General Electric Aircraft Engines

Cincinnati, Ohio

HPT Clearance Control

Intelligent Engine Systems—Phase 1

NASA/CR—2005-213970

October 2005
The NASA STI Program Office . . . in Profile

Since its founding, NASA has been dedicated to

the advancement of aeronautics and space

science. The NASA Scientific and Technical

Information (STI) Program Office plays a key part

in helping NASA maintain this important role.

The NASA STI Program Office is operated by

Langley Research Center, the Lead Center for

NASA’s scientific and technical information. The

NASA STI Program Office provides access to the

NASA STI Database, the largest collection of

aeronautical and space science STI in the world.

The Program Office is also NASA’s institutional

mechanism for disseminating the results of its

research and development activities. These results

are published by NASA in the NASA STI Report

Series, which includes the following report types:

• TECHNICAL PUBLICATION. Reports of

completed research or a major significant

phase of research that present the results of

NASA programs and include extensive data

or theoretical analysis. Includes compilations

of significant scientific and technical data and

information deemed to be of continuing

reference value. NASA’s counterpart of peerreviewed formal professional papers but

has less stringent limitations on manuscript

length and extent of graphic presentations.

• TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or

of specialized interest, e.g., quick release

reports, working papers, and bibliographies

that contain minimal annotation. Does not

contain extensive analysis.

• CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

• CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or cosponsored by

NASA.

• SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions,

often concerned with subjects having

substantial public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific

and technical material pertinent to NASA’s

mission.

Specialized services that complement the STI

Program Office’s diverse offerings include

creating custom thesauri, building customized

databases, organizing and publishing research

results . . . even providing videos.

For more information about the NASA STI

Program Office, see the following:

• Access the NASA STI Program Home Page

at http://www.sti.nasa.gov

• E-mail your question via the Internet to

help@sti.nasa.gov

• Fax your question to the NASA Access

Help Desk at 301–621–0134

• Telephone the NASA Access Help Desk at

301–621–0390

• Write to:

 NASA Access Help Desk

 NASA Center for AeroSpace Information

 7121 Standard Drive

 Hanover, MD 21076
General Electric Aircraft Engines

Cincinnati, Ohio

HPT Clearance Control

Intelligent Engine Systems—Phase 1

NASA/CR—2005-213970

October 2005

National Aeronautics and

Space Administration

Glenn Research Center

Prepared under Contract NAS3–01135, Work element 3.4, Task order 23
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov
NASA/CR—2005-213970 iii

Contents 

List of Figures ........................................................................................................................................... iv 

1.0 Executive Summary......................................................................................................................... 1 

1.1 Background......................................................................................................................... 1 

1.1.1 Cruise performance considerations........................................................................ 1 

1.1.2 Conceptual design of an advanced HPT ACC....................................................... 1 

1.1.3 Mission .................................................................................................................. 1 

1.2 Study Results ...................................................................................................................... 3 

1.3 Conclusions......................................................................................................................... 3 

2.0 HPT Blade Tip Clearance Control Systems..................................................................................... 3 

2.1 Fan and Compressor Air based System .............................................................................. 3 

2.2 Enhanced Stator Thermal Expansion System ..................................................................... 3 

2.3 Solid-Liquid Phase Change System.................................................................................... 3 

2.4 Mechanically Actuated Stator System................................................................................ 5 

3.0 HPT Clearance Control System Scorecard ...................................................................................... 5 

4.0 Engine Size and Flight Cycle Selection........................................................................................... 6 

4.1 General Considerations....................................................................................................... 6 

4.2 Performance Considerations ............................................................................................... 6 

4.3 Emissions Considerations ................................................................................................... 6 

5.0 Process for Evaluating HPT ACC Systems ..................................................................................... 6 

6.0 Summary of Current Work .............................................................................................................. 7 

7.0 Future Work Recommendations ...................................................................................................... 7 

 7.1 Phase II Work ..................................................................................................................... 7 

 7.2 Phase III Work.................................................................................................................... 7 
NASA/CR—2005-213970 iv

List of Figures 

Figure 1.—Current HPT ACC System—New and Deteriorated Engine Cruise Clearances ........................ 2 

Figure 2.—Advanced HPT ACC System—SFC for New and Deteriorated Engines................................... 2 

Figure 3.—Thermal Based HPT ACC System Schematic............................................................................ 4 

Figure 4.—Thermal Based HPT ACC System Hardware Configuration ..................................................... 4 

Figure 5.—Mechanical Based HPT ACC System Schematic....................................................................... 5 
NASA/CR—2005-213970 1

1.0 Executive Summary 

1.1 Background 

HPT (high pressure turbine) blade tip clearance has a significant impact on fuel burn and emissions. 

Hence, developing an HPT clearance control system that can adapt to changing environment/requirements 

is of paramount importance. This will enable a pre-eminent U.S. position in the aerospace industry well 

into the 21st century. 

When the HPT module of an engine is assembled, there is a certain amount of HPT blade tip 

clearance (i.e., gap between blade tip and shroud). This is called the assembly clearance or the cold build 

clearance. Its magnitude depends on: 

• anticipated thermal and mechanical radial deflections of rotor and stator during flight cycle 

• desired clearances during takeoff to achieve targeted performance parameters and to avoid blade tip 

rubs 

• target operating clearance during cruise, and 

• margin for protection against blade tip rubs due to dynamic events, hot rotor reburst, etc. 

The mechanical deflections occur almost instantaneously – the stator reacts to pressure force and the 

rotor (disk) to centrifugal force. The blade too reacts rapidly to centrifugal and thermal forces. On the 

other hand, the disk and the case thermal deflections occur at a slower rate, which is governed by the time 

constant of each component. Although slow, the thermal deflections are significant in magnitude. 

Typically, blades have the smallest time constant followed by that of the stator, and then by that of the 

disk. This difference in time constants makes it difficult to set the clearances precisely. 

1.1.1 Cruise performance considerations 

Since cruise forms the longest segment of a flight cycle, SFC (specific fuel consumption) is the 

driving factor in setting the target cruise clearance. A current state-of-the-art HPT ACC (active clearance 

control) system is shown in figure 1. New engine build clearance is shown in figure 1(a). At cruise, both 

stator and rotor have grown radially out (thermally and mechanically) as shown in figure 1(b). HPT ACC 

system is used to cool the stator and bring it radially in, thus reducing the blade tip clearance. 

On new engines, the system can close the gap to achieve design operating cruise clearance as shown 

in figure 1(c). 

The engine deteriorates over time due to amongst other things blade tip loss. The current HPT ACC 

systems do not intelligently compensate for the blade tip loss over time. This leads to an increase in 

operating clearance as shown in figure 1(d) and hence SFC is impacted. 

1.1.2 Conceptual design of an advanced HPT ACC 

To overcome the shortfall of existing HPT ACC systems under deteriorated engine conditions, the 

blade tip clearances would need to be measured during engine operation and an HPT ACC system capable 

of maintaining these clearances at or near the new engine levels is required. Such a system can be thermal 

or mechanical. 

A schematic of the HPT ACC concept at cruise conditions is shown in figure 2. As shown in 

figure 2(d), the blade tip has deteriorated causing increased clearances; but the HPT ACC system is able 

to compensate for the blade tip rub to close the gap back and retain the clearances at design level. Such a 

system will help maintain the SFC in spite of engine deterioration. 

1.1.3 Mission 

The current investigation “Propulsion 21 – HPT Clearance Control” (Work Element 3.4 of Task 

Order no. 23) is to analyze and develop an advanced HPT ACC system that will decrease cruise SFC. 
NASA/CR—2005-213970 2

Figure 1.—Current HPT ACC System—New and deteriorated engine cruise clearances. 

Figure 2.—Advanced HPT ACC system—SFC for new and deteriorated. 

Shroud

Blade

Shroud

Blade

Shroud

Blade

SFC SFC

New engine

build

clearance

New

Engine

(Cold)

New

Engine

(cruise)

w/o ACC

New engine

(shroud & blade

grown out)

Deteriorated

Engine

(cruise)

w ACC

Deteriorated engine

clearance at cruise

Blade

Tip Loss

Shroud

Blade

SFC

New

Engine

(cruise)

w ACC

New engine

design

clearance

at cruise

Shroud

brought in

(a) (b) (c) (d)

Shroud

Blade

Shroud

Blade

Shroud

Blade

SFC SFC

New engine

build

clearance

New

Engine

(Cold)

New

Engine

(cruise)

w/o ACC

New engine

(shroud & blade

grown out)

Deteriorated

Engine

(cruise)

w ACC

Deteriorated engine

clearance at cruise

Blade

Tip Loss

Shroud

Blade

SFC

New

Engine

(cruise)

w ACC

New engine

design

clearance

at cruise

Shroud

brought in

(a) (b) (c) (d)

SFC

Deteriorated

Engine

(cruise)

w ACC

Shroud

Blade

New engine

build

clearance

New

Engine

(Cold)

SFC

New

Engine

(cruise)

w ACC

Blade

New engine

design

clearance

at cruise

Blade

Deteriorated engine

clearance at cruise

Shroud

Shroud

Shroud moved in

to compensate

for blade tip rub

(a) (c) (d)

SFC

Deteriorated

Engine

(cruise)

w ACC

Shroud

Blade

New engine

build

clearance

New

Engine

(Cold)

SFC

New

Engine

(cruise)

w ACC

Blade

New engine

design

clearance

at cruise

Blade

Deteriorated engine

clearance at cruise

Shroud

Shroud

Shroud moved in

to compensate

for blade tip rub

(a) (c) (d)
NASA/CR—2005-213970 3

1.2 Study Results 

The first step was to downselect an engine type to use as a baseline for further study. Impact of HPT 

clearances on engine performance parameters was evaluated for both small and large engines. Significant 

benefits in cruise fuel burn, range, and payload capability are achieved for the long range aircraft. This 

evaluation narrowed the candidate engines to large commercial engines. An engine with current state-ofthe-art HPT ACC system was selected as the baseline engine/system. 

Preliminary conceptual designs of potential advanced HPT ACC systems were made. The designs 

included thermal and mechanical systems. A standardized template was developed to evaluate and 

compare existing and potential systems on the same basis and to downselect promising systems for 

further study. 

1.3 Conclusions 

An advanced HPT ACC system can maintain cruise SFC despite engine deterioration at or near new 

engine levels. 

2.0 HPT Blade Tip Clearance Control Systems 

HPT clearance control systems can be categorized as passive and active, active being controlled via a 

hydro-mechanical control or via FADEC (full authority digital engine control). The systems can be 

further classified as either thermal or mechanical. 

2.1 Fan and Compressor Air Based System 

A thermal system uses temperature differential between fan/compressor air and flowpath air to 

control radial displacement of shrouds. A schematic of such a system is shown in figure 3. As shown, 

HPT ACC air (from fan/compressor) is used to control the radial deflection of shroud. The external view 

of such a system is shown in figure 4. 

2.2 Enhanced Stator Thermal Expansion System 

This system uses a modified shroud support made from a low α (coefficient of thermal expansion) 

material resulting in reduced operating clearances. This system has the capability to provide additional 

closure at altitude for deteriorated engines. 

2.3 Solid-Liquid Phase Change System 

This is a passive system without any moving parts. The concept behind this system is to use phase 

change material (PCM) to control stator time constant. Since additional heat is required to melt /solidify 

the PCM and since temperature does not change during phase change, the stator time constant increases 

during both accel and decel. 

Advantages of this system are: 

• Provides a 360° uniform stator temperature 

• No changes to any of the existing flow circuits 

• Capable of being combined with other systems 
NASA/CR—2005-213970 4

Figure 3.—Thermal based HPT ACC system schematic. 

Figure 4.—Thermal based HPT ACC system hardware configuration. 

HPT ACC Air

HPT Stg 1 Shroud HPT Stg 1 Blade

ACC 

air 

supply 

pipe

ACC 

manifold

FWD AFT
NASA/CR—2005-213970 5

2.4 Mechanically Actuated Stator System 

The concept behind this system is to control HPT blade tip clearance by controlling the shroud 

position by means of mechanical actuators. A schematic of such a system (United States Patent 

5,056,988) is shown in figure 5. 

Together with clearance sensors, this system can: 

• Provide on demand clearance control 

• Have a significant clearance closure capability 

• Manage stator ovalization 

• Work equally effectively on both new and deteriorated engines 

• Provide rotor reburst protection 

3.0 HPT Clearance Control System Scorecard 

In order to benchmark current HPT clearance systems and to compare new systems to the existing 

ones, it is necessary to develop an overall system scorecard. 

Such a scorecard has been developed to address all major items for comparing existing and potential 

systems. The items addressed are: 

• System Level 

• Performance 

• Thermal 

• Mechanical 

CTQs (critical to quality) items from each category have been identified and a scoring method has 

been established. All HPT ACC systems will be scored based on this for a data driven comparison of the 

systems. 

Figure 5.—Mechanical based HPT ACC system schematic.
NASA/CR—2005-213970 6

4.0 Engine Size and Flight Cycle Selection 

All existing and potential HPT ACC systems have been reviewed. It is now necessary to identify the 

engine/aircraft system on which such a system will be most beneficial. This engine/aircraft system will be 

used as baseline to compare all HPT ACC systems on. 

After a careful evaluation of performance and emissions benefits associated with a +1 percent HPT 

efficiency gain, a “large engine”/“long range twin isle aircraft” system was chosen as the base 

engine/aircraft system. 

4.1 General Considerations 

Fuel Burn and Range 

• Generate engine performance data for the reference engine and the reference engine with 

+1 percent HPT efficiency. 

• “Fly” each set of engine data and identify fuel burn, range, and payload benefit from improved 

HPT efficiency. 

Emissions 

• Generate emissions data for the reference engine the reference engine with +1 percent HPT 

efficiency. 

• Measure engine data and identify impact on emissions. 

4.2 Performance Considerations 

For a 1 percent HPT efficiency increase on a “large engine/long range aircraft” system: 

• Fuel burn decreases by 0.93 percent. 

4.3 Emissions Considerations 

For a 1 percent HPT efficiency increase on a “large engine/long range aircraft” system: 

• The NOx fraction decreases by ~10 percent 

• The CO fraction decreases by ~ 16 percent 

• The HC fraction increases by ~ 49 percent 

Although the increase in HC fraction is large on a percent basis, it is not of concern since its EI 

(emissions index) for HC is insignificant (<1). 

5.0 Process for Evaluating HPT ACC Systems 

A process has been developed to evaluate various concepts and existing systems to identify the best 

system. 

There are three legs to this process initially. Two of them are more or less a one time event. They are: 
NASA/CR—2005-213970 7

• Establish a scorecard to benchmark existing HPT ACC systems on GE engines in the field, at 

systems, performance, thermal, and mechanical levels 

3⁄4 develop a scoring criteria to score items on the scorecard. This weighted scoring is to highlight 

the CTQs (critical to quality). 

• Select a baseline engine model to compare all systems against. 

3⁄4 select a transient cycle/mission to evaluate the concepts. 

The third leg, “first pass template” is to describe each new process in enough detail and is particular 

to the system under consideration. 

6.0 Summary of Current Work 

The following work has been completed to satisfy the Phase I Deliverables for the “HPT Clearance 

Control” project under NASA–GRC’s “Intelligent Engine Systems” program. 

• Need for the development of an advanced HPT ACC system has been very clearly laid out. 

• Several existing and potential clearance control systems have been reviewed. 

• A scorecard has been developed to document the system, performance (fuel burn, range, payload, 

etc.), thermal, and mechanical characteristics of the existing clearance control systems. 

• Engine size and flight cycle selection for the advanced HPT ACC system has been reviewed with 

“large engine”/“long range mission” combination showing the most benefit. 

• A scoring criteria has been developed to tie together performance parameters for an objective, 

data driven comparison of competing systems. 

• The existing HPT ACC systems have been scored based on this scoring system. 

7.0 Future Work Recommendations 

7.1 Phase II Work 

Starting from the technological foundation laid in the current phase of the “HPT Clearance Control” 

project under NASA–GRC’s “Intelligent Engine Systems” program, the following work needs to be 

completed to design and develop an advanced HPT ACC system. This system is to address the 

performance degradation associated with engine deterioration. 

• Score the selected HPT ACC concepts 

• Compare the scores of concept systems and existing systems 

• Identify an optimum HPT ACC system design based on overall performance benefit, reliability, 

maintainability, cost, and weight. 

• Perform detailed design and development of the system. 

• Design and develop a FADEC system to monitor and control the HPT ACC system. 

7.2 Phase III Work 

• The advanced HPT ACC system designed and developed in Phase II will be fabricated and tested 

on a large commercial engine with the corresponding long range flight cycle. 

• Back to back engine tests (sea level static and flight) will be performed on new and deteriorated 

engines to demonstrate improvement in overall engine performance. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

REPORT DOCUMENTATION PAGE

2. REPORT DATE

19. SECURITY CLASSIFICATION

 OF ABSTRACT

18. SECURITY CLASSIFICATION

 OF THIS PAGE

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102

Form Approved

OMB No. 0704-0188

12b. DISTRIBUTION CODE

8. PERFORMING ORGANIZATION

 REPORT NUMBER

5. FUNDING NUMBERS

3. REPORT TYPE AND DATES COVERED

4. TITLE AND SUBTITLE

6. AUTHOR(S)

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT

13. ABSTRACT (Maximum 200 words)

14. SUBJECT TERMS

17. SECURITY CLASSIFICATION

 OF REPORT

16. PRICE CODE

15. NUMBER OF PAGES

20. LIMITATION OF ABSTRACT

Unclassified Unclassified

Final Contractor Report

Unclassified

1. AGENCY USE ONLY (Leave blank)

10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546–0001

Available electronically at http://gltrs.grc.nasa.gov

October 2005

NASA CR—2005–213970

E–15288

WBS–22–714–92–50

NAS3–01135

Work element 3.4, Task order 23

15

HPT Clearance Control

Intelligent Engine Systems—Phase 1

General Electric Aircraft Engines

Propulsion systems (aircraft)

Unclassified -Unlimited

Subject Category: 07

General Electric Aircraft Engines

One Neumann Way

Cincinnati, Ohio 45215–1915

Project Manager, Clayton L. Meyers, Aeronautics Division, NASA Glenn Research Center, organization code PRV,

216–433–3882.

The following work has been completed to satisfy the Phase I Deliverables for the "HPT Clearance Control" project under

NASA—GRC's "Intelligent Engine Systems" program: (1) Need for the development of an advanced HPT ACC system

has been very clearly laid out, (2) Several existing and potential clearance control systems have been reviewed, (3) A

scorecard has been developed to document the system, performance (fuel burn, range, payload, etc.), thermal, and

mechanical characteristics of the existing clearance control systems, (4) Engine size and flight cycle selection for the

advanced HPT ACC system has been reviewed with "large engine"/"long range mission" combination showing the most

benefit, (5) A scoring criteria has been developed to tie together performance parameters for an objective, data driven

comparison of competing systems, and (6) The existing HPT ACC systems have been scored based on this scoring

system.