# Lesson 17 High Lift Devices

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Lesson 17 High Lift Devices
Aero Engineering 315 Lesson 17 High Lift Devices

Homework #20 (BSBW A4.1) Consider a flying wing aircraft made using a NACA 2412 airfoil with a wing area of 250 ft2, a wing span of 50 ft, and a span efficiency factor of 0.9. If the aircraft is flying at 6 deg angle of attack and a Reynolds number of approximately 9 x 106, what are CL and CD for the flying wing? b. If the flying wing is flying at sea level, standard day at V¥ = 280 ft/s, how much lift and drag is it experiencing?

NACA 2412

High Lift Device Objectives
Calculate stall velocity Describe typical high lift devices and their purpose Flaps, strakes, BL control, powered/blown lift and thrust vectoring Draw the effects of flaps and boundary layer control devices on lift and drag curves

High-Lift Devices Method to improve CL in certain circumstances

For straight level unaccelerated flight (SLUF)
Velocity for lift Remember our forces? If Lift = Weight, then , and the velocity to maintain lift is For straight level unaccelerated flight (SLUF) Lift Thrust Drag Weight So, to fly slower at a given altitude you either jettison wt or increase CL

Stall Velocity The velocity at stall, Vstall, occurs at CLmax
Recall the definition of equivalent airspeed? The minimum Ve to avoid stall is

Vstall Problem 2 = S W V C r 2 = S W V C rSL = V = V = V = V
You are landing your F-16 at Luke AFB with a density altitude of 6,000 feet. Your gross weight is 25,000 lbs. CLmax is 1.2 and the whole aircraft planform area is 428 ft2. What is your stalling velocity in knots? 2 = S W V max C L Stall r 2 = S W V max C L e-Stall rSL = V Stall ( )(428)(1.2) 2(25,000) = V e-Stall ( )(428)(1.2) 2(25,000) = V Stall 221 ft/sec = 131 KTAS = V e-Stall 202 ft/sec = 120 KEAS

Why High-Lift Devices? Low camber good for high speed
A/C needs to operate at low speeds also! High lift devices allow increase in CLmax Which lowers Vstall Flaps improve visibility on final by creating an angle of incidence Landing speeds (1.3 Vstall) limited by braking, tires, runway distance and condition… Must dissipate KE (1/2mV2) Take-off speed (1.2 Vstall) limited by max tire spin up speeds, runway length, thrust available…

Trailing Edge Flaps Plain flap Split flap Slotted flap Fowler flap

Effect of Flaps on Lift and Drag Curves
Wing with Flap C C D Wing with Flap L Basic Wing Section Basic Wing Section C a L

Typical Flap Impact d d = 500 With flaps d = 150 No flaps d = 0
a (degrees) Anderson, J. D., Introduction to Flight, 4th Edition, page 314

KC-10 Slotted Flap

Anderson, J. D., Introduction to Flight, 4th Edition, page 315

Leading Edge Devices and Boundary Layer Control
Leading-edge flap by upper surface suction Fixed slot Boundary Layer Control by tangential blowing Slat

F-16XL upper surface suction

Boundary layer control devices affect lift curve like higher Rec
Wing with Leading-Edge Flap or Slat or Boundary Layer Control C L Basic Wing Section a

Powered (blown) lift Internally Blown Flap

Powered (blown) lift Externally Blown Flap

C-17 externally blown flap

Powered (blown) lift Upper-Surface Blowing Example: YC-14

YC-14 - Upper Surface Blown Flap

Powered (vectored) lift
Vectored Thrust

AV-8B -- Vectored Thrust

C-130 JATO - Vectored Thrust

Strakes Energize the flow over the wing, delaying separation
Cause more lift (especially at high AOA) due to lower pressure inside vortex Increase the amount of lifting surface Cause lift curve to rotate up and extend

Strakes Strakes

Strakes Strakes Strake Vortices

F-16 Strakes

F-16 Strakes

F-16 vortex rollup sequence

F-18 HARV LEX effects

Effect of strakes on lift curve
Wing with strake C L Wing with no strake a

Next Lesson (18)… Prior to Class In Class Read 4.5, 4.6, 3.5.10
Complete all problems through #21 In Class Discuss how lift and drag of an aircraft differ from that of a wing Discuss the aircraft drag polar

To get numbers (i.e. 3-D wing lift and drag), we need the 3-D lift-curve slope
Airfoil a cl cla and CL Wing CLa For a in radians

Calculating 3-D Lift and Drag Coefficients
per deg Determine Rec Find 3-D lift curve slope ( from chart) Find 3-D lift coefficient ( from chart) Find induced drag coeff. Find profile drag coeff. (c from chart) L=0 d c l