understanding high advance ratio flight

Alfred Gessow Rotorcraft Center

University of Maryland

Understanding

High Advance Ratio Flight

Graham Bowen-DaviesGraduate Research Assistant

Adviser: Inderjit Chopra

Alfred Gessow Professor and Director



Name: Graham Bowen-DaviesBorn in: Bulawayo, Zimbabwe (1984)

Never snowsWe had one Winter

Olympian

I was born here

Population: 11 MillionTemperate: 50°F-90°F



Name: Graham Bowen-DaviesBorn in: Zimbabwe (1984)High school: St. Georges College

Harare (Capitol City)

Jesuit collegeOldest all boys school (115)



Name: Graham Bowen-DaviesBorn in: Zimbabwe (1984)High school: St. Georges CollegeUnder Graduate:

Mechanical Engineering Degree University of Cape Town

2003-2007



Name: Graham Bowen-DaviesBorn in: Zimbabwe (1984 )High school: St. Georges CollegeUnder Graduate:


2003-2007Currently working on:

Ph.D. in Aerospace EngineeringUniversity of Maryland

2008 – 2014?)

Proud team leader of the Gamera Human Powered Helicopter Team



Name: Graham Bowen-DaviesBorn in: Zimbabwe (1984 )High school: St. Georges CollegeUnder Graduate:


2003-2007Currently working on:

Ph.D. in Aerospace EngineeringUniversity of Maryland

2008 – 2014?)

Proud team leader of the Gamera Human Powered Helicopter Team

My research topic is:

Investigating the Aeromechanics of Variable Rotor Speed (With some morphing rotor studies)

Motivation

Helicopters are very good

(at what they do)

Insertion and extraction News and reporting

Resupply and support Search and rescue

What about high speed?

Helicopter: Westland Lynx – G-LYNX• Max Speed: 216 knots• Year 1986

G-LYNX

Fixed wing: Curtis R2C - 1• Max Speed: 224 knots• Year 1923

Fixed wing: Lockheed SR-71 Blackbird• Max Speed: 1905 knots• Year 1976

Helicopter Aerodynamics 101:

μ = 0.0

Hover

μ = 0Vadv = ΩR

Vret = ΩR

Hover

Cruise

μ = 0.2 (100 kts)Vadv = ΩR(1 + μ)

Vret = ΩR(1 – μ)


μ = 0.2

Hover

Cruise

μ = 0.40 (170 kts)


μ ~ 0.4

Vadv = ΩR(1 + μ)

Vret = ΩR(1 – μ)

Retreating blade stall- Vibrations and power

Lift limited on retreating side- No longer able to trim

Compressibility- Power requirements rise- Vibrations increase dramatically

Increasing parasitic drag- Limited installed power


Roadblocks at High Advance Ratio


Slowed rotor solution

μ = 0.5 (220 kts)

Vret = ΩR - μVret = 0

μ = 1.0 (220 kts)

100 % RPM 50 % RPM

Vadv = ΩR(1 + μ) Vadv = ΩR(1 + μ)

V

Lift

Slowed Rotor Helicopters

XH-51A (1965)

X2TD(2010)

V-22 (1989)

260 knotsSlowed the rotor by 5%Stability issues

250+ knotsSlowed the rotor by 20%

Limited payload

Goals of this research are:1) Understand the performance and loads at high

advance ratios

2) Validate predictive capability with test data

Description of Rotor

Articulated4 BladesRadius: 2.78 ft (1/9

th of UH-60)Rotor speed: 2300 RPMMach No: 0.6

Tests RunRotor speed: 30%Advance Ratio: 1.4

Comprehensive analysisBased on the analysis code – UMARC

CapabilityStructural Model: Elastic – Flap, Lag and TorsionAerodynamics: Table look upInflow model: Freewake (Bagai-Leishman)Solution type: Periodic, steady state trim

Additional CapabilityTrailing edge flap and leading edge slatsVariable radius and RPM modelingCoaxial (in progress)Stability analysis, bearingless rotor

Analysis Description

Lift Distribution

Results: High advance ratio

0° Shaft Tilt, 0° Collective

Why is the model generating thrust at zero collective?What is the analysis missing?

Wind tunnel tests

UMARC analysis

Thru

st C

T/σ

Does the fuselage matter?

UH-60Maryland

No fuselage model

No fuselage model

Both the Maryland and UH-60A thrusts are underpredicted

Fuselage Representation

Results on HART rotor by Amiraux et al. (2013)• Uniform upwash over fuselage nose• Assume linear increase with advance ratio• Calibrated to HART rotor result

Upwash in rotor plane:μ = 0.15αs = 4°

Does the fuselage matter?

UH-60Maryland

No fuselage model

With fuselage model

Fuselage model improves thrust correlationA higher fidelity model of the fuselage is important for low thrusts.

Thrust vs. Advance Ratio

ExperimentAnalysis

Shaft angle = 4°

0° CollectiveAdvance Ratio = 0 – 1.4

Shaft angle

Thru

st, C

T/σ

0 knots 170 knots

Shaft angle = 2°

Analysis does very well for aft shaft tilt.Fuselage is less important for high thrusts and aft shaft tilt.

Results: Thrust Reversal

UM2013

Analysis

Thrust Reversal Prediction

αs = 0°, μ = 0.25

μ = 0.25

Normal force map

Stall map

Angle of attack below stall

Angle of attack above stall

V

V

UM2013

Analysis


αs = 0°, μ = 0.41

μ = 0.41

Stall map

Airfoil unstalled in reverse flow

Angle of attack above stall

V

V

V

Lift

UM2013

Analysis


αs = 0°, μ = 0.58

μ = 0.58

Stall map


V

V

UM2013

Analysis


αs = 0°, μ = 0.82

μ = 0.82

Stall map


V

V

UM2013

Analysis


αs = 0°, μ = 1.02

μ = 1.02

Stall map


V

V

Summary

The University of Maryland Advanced Rotorcraft Code (UMARC) has been developed to understand high advance ratio aerodynamics

Fuselage modeling can be importantAt low thrusts and for small shaft angles

Thrust is well predicted up to an advance ratio of 1.4

Thrust reversal at high advance ratio is predicted wellReverse flow airfoil characteristics are important

Summary

Future Work

Continue to evaluate the Maryland wind tunnel dataPressure gauge readingsBlade bending momentsNew wind tunnel test

Compare UH-60 and Maryland rotors at high advance ratio

Incorporate a higher fidelity fuselage model

Acknowledgements

Ben Berry for performing the wind tunnel tests and making the data available.

Dr. Tom Norman for making the UH-60A data available.

understanding high advance ratio flight

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