able to differentiate between laminar and turbulent flow due to differences in the heat transfer between the two boundary-layer states.

The baseline wing airfoil section was designed to achieve a certain amount of laminar flow. A main benefit of laminar flow is reduced drag, which translates into reduced power requirements and/or increased range. Due to the complex interaction between the periodic blade wake and the wing airfoil, with different regions of upwash and downwash, it’s extremely difficult to reliably predict the actual amount of achievable laminar flow. The use of IRT thus allows the amount of laminar flow to be quantitatively visualized during the test for the baseline airfoil section and also for the wing airfoil under the influence of the propeller at different thrust conditions.

THE FINDINGS

Archer uses state-of-the-art Computational Fluid Dynamics (CFD) to design and predict performance for our eVTOL aircraft. As mentioned above, there are certain flight conditions that are difficult to accurately predict with today’s CFD capabilities. The interactions of multiple propellers in different orientations and the effects they have upon the airframe are extremely complex. It’s why we complement analytical prediction methods with this type of wind tunnel testing. 

The data our team collected by testing in the wind tunnel will be instrumental in calibrating our tools and in verifying the CFD-based predictions we are using to design and develop our production eVTOL aircraft. Detailed comparisons between the wind tunnel data and our CFD-based predictions give us confidence in our design tools, methods, and processes. Results from both the isolated and installed wing/boom test campaigns have compared well with our CFD predictions. 

These tests are only part of the journey, but they’re integral to turning our vision into reality. We are on a path to revolutionize air travel and bring urban air mobility to the masses. Going into the wind has brought us ever closer to taking our production aircraft to the skies. And we’re continuing to study the data that will make venturing further possible.