Electric versus Hydraulic Flight Controls: Assessing Power Consumption and Waste Heat Using Stochastic System Methods

Of all aircraft power and thermal loads, flight controls can be the most challenging to quantify because they are highly variable. Unlike constant or impulsive loads, actuator power demands more closely resemble random processes. Some inherent nonlinearities complicate this even further. Actuation power consumption and waste heat generation are both sensitive to input history. But control activity varies considerably with mission segment, turbulence and vehicle state. Flight control is a major power consumer at times, so quantifying power demand and waste heat is important for sizing power and thermal management system components. However, many designers sidestep the stochastic aspects of the problem initially, leading to overly conservative system sizing. The overdesign becomes apparent only after detailed flight simulations become available. These considerations are particularly relevant in trade studies comparing electric versus hydraulic actuation. These two actuation types use power in fundamentally different ways. This paper provides methods to quantify power consumption and waste heat, by applying stochastic system methods. Both electric (electromechanical and electrohydrostatic) and conventional hydraulic actuation are discussed. Formulas are derived to quantify average and peak power demand. A complete set of waste heat mechanisms is also discussed, and methods are provided to quantify each one. For electric actuation, a method is provided to estimate regenerative power. Approximate methods are also addressed, to facilitate quick engineering estimates. In addition, the physical locations of waste heat generation are identified because these can impact thermal management system architecture.