A 7-seater mid-sized helicopter to be used for transportation was aimed. This design gives a good room for a well-equipped 7 seats cabin and also comes with a skid landing gear just because …Full description
A 7-seater mid-sized helicopter to be used for transportation was aimed. This design gives a good room for a well-equipped 7 seats cabin and also comes with a skid landing gear just because of its ...
Conceptual Helicopter Design • On the other hand design technology for the military market is driven by: – Operational – Operational flexibility and adaptability – Long – Long operational life – Upgradeable – Upgradeable components Vulnerability and Survivability – Vulnerability –
• Emphasis is being placed on the dual use of military and civilian technology. This has benefits for the customer and manufacturer
Conceptual Helicopter Design Dual use of military and civilian technology
EC 135 Civil EC 635 Military
Conceptual Helicopter Design • The general design requirements will include – Hover – Hover capability – Maximum – Maximum payload – Range/Endurance – Range/Endurance – Cruise – Cruise or maximum level flight speed – Climb – Climb Performance – “Hot and High” performance performance and other environmental issues – Manoeuvrability – Manoeuvrability and agility
Conceptual Helicopter Design • The general constrain by:
design
requirements
will
be
– 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
Conceptual Helicopter Design • 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 P T
C P C P C T C P R R R C T C C 2 T T C T C d 0 R 8 C 2 T C P
i
0
0
1
2
• And the C T for the best PL (minimum P/L) C T Best PL
1 C d 0
2
2 3
Main Rotor Diameter • The disk loading for minimum power loading is: DL
T
1
2 C d 0
2
3
W
R A 2 A
• We can then obtain the optimum radius for maximizing the power loading. •
DL
T R
2
R
Single rotor
W DL
or R
1 2
W DL
Dual rotor
Main Rotor Diameter • We have also seen that the PL is proportional to: PL actual
T P
FM DL
• So the rotor should operate a maximum FM
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
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
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
N b cR R
2
N b c R
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):
C T • Or
1 2
1
0
r C l dr
C L 6
2
1 2
1
0
r C L dr 16 C L 2
C T
• Typically C L is found to be on the range of 0.4 to 0.7.
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 N b.
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 P i requires λ=const. (uniform inflow) α1 • Minimum P 0 requires α= α(min C d /C )= l
• Then for minimum induce power θ= θ tip /r and each blade element must operate at α1
C l C l 2 2 dC T 1r dr tip r dr 2 2 r • With (BEMT) dCT=4 λ2rdr then:
rC l 1 8
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.
N b r const cr R
Blade Planform • The previous situation is achieved when
cr
ctip r
or
r
tip r
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 compressibility effects increases the profile power
– Effects the Tip vortex formation • Vortex strength • Vortex trailed location
the
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