Introduction to Aircraft Icing and NASA's Approach to Understanding It

Introduction to Aircraft Icing

and NASA’s Approach to Understanding It

Tadas Bartkus, Ph.D.

Ohio Aerospace Institute NASA Glenn Research Center – Icing Branch Cleveland Engineering Society Leadership Breakfast Series October 17, 2024 This material is a work of the U.S. Government and is not subject to copyright protection in the United States

Overview

• What is Aircraft Icing?

• Types of icing • Airframe icing (supercooled liquid) • Introductory icing physics • Engine icing (ice crystal) • Rotorcraft icing • NASA’s Approach to Understanding • Flight Tests • Wind tunnel Tests • Computational Tools • The Future of Aviation and Icing 2 2

What Is Icing And Why Is NASA Studying It?

What is Icing?

4 4

What is Aircraft Icing?

Windshields Propeller/Rotor Blades Wings and Control Surfaces External Probes/Antennas 5 5

Why NASA studies aircraft icing

• Challenges • Aircraft icing can be a safety hazard • Meteorological conditions that result in aircraft icing will • NASA’s role always be present • It is expensive for any single entity to study it and provide • An agency that can provide publicly solutions.

available models and validating datasets • Newer icing regulations will have large impacts on new • Mature testing facilities airframes and propulsion systems design, cost, and certification • Decades of institutional icing knowledge • Community enduring need is for NASA to continue performing foundational icing • Needs physics studies in the public to improve • Mitigate icing safety risks current capabilities • Reduce amount of expensive/dangerous icing flight testing • Improve computational icing tools to aid certification • Provide publicly available data to validate tools • Develop technologies like icephobics, low power and weight Ice Protection Systems

Airframe Icing - Supercooled Liquid Icing

Impact on Aerodynamic Performance

• What factors affect airframe icing?

▪ Water in the cloud (not ice) ▪ Temperature ▪ Size of the water droplets ▪ Type of clouds ▪ Airfoil geometry ▪ Airspeed ▪ Time the aircraft is in icing condition • What does ice do to an aircraft?

8 8

Where to find supercooled liquid icing clouds Cumulus – Large Drops Stratus – Small Drops

Physics of Supercooled Liquid Water

O T < 0 C air

T

particle O 1. Sensible cooling of liquid water to below the equilibrium freezing temperature (T = 0 C ) until homogeneous/heterogeneous freeze crystallization temperature ( T ) is reached. T dependent on droplet impurity content.

hc hc 2. Latent heat release of supercooled water where rapid crystallization of a particle occurs, along with an increase in particle te mperature.

O The particle will partially freeze and reach its equilibrium freezing temperature of 0 C.

O 3. Latent freezing where heat is transferred from the partially frozen particle at a constant temperature of 0 C until it has completely frozen.

4. Sensible cooling of the solid ice particle until the temperature is reduced to the wet - bulb temperature.

Glaze vs Rime Icing (Video)

Glaze Ice

Rime Ice

Glaze Rime

Clear/translucent Opaque Ice shapes protrude Streamlined ice from the leading edge shapes O O T < - 10 C T = - 2 to - 10 C 11 11

Impact on Aerodynamic Performance

2 min 6 min 22 min 12 12

Deicing Boot (Video) 13 13

Meteorological Requirements for Icing

Aircraft icing requires two conditions to occur: 1. Temperatures near or below freezing • Icing is more frequent when the static air temperature (SAT) is between +2 ° C and - 20 ° C.

2. Existence of water droplets • Liquid water must be present in the air for ice accretion to occur.

• Water in the form of vapor, snow, or ice will generally not stick to an airplane’s external surface and contributes little to the overall ice buildup.

• If there is sufficient liquid water in the air to pose an icing threat, it will generally be visible in the form of a cloud or liquid precipitation.

14 14

Meteorological Conditions that Influence

Icing Severity

The main meteorological conditions that influence icing severity include temperature , liquid water content (LWC) , and droplet size .

Temperature • Temperature effects both severity and type of icing.

• Most icing occurs between 0 ° C and - 20 ° C. Icing is not severe below - 40 ° C because this is the temperature droplets freeze without an icing nuclei.

Liquid Water Content (LWC) • Defined as the density of liquid water in a cloud expressed in grams of water per cubic meter (g/m ) • In general, the higher the LWC, the greater the icing severity.

Continued… 15 15

Meteorological Conditions that Influence

Icing Severity

Droplet Size • Droplet diameter is usually expressed in microns (μm) and represented by an average value called Median Volumetric Diameter (MVD) .

• Droplet size affects the collection of water drops on the surface of the aircraft; smaller drops tend to impact near the leading edge of a wing whereas larger droplets can impact farther aft.

16 16

Non - meteorological Factors that Influence

Icing Severity

Airspeed and collecting surface geometry are important non - meteorological factors that influence icing type and severity.

Airspeed • The higher the airspeed, the higher the rate of ice accumulation.

• Airspeed also determines where ice collects on a surface; higher airspeeds allows for more droplets to impinge farther aft on an airfoil.

Collecting Surface Geometry • Often described by the radius of curvature of the leading edge.

• Surfaces with smaller radiuses of curvature have greater collection efficiency's and thus collect more droplets.

17 17

Collection Efficiency ( β )

Not all water droplets in the free air stream will strike the airframe. Droplets with small inertial forces will closely follow the streamlines and avoid striking the airframe.

Free Air Stream The ratio of the water mass striking the airframe to the water mass in the free airstream is known as the Collection Efficiency ( β ) .

➢ As collection efficiency increases, so will the rate of ice accretion .

18 18

Collection Efficiency ( β )

Not all water droplets in the free air stream will strike the airframe. Droplets with small inertial forces will closely follow the streamlines and avoid striking the airframe.

Miss!

Miss!

Hit!

Miss!

Miss!

The ratio of the water mass striking the airframe to the water mass in the free airstream is known as the Collection Efficiency ( β ) .

➢ As collection efficiency increases, so will the rate of ice accretion .

19 19

Collection Efficiency ( β )

Collection Efficiency ( β ) is influenced by: • Airspeed (increase in airspeed = increase in β ) • Droplet Size (increase in droplet size = increase in β ) • Collecting Surface Geometry (smaller radius of curvature = increase in β ) Miss!

Hit!

Hit!

Hit!

Miss!

Due to having greater inertia, droplets with higher mass and/or velocity are more ballistic and more likely to strike the object.

20 20

Dry - Bulb Temperature vs

Wet - Bulb Temperature

Water droplet (small) Metal ball Convection Evaporation Convection T = T = f( T , RH, P) T = T = f( T ) SS WB air SS DB air T = Steady State Temperature, T = dry bulb temperature, T = wet bulb temperature SS DB WB If RH = 100% → No evaporation → T = T WB DB If RH < 100% → Evaporation → T < T WB DB 21 21

Dry - Bulb Temperature vs Wet - Bulb Temperature

Metal ball Water droplet (small) Convection Convection Evaporation ( dT driven) ( dT driven) (dH O vapor conc. driven) O T 20 C O air = O T 50 C O T 20 C ball = air = T = 50 C RH = 60% droplet dT = 0 so Evaporation Steady State = no Convection No energy flux O O T 20 C = T T = 20 C ball = dry bulb droplet Evaporation Convection Steady state (thermodynamic Convection = Evaporation equilibrium) O T = 15 C = T droplet wet bulb

Engine Icing – Ice Crystal Icing

Where to find ice crystals

High Clouds – Ice Crystals

Engine Cross - Section

174 reported jet engine icing events worldwide and documented in Boeing database as of January 2019 Bravin, SAE Technical Paper 2019 - 01 - 1964 ice crystals Generic Engine Compression System

Engine Icing (Video) 26 26

Engine Icing Tests

Honeywell ALF02 LF11 Engine icing test Generic engine schematic EGV Flow EGV 27 27

Engine Icing Tests (video)

Honeywell ALF02 LF11 Engine icing test Generic engine schematic EGV Flow EGV 28 28

Engine Icing Tests

Goal: Understand the mechanisms of ice crystal icing inside engine core flow path Engine icing tests provide valuable data but… • Difficult to instrument jet engine with probes and cameras • Difficult to quantify accreted ice mass Typical Performance Data Ice Accretion in the Engine Rollback Indicators - FLT850 EGV Flow EGV Average Load (lbf) Temperature (F), %N, Cloud EGV Cloud ON Cloud OFF Time EGV1LE LF11 EGV1LE LF01 EGV1TE LF11 EGV1TE LF01 EGV2LE LF11 EGV2LE LF01 EGV2TE LF11 EGV2TE LF01 cloud LF11 cloud LF01 N2 LF11 N2 LF01 29 29 OS_AVG LF11 OS_AVG LF01 N1 LF11 N1 LF01 Load LF11 Load LF01

Component - level Icing Test with SIDRM Test Article

SIDRM = S imulated I nter - compressor D uct R esearch M odel Thin film surface heaters

Ice Crystal Icing Tests with SIDRM Test Article

Results In 2022 , conducted 61 icing tests, providing validation data for 3 D computational icing tools .

Significance These tests represent the first time that ice accretions were measured on a 3 D test article at IRT . It also marks at first time that ice crystal ice accretions were generated utilizing heated surfaces . Using an open geometry configuration, these tests recreated ice crystal icing features characteristic of those seen in previous engine icing tests .

Supercooled liquid ice Ice crystal icing utilizing accretion on SIDRM SIDRM’s heated panels https://arc.aiaa.org/doi/abs/10.2514/6.2022 - 3700 https://ntrs.nasa.gov/citations/20220006460

Rotorcraft Icing

Rotorcraft Icing

➢ Rotorcraft are often required by mission objectives to operate in icing conditions for prolonged periods of time.

Military Operations Search and Rescue Urgent Transportation Operate at lower altitudes ➢ Current certification process is time consuming and expensive.

Flight campaigns in natural icing conditions Simulated icing conditions using a tanker ➢ Conventional electrothermal deicing systems can be unreliable and consume a lot of power.

➢ Severe vibration or damage can occur due to ice shedding from rotor blades.

33 33

Rotorcraft Icing Studies 34 34

Rotor Ice Shedding 35 35

Characterization of Low Ice Adhesion Materials

PROBLEM In order to develop effective ice - resistant coatings for aircraft, we must first have a consistent, reliable method for quantifying adhesion strength of accreted ice.

OBJECTIVE Design, fabricate and demonstrate robust, efficient adhesion test method(s) which can be used in our facilities.

APPROACH Utilize various test methods: Centrifugal test, lap joint method.

ACCOMPLISHMENTS Determined factors driving the ice adhesion strength are icing cloud conditions and stress concentrations.

SIGNIFICANCE Evaluate coatings. Develop adhesion strength database. Exploring the Ice adhesion shear test using lap joint rig effect(s) of grain structure on ice adhesion.

Shear test sample and accreted ice grain structure Centrifuge shear test and ice coupon on centrifuge arm

NASA’s Approach to Understanding Icing

NASA’s Approach to Understanding Icing

Wind

Tunnel Tests

Simulate the Environment

Flight Tests

Computational

Understand the

Tools

Environment Model the Environment 38 38

Icing Flight Tests

High Ice Water Content Flight Test Campaign

Problem • Turbofan engine power - loss or damage events have been attributed to the ingestion of ice crystals .

Objective • Measure ice crystal cloud properties in high - aerosol environment to determine globally representative and data - driven engine certification envelope limits .

Approach • NASA's DC - 8 , instrumented with icing cloud and aerosol sampling instruments, Early morning takeoff. Inset shows icing was flown at high altitudes through deep convective clouds and below clouds to instruments on DC - 8 wing pylon measure the aerosols that interacted with cloud and ice crystals .

• Radar development to identify and avoid hazardous icing conditions Fl ight - track through regions identified for ice cloud measurement. West to East runs were Results approximately 100 miles in cloud • Research flight campaign conducted with over 60 flight - hours of data collected .

Significance • Once the data is analyzed, the regulations for ice crystal icing certification will be assessed and updated for engine certification and operability .

• Measured conditions in Nature used to recreate icing conditions in ground - based icing wind tunnels

Icing Wind Tunnel Facilities: Icing Research Tunnel Propulsions Systems Laboratory Adaptive Icing Tunnel

Icing Research Tunnel (IRT)

Icing Research Tunnel

Icing Research Tunnel Quick Facts Test Section 6 ft tall by 9 ft wide by 20 ft long Airspeeds 50 to 300 knots Temperatures As low as - 35 ° C

IRT Test Section

Droplet Size 15 to 275 microns Water Content 0.15 to 4.0 g/m 43 43

Icing Research Tunnel (IRT)

Spray Bars (Video) 44 44

Laser Scanner Technology

Document ice accretion geometry with high fidelity 45 45

Propulsion Systems Laboratory (PSL)

Propulsion Systems Laboratory (PSL)

PSL - 3

Specification Min Max Engine / Rig Dia. (in | cm ) 24 | 60 _ 72 | 180 Air Flow Rate (lbm/s | kg/s ) 10 | 5 _ 330 | 150 Altitude, pressure (kft | km ) - _ 4 | 1.2 50 | 15 Total Temp ( ° F | ° C ) - 60 | - 50 50 | 10 Mach Number 0.15 0.80 TWC ( g/m ) 0.5 8.0

47 Spray Bars

Particle Size - MVD (um) 15 >100

Propulsion Systems Laboratory (Video – 1 min) 48 48

Adaptive Icing Tunnel (AIT)

What is the Adaptive Icing Tunnel (AIT)

• Closed loop, vertical refrigerated icing wind tunnel Lower - cost capability for instrument evaluation and proof - of - concept testing.

• Test section: • 1’ x 1’ cross section • 2’ long • Flow speeds of ~210 knots (~110 m/s) • Temperatures as cold as - 20 ° C • Walk - in freezer surrounding test section • Scheduled for installation in FY23 followed by tunnel characterization

Icing Tunnel Cloud Measurement and Calibration

IRT Calibration and Maintenance

• SAE's ARP5905 "Calibration and Acceptance of Icing Wind Tunnels" • Regular calibrations • Interim and full calibrations • Statistical Analysis and Statistical Process Control

Calibration Instrumentation

• Uniformity • 6ft x 6ft grid, extends floor to ceiling, mesh elements spaced every 6 in, digital calipers used on vertical elements to measure ice thickness • Liquid Water Content • Multi - Element Sensor ("Multi - Wire"), Science Engineering Associates, Inc., 3 sensing elements of different size, designed for response of varying conditions • Icing Blade, 1/8" x 6" x ¾" stainless steel blade used to confirm accuracy of MW

Calibration Instrumentation

• Drop Size • Cloud Droplet Probe (CDP), 2 - 50 um • Optical Array Probe, OAP - 230X, 15 - 450 um • Optical Array Probe, OAP - 230Y, 50 - 1525 um • Drop size distributions from the CDP are combined with the OAP - 230X and OAP - 230Y to calculate the particle size distribution Particle size measurements

Cloud Characterization Instrumentation

Test Section Instrument Measurement Multiwire (MW) Melt ratio & recirculation Isokinetic Probe 2 (IKP2) Total water content & recirculation Rearward Facing Probe (RFP) Air temperature and humidity Particle Imaging – Ice Crystal probe Particle size distribution Particle Tracking Velocimetry probe Particle velocity TAT probe (TAT) Air temperature Ice Detector probe Glaciation (liquid presence) Background Humidity Bent Tube probe (BHBT) Humidity Light Extinction Probe Recirculation

Flow

Various probes installed in the NASA IRT test section during Ice crystal cloud characterization tests in Feb 2022

NASA Computational Icing Tools

Icing Computational Tools

Goal : Develop a system of codes that can model the accretion process in current and future engine and airframe designs and characterize the accretion risk Sample Output • Ice shape geometry • Collection efficiencies • Heat transfer values • Surface Temperatures 57 57

NASA Icing Tools

LEWICE • 2D tool that evaluates the freezing process thermodynamics that occur when super - cooled droplets impinge on a body and generate a 2D ice shape.

LEWICE3D • Quasi - 3D tool that computes the trajectories and impingement in 3D but uses a strip - theory assumption to compute the mass/energy balance and ice growth on user specified cut planes.

COMDES - MELT: A Turbofan Engine Icing Risk Analysis Tool • Mean - line compressor analysis code coupled with an ice crystal thermodynamic state code.

TADICE • One dimensional (1D) numerical model simulates icing wind tunnels by modeling the thermodynamic interactions between the water/ice particles of an icing cloud and the flowing air.

GlennICE • 3D ice accretion simulation; multi time step . Currently the most supported icing code.

The Future of Aviation and Icing

Transonic Truss - Braced Wing

High aspect wing ratio can enable fuel burn benefit of 5 - 10% High - Speed Unique Noise Key Icing Objectives: Buffet Sources • Provide a validated capability to determine the High - Lift Integration impact of ice accumulation on the Transonic Truss V4 2/18/21 Thin - Wing Design Braced Wing aircraft • Reduce conservatism while maintaining safe operation in an icing encounter Icing • Identify potential impact of ice protection system Protection on fuel burn objective Critical Structural Challenges • Develop of potential materials that are both durable Joints and icephobic

Advanced Air Mobility (AAM)

NASA’s Vision for

Advanced Air Mobility

Advanced Air Mobility (AAM): An air transportation system that moves people and cargo between places previously not served or underserved by aviation.

Why AAM?

• Decreased surface traffic congestion • Fast and reliable transportation 61 61

Planned AAM Icing Tests

Problem • AAM vehicles will need to be certified for safe flight Hub Ø 36” in icing conditions. Icing engineering tools and Angle max blade methods for means of compliance are mature for 0 - 95 ° diameter existing legacy aircraft but there are no accepted icing engineering tools specifically developed for AAM class vehicles.

6 - Axis Load Cell Objective • Develop an experimental and computational icing simulation capability for AAM vehicles.

Experimental Test Plans • Simulate an Advanced Air Mobility representative rotor in forward flight conditions in the IRT 24 - 36” • Acquire rotor performance data in icing conditions • Characterize ice accretion on rotor (3D ice scans, ice mass measurements)

AAM Rotor Test Stand

Thank You 63 63