Airframe

Question 1

Length

Answer 1

42.45 ft (12.94 m) from main-rotor tip to the upper tip of the vertical stabilizer.

From nose to tail, the airframe is 35.86 ft (10.93 m)

Question 2

Height

Answer 2

Top of the Starflex main-rotor hub is a half an inch shy of 11 ft tall (3.34 m)

Question 3

Engine

Answer 3

Turbomeca Arriel 1D1

• Free-turbine engine (clutchless drive) with integral freewheel
• Modular design
• Cooling system and externally fitted oil tank

Question 4

Arriell 1D1 weight

Answer 4

287 lbs/ 130 Kg

Question 5

Main Rotor Hub (MRH)

Answer 5

• Starflex semi-rigid
• Bearingless hub (laminated glass-resin star) without a drag damper
• No grease nipples
• Modular, fail-safe design

Question 6

Main Rotor Blades

Answer 6

• Spar made of fiberglass roving
• Glass fabric skin and foam core
• Fail- safe design

Question 7

Main Rotor Mast

Answer 7

• Removable subassemblies
• Mast casing attached by 4 suspension bars which “support” the helicopter
• Includes the servo actuators

Question 8

Main Gearbox

Answer 8

• Modular design
• Attached by flexible bidirectional suspension
• Two reduction gear stages (1 bevel gear drive, 1 epicylic gear train)
• Pressure lubrication with oil cooling system
• Includes rotor brake and hydraulic pump drive

Question 9

Structural Subassemblies

Answer 9

A FEW WORDS ABOUT THESE NEW MATERIALS
They are synthetic resins divided into 2 main classes:
- Thermoplastics which soften when heated and harden when cooled, e.g. polyamides (Nylon, Rilsan), polycar- bonates, etc.
- Thermosetting resins which, under the combined ac- tion of heat and a hardener, hot-cure irreversibly to form a new product, e.g. epoxy resins, silicone, etc. Laminates and laminated honeycomb are reinforced plas- tics with very good mechanical strength properties. Laminated materials are produced from thermosetting resins and reinforcing materials (glass, carbon, graph- ite, boron or other fibers).

Question 10

Tail Boom Strake

Answer 10

In sideways flight to the left, the main rotor downwash is deflected and accelerated over the RH side of the tail boom, which induces a negative pressure of approxi- mately 1 mbar/cm2 along the entire tail boom. This re- duces the effect of the tail rotor by roughly 5%. A strake added at 45° causes the main rotor downwash flow to separate and restores the pressure to the static value. The effect of the strake is thus to regain the 5% moment and to improve the tail rotor efficiency (including in hover).

The strake is secured longitudinally from the forward frame to the horizontal stabilizer. It is designed to gener- ate a pressure equal to the static pressure on the RH side of the tail boom.

Question 11

Tail Rotor

Answer 11

• Hingeless, greaseless seesaw rotor with glass roving spar
• Pitch change by spar twisting
• Fail- safe design

Question 12

Tail Rotor Gearbox (TGB)

Answer 12

Angle reduction gear with splash lubrication

Question 13

Tail Guard

Answer 13

Protects the ventral fin

Question 14

Vertical Fins

Answer 14

Dorsal (upper)

Ventral (lower)

• In cruise flight, the asymmetric NACA airfoil of the dorsal fin generates an aerodynamic force that opposes the main rotor’s counter torque and thus reinforces the tail rotor thrust.
• The ventral fin has a symmetric NACA airfoil to stabilize the helicopter about its yaw axis.

Question 15

Horizontal Stabilizer

Answer 15

An asymmetric NACA airfoil, set at negative angle to the horizontal datum; creates nose-up moment with a relative wind

Question 16

Landing Gear

Answer 16

Supports the helicopter

Protects the airframe on landing

Damps out vibration when the helicopter is on the ground with the rotor spinning

The landing gear assembly comprises:

• front and rear cross tubes
• two skids
• two hydraulic shock absorbers

Question 17

Flexible Steel Strip behind Skid

Answer 17

A flexible steel strip bent downwards behind the skid in- creases the landing gear stiffness and changes its natu- ral frequency so that ground resonance can never occur.

Question 18

Ground Resonance

Answer 18

When the helicopter is on the ground the vibrations have a support point via the landing gear

If the natural frequency of the landing gear coincides with the principal vibrational frequencies of the main rotor, the vibrations are augmented every blade revolution as they receive a new “reflected” impulse

The vibration amplitude then increases very rapidly

The vibration becomes divergent and the resulting oscillations can destroy and overturn the helicopter.

Question 19

Main Rotor Drive System

Answer 19

Transmits engine power to the MR and TR drive shaft

Consists of:

• the engine/MGB coupling
• the main gearbox (MGB)
• the MGB suspension

Question 20

Engine Controls

Answer 20

2 mechanical controls:

• The FFC operated by the pilot
• An automatic engine governor compensation control, coupled to the collective pitch control.
• Meters the fuel quantity to match the power demand (collective pitch function)
• At the same time, keeps the free turbine rpm constant

Question 21

Engine Governing Control

Answer 21

Acts on the free turbine governor; the control reacts automatically to collective pitch variations and:

• it compensates for the STATIC DROOP of the centrifugal governor, by maintaining a CONSTANT rotor rpm (NR) irrespective of the fuel flow and hence irrespective of the power demand
• it has a very fast RESPONSE TIME to prevent surging during sudden accelerations and flameout during sud- den decelerations. This is why this control is also called an ANTICIPATOR since it acts prior to the normal reac- tion of the centrifugal governor.

Question 22

Static Droop

Answer 22

Paraphrased: the small RPM difference due to the centrifugal governor’s reactions to power demand changes. Anticipator (attached to collective) acts prior to centrifugal governor’s normal reactrions.

The function of the free turbine governor is to maintain the Nf (and hence Nr) constant.

A simple Watt type governor, consisting of a directly acting, flyweight centrifugal governor operating in an “open loop”.

Open Loop: detects rpm variations and counteracts them but it does not check or correct the results of its action. Cannot operate “intelligently” because it is not informed of the effects of its action. In cybernetics, such a governing system is termed “open loop”, as opposed to “closed loop” systems where there is a feedback from the sensing element which compares the result with a reference value and modifies its magnitude.

Consequently, Nr is not strictly constant: compared to the selected governor speed, the rpm drops slightly when the power demand increases and rises slightly when the power demand decreases. This small rpm difference is called “static droop”.

Question 23

Engine Power Parameters/Limitations

Answer 23

• NG - gas generator rpm: Power produced by the engine varies with NG, which directly depends on the amount of fuel ignited.
• T4 - gas temperature at the free turbine inlet depends mainly on the quantity of fuel ignited
• Torue (Cm): torque transmitted to the rotors by the free turbine; represents the power absorbed by the rotors. Power varies with the collective pitch.

Now the power absorbed by rotors = Cm x ω

As ω (rotor rpm) = constant, the power absorbed by the rotors is proportional to the engine torque.

NG and T4 limitations to protect the engine.

Torque (Cm) limitations to protect the MGB

Question 24

Engine Torque Monitoring

Answer 24

measured at the intermediate pinion of the engine reduction gear

pinion has helical teeth and it is therefore subject to an axial thrust (PA) proportional to the engine torque and to an axial reaction RA equal to PA, i.e. the reaction itself is also proportional to the engine torque. The engine torque is then measured using the axial displacement of the pinion due to RA.