Electric Aircraft SymposiumMay 23, 2007

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Benefits of Electric Propulsion

• Public Policy:– Diversion from use of gasoline– Reduced noise (radiated)– Enhanced safety and performance– Reduced air pollution– Further increase efficiency of energy use

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4

ConceptSource: Don Galbraith, 1979

High Lift Technology - Wind Turbine Technology

C.P. (Case) van Dam

Department of Mechanical and Aeronautical Engineering

UC Davis

6

Outline

• Introduction • High-Lift Technology• Wind Turbine Technology• Concluding Remarks

7

Historical Jet Fuel Prices

Price information as of 15 May 2007:

• NY Harbor $2.0904

• US Gulf Coast $2.0479

• LA $2.1179

• Rotterdam $1.9919

• Singapore $1.9321

8

Fuel Share of Direct Operating CostSchrauf (2006)

Fuel: $1.80/US gallon

Trip length = 6000nm,

Fuel: $0.60/US gallon

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Impact of Fuel Efficiency

• Impact of fuel on aircraft operating cost is significant and is main component of DOC for long missions at current fuel prices

• Reduction of fuel usage also has beneficial on air pollution with X% reduction of fuel burn for a given mission resulting in X% reduction in emissions– European Aeronautics Vision 2020 includes:

• 50% reduction in fuel consumption (i.e., 50% reduction in CO2)• 80% reduction in NOx

• 50% reduction in perceived noise

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0

20

40

60

80

100

Trip Fuel per unit Distance

Current technology status

Aerodynamics

EnginetechnologyStructures,materials,systemd

L/D SFC W

36 %

23 %

8%

Potential Reduction in Fuel ConsumptionSzodruch&Hilbig (1998)

Conclusion: > 50% reduction in fuel consumption is feasible

Time frame ~ 20 years

Trip Fuel/Distance ~W/(L/D)SFC/M∞

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Potential Improvement in Aerodynamic EfficiencySchrauf (2006)

Conclusion: Total improvement of 25% in L/D is feasible

Shock controlNovel configurations

Shape optimizationAdaptive wing devicesWingtip devicesLoad control

Laminar flow technologyTurbulence controlFlow separation control

-3%

-7%

-15%

Drag Decomposition

Technologies Drag ReductionPotential

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Breguet Range Equation for Transport Jets

Range performance of a transport jet at constant lift coefficient and speed (cruise-climb cruise) is governed by:

where: a∞ = speed of sound

TSFC = thrust specific fuel consumption

M∞ = Mach number

L/D = airplane lift-to-drag ratio

W0 = initial weight

W1 = final weight = W0 - fuel weight

€

R =a∞

TSFCM∞

L

Dln

W0

W1

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Performance Characteristics of Transport Jets in Cruise

Note:• Application of laminar flow reduces CD0

and, hence, CL(CDmin)

• Increase in aspect ratio, AR, or Oswald’s efficiency factor, e, increases CL(CDmin

)• Combined effect necessary in order to maximize beneficial effects

of laminar flow and high AR for given cruise condition and wing loading

€

T = D = CD1

2 γp∞M∞2S

W = L = CL1

2 γp∞M∞2S

€

T

W

⎛

⎝ ⎜

⎞

⎠ ⎟min

=1

(L /D)max

Simple parabolic drag polar:

Minimum required thrust condition:

€

CD = CD0+ CL

2 /(π ⋅ AR ⋅e)

€

CL(CDmin) = CD0

⋅π ⋅ AR ⋅e

CDmin = 2CD0

L

D

⎛

⎝ ⎜

⎞

⎠ ⎟max

=1

2

π ⋅ AR ⋅eCD0

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Aerodynamic Efficiency Evolution of Long-Range CTOL AirplanesAirplane range > 4,500 n.mi., Callaghan & Liebeck (1990)

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Design Space for Long-Haul CTOL TransportsInitial cruise conditions: M∞ = 0.80 at h = 30,000 ft (SA)

0

20

40

60

80

100

0 50 100 150 200 250 300 350

Wing Loading W/S, lb/ft2

ConventionalC

Do = 0.0145

AR = 7.5e = 0.80(L/D)

max = 18.0

W/S = 147 lb/ft2

20

15

10

AR.e = 5

CDo

= 0.0025

0.0050

0.0100

0.0150

0.0200

Increased effective aspect ratio

Reduced zero-lift drag

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Design Space for Personal Air VehiclesInitial cruise conditions: h = 8,000 ft at 200 KTAS (SA)

0

20

40

60

80

100

0 20 40 60 80 100

Wing Loading W/S, lb/ft2

20

15

10

AR.e = 5

CDo

= 0.0025

0.0050

0.0100

0.0300

0.0200

Increased effective aspect ratio

Reduced zero-lift drag

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CurrenttechnologyW/S and (L/D)

max

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Stall Speed and Maximum Lift for GA AircraftHolmes (1982)

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Modern PAV DesignLancair Legacy, Source: http://www.cafefoundation.org/

(W/S)TO = 26.66 lb/ft2

MTOW = 2,200 lb

S = 82.5 ft2

CLmax,clean = 1.51

CLmax,flaps = 2.35

Vs = 56.9 kts

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PAV Design Observations

• To achieve higher performance plateaus, higher wing loadings are required.

• To maintain acceptable stall speeds (61-knot rule), higher maximum lift coefficients are required.

• Lower viscous drag (lower CDo) can significantly

mitigate the negative impact of higher wing aspect ratio on wing loading.

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Evolutionary Versus Revolutionary Progress

Value

Time

• Technical progress for a product tends to occur along an S-curve

• The S-curve models the evolutionary changes in technology

• Leaps in technology are modeled in the form of jumps between S-curves

• These jumps model revolutionary changes in technology

• Leaps in technology are needed to extend the capabilities of the product to a new level

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Circulation Control Wing (CCW) ConceptLinton (1994)

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Circulation Control Airfoils and Wings

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Effect of Circulation Control on Liftt/c = 0.17 supercritical airfoil, Englar et al (1993)

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Modifications to A-6/CCW AirplaneNichols (1979)

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Trimmed Lift Curves of A-6/CCW AirplaneNichols (1979)

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Flight Test Results of A-6/CCW AirplaneNichols (1979)

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CCW Concept Advantages and Disadvantages

• Advantages– Higher maximum lift than obtainable with conventional multi-element

mechanical high-lift systems resulting in:• Improved takeoff/landing performance• Higher design wing loadings and, hence, improved cruise

performance– Reduced complexity of high-lift system– Reduced complexity of control system by combining high-lift, direct-

lift control and roll control into single multipurpose pneumatic surfaces

• Disadvantages– Reduced efficiency due to (bleed-)air requirements– Airplane trim requirements may significantly reduce attainable

maximum lift coefficient

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Wind Turbine Technology

• Wind-based electric energy has become cost effective• Cost effectiveness has spurred large companies (FPL, GE, Siemens, etc.) to

enter the market• Push to drive cost of energy down has resulted in much larger wind turbines• Square-cube law (rotor power goes by diameter-squared, rotor mass goes

by diameter-cubed) has resulted in significant lighter composite rotor structures

• Modern rotors have efficiency of approx. 52% vs. theoretical maximum of 59.7%

• Modern rotors use variable speed to maximize efficiency and variable pitch to control torque/power

• Technology developments are focused on lighter structures for given energy capture or increased energy capture through longer blades for unchanged rotor mass

• Technology is bifurcating:– Land-based utility-scale wind turbines (blade length ≤ 45 m, P ≤ 3 MW)– Off-shore wind turbines (P ≥ 5 MW)– Wind turbines for residences and small businesses (P = 0.5 - 50 kW)

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Trend inTurbine Size and Power Output

S. Johnson

QuickTime™ and aMotion JPEG OpenDML decompressor

are needed to see this picture.

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Innovations in Utility Scale Wind Turbines

Advanced Drive Trains

Advanced Blades

Advanced Tower Designs

Jack-up concept Telescoping concept

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Design Advancement Opportunities Source: Ashwill, SNL

– Load alleviation • Passive - twist coupling

– Sweep twist• Active devices - microtabs

– Performance enhancement & control devices

• Pitch – collective & individual• Flaperons, ailerons• Active devices• Variable diameter rotor

– Efficient internal blade architecture• Anti-buckling concepts

– Slender blades– Integrated structural/aerodynamic design

• Thickened airfoils

– Safety factor shakeouts

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Conclusions

• It is timely to (re-)consider electric aircraft• In order for these vehicles to achieve acceptable flight

performance, they must have high L/D, and capable of generating high lift coefficients

• Circulation control wing concept is proven technology to provide high lift performance

• As we move forward and reduce our oil dependency and emissions, wind power may be able to provide a large percentage (>20%?) of electric energy needs in USA

• Wind turbine technology is rapidly becoming very sophisticated