Recent NASA Wake Surfing Flight Research

Recent NASA Wake Surfing Flight Research

EXPERIMENTAL MEASUREMENTS OF FUEL SAVINGS DURING AIRCRAFT WAKE SURFING

C U R T H A N S O N , J O E PA H L E , J A M E S R E Y N O L D S , S T E P H A N I E A N D R A D E , N E L S O N B R O W N

N A S A A R M S T R O N G F L I G H T R E S E A R C H C E N T E R

EXPERIMENTAL MEASUREMENTS OF PASSENGER RIDE QUALITY DURING AIRCRAFT WAKE SURFING

C U R T H A N S O N , S T E P H A N I E A N D R A D E , J O E PA H L E

N A S A A R M S T R O N G F L I G H T R E S E A R C H C E N T E R

introduction

1

Wake Surfing Background

WAKE SURFING EXTRACTS ENERGY FROM THE

UPWASH OF ANOTHER AIRCRAFT’S WAKE VORTEX.

Wieselsberger, C., “Contribution to the Explanation of Angled Flight Patterns of Some Migratory Birds,” 1914.

REDUCE DRAG, FUEL USE, AND EMISSIONS

• AIR CARGO OPERATORS

• CIVILIAN PASSENGER AIRCRAFT PAIRS

• DISSIMILAR AIRCRAFT PAIRINGS

ex: fighter-tanker missions

• 3-SHIP STAGGERED V FORMATIONS

• 4+ AIRCRAFT FORMATIONS

- string stability- downstream wake effects

• HALE• SMALL UAVS

Q: what is the lower size limit?

Extended Formation Flight ResearchClose Formation Flight Research

2

Prior Wake Surfing Flight ResearchF

ue

l S

avin

gs (

lbs)

Flight Time

Ray, Ronald J., et al, “Flight Test Techniques Used to Evaluate Performance

Benefits During Formation Flight,” 2002Bieniawski, Stefan R., et al, “Summary of Flight Testing and Results

for the Formation Flight for Aerodynamic Benefit Progam,” 2014

3

Wake Surfing Challenges

DR. ERBSCHLOE, USAF AMC CHIEF SCIENTIST (2008):

“WE WILL ONLY BE INTERESTED IN FORMATION FLYING

FOR AERODYNAMIC BENEFIT IF IT IS:

• SAFE.• AIRCREW FRIENDLY.• AIRCRAFT FRIENDLY.• MAKES BUSINESS SENSE.• MAKES OPERATIONAL SENSE.”

OTHER CHALLENGES IDENTIFIED BY THE USAF:

• PILOT TRAINING

• PILOT TACTICAL DUTY DAY RESTRICTIONS

• EQUIPAGE FOR AIRCRAFT OTHER THAN THE C-17• DOMESTIC AND FOREIGN AIR TRAFFIC CONTROL

INDUSTRY PERSPECTIVES, WAKENET USA 2013:• AIR CARGO COMPANIES

• MAJOR CARRIERS AND REGIONAL AIRLINES

• AIRLINE PILOT ASSOCIATION

CHALLENGES IDENTIFIED BY INDUSTRY:

• LACK OF CIVILIAN AIRFRAME DATA

• PASSENGER DISCOMFORT

• WAKE CROSSING PREVENTION

• COST OF EQUIPAGE

• AIR TRAFFIC CONTROL PROCEDURES

• FAA APPROVAL

DR. ERBSCHLOE, USAF AMC CHIEF SCIENTIST (2008):

“WE WILL ONLY BE INTERESTED IN FORMATION FLYING

FOR AERODYNAMIC BENEFIT IF IT IS:

• SAFE.• AIRCREW FRIENDLY.• AIRCRAFT FRIENDLY.• MAKES BUSINESS SENSE.• MAKES OPERATIONAL SENSE.”

OTHER CHALLENGES IDENTIFIED BY THE USAF:

• PILOT TRAINING

• PILOT TACTICAL DUTY DAY RESTRICTIONS

• EQUIPAGE FOR AIRCRAFT OTHER THAN THE C-17• DOMESTIC AND FOREIGN AIR TRAFFIC CONTROL

INDUSTRY PERSPECTIVES, WAKENET USA 2013:• AIR CARGO COMPANIES

• MAJOR CARRIERS AND REGIONAL AIRLINES

• AIRLINE PILOT ASSOCIATION

CHALLENGES IDENTIFIED BY INDUSTRY:

• LACK OF CIVILIAN AIRFRAME DATA

• PASSENGER DISCOMFORT

• WAKE CROSSING PREVENTION

• COST OF EQUIPAGE

• AIR TRAFFIC CONTROL PROCEDURES

• FAA APPROVAL

4

NASA G-III Wake Surfing Flight Experiment

LEAD AIRPLANE:

• NASA G-III

• PRODUCTION AVIONICS WITH

ADS-B OUT

• CABIN VIBRATION SENSORS

TRAIL AIRPLANE:

• NASA C-20A (G-III MILITARY VARIANT)

• PRODUCTION AVIONICS AUGMENTED WITH:

• EXPERIMENTAL PROGRAMMABLE AUTOPILOT

• PILOT TABLET DISPLAYS

• COMMERCIAL ADS-B IN

• VIDEO RECORDING OF FUEL FLOW

• CABIN VIBRATION AND NOISE SENSORS

5

ADS-B Enabled Experimental Autopilot

1090 MHz ADS-B Data Link

• Non-secure data link

• Broadcast twice-per-second at random variations

• Horizontal resolution ~16.7 feet / 1 knot

• Vertical resolution ~25 feet / 64 ft per min

• Accuracy dependent on transmitting avionics

• No wind or weather information

Research Autopilot

• Wake drift and descent predictions

• Wake-relative navigation

• Trajectory control

• Analog ILS localizer and glideslope commands

• Throttle cues to pilot display

Operator Interfaces

• Lead aircraft selection

• Controller gains and parameters

• 3-axis wake-relative position commands

• Arm / engage / disengage

TEST OPERATIONS:

• MACH 0.7, 35,000 FEET

• 4000 FEET IN TRAIL

Experiment Conditions6

TEST CONDITIONS

• DAY VMC

• CALM TO LIGHT TURBULENCE

• 30-40 MINUTE TEST LEGS

• W-291 RESTRICTED AIRSPACE

OVER THE PACIFIC OCEAN

• CONTRAILS PREFERRED BUT

NOT REQUIRED

TEST METHOD:

Experiment Methodology7

10 fps

5 f ps

2 f ps

1 f ps

0.5 f ps

10 fps

5 f ps

2 f ps

1 f ps

0.5 f ps

Reported Lead

Location

Predicted

Wake

Location

wind drift wake

descent

Cross-Track, ft

Vert

ical

Posit

ion

, ft

-100 0 100 200 300 400

-100

-50

0

50

1001. WAKE-FREE TARE

(MINIMUM OF 3 MINUTES)

2. WAKE INGRESS / MAPPING

3. STABILIZED WAKE SURFING:PERFORMANCE DWELL AND

RIDE QUALITY EVALUATION


1

3

2

4

The autopilot computes a wind- and descent-corrected trajectory for the trail airplane. This trajectory is relative to the lead airplane’s wake.

One knot of error in cross-track wind speed adds 10 ft of error to the predicted wake location.

4. WAKE-FREE TARE


performance benefits

8

ሶ𝑚0 𝑚 = ሶ𝑚𝑅 𝑚 −𝜕 ሶ𝑚

𝜕𝑉Δ𝑉 −

𝜕 ሶ𝑚

𝜕 ሶ𝑉ሶ𝑉

cockpit fuel flow meters

Fuel Flow Estimation

1 2 3 4 5 6 7

-5

0

5

10

Test Point

Off

-tri

m A

irsp

ee

d,

kca

s

Analysis Point Airspeed, mean

Analysis Point Airspeed, range

-0.1 -0.05 0 0.05 0.1

30

32

34

36

38

Airspeed Rate, kcas/sec

Fue

l F

low

, P

PH

/100

Flight Measurement

Linear Fit

Trim Fuel Flow Estimate

9

Fuel Flow Estimation

8101214161820

32

33

34

35

Fuel Quantity, lb x 1000

Fue

l F

low

, P

PH

/100

Tare Point Trim Estimates

Estimate Uncertainty

Tare Quadratic Fit

-0.2 -0.1 0 0.1 0.2

-8

-4

0

4

8

Off

-Tri

m F

ue

l F

low

, P

PH

/10

0

Airspeed Rate, kcas/sec

Airspeed Rate Correction

Tare Point Data

0 50 100 150 200 250 300 350 400 450

28

30

32

34

36

38

40

Fue

l F

low

, P

PH

/100

0 50 100 150 200 250 300 350 400 450

-0.1

-0.05

0

0.05

0.1

0.15

Air

sp

ee

d R

ate

, kcas/s

ec

Time, sec

Wake-Based Correction

Tare-Based Correction

Mean Tare-Corrected

Measurement

Steady-State Time Segments

𝜕 ሶ𝑚

𝜕 ሶ𝑉for all tare pointsvalue of

tare fuel flow vs. fuel quantity

Example fuel flow corrections for in-wake

performance point.

(local linear fit vs. tare-based correction)

810121416182028

29

30

31

32

33

34

35


Fue

l F

low

, P

PH

/100

Tare Point Trim Estimates

Estimate Uncertainty

Tare Quadratic Fit

10

Fuel Flow Reduction Results

Seven test points were

completed on the final flight.

810121416182028

29

30

31

32

33

34

35

2%

5%

8%

10% 2%

5%

8%

10%

TP1 TP2 TP3 TP4 TP5 TP6 TP7


Fue

l F

low

, P

PH

/100

Tare Quadratic Fit

Wake Turbulence Onset

In Wake Effect, 1 Hz Estimate

Wake Ingress, Segment Mean

Wake Surfing, Segment Mean

Wake ingress was stopped when

wake effects (rumbling) were felt

in the cabin. Post-flight analysis

showed this occurred around

3.5% fuel flow reduction.

: Points of constant throttle

setting (20-plus seconds).

This potentially limited the

maximum measured benefit.

Fuel Flow Reduction Results

The steep gradient of wake

effects vs. position prevented the

controller from stabilizing at a

single location within the wake

for extended periods.

Two of the test points (2 and 7)

significantly exceeded the 3.5%

ride quality threshold.

A maximum performance benefit

of >8% was achieved briefly,

consistent with previous wake

surfing results.

11

810121416182028

29

30

31

32

33

34

35

2%

5%

8%

10% 2%

5%

8%

10%

TP1 TP2 TP3 TP4 TP5 TP6 TP7


Fue

l F

low

, P

PH

/100

Tare Quadratic Fit

Wake Turbulence Onset

In Wake Effect, 1 Hz Estimate

Wake Ingress, Segment Mean

Wake Surfing, Segment Mean

Wake Effect Map

80 100 120 140

-20

-10

0

10

20

8%6%

4%2%

Cross Track, ft

Vert

ical

Tra

ck,

ft

Test Point 2

80 100 120 140

-20

-10

0

10

20

8%

6%

4%2%

Cross Track, ft

Test Point 7

theory 2-4% 4-6% 6-8% 8+ %

An independent measure of the

wake location was unavailable

for verification of the wake

prediction algorithm.

In general, the largest benefits

were measured closest to the

predicted core location.

The gradients of the flight

measurements appear to be

more steep than predicted.

12

Secondary Effects

0 1 2 3 4 5 6 7 8

-0.4

-0.2

0

0.2

Fuel Flow Reduction, %

Devia

tio

n f

rom

tri

m,

deg

Angle of Attack, Theory

Mean Pitch Angle, TP 2

Mean Pitch Angle, TP 7

0 2 4 6 80

0.5

1

1.5

2

2.5


Defl

ect

ion

, d

eg

Ailerons

0 2 4 6 80

0.5

1

1.5

2

2.5


Spoilers

Mean - TP 2

Mean - TP 7

Combined Peak

Pitch Trim

Wake-induced drag savings are

accompanied by a reduction in trim angle

of attack. The flight-measured change in

pitch trim vs. fuel flow reduction matches

theoretical predictions.

Roll Trim

The wake field produces an asymmetric

lift distribution across the wing, resulting

in increased roll trim with higher fuel

savings. The measured aileron and spoiler

deflections show a correlation with

measured fuel flow reduction.

13

passenger ride quality

Passenger Ride Quality Instrumentation:

• Accelerometers mounted to the seat rails of both airplanes• 3-axis accels sampled at 200 Hz• separate accels for low and high frequency measurements• internal data logging with time stamp

• Sound dosimeter with microphone at approximate passenger ear location• records and logs 1-minute time-average sound levels• 100 Hz to 5 kHz, 40-140 dB

• Pre-flight and post-flight surveys of pilots and research crew

• An additional accelerometer was mounted to the ceiling of the aft baggage compartments of both airplanes to measure tail buffeting

Passenger Ride Quality: Cabin Vibration

14

• Occurred in the strongest part of the wake

• Strong variation with fore-aft cabin location

• Described as “rumbling”, compared to light turbulence or a driving on a washboarded road


15

0 200 400 600 800 1000 1200-0.2

0

0.2

Fw

d C

ab

in

Test Point 2 Peak Acceleration, gs

Vertical Lateral Longitudinal

0 200 400 600 800 1000 1200-0.2

0

0.2

Mid

Cab

in

0 200 400 600 800 1000 1200-0.2

0

0.2

Tare Point Wake Ingress Wake Surfing

Time, sec

Aft

Bag

ga

ge


16

0 20 40 60 80 100

-50

-40

-30

-20

-10

0

Pow

er/

fre

qu

ency (

dB

/Hz) Fwd Cabin, Lateral

Calm Air

Turbulence

0 20 40 60 80 100

-50

-40

-30

-20

-10

0

Mid Cabin, Lateral

Lead Aircraft

Wake Surfing

Trail Aircraft

0 20 40 60 80 100

-50

-40

-30

-20

-10

0

Frequency (Hz)

Pow

er/

fre

qu

ency (

dB

/Hz) Fwd Cabin, Vertical

0 20 40 60 80 100

-50

-40

-30

-20

-10

0

Frequency (Hz)

Mid Cabin, Vertical

0 20 40 60 80 100

-50

-40

-30

-20

-10

0

Aft Baggage, Lateral

0 20 40 60 80 100

-50

-40

-30

-20

-10

0

Frequency (Hz)

Aft Baggage, Vertical

• Wake-induced vibrations are similar to those of light turbulence at higher frequencies

• Light turbulence contains low frequency content not found in the wake

• Cabin vibrations on the lead airplane during wake surfing were similar to non-turbulent conditions, suggesting measured effects on the trail airplane were due to flight within the wake

Calm Air Wake Ingress Wake Surfing Turbulence 80

81

82

83

84

Ave

rage

No

ise L

eve

l, d

B

Test Point 2

Test Point 7

Passenger Ride Quality: Cabin Noise

17

• A similar increase in noise was also recorded during the more severe of the two “light turbulence” turbulent tare points.

• Slighty increased cabin noise levels were recording during wake surfing, as compared to flight in calm air and in the weaker portions of the wake.

Dosimeter noise recorder,trail aircraft cabin installation

In the 1970s, Jack Leatherwood and others at NASA LaRC conducted a series of studies to develop a criteria to predict passenger discomfort due to vibration and noise.

• Vibration Tests• 2200 test subjects• motion simulator fitted with six tourist-class aircraft seats• 10 - 15 second excitations• lateral, vertical, longitudinal, roll, and pitch vibrations• rated as “comfortable” or “uncomfortable”

• Noise and Vibration Tests• 60 test subjects• combinations of noise and vibration• 4 sound levels, 6 octave bands

NASA Passenger Ride Quality Metric

18

from “Human Discomfort Response to Noise Combined With Vertical Vibration,” Leatherwood, April 1979


19

• Frequency-weighted acceleration measurements are combined to form a Discomfort Metric: DISC.

• For sinusoidal vibrations, the DISC metric was developed with the following excitations:

Vertical: 1 - 30 Hz | 0.04 - 0.34 g

Lateral: 1 - 10 Hz | 0.04 - 0.34 g

Roll: 1 - 4 Hz | 0.23 - 2.62 rad/s2

• A DISC of 1 predicts that 50% of passengers will find the ride uncomfortable.

• Note: Leatherwood’s Noise and Duration corrections were not applied for the following results.

from “A Design Tool for Estimating Passenger Ride Discomfort Within Complex Ride Environments,” Leatherwood, Dempsey, and Clevenson, Human Factors, June 1980

the lateral 10-Hz weigting was applied to all data above 10 Hz

0 2 4 6 80

0.5

1

1.5

2

3.5

%

onset established


Dis

co

mfo

rt M

etr

ic

Fwd Cabin, onset

Mid Cabin, onset

Fwd Cabin, established

Mid Cabin, established


20

0 50 1000

1

2

3

4

Passengers Uncomfortable, %

Dis

co

mfo

rt M

etr

ic

Fwd Cabin

Calm Air

Peak In-Wake

Turbulence

0 50 1000

1

2

3

4

Passengers Uncomfortable, %

Mid Cabin

NASA Criteria

Lead Aircraft

• DISC metric values were calculated for lateral and vertical vibrations recorded at the forward and mid-cabin locations.

• DISC plotted vs. fuel flow reduction shows the gradual onset of wake discomfort below 3.5%.

• Above 3.5% the DISC is consistently high.

• Using the Leatherwood criteria, the peak DISC values calculated at the two cabin locations during wake surfing fall within the region of values measured for light turbulence.

• Even in calm air, the DISC values are quite high, suggesting this metric may over-predict passenger discomfort.

ISO-2631 Passenger Ride Quality Metric

21

• Frequency-weighted acceleration measurements are combined to form a Vibration Dose Value: VDV.

• The ISO metric addresses the following frequency ranges:

0.5 - 80 Hz: health, comfort, and perception

0.1 - 0.5 Hz: motion sickness

• VDV-based human comfort rating predictions increase with exposure time raised to the ¼ power.

• Motion sickness increases with the square root of the exposure time.

• The ISO standard gives a relationship between VDV and descriptive “likely reactions” in terms of comfort value.

ISO-2631 Passenger Ride Quality Metric

22

0 5 100

1

2

3

Exposure, hours

Vib

ratio

n D

ose V

alu

e

Fwd Cabin

Calm Air

Turbulence

Wake Surfing

0 5 100

1

2

3

Exposure, hours

Mid Cabin

0 2 4 6 8 100

5

10

15

Exposure, hours

Moti

on S

ickn

ess

Ra

te,

%

Calm Air

Turbulence

Wake Surfing

• The ISO metric predicts increased passenger discomfort due to wake surfing vs. calm air, although not as severe as light turbulence.

• Even the calm air predictions are solidly ‘uncomfortable’ for flights longer than 5 hours, which may indicate over-prediction of passenger discomfort by this metric.

uncomfortable

veryuncomfortable

extremely uncomfortable

• The ISO metric predicts no appreciable increase in passenger motion sickness due to wake surfing vs. flight in calm air, and significantly less motion sickness than flight through light turbulence.

Summary of the post-flight questionnaires:

• 9 participants (2 pilots, 6 engineers, 1 videographer); majority are frequent flyers

• Wake Surfing Comfort Response:• “Comfortable”: 45% (4 of 9)

• “Neutral”: 45% (4 of 9)

• “Uncomfortable”: 10% (1 of 9)

• 10% reported “Writing” would be difficult

• 33% reported “Sleeping” would be difficult

Comments:

• “Similar to light turbulence”

• “Rhythmic, pulsing sound - not unpleasant but noticeable”

• “Like driving over a slightly-washboarded road”

• “I found the view of contrails outside my window unsettling”

• “The appearance of the wake was larger than I had originally imagined”

Passenger Ride Quality Survey

23

Conclusions:

1. ADS-B is adequate for moderate wake surfing benefits.

2. Accurate wind estimates are critical for wake prediction.

3. Sustained fuel savings are possible above 5% for wake surfing at extended trail distances.

4. There is significant ride quality degradation at higher fuel flow savings.

5. Automatic control is a necessity, including throttles.

Recommendations:

1. Develop and test robust wake estimation, performance optimization, and wake-crossing prevention algorithms.

2. Through modeling and flight research, improve understanding of the causes of ride quality degradation.

3. Characterize wake strength, descent, and decay downstream of the trail airplane.

4. Develop routing and scheduling algorithms for civil operators, and meta-aircraft operations for air traffic control.

Summary

24

Questions?

B-757NASA SUCCESS Mission Video

DC-8NASA ACCESS Mission Video

Examples of Wake Dynamics

B-737© B. Whittaker

B-767A. Brown, AIAA 2007-289

Relative Navigation and Wake Prediction

Wake prediction functions in the autopilot compute a wind-corrected trajectory for the trail airplane. This trajectory is relative to the lead airplane’s wake.

Timing uncertainty in ADS-B messages results in larger errors in along-track vs. cross-track.

The trail airplane flies a wake-relative trajectory.

-50 0 50-250

-200

-150

-100

-50

0

50

100

150

200

Cross-Track Error, ft

Alo

ng

-Tra

ck E

rro

r, f

t

1-

2-

4%6%8%

Drag Reduction Error Budget

One knot of error in cross-track wind speed adds 10 ft of error to the predicted wake location.

N

Simplified Wake Location Prediction

ξ

ψ

Gt

Xtrack

Ltrackξ

ξ = ψ - Gt

+

+

+

+

++

+

+

+

+

+

+

+

+

++

+

+

Parallel ground tracks

Estimated Vortex

Formation Axis

N

ξ

WE

Despite good results in the

piloted sim, the pilots initially

found the throttle cues

“Unsatisfactory” in flight.

For the final flight, the pilot

along-track error cue was re-

designed with an increased

range of view, and a relaxed

acceptable error criteria.0 0.05 0.1 0.15 0.2

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Duty Cycle

Agg

ress

iven

ess

ACT Pilot Throttle Workload Metric

Config A Tare

Config A Wake

Config B Tare

Config B Wake

The modified display reduced the pilot

workload to “Satisfactory” and improved

post-flight calculation of fuel flow savings.

Pilot Throttle Cue and Wake Display

Documents

Recent NASA Wake Surfing Flight Research