UH-60/VTDP Compound Helicopter Research

Introduction

The main objective of this research is to devise a method for optimizing the redundant controls on a compound helicopter and to incorporate the results of the optimization into a fly-by-wire controller. The aircraft studied is the UH-60/VTDP compound helicopter currently being built by Piasecki Aircraft Corp. In addition to the prominent main rotor, a fully compounded helicopter has a wing and an auxiliary thruster. On the UH-60/VTDP, the wing is just above the side door, and the auxiliary thruster replaces the tail rotor. The thruster has clamshell doors that can be deployed to turn the thrust 90 deg to serve as anti-torque (as a tail rotor would function). The wing and thruster both have control surfaces, and in addition to the main rotor controls, the pilot ends up with more controllable forces than control axes. Discovering the best way to set these 'redundant' control forces is a major research focus. The figure below shows the various redundant controls on the UH-60/VTDP. Main Controls: Longitudinal Cyclic Lateral Cyclic Collective Pitch Auxiliary Controls: Variable Pitch Propeller Elevator Rudder Sector Symmetric Flaperons Differential Flaperons A compound helicopter has several advantages over a conventional one. The auxiliary thruster and wing unloads the rotor at high speed, allowing the compound to fly more efficiently or reach a higher top speed. The redundant controls allow the compound to change its pitch attitude while maintaining altitude and airspeed, something the conventional helicopter cannot do. Reduced vibration levels and controlling structural loads in maneuvers are also possible with the compound helicopter.

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Overview

The figure to the right graphically shows the optimization problem. This plot show the power required vs. vibration index (a relative measure of vibration levels) for various airspeeds at different propeller pitch settings. Increased propeller pitch results in increased thrust from the ducted fan, which means that the rotor is not required to provide as much horizontal thrust. This reduction of rotor thrust leads to reduced vibration levels at moderate to high speed. The plot shows a small cost in power to achieve this vibration reduction, however for extreme reduction in vibration, the power increment becomes increasingly large. The optimization objective is to achieve the maximum vibration reduction for a minimum power increase. The approach used is a simple parametric search across all reasonable and possible auxiliary control combinations. Variations in main rotor speed are also examined. This cross section of control settings resulted in approximately 200,000 specific trim cases per rotor speed. Six rotor speeds are tested from 100% to 90% Nr, resulting in a large database of trim cases. This database is then examined per airspeed to find the cases that best satisfy the objective. These best cases are collected and assembled into an auxiliary control schedule. The schedule represents the most desirable auxiliary control settings to use (in terms of vibration and power levels) at a particular airspeed, ambient condition, and helicopter loadout. Once the auxiliary control schedule is generated, it is used in a fly-by-wire controller coded into the Penn State version of GENHEL (a non-linear helicopter simulation). The controller is capable of flying the schedule automatically to reduce pilot workload. The pilot can control longitudinal acceleration, pitch attitude (ACAH), roll attitude/turn rate, and sideslip. Vertical speed control is also used. The figure to the right shows an overview of the entire process (click for a larger version).

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Results

Included in this section are a few samples of the results of this research. Please refer to the Related Literature section for a complete list of all results. The figure below shows the optimal auxiliary control schedule generated for a UH-60/VTDP with an assault mission loadout at sea level standard ambient conditions. Note that in hover and low speed flight, the flap are deflected fully downward to reduce wing loading from the rotor. The following figure shows the vibration reduction achieved at moderate to high speeds. The 'Baseline' label on the plot refers to the conventional UH-60 helicopter. The 'Baseline with drag reduction' label refers to the fact the baseline equivalent flat plate drag area was reduced to be competitive with the compound UH-60 to show the benefits of compounding. This final plot shows the ability of the compound helicopter to reduce structural loads in maneuvering flight. A 3.3g vertical pullup maneuver was performed with differing ratios of longitudinal cyclic and elevator. Note that high ratios represent increased elevator use and decreased longitudinal cyclic use. It was found that by favoring elevator use in the pullup maneuver, the fuselage bending moment at station 600 (in the tail section) was reduced.

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Related Literature

Horn, Joseph F., Geiger, Brian R., Piasecki, Fred W., Greenjack, Andrew, "Trim and Maneuver Optimization Methods for a Compound Rotorcraft", Proceedings of the 60th Annual Forum of the American Helicopter Society, June 2004 Geiger, Brian R., Flight Control Optimization on a Fully Compounded Helicopter with Redundant Control Effectors, M.S. Thesis, Dept. of Aerospace Engineering, The Pennsylvania State University, May 2005 Swartzwelder, Matthew A., Trim and Control Optimization of a Compund Helicopter Model, M.S. Thesis, Dept. of Aerospace Engineering, The Pennsylvania State University, August 2003

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