conceptual helicopter design - autenticação · helicopters / filipe szolnoky cunha conceptual...

Helicopters / Filipe Szolnoky Cunha Slide 1 Conceptual Helicopter Design

Conceptual Helicopter Design

• Helicopter design will depend on:

– Aerodynamics

– Structural Dynamics

– Aeroelasticity

– Materials

– Weight

– Flight Dynamics

• Design starts with:

– Potential customer specifications (civil)

– Mission requirements (military)



• Design technology for the civilian market is

driven by:

– Reduced acquisition

– Reduced operating costs

– Increased safety

– Reduced cabin noise

– Increased passenger comfort

– Better mechanical reliability and maintainability

– Reduced external noise



• On the other hand design technology for the

military market is driven by:

– Operational flexibility and adaptability

– Long operational life

– Upgradeable components

– Vulnerability and Survivability

• Emphasis is being placed on the dual use of

military and civilian technology. This has

benefits for the customer and manufacturer



Dual use of military and civilian technology

EC 135 Civil

EC 635 Military



• The general design requirements will include

– Hover capability

– Maximum payload

– Range/Endurance

– Cruise or maximum level flight speed

– Climb Performance

– “Hot and High” performance and other environmental

issues

– Manoeuvrability and agility



• The general design requirements will be constrain by:

– Maximum main rotor disk loading

– Maximum physical size

– One engine inoperative performance

– Autorotative capability

– Noise issues

– Maintenance issues

– Crashworthiness

– Radar cross section and detectability (Vulnerability)

– Civil/Military Certification



• The objective will be:

– Smallest Helicopter

– Lightest Helicopter

– Least expensive

• All with the minimum cost (design)

• Simple analytical models


Design of the Main Rotor

• The Main Rotor is the most important component

of the helicopter.

• Small improvements in the Main Rotor efficiency

can potentially result in significant increases in:

– Aircraft payload

– Manoeuvre margins

– Forward flight speeds


Design of the Main Rotor

• The preliminary design of the Main Rotor must take into consideration:

– General sizing

• Rotor diameter

• Disk Loading

• Tip Speed

– Blade Planform

• Chord

• Solidity

• Blade twist

– Airfoil Sections


Main Rotor Diameter

• Large diameter required by:

– Autorotational capabilities

– Hover performance

• Advantages of a large rotor:

– Lower disk loadings

– Lower average induced velocities

– Lower induced power requirements


Main Rotor Diameter

• From the modified momentum theory we have

obtained

• And the CT for the best PL (minimum P/L)

T

P

T

P

C

CR

T

PP

C

CCR i 0

T

PT

C

CCR 0

2

T

0dT

C8

C

2

CR

21

32

0

2

1

d

PLBestT

CC


Main Rotor Diameter

• The disk loading for minimum power loading is:

• We can then obtain the optimum radius for

maximizing the power loading.

• or

A

WCR

2

1

A

TDL

32

0d2

2R

TDL

DL

WR

DL

WR

2

1

Single rotor Dual rotor


Main Rotor Diameter

• We have also seen that the PL is proportional to:

• So the rotor should operate a maximum FM

DL

FM

P

TactualPL


Main Rotor Diameter

• Other factors influence the rotor diameter:

– An aircraft operating in unprepared runway must

have low induced velocity, therefore limited disk

loading (high rotor diameter)

– Large diameter also means higher inertia, better

autorotative characteristics


Main Rotor Diameter

• The rotor diameter will be constrained by:

– Overall helicopter size

• Storage

• Transport

– Weight

– Cost

– Gearbox torque limit

– Speed

– Manoeuvrability

– Static droop of the blades

• Normally the radius is kept smaller than 12m


Main Rotor Diameter


Disk Loading

• We can therefore conclude that for the low disk

loading the advantages are:

– Low induced velocities

– Low autorotative rate of descent

– Low power required in hover

• Advantages of high disk loading:

– Compact size

– Low empty weight

– Low hub drag in forward flight


Tip Speed

• A high tip speed is necessary for:

– Decreases the AOA of the retreating blade

– High kinetic energy

• Reduces design weight

– The rotor torque is lower (Since P=ΩQ)

• Lighter gear box

• Lighter transmission


Tip Speed

• High tip speed also means:

– Compressibility effects

– Noise (rapidly increasing with tip mach number)

• Low tip speed: noise resulting from steady and harmonic

loading is dominant

• High tip speed noise cause by the blade thickness effects

becomes important


Tip Speed


Rotor Solidity

• Definition:

– Ratio between the blade area with the rotor area. For

a rectangular blade:

• Typical values:

– From 0.08 to 0.12

R

cN

R

cRN bb

2


Rotor Solidity

• The average lift coefficient is defined to give the

same lift coefficient when the blade is operating at

the same local lift coefficient (optimum rotor):

• Or

• Typically is found to be on the range of 0.4 to

0.7.

1

0

2

21 drCrC lT

1

0

2

21 drCr L LC

61

T

L

CC 6

LC


Rotor Solidity

• Certification requires that load factors (1.15g)

and bank angles (30º) must be demonstrated

without rotor stalling.

• Therefore the selection of rotor solidity must

have into consideration the blade stall limits.

• Rotor designs for high speed or high

manoeuvrability helicopters must have a high

solidity for a given diameter and tip speed.


Rotor Solidity

• To avoid using a high solidity we can choose an

airfoil with a high maximum lift coefficient that

would allow a lower tip speed.

• Remember all other factors remain constant.


Rotor Solidity


Rotor Solidity

• Lower solidity means lower profile power

• But lower solidity also means:

– Reduced blade lifting area

– Increases the blade loading coefficient

– Increases the local and mean blade lift coefficient

• Therefore decreasing the solidity also decreases

the stall margins.


Rotor Solidity

• Since the onset of stall sets the performance

limits for a rotor its is important to have a big

stall margin :

– Allow for manoeuvres

– Allow for gusts in turbulent air

• A highly manoeuvrable combat helicopter will

require a larger stall margin than a civilian

transport


Rotor Solidity

• The onset of stall in the retreating blade also

limits the rotor performance


Rotor Solidity


Number of blades

• The selection of the number of blades is based

more on dynamic issued than on aerodynamic

issues.

• Following the experimental study performed by

several investigators the conclusion was reached

that the hover performance is primarily affected

by the rotor solidity σ and only secondarily by

the number of blades Nb.


Number of blades

• For a high number of blades:

– Lower vibration levels

– Lower induced tip looses

• The effect on induced power for large aspect ratio blade is

small

– Weaker tip vortex (for the same thrust)

• Reducing the airloads of potential BVI


Number of blades

• Reducing the number of blades:

– Lower weight

– Smaller hubs

• Lower weight

• Lower drag

– Better maintainability

– Less number of BVI


Number of blades

• Typically a light weight helicopter will have 2

blades

• A heavy lift helicopter will have 4, 5 even 7 or 8

blades


Blade Twist

• Using the BEMT we have seen that negative

(nose down) pitch can redistribute the lift over

the blade and help reduce the induced power.

Proper use of the

blade twist can

therefore improve

the FM in hover.


Blade Twist

• In forward flight blades with high nose down

blade twist may suffer some performance loss:

•Reduced AOA on the

tip of the advancing

blade

•Reduced or even

negative lift

•Loss of rotor thrust

and propulsive force


Blade Twist

• Existing helicopter have a negative linear blade

twist of 8º to 15º

• The twist range is a compromise between

maximizing the hover FM and maintaining good

forward flight performance

• Some manufacturers used a non-linear or double

linear twist here the effective twist near the tip is

reduced or even reversed


Blade Planform

• We have already seen that small amounts of taper

over the blade tip can help improve the FM in

hover:


Blade Planform

• Minimum Pi requires λ=const. (uniform inflow)

• Minimum P0 requires α= α(min Cd/Cl)= α1

• Then for minimum induce power θ= θtip/r and

each blade element must operate at α1

• With (BEMT) dCT=4λ2rdr then:

drrC

drrr

CdC

l

tip

l

T

2

1

2

22

8

1 l

rC


Blade Planform

• We have seen that the minimum induced power

requires a uniform inflow. Therefore the previous

equation is constant over the disk.

• Let’s assume that α1 is the same for all airfoils

along the blade and is independent of Re and M

• From the equation since α1 =const and we now

that λ=const then σr must be constant too.

cr

R

Nconstr b


Blade Planform

• The previous situation is achieved when

r

rr

crc

tiptip or


Blade Planform

• However for the benefit is lost for higher taper

ratios since the tip will be operating at smaller

chord Reynolds number and therefore at higher

profile drag coefficients.


Blade tip shape

• The tip of the blades play a very important role

in the aerodynamic performance of the rotor

• The blade tip encounter

– The highest dynamic pressure

– The highest mach number

– The strong trailed tip vortex

• It is very important then to have a properly

design blade tip


Blade tip shape


Blade tip shape

• Anhedral

– Can improve the rotor FM

• Sweeping the leading edge

– Reduces de Mach number normal to the leading edge

• Higher velocities can be achieved before the

compressibility effects increases the profile power

– Effects the Tip vortex formation

• Vortex strength

• Vortex trailed location


Blade tip shape

• Sweep angle

– Constant

– Progressively varying

– Keep low (<20º)

• No inertial coupling in the blade dynamics introduced by an

aft centre of gravity

• No aerodynamic coupling caused by an rearward centre of

pressure


Blade tip shape

• Progressively sweep angle

– Choose a sweep angle that is just sufficient to

maintain a constant incident Mach number

perpendicular to the leading edge:

– The normal velocity to the leading edge Un:

cossinrRUn

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