L6.1_Advanced Helicopter Designs

rotor mast

swashplate

Helicopter Rotors consists of two or more blades and a hub.

The hub attaches the blades to the _____ and includes a control system, often with a _____ or mechanism to feather the rotor blades.

forward flight

pitch and roll

In additional to lifting the helicopter, the main rotor propels the helicopter in _____ and provides _____ and _____ control.

feathering and flapping

To understand how the main rotor provides propulsion, pitch and roll control, you need to understand _____ and _____.

Main rotor

is powered by a shaft, which itself is power by the helicopter engine(s).

blades

The spinning of this shaft keeps the _____ rotating at a constant speed.

The movement of these blades through the air creates aerodynamic forces that are transferred back to the helicopter.

multirotor or multicopter

is a rotorcraft with more than two lift-generating rotors.

Multirotors

frequently use fixed-pitch blades, as compared to single and double rotor helicopters, which use complex variable pitch rotors whose pitch changes as the blade rotates for flight stability and control.

Vehicle motion

is controlled by changing the relative speed of each rotor to change the thrust and torque produced by each.

Synchropter

a helicopter with two rotors turning in opposite directions and mounted on two masts slightly inclined towards each other, so that the blades intermesh (without colliding).

Synchropter

two rotors mesh with one another, like a gearwheel.

Synchropter

this does not require a tail rotor, since the anti-torque action is performed by the counter-rotation of the rotors.

Synchropter

have high stability and powerful lifting capabilities.

Tandem Rotor

has two main rotor systems and no tail rotor.

Usually the rear rotor is mounted at a higher position than the front rotor, and the two are designed to avoid the blades colliding, should they flex into the other rotor’s pathway.

Tandem Rotor

used for larger helicopters, has the advantage of being able to support more weight with shorter blades.

Tandem Rotor

are typically powerful and fast.

Coaxial Rotor

are two main rotors mounted on one mast, sharing the same axis of rotation but turning in opposite directions, one on top of the other.

Coaxial Rotor

drag produced by the rotors is quite large due to the interference of airflows, so these helicopters do not normally have a high cruising speed.

Coaxial Rotor

Mounting rotors closer together, which is possible only with rigid rotors, reduces the amount of drag produced.

Tilt Rotor

combines the vertical lift capability of a helicopter with the speed and range of a turboprop airplane.

Tilt Rotor

their two engines, which tilt on the wings, move the "proprotors" in a counter rotating way to counteract the torque of the body.

Tilt Rotor

differs from a tiltwing in that only the rotor pivots rather than the entire wing.

This method trades off efficiency in vertical flight for efficiency in STOL/STOVL operations.

Tricopter

all three rotors rotated in the same direction.

Tricopter

was chosen out of concern that a single rotor would not generate enough lifting power.

Tricopter

already in 1926, this configuration was patented by Nicolas Florine in Belgium.

Tricopter

vibrations and poor flight control were its main flaws.

Tricopter

control of the helicopter was very marginal when making left turns.

Cierva W.11 Air Horse

Tricopter

was a helicopter developed by the Cierva Autogiro Company in the United Kingdom during the mid-1940s.

Cierva W.11 Air Horse (Tricopter)

is the only helicopter of its type ever built and included three lifting rotors all turning in the same direction.

large

The adoption of three rotors was due to concerns over the capability of a single _____ rotor to generate the required lift

W.11 G-ALCV (Tricopter)

made its first flight on 7 December 1948 and was displayed at the Farnborough Air Show in 1949.

G-ALCV (Tricopter)

crashed on 13 June 1950.

The cause of the crash was due to fatigue failure of a swashplate carrier driving link in the front rotor hub.

G-ALCW

Approximately one year later (1951), the second W.11, _____ was scrapped.

Quadcopter or Quadrotor

is a type of helicopter with four rotors.

Quadcopter or Quadrotor

each rotor produces both lift and torque about its center of rotation, as well as drag opposite to the vehicle's direction of flight.

cyclic pitch control

Unlike conventional helicopters, quadcopters do not usually have _____, in which the angle of the blades varies dynamically as they turn around the rotor hub.

Quadcopters

were seen as a possible solution to some of the persistent problems in vertical flight.

Quadcopters

torque-induced control issues can be eliminated by counter rotation, and the relatively short blades are much easier to construct.

de Bothezat helicopter, also known as the Jerome-de Bothezat Flying Octopus

was an experimental quadrotor helicopter built for the United States Army Air Service by George de Bothezat in the early 1920s, and was said at the time to be the first successful helicopter.

de Bothezat helicopter, also known as the Jerome-de Bothezat Flying Octopus

although its four massive six-bladed rotors allowed the craft to successfully fly, it suffered from complexity, control difficulties, and high pilot workload, and was reportedly only capable of forward flight in a favorable wind.

The Army canceled the program in 1924, and the aircraft was scrapped.

Volocopter

a series of German prototype electric multicopters with 18 rotors, the first electric multicopter in the world to achieve crewed flight.

Volocopter

large number of low-cost motors make it economical, quiet and provide redundancy with ability to maintain control with up to four failed motors.

Volocopter GmbH

is a German aircraft manufacturer based in Bruchsal.

The company specializes in the design of electric multirotor helicopters in the form of personal air vehicles, designed for air taxi use.

Summary of all products designed and built:

1. Volocopter VC1 (prototype)

2. Volocopter VC2 (prototype)

3. Volocopter VC200 (prototype)

4. Volocopter 2X (prototype)

5. Volocopter VoloCity (2021 two-seater eVTOL aircraft)

6. Volocopter VoloConnect (2021 announced new eVTOL aircraft)

Allgemeiner Deutscher Automobil-Club (ADAC)

In January 2021, the company confirmed that the _____ had reserved two of its VoloCity aircraft for operational testing in 2023.

January 2021

Volocity

Washington, DC

Also in _____, the company announced that the FAA had accepted its application to concurrently validate the EASA type certification it expects secure within the next three years.

In the same announcement, the company claimed it was exploring launching _____ within the United States to provide intra-city air taxi services in major metropolitan areas such as New York, Los Angeles, San Francisco, and _____.

VS-300

Ever since the development and flight of the first practical helicopter, the _____ in 1943 by Igor Sikorsky, the father of modern helicopters, thousands of man-hours and billions of dollars have been spent on improving the helicopter design in order to make it fly farther and faster.

150 knots

350 nautical miles

Due to the engineering and aerodynamic constraints related to helicopters, the design approaches over the years have not yielded any significant results.

Most of the rotary-wing machines today are limited to speeds of approximately _____ at the top end and a range of around _____.

retreating blade stall

most significant aerodynamic factors limiting the speed of a helicopter is a phenomenon known as ‘_____.’

blade tip, the area of the blade rotating the fastest

As the helicopter’s forward speed increases, the retreating blade will reach a point where the _____ reaches its critical angle and stalls, causing loss of lift, buffet, vibrations and ultimately loss of control.

Bell

Sikorsky

Concerted efforts have been made by major helicopter giants like ____, _____, Boeing and Airbus to overcome this ‘retreating blade stall’ in the last decade plus, but with limited success.

Tilt-Rotor

If the designer has to rely on a rotating blade to provide the majority of lift for the rotorcraft, retreating blade stall will produce a barrier to top speed, efficiency and range – hence the only way to overcome this problem was to look at the concept of merging the attributes of a helicopter with a fixed-wing aircraft.

This meant that during cruise flight, the aircraft should rely on a wing to produce the lifting force and the rotors must morph from providing all of the lift while in the hover, to producing all of the propulsive force in wing-borne cruise flight – basically called the _____.

maneuverability

Advantages of Tilt-Rotor

The ease with which the aircraft can be converted from one flight mode to another enhances its _____ and permits the aircraft to be configured to meet mission requirements.

flexibility

Advantages of Tilt-Rotor

It provides a flight envelope that overlaps the helicopter and airplane flight envelopes, increasing mission _____.

VTOL

Advantages of Tilt-Rotor

Operating as a _____ aircraft, it can take off like a helicopter and deliver payloads on less fuel than consumed by a helicopter.

STOL

Advantages of Tilt-Rotor

In the _____ mode, tiltrotor aircraft are ideal for long distance transport of heavy cargos into remote areas. Only short runways are necessary.

twin or tandem-rotor

In vertical flight, the tiltrotor uses controls very similar to a _____ helicopter.

Yaw

is controlled by tilting its rotors in opposite directions.

Roll

is provided through differential power or thrust.

Pitch

is provided through rotor blades cyclic, or nacelle, tilt.

Vertical motion

is controlled with conventional rotor blade pitch and either a conventional helicopter collective control lever or a unique control similar to a fixed-wing engine control called a thrust control lever (TCL).

thrust control lever (TCL)

Vertical motion is controlled with conventional rotor blade pitch and either a conventional helicopter collective control lever or a unique control similar to a fixed-wing engine control called a _____.

2017 Bell V-280 Valor

is Bell's third generation tilt-rotor designed for the the US Army’s Joint Multi-Role (JMR) / Future Vertical Lift (FVL) programme which search for a UH-60 Black Hawk replacement from 2030.

2017 Bell V-280 Valor

will cruise at 280kts (519km/h) carrying 11 passengers with a mission radius of over 250nm (463km).

2017 Bell V-280 Valor

Prototype performed first flight on December 18, 2017.

Coaxial rotors or coax rotors

are a pair of helicopter rotors mounted one above the other on concentric shafts, with the same axis of rotation, but turning in opposite directions (conta-rotating).

main rotor

The problems with any single set of rotor blades is the torque (rotational force) exerted on the helicopter fuselage in the direction opposite to the rotor blades.

This torque causes the fuselage to rotate in the direction opposite to the rotor blades.

In single rotor helicopters, the antitorque rotor or tail rotor counteracts the _____ torque and controls the fuselage rotation.

Coaxial rotors

solve the problem of main rotor torque by turning each set of rotors in opposite directions.

collective pitch

Coaxial rotors

The opposite torques from the rotors cancel each other out.

Rotational maneuvering, yaw control, is accomplished by increasing the _____ of one rotor and decreasing the _____ on the other.

This causes a controlled dissymmetry of torque.

Dissymmetry of lift

is an aerodynamic phenomenon caused by the rotation of a helicopter's rotors in forward flight.

Rotor blades

provide lift proportional to the amount of air flowing over them.

Dissymmetry of lift

is the differential (unequal) lift between advancing and retreating halves of the rotor disk caused by the different wind flow velocity across each half.

Coaxial rotors

avoid the effects of dissymmetry of lift through the use of two rotors turning in opposite directions, causing blades to advance on either side at the same time.

tail rotor

typically wastes some of the available engine power that would be fully devoted to lift and thrust with a coaxial design.

Reduced noise

is the main advantage of the configuration; some of the loud "spanking" sound associated with conventional helicopters arises from interaction between the airflows from the main and tail rotors, which in some designs can be severe

Coaxial Rotors

Helicopters using _____ tend to be more compact.

It also increased safety on the ground; the absence of a tail rotor eliminates the major source of injuries and fatalities to ground crews and bystanders.

Some Hazards of Helicopter Flight

Retreating blade stall

forward flight

hovering

The U.S. Department of Transportation has published a “Basic Helicopter Handbook”.

One of the chapters in it is titled, “_____”.

10 hazards have been listed to indicate what a typical single rotor helicopter has to deal with.

The coaxial rotor design either reduces or completely eliminates many of these hazards. The following list indicates which:

1. Settling with power: Reduced

2. _____: Reduced

3. Medium frequency vibrations: Reduced

4. High frequency vibrations: None

5. Anti torque system failure in _____: Eliminated

6. Anti torque system failure while _____: Eliminated

The reduction and elimination of these hazards are the strong points for the safety of coaxial rotor design.

Coaxial Rotors

there is an increased mechanical complexity of the rotor hub.

Coaxial Rotors

The linkages and swashplates for two rotor systems need to be assembled atop the mast, which is more complex because of the need to drive two rotors in opposite directions.

Coaxial Rotors

because of the greater number of moving parts and complexity, it is more prone to mechanical faults and possible failure.

whipping

self-collision

Coaxial helicopters are also more prone to the "_____" of blades and blade _____.

2015 Sikorsky S-97 Raider

includes variable speed rigid coaxial main rotors and a variable-pitch pusher propeller, making the it a compound helicopter.

2015 Sikorsky S-97 Raider

has fly-by-wire control and dynamic anti-vibration actuators to cancel out shaking.

hingeless

stiff

hubs

stiff blades

propulsion

2015 Sikorsky S-97 Raider

The main rotors have _____ hubs and _____ blades, to improve low-speed handling and efficiency of hover.

At high speeds, the close spacing of the _____ reduces drag.

The _____ allow each rotor to have low lift on the retreating side of its rotor disk (reducing drag), whereas each rotor on a conventional coaxial rotor with "floppy" blades needs nearly equal lift distribution.

The propeller relieves the rotor of _____, further reducing drag.

2015 Sikorsky S-97 Raider

capable of carrying up to six passengers, in addition to a flight crew of two in a side-by-side cockpit.