U5AEA15 AIRCRAFT STRUCTURES-II

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U5AEA15 AIRCRAFT STRUCTURES-II
PREPARED BYMr.S.KarthikeyanDEPARTMENT OF
AERONAUTICALENGINEERINGASSISTANT PROFESSOR
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We have looked at..

• Airfoil Nomenclature
• Lift and Drag forces
• Lift, Drag and Pressure Coefficients
• The Three Sources of Drag
• skin friction drag in laminar and turbulent flow
• form drag
• wave drag

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Airfoil Drag Polar Cd vs. Cl
Rough airfoils have turbulent flow over them,
high drag.
Smooth airfoils have laminar flow over at least a
portion of the surface. Low Drag.
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Form Drag
Form drag may be reduced by proper design,
and streamlining the shape.
Source http//www.allstar.fiu.edu/aerojava/flight
46.htm
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Supersonic wave Drag
For a given airfoil or wing or aircraft, as the
Mach number is increased, the drag begins to
increase above a freestream Mach number of 0.8 or
so due to shock waves that form around the
configuration.
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Shock waves
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How can shock waves be minimized?

• Use wing sweep.
• Use supercritical airfoils, which keep the flow
  velocity over the airfoil and the local Mach
  number from exceeding Mach 1.1 or so.
• Use area rule- the practice of making the
  aircraft cross section area (from nose to tail,
  including the wing) vary as smoothly as possible.

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How can shock waves be minimized?
Use sweep.
0.8cos30?
30 ? sweep
M 0.8
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In your design...

• The Maximum Mach number is 0.85
• Wings for supersonic fighters are designed to
  reduce wave drag up to 80 of the Maximum speed.
• In our case, 80 of 0.85 is 0.68.
• If we use a wing leading edge sweep angle of 30
  degrees or so, the Mach number normal to the
  leading edge is 0.68 cos 30 0.6

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Effect of Thickness and Sweep on Wave Drag
Source http//www.hq.nasa.gov/office/pao/History
/SP-468/ch10-4.htm
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Supercritical Airfoils
Their shape is modified to keep the Mach number
on the airfoils from exceeding 1.1 or so, under
cruise conditions.
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Conventional vs. Supercritical Airfoils
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Wing Drag

• Since a wing is made up of airfoils, it has
• skin friction drag
• profile drag
• wave drag at high speeds, and
• Induced drag due to tip vortices

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TIP VORTICES
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Effect of Tip Vortices
Downwash
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Induced Drag
Induced drag is caused by the downward rotation
of the freestream velocity, which causes a
clockwise rotation of the lift force. From AE
2020 theory,
e Oswald efficiency factor
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Variation of Drag with Speed
Induced drag decreases as V increases, because we
need less values of CL at high speeds. Other
drag forces (form, skin friction , interference)
increase. Result Drag first drops, then rises.
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At High Values of a Wings Stall
We need high CL to take-off and land at low
speeds.
http//www.zenithair.com/stolch801/design/design.h
tml
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Achieving High Lift
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One form of flaps, called Fowler flaps increase
the chord length as the flap is deployed.
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How do slats and flaps help?
1. They increase the camber as and when needed-
during take-off and landing.
High energy air from the bottom side of the
airfoil flows through the gap to the upper side,
energizes slow speed molecules, and keeps the
flow from stalling.
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Leading Edge SlatsHelp avoid stall near the
leading edge
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High Lift also Causes High Drag
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We have looked at..

• Airfoil aerodynamics (Chapter 5)
• Sources of Drag (Chapter 5)
• Induced Drag on finite wings (Chapter 5)
• Wave Drag, Profile Drag, Form drag
• Airfoil and Aircraft Drag Polar
• High Lift Devices

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AERODYNAMIC PERFORMACE

• Performance is a study to see if the aircraft
  meets all the requirements.
• Level Flight (Is there enough thrust and/or
  power?)
• Climb Performance (Will it meet the requirement
  that the aircraft can gain altitude at a required
  rate given in feet/sec?)
• Range (How far can it fly without refueling?)
• Takeoff and Landing Requirements
• Others (e.g. Turn radius, Maneuverability)
• You will learn to evaluate aircraft performance
  in AE 3310.
• Performance engineers are hired by airlines,
  buyers, and aircraft companies.

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Your Fighter Has Certain Requirements

• Level Flight at a Maximum Speed of Mach 2 at
  30,000 feet altitude.
• Range (1500 Nautical Mile Radius with 45 Minutes
  of Fuel Reserve)
• Takeoff (6000 foot Runway with a 50 foot obstacle
  at the end)
• Landing (6000 foot Runway)
• Will your fighter do the job?

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Your transport aircraft has certain requirements,
say..

• Payload150 passengers weighing 205 lb. each
  including baggage.
• Range1600 nautical miles, with 1 hour reserve.
• Cruise Speed M0.82 at 35,000 feet.
• Takeoff/Landing FAR 25 field length
• 5000 feet at an altitude of 5,000 feet on a 95
  degrees F day.
• Aircraft should be able to land at 85 of
  Take-off weight
• Performance calculation is the process where you
  determine if your design will do the job.

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Level Flight Performance

• We assume that the gross weight GW is available.
  You will know this for your aircraft after
  Homework Set 4. An estimate of wing area S is
  assumed to be known (Homework, later in the
  course).
• Select a cruise altitude. Compute the speed of
  sound
• Select a set of M? 0.4, 0.6, 0.8.
• Find Aircraft Speed M ? times a?
• Find CL GW / (1/2 r? V?2 S)
• Find CD CD,0 CL2/(p AR e) (this info is
  given in our course)
• Find Thrust required T CD (1/2) r? V?2
  S
• Plot Power Required (T times V) or thrust
  required vs. Speed
• Plot Power Available for your Engine (number of
  engines times T times V) or thrust available at
  this altitude and Speed (Supplied by Engine
  Manufacturer)
• Where these two curves cross determines maximum
  and minimum cruise speeds.

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Level Flight Performance
Power Required
Power Available with all engines
Power HP
Excess Power
Aircraft Speed (Knots)
Best speed for longest endurance flights since
the least amount of fuel is burned
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Maximum Rate of Climb
Power HP

• Find Excess Power from previous figure.
• This power can be used to increase aircraft
  potential energy or altitude
• Rate of ClimbExcess Power/GW

Excess Power
Aircraft Speed (Knots)
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Absolute Ceiling
Power HP

• Absolute ceiling is the altitude at which Power
  available equals power required only at a single
  speed, and no excess power is available at this
  speed.
• Rate of climb is zero.

Power required
Power available
Aircraft Speed (Knots)
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Equilibrium Gliding Flight
L
D
Glide Angle, q
W cosq L W sinq D
q
Flight Path
W
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Gliding Distance
Glide Angle, q
Flight Path
Altitude h
Gliding Distance h/tanq h L/D
Ground
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Gliding Flight

• DW sinq where q is the equilibrium glide angle.
• L W cosq
• Tanq D/L
• Glide distance h/ tanq h ( L/D).

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Cruise Speed for Maximum Range
V? L/D
Speed for maximum range
Aircraft Speed (Knots)
From your level flight performance data plot V?
L/D vs. V? As will be seen later, the speed at
which V? L/D is maximum gives maximum range.
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Calculation of Range
We have selected a cruise V?. Over a small
period of time dt, the vehicle will travel a
distance equal to V? dt The aircraft weight will
decrease by dW as fuel is burned. If we know the
engine we use, we know the fuel burn rate per
pound of thrust T. This ratio is called
thrust-specific fuel consumption (Symbol used
sfc or just c). dt Change in the aircraft
weight dW/(fuel burn rate) dW / (Thrust
times c) dW/(Tc) Distance Traveled during
dtV?dW/(Tc) V? W/T(1/c) dW/W
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Calculation of Range (Contd)

• From previous slide
• Distance Traveled during dtV?W/T(1/c) dW/W
• Since TD and WL, W/T L/D
• The aircraft is usually flown at a fixed L/D.
• The L/D is kept as high as possible during
  cruise.
• Distance Traveled during dt V?L/D(1/c) dW/W

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Calculation of Range (Contd)

• From previous slide
• Distance Traveled during dt V?L/D(1/c) dW/W
• Integrate between start of cruise phase, and end
  of cruise phase. The aircraft weight changes from
  Wi to Wf.
• Integral of dx/x log (x) where natural log is
  used.
• Range V?L/D(1/c) log(Wi/Wf)

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Breguet Range Equation
Structures Weights Group/ Designer
Responsibility to keep Wfinal small.
Propulsion Group/ Designer Responsibility to
choose an engine with a low specific fuel
consumption c
Aerodynamics Group/ Designer Responsibility to
maximize this factor.