Unit 15 Aircraft Propulsion Technology
Outcome 1.1

1) T=m(V0 -V1) m= 1000kg/s T=1000(120-100) V0 =120m/s T=20,000N V1 =100 m/s

2) Total thrust =Thrust of core engine + thrust of fan

Thrust = Mass airflow x (bypass velocity – aircraft velocity) + Mass airflow x (exhaust velocity – aircraft velocity)

Thrust = 300 x (180-120) + 200 x (220-120) = 300 x 60 + 200 x100 = 1800 + 2000 = 38,000N

3) Cross reference to Unit 17 Gas turbine science outcome 2 assignment 2 Q1

4) Cross reference to Unit 17 Gas turbine science outcome 2 assignment 1 Q1

Outcome 3.2

Materials used in gas turbines have gone through many incremental improvements since the first practical turbines were developed in the 1940s. Most recent efforts have led to improved steel alloys for use in turbine vanes, blades, and inlet blocks. material improvements have led to an increase in rotor life and reliability.

Progress in gas turbine material development often came in the form of alternative stainless steel or metal alloys that had improved heat characteristics. Different parts of gas turbines use a variety of alloy metals, including varying quantities of cobalt, nickel, and chromium. In turbine compressors, manufacturers vary in their metals and manufacturing methods, but initial blades are often made with stainless steel because it is strong and easy to machine.

Materials in other parts of the turbine have been changed more frequently as the state of the art advanced. This is certainly the case with stationary turbine blades (or vanes). Some early stationary blade designs used welded structures in austenitic stainless steel that had excellent resistance to both corrosion and to oxidation at elevated temperatures, but which had limited strength capabilities. Some turbojets then switched to higher strength, nickel-based alloys. In the1960s, engineers began to design these vanes with cobalt alloys for two reasons. First, cobalt alloys have high heat tolerances and can withstand high firing temperatures and corrosion with less cracking or warping. Second, cobalt alloys tend to have favourable welding characteristics.
The welding ease of this metal can be extremely important when facing the inevitable fact that turbine vanes will occasionally crack with time and use. Having the ability to adequately repair a vane through welding is far preferable and less costly than having to replace the whole component. In this sense, material improvements in stationary blades and vanes have improved heat characteristics and increased rotor life by reducing turbine damage and allowing easier maintenance. Cobalt alloys are still used today, although the type of alloy has been improved to increase creep and oxidation resistance.

Some turbine manufacturers have also increased their use of titanium, a particularly strong but expensive metal, in their gas turbine components. Rotating turbine blades have also improved with progressions in their materials. These rotating blades tend towards nickel alloys, which also display improved properties with iterative change. Early designs used a variety of nickel-based alloys and even some 12% chrome material similar to that used for compressor blades. Development led to the more widespread use of some standard Inconel nickel alloys, which were necessary as firing temperatures increased.

Future gas turbines may be able to make better use of ceramics materials. The introduction of ceramic parts, with their excellent abilities to withstand heat and corrosion, has the potential to be a great technological breakthrough in the future. Unfortunately, the brittleness of ceramics have prevented their widespread use and dampens the enthusiasm of many engineers for the prospects of this material. So far, attempts have yielded mixed results.

Outcome 4.3

Engine Firewall

Propeller driven piston engines installed along the fuselage centre line are mounted to a piece of structure called a firewall. The firewall provides support for the engine as well as maintaining a safe distance between any engine fire and the occupants in the cockpit and fuel tanks. Fig 1 illustrates a firewall which is employed for a piston engine installation in a single engine light aircraft.

FIG1

Each firewall and shroud must be fireproof, constructed so that no hazardous quantity of air, fluid, or flame can pass from the compartment to other parts of the airplane and constructed so that each opening is sealed with close fitting fireproof grommets, bushings, or firewall fittings.

The vertical position of a single prop driven engine must be below the horizon pilot view as well as prop ground clearance.

Engine Cowlings

Engine cowlings are designed to be stream lined and keep external form drag and cooling drag to a minimum, the cowlings reduce external form drag by reducing the surface area, having a smooth surface which leads to laminar air flow. Engine cowlings typically have a nose cone shape, which prevents early flow separation. Engine cowlings must induce an internal air flow to effectively cool the engine, during this process the air slows and exits the cowling at a slower speed than the air passing around the cowling this creates cooling drag. The inlet and nozzle of engine cowling are designed to keep cooling drag to a minimum.

Acoustic absorbent panel

When aircraft engines are operating acoustic energy is released in the form of noise and vibration this is known as structure borne vibration energy. The noise creates an unpleasant environment to be in for both passengers and crew members, to address these aircraft designers will use some sort of noise control system in their design.

All noise control systems use at least one of the following control types: * Barriers-APU enclosures, overframe blankets, interior trim. * Absorption materials-polyimide foams, fibrous batts or blankets, acoustical tiles. * Vibration isolators-trim panel isolators, engine mounts. * Damping materials-elastomeric composites, adhesive films.

Effective noise control incorporates the use of both absorption and barriers for airborne noise created by aircraft engines. Typical absorption materials for an aircraft are bagged fiberglass or polyimide foam. While these products provide some degree of absorption at nearly all frequencies, performance at low frequencies typically increases with increasing material thickness.
A sound barrier is usually a solid material which, by virtue of its mass, acts as an acoustical reflector, interrupting the path of a sound wave. A barrier may be a rigid structure, such as a trim panel, or a limp sheet material, such as an overframe blanket/barrier. For most installations, it is not the stiffness of the barrier that produces the noise reduction, but the mass.

Engine Mounts

Aircraft engine mounts are very important to the characteristics of how the engine performs. Collapsed or badly positioned mounts can cause large vibrations resulting in engine or even aircraft failure.

Normally bonded sandwich assemblies made of natural rubber or specially blended synthetic compounds bonded to two plates. The normal installation requires four assemblies, each consisting of two sandwich mountings and one spacer. Satisfactory performance requires that the spacer be designed to the correct length to pre-compress each mounting to guarantee proper positioning of the mountings. When properly installed, these mountings provide excellent isolation of engine vibration, resulting in smoother, quieter flight.

Operational Controls

Aircraft engine controls provide a means for the pilot to control and monitor the operation of the aircraft's powerplant. The basic controls for a piston driven propeller aircraft are as follows;

* Master Switch – Commonly two separate switches, the Battery Master and the Alternator Master. The Battery Master activates a relay which connects the battery to the aircraft's main electrical bus. The alternator master activates the alternator by applying power to the alternator field circuit. With these two switches selected on power is provided to all aircraft systems. * Throttle - Sets the desired power level. Depending on the type of engine the throttles control either mass flow rate of air (fuel injected engines) or air/fuel mixture (carbureted engines). * Propeller Control - Adjusts the Constant Speed Unit, which in turn adjusts the propeller pitch and regulates the engine load as necessary to maintain the set R.P.M. * Mixture Control - Sets the amount of fuel added to the intake airflow. When aircraft are at higher altitudes pressure and oxygen levels are lower in order to maintain the correct air/fuel mixture the fuel volume must be lowered accordingly. This process is known as leaning * Ignition Switch - Activates the magnetos which send high-voltage output to the spark plugs. In piston aircraft engines. Magnetos are connected to the engine by gearing. When the crankshaft turns, the magnetos turn mechanically generating voltage for spark. * Tachometer - A gauge to indicate engine speed in revolutions per minute (RPM) or percentage of maximum. * Manifold Pressure (MP) Gauge - Indicates the absolute pressure in the intake manifold. * Oil Temperature Gauge - Indicates the engine oil temperature. * Oil Pressure Gauge - Indicates the supply pressure of the engine lubricant. * Exhaust Gas Temperature (EGT) Gauge - Indicates the temperature of the exhaust gas after combustion. EGT information allows scheduling of fuel air mix though temperature datum control or engine monitoring systems. * Cylinder Head Temperature (CHT) Gauge - Indicates the temperature of at least one of the cylinder heads. This also is used to set the correct fuel/air mix. * Carburetor Heat Control - Controls the application of heat to the carburetor venturi to prevent ice formation at the throat of the carburetor.

Outcome 4.4

After an engine has been installed into an aircraft it needs to be properly tested both on the ground and in flight before the aircraft can be released for normal operations. The purpose of a ground test of an aircraft is to assure that the engine meets all specifications, RPM, manifold pressure, fuel flow and oil pressure. The oil cooler system must hold oil temperatures within limits shown in the Manufactures Operating Manual.

The following procedure provides a guideline for testing a piston engine that has been installed onto an aircraft (Referenced from Lycoming) after each test is an explanation of the test.

GROUND TEST.

1. Face the aircraft into the wind.

2. Start the engine and observe the oil pressure gage. If adequate pressure is not indicated within 30 seconds, shut the engine down and determine the cause. Operate the engine at 1000 RPM until the oil temperature has stabilized or reached 140°F. After warm- up, the oil pressure should not be less than the minimum specified in the applicable operator’s manual.

* If adequate oil pressure is not indicated within 30 seconds, this is an indication of either a faulty pressure transmitter or oil pump failure and would be required to be fixed prior to another engine start to prevent damage to the engine. Once the engine has stabilized and is at normal operating temperature the engineer is checking oil pressure is within the manufactures specified limits proving correct operation of oil system components.

3. Check magneto drop-off as described in the latest revision of Service Instruction No. 1132.

* The mag drop check is carried out to ascertain if either magneto is equally capable of sustaining ignition in flight.

4. Continue operation at 1000/1200 RPM for 15 minutes. Insure that cylinder head temperature, oil temperature and oil pressure are within the limits specified in the operator’s manual. Shut the engine down and allow it to cool if necessary to complete this portion of the test. If any malfunction is noted, determine the cause and make the necessary correction before continuing the test.

* This is a continuation of check 2 and is proving that the oil cooling systems is able to control temperature preventing the oil from overheating.
5. Start the engine again and monitor oil pressure. Increase engine speed to 1500 RPM for a 5 minute period. Cycle propeller pitch and perform feathering check as applicable per airframe manufacturer’s recommendation.

* This check confirms horsepower/torque at various propeller blade angles and confirms the propeller feathers when commanded within a set time limit as per manufactures instructions.

6. Run engine to full- static airframe recommended power for a period of no more than 10 seconds.

* This check is to confirm take off power

7. After operating the engine at full power, allow it to cool down moderately. Check idle mixture adjustment prior to shutdown.

* Checks efficiency of engine, mixture can be adjusted if engine is running to rich or lean.

8. Inspect the engine for oil leaks.

* This is to ensure all oil connections during engine installation have been made correctly and all seals are sealing.

9. Remove the oil suction screen and the oil pressure screen or oil filter to determine any contamination. If no contamination is evident, the aircraft is ready for flight testing.

* Confirms correct operation of lubrication system contamination could indicate the early stages of a bearing or oil pump failure which would require investigation.

FLIGHT TEST.
1. Start the engine and perform a normal preflight run- up in accordance with the engine operator’s manual. * Standard Operating Procedures (SOP) to ensure all essential systems are operating correctly.

2. Take off at airframe recommended take off power, while monitoring RPM, fuel flow, oil pressure, oil temperature and cylinder head temperatures.

* SOP’s indications may be slightly higher if piston rings have been replaced on the installed engine until engine break in has been completed.

3. As soon as possible, reduce to climb power specified in operator’s manual. Assume a shallow climb angle to a suitable cruise altitude. Adjust mixture per pilot’s operating handbook (POH).

* SOP’s pilot will adjust mixture as per his POH so that the engine is running efficiently during the climb.

4. After establishing cruise altitude, reduce power to approximately 75% and continue flight for 2 hours. For the second hour, alternate power settings between 65% and 75% power per operator’s manual. NOTE
If the engine is normally aspirated (non-turbocharged), it will be necessary to cruise at the lower altitudes to obtain the required cruise power levels. Density altitude in excess of 8,000 feet (5,000 feet is recommended) will not allow the engine to develop sufficient cruise power for a good break- in.

* This procedure is to allows the for a good break in (breaking in is where the newly fitted piece of equipment is given an initial running in period under a light load during this period parts wear against each other producing the last small size and shape adjustment which will remain for the rest of the engines working life) 5. Increase engine power to maximum airframe recommendations and maintain for 30 minutes, provided engine and aircraft are performing within operating manual specifications.

* This is to check maximum engine power being produced is within limits.

6. Descend at low cruise power while closely monitoring the engine instruments. Avoid long descents at low manifold pressure. Do not reduce altitude too rapidly or the engine temperature may drop too quickly.

* This is to ensure engine operating correctly during descents, during descents fuel is trimmed as the air pressure alters to maintain a steady power setting proven by the gauges.

7. After landing and shutdown, check for leaks at fuel and oil fittings and at engine and accessory parting surfaces. Compute fuel and oil consumption and compare the limits given in operator’s manual. If consumption exceeds figures shown in manual, determine the cause before releasing the aircraft for service.

* A secondary check to ensure all oil connections during engine installation have been made correctly and all seals are sealing.

8. Remove oil suction screen and oil pressure screen or oil filter to check again for contamination.

* A secondary check to confirm correct operation of lubrication system contamination could indicate the early stages of a bearing or oil pump failure which would require investigation.

AFTER FLIGHT – ON GROUND.
1. Inspect engine for leaks.

* A third check to ensure all oil/ fuel connection are made correctly and there are no evidence of leaks.
2. Compute fuel and oil consumption. If figures exceed limits, determine cause(s) and correct before releasing aircraft.

* A record of fuel and oil consumption is kept this enable’s excessive usage to be quickly identify a possible fault.

3. Remove oil suction screen and pressure screen (or oil filter). Inspect for contamination.

* A third check to ensure correct operation of the lubrication system for any evidence of component wear.

4. After reinstalling the suction screen and pressure screen (or new filter) to proper torque, service engine with correct grade and quantity of oil. After the aircraft has been released, the engine must be operated on straight mineral oil during the first 50 hours of operation or until the oil consumption stabilizes. During this time, maintain engine power above 65% and insure that all aircraft and engine operating temperatures and pressures are monitored and maintained within limits.

* Although engine break in has been carried out the engine continues to bed in overtime and oil consumption and operating temperatures must be closely monitored during the first 50 hours to ensure correct breaking in.