Supplier Spotlight

Video Exclusives

Plasma wind tunnel testing

Watch the first simulated re-entry of a full-sized spacecraft in a plasma wind tunnel as the The Scirocco facility tests the Qarman cubesat.

Click here to watch video

18 July, 2018

Sonic boom acoustic tests

This video, from a NASA Social event at Armstrong Flight Research Center, shows an F/A-18 producing a regular sonic boom at 0:43 and and then a low “boom” by performing the dive maneuver at 02:34. NASA researchers are preparing for public tests of technology designed to reduce the noise of sonic booms

Click here to watch video

03 July, 2018

Autonomous vehicles explained

Safran’s SimplyFly! light hearted series of video’s takes on autonomous vehicles - how will they work and what are the main challenges? Learn more about the topic by watching this video.

Click here to watch video

20 June, 2018


To watch more videos, click here

With Airbus' Zephyr drone pushing the limits of aviation to reach an altitude of 74,000ft in the stratosphere during test flights, we would like to ask - where do you think the atmosphere ends and space begins?

Web Exclusive Articles

« back to listing

Pegasus - flying horsepower

Supporting the development of a new Pegasus LPC1 fan blade for the US Marine Corps Harrier upgrade led to the revival of a mothballed testbed


When the US Marine Corps asked Rolls-Royce to improve the performance of fan blades for its Pegasus engine for the USMC Harrier (see figure above), it set a test challenge that involved some sophisticated vibration modeling and trials, even including the reactivation of a mothballed test facility. 

The Rolls-Royce Pegasus engine powers the Harrier aircraft operated by the USMC as well as the Italian, Indian and Spanish navies. In 2012, the USMC awarded Rolls-Royce a contract under the Component Improvement Program (CIP) to develop a modified fan blade to improve the robustness of the engine.

Problem control
Rolls-Royce created a number of finite element (FE) stress models to understand the dynamic behavior of the blade and establish the root cause of some of the problems that had been experienced in initial operations. The fan blade, as shown in the figure on the right, is manufactured from titanium alloy and rotates at in excess of 8,000rpm. It is retained within the engine by a feature commonly referred to as a dovetail.

The blade features a ‘snubber’ at around two-thirds height, which is intended to control the flap and torsional response of the blade. After the analysis was completed, it was established that issues could be caused by high stresses within the blade, which were often exacerbated by damage from material drawn into the engine during operation. From completed analysis, it was determined that the dynamic mode most likely to cause issues was a part speed stall flutter mode, which gives rise to high stress levels at several locations on the blade.

The importance of this mode was further underlined by an analysis of the operating regime of the Harrier aircraft, which indicated that considerable time was spent operating in the speed range at which this mode was prevalent.

Analysis and practical testing was able to show that control of this flutter mode was highly dependent on the tribology of the snubber faces. During operation, the snubbers can become locked due to centrifugal and aerodynamic loads on the blade. Excitation of the stall flutter mode can reverse the loading on the snubbers, leading to a situation whereby higher-order modes become excited due to stick-slip motion as the snubber faces move relative to each other.

The snubber faces are coated with tungsten carbide and a dry film lubricant, and will lock when the critical coefficient of friction, determined by the angle of the snubber face is exceeded. For the existing design, having a snubber angle of 20°, this value is 0.36. It was indicated that loss or lack of lubrication between the two faces can impact the behavior of the mode.

In order to understand this phenomenon, a friction wear test was initiated at the Rolls-Royce sponsored University Technology Centre (UTC) at Oxford University. This test was set up to simulate the relative motion between two snubbers in service and was performed at a frequency in line with what would be seen in service. The rig used is shown in the image on the left. 

The results from the rig enabled an understanding of the bedding-in process and indicated that the level of friction between the two faces increases as the dry film lubricant wears away and debris builds up. The company proposed a number of design changes to the blade to mitigate this issue: the angle of the snubber face was increased to 25° to make the adjacent faces slide more easily and an improved under-platform damper was introduced to control the dynamic response of the blade. 

Validation program

Having identified the design solution, the company’s efforts turned to proving the change. It was established that the design changes would be validated by both rig testing and engine running. The validation process commenced with a frequency survey of the revised blade standard and the results compared with the data previously available from the existing standard. With the results of the frequency survey in line with pre-test FE predictions, the next step was a fatigue test of the new blade. Six sample blades were tested to establish each of their failing fatigue strengths.

Each blade was mounted on a vibration table and restrained at the inner dovetail fixing and the snubber to accurately represent engine conditions. The rig used is shown in the figure below right.

From the testing, it was demonstrated that the basic strength of the new blade was within the scatter of the legacy blades and that no new potential failure modes had been introduced. 

Engine trials
Rolls-Royce followed a structured process to establish the verification and validation requirements for the revised blade. It was determined by the engineers that a number of engine tests would be necessary to gather all the evidence required to clear the new blade for operational duty.

The last production engine was shipped from the Rolls-Royce Filton factory in 2005 and the testbed was mothballed in 2010. Therefore establishing an engine test capability was the first major challenge. The first of the engine tests, to calibrate the testbed, was completed in May 2013.

With the testbed deemed serviceable, the program to validate the design change could begin in earnest. The first test of the structured validation program was a build of the engine, incorporating its existing baseline configuration, against which the intended design improvements could be assessed. The build of the engine incorporated two systems to measure the dynamic response of the blade. 

The first system is the blade tip timing, which uses optical probes to detect the arrival time of the tip of the blade at a number of points around the casing. The system measures the speed of the engine spool and differences in the time at which the tip of the blade passes the probe compared with the expected timing, and can be used to determine the amplitude and frequency of any blade resonance. 

The second system is the FM grid, and is similarly used to measure blade dynamic response with a magnet attached to the blade tip and a conductive wire grid around the engine casing. The motion of the blade tip causes a signal to be induced in the grid, which is proportional to the frequency and displacement of the blade tip and from which the blade dynamic characteristics can be deduced. Subsequent engine builds and testing were aimed at understanding the impact of the individual improvements in the proposed design changes.

As discussed earlier, the propensity for flutter is dependent on the lubrication of the snubber and other contact faces on the blade. The gap between adjacent snubber faces is also a major contributor to blade lock up. In order to promote flutter during the test, the fan assembly was built with dry film lubricants removed and snubber gaps at the bottom limit of tolerance. Fan blade flutter can be influenced by a number of factors, the two principal being engine inlet distortion and blockage downstream of the fan. During the test program the blockage in the front (cold) nozzles of the engine was varied to further promote flutter. 

The first of these tests was completed in September 2013 and featured the existing blade fitted with the revised damper. The purpose of the test was to establish the individual benefit of the damper, with the testing completed in the flutter promoting configuration.

The next test, again in the flutter promoting configuration, featured both the revised blade and the damper. The final test, completed in August 2014, featured the revised blade and damper in the production standard configuration with all surface lubrication reapplied. The results of the test program clearly showed that the redesign of the blade and the incorporation of the new damper led to a great reduction in flutter amplitude of the blade. In the final production standard configuration, flutter has been all but eliminated. 

Despite the twin technical and logistical challenges, the Rolls-Royce team compiled a comprehensive understanding of the issue and devised a substantial design improvement, which is currently being evaluated by the customer.

Tim Williams is chief engineer – Pegasus, transport and helicopter engines, with Rolls-Royce plc, based in Filton, UK



Your email address:

Read Latest Issue

Read Latest Issue

Web Exclusives

Customers appreciate the Global 6000’s smooth ride, but it wasn’t until Bombardier conducted a series of innovative tests that there was quantifiable data to support their claims
Click here to read more

Marine Dumont, business development manager, Kistler Instruments outlines how innovation in dynamometers is enabling clearer images of Earth from satellites 
Click here to read more

Steve Drake, NDT market manager for Ashtead Technologies, describes how the rapid deployment of the latest borescope equipment to Africa helped a business aviation operator make tough decisions about a grounded aircraft 
Click here to read more

Hardware-in-the-loop systems have helped to reduce flight testing of Saab's latest fighter jet, writes National Instruments in this case study
Click here to read more

An experiment is set to be launched from the International Space Station this month which turns the normal approach to space on its head – instead of putting things into space, this experiment aims to test ways to take things out of space. 
Click here to read more

Submit your industry opinion

Industry BlogDo you have an opinion you'd like to share with the aerospace testing community? Good or bad, we'd like to hear your views and opinions on the leading issues shaping the industry. Share your comments by sending up to 500 words to

Submit Your Recruitment Ad

Recruitment AdTo send us your recruitment advertising or to receive information on placing a banner please email

Supplier Spotlight

Supplier SpotlightClick here for listings and information on leading suppliers covering all aspects of the aerospace testing industry. Want to see your company included? Contact for more details.