Stability Pt2

8/3/2019 Stability Pt2

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ME 380Aircraft Design

Stability and Control, Pt. 2

Energy Maneuverability Diagram

Puts all of the maneuver information in one compact

diagram

Flight velocity, turn rate, turn radius, & load factor

However, requires a diagram for each altitude and

weight


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Stability

In order of importance: Longitudinal stability

Stability about the pitch axis: horizontal stabilizer

Lateral stability

Stability about the roll axis: bi-lateral symmetry, wing design

(dihedral), ailerons, keel effect,

Directional stability

Stability about the yaw

axis: vertical stabilizer

Note on axes:

Trim Point Location

Compare two aircraft below

Cmcg

+

(nose up)

(nose down)

Aircraft 1

Aircraft 2

Airplane 1

Trimmed at point B

Cm=0 for > 0

Cmcg = 0Trim Pt.

Airplane 2

Cannot be trimmed to point B

Cm=0 for < 0

B CA

For stability,

Cm =dCmcg

d< 0

Cmcg = 0 at > 0


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Balance

The moment about the c.g. is the sum of these moments

Mcg =Mc/ 4 L(xc/ 4 xcg ) +Lt(x txcg )

Mcg = 0

Static Margin & CG Travel

By looking at CL and Cm we can define the static

margin

This is a measure of an

aircrafts stability - this

value should be between

0.03 (low) to 0.1 (high);

0.05 is a good value to

aim for

=Cm

CL

=xc/ 4 xcg

c

c.g. travel must bewithin SM limits


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Pitch Stability

From these we can determine the limits for c.g.(forward, XF, and aft, XR) - note that a larger tail

provides a larger range of c.g. travel

Pitching Tendencies in Stall

Low-tail aircraft pitch down in stall; recovery easier

T-tail aircraft pitch up in stall; tail in stalled wake,

recovery more problematic

cruciform T-tail


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Effect of Sweep on Stall Angle

Sweep reduces drag, but also increases stability atthe expense of lower lift

For example,

Effect of Elevator on Pitch Stability

Shifts stability curve up and down


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Phugoid

The phugoid is the traditional pitch behavior of anaircraft responding to a disturbance

Directional Stability

Cn

> 0

Stability reqmts


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Requirements for Direction Control

Adverse Yaw

When an airplane is banked to execute a turning maneuver, the aileronsmay create a yawing moment that opposes the turn (adverse yaw). Therudder must be able to overcome the adverse yaw so that a coordinatedturn can be achieved.

This usually occurs during slow flight (high CL).

Crosswind landings

To maintain alignment with the runway during a crosswind landing the

pilot must fly at a non-zero sideslip angle. The rudder must be powerful

enough to permit the pilot to trim the airplane for specified crosswinds.

Max. crosswind design value typically 15.5 m/s (51 fps).

Asymmetric power condn

When one engine fails on a multi-engine plane, a critical asymmetric

power condition occurs. The rudder must be able to overcome the yawingmoment produced by the asymmetric thrust arrangement.

The farther an engine is away from the centerline, the greater theasymmetric power control requirements are.

Asymmetric power & Stall into Spin

Spin Recovery

The primary control for spin recovery in most airplanes is the

rudder. The rudder must be powerful enough to oppose the spin

rotation.

Rectangular wing

Stall seen inboard; tail

blanked, but aileron control

still available

Swept wing

Stall outboard; tail available

but ailerons may not be


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Dorsal Fin

Addition of dorsal

fin delays tendency

of tail to stall at high

sideslip angles w/

reduced parasite drag

Forces on Aircraft in Roll


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Roll Stability

Cl

< 0

Stability reqmts

Fuselage Contributions

High wings more stable due to stabilizing roll

moment; low wings typically include dihedral to

counteract the destabilizing moment


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Dihedral & Roll Stability

Dihedral angle denoted by

, typically +3-5o

for a lowwing plane, 0 or slightly negative for a high wing

Dihedral Effect

When an airplane is disturbed from wings level attitude it will

begin to sideslip.

During sideslip, an additional velocity component is present -

The leading wing experiences an increased angle of attack, hence

increased lift.

The trailing wing experiences a decreased angle of attack, hence

decreased lift.

This results in a restoring force.

Wing Dihedral: Simplified Explanation


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Lateral Control and Roll Control Power

MROLL = Ly

Cl =L

qSb=

Clcydy

Sb

Cla =dCl

da

=2Clw

Sbcydy

y1

y 2

Common Coupled Dynamics

Spiral divergence (graveyard spiral); occurs

when static directional stability is large

compared to static lateral stability - solved

with addition of dihedral

Directional divergence; sideslip coupled with

yaw

Dutch roll; occurs when dihedral effect is

large compared to directional stability


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Dutch Roll

Commonly seen in low speed flight or with too muchdihedral

Slipstream Rotation

Slipstream rotation

from prop yaws

aircraft; most critical

at high power/low speed

scenarios (landing

and takeoff)


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Wing Rock

All Coefficients

Each coefficient (3 forces, 3 moments) has a

derivative in each direction and each angle, plus a

derivative with each rate (such as d/dt or q)

In general, a handful of these may be important for

any particular aircraft -

usually determined by

software (including

numerical models)

See Phillips (Mechanicsof Flight) or Etkin &

Reid (Dynamics of

Flight) for more details


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SomeStability Derivatives

Longitudinal derivatives (Etkin & Reid, Table 5.1)

Lateral derivatives (Etkin & Reid, Table 5.2)

Blue denotes tail only, wing-body

formula not available

No formula available

No formula available

Control


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Control Surfaces

Control Surface Deflections

CL =dCL

dee

CL =dCL

d+

dCL

dee

= a+ CLee


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Flap Effectiveness

CL =dCL

de=

dCL

dt

dtde

=CLt

How much extra lift is added by a control surface?

Trim AoA

Hinge Moments & Trim

He = Che1

2U2Sc

To size a servo, we need to note the required moment

to move a control surface


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Stick Forces

Flss =Hee

F=elss

He

= GHe

= GChe12U2Sc

Servo motors on control surfaces easily sized oncehinge moments are determined. Use moment

balance (even if modern control system is used).

Can get flap forces from Xfoil

Stick Fixed v. Stick Free

When the elevator is set free, the stability and control

characteristics change.

Typically, when the AoA

is increased, the elevator

floats upwards.

Regardless, the location

of the stick fixed and stick

free neutral points sets an

aft limit to the center ofgravity travel for the plane.


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Fixed vs. Free Static Margin

stick fixed static margin=xNP

c

xcg

c

stick free static margin=xNP

c

xcg

c5% (0.05c) is the general design rule of thumb for static margin

Static margin is a way of measuring the static stabilityof an aircraft

Neutral point is location of c.g. where stability goes to 0 (neither +

nor -)

Neutral point (NP) is usually the aerodynamic center (AC), or

where the lift vector acts

Stick Force or Speed Stability

Negative stick force gradient provides pilot with

speed stability; once trimmed, the velocity will return

to trimmed speed if perturbed


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Control Issues: e.g., Aileron Reversal

As an example of the many control issues one mayencounter, aileron reversal is one commonly seen at

higher speeds

Control: Open & Closed Loop

Example: wing leveling autopilot

Practically all aircraft are closed-loop control

Classic: pilot gets feedback from stick forces and instruments

Modern: digital autopilot corrects/enhances pilot input


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Types of Control Systems

Direct Push-rod

Cable-and-pulley

Indirect

Hydraulic

Fly-by-wire

Fly-by-light

FBW Philosophy

The computer should have final authority on the

commands sent to the control system. The pilots

inputs should be limited by the computer (hard limits

or protections) to prevent exceeding the physical

design limits of the aircraft (e.g., angle of attack, g-

loads, etc.) to protect the integrity and dynamics of

the aircraft.

The pilot should have final authority of the commands

sent to the control system. The computer shouldmonitor the pilots inputs for limits (soft limits) and

warn when they exceed the physical design limits of

the aircraft, but carry out the commands even if that

would endanger the aircraft integrity or flight.


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Handling Qualities

Controls must feel right to pilot Control parameters (gains, damping, etc.) are unique

to each aircraft and thus must be tuned, typically

through wind tunnel and flight tests

Cooper-Harper Scale


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Weather

Typical Storm Wind Patterns


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Landing in Wind Shear

From headwind to tailwind

1. Normal approach

2. Increasing downdraft and

tailwind

3. Airspeed decreases, pitch down

4. Aircraft crashes short of runway

From tailwind to headwind; hard

landing or overshoot

Wind Shear


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Wind Shear

1- & 2-D Gusts


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Boundary Layer Effect

Stability Pt2

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Transcript of Stability Pt2