Proposal – Analysis of Ice Accretion Physics for Rotorcraft Engine Inlet
Systems
by
Daniel Shields
Approved:
_________________________________________
Ernesto Gutierrez-Miravete, Thesis Adviser
Rensselaer Polytechnic Institute
Troy, New York
May 2010
(For Graduation August 2010)
© Copyright 2010
by
Daniel Shields
All Rights Reserved
ABSTRACT
The purpose of this thesis is to explore the effects of supercooled water drops impinging
on a rotorcraft engine inlet. An analytical model of an existing rotorcraft engine inlet will
be created and analyzed in 2D and 3D using LEWICE accretion computer code for
comparison to wind tunnel data taken from The National Research Council (NRC)
Propulsion Icing Wind Tunnel for the S-76D program. The resulting ice accretion data
will then be compared to existing icing adhesion properties gathered from the references
below to determine the amount of force/displacement to deice the surface. A structural
analysis of the engine inlet system using the structural analysis code ANSYS will
determine the amount of mechanical energy an electro-expulsive device would need to
apply to the composite inlet components to induce ice shedding.
Introduction
The formation of ice on aircraft is the most critical natural hazard affecting the safe
operation of aircraft and it has been a concern since the early days of aviation. Several
rotorcrafts are certified for operation in forecast icing conditions including the Sikorsky
S-92A and the Agusta Westland AW139; however, the method of protection may differ
greatly. The design and development of technologies that will protect rotorcraft depends
upon knowledge of atmospheric conditions, icing properties, water drop kinematics, and
characteristics and heat transfer and fluid flow mechanics.
Accretion of ice on aircraft surfaces is a result of a tendency for water drops in stratiform
and cumulus atmospheric clouds to stay in a liquid state even at temperatures as low as 40°C. These drops, referred to as supercooled drops, will crystallize in the presence of a
seed crystal such as another ice crystal, snow flake, or dust and dirt. In the absence of
any seeds, the water drops can remain in the liquid phase until they impinge on aircraft
structure and begin to freeze forming an ice accretion. The accretion is mainly governed
by the heat transfer, which includes kinetic heating, convective cooling, evaporative
cooling, and latent heat of freezing, from the aircraft surface. The two major types of ice
accretion considered are rime ice and glaze ice.
Rime ice occurring at very cold
temperatures is where the water freezes almost instantaneously resulting in smaller ice
accretions in the general vicinity of the impact.
Glaze icing occurs at warmer
temperatures. For glaze ice accretion, the supercooled droplets do not freeze instantly
and some of the water will flow along the surface forming a larger icing ridge or a double
horn shape.
Supercooled droplet trajectory calculations are used to determine where and how much
ice will accrete on a surface. The volume of water that impacts the aircraft surface and
freezes is a function of the efficiency that the body that collects the water or water catch
efficiency, the liquid water content of the cloud, the diameter of the drops, the speed of
the body as it moves through the cloud, ambient pressure and temperature. These terms
are some of the inputs used in ice prediction models to determine ice accretion.
LEWICE, first developed in the 1980’s, uses the above parameters along with Lagrangian
droplet trajectory calculations to determine the path of the droplet as it impacts the
structure. Using a CFD flow solution the droplet trajectory is calculated at the discrete
node positions of the flow solution using Newtonian physics of kinematics. The second
part of the calculation involves an energy balance at the structure surface to determine the
rate of freezing and amount of run back. The energy balance at the surface includes
terms for, heat lost due to convection, sublimation and sensible heat absorbed by the
warming of the drop, and heat gained due to the latent heat of fusion, frictional heating
and kinetic heating. The resulting energy balance accompanied with a mass balance
determines the overall ice shape on the structure.
The detailed physics of how accreted ice adheres to an aircraft surface is not well
understood. There is, however, some experimental data available which helps describe
the measurements of the adhesive and cohesive properties of ice. The measurements
include the shear force required to break the bond between the ice and surface.
The airframe structure is a composite based carbon fiber laminate with Nomex
honeycomb core panels. The analysis of the composite structure will assume that the
laminate is assembled from plates and shells. Plate and shell structures are more efficient
when carrying membrane loads. ANSYS finite element code will be used to analyze the
orthotropic composite structures at the lamina level to determine the amount of force and
deflection required to break the bond of ice.
Problem Description
The intent of this thesis is to correlate existing icing wind tunnel data from a rotorcraft
engine inlet icing test with results from NASA’s ice accretion code LEWICE in 2D and
3D for rime and glaze icing conditions. From these data, an expulsive protection system
will be designed by first determining the ice adhesion properties based on recorded
experimental data. The composite engine inlet structure will then be analyzed using
ANSYS finite element code to determine the force and deflection required to break the
bond between the ice and structure.
Methodology
2-dimensional inlet lip modeling:
Preliminary research will be conducted to determine typical methodology in ice accretion
code to determine the effectiveness of LEWICE. An inlet lip cross section of the ice
accretion from a wind tunnel experiment was measured and will be replicated using
LEWICE 3.2.
3-dimensional inlet lip modeling:
Upon completion of the 2-dimensional modeling, the same inlet geometry will be
analyzed using LEWICE 3D in an attempt to data match test results for rime ice and
glaze ice conditions.
Inlet lip protection:
Using a combination of the analytical and experimental results, an expulsive protection
system will be designed by determining the stiffness of the composite airframe structure
using ANSYS finite element code and then evaluating the adhesion properties of the ice
to determine if the ice will shed.
Milestones
Preliminary research (17 Feb 10)

Typical ice accretion physics

Ice accretion modeling code set-up

Expulsive ice protection systems

Selection of two experimental results for analysis
2-D LEWICE development (17 Mar 10)

Develop cross section of inlet lip for analysis

Mesh of inlet lip geometry

Run 2-D analysis
3-D LEWICE development (26 Mar 10)

Develop engine inlet lip geometry

Mesh of inlet geometry

Run 3-D analysis
Develop expulsive protection system (16 Apr 10)

Identify ice adhesion properties for structure based on surface roughness

Size expulsive device for composite inlet structure
Thesis submittal (28 Apr 10)
References
Gent, R.W., Dart, N.P., Cansdale, J.T., “Aircraft Icing,” Philosophical Transactions:
Mathematical, Physical and Engineering Sciences, Vol. 358, No. 1776, November 2000,
pp. 2873-2911.
Messinger, B.L., “Equilibrium Temperature of an Unheated Icing Surface as a Function
of Air Speed,” Journal of The Aeronautical Sciences, Vol. 20, No. 1, January 1953, pp.
29-42.
Naterer, G.F., “Energy Balances at the Air/Liquid and Liquid/Solid Interfaces With
Incoming Droplets at a Moving Ice Boundary,” International. Journal of. Heat and Mass
Transfer, Vol. 29, No. 1, 2002, pp. 57-66.
Myers, T.G., Hammond, D.W., “Ice and Water Film Growth from Incoming Supercooled
Droplets,” International Journal of Heat and Mass Transfer, Vol. 42, 1999, pp2233-2242.
Raraty, L.E., Tabor, D., “The Adhesion and Strength Properties of Ice,” Proceedings of
the Royal Society, Vol. XXA245, No. 1241, June 1958, pp. 184-201.