Actuator Prior Art

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Report 52-6-06 Electric Flap Actuator - Prior Art Copyright PreMag

ELECTRIC FLAP ACTUATOR CONFIGURATIONS

BY DANTAM RAO

PRECISION MAGNETIC BEARING SYSTEMS INC.,

ALBANY, NY.

REPORT NO. 52-6-06STARTED 8-1-06

ENDED 9-3-06

rev date Description

This document is Proprietary to PreMag. Disclosure to thrid parties is prohibited. 1

PRECISION MAGNETIC BEARING SYSTEMS, INC.25 Walker Way, Suite 2A, Albany, NY 12205 Tel: 518-218-0477 Fax: 518-218-0866

E-Mail: [email protected] Web: www.premag.com


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ABSTRACT

The goal of this paper is to document the prior art of flap actuators. The scope includes linear forceactuators and torque actuators. It describes various configurations of the force or torque actuators.

It assesses their advantages and limitations and ranks them. A brief summary of failure modes of

both force and torque actuators and lessons learned is included.

Major specifications of a torque actuator planned by the Navy are summarized. Next paper willdeal with description and advantages of PreMag's geared actuator configuration.

Lockheed believes that current LEFAS actuator is satisfactory. However, government believes

(from recent tests) that the long high speed shaft used in LEFAS is its Achille's heel and is flawed.It wants to develop Rotary Gear Transmission that uses only motors and gear train to eliminates

the high speed shaft as well as servovalue and hydraulic drive unit.



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TABLE OF CONTENTS

1 INTRODUCTION...........................................................................................................................4

2 EMA .................................................................................................................................................7

3 EHAS ................................................................................................................................................8

4 LEFAS ..............................................................................................................................................8

5 RGA TORQUE ACTUATOR......................................................................................................11

5.1 BACKGROUND.............................................................................................................................115.2 RGA TORQUE ACTUATOR...........................................................................................................12

5.3 LOW CHANCEFORPH II..............................................................................................................12

6 RANKING ACTUATORS ...........................................................................................................12

6.1 EMA........................................................................................................................................13

6.2 LEFAS........................................................................................................................................136.3 RANKINGTHEACTUATORS............................................................................................................13

7 CONCLUSION..............................................................................................................................14



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1 INTRODUCTION

All military aircraft have several control surfaces which are rotated to stabilize it when flying.

Ones on the wing are calledflaps, while those on the tail are called rudders. Fig. 1 shows F-18Aaircraft and its flaps. The flap are the hinged parts of the wing used to increase lift at reducedairspeeds1. The specific flap that is attached to the leading edge of the wing is called the leading

edge flap; the aircraft has two leading edge flaps, one on each wing. (that attached to the trailing

edge is called the trailing edge flap2,but this is not the focus of this report). In all military aircraft

flaps rotate during extreme manevours continuously and rapidly to adjust lift loads. That is whythey are called maneuvering flaps, meaning they are always moving always. (In commercial

aircraft they are mostly stationary, and move only during landing and takeoff). The performance

of all military aircraft relies heavily on the ability of these actuators 3to rotate the flaps rapidly andprecisely under the command of the pilot.

Figure 1. F-18A Aircraft Leading Edge Flap Actuators.

Actuatoris device that applies linear force or rotary torque to the flap to rotate it over a limited

angle against wind load, and provide the needed lift. The force is applied to a lever, while the1 Quest for Performance,The Evolution of Modern Aircraft, http://www.hq.nasa.gov/office/pao/History/SP-468/ch10-5.htm/2 Sometimes the part on the leading edge is called a slat, while the part on the trailing edge is called a flap, see http://www.grc.nasa.gov/WWW/K-

12/airplane/flap.html3 Flap Actuator Maket is shared by Lucas [30%], Liebherr [25%], Curtis-Wright [25%], Moog [20%] and Dowty [10%], see



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torque is applied to a hinge. They are called force actuator or torque actuator respectively. They

are located within the belly of the wing section.

Moogappears to be the monopoly and sole-source supplier for all flap actuators in all military

aircraft and most commercial aircraft. Another firm in Israel and Vikram Sarabhai Space Center,

Trivendrum are reportedly developing Electro Hydraulic Actuators.

Fast airplanes require small, thin wings to achieve eye-popping speeds far beyond Mach 2. The

thickness of the wing however is limited by the diameter of the actuator (smaller the actuator,thinner is the wing, higher is the aircraft speed). About 20 years back the flap actuators used to be

20 cm (7.87 in) in diameter - hence wings used to be thicker than 20 cm, so aircraft used to have

limited speed. Recent developments enabled actuator diameter to be reduced to 15 cm (5.8 in),

enabling thinner wings and hence faster aircraft such as JSF. The goal of this project to reduce theactuator diameter further down to 10 cm (4 in), thereby. achieving higher speeds.

Fig. 2 shows the leading edge flap drive systems in F/A-18; it is actuated4 by servo hydraulic

actuators. Current wing flaps have hydraulic fluid temperatures limits of 250 degrees and airwithin the wing can be as hot at 100 C.They are however, very heavy, bulky, complicated, difficult

to maintain and leak-prone5. Once a hydraulic line leaks, it could lead to loss of expensive plane.As a result, multiple hydraulic circuits are used to maintain high degree of reliability, increasing

the weight further.

Over past 20years Air Force built few experimental aircraft that used electric power to actuate the

flaps. It essentailly uses electric power instead of hydraulic power to rotate the flight control surfaces.

The schemes asre are called Power By Wire schemes. Most of them incorporate electric motors to

generate linear force. Power by Wire concept was originated in Air Force Research Lab, Dayton OH inearly 1990's.

It is widely recognized that PBW actuation is the next major breakthrough in aircraft control. Justas the fly-by-wire flight control system eliminated the need for mechanical interfaces, power-by-

wire actuators can eliminate hydraulic systems. Control power comes directly from the aircraft

electrical system instead of via hydraulics. This can reduce the weight of aircraft, size of actuator,and maintenance cost6. It can improve safety, efficiency, reliability and maintainability. Removing

hydraulic systems would greatly reduce the amount of support equipment and personnel required

to maintain and operate.

Government studies indicate that this new approach could deliver up to a 5% reduction in

procurement costs, 3% in lower life-cycle costs, and a 6% decrease in gross takeoff weight. The

emergency and secondary power system is much simpler and cleaner if you go with the electricapproach, enabling a much smaller and simpler airplane. This can reduce aircraft weight by as

much as 700 lb because weight is taken out of the hydraulic system, out of the secondary power

system, out of the thermal management system7. Because it is not generating as much heat, it also

4 M.I.T., Aircraft Systems Engineering, http://ocw.mit.edu/NR/rdonlyres/Aeronautics-and-Astronautics/16-885JFall-2004/1ABA501E-4F31-4EEE-AEEB-123274492635/0/flight_controls_1.pdf5Ford, T., Actuation systems development, J. Aircraft Engg and Aerosp.Tech. Aug 1998, Vol. 70, No. 4, pp. 265 - 2706 Navy Electrical Power and the Future: http://www.nap.edu/html/tech_21st/t8.htm7 "Power by Wire", Avionics Magazine, May 1, 2001.



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simplifies maintenance on the aircraft. Going to an electrical distribution and electrical actuation

system simplifies the power and thermal management systems, leading to significant savings.

PBW actuators are more efficient than their hydraulic counterparts8. A hydraulic system in F-16

may generate ~ 3000 watts of heat at all times, regardless of whether flaps are lifted or not. In

contrast, in the PBW approach, actuators consume power only when flaps are activated, and eventhen they generate ~ 500 watts of heat. With the PWB if you are not actuating, you do not

generate heat, and you dont have to dissipate it. As a result, we dont have to provide a secondary

and emergency sources for cooling. This saves weight of thermal management system making theaircraft smaller, lighter and cheaper.

At present, three types of actuators EMA, EHAS, LEFAS - are used to drive the flaps and other

control surfaces in military aircraft. All of them use 3-phase PM motors. An experimental F-18Aused EMA9to drive flaps it uses two motors that drive a single ball screw through a differential

gear; it eliminates oil pump and leak-prone oil lines. Some F-35s use10 EHAS to drive rudders it

uses a motor to drive an oil pump that drives hydraulic pistons; it eliminates leak-prone oil lines.

The F18A Lefas, uses an electric motor to pump a hydraulic motor which spins a gearedtransmission; . In F-35 Lefas11, a motor drives a shaft with transmission, eliminating oil pumps,

hydraulic motor and oil lines. Recently PreMag proposed to Lockheed to embed motor andtransmission into one single unit; this eliminates failure prone shaft12 in addition to oil pumps and

oil lines.

A force actuator can use an electric motor in two ways. In one called Electro Hydro Static

Actuator (EHA), the motor drives an oil pump and oil pressure drives a hydraulic cylinder. In

another called Electro Mechanical Actuator (EMA), the motor drives a gear train which then drives

a ball screw that reciprocates. They are shown in Fig. 1.

8 Jensen et al.,Flight test experience with an electromechanical actuator on the F-18 systems research aircraft, 19th Digital Avionics Systems

Conf., Oct. 7-13, 2000, Philadelphia, PA9 Jenssen, S.C., et al., Fight test experience with an electromechanical actuator on the F-18 systems research aircraft,10 Phillips, E. H., Big step for F-35A, Aviation Week & Space Technology , Jan. 9, 2006, p. 44-45.11 Curtiss-Wright, http://www.cwcontrols.com/pdf/pressrelease20061219.pdf12 PreMag, Geared Motor Actuator configuration for Leading Edge Flap of JSF aircraft, report no. 0001AC-1, Mar. 2007.



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Figure 2. Prior Art Force Actuator Concepts.

2 EMA

EMA consists of 2 small motors, a geartrain and a ball screw ram. The motor drives

the gear train which drives a ball screw as

shown in Fig. 1b and 2. NASA has flown itin a military aircraft with this EMA. It

claims that, compared to an EHA, the EMAis lighter, smaller, and less complex becauseof the absence of an internal hydraulic

system. EMA's motor is made by MPC

Products, Skokie, IL, inverter is made byDynamics Controls Inc., Dayton, OH and

the Controller is made by British Aerospace

in Johnson City, NY. The motor appears to


Figure 3. EMA Force Actuator built for Air Force


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overheat, and it appears that effort to transfer the heat from motor to aircraft skin failed13. It appears

that EMA technology was not chosen for the JSF by Lockheed or the Government. The reason for

its disfavor inspite of extensive test experience is not known.

3 EHAS

In EHAs the motor drives an oil pump. EHASuses high speed reversible variable pressure

(3000 psi) pumps pump self-contained

hydraulic fluid to a piston. This drives the ram in

the same fashion as a standard hydraulic actuator(Figure 1(a)). The motors need high bandwidth

or fast response time. For example JSF uses

force actuators called EHAS (ElectroHydrostaticActuation System) to deflect tail rudders and

trailing edge flaps. Lockheed14integrated the 6

EHAS actuators built by Moog and PerkinsHannifin, CA with motors, control electronics

and software made by Hamilton Sundstrand.

Fig. 3 shows the EHAS on a test bed. The

EHAS weigh 770 lb to 865 lb per Trimboli, Director of JSF Vehicle Systems, LockeedAeronautics, Dallas15.

4 LEFAS

The Leading Edge Flap Actuation System (LEFAS) rotates the leading edge flap. The hardware is brokeninto a mechanical part and electrical part. Fig. 2 shows the mechanical part. It consists of a drive unit, a

long shaft and two gear reducers, a torque limiter and a power off brake (stop module). The drive unitspins a 20 ft long slender shaft (called torque tube) at a high speed of 3000 rpm to power two gear

reducers (called inboard transmission and outboard transmission, or Rotary Gear Actuators, RGAs) The

transmissions convert low torque high speed shaft motion into high torque low speed flap motion.

In F-18A, the LEFAS used a hydraulic motor to power the torque tube. This consisted of an

electric pump motor that drive servovalves which drive a rotary Hydraulic Drive Unit In F-35 or

JSF, they replaced the hydraulic motor with a big electric motor, thereby eliminating high pressurehydraulic lines. The drive is powered by a power source and controller. The power source

consists of a triple redundant 270 Vdc electrical system with 2 generators, a battery. The controller

consists of a power conversion system and a control system.

They eliminate the ball screw, and use only motor and gearbox. The EHAS and LEFAS eliminate

hydraulic lines running throughout the plane. They do not eliminate hydraulic oil completelyhowever. It does reduce weight, complexity and risk of battle damage. These actuators weigh more

than flight control systems used in legacy fighters such as the F-15 per an article in Aerospace

Weekly.

13 Oscillating heat pipe for actuator cooling, http://www.pr.afrl.af.mil/mar/1999/oct1999.pdf#search=%22EMA%2214 Ramsey, J. W., Power by Wire, Avionics, May 2001, http://www.avionicsmagazine.com15 Phillips, E. H., Big step for F-35A, Aviation Week & Space Technology , Jan. 9, 2006, p. 44-45.


Figure 4. EHAS Actuator on Test Bed


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LEFAS however is heavy and failure-prone. In JSF , the flap is 20 ft long. LEFAS that drives this

long flap contains a single variable speed hydraulic drive unit (HDU) and 2 gear transmissions,all produced by16Moog, Curtiss-Wright and Parker. The HDU drives a long high speed shaft

(3000 rpm, 20 ft).

Figure 5. LEFAS relies on a 20ft long and slender torque tube.

Figure 5 shows the general layout of the LEFAS. It consists of HDU with torque bar, inboard

transmission, torque limiter and stop module, pillow bloc, universal joint, output transmission and controlunit. Hydraulic Drive Unit HDU, driven by a servovalve spins a 20 ft long torque tube at high speed.This goes through two sets of gear trains called inboard and outboard transmissions.

This shaft drives 3 transmissions, called RGA. Each RGA is 1.5 ft long and is made of 4 or moreslices. Each slice houses independent planetary gear train set. Each planetary gear slice reduces

speed from 3000 rpm to 5 rpm and has its hinges rotate the flap at slow speed. The diameter of the

3 RGAS steps down from 5.8, 4.2, 2.5 as one travels from root to tip. The inboard transmission has

5 "nodes" that attach to the wing; inner node are made of 2 tabs, while outer most hinge has only onetab, 7 total. It also has 4 " hinges"; each hinge is made of 2 tabs, each tab has 2 eyes. The hinges rotate

the flap. The outboard transmission has 8 "nodes" made of 14 tabs to attach to wing, and 7 "hinges"

made of 14 tabs to rotate the flap. The connection in the middle is a universal joint allowing the wingto fold. Each of the hinge attaches to each wing rib and flap part of the rib. Each of the transmissions

converts high speed low torque from torque tube to low speed high torque that moves the leading edge

flap. Figure 5 shows the Lefas system for FA18 air craft. From this, one can see that the entire LEFAS

16 In JSF (F-35) Curtiss-Wright supplies Leading Edge Flap Rotary Geared Actuators and Torque Tubes. These areDeveloped in partnership with Moog Inc.,(the LEFAS Integrator), these mechanisms position the leading edge flaps during take-off, landing

and during flight to provide additional lift for maneuverability and reduced take-off and landing speeds. See

http://www.cwcontrols.com/pdf/pressrelease20061219.pdf



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is system has at least 20 parts, is surprisingly very complex and expensive.

Figure 6. LEFAS flight actuator system for FA18 (From Moog)



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5 RGA TORQUE ACTUATOR

5.1 BACKGROUND

Figure 7 Goal is to replace the HDU, transmission and brake with a Geared Motor Actuator.

Figure 8 depicts the parts that are to be replaced in Phase I. These include the drive unit (whetherhydraulic or electric), inboard transmission with 4 slices, the torque tube and stop module. This

transmission consists of 4 gear reducers or slices. Each slice has a stationary tab pair (that fixes its house

to the wing) and a moving tab pair (that attach it to the hinge on the leading edge flap). Current Lefasslices rely on compound differential planetary gear set; they employ a torque limiter and brakes to

prevent the system from self-damage; these fail-safe features ensure that the flap will not spin

uncontrolled should a component fail or rupture.

In LEFAS the shaft is 20 ft long and can spin up to 3000 rpm at full no load rate. It is like a long

propeller shaft that connects all transmissions. The rotor dyanmics of the flimsy shaft is complex and

could yield several resonant failure modes. A full wing may have as many as 5 transmissions on eachwing. The diameter of each transmission set gets smaller as one moves from wing root to wing tip

because the wing gets thinner.



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5.2 RGA TORQUE ACTUATOR

Rotary Gear Actuator (RGA) essentially uses electric motor and gear train to drive the flap. It will have

no high speed shaft, no servovalve, no hydraulic motor or ball screw. The intent is to replace all partsshown in Figure 5. This eliminates the troublesome high speed shaft and Servovalve and

Hydraulic Drive Unit as well as inboard transmission. In its place will be one RGA. RGA will

consist of 4 slices. Each slice is made of an electric motor and gear train, and drives one hingejoint of the flap. The motors and gear trains are housed in one unit while the controller is housed ina separate unit.

In Phase I, only one slice, i.e., geared motor that drives one hinge joint of the transmission will beconsidered. Notionally, one motor per hinge or one motor for all four-hinges are possible

configurations. The intent is to allow redundant operation if one of the motors fails. The motor

design should be scalable down to the smaller outboard rotary transmission also. More details of theinboard rotary transmission is shown in Figure 4.

5.3 LOW CHANCE FOR PH II

In Phase II, we propose to perform detailed design of the complete actuator system and fabricate a

prototype. Phase II detailed design will address what to do with the heat produced. In this phase wewill design the cooling system to minimize the hot spot temperatures. We will work with the

government and prime contractor to develop thermal management strategies that will transfer the

heat into identified heat sink (viz., surrounding cabin air or wing structure or to outside ambientair). We will use multiple paths, extended surfaces and high thermal conductivity materials to

siphon the heat in this process. It will also address detail control design and integration. It will also

address detail failure modes and redundancy operations. In phase II we will test the performance of

components in the laboratory, and will ship the tested prototype to the JSF aircraft builder(Lockheed Aeronautics) for testing in his Vehicle Systems Integration Facility.

Navy will however rely on the advice of Lockheed to go into Ph II. To get Ph II money, we have topass through 4 hoops. First, Lockheed has to verify and concur that PreMag's torque actuator

configuration is lighter than current LEFAS design (or it has other performance advantages).

Second, Lockheed thinks that the current electrohydraulic LEFAS "works", and sees no reason tochange from it, so it has no incentive to support the fundamental premise that Lefas design is

faulty. Third, Lockheed does not believe that there is a motor so small that it will fit in the space

and generate the huge torque. Fourth, Lockheeds current plans for LRIP from 2008 will becompletely disrupted if it were to admit that lefas is defective and embark on plan for redesign. In

short, Lockheed and Moog has all the incentives to actively oppose any funding of further

research, and they will lose heavily if it supports this research direction. Governments has good

intent is to support development, but without Lockheeds blessing it cannot move into Ph II. Assuch chances for funding Phase II appear to be low.

6 RANKING ACTUATORS



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the aircraft speed. The planned RGA, which uses only electric motor and gear train, appear to eliminate

hydraulic drive and servo valve and hence could potentially save significant weight and space.

7 CONCLUSION

In this paper we have reviewed various configurations of the Power By Wire actuators used torotate the control surfaces of military aircraft. We have identified 4 configurations

Electrostatic Hydraulic Actuator (EHA),

Electro Mechanical Actuator (EMA)

Leading Edge Flap Actuator (LEFAS)

Rotary Gear Actuator (RGA).

EHA and EMA are force actuators. LEFAS and RGA are torque actuators. EHA uses oil pumpmotor and linear hydraulics. EMA uses geared motor driving a ball screw. LEFAS used oil pump

motor and rotary hydraulic drive, a long high speed shaft and gear trains and is the most complex.

RGA uses electric motors and gear train and has fewest parts.

Development of RGA forms the subject matter of the Ph I study effort. We presented

specifications of RGA for a notional aircraft. Lockheed believes that current LEFAS actuatorworks satisfactorily. However, government believes (from recent tests) that the long high speed

shaft used in LEFAS is its major weakness. It wants to develop RGA which will eliminate it.

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