Final Report Summary - RIFPA (Grooved paint surface manufacturing for aerodynamic drag reduction testing)
Executive Summary:
Experiences had been conducted in the 90's showing that longitudinal grooves actually reduce the aerodynamic drag. However, plastic stickers used in that period suffered inconvenient.
In this programme, it is proposed to realize various shapes of grooved surfaces thanks to a new casting method, developped by the applicant.
The project aims at providing ONERA with models covered with grooved paint of several profiles, in order to qualify the aerodynamic performance.
After an initial study and process definition, paint films of different shapes will be designed and realized. Then, films will be applied onto a model, to be further tested into a wind tunnel.
The paint film process will use approved aeronautical paints with as little modification as possible, so as to prepare future application on aircrafts.
Project Context and Objectives:
Reducing the friction drag is a major stake for subsonic aircrafts. For modern transonic commercial aircraft, friction contribution represents half of the total drag.
Among possible solutions, passive surface geometry seems to be particularly interesting. The effect of riblets or grooved surfaces has been demonstrated by ONERA through studies in wind tunnels and models [1].
In the 1990’s, a scale one experiment was engaged by Airbus Industry on an aircraft with a vinyl coating adhesive designed by 3M company.
However, plastic stickers revealed difficult to apply and maintain, with a loss of sharpness on the edges.
Grooves directly made of paint should have a far better duration and performance than plastic stickers. Aeronautical paintings are especially aggressive and represent a significant weight compared to the useful load of airplanes.
Removing and maintaining paints are also very pollutant. Costs and immobilizations of planes for painting operations are quite important.
Corso Magenta brings a new approach [2]: beside liquid and powder, paint can also be considered as a finished layer, machine under controlled conditions, then applied as a protective or decorative coating.
Corso Magenta process provides an important reduction in the environmental impact: no losses during application, no VOC evaporation during application, no drying time …
Corso Magenta contribution to the JTI Clean Sky SFWA-ITD is to provide ONERA with new innovative micro grooved (riblets) painted films to evaluate the aerodynamic performances of riblet materials under transonic flow conditions.
Two different cross-sections have been produced: a “saw-tooth V-type” with an aspect ratio s/h equal to 1 (rib spacing, s=50 µm; rib height h=50µm) and a “trapezoidal-type” with s/h equal to 2, h=25µm and a peak thickness, t/s= 0.025.
Geometry is extremely precise: it represents a real issue and values are closed to expect ones in the range of accepted tolerances (Figure 1). Sample test films have been designed with a self-adhesive layer.
(a) (b)
Figure 1: SEM images of the paint film with riblet shapes (a) saw-tooth “V-type” cross section; (b) “trapezoidal-type” cross section.
The goal of RIFPA project being to perform wind tunnel campaigns, CORSO MAGENTA attends part of the tests in Modane from October 21th 2013 to October 25th 2013.
The tests have been performed at ONERA S3MA transonic wind tunnel with the existing ogive-cylinder model tested in the former ONERA T2 transonic wind tunnel which is now closed.
During the test different Mach numbers (from 0.3 to 0.8) and different stagnation pressures (from 1 to 2.5 bars) have been studied to measure associated drag friction coefficient.
To highlight the riblet effects on drag friction coefficient, a smooth cylinder has been tested as the reference.
The aerodynamic benefit of the “trapezoidal-type” riblet profile has been put in evidence qualitatively in the range of Reynolds number considered.
So, the riblet technology could be a very promising enabler for reducing fuel burn.
[1] E. Coustols and J. Cousteix, AIAA Journal, vol.32 n°2, pp 431-433
[2] Corso Magenta Patent: FR 2881681 filed in 2005
Project Results:
1. Corso Magenta objectives
Actually, CORSO MAGENTA contribution to the JTI Clean Sky SFWA-ITD is to build an effective aeronautical solution of paint riblets with a new geometry.
This solution will further be tested in a wind tunnel at the ONERA Centre of Modane, in order to measure the drag reduction and compare it to former results obtained by ONERA R&D teams in the late 1980’s.
CORSO MAGENTA has provided to ONERA three dural cylinders equipped with grooved paint films with the following cross-sections of riblet samples:
Profile 1: “saw-tooth V-type” with s/h=1, h=50µm
Profile 2: “trapezoidal-type” s/h=2, h=25µm, t/s ≤ 0.025
Profile 3: “trapezoidal-type” with imperfections
The riblet material R1 (profile 1), R2 (profile 2) and R3 (profile 2 degraded) have been delivered to ONERA Modane on October 15th: they were considered to be satisfying by ONERA.
2. Measurements in the wind tunnel
Tests were performed in the transonic ONERA S3MA wind tunnel at the ONERA Center of Modane.
S3MA is a supersonic wind tunnel with run time from 10 seconds to 15 minutes.
The blow down wind tunnel is a rectangular section 0.80 m x 0.76 m maximum. Mach number may vary from 0.1 to 5.5.
The stagnation pressure varies from 0.2 bars to 7.5 bars depending on nozzle and Mach number. The maximum generated stagnation temperature is 530 K (257°C).
The objective of the wind tunnel tests is to simulate flight conditions of an airplane.
When the aircraft is flying at cruising altitude (between 8000 and 10000 meters) at Mach number equal to 0.7 the static pressure Ps which depends on the altitude, is equal to 0,356 bars to 0,264 bars for 8000 m and 10000 m respectively. The associated stagnation pressure Pi is equal to 0.5 and 0.4 bars respectively.
Such stagnation pressures cannot be generated at Ma=0.7 with the S3MA wind tunnel, so to simulate flying conditions it is necessary to decrease Mach number value.
The tests campaign duration was 7 working days: from October 17th 2013 to October 25th 2013. CORSO-MAGENTA attended part of the tests (from October 21th to October 25th), as suggested by ONERA partners and agreed by the Scientific Officer of SFWA-ITD.
2.1 Specifications of the delivered riblet materials
The dural cylinders which were equipped with paint films have an upstream portion with shoulder and a downstream portion without shoulder.
To validate profiles and sole + glue coating thickness, SEM measurements were performed on paint films after implementation on the cylinders. Samples measured paint films were taken at the shoulder side of the cylinder on four areas along the perimeter.
2.1.1 Riblet material R1
The cylinder equipped with riblets n°1 is an existing ONERA cylinder referenced “Old-1: NPG 284” with a measured external diameter (upstream part) equal to 79.78 ± 0.03 mm.
Pictures of the material R1 can be seen in Figure 1.
(a)
(b) (c)
Figure 1: Pictures of the material R1: shoulder of the cylinder (a); rear of the cylinder (b); SEM image of the profile 1 (c).
SEM images of the paint film for profile 1 can be observed in Figure 2 and obtained values have been reported in Table 1.
These values are those expected in the range of accepted tolerances.
Figure 2: SEM pictures and values obtained for profile 1.
S (µm) h (µm) Paint sole thickness (µm) Glue thickness (µm) Ts (paint + glue) (µm)
Target 50 50 100 ± 10
R1 50.6 ± 1.2 49.2 ± 0.8 65.1 ± 3.5 25.8 ± 4.3 92.4 ± 5.3
Table 1: SEM values obtained for profile 1 (material R1).
2.1.2 Riblet material R2
The cylinder equipped with riblets n°2 and referenced “2013-2” has been manufactured by the AIRBUS model-shop in April-May 2013. The measured external diameter (upstream part) is equal to 79.79 ± 0.01 mm.
Pictures of the material R2 are shown in Figure 3.
(a) (b)
(c) (d)
Figure 3: Pictures of the material R2: equipped cylinder (a); shoulder of the cylinder (b); rear of the cylinder (c); SEM image of the profile 2 (d).
SEM images of the paint film for profile 2 can be observed in Figure 4 and obtained values have been reported in Table 2.
These values are those expected in the range of accepted tolerances.
Figure 4: SEM pictures and values obtained for profile 2.
S (µm) h (µm) V (µm) b (µm) Paint sole thickness (µm) Glue thickness (µm) Ts (paint + glue) (µm)
Target 50 25 29.3 20.7 100 ± 10
R2 49.1 ± 1.2 24.9 ± 0.6 29.3 ± 0.8 20.2 ± 0.8 50.6 ± 1.2 23.5 ± 6 90.8 ± 8.1
Table 2: SEM values obtained for profile 2 (material R2).
2.1.3 Riblet material R3 (degraded profile 2)
Initially, it has been intended to degrade riblet material R2. However, CORSO MAGENTA has also provided a third riblet material R3 equipped with profile 2 but not corresponding to the expected criteria (lack of paint in some areas).
It was then decided to keep intact riblet material R2 and to degrade riblet material R3.
This third cylinder referenced “2013-1” has been manufactured by AIRBUS model-shop and has a measured external diameter equal to 79.79 ± 0.01 mm.
SEM measurements were not made because it was not planned to test this riblet material in wind tunnel.
Degradation involved removing strips of paint film. Figure 5 illustrates the process: the cylinder was heated to 80°C (temperature at which the adhesive softens) with a heat gun. First, a longitudinal band 2.5cmx55cm was removed (10% of the surface) and then a wider strip of 12.5cmx55cm (50% of the surface).
(a) (b)
Figure 5: Pictures of the material R3: degradation by heating the cylinder at 80°C (a); aspect of degraded riblet material (10% area) (b).
2.2 Testing program and results
Different types of measurements have been done:
1) Values inside the test section of the wind tunnel:
- Ps = static pressure in the test section
- Pi = total pressure
- Ti = total temperature
- M0 = test section Mach number = f (Ps/Pi)
- Humidity
There are some calibrated test section corrections on the Mach number (M0c instead M0).
2) Static pressure distribution Ps on the 14 pressure taps along the generatrix of the body of revolution: eight taken on the ogive and six taken on the smooth cylinder.
3) Steady pressure in the internal cavity of the cylinder: Pint
4) Static pressure difference P between the internal cavity and the static pressure measured by a differential sensor with high sensitivity (accuracy of ± 10 to ± 20 Pa): P = Ps-Pint. The measurement is used for correcting the drag force.
5) Drag force Fx measured by the balance (accuracy of at least ± 0.25 N). There are some calibrated test section corrections on drag force obtained by subtracting the pressure difference P: Fxc = Fx - P.S where S is the frontal area of the cylinder (S = 0.0050266 m2).
The measurements were made in transonic flow with an incidence angle of 0°. Incidence angles of 5° and -5° were also tested in some cases.
The duration of the bursts was 2-3 minutes.
The measurements were made for different Mach number with different Pi. The Mach numbers were varied from 0.3 to 0.8 and stagnation pressures from 1 bar to 2.5 bars.
Measurement accuracy for low stagnation pressures and high Mach numbers is good enough. However, for low stagnation pressures and low Mach numbers, measurement accuracy is low.
Results are analyzed by plotting the drag coefficient Cx versus the unit Reynolds number.
Through first qualitative analysis, the drag results are those expected by ONERA in the Reynolds number range of the present measurements:
- For Profile 1 (R1) the drag is increased at high Reynolds number and at low Reynolds number, drag remains very near of the smooth case value.
- For Profile 2 (R2) the drag is increased at high Reynolds number (but less than for Profile 1). However, there is a significant domain at low Reynolds number where the drag is slightly decreased in comparison to the smooth case value.
- Concerning the R3 cylinder (with Profile 2):
- The first drag results without and before any degradation of the coating are very near from the R2 ones;
- When 50% of the riblets coating is taken off (see § 7.1.3) the drag curve appears to be located between the R2 case and the smooth case.
It is important to note that this first information on results is only indicative. The final analysis will be done by ONERA. It will take in account the uncertainty and dispersion of the drag measurements.
2.2 Diameters measurements
After wind tunnel tests, diameters measurements of film paint equipped cylinders were made by ONERA using an optical probing mounted on a coordinate measuring machine "Olivetti".
For each cylinder, four diameters were measured: two diameters perpendicular to each other (Φ1, Φ2) at 25 mm of the edge on shoulder side and two diameters on rear side. One of the two perpendicular diameters is located at 10 mm from the joint (Figure 6).
Results have been reported in table 3.
Figure 6: Drawing of two perpendicular diameters to each other measured with the optical probing. Picture shows the shoulder side of the cylinder. The same diameters have been considered on the rear side.
Coated cylinder Shoulder side Rear side
Diameter Φ1 (mm) Diameter Φ2 (mm) Diameter Φ1 (mm) Diameter Φ2 (mm)
R1 80.1380.1480.0580.06
R2 80.0280.0280.0179.99
R3 79.9379.9379.9279.93
Table 3: Results obtained with the optical probing for the four diameters. Measurement accuracy is equal to ± 0.02 mm (± 20 µm).
Total thickness Ts (paint sole + glue thickness) can be extrapolated from diameters measurements with the following equation:
Φ1,2 = Φc + 0.002 (Ts + h) (1)
Where Φc is the external diameter of the cylinder without paint film and h is the riblets height.
To determine Ts, we have considered values of h obtained with SEM. Results are listed in Table 4 and also results obtained with SEM in shoulder side for comparison.
Coated cylinder Shoulder side Rear side
Φ1 (mm)Φ2 (mm)Ts1 (µm) Ts2 (µm) SEM Ts value (µm) Φ1 (mm)Φ2 (mm)Ts1 (µm) Ts2 (µm)
R1 80.1380.14125.8130.892.4 ± 5.3 80.0580.0685.890.8
R2 80.0280.0290.190.190.8 ± 8.1 80.0179.9985.175.1
R3 79.9379.9345.145.1- 79.9279.9340.145.1
Table 4: Results obtained from equation (1) for total thickness Ts. Measurement accuracy is equal to ± 25.8 µm for R1 and ± 15.6 µm for R2 and R3.
The optical probing measurements indicate that there is a thickness gradient between the shoulder side and the rear side for riblet material R1.
Total thickness values in contact with the adjacent smooth ogive part (shoulder side) suggest that there is a facing step effect. However, taking into account the large uncertainties on measurements of Ts with optical probing (± 25.8 µm), it is difficult to conclude such a think especially considering the more accurate values obtained by SEM.
Indeed, the total thickness value measured with SEM, Ts = 92.4 ± 5.3 µm, indicates that there is no facing step effect: there is a downward step of 7.6 µm.
In this case, it looks like the riblet cross-section was a trapezoidal with h = 43 µm, h/S = 0.8 and a valley V = 6.4 µm.
For riblet material R2, values obtained with optical probing and SEM, are close to each other.
The total thickness value Ts = 90.8 ± 8.1 µm indicates that there is no facing step effect: there is a downward step of 9.2 µm.
In this case, it looks like the riblet cross-section was a trapezoidal with h = 15.7 µm, h/S = 0.3 and a valley V = 36.6 µm.
3. Conclusion
CORSO MAGENTA has delivered to ONERA three cylinders covered with riblet paint application samples: one with a “saw-tooth V-type” profile (R1) and two with a “trapezoidal- type” profile (R2, R3).
Characteristics of grooved paint application samples (cross-section, groove height, grooves spacing, peak thickness …) have been qualified from SEM measurements.
Characteristics of the two riblet geometries (saw-tooth V-type and trapezoidal- type) correspond to those provided and expected by ONERA.
ONERA has validated the three riblet materials R1, R2 and R3.
The goal of RIFPA project being to perform wind tunnel campaigns, CORSO MAGENTA attends part of the tests in Modane.
The objectives were to evaluate the aerodynamic performances of riblet materials provided by CORSO MAGENTA under transonic flow conditions.
The tests have been performed at ONERA S3MA transonic wind tunnel with the existing ogive-cylinder model tested in the former ONERA T2 transonic wind tunnel which is now closed.
During the test different Mach numbers (from 0.3 to 0.8) and different stagnation pressures (from 1 to 2.5 bars) have been studied to measure associated drag friction coefficient.
To highlight the riblet effects on drag friction coefficient, a smooth cylinder has been tested as the reference.
The aerodynamic benefit of the “trapezoidal-type” riblet profile has been put in evidence qualitatively in the range of Reynolds number considered.
So, the riblet technology could be a very promising enabler for reducing fuel burn.
Potential Impact:
The present study is expected to allow ONERA to prove a high level of performance in drag reduction. As mentioned above, friction contributes to about half of the total drag of an aircraft. Would the studies be successful, a reduction of 5% - 8% of the drag could be obtained thanks to a riblets covered surface.
CORSO MAGENTA contribution is to use paint as a film for getting these new surface aspects.
Thanks to its know how and patents, CORSO MAGENTA is in position to transform into films already approved aeronautic paints, and to mould their surface with defined shapes.
These paint films will thus get the same high level of performance in strength, flexibility and shock resistance.
However, it seems reasonable to consider that before applying a grooved coating onto an aircraft, several questions will have to be answered:
- ageing of riblets
- influence of surface elements on dust, and effect of cleaning brushes on the sharpness of riblets
- influence on the frost point
- maintenance and replacement
Nevertheless, the present program will be an introduction for paint films in aircrafts, with probably a number of possible new applications.
List of Websites:
Contact:
Maria Rosanne
mrosanne@corso-magenta.com
+33 6 13 50 90 57 / + 33 9 72 35 13 90
