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