The Next Frontier

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15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Farhan Gandhi

Professor of Aerospace Engineering

Deputy Director, Vertical Lift Research Center of Excellence (VLRCOE)

The Pennsylvania State University

Reconfigurable “Morphing” Rotors



The Next Frontier

Keynote Lecture presented
at the NATO Symposium on Morphing Aircraft

April 20
-
23, 2009,
Evora
, Portugal


Snake/eel robots, fish robots etc. show large shape change


In the aerospace field the Wright Flyer was controlled by
wing morphing

What sets Modern Morphing Aircraft apart
from other Shape
-
Changing Systems?

Snake/eel/fish robot type shape
-

changing structures are non load
-
bearing


For early aircraft like the Wright
Flyer, wing loading was very low

Modern aircraft, with high
-
wing loading designed to be v. stiff.

Energy required to change the shape of such a rigid structure
would be very high.

Making it flexible would reduce load
-
bearing capability.

Need designs that are rigid enough to be load bearing, yet
flexible enough so morphing energy required is reasonable


Rotors


Centrifugal Force is MUCH larger than aerodynamic
loads


about 1000 g’s at the blade tips.
So we are trying to
deform a structure that has to be rigid enough to carry the
aerodynamic and the CF loads


What sets Morphing Rotors apart from
Morphing Fixed
-
Wing Aircraft?

Fixed
-
Wing Aircraft


ability to morph while being rigid
enough to carry the aerodynamic loads.

Terms frequently used in fixed
-
wing morphing

gross morphing:

span, chord, sweep, twist change; big things!

fine morphing:

camber, leading
-
edge droop, subtler changes!

In the rotary
-
wing world

Span, chord, twist
-
change, maybe anhederal


gross morphing

camber, leading
-
edge droop


smaller changes made per rotor
revolution


rotor active control
. Same technologies are used
for noise/vibration reduction can provide some (modest)
performance improvement, stall alleviation, etc.

(Quasi
-
steady)

Morphing Rotors

As in the fixed
-
wing world, gross
-
morphing is a game
-
changer


Helicopter rotors



morphing a structures that in additional to
lift and drag, has to carry
CF loads

(many times larger than the
aero forces).
Can you USE the forces present to morph the
rotor, rather than OVERCOMING them?


What Will Morphing Rotors Buy Us?


--

Performance improvement over the flight regime


--

Envelope Expansion


--

Operational Flexibility


… and, at what cost?


Types of Morphing Discussed


Span Morphing


Chord Morphing


Twist Morphing

Rotor Span Morphing

Recent Work (2005


2007)


Centrifugally Actuated Variable Span Morphing Rotor

Previous Work (Early 1990’s)


Sikorsky Variable Diameter Tiltrotor (VDTR)


Optimal Rotor


large, “lightly” twisted, higher RPM

Propeller


smaller, highly twisted, lower RPM


Actuation using motor at the hub or differential gears

Blade moving on a jackscrew; tension strap to carry CF

Design was complex and heavy

Fixed inner
section of
blade

Sliding outer
section

Restraining
Spring

High
Ω

Low CF Force

Little extension

High CF Force

Large extension

Retracted or short configuration

Extended or long configuration

Low
Ω

L
-

Position of center of mass of sliding
section + end cap


u
-

Deformation

Centrifugal Force Actuated Variable Span Rotor

SIMPLICITY


Compare to VDTR

Centrifugal Force Actuated Variable Span Rotor

This figure (of a rotor test) in lieu of a movie

Low RPM

No Extension
High RPM

Rotor Extended
Extension

Extension

Movie showing this structure span morphing available with Gandhi

Span

RPM Dependence, Is it a Problem?

Normally small rotors spin fast and large rotors spin slow




But not always!

Application 1


Tiltrotors


Propeller Mode


low RPM, compact


Rotor Mode


increase RPM, expand. Lower disk loading,

induced power, downwash, etc.

Application 2


Slowed Rotor Compound for High Speed Flight


Aux Lift and Propulsion available in H/S flight

Rotor slowed to avoid compressibility

Ideally, you’d like it to disappear (it’s a source of drag)

Unloaded rotor susceptible to gust and
aeroelastic

instability


Slow down the rotor and automatically have it contract.

A lot of the problems diminish

CF Actuated Variable Span Rotor


Applications (contd.)

Application 3


High
-
Speed Coaxial Rotors (Sikorsky X
-
2)


Higher the max cruise speed, the more the rotor needs to be
slowed down. Beyond a certain point, Variable Speed
Transmission required (heavy and complex). It is the TIP
SPEED that really needs to be controlled, if a small reduction in
RPM simultaneously reduces span, a much larger reduction in tip
speed realized without a
VST

Application 4


Operation in Confined Spaces


Shipboard Operations, Urban Canyons, etc.


Rotor compact and operated at high angle of attack.

Not the most efficient rotor, but it CAN operate.

It gives you operational flexibility.


Expand to a more efficient rotor when you’re not space
constrained.

RPM
-
Span Dependence not that Restrictive

0
5
10
15
20
25
30
35
40
45
50
0
2
4
6
8
10
Spring Force (lbs)

Extension (in)

Spring Force vs. Ext

Design non-linear spring
Linear spring
0
1
2
3
4
5
6
0
50
100
150
200
250
300
350
400
Extension (in)

Rotational Speed (RPM)

Design non-linear spring
Linear Spring
Extremely stiff to a critical
force, then displays soft
linear behavior

Virtually no extension up to a
certain RPM, followed by a large
extension over a very small
increase in RPM

Nonlinear Springs give you ENORMOUS Flexibility

Variable Span Rotors with Locking Mechanism

CF actuated Var Span Rotor


equipped with locking mechanism


RPM change used as the actuator;
once the rotor is locked,
then the RPM can be changed as desired and span cannot
change


so RPM and span decoupled.

This is very empowering.


Now the small rotor
can spin faster, not
at extreme pitch
near stall margin;
large rotor can spin
slower, not at too
high tip speeds



of much greater
interest for
conventional rotors

Required Power Vs. Lift
Locked Rotor Vtip = 650 ft/s
400
500
600
700
800
900
1000
5000
5500
6000
6500
7000
7500
8000
8500
9000
9500
Lift (lb)
Power (HP)
R = 16 Omega = 40
R = 14 Omega = 46.4285
R = 12 Omega = 54.1666
Max Power = 840 HP
Takeoff Weight = 5500 lb
10 deg
8 deg
9 deg
Theta = 12 deg
12 deg
13 deg
~10% increase in lift at 700 HP

~10% reduction in
power at 6,500 lbs

Lift (lbs)

Main Rotor Power (HP)

The Case for CHORD MORPHING

Extra Chord for High Payload, High
-
Altitude Ops

4th Generation Blade Shape with Anhedral Tip End

4000lbs of additional payload just

through blade design

US Marine Core

CH
-
53K 4
th

Generation Blades

Bonded Tab Rotor


Increased solidity, Hover payload improvement

MV
-
22 Potential Rotor Improvements

Increase chord AS NEEDED


for high altitude, high payload,
very high
-
speed operation
(anytime rotor is close to stall boundary).


No need to pay drag penalty when extra solidity not required

In Afghanistan, Chinooks doing the work of Blackhawks

Effective Chord Increase through
Extendable TE Plate

Modified shape doesn’t present significant aerodynamic penalty

Aerodynamic Benefits of Extendable TE Plate

In terms of lift increment per unit drag increment, Extendable
TE plate does better than other high
-
lift devices such as
Trailing
-
Edge Flaps (Variable Camber) or Gurney Flaps


Actuation force required is low (no pressure differential, “hinge
moment” to overcome)

Envelope Expansion with Extendable TE Plate

Max Altitude and Speed both increased significantly

24,000 lbs Gross Weight

(installed 4000 HP SL)

Power Reduction at Envelope Boundary with
Extendable TE Plate

Possible to trade
-
off power reduction instead of increase
in max speed or altitude

24,000 lbs

Gross Weight


8,000 ft.

Rotor Power v/s Gross Weight with
Extendable TE Plate (Stall Alleviation)

Payload (or fuel/range) can be increased for given power,

Power can be reduced significantly at a given high gross weight

8,000 ft.


112 kts.

Rotor Power v/s Gross Weight with
Extendable TE Plate (Stall Alleviation)

+10% chord

+30% chord

Extendable TE Plate within the Flight Envelope

All the benefits seen, so far, with extendable TE plate were at
the envelope boundaries.

Max Altitude, Cruise Speed, Gross Weight could be increased

Alternatively Power could be reduced


at envelope boundary


What if envelope expansion was not a requirement?


Does the extendable TE help within the envelope?


With the extendable TE, the fixed chord could be reduced.

This implies lower profile drag, increased range, endurance, etc.


The lower rotor solidity would shrink the envelope


now the
extendable TE plate can push the boundary back to where it
was.

Extendable TE Plate


Deployed using a
Morphing Truss

Stowed Configuration

Deployed

Configuration

Slit trailing edge over section

Bounded by ribs.

Morphing Truss anchored to

the aft of the LE D
-
spar.

Chordwise extension of the

morphing truss leads to deployment

of the TE plate aft of the “normal”

blade trailing
-
edge

Can TE plate deployment be achieved by small change in RPM?

Morphing Truss actuated
mechanically in the current

model.

Extendable Trailing
-
Edge Plate


Hardware

Upper Skin Removed, showing
morphing truss in various states.

Plate retracted or stowed,
and extended through slit TE

Extendable Trailing
-
Edge Plate

deployed using Bi
-
Stable Arc

Spar

Hub

Plate

Roller

Arc

SMA wires (actuators)

Bi
-
stable arc

Continuously Extendable Chord
using Cellular Structures

Flexible face
-
sheet over extendable chord section not depicted

Movie showing this blade section chord morphing available with Gandhi

Twist Morphing of Rotor Blades

LOT of work done on active
-
twist rotors by various groups.


Most focuses on piezoelectric actuation.


Typically twist of the order of +/
-
2 deg, at frequencies in the
range of ~20 Hz


Shown to be quite effective for vibration/noise reduction


Also possible to actuate at 5
-
10 Hz (1P, 2P)


can potentially
result in
small

reductions in rotor power.

Rotor twist morphing implies
large
,
quasi
-
static

twist change
for optimal performance at different conditions.


10
-
15 deg twist change for conventional rotor (hover v/s H/S)

30
-
45 deg change for a
tiltrotor

(rotor v/s propeller mode)

This is NOT morphing, this is rotor active control

Twist Morphing of Rotor Blades

All prior rotor twist morphing attempts have focused on the
use of Shape
-
Memory Alloys.


Most recent was ONR funded effort by Boeing


used SMAs to
change the twist of a V
-
22 blade. Twist change obtained was…


So what do you do?


If you make the structure compliant, you do so at some peril.

Low
torsional

stiffness can increase susceptibility to flutter,
high loads and vibration, etc.

Baseline structure was incredibly stiff.


If the compliance isn’t increased at all, the force and power
required to actuate are going to be enormous!!


Assuming such force and power are available, someone should
try and calculate the strains in the stiff metallic skin!!

One such concept exploits the idea of
warp
-
induced twist,
facilitated by a threaded rod actuated by a motor.


Trailing
-
edge is slit, a threaded rod runs

Through threaded/unthreaded housings

Alternately connected to the inside

Of the upper and lower skins.


Rod rotation causes





Very low torsion

top skin to warp





stiffness during

relative to bottom




morphing (low actuation)

and induces twist




High torsion
-
stiffness




otherwise, for aeroelastic stability and loads.


Large Twist Actuation of Rotor Blades

Is it possible to design a blade whose torsion stiffness under
aerodynamic loads is high, but actuation requirements to twist
the blade are low?

Twist Morphing of Rotor Blades

Movie showing this blade section twist morphing available with Gandhi

Twist Morphing of Rotor Blades

Twist Morphing of Rotor Blades

Could be implemented over entire blade, or over outer section
of the rotor blade.


Is it worth the trouble?


When considering a morphing solution you need to look at the
benefits of morphing over a sub
-
optimal compromise solution


Conventional (edgewise) rotors

-
18 deg twist might be optimal for hover, and
-
6 deg optimal
for high
-
speed forward flight. What if blade has compromise
-
12 or
-
14 deg twist. What is the power penalty at hover, and
at high
-
speed. Turns out


not that much!


There may be an advantage for vibration reduction (that’s a
different story)


Tiltrotors



different ballgame, advantages are unquestionable

For each of these concepts there is
IMPACT
and
Risk/Complexity (RICO)

to be considered.


Each Concept ranked from 1 (lowest)


3 (highest), in both categories




IMPACT

RICO


IMPACT/RICO


Span



3



3



1



Chord



2



1



2


Twist



1



2



1/2


Rotor Morphing


Span, Chord or Twist

If just ONE technology had to be chosen


CHORD
MORPHING, appears to give maximum bang for the buck.


A blade whose chord could increase 20
-
30% over some
spanwise

section

Some Additional Thoughts on Rotor Morphing

Real
-
estate in the rotor blade aft of the LE spar is very limited


Transfer of power is an issue


hydraulics not likely to be used,
even for electric power transferring KVolts through an electric
slipring is challenging


Cannot use classical structures


some increase in compliance is
inevitable.


We haven’t called RPM change morphing, but lot of opportunity
for synergy here. CF force is the 10,000 lb gorilla in the room.
If change in CF force can be used to morph


USE IT!! Don’t
fight it! Think martial arts…..


CF actuated chord variation is likely possible


What about anhedral?



Some Additional Thoughts on Rotor Morphing

Flexible Skins


maybe nice, but not critical. Discrete shape
change (rather than continuous) may be adequate.


Promising Technologies:


Bi
-
Stable
Structures and Cellular Structures have very
interesting possibilities


Do not get too attached to a specific way to implement a
certain type of morphing (ex. SMAs for twist morphing)


Do not get too attached to a specific morphing solution for a
system level objective (ex. Leading
-
edge droop for stall
alleviation).



Through Configuration Change (Morphing)


Can we expand the flight envelope

(higher max speed, altitude, payload),


Improve operational flexibility

(operate in tight spaces)


Improve performance at multiple points

(compared to a sub
-
optimal compromise)

Thank you for your attention!

Questions ??