U5AEA15 AIRCRAFT STRUCTURES-II

PREPARED BYMr.S.Karthikeyan

DEPARTMENT OF AERONAUTICALENGINEERINGASSISTANT PROFESSOR

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

Airfoil Drag Polar Cd vs. Cl

Rough airfoilshave turbulent flowover them, high drag.

Smooth airfoils havelaminar flow overat least a portionof the surface.Low Drag.

Form Drag

Source: http://www.allstar.fiu.edu/aerojava/flight46.htm

Form drag may be reducedby proper design, andstreamlining the shape.

Supersonic wave Drag

For a given airfoil or wing or aircraft, as the Mach numberis increased, the drag begins to increase above afreestream Mach number of 0.8 or so due to shock waves thatform around the configuration.

Shock waves

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.

How can shock waves be minimized?

Use sweep.

M= 0.80.

8cos

30

30 sweep

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

Effect of Thickness and Sweep on Wave Drag

Source: http://www.hq.nasa.gov/office/pao/History/SP-468/ch10-4.htm

Supercritical Airfoils

Their shape is modified to keep the Mach number on the airfoilsfrom exceeding 1.1 or so, under cruise conditions.

Conventional vs. Supercritical Airfoils

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

TIP VORTICES

Effect of Tip Vortices

Downwash

Induced Drag

Induced drag is caused by the downward rotation of the freestream velocity, which causes a clockwise rotation of the liftforce.

From AE 2020 theory, e

2

, AR

CC L

iD

e= Oswald efficiency factor

Variation of Drag with Speed

Induced drag decreases as Vincreases, because we need lessvalues of CL at high speeds.

Other drag forces (form, skin friction , interference) increase.

Result: Drag first drops, then rises.

At High Values of Wings Stall

We need high CL to take-off and land at low speeds.

http://www.zenithair.com/stolch801/design/design.html

Achieving High Lift

One form of flaps, called Fowler flaps increase the chord length asthe flap is deployed.

High energy air from the bottom side of the airfoilflows through the gap to the upper side, energizes slow speedmolecules, and keeps the flow from stalling.

How do slats and flaps help?

1. They increase the camber as and when needed- duringtake-off and landing.

Leading Edge SlatsHelp avoid stall near the leading

edge

High Lift also Causes High Drag

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

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.

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?

Your transport aircraft has certain requirements, say..

– Payload:150 passengers weighing 205 lb. each including baggage.

– Range:1600 nautical miles, with 1 hour reserve.– Cruise Speed: M=0.82 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.

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 * * V2 * S)

• Find CD = CD,0 + CL2/( AR e) (this info is given in our course)

• Find Thrust required T = CD * (1/2) * * V2 * 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.

1.4 where RTa

Level Flight Performance

Aircraft Speed (Knots)

PowerHP

Power Required

Power Availablewith all engines

Best speed for longest endurance flightssince the least amount of fuel is burned

Excess Power

Maximum Rate of Climb

• Find Excess Power from previous figure.

• This power can be used to increase aircraft potential energy or altitude

• Rate of Climb=Excess Power/GW

Aircraft Speed (Knots)

PowerHP

Excess Power

Absolute Ceiling

• 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. Aircraft Speed (Knots)

PowerHP

Power available

Powerrequired

Equilibrium Gliding Flight

Glide Angle, W cos = LW sin = D

D

W

L

Flight Path

Gliding Distance

Glide Angle,

Flight Path

Ground

Altitude h

Gliding Distance = h/tanh * L/D

Gliding Flight

• D=W sinwhere is the equilibrium glide angle. • L= W cos• Tan = D/L• Glide distance = h/ tan = h ( L/D).

Cruise Speed for Maximum Range

Aircraft Speed (Knots)

V L/D

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 maximumgives maximum range.

Speed for maximum range

Calculation of RangeWe 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 rateper pound of thrust T. This ratio is called thrust-specificfuel 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 dt=VdW/(Tc) =V [W/T](1/c) dW/W

Calculation of Range (Contd…)

• From previous slide: – Distance Traveled during dt=V[W/T](1/c) dW/W

• Since T=D and W=L, 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

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)

Breguet Range Equation

final

initiale W

W

D

LV

cRange log

1

Propulsion Group/Designer Responsibilityto choose an enginewith a low specificfuel consumption c

Aerodynamics Group/Designer Responsibilityto maximize this factor.

Structures & WeightsGroup/Designer Responsibilityto keep Wfinal small.