How Stratos Aircraft cleared a transonic flight envelope without breaking the budget

Flutter testing is a safety-critical requirement for any new aircraft program. For large aerospace manufacturers, the cost is manageable. For aviation startups like Stratos Aircraft, standard flutter testing methods can be prohibitively expensive. enDAQ's wireless, self-contained sensors enabled Stratos to take a novel, objective approach—distributing sensors across their PoC airframe to capture bending and torsional modes during both ground and in-flight testing.

Flutter testing is essential—but standard approaches were out of reach

Flutter is a self-excited vibration caused by the interaction of aerodynamic, inertial, and elastic forces acting on an aircraft wing. If not designed for and validated through testing, flutter can lead to catastrophic structural failure. Every new aircraft program must address flutter—both on the ground and in flight.

For Stratos Aircraft, a jet manufacturer from Redmond, Oregon developing an Experimental-designated single-engine transonic jet, the standard approach to flutter testing was financially prohibitive. To compensate for smaller budgets, many Experimental/Amateur-Built (E/AB) aircraft manufacturers fall back on pilot perception of stick and rudder pedal raps—a method Stratos deemed too risky and too subjective for their complex transonic flight envelope.

• Standard flutter certification testing is cost-prohibitive for aviation startups.
• Pilot-perception methods are subjective and do not produce objective, repeatable data.
• The flight envelope included transonic speeds, increasing the complexity and risk of flutter events.
• Any sensor solution needed to avoid altering aircraft mass or structural properties.

Distributed wireless sensors across the airframe—no wiring, no mass impact

enDAQ provided customized wireless sensors based on the W8-E25D40 platform. The customization allowed the devices to be affixed to the aircraft without altering mass or structure—a critical requirement for valid flutter testing. Stratos mounted the sensors at discrete locations on the PoC airframe, enabling simultaneous monitoring of the wing's main bending and torsional modes.

enDAQ accelerometer in-situ set-up and diagram of placement on body of aircraft.

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With onboard GPS and wireless capabilities, the sensors generated timestamped data that allowed exact synchronization across all devices. This synchronization was crucial to differentiating between symmetric and antisymmetric motion—a key requirement for flutter envelope clearance.

To excite the PoC aircraft's structure, pilots conducted stick and pedal raps with a dead blow hammer, stomped on the pedals, and deployed airspeed brakes during flight. The resulting data revealed the frequency content of each excitation type across different altitudes and airspeeds.

• Stick and pedal raps excited structural modes up to 15 Hz—higher than previously expected.
• Airbrake deployment produced broadband excitation of the entire airframe from 15 to 40 Hz.
• GPS-based time synchronization enabled symmetric vs. antisymmetric mode differentiation.
• Pressure transducer data provided altitude synchronization across sensor devices.
  
  

Left: Pilot performing stick and pedal rap testing with a dead blow hammer. Right: Air brakes deployed on PoC aircraft.

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Flutter-free envelope clearance—and a new framework for E/AB testing

The data collected by enDAQ's distributed sensors confirmed no trends indicative of flutter within the PoC aircraft testing data, leading to successful flight envelope clearance. The testing also produced insights of broader value to the E/AB aircraft community.

Stick and pedal rap frequency response (up to 15 Hz) proved more valuable for flutter testing than previously thought, while airbrake deployment was highly effective at exciting the 15–40 Hz range across the entire airframe. Both findings were published as part of an AIAA technical article, contributing to the broader body of knowledge on E/AB flutter clearance.

enDAQ's self-contained sensors made this novel approach possible—providing accurate, high-quality data with minimal installation requirements and no interference with the aircraft's structure. Stratos's results indicate that this methodology may serve as an alternate means of flutter clearance for other E/AB manufacturers.

• No flutter trends found across any test condition—full PoC flight envelope cleared.
• Stick/pedal rap excitation proven effective up to 15 Hz for flutter testing purposes.
• Airbrake excitation covered 15–40 Hz, complementing stick/pedal rap data.
• Findings published in an AIAA peer-reviewed technical article.
• Approach validated as a cost-effective alternative for E/AB flutter clearance programs.