December 2011 

NASA/TM–2011-217316 

Deployment of a Pressure Sensitive Paint 

System for Measuring Global Surface Pressures 

on Rotorcraft Blades in Simulated Forward 

Flight

Preliminary PSP Results from Test 581 in the 

14- x 22-Foot Subsonic Tunnel 

Anthony Neal Watkins, Bradley D. Leighty, and William E. Lipford 

Langley Research Center, Hampton, Virginia 

Oliver D. Wong 

U.S. Army Aeroflightdynamics Directorate, Joint Programs Office, Hampton, Virginia 

Kyle Z. Goodman 

ATK, Hampton, Virginia 

James Crafton, Alan Forlines, and Larry Goss 

Innovative Scientific Solutions, Inc., Dayton, Ohio 

James W. Gregory and Thomas J. Juliano 

Ohio State University, Columbus, Ohio 
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December 2011 

NASA/TM–2011-217316 

Deployment of a Pressure Sensitive Paint 

System for Measuring Global Surface Pressures 

on Rotorcraft Blades in Simulated Forward 

Flight

Preliminary PSP Results from Test 581 in the 

14- x 22-Foot Subsonic Tunnel 

Anthony Neal Watkins, Bradley D. Leighty, and William E. Lipford 

Langley Research Center, Hampton, Virginia 

Oliver D. Wong 

U.S. Army Aeroflightdynamics Directorate, Joint Programs Office, Hampton, Virginia 

Kyle Z. Goodman 

ATK, Hampton, Virginia 

James Crafton, Alan Forlines, and Larry Goss 

Innovative Scientific Solutions, Inc., Dayton, Ohio 

James W. Gregory and Thomas J. Juliano 

Ohio State University, Columbus, Ohio
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

Acknowledgments 

The authors would like to thank the staff from the 14- x 22-foot Subsonic Tunnel for their 

efforts in making this test possible. Funding for this test was provided by the Subsonic 

Rotary Wing project under the Fundamental Aeronautics Program. 

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not 

constitute an official endorsement, either expressed or implied, of such products or manufacturers by the 

National Aeronautics and Space Administration. 
Abstract

This report will present details of a Pressure Sensitive Paint (PSP) system for measuring global surface 

pressures on the tips of rotorcraft blades in simulated forward flight at the 14- x 22-Foot Subsonic Tunnel. 

The system was designed to use a pulsed laser as an excitation source and PSP data was collected using 

the lifetime-based approach. With the higher intensity of the laser, this allowed PSP images to be 

acquired during a single laser pulse, resulting in the collection of crisp images that can be used to 

determine blade pressure at a specific instant in time. This is extremely important in rotorcraft 

applications as the blades experience dramatically different flow fields depending on their position in the 

rotor disk. Testing of the system was performed using the U.S. Army General Rotor Model System 

equipped with four identical blades. Two of the blades were instrumented with pressure transducers to 

allow for comparison of the results obtained from the PSP. This report will also detail possible 

improvements to the system. 

I. Introduction 

The accurate determination of spatially continuous pressure and temperature distributions on 

aerodynamic surfaces is critical for the understanding of complex flow mechanisms and for comparison 

with computational fluid dynamics (CFD) predictions. Conventional pressure measurements are based on 

pressure taps and electronically scanned pressure transducers or embedded pressure transducers. While 

these approaches provide accurate pressure information, pressure taps/transducers are limited to providing 

data at discrete points. Moreover, the integration of a sufficient number of pressure taps/transducers on a 

surface can be time and labor intensive and expensive. 

This is especially true in rotorcraft research, where the examination of pressure distributions on the 

blade is vital to advance analytical prediction methods for rotorcraft aerodynamics, acoustics, and 

interactional effects. There has been considerable research involving pressure measurements on rotor 

blades.1-4 However, these measurements typically lack the spatial resolution necessary to capture 

phenomena such as the nascent tip vortex or dynamic stall. Instrumenting the blades with additional 

transducers to increase spatial resolution can quickly become prohibitive due to the cost and practicality 

of fitting a large number of sensors into a small area. In addition, the added centrifugal loads of the 

pressure transducers can rapidly become unmanageable. 

Applying pressure sensitive paint (PSP) to the surface may enable high spatial resolution surface 

measurements on helicopter rotor blades, thus allowing more accurate analytical prediction methods to be 

developed. The PSP technique5-9 exploits the oxygen (O2) sensitivity of luminescent probe molecules 

suspended in gas-permeable binder materials. If the test surface under study is immersed in an 

atmosphere containing O2 (e.g. air), the recovered luminescence intensity can be described by the SternVolmer relationship10 

SV O2

0 1 K (T)P I

I   (1) 

where I0 is the luminescence intensity in the absence of O2 (i.e. vacuum), I is the luminescence intensity at 

some partial pressure of oxygen PO2, and KSV is the Stern-Volmer constant. 

Since it is a practical impossibility to measure I0 in a wind tunnel application, a modified form of the 

Stern-Volmer equation is typically used. This form replaces the vacuum calibration (I0) with a reference 

standard

REF

REF

P

P A(T) B(T) I

I   (2) 
2

where IREF is the recovered luminescence intensity at a reference pressure, PREF. The coefficients A(T) 

and B(T) are temperature dependent constants for a given PSP formulation and are usually determined 

beforehand using laboratory calibration procedures. 

There are two methods for acquiring PSP data. The most common method used for data acquisition 

is an “intensity-based” technique. During intensity-based PSP experiments, IREF is typically acquired 

while the wind tunnel is off or at very low speed and PREF is the static pressure when no wind is applied. 

Thus IREF is referred to as the “wind-off” intensity. I is the recovered luminescence intensity at some 

pressure P. Since this data is collected at a specific condition in the wind tunnel, I is also referred to as 

the “wind-on” intensity. A(T) and B(T) are temperature dependent constants for a given PSP formulation 

and are usually determined beforehand using laboratory calibration procedures. 

A second method of PSP data acquisition is known as “lifetime-based” PSP.11-15 In the lifetime-based 

technique, excitation of the PSP is accomplished using a modulated light source (e.g. laser, flash lamp, or 

pulsed LED arrays). A fast framing camera (intensified CCD or interline transfer CCD) is used to collect 

the excited state luminescence decay. Typically the decay is approximated by acquiring two or more 

images at different delay times during and/or after 

the pulsed excitation and integrating photons for 

fixed periods of time (i.e. gate widths) that have 

been predetermined to maximize the pressure 

sensitivity, as demonstrated in Fig. 1. The first 

image (Gate 1) usually consists of a short gate 

width and is collected either during the excitation 

pulse or shortly after it ends. This can be thought 

of as the reference image because the excited-state 

decay has the least pressure sensitivity. The 

second image (Gate 2) is taken at a later time after 

the excitation pulse and usually has a longer gate 

width, ensuring maximum pressure (and 

temperature) sensitivity. More information on the 

lifetime technique used in this work can be found 

in Watkins et al.15

Over the last several years, the U.S. Army 

Aeroflightdynamics Directorate, Joint Research 

Program Office, and the NASA Subsonic Rotary 

Wing Project have partnered to develop the PSP 

measurement technique for use on rotor blades. 

This work included an initial proof of concept work in 200316 which resulted in the development of 

instrumented pressure blades for more extended testing in 2008.17 From these results, a new PSP system 

based on the previously described system was developed with several modifications for use with rotating 

test articles. This report will detail these modifications as well as present some preliminary data from the 

deployment of this system in the 14- x 22-Foot Subsonic Tunnel (hereby abbreviated 14x22) in 2011. 

II. Experimental 

A. Paint Formulation and Calibration 

The two blades that were painted with PSP were coated with a porous polymer formulation that has 

been described previously.18-19 This binder can routinely measure dynamic pressure fluctuations at 5 kHz 

and has been demonstrated to potentially measure fluctuations up to 20 kHz (depending on a variety of 

factors, including thickness and luminophore). A more detailed review of this formulation and other PSP 

formulations capable of operating at elevated frequencies can be found in Gregory et al.20 The oxygen 

sensitive luminophore chosen was platinum meso-tetrakis(pentafluorophenyl) porphine (abbreviated 

Figure 1. Schematic representation of lifetimebased data acquisition showing excitation (blue) 

and measured emission (red). The gate regions 

represent example Gate 1 (during excitation) 

and Gate 2 (after excitation). 
3

Pt(TfPP)), which is a common luminophore for PSP applications. A typical application of the PSP 

involved initially applying the porous polymer binder to a basecoat (usually white to maximize intensity 

collection efficiency) using conventional spraying techniques. After the binder dries, a solution of the 

luminophore is then typically over-sprayed onto the binder. This helps to ensure that the luminophore is 

resting on the surface for maximum interaction with oxygen (thus increasing the frequency response). 

The disadvantage of this is that the luminophore can degrade fairly quickly. However, this can be 

alleviated by simply over-spraying with additional luminophore solution. For this work, it was found that 

over-spraying once a day before running was sufficient for data acquisition. 

Calibration of the paint formulation was performed separate from the wind tunnel in a laboratory 

calibration chamber. This chamber is only capable of measuring pressure and temperature sensitivities; 

no attempt to determine the frequency response of this paint was attempted. However, as mentioned 

above, previous testing has shown that this formulation can respond to 5 kHz, well above the frequency 

range needed for this test. For calibrations, the PSP was applied to 3-inch diameter aluminum coupons 

that were then placed in the calibration chamber. Illumination of the PSP and acquisition of the 

luminescent intensity was accomplished using the same system as used in the tunnel. 

The PSP formulation was calibrated over a pressure range of 6 to 14.7 psia (41 to 101 kPa) at 

temperature ranging from 77 to 140 o

F (25 to 60 oC). A calibration model for the coating was derived by 

solving Eq. (2) for normalized pressure in terms of the normalized temperature and the gate intensities 

acquired form the images as described in the previous section. The calibration data showed a multidimensional dependence on both pressure and temperature, which can be attributed to the complex nature 

of oxygen diffusion into the paint binder.7-9 A linear least squares algorithm was used to fit the data to a 

modified and expanded version of Eq. (2) above assuming a second order relationship in both temperature 

and pressure 

2

1 2

2

31 32 REF 33 REF

1 2

2

21 22 REF 23 REF

2

REF 11 12 REF 13 REF

(a a (T/T ) a (T/T ) )(G /G )

(a a (T/T ) a (T/T ) )(G /G )

(P/P ) (a a (T/T ) a (T/T ) )

 

  

   

 (3) 

where P and PREF are the pressures, T and TREF are the temperatures, G1 and G2 are the intensities in the respective 

gates (analogous to IREF [G1] and I [G2]), and axy are the calibration coefficients. A typical calibration is shown in 

Figure 2.

B. Model and Facilities

The rotor blades that were tested have 

been constructed from carbon fiber, 

fiberglass, and aromatic nylon fiber 

honeycomb trailing-edge core. Each blade 

has been painted with a white basecoat to 

enhance the PSP luminescent output (by 

reflecting the luminescence away from the 

surface and to the camera) as well as to seal 

the blade to protect the blade structure from 

the solvents used in the painting process. 

The blades are constant chord with a swepttapered tip and a 14 degree linear twist 

distribution, using the RC family of 

airfoils.21-22 The upper portion of Figure 3 

shows the distribution of airfoils and the 

dimensions of the blades (in inches). Of the 

Figure 2. Calibration of PSP formulation for various 

temperatures and pressures. 
4

four blades, two are pressure instrumented using Kulite pressure sensors. The first instrumented blade 

has two rows of chord-wise transducers, with rows located at the 93% and 99% radial stations. The 

second has one chord-wise row at 93% radius. Each row has 10 pressure transducers located on the upper 

surface, as shown in the lower portion of Figure 3. 

The forward flight testing was conducted in the 14x22 facility at NASA Langley Research Center. 

The tunnel is an atmospheric, closed return tunnel with a test section 14.5 ft (4.4 m) high, 21.75 ft (6.6 m) 

wide, and 50 ft (15.2 m) long. The tunnel can reach a maximum velocity of 348 ft/s (106 m/s) with a 

dynamic pressure of 144 psf (6.9 kPa). The achievable Reynolds number of the tunnel ranges from 0 to 

2.2 x106

 per foot (0 to 7.2 x 106 per meter). Test section airflow is produced by a 40 ft (12.2 m) 9 bladed 

fan driven by a 12,000 Hp (8.9 MW) main drive. 

The rotor blades were mounted to the General Rotor Model System (GRMS) and a modified ROtor 

Body INteraction (ROBIN) fuselage. GRMS is a generic rotor drive system that allows testing of 

different rotor and fuselage configuration. GRMS is powered by two 75 Hp (55.9 kW) water-cooled 

electric motors that drive a 5.47:1 transmission. Two six component strain gage force and moment 

balances are contained within GRMS to enable separate measurement of rotor and fuselage loads. The 

rotor hub is a four bladed fully articulated hub. One blade cuff is instrumented to measure cuff pitch, lead 

lag, and flapping. Additional instrumentation on GRMS includes an encoder to provide 1/rev and 

1024/rev timing signals and accelerometers to monitor machine health. The fuselage is similar to the 

original ROBIN fuselage with the exception of a rear ramp section. The ROBIN fuselage is an 

analytically defined representative generic helicopter fuselage that has been used in previous work.23 The 

modified ROBIN fuselage used in this test uses the same family of super-ellipse equations as the original 

ROBIN fuselage while employing a modified set of coefficients to generate the ramp section. 

All PSP instrumentation was mounted on the ceiling of the 14x22 so that illumination and image 

acquisition were performed though AcryliteTM OP-4 windows. OP-4 is a brand of acrylic plastic that can 

transmit UV light. It also has a high clarity, transmitting ~90% of visible light. 

Figure 3. Rotor blades for use with PSP. The upper diagram shows the distribution of the airfoils 

and the dimensions of the blades (in inches). The lower diagram shows the rotor instrumentation 

locations.
5

C. Instrumentation 

For this test, the light emitting diode (LED) based arrays used in the previous work were exchanged 

for frequency doubled Nd:YAG lasers (532 nm). A laser-based illumination system was used to attempt 

to acquire the PSP images needed in one single laser pulse as opposed to several hundred LED flashes 

(with one flash per revolution). This would provide instantaneous pressure data on the blade while also 

alleviating issues with the dynamic nature of rotorcraft flight (i.e. blade lead-lag and flap motion). The 

laser employed was a rugged, compact dual laser head system originally designed for Particle Image 

Velocimetry (PIV) applications. Because of this, the lasers have been pre-aligned so that the laser path 

from each head is co-linear and the timing can be manipulated so that both heads fire at the same time. 

The lasers employed had a nominal power of 150-200 mJ per pulse per head. 

An optics stack was designed and built to steer and condition the beam for blade illumination. The 

optics stack generally consisted of a mirror to steer the beam and a negative lens to expand the beam to a 

suitable spot size. In addition to the lens, a polycarbonate diffuser was also placed after the lens to 

remove the fringes caused by the lens and make the illumination spot more uniform. The stack was 

housed in a holder that allowed for a compact system that could be easily mounted above the windows in 

the test section ceiling. The typical components of the optics stack are shown in Figure 4a and a 

completed holder showing the nominal beam path is shown in Figure 4b. 

PSP data images were acquired using a specialized interline transfer camera. This was developed 

specifically for use in PIV applications and operated by masking every other line of the chip, allowing for 

charge to be transferred quickly (~200 nm transfer time) from the unmasked to the masked region for 

Figure 4. Optics stack components and assembled unit. (A) Individual components and accompanying 

Thor part numbers. (B) Assembled unit showing optical path. 
6

either storage or readout. This allows for the rapid collection of image pairs with a minimal time delay 

between images (the interline transfer time above). The camera is also equipped with a fast electronic 

shutter capable of acquiring an image with an exposure time of as little as 1 μs. The camera employs a 

CCD chip with an active area of 1600 x 1200 pixels with peak quantum efficiency greater than 50% at 

650 nm. The camera has 14-bit digitization as well as on-board memory that will allow it to rapidly store 

images on the camera, making it possible to run multiple cameras simultaneously from the same computer 

platform. The camera was binned to 800 x 600 pixels to improve collection efficiency and increase data 

collection rates. 

Due to the testing and safety requirements, it was necessary to have nearly full remote control of pan 

and tilt as well as focus of the cameras during the test. The cameras were mounted onto a commercial 

pan/tilt head that was capable of 

being controlled at distances of 

several hundred feet. Due to the 

size of the CCD chip (2/3 inch) and 

the need for maximum light 

collection efficiency (or maximum 

aperture), remote focus and zoom 

lenses were impractical. Instead, 

large aperture SLR lenses were used 

to maximize collection efficiency. 

Custom designed and built systems 

were utilized to enable remote 

focusing during the test. An image 

showing the camera on the pan/tilt 

stage as well as with the remote 

focusing attachment is shown in 

Figure 5. Full details of the 

mechanical setup of the camera 

system will be published at a later 

date.

D. Data Acquisition 

All image acquisition was accomplished using the lifetime-based approach, which was found to be 

essential in previous test.16-17 However, these previous tests employed LED-based arrays and functioned 

by on-chip accumulation of several images to build the necessary data. This was shown to suffer from 

excessive blurring due to variations in rotational speed, flapping, and lead lag of the blade. Thus, a 

method to acquire the data in one single rotation was needed to account for this. Using the high powered 

pulse laser provided sufficient levels of illumination and operating the cameras in the double exposure 

mode described above allowed the acquisition of the two gate images from one laser pulse. In addition, 

there was a requirement to synch the actual PSP data acquisition with the wind tunnel dynamic data 

acquisition system to be able to compare the dynamic pressure transducer measurements with the PSP at 

the correct azimuthal positions. Timing for the acquisition was accomplished using a custom designed 

and built system based on a configurable counting board and software interface (Rotor Azimuth 

Synchronization Program, or RASP) and the signals from the 1/ref and 1024/rev encoders on the GRMS. 

The RASP allowed for accurate and reproducible alignment of the blades with a specific azimuth location 

in the rotor disk. Programmable delay generators were also used to synch the camera acquisition with the 

flashlamp and Q-switch firing of each laser head. The overall control of the data acquisition was 

accomplished via an external signal sent from the wind tunnel dynamic data acquisition system. Each 

individual firing of the Q-switch was also recorded by the dynamic data acquisition system to enable 

comparison between the pressure transducer data with the PSP data at the same rotor azimuth. A 

simplified diagram of the timing setup is shown in Figure 6. 

Figure 5. Interline transfer camera affixed to pan/tilt stage and 

modified with remote focusing device. 
7

The actual acquisition of the PSP data was 

acquired using a double frame imaging 

technique in which a short exposure image 

was taken followed immediately by a longer 

exposure image as detailed by Juliano et al.

24

The longer exposure image was started after 

the interline transfer time of the pixels (200 

ns) and lasted as long as it took for the first 

image to be read into the on-board RAM of 

the camera. With the arrangement, data could 

be acquired approximately every 400 ms, and 

with the current rotor speed, this corresponds 

to one PSP image pair acquired every 8 

revolutions. For an image pair, the camera 

was set for an initial exposure time of slightly 

more than 200 μs, corresponding to the 

optimal delay between flash lamp and Qswitch firing. The initiation of the camera 

exposure also triggered the programmable 

delay generator to trigger the flash lamp and 

Q-switch at the desired times. These times 

were set to ensure that the laser flash occurred 

just before the end of the first exposure, exciting the paint. Then the second image was collected so that 

the remainder of the excited state decay occurred in this frame. A diagram of the nominal PSP imaging 

process is shown in Figure 7. 

For this test the rotor shaft angle was maintained at -3 degrees and there was no yaw in the model. 

PSP images were acquired on the Advancing Blade Side (ABS) at an approximate rotor azimuth of 98 

degrees and on the Retreating Blade Side (RBS) at an approximate rotor azimuth of 258 degrees. The 

ABS is the side where the blade is advancing into the freestream velocity and the RBS is the side where 

the blade is moving in the same direction as the freestream. All data was acquired from the same blade 

and rotation speed was 1150 rpm. 

Figure 6. Timing schematic for controlling up to 

four separate camera/laser systems. PG: Pulse 

Generator; FL: Flash Lamp; QS: Q-Switch; LC880: 

Programmable logic gate controller for throttling data 

acquisition; DDAS: Dynamic Data Acquisition 

System. 

Figure 7. Schematic representation of data acquisition using dual frame imaging and laser 

pulse excitation. Laser pulse width and delay between images is exaggerated to show 

difference. 
8

E. Data Analysis 

Data analysis for this work followed the standard procedure for analysis of PSP data acquired using 

the lifetime-based data acquisition procedures with some exceptions. Usually the lifetime-based data 

analysis is simply dividing Gate 1 by Gate 2 to form an IREF/I image. However, the chosen paint 

formulation (the porous polymer) displays a significant change in performance that is tied to the 

application process. This phenomenon has been observed previously in many PSP formulations25-27 but is 

very pronounced in this formulation. Essentially, the excited state lifetime of the Pt(TfPP) shows 

heterogeneity with application, where the lifetime can change dramatically based on the relative localized 

concentration of the probe. To solve this, a single wind-off image set was acquired immediately after the 

overspray. Since the overspray was done each morning, this wind-off image set was also acquired each 

morning. The wind-off image pair served as a further reference for the lifetime data and can account for 

much of the non-homogeneity effects. The basic data analysis followed the following protocol: 

1. Background correction of all images 

2. Correlation of wind-on images Gate 1 and Gate 2 to the second gate image of the wind-off pair 

3. Creating a “ratio of ratios” image using the wind-off image pair 

4. Mapping the resultant image to the surface grid using the previously determined three dimensional 

coordinates of registration marks added to the blade 

5. Final calibration of the image to convert to pressure. 

Several of these steps will be explained below. 

Correlation of Images and Surface Mapping: For static and quasi-static testing, correlation of the 

gate images is typically not required when data is acquired in the lifetime mode because both the 

reference and pressure images are acquired at the same conditions. This usually means that any 

deformation or model motion that could occur would be minimal. However, in the case of rotor blades, 

this is not the case. Due to the high degree of aeroelastic bending as well as the dynamic nature of the 

testing, the two gate images are acquired in one laser pulse. This results in a slight blurring of the second 

gate image due to the finite excited state lifetime of the luminophore. This blur can be seen in Figure 8, 

which compares the first gate image (in which the laser pulse fires at the end of the first exposure) with 

the second gate image (which contains the vast majority of the excited state decay). This blur is caused 

by both the finite time between the first and second image (~200 ns) as well as the excited state lifetime 

of the Pt(TfPP) in the binder, which is on the order of 5 μs. The total effect of this is to cause a blur of 

~2-3 pixels in the second image. While the actual blurring is not accounted for in this report (please see 

the Lessons Learned section), it does require a correlation of the image to the reference image for 

alignment. Furthermore, as a wind-off image pair is used to account for lifetime heterogeneity, a further 

Figure 8. Comparison of first gate (left) and second gate (right) images highlighting the rotational 

blur that occurs in the second gate (note leading edge). 
9

correlation to these images must also be done. 

This correlation was accomplished using registration marks that were placed on the blade using a 

Sharpee marker. With the registration marks, one can apply an image correlation technique based on a 

mathematical transformation to empirically map the Gate 2 image to the Gate 1 image, as described in Liu 

and Sullivan.9

 After the test, the three dimensional locations of the registration marks were measured 

using the VSTARS technique with respect to the model. With this information it was possible to solve 

the collinearity equations and transform the image coordinates to physical model coordinates (analogous 

to model deformation measurements).9

 This allowed the mapping of the PSP image data to a pre-defined 

surface grid of the blades for later interrogation in software packages such as TecPlotTM as well as easing 

the direct comparison with computational data. 

Ratio and Calibration: After the correlation of the images to the wind-off image pair, the pressure 

image was calculated by simply ratioing the wind-on images (Gate 1/Gate 2) to form an IREF/I image and 

then correcting this by dividing the wind-off IREF/I image. Conversion to pressure units was performed 

using an a priori calibration determined above. For this calculation, a temperature on the blade was 

assumed and then the pressure transducers values were used as a correction factor. This hybrid type of 

calibration has been described previously.9

III. Results and Discussion 

A. Improvements from Using Laser-Based Data Acquisition 

The benefits from using the laser-based data acquisition 

technique were apparent from the start. The greatest 

improvement was the clarity of the images. This is 

depicted in Figure 9, which shows a comparison of a raw 

image taken using the LEDs and integrating over multiple 

revolutions and a raw image from this test. Because of the 

multiple revolutions that were required for the LED-based 

approach, the image has noticeable blur around the pressure 

transducers, especially when compared with the laser-based 

data acquisition technique. An additional benefit is the 

greater increase in efficiency. Acquiring a data image over 

multiple revolutions required data acquisition times on the 

order a minute to acquire a single image pair. This 

precluded many of the advantages in signal-to-noise that 

can be achieved with averaging. Additionally, the 

comparison with pressure transducers would become 

tenuous as only an ensemble average could be used over 

that time frame, severely mitigating any dynamic effects that may exist. Alternatively, with the laserbased data acquisition technique, an image pair can be obtained in single laser flash, corresponding to a 

single rotation. Now, the comparison with pressure transducers is much cleaner as the image is collected 

at a single point in time. Additionally, with the current setup, as many as 30 image pairs could be 

obtained in a single test point collection from the rotorcraft dynamic data acquisition system, which 

required approximately 15 seconds. 

B. Forward Flight Test Results 

As mentioned in the Data Analysis section above, a single wind-off image pair was needed to correct 

some anomalies that happen with this particular paint formulation (the porous polymer PSP). 

Additionally, there was a contamination element that occurred near the pressure transducers. To protect 

the pressure transducers from clogging or becoming damaged during the painting process, a thin strip of 

Kapton tape was placed over the tap rows. While this would necessarily limit some of the data in regions 

Figure 9. Raw images from LEDbased (left) and laser-based (right) data 

acquisition techniques showing the 

reduced blurring. 
10

directly at the transducers, another effect was seen. This is shown in Figure 10. In the first gate image 

(left), the effect is not very noticeable. However, in the second gate image (right) there are significant 

variations at the tap row at 99% chord. This image contains most of the excited state decay, thus any 

variations in the lifetime should be convoluted with the actually excitation field. It is currently postulated 

that this is a contamination from the adhesive of the tape that is dissolved during the overspray process (in 

which the Pt(TfPP) is applied in a solution of toluene), which may have been applied a bit too heavily at 

the tip (resulting in a “wetter” overspray). Additionally, the wind-off image pairs were acquired almost 

immediately after the overspray, so the solvent may not have been allowed to dry sufficiently. 

An attempt to mitigate this issue was made by simply “patching” this area of the paint with small 

regions near the 99% chord row. A comparison of the wind-off IREF/I image before and after the 

“patching” is shown in Figure 11. Ideally, this image should have a uniform appearance, but lifetime 

variations in the paint (again, usually caused by application) can be seen. In the unpatched image (left), 

these variations at the 99% chord are extreme. Patching (right) can remove much of this effect. 

Obviously this can bias the results in this region, so further study on the effects needs to be carried out as 

well as strategies to mitigate the effect from happening in the first place. All data analysis was 

accomplished using the patched reference images. 

For the hybrid calibration mentioned in the Data Analysis section, the location of the pressure 

transducers was virtually moved on the surface grid away from the taped regions. If the pressure 

Figure 10. Raw wind-off images. (Left) The First Gate image taken at the laser flash; (B) the 

second gate image encompassing the majority of the excited-state decay. The contamination 

from the tape is most noticeable as the blue regions near the 99% chord. 

Figure 11. Wind-off IREF/I images. (Left) Unpatched image showing contamination; (right) 

patched image showing nearly complete removal of the contamination. 
11

transducers would have been covered with only a small piece of tape individually, this probably would 

not have needed to be done. However, the tape strip afforded the maximum protection to the transducers, 

as well as significantly reduced the amount of time needed for application. For the final data analysis, the 

transducers were moved toward the hub about 0.3 inches (the original location was on a station at 108.6” 

and they moved to 108.3”). This move was also structured to maintain the same locations in x/c as were 

in the original. This moved the transducers a significant (> 5 pixels) distance from the tap to allow their 

use in calibration (the spatial resolution in the blade is ~0.03”/pixel) while keeping a close proximity to 

their actual location. 

With the transducers virtually moved to a clear region, they can now be used to anchor the a priori

calibration, which was calculated using an assumed temperature. A comparison of the a priori calibration 

with the hybrid calibration is shown in Figure 12. The comparison between the pressure tap 

measurements and the PSP data is also included and shows that the hybrid calibration does bring the PSP 

data closer in line to the transducers. It should also be noted that the PSP data at the extremely low and 

high x/c locations is probably biased due to blurring in the second gate (as mentioned above). Even with 

this, the PSP shows relatively good agreement with the transducers. For consistency, all final calibrations 

were done using the hybrid calibration technique to anchor the a priori calibration. 

The first set of runs was acquired at a constant velocity of 138 knots (71.0 m/s) and at four thrust 

coefficients. A comparison of a PSP image at each thrust condition is shown in Figure 13. This is the 

ABS and shows good qualitative agreement with what should be expected. It also shows that the pressure 

on the blade at this position has little dependence on the thrust coefficient. The comparison between the 

pressure transducers and the PSP is also shown in Figure 14, and shows the same result. The PSP does 

not agree as well as the previous figure, most likely due to the smaller pressure changes on the model as 

well as not accounting for the blur. However, both the PSP and the transducers show the higher pressure 

region at the extreme leading edge with the pressure decreasing as the flow accelerates over the center of 

the blade, flowed by a gradual return to higher pressure at the trailing edge. As with the PSP data, the 

pressure tap measurements also show little dependence on the thrust coefficient. 

Figure 12. Comparison of PSP data calibrated using the a priori calibration (left) and an assumed 

temperature and the hybrid calibration (right) using the pressure taps to “anchor” the a priori

calibration. The comparison between the taps and the PSP is shown below each image. The black 

region on the blade is unmapped data. 
12

However, the same cannot be said of 

the blade in the “retreating” position (the 

blade is moving with the air flow in the 

tunnel). Figure 15 shows PSP data taken 

from the same blade during the same 

rotation, but retreating from the air flow. 

The PSP data shows that there is a much 

larger dependence on the thrust 

coefficient, as evidenced by the lower 

pressure region near the leading edge of 

the blade. Additionally, there is 

evidence of a flow phenomenon near the 

blade tip, such as a vortex shedding off 

the tip. This is highly dependent on the 

thrust coefficient, and evidence of it can 

be seen from CT greater than 0.006. The 

larger pressure differentials are also 

evident from the pressure tap and PSP 

comparisons, which is shown in Figure 16. As with the previous data, the tap agreement is very good 

(except near the leading and trailing edge). However, the flow phenomenon that is seen in the PSP at the 

tip does not appear in the tap data. From visual inspection of the PSP data, it seems that the phenomenon 

flows just past the last pressure tap, or possibly between two transducers. This does show one of the 

greatest advantages to using PSP: the ability to visualize and measure global pressure distributions as 

opposed to localized pressure measurements as is acquired from pressure transducers. 

A test condition was also run to maximize the suction peak on the blades by increasing the thrust 

coefficient to CT = 0.012. In addition the forward speed was reduced to 120 knots (61.7 m/s). the results 

from both the advancing and retreating position of the blade as well as the pressure tap comparisons are 

Figure 13. PSP images acquired from the ABS. The 

arrow represents the direction of the tunnel flow. The 

black regions on the blade are unmapped data. 

Figure 14. ABS comparisons between PSP data and pressure tap measurements from Figure 13. 
13

shown in Figure 17. As was the 

case in the previous testing, the 

advancing blade position shows 

little dependence on the thrust 

coefficient. The retreating blade 

position shows much the same 

behavior as well, though the overall 

pressure at the leading edge is lower 

due to increased thrust coefficient.

However, the flow phenomenon at 

the tip is much more pronounced at 

this condition with possibly some 

flow structure inside observable. 

Finally, because of the large 

increase in data collection efficiency 

of the laser-based data acquisition 

technique, it is now possible to 

visualize and measure the variation 

in pressure on the blade during each 

rotation. This is illustrated in Figure 18 in which three individual sets of data from one data point are 

interrogated. This is the same condition from Figure 17 and only the retreating blade position is 

considered. These data were taken approximately 80 revolutions apart and represent approximately 66% 

of the time used to collect a data point by the dynamic data acquisition system. From the images, it is 

readily apparent that there is variation across the blade, especially in the magnitude of the pressure. This 

is borne out in comparison between the three images. In this comparison, only the PSP data is considered 

and shows a variation of about 1 psi through the center of the blade. Once the rotational blur is accounted 

for, analysis of the edges can also be carried out. Additionally, the flow phenomenon that occurs at the 

tip does seem dynamic in nature as it is visible changing throughout the run. 

Figure 16. RBS comparisons between PSP data and pressure tap measurements from Figure 15. 

Figure 15. PSP images acquired from the RBS. The arrow 

represents the direction of the tunnel flow. The black regions 

on the blade are unmapped data. 
14

IV. Lessons Learned/Future Directions 

This section will detail several of the lessons learned in this test as well as possible solutions to these 

challenges. It is anticipated that another test will be forthcoming in FY12 and further development of the 

technique will find much interest in current rotorcraft designers. The major challenges stem from the 

paint and its application and accounting for the rotational blur that is inherent in this technique. 

A. Paint 

The porous polymer employed in this test is a vast improvement over the sol-gel-based formulation 

used in the previous testing.16-17 However, there are still some limitations of this paint, most notably in 

the application and the temperature sensitivity. The ceramic compound that is the major component of 

the binder is very abrasive, thus requiring the protection of the pressure transducers. In conventional 

testing using static pressure transducers, this can be easily accomplished by simply purging the pressure 

tap orifices using compressed shop air. This has been used historically and has always shown good 

results. However, this test required the use of dynamic pressure transducers. These transducers are 

essentially thin diaphragms that need to be mounted on or very near the surface. As a consequence, the 

typical purging methods are no longer applicable, nor are methods such as blocking the orifices with wire. 

For this test, the only solution was to use a strip of Kapton tape. While this protected the transducers (as 

evidenced by their continued functionality), two issues were introduced that needed to be overcome. 

First, there was no PSP data directly at the pressure transducers. This could be mitigated by simply 

adjusting the virtual positions of the transducers on the surface grid. While not ideal, this is a valid 

procedure if the area in question is large enough and there are no discrete surface events occurring (like 

shocks or vortices). 

The bigger issue that occurred was the contamination that was evident near the taped region. While 

there is more laboratory work to be done on this, it appears that this is most likely a result of the 

overspray process itself. The solvent used in the overspray could be dissolving some of the adhesive that 

Figure 17. PSP images and pressure tap comparison from the advancing and retreating blade 

position at a CT = 0.012. The arrow represents the direction of the tunnel flow. The black regions on 

the blade are unmapped data. 
15

is in turn flowing into the binder causing a change in the local environment of the luminophore. This 

leads to lifetime heterogeneity in the paint that needs to be addressed. Since it appears to manifest itself 

most in the wind-off image, this portion was patched to remove the contamination. This is definitely not 

an ideal solution and laboratory work to identify the cause and how to mitigate this are currently 

underway. 

Finally, the temperature sensitivity of the porous polymer paint is one of its main limitations. As can 

be seen in Figure 2, there is a large spread in the sensitivity curves due to temperature. While this 

conceivably could be accounted for in processing, this would require a pixel-by-pixel knowledge of the 

temperature to do it most accurately. In this case, assuming a single temperature seems to work (at least 

close to the transducers), but this will not always be the case. This temperature sensitivity is mostly due 

to the binder itself and how the diffusion of oxygen into (and out of) the binder is affected by 

temperatures. Even in an ideal paint, there will be a temperature dependence due to the physical nature of 

luminescence. However, that dependence is up to an order of magnitude smaller than the binder effect. 

The temperature sensitivity issue with PSP formulations has been an ongoing development area and to 

date, there are few formulations that can approach the limit of temperature sensitivity. One of these 

formulations has also been modified to exhibit a high frequency response. Though not as high as the 

porous polymer, it should be sufficient for most rotorcraft applications. This binder is based on a fluoroisopropyl-butyl (FIB) copolymer. A sample calibration curve compared to the porous polymer 

formulation is shown in Figure 19, showing that the FIB formulation (circles) has a much narrower 

distribution than the porous polymer (squares). Further investigation of this formulation for dynamic PSP 

applications is currently under study. 

B. Rotational Blur 

Owing to the finite excited-state lifetime and the inherent time between exposures, the second gate 

typically exhibited a blur of 2-3 pixels in the image as shown previously in Figure 8. This blur is 

especially evident at the leading edge. To account for this, several algorithms are currently under 

development and are being tested. These are based on a variety of deconvolution algorithms. Once 

tested, these data will be reanalyzed to see the effects, especially near the leading and trailing edge. This 

Figure 18. Three separate PSP images acquired at the same conditions as Figure 17. These images 

were acquired approximately 80 revolutions apart. The arrow represents the direction of the tunnel 

flow. The black regions on the blade are unmapped data. 
16

should enable a better visualization of 

these areas where larger pressure 

gradients are expected to be seen.

This will also greatly decrease the 

discrepancies between the PSP and 

the pressure transducers in these 

regions as seen in Figures 14 and 16. 

V. Conclusions 

This report details a test of using 

PSP for the global pressure 

determination on the tips of rotorcraft 

blades in forward flight. This test 

was performed using the General 

Rotor Model System installed in the 

14- x 22-Foot Subsonic Tunnel at 

NASA Langley Research Center. 

Two rotor blades were painted with a 

porous polymer PSP formulation 

capable of frequency responses on the 

order of 20 kHz. The blades were 

instrumented with pressure 

transducers, with the actual blade used in the measurement instrumented with a row at 93% and 99% 

chord, respectively (though only the 99% chord was functional). The blades were tested at various 

forward velocities and thrust coefficients. 

For this test, a laser-based data acquisition system was designed and deployed. This system was 

capable of measuring a single blade through up to four different positions in the rotor disk thorough a 

single revolution. This was accomplished by exciting the paint with the laser and using an interline 

transfer camera to take a pair of images. With correct timing, the laser flash occurs at the end of the first 

gate with the majority of the excited-state decay being recorded by the second gate. This is analogous to 

the traditional lifetime-based approach in which two images are collected, one during the excitation pulse, 

and one after the pulse. However, with the power of the laser, all data could be acquired in one rotation 

with one laser pulse. Thus, data collection is inherently more efficient as well as the possibility of 

recording dynamic pressure data is a possibility. 

Analysis of the data shows fairly good agreement (within 10%) of the pressure tap measurements, 

though there are some issues that were encountered. To protect the pressure transducers, Kapton tape was 

placed over the transducers. During the application of the PSP, the solvent seems to have caused some of 

the adhesive to dissolve and leach into the binder, leading to some significant effects in this region. This 

was patched with nearby data to mitigate the results. Additionally, the temperature sensitivity of the PSP 

is a limiting factor in the accuracy of the data, though this can be lessened using the pressure tap reading 

to correct the bias. Finally, there was no attempt to account for the rotational blur seen in these data, 

resulting is suspect results near the leading and trailing edge. 

Even with these limitations, the data agreed both qualitatively and somewhat quantitatively with the 

expected results. In addition, there is evidence of a vortex shedding or other flow phenomenon that can 

be seen dependent on both blade position (retreating or advancing) and thrust coefficient. To date, this is 

one of the first successful tests of PSP on flexible rotating surfaces capable of measuring dynamic 

phenomena. 

Finally, several methods to further improve the measurement technique have been discussed. These 

include optimizing paint application strategies as well as employing a different formulation that exhibits 

Figure 19. Representative calibration comparison 

between a FIB binder (circles) and a porous polymer 

binder (squares) highlighting the lower temperature 

sensitivity of FIB. Courtesy of Innovative Scientific 

Solutions, Inc. 
17

much lower temperature sensitivity. Additionally, algorithms to account for the rotational blur in the 

images while maintaining data integrity are also under development and will be tested in the near future. 

References 

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and Airflow of an Extensively Instrumented Model Helicopter Rotor,” Proceedings of the 45th Annual 

Forum of the American Helicopter Society, Boston, MA, May 1989. 

2. Lal, M.K., Liou, S.G., Pierce, G.A., and Komerath, N.M., “Measurements around a Rotor Blade 

Excited in Pitch, Part 2: Unsteady Surface Pressure,” Journal of the American Helicopter Society, Vol. 

39, No. 2, 1994, pp. 13-20. 

3. Gorton, S.A., Poling, D.R., and Dadone, L., :Laser Velocimetry and Blade Pressure Measurements 

of a Blade-Vortex Interaction,” Journal of the American Helicopter Society, Vol. 40, No. 2, 1995, pp. 15-

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4. Lorber, P.F., “Aerodynamic Results of a Pressure-Instrumented Model Rotor Test at the DNW,” 

Proceedings of the 46th Annual Forum of the American Helicopter Society, Washington, DC, May 1990. 

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McLachlan, B., “Luminescent Barometry in Wind Tunnels,” Review of Scientific Instrumentation, Vol. 

61, No. 11, 1990, pp. 3340-3347. 

6. Morris, M.J., Benne, M.E., Crites, R.C., and Donovan, J.F., “Aerodynamic Measurements Based 

on Photoluminescence,” 31st Aerospace Sciences Meeting, AIAA, Reno, NV, 1993, Paper 93-0175. 

7. McLachlan, B., and Bell, J., “Pressure-Sensitive Paint in Aerodynamic Testing,” Experimental

Thermal and Fluid Science, Vol. 10, No. 4, 1995, pp. 470-485. 

8. Liu. T., Campbell, B., Burns, S., and Sullivan, J., “Temperature- and Pressure-Sensitive 

Luminescent Paints in Aerodynamics,” Applied Mechanics Reviews, Vol. 50, No. 4, 1997, pp. 227-246. 

9. Liu, T., and Sullivan, J., Pressure and Temperature Sensitive Paints (Experimental Fluid 

Dynamics), Springer-Verlag, Berlin, 2004. 

10. Lakowicz, J., Principles of Fluorescence Spectroscopy, 2nd ed., Kluwer Academic/Plenum 

Publishers, New York, 1999, pp. 239-242. 

11. Engler, R., and Klein, C., “DLR PSP System: Intensity and Lifetime Measurements,” 17th

International Congress on Instrumentation in Aerospace Simulation Facilities, IEEE, Pacific Grove, CA, 

1997, pp. 46-56. 

12. Holmes, J., “Analysis of Radiometric, Lifetime, and Fluorescent Imaging for Pressure Sensitive 

Paint,” Aeronautical Journal, Vol. 102, No. 1014, 1998, pp. 189-194. 

13. Bell, J.H., Schairer, T.E., Hand, L.A., and Mehta, R.D., “Surface Pressure Measurements Using 

Luminescent Coatings,” Annual Review of Fluid Mechanics, Vol. 33, 2001, pp. 115-206. 

14. Mitsuo, K., Egami, Y., Asai, K., Suzuki, H., and Mizushima, H., “Development of Lifetime 

Imaging System for Pressure-Sensitive Paint,” 22nd AIAA Aerodynamic Measurement Technology and 

Ground Testing Conference, AIAA, St. Louis, MO, 2002, Paper 2002-2909. 

15. Watkins, A.N., Jordan, J.D., Leighty, B.D., Ingram, J.L., and Oglesby, D.M., “Development of 

Next Generation Lifetime PSP Imaging Systems,” 20th International Congress on Instrumentation in 

Aerospace Simulation Facilities, IEEE, Gottingen, Germany, 2003, pp. 372-382. 

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Scale Rotor Blades in Hover,” 35th AIAA Fluid Dynamics Conference, AIAA, Ontario, Canada, 2005, 

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Pacific Grove, CA, 2007, and published in the proceedings. 
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Luminescence-Based Pressure and Temperature Measurement in Aerodynamic Applications, MS Thesis, 

School of Material Science and Engineering, Purdue University, 1999. 

20. Gregory, J.W., Asai, K., Kameda, M., Liu, T., and Sullivan, J.P., “A Review of PressureSensitive Paint for High-Speed and Unsteady Aerodynamics,” Proceedings of the Institution of 

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21. Noonan, K.W., “Aerodynamic Characteristics of Two Rotorcraft Airfoils Designed for 

Application to the Inboard Region of a Main Rotor Blade,” NASA-TP-3009, AVSCOM-TR-90-B-005, 

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22. Noonan, K.W., “Aerodynamic Characteristics of Two Rotorcraft Airfoils Designed for the Tip 

Region of a Main Rotor Blade,” NASA-TM-4264, AVSCOM-TR-91-B-003, 1991. 

23. Wong, O.D. and Watkins, A.N., “Blade Tip Pressure Measurements Using Pressure Sensitive 

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24. Juliano, T.J., Kumar, P., Peng, D., Gregory, J.W., Crafton, J., and Fonov, S., “Single-Shot, 

Lifetime-Based Pressure-Sensitive Paint for Rotating Blades,” Measurement Science and Technology,

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25. Bell, J.H., “Accuracy Limitations of Lifetime-Based Pressure-Sensitive Paint (PSP) 

Measurements,” 19th International Congress on Instrumentation in Aerospace Simulation Facilities,

IEEE, Cleveland, OH, 2001, pp. 5-16. 

26. Ruyten, W. and Sellers, M., “Lifetime Analysis of the Pressure-Sensitive Paint PtTFPP in FIB,” 

42nd Aerospace Sciences Meeting, AIAA, Reno, NV, 2004, Paper 2004-881. 

27. Ruyten, W., Sellers, M.E., and Baker, W.M., “Spatially Nonuniform Self-Quenching of the 

Pressure-Sensitive Paint PtTFPP/FIB,” 47th Aerospace Sciences Meeting, AIAA, Orlando, FL, 2009, 

Paper 2009-1660. 
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

Deployment of a Pressure Sensitive Paint System for Measuring Global 

Surface Pressures on Rotorcraft Blades in Simulated Forward 

Flight - Preliminary PSP Results From Test 581 in the 14- x 22-Foot 

Subsonic Tunnel 

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Watkins, Anthony N.; Leighty, Bradley D.; Lipford, William E.; Wong, 

Oliver D.; Goodman, Kyle Z.; Crafton, James; Forlines, Alan; Goss, Larry 

P.; Gregory, James; Juliano, Thomas J.

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

NASA Langley Research Center

Hampton, VA 23681-2199

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National Aeronautics and Space Administration

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NASA

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Unclassified - Unlimited

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Availability: NASA CASI (443) 757-5802

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14. ABSTRACT

This report will present details of a Pressure Sensitive Paint (PSP) system for measuring global surface pressures on the tips of

rotorcraft blades in simulated forward flight at the 14- x 22-Foot Subsonic Tunnel. The system was designed to use a pulsed 

laser as an excitation source and PSP data was collected using the lifetime-based approach. With the higher intensity of the 

laser, this allowed PSP images to be acquired during a single laser pulse, resulting in the collection of crisp images that can be 

used to determine blade pressure at a specific instant in time. This is extremely important in rotorcraft applications as the 

blades experience dramatically different flow fields depending on their position in the rotor disk. Testing of the system was 

performed using the U.S. Army General Rotor Model System equipped with four identical blades. Two of the blades were 

instrumented with pressure transducers to allow for comparison of the results obtained from the PSP. This report will also 

detail possible improvements to the system. 

15. SUBJECT TERMS

Dynamic pressure measurements; Non-intrusive measurements; Optical diagnostics; Pressure Sensitive Paint

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01- 12 - 2011