aero engineering 315 lesson 30 turn performance. “turning” the tables…

Aero Engineering 315

Lesson 30

Turn Performance

“Turning” the tables…

Turn Performance Overview

Define bank angle and load factor Calculate load factor needed to sustain

a level turn at a given bank angle Calculate initial turn rate and radius for

level turns, pull-ups and pull downs

Turn Performance Definitions

Load factor – Lift/Weight (measured in g’s)

n = L/W Bank angle () – angle created

by rotating the aircraft about the aircraft reference line

Turn rate () – radians/second or degrees/second

Turn radius (r) – feet

L

W

Level Turns

Level flight means constant altitude (no vertical acceleration)

In level flight, bank angle creates a specific loadExample: level turn at

= 60°

n = 1/cos 60° = 2

L cos

L sin

Level Turn Load Factor

n LW

1cos

If bank angle is known, you can find load factor and vice versa for a level turn

In a level turn:

Bank AngleL

evel

Tu

rn L

oad

Fac

tor

0

2

4

6

10 20 30 40 50 60 70 80

Level Turns

Constant velocity turn Stall speed in a turn:

Stall speed increases as n½

Extra pilot care when performing high bank angle turns near the ground

L cos WL sin

L sin

r

L = nW = CLMAX VSTALL S/22

VSTALL = 2nW

CLMAX S

L = nW

Level Turn Performance Concepts

For good turn performance we want:Small turn radiusHigh turn rate

To achieve this we should turn withLow velocityHigh load factor

Note: Turn radius and rate are functions only ofvelocity and load factor. A B-52 and an F-16 atthe same airspeed and “g” have identical turn radiiand rates.

To get small turn radius (r) and large turn rate () Maximize “g” loading

Structural considerations (max g for the structure) Physiological considerations (human g limits) Aerodynamic considerations (stall limitations)

Minimize wing loading, W/S (n = CL q S / W) Maximize lift coefficient

High lift devices Pitch vectored thrust

Maximize thrust-to-weight ratio (T/W) High “g” creates very high induced drag

Level Turn Design Considerations

F-16 Turn Performance Limits*

*F-16 COMBAT AIRCRAFT FUNDAMENTALS, page 46

Stall limits

Human/aircraft structural limits

Aeroelastic limits

Homework #36

An SR-71 is in a 20° banked level turn at Mach 3.0 and an altitude of 80,000 ft. Assume the ambient temperature is 390°R.a. What is the aircraft’s turn radius in NM? b. What is the aircraft’s turn rate in deg/s?c. If the pilot increases the bank angle to 45°

while maintaining a level turn at Mach 3.0, what would the new turn radius be in NM? What would happen to the aircraft load factor?

SR-71 Mach 3 80,000 ft

20o bankr = 118 nm

10o bankr = 244 nm

45o bankr = 43 nm

Vertical turns (loops)

r

L

L

L

W

W

W

D

D

D T

T

T

Pull-Down

Pull-Up

WV2

g rFR = = L -W

= V/r =g(n-1)

V

WV2

g rFR = = L

= V/r =g nV

WV2

g rFR = = L +W= V/r =

g(n+1)V

Pull-down

Pull-up

Vertical Portion

4.0 GR4.0 GR

3.0 GR 3.0 GR

5.0 GR

3.3 GR

4.7 GR4.7 GR

r =V2

g(n+1)

r =V2

g(n-1)

r =V2

gnr =

V2

gn

GR = denominator term in g’s

Next Lesson (31)…

Prior to class Read text 5.12 Complete homework #36-37 All sections of Fighter Design Project

In class Discuss V-n diagrams