Far 23 PROPELLER ENGINE INTERFERENCE DESIGN

The Federal Aviation Regulations

FAR

SUBMITTED BY PAULRAJ J OJT BATCH -6

FARs, are rules prescribed by the Federal Aviation Administration

governing all aviation activities in the United States.

PART 23 It contains airworthiness standards for

airplanes in the normal, utility, aerobatic, and commuter categories.

It dictates the standards required for issuance and change of type certificates.

E.g., the maximum takeoff weight of an airplane in the normal, utility or acrobatic category cannot exceed 12,500 lb,

commuter category it cannot exceed 19,000 lb.

This part has a large number of regulations to ensure airworthiness in areas such as structural loads, airframe, performance, stability, controllability, and safety mechanisms, how the seats must be constructed, oxygen and air pressurization systems, fire prevention, escape hatches, flight management procedures, flight control communications, emergency landing procedures, and other limitations, as well as testing of all the systems of the aircraft.

It also determines special aspects of aircraft performance such as stall speed (e.g., for single engine airplanes – not more than 61 knots), rate of climb (not less than 300 ft/min), take-off speed (not less than 1.2 x VS1), and weight of each pilot and passenger (170 lb for airplanes in the normal and commuter categories, and 190 lb for airplanes in the acrobatic and utility categories).

POWER PLANTS

An aircraft engine is the component of the propulsion system  that generates mechanical power.

Aircraft engines are almost always either lightweight piston engines or gas turbines.

EXAMPLES FOR PISTON

ENGINES

PROPELLER

propeller converts rotary motion from an engine to provide propulsive force.

It comprises a rotating power-driven hub, to which are attached several radial airfoil-section

The blade pitch may be fixed, manually variable to a few set positions, or of the automatically-variable "constant-speed" type.

Introduction

It attaches to the power source's driveshaft either directly  reduction gearing.

Materials only suitable for use at subsonic airspeeds up

to around 480 mph (770 kph), above this speed the blade tip speed begins

to go supersonic, with the consequent shockwaves causing high drag and other mechanical difficulties.

PROPELLERS

Nieuport N.28C-1

C-130

Counter-rotating propellers (handed propellers)

 balance out the torque effects of high-power piston engine  gyroscopic precession effects (p-factor) during flight manoeuvres, eliminating the problem of the critical engine.

Contra-rotating propeller

The forward propeller provides the majority of the thrust,

The rear propeller also recovers energy lost in the swirling motion of the air in the propeller slipstream.

Provide good circulation strength. increases the ability of absorb power from

engine, without increasing propeller diameter. added cost, complexity, weight and noise of

the system it is only used on high-performance types

where ultimate performance is more important than efficiency.

The earliest references for vertical flight came from China.

Since around 400 BC, Chinese children have played with bamboo flying toys.

HISTORY

Pioneered the twisted aerofoil shape of modern aircraft propellers . 

The Wrights realized that a propeller is essentially the same as a wing, and were able to use data from their earlier wind tunnel experiments on wings. This was necessary to ensure the angle of attack of the blades was kept relatively constant along their length

Wright brothers

early pioneer, make a propeller with a steel shaft and aluminium blades for his 14 bis biplane.

Some of his designs used a bent aluminium sheet for blades, thus creating an airfoil shape.

They were heavily undercambered, absence of lengthwise twist made them less efficient than the Wright propellers.

Alberto Santos Dumont

Theory and design of aircraft propellers

A well-designed propeller typically has an efficiency of around 80% when operating in the best regime.

 The efficiency of the propeller is influenced by the angle of attack (α). This is defined as

α = Φ – θ where θ is the helix angle (the angle

between the resultant relative velocity and the blade rotation direction)

Φ is the blade pitch angle.

Propellers are similar in aerofoil section to a low-drag wing

Efficiency is poor in when at other than their optimum angle of attack.

Therefore, some propellers use a variable pitch mechanism to alter the blades' pitch angle as engine speed and aircraft velocity are changed.

How lift is generatedPROPELLER SYSTEM

In this example

Pressure Remains Constant here

Pressure Decreases here In this direction

The result is LIFT

How lift is generatedPROPELLER SYSTEM

Small Pressure Increase here

Greater Pressure Decrease here

The result isMORE LIFT

How lift is increasedPROPELLER SYSTEM

Direction of travel

Aerofoil incline

The difference in direction of travel and aerofoil incline is called:-

The ANGLE of ATTACK

How lift is increasedPROPELLER SYSTEM

How does lift apply to PROPELLORS?

On Propellers, LIFT is called THRUST

And propeller Blades work the same way as aircraft wings

When a propeller spins and the aircraft moves forward, the tips of the propeller blades move in a ‘corkscrew’ path

This path is called a HELIX PROPELLER SYSTEM

How the HELIX ANGLE is generated

How the blade tip travel produces the HELIX ANGLE PROPELLER SYSTEM

How the blade tip travel produces the HELIX ANGLE PROPELLER SYSTEM

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

How the blade tip travel produces the HELIX ANGLE PROPELLER SYSTEM

Forward Speed - Distance Travelled over One Minute

Rotation - Number of Rotations per Minute

Forward Speed

RPM

How the blade tip travel produces the HELIX ANGLE PROPELLER SYSTEM

PROPELLER SYSTEM

Forward Speed

RPM

How the blade tip travel produces the HELIX ANGLE

Forward Speed

RPM

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

Forward Speed

RPM

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

Forward Speed

RPM

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

Forward Speed

RPM

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

Forward Speed

RPM

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

How the HELIX ANGLE is changed by engine rpm and forward speed

Forward Speed

RPM

How an increase in RPM changes the Helix Angle

Changes in FORWARD SPEED and RPM will change the Helix Angle

Faster RPM

PROPELLER SYSTEMHow the blade tip travel produces the HELIX ANGLE

Forward Speed

RPMRPM

Faster Forward Speed

Changes in FORWARD SPEED and RPM will change the Helix Angle

How an increase in FORWARD SPEED changes the HELIX ANGLE PROPELLER SYSTEM

Let’s take a closer look at the blade aerofoil and the Helix Angle and thrust (lift) generation

If the Helix Angle changes, then we need to change the

blade angle.

Remember (from the comparison with the aircraft wing), the optimum Angle of Attack is required to maintain most efficient thrust generation.

This is the Helix Angle

This is the Angle of AttackDirection of

rotationDirection of blade through the air with forward speed

PROPELLER SYSTEM

Mechanical STOPS and blade angles

All propeller blades are actuated by the same mechanical linkage

PROPELLER SYSTEM

Sliding Piston

Hard Stops

Fine Pitch

Coarse Pitch

Direction of

Rotation

Direction of Flight

Propeller Blade

Actuating Lever

Actuating Link

Note: - blade angle is relative to piston travel

Fine pitchCoarse pitch

Or‘Feathered’

Piston travels between ‘hard’ stops

DirectionOf

Rotation

Maximum resistance to rotation

Minimum resistance to forward

speed

Minimum resistance to

rotation

Maximum resistance to forward

speed

The blade angle changes through 90deg with piston travel

At this hard stop the blade is in this

position

At this hard stop the blade is in this

position

PROPELLER SYSTEM

Easier Starting of engine

Direction of travel

Direction of Rotation

Good for:-

Running engine with no/minimal thrust

Bad for:-

In-flight – loss of control

High drag – braking effect on ground

Zero pitch – or Ground Fine Pitch

In-flight engine failure – loss of control andengine disintegration

PROPELLER SYSTEMImportance of set blade angle

Fine pitch

Minimum resistance to

rotation

Maximum resistance to forward

speed

Maximum resistance to rotation

Minimum resistance to forward

speed

Starting of engine

Direction of travel

Direction of Rotation

Bad for:-

Could cause engine burn-out if running

Low drag – NO braking effect on ground

Maximum pitch – or Feathered

Good for:-

In-flight – loss of control

In-flight engine failure – control maintained and engine stops rotating minimizing damage

PROPELLER SYSTEMImportance of set blade angle

Minimal resistance to

rotation

Air pushed forward giving reverse thrust

Direction of travel

Direction of Rotation

Used for:-

Bad for:-

In-flight – loss of forward speed, aircraft stalls

High drag – high braking effect on ground

Reverse Pitch

In-flight engine failure – loss of control and reverse rotation increasingengine disintegration

Usually for military aircraft only

PROPELLER SYSTEMImportance of set blade angle

Direction of travel

Direction of Rotation

Used for:-

Low drag on final approach

Flight Fine and Cruise Pitch

Used for:-

In-flight descent – faster forward speed thanfinal approach

Flight Fine pitch

Cruise pitch

Both give minimal drag at low power settings

PROPELLER SYSTEMImportance of set blade angle

Blade Twist

DISTANCE TRAVELLED BY ROOT, MID-SPAN AND TIP

THICK FOR STRENGTH

PROPELLER SYSTEMBlade Twist

ROOT MID-SPAN TIP

THINNER FOR STRENGTH AND

THRUST

THIN FOR THRUST

COARSE ANGLE

MEDIUM ANGLE

FINE ANGLE

BLADE ANGLE RELATIVE TO DISTANCE (AND THEREFORE SPEED) TRAVELLED BY ROOT, MID-SPAN AND TIP

Typical Blade

Typical 3 Blade Prop

NO OF BLADES Increasing the aspect ratio  The blades reduces drag but the amount of thrust

produced depends on blade area, so using high-aspect blades can result in an excessive propeller diameter.

smaller number of blades It reduces interference effects between the blades, but to

have sufficient blade area to transmit the available power within a set diameter means a compromise is needed.

Increasing the number of blades It decreases the amount of work each blade is

required to perform limiting the local Mach number - a significant

performance limit on propellers.

A propeller's performance suffers as the blade speed nears the transonic.

As the relative air speed at any section of a propeller is a vector sum of the aircraft speed and the tangential speed due to rotation

propeller blade tip will reach transonic speed well before the aircraft does.

propeller's performance transonic speed

When the airflow over the tip of the blade reaches its critical speed, drag and torque resistance increase rapidly and shock waves form creating a sharp increase in noise.

Aircraft with conventional propellers, therefore, do not usually fly faster than Mach 0.6.

There have been propeller aircraft which attained up to the Mach 0.8 range, but the low propeller efficiency at this speed makes such applications rare.

 The 'fix' is similar to that of transonic wing design. The maximum relative velocity is kept as low as possible by careful control of pitch to allow the blades to have large helix angles; thin blade sections are used and the blades are swept back in a scimitar shape (Scimitar propeller).

The propellers designed are more efficient than turbo-fans and their cruising speed (Mach 0.7–0.85) is suitable for airliners, but the noise generated is tremendous (see the Antonov An-70 and Tupolev Tu-95 for examples of such a design).

Forces acting on a propeller

Thrust bending force Centrifugal and aerodynamic

twisting forces Centrifugal force Torque bending force

Variable pitch

Early pitch control settings were pilot operated, either with a small number of preset positions or continuously variable.

Following World War I, automatic propellers were developed to maintain an optimum angle of attack.

This was done by balancing the centripetal twisting moment on the blades and a set of counterweights against a spring and the aerodynamic forces on the blade.

Automatic props had the advantage of being simple, lightweight, and requiring no external control, but a particular propeller's performance was difficult to match with that of the aircraft's powerplant.

Feathering

On some variable-pitch propellers, the blades can be rotated parallel to the airflow to reduce drag in case of an engine failure. This uses the term feathering,

On single-engined aircraft, whether a powered glider or turbine-powered aircraft, the effect is to increase the gliding distance.

On a multi-engine aircraft, feathering the propeller on a failed engine helps the aircraft to maintain altitude with the reduced power from the remaining engines.

Reverse pitch In some aircraft, such as the C-130 Hercules, the

pilot can manually override the constant-speed mechanism to reverse the blade pitch angle, and thus the thrust of the engine (although the rotation of the engine itself does not reverse).

This is used to help slow the plane down after landing in order to save wear on the brakes and tires, but in some cases also allows the aircraft to back up on its own - this is particularly useful for getting floatplanes out of confined docks. See also Thrust reversal.

Thrust

Power

Activity Factor Activity factor (AF) is a design parameter

associated with the propeller blade’s geometry. The more slender the blade (larger radius, smaller chord), the lower the AF value:

xxdcAF

hx p

d16

100000 31

pdcAF 1563

Typically see higher AF props on turboprop engines.

propeller's efficiency

RULES AND REGULATIONS Each propeller must have a type

certificate. Engine power and propeller shaft rotational

speed may not exceed the limits for which the propeller is certificated.

Each featherable propeller must have a means to unfeather it in flight.

The propeller blade pitch control system must meet the requirements of §§35.21, 35.23, 35.42 and 35.43 of this chapter.

All areas of the airplane forward of the pusher propeller that are likely to accumulate and shed ice into the propeller disc during any operating condition must be suitably protected to prevent ice formation, or it must be shown that any ice shed into the propeller disc will not create a hazardous condition.

Each pusher propeller must be marked so that the disc is conspicuous under normal daylight ground conditions.

If the engine exhaust gases are discharged into the pusher propeller disc, it must be shown by tests, or analysis supported by tests, that the propeller is capable of continuous safe operation.

All engine cowling, access doors, and other removable items must be designed to ensure that they will not separate from the airplane and contact the pusher propeller.

Propeller vibration and fatigue. This section does not apply to fixed-pitch wood propellers

of conventional design. The applicant must determine the magnitude of the

propeller vibration stresses or loads, including any stress peaks and resonant conditions, throughout the operational envelope of the airplane by either:

(1) Measurement of stresses or loads through direct testing or analysis based on direct testing of the propeller on the airplane and engine installation for which approval is sought.

(2) Comparison of the propeller to similar propellers installed on similar airplane installations

(b) The applicant must demonstrate by tests, analysis based on tests, or previous experience on similar designs that the propeller does not experience harmful effects of flutter throughout the operational envelope of the airplane.

(c) The applicant must perform an evaluation of the propeller to show that failure due to fatigue will be avoided throughout the operational life of the propeller using the fatigue and structural data obtained in accordance with part 35 of this chapter and the vibration data obtained from compliance with paragraph (a) of this section.

The propeller includes the hub, blades, blade retention component and any other propeller component whose failure due to fatigue could be catastrophic to the airplane. This evaluation must include:

(1) The intended loading spectra including all reasonably foreseeable propeller vibration and cyclic load patterns, identified emergency conditions, allowable overspeeds and overtorques, and the effects of temperatures and humidity expected in service.

(2) The effects of airplane and propeller operating and airworthiness limitations.

Propeller clearance. Unless smaller clearances are substantiated, propeller

clearances, with the airplane at the most adverse combination of weight and center of gravity, and with the propeller in the most adverse pitch position, may not be less than the following:

(a) Ground clearance. There must be a clearance of at least seven inches (for each airplane with nose wheel landing gear) or nine inches (for each airplane with tail wheel landing gear) between each propeller and the ground with the landing gear statically deflected and in the level, normal takeoff, or taxing attitude, whichever is most critical.

In addition, for each airplane with conventional landing gear struts using fluid or mechanical means for absorbing landing shocks, there must be positive clearance between the propeller and the ground in the level takeoff attitude with the critical tire completely deflated and the corresponding landing gear strut bottomed. Positive clearance for airplanes using leaf spring struts is shown with a deflection corresponding to 1.5g.

(b) Aft-mounted propellers. An airplane with an aft mounted

propeller must be designed such that the propeller will not contact the runway surface when the airplane is in the maximum pitch attitude attainable during normal takeoffs and landings.

(c) Water clearance.  There must be a clearance of at least 18

inches between each propeller and the water, unless compliance with §23.239 can be shown with a lesser clearance.

(d) Structural clearance. There must be— (1) At least one inch radial clearance between

the blade tips and the airplane structure, plus any additional radial clearance necessary to prevent harmful vibration;

(2) At least one-half inch longitudinal clearance between the propeller blades or cuffs and stationary parts of the airplane; and

(3) Positive clearance between other rotating parts of the propeller or spinner and stationary parts of the airplane.