p r o p e l l e r s

fig 6 - 4 cross section of a propeller. An a (alpha) denotes angle of attack of airfoil sections

Propellers may be classified as to whether the blade pitch is fixed or variable . The demands on the propeller differ according to circumstances. For example, in takeoffs and climbs more power is needed, and this can best be provided by low pitch. For speed at cruising altitude, high pitch will do the best job. A fixed-pitch propeller is a compromise.

f i x e d p i t c h p r o p e l l e r s

The propeller is made in one piece. Only one pitch setting is possible and is usually two blades propeller and is often made of wood or metal. 

Wooden Propellers: Wooden propellers were used almost exclusively on personal and business aircraft prior to World War II .A wood propeller is not cut from a solid block but is built up of a number of separate layers of carefully selected .any types of wood have been used in making propellers, but the most satisfactory are yellow birch, sugar maple, black cherry, and black walnut. The use of lamination of wood will reduce the tendency for propeller to warp. For standard one-piece wood propellers, from five to nine separate wood laminations about 3/4 in. thick are used.

Metal Propellers: During 1940, solid steel propellers were made for military use. Modern propellers are fabricated from high-strength , heat-treated, aluminium alloy by forging a single bar of aluminium alloy to the required shape. Metal propellers is now extensively used in the construction of propellers for all type of aircraft. The general appearance of the metal propeller is similar to the wood propeller, except that the sections are generally thinner.

v a r i a b l e p i t c h p r o p e l l e r s

There are two types of variable-pitch propellers adjustable and controllable. The adjustable propeller's pitch can be changed only by a mechanic to serve a particular purpose-speed or power.

The variable pitch propeller permits pilots to change pitch to more ideally fit their requirements at the moment. In different aircraft, this is done by electrical or hydraulic means.

Two-position: A propeller which can have its pitch changed from one position to one other angle by the pilot while in flight.

Controllable pitch: The pilot can change the pitch of the propeller in flight or while operating the engine by mean of a pitch changing mechanism that may be operated by hydraulically.

C o n s t a n t s p e e d p r o p e l l e r s

In modern aircraft, it is done automatically, and the propellers are referred to as constant-speed propellers. As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm. If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle. This acts as a brake on the crankshaft. If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase. The constant-speed propeller thus ensures that the pitch is always set at the most efficient angle so that the engine can run at a desired constant rpm regardless of altitude or forward speed.

Constant-speed propellers may have a full-feathering capability. Feathering means to turn the blade approximately parallel with the line of flight, thus equalizing the pressure on the face and back of the blade and stopping the propeller. Feathering is necessary if for some reason the propeller is not being driven by the engine and is wind-milling, a situation that can damage the engine and increase drag on the aircraft.

Some controllable-pitch and constant-speed propellers also are capable of being reversed. This is done by rotating the blades to a negative or reverse pitch. Reversible propellers push air forward, reducing the required landing distance as well as reducing wear on tires and brakes.

Beta Control: A propeller which allows the manual repositioning of the propeller blade angle beyond the normal low pitch stop. Used most often in taxiing, where thrust is manually controlled by adjusting blade angle with the power lever.

Propeller theory

The forces. Propeller blades are constructed using aerofoil sections to produce an aerodynamic force, in a similar manner to a wing. Consequently the blades are subject to the same aerodynamics – induced drag, parasite drag, wingtip vortices, lift/drag ratios at varying aoa, pressure distribution changing with aoa etc. There is a difference in application because, in flight, the propeller has rotational velocity added to the translational [forward] velocity thus the flight path of any blade section is a spiral – a helical flight path.

The diagram at left represents a blade section in flight and rotating around the shaft axis. Because of the different application it doesn't serve much purpose to express the resultant aerodynamic force as we would for a wing, with the components acting perpendicular (lift) and parallel (drag) to that flight path, as in the upper figure. So we represent the aerodynamic force component acting forward and aligned with the aircraft's longitudinal axis as the thrust force, and that component acting parallel to the direction of rotation as the propeller torque force. 

As you see in the lower figure the component of the lift acting in the rotational plane has now been added to the drag to produce the 'propeller torque force' vector. The remaining forward acting portion of lift is then the thrust. That is why propeller efficiency is usually no greater than 80 – 85%, not all the lift can be used as thrust and the propeller torque force consumes quite a bit of the shaft horse power. The propeller torque and the engine torque will be in balance when the engine is operating at constant rpm in flight. 

There are other forces acting on the blades during flight, turning moments that tend to twist the blades and centrifugal force for example. The air inflow at the face of the propeller disc also affects propeller dynamics.

B l a d e a n g l e a n d p i t c h

Although all parts of the propeller, from the hub to the blade tips, have the same forward velocity, the rotational velocity – and thus the helical path of any blade station – will depend on its distance from the hub centre. Consequently, unless adjusted, the angle of attack, will vary along the length of the blade. Propellers operate most efficiently when the aoa at each blade station is consistent (and, for propeller efficiency, that giving the best lift drag ratio) over most of the blade, so a twist is built into the blades to achieve a more or less uniform aoa.

The blade angle is the angle the chord line of the aerofoil makes with the propeller's rotational plane and is expressed in degrees. Because of the twist the blade angle will vary throughout its length so normally the standard blade angle is measured at the blade station 75% of the distance from the hub centre to the blade tip. The angle between the aerofoil chord line and the helical flight path (the relative airflow) at the blade station is, of course, the angle of attack and the angle between the helical flight path and the rotational plane is the angle of advance or helix angle. The aoa and helix angle vary with rotational and forward velocity. 

The basic dimensions of propellers for light aircraft are usually stated in the form of number of blades, diameter and pitch with the latter values given in inches. e.g. 3 blade 64" × 38". The pitch referred to is the geometric pitch which is calculated, for any blade station but usually the 75% radius position, thus:

Geometric pitch = the circumference (2 π r) of the propeller disc at the blade station multiplied by the tangent of the blade angle. Thus it is the distance the propeller – and aircraft – would advance during one revolution of the propeller if the blade section followed a path extrapolated along the blade angle. e.g. For a blade station 24 inches from the hub centre [0.75r] and a 14° blade angle, the circumference = 2 × 3.14 × 24 = 150 inches and tangent 14° = 0.25. Thus the geometric pitch is 150 × 0.25 = 38 inches. Propellers are usually designed so that all blade stations have much the same geometric pitch. 

Designers may establish the ideal pitch of a propeller which is the theoretical advance per revolution

which would cause the blade aerofoil to be at the zero lift aoa; thus it would generate no thrust and, ignoring drag, is the theoretical maximum achievable aircraft speed. 

The velocity that the propeller imparts to the air flowing through its disc is the slipstream and slip used to be described as the difference between the velocity of the air behind the propeller ( i.e. accelerated by the propeller) and that of the aircraft. Nowadays slip has several interpretations, most being aerodynamically unsatisfactory, but you might consider it to be the difference, expressed as a percentage, between the ideal pitch and the advance per revolution when the the propeller is working at maximum efficiency in conversion of engine power to thrust power. Slip in itself is not a measure of propeller efficiency; as stated previously propeller efficiency is the ratio of the thrust power (thrust × aircraft velocity) output to the engine power input.

P i t c h a n d v e l o c i t y

The performance of aircraft fitted with fixed pitch or ground adjustable propellers is very much dependent on the chosen blade angle. Fixed pitch propellers limit the rpm developed by the engine at low forward velocity, such as occurs during the take-off ground roll and may also allow the engine rpm to exceed red-line maximum when the load on the engine is reduced, such as occurs in a shallow dive. Fixed pitch propellers operate at best efficiency at one combination of shaft power and airspeed. Blade angle is usually chosen to produce maximum performance at a particular flight condition, for example:       •  Vy climb i.e. a climb propeller       •  Vc cruise i.e. a cruise propeller       •  High speed.

The climb propeller is usually chosen when the aircraft normally operates from a restricted airfield or in high density altitude conditions. The climb propeller will produce maximum efficiency at full throttle around the best rate of climb airspeed and will perform fairly well at take-off, but during the initial take-off acceleration even the climb propeller may restrict the engine rpm to less than 75% power. The cruise propeller will achieve maximum efficiency at 75% power at airspeeds around the design cruising speed but aircraft take-off and climb performance will not be the optimum. The cruise propeller usually has a little more pitch than the standard propeller fitted to the aircraft. A high speed propeller might be fitted when the aircraft is intended to be operating at, or above, rated power for short periods – in speed competition for example. 

A constant speed propeller allows the engine to develop maximum rated power and rpm during the ground roll and to develop full power throughout its normal rpm range. With a constant speed propeller the pilot controls inlet manifold air pressure [MAP] with the throttle lever and the engine rpm with the rpm control lever or knob/switches. The pilot has several combinations of rpm/MAP to achieve a particular power setting. For example, in one particular aircraft, the recommended combinations for 65% power at sea level are 2100 rpm + 26 inches Hg MAP or 2200 + 25 inches or 2300 + 24 inches or 2400 + 23 inches. So you can use low rpm and high MAP or high rpm and low MAP to achieve exactly the same power output. The low rpm / high MAP combination probably gives more efficient cylinder charging and better combustion plus less friction. The high MAP also acts as a cushion in the cylinders, reducing engine stress. MAP is usually measured in inches of mercury [Hg] rather than hectopascals. Standard sea level barometric pressure is 29.92 inches Hg or 1013.2 hPa.

T h e w i n d m i l l i n g p r o p e l l e r

The angle of attack of a fixed pitch propeller, and thus its thrust, depends on the forward speed of the aircraft and the rotational velocity. Following a non catastrophic engine failure the pilot tends to lower the nose so that forward airspeed is maintained while at the same time the rotational velocity of the engine/propeller is winding down. As the forward velocity remains more or less unchanged while the rotational velocity is decreasing the angle of attack must be continually decreasing and at some particular rpm the angle of attack will become negative to the point where the lift component becomes negative ([reverses) and the propeller autorotates, driving the engine. This acts as greatly increased aerodynamic drag which seriously affects the aircraft's L/D ratio and thus glide angles. The drag (including the negative lift) is much greater than that of a stationary propeller, also the engine rotation may cause additional mechanical problems if oil supply is affected.

If the forward speed is increased windmilling will increase, if forward speed is decreased windmilling will decrease, thus the windmilling might be stopped by temporarily reducing airspeed, probably to near stall, so that the negative lift is decreased to the point where internal engine friction will stop rotation. This is not something which should be attempted without ample height. 

In the diagram the upper figure shows the forces associated with a section of a propeller blade operating normally. The lower figure shows the forces and the negative angle of attack (aoa) associated with the propeller now windmilling at the same forward velocity. A variable pitch propeller may have a feathering facility which turns the blades to the minimum drag position (i.e. the blades are more or less aligned fore and aft) and halts windmilling when the engine is stopped. 

T h e r u n a w a y p r o p e l l e r

As a propeller system increases in complexity then the possibilities for malfunction increase. A problem associated with constant speed propellers is governor failure during flight which, in most installations, will cause the propeller blades to default to a fine pitch limit. This greatly reduces the load on the power plant, and the engine will immediately overspeed, particularly if in a shallow dive. The rpm of an overspeeding engine – sometimes referred to as a 'runaway prop' – will quickly go way past red-line rpm and, unless immediate corrective action is taken, the engine is likely to self destruct and/or the propeller blades depart the hub due to the increased centrifugal force. 

The corrective action is to immediately close the throttle and reduce to minimum flight speed by pulling the nose up. . Once everything is settled down fly slowly, consistent with the fine pitch setting, to a suitable airfield using minimum throttle movements. (The constant speed propeller fitted to a

competition aerobatic aircraft usually defaults to the coarse pitch limit to prevent overspeeding but an immediate landing is required.)

Ketika membahas mesin turboprop tidak dapat dipisahkan dari pembahasan masalah pesawat udara sebab mesin ini banyak dipakai pada mesin propulsi. Mesin propulsi adalah mesin yang dipasang pada pesawat udara yang berfungsi untuk memproduksi gaya dorong (thrust). Prinsip kerja mesin ini adalah merubah energi kimia yang terkandung di dalam bahan bakar menjadi energi mekanik. Thrust digunakan untuk mendorong pesawat udara sehingga dapat bergerak maju. Mesin turboprop adalah salah satu jenis mesin turbin yang diterapkan di pesawat udara. Mesin turbin adalah mesin yang cara kerjanya menerapkan Siklus Brayton sehingga proses kompresi, pembakaran, dan ekspansi terjadi pada tempat yang berbeda. Sedangkan mesin piston atau reciprocating adalah mesin yang menerapkan Siklus Otto sehingga proses kompresi, pembakaran, dan ekspansi dilakukan di tempat yang sama. Mesin turbin ada beberapa macam jenisnya antara lain: ramjet, scramjet, turbojet, turbofan, turboshaft, propfan, dan turboprop. Daya mesin turboprop dipergunakan untuk menggerakkan baling-baling (propeller). Hal ini mirip dengan pada mesin piston dimana daya mesin juga digunakan untuk memutar baling-baling. Kelebihan mesin turbin adalah kompak (ringkas), ringan, memiliki daya yang besar, dan bebas vibrasi karena tidak ada bagian mesin yang bergerak translasi. Kelebihan inilah yang menyebabkan mesin turboprop banyak dipakai di pesawat khususnya di pesawat transport dan latih. Sedangkan kelebihan mesin piston adalah lebih irit dalam pemakaian bahan bakar. Mesin piston hanya dipakai di pesawat kecil dan tidak dipakai pada pesawat transport yang besar karena mesin ini memiliki berat yang cukup besar sehingga secara ekonomi tidak menguntungkan karena mengurangi beban yang menguntungkan (payload) yaitu penumpang dan barang. Prinsip kerja mesin turboprop mirip dengan mesin turbojet namun ada perbedaan yang cukup prinsip, pada mesin turboprop terdapat baling-baling sedangkan pada mesin turbojet tidak terdapat baling-baling. Biasanya mesin turboprop dipakai pada pesawat dengan kecepatan subsonik rendah. Komponen utama pada mesin turboprop adalah: intake, kompresor, ruang bakar, turbin, and nozzle. Cara kerja mesin ini pada awalnya udara masuk dari atmosfer ke dalam intake. Kemudian tekanan udara tersebut dinaikkan dengan menggunakan kompresor. Tujuan peningkatan tekanan adalah untuk meningkatkan efisiensi pembakaran sebab pada saat pesawat udara beroperasi yaitu terbang di ketinggian maka temperatur udaranya sangat rendah sehingga sangat sulit untuk dilakukan pembakaran. Selanjutnya udara bertekanan tinggi diumpankan ke ruang bakar dan dicampur dengan bahan bakar kemudian dilakukan pembakaran. Selanjutnya gas panas hasil pembakaran diumpankan ke turbin. Turbin berfungsi merubah energi panas (thermal) menjadi energi mekanik. Selain memutar kompresor, turbin juga memutar baling-baling melalui roda gigi reduksi. Dan akhirnya gas sisa pembakaran dibuang ke atmosfer melalui nozzle. Gambar 1.1 menunjukkan bagian-bagian dan cara kerja dari mesin Turboprop.

Gambar 1.1: Diagram Skema Mesin Turboprop

Keterangan gambar 1.1 adalah sebagai berikut :

UA : Udara Atmosfer RB : Ruang BakarI : Intake T : TurbinK : Kompresor N : NozzleBB : Bahan Bakar GB : Gas Buang Gambar 1.1 menunjukkan bahwa aliran udara atmosfer yang berwarna biru setelah melewati propeller dibagi menjadi 2 (dua) alira yaitu aliran di luar mesin dan aliran di dalam mesin. Ketika udara melewati ruang bakar berubah menjadi gas setelah melalui proses pembakaran. Gas hasil pembakaran ditunjukkan dengan warna ungu. Di dalam gear box terdapat roda gigi reduksi yang berfungsi untuk meningkatkan putaran propeller sehingga putaran propeller akan lebih cepat dibandingkan dengan putaran turbin. Namun demikian putaran propeller harus dibatasi dengan menggunakan governor. Posisi governor berada di dekat gear box. Kecepatan propeller adalah jumlah dari kecepatan pesawat ditambahkan secara vektor dengan kecepatan akibat putaran propeller. Governor membatasi putaran propeller supaya kecepatan ujung dari propeller tidak mencapai kacepatan sonic atau supersonic. Jika kecepatan total propeller mencapai kecepatan sonic atau supersonic akan terjadi gelombang kejut (shock wave) yang mengakibatkan dragnya membesar sehingga efisiensi propeller menurun. Mesin propulsi dapat berupa mesin piston, mesin turbin, dan roket. Saat ini mesin turbin pemakaiannya sangat luas baik pada pesawat transport maupun pesawat militer seperti ditunjukkan pada gambar 1.2 di bawah ini:.

Mesin turboprop banyak dipakai pada pesawat udara khususnya yang beroperasi pada bilangan Mach kurang dari 1.Koridor terbang beberapa jenis mesin propulsi ditunjukkan pada gambar 1.3 di bawah ini.

Gambar 1.3: Koridor Terbang Beberapa Jenis Pesawat Udara Gambar 1.3 menunjukkan bahwa wilayah kerja mesin turboprop hampir sama dengan mesin piston dan helikopter yaitu pada bilangan Mach dan ketinggian terbang yang rendah. Hal ini disebabkan karena ketiga mesin tersebut memperoleh thrust dari putaran baling-baling dimana efisiensi propulsinya sangat ditentukan oleh kerapatan udara (air density). Berbeda dengan mesin turbojet,

ramjet, dan roket yang tetap dapat beroperasi dengan efektif di ketinggian yang cukup besar sebab mesin jenis ini memproduksi thrust dengan cara melontarkan gas buang sekuat-kuatnya. 

Gambar 1.4: Efisiensi Propulsi BeberapaSistem Propulsi Subsonik Gambar 1.4 menunjukkan efisiensi propulsi terhadap blangan Mach dari beberapa mesin propulsi antara lain mesin piston/torak, turboprop, turbofan, dan turbojet. Dari gambar 1.4 dapat disimpulkan urut-urutan mesin propulsi jika dilihat dari efisiensi propulsinya dari yang terbesar adalah mesin piston/torak, turboprop, turbofan, dan terakhir adalah mesin turbojet. Pada mesin piston dan torboprop memiliki efisiensi propulsi yang terbesar dikaitkan dengan grafik pada gambar 1.3 yang menyatakan kedua mesin tersebut beroperasi di ketinggian rendah sehingga berada di lingkungan dengan kerapatan udara yang terbesar sehingga produksi thrust lebih mudah dibandingkan jenis mesin lainnya yang beroperasi di ketinggan besar. Dari gambar 1.5 dapat disimpulkan bahwa mesin turboprop tergolong mesin yang hemat bahan bakar walau masih kalah hemat jika dibandingkan dengan mesin piston. Sedangkan gambar 1.6 menunjukkan bahwa thrust yang dihasilkan mesin turboprop tergolong kecil. Hal ini cukup logis sebab mesin turboprop sangat irit dalam pemakaian bahan bakar sehingga tenaga yang dihasilkan juga lebih kecil dibandingkan mesin turbojet yang lebih boros dalam pemakaian bahan bakar. 

Gambar 1.5: Batas Kecepatan Sistem Propulsi

Gambar 1.6: Karakteristik Gaya Dorong Spesifik

Beberapa Mesin Pesawat Udara

Gambar 1.7: Karakteristik Pemakaian Bahan Bakar Spesifik Gaya Dorong Beberapa Mesin Propulsi

Gambar 1.8 menunjukkan bahwa efisiensi thermal, propulsi, dan total (overall) mesin turboprop dan propfan yang terbesar dibandingkan mesin propulsi jenis lainnya.

Gambar 1.8: Karakteristik Efisiensi Beberapa Mesin Propulsi Sejarah perkembangan mesin turboprop dimulai dari mesin turboprop yang pertama yang diberi nama Jendrassik . Mesin ini dirancang oleh seorang insinyur mesin berkebangsaan Hungaria bernama György Jendrassik. Ia membuat dan melakukan pengujian di Pabrik Ganz di Budapest sekitar tahun 1939 - 1942. Mesin ini rencananya akan dipasang pada pesawat bomber Varga RMI-1 X/H buatan László Varga tapi proyek ini akhirnya gagal pada tahun 1940. 1Cs− Inggris pertama kali mengembangkan mesin turboprop Rolls-Royce RB.50 Trent seperti ditunjukkan oleh gambar 1.9. Mesin ini memiliki baling-baling dengan diameter 7 feet 11 inchi. Gambar ini diambil pada saat pengujian di Hucknall pada Maret 1945

Gambar 1.9: Mesin Rolls-Royce RB.50 Trent Uni Soviet mengembangkan contra-rotating propellers yang dipasang pada pesawat bomber Tu-95 'Bear'. Pesawat ini dapat terbang mencapai kecepatan jelajah 575 mph. Pada waktu itu kecepatan pesawat ini lebih cepat daripada pesawat jet yang pertama. Pesawat ini menjadi simbol kesuksesan Uni Soviet dalam mengembangkan pesawat militer pada akhir abad 20. USA mengembangkan pesawat Convair XFY Pogo dan Lockheed XFV Salmon pada tahun 1950 yang juga bermesin contra-rotating turboprop. Mesin turboprop pertama yang dikembangkan Amerika adalah General-Electric T-31.

Saat ini produsen mesin turboprop yang paling populer adalah Pratt & Whitney Canada PT6. Produk dari perusahaan ini sudah mendunia. Bangsa Indonesia adalah salah satu konsumen Pratt & Whitney ketika merancang pesawat N250. (STTA/KI)

A well-designed propeller typically has an efficiency of around 80% when operating in the best regime.[5] The efficiency of the propeller is influenced by the angle of attack (α). This is defined as α = Φ - θ,[6] where θ is thehelix angle (the angle between the resultant relative velocity and the blade rotation direction) and Φ is the blade pitch angle. Very small pitch and helix angles give a good performance against resistance but provide little thrust, while larger angles have the opposite effect. The best helix angle is when the blade is acting as a wing producing much more lift than drag.

A propeller's efficiency is determined by

Propellers are similar in aerofoil section to a low-drag wing and as such are poor in operation 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.

The three-bladed propeller of a light aircraft: the Vans RV-7A

A further consideration is the number and the shape of the blades used. Increasing the aspect ratio of 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. A further balance is that using a smaller number of blades 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 also 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, a propeller blade tip will reach transonic speed well before the aircraft does. 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.

There have been efforts to develop propellers for aircraft at high subsonic speeds.[7] 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); a large number of blades are used to reduce work per blade and so circulation strength; contra-rotation is used. 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-70and Tupolev Tu-95 for examples of such a design).

Forces acting on a propeller[edit]

Five forces act on the blades of an aircraft propeller in motion, they are:[8]

Thrust bending force

Thrust loads on the blades act to bend them forward.

Centrifugal twisting force

Acts to twist the blades to a low, or fine pitch angle.

Aerodynamic twisting force

As the centre of pressure of a propeller blade is forward of its centreline the blade is twisted towards a coarse pitch position.

Centrifugal force

The force felt by the blades acting to pull them away from the hub when turning.

Torque bending force

Air resistance acting against the blades, combined with inertial effects causes propeller blades to bend away from the direction of rotation.

Curved propeller blades[edit]

Since the 1940s, propellers and propfans with swept tips or curved "scimitar-shaped" blades have been studied for use in high-speed applications so as to delay the onset of shockwaves, in similar manner to wing sweepback, where the blade tips approach the speed of sound. The Airbus A400M turboprop transport aircraft is expected to provide the first production example: note that it is not a propfan because the propellers are not mounted direct on the engine shaft but are driven through reduction gearing.

Propeller control[edit]

Variable pitch[edit]

Cut-away view of a Hamilton Standard propeller. This type of propeller was used on many American fighters, bombers and transport aircraft of World War II

The purpose of varying pitch angle with a variable pitch propeller is to maintain an optimal angle of attack (maximum lift to drag ratio) on the propeller blades as aircraft speed varies. Early pitch control settings were pilot operated, either two-position or manually 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. An improvement on the automatic type was the constant-speed propeller. Constant-speed propellers allow the pilot to select a rotational speed for maximum engine power or maximum efficiency, and a propeller governor acts as a closed-loop controller to vary propeller pitch angle as required to maintain the selected engine speed. In most aircraft this system is hydraulic, with engine oil serving as the hydraulic fluid. However, electrically controlled propellers were developed during World War II and saw extensive use on military aircraft, and have recently seen a revival in use onhomebuilt aircraft.

Feathering[edit]

A propeller blade in feathered position

On some variable-pitch propellers, the blades can be rotated parallel to the airflow to reduce drag in case of an engine failure. This is called 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 allows the aircraft to maintain altitude with the reduced power from the remaining engines.

Most feathering systems for reciprocating engines sense a drop in oil pressure and move the blades toward the feather position, and require the pilot to pull the propeller control back to disengage the

high-pitch stop pins before the engine reaches idle RPM. Turboprop control systems usually utilize a negative torque sensor in the reduction gearbox which moves the blades toward feather when the engine is no longer providing power to the propeller. Depending on design, the pilot may have to push a button to override the high-pitch stops and complete the feathering process, or the feathering process may be totally automatic.

Reverse pitch[edit]

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. See also Thrust reversal.

Contra-rotating propellers[edit]Main article: Contra-rotating propellers

Contra-rotating propellers of a modified North American P-51 Mustang fitted with a Rolls-Royce Griffon

Contra-rotating propellers use a second propeller rotating in the opposite direction immediately 'downstream' of the main propeller so as to recover energy lost in the swirling motion of the air in the propeller slipstream. Contra-rotation also increases power without increasing propeller diameter and provides a counter to the torque effect of high-power piston engine as well as the gyroscopic precession effects, and of the slipstream swirl. However on small aircraft the added cost, complexity, weight and noise of the system rarely make it worthwhile.

Counter-rotating propellers[edit]Main article: Counter-rotating propellers

Counter-rotating propellers are sometimes used on twin-, and other multi-engine, propeller-driven aircraft. The propellers of these wing-mounted engines turn in opposite directions from those on the other wing. Generally, the propellers on both engines of most conventional twin-engined aircraft spin clockwise (as viewed from the rear of the aircraft). Counter-rotating propellers generally spin clockwise on the left engine, and counter-clockwise on the right. The advantage of counter-rotating propellers is to balance out the effects of torque and p-factor, eliminating the problem of the critical engine. These are sometimes referred to as "handed" propellers since there are left hand and right hand versions of each prop.

Aircraft fans[edit]Main articles: Propfan and Ducted fan

A fan is a propeller with a large number of blades. A fan therefore produces a lot of thrust for a given diameter but the closeness of the blades means that each strongly affects the flow around the others. If the flow is supersonic, this interference can be beneficial if the flow can be compressed through a series of shock waves rather than one. By placing the fan within a shaped duct, specific flow patterns can be created depending on flight speed and engine performance. As air enters the duct, its speed is reduced while its pressure and temperature increase. If the aircraft is at a high subsonic speed this creates two advantages: the air enters the fan at a lower Mach speed; and the higher temperature increases the local speed of sound. While there is a loss in efficiency as the fan is drawing on a smaller area of the free stream and so using less air, this is balanced by the ducted fan retaining efficiency at higher speeds where conventional propeller efficiency would be poor. A ducted fan or propeller also has certain benefits at lower speeds but the duct needs to be shaped in a different manner than one for higher speed flight. More air is taken in and the fan therefore operates at an efficiency equivalent to a larger un-ducted propeller. Noise is also reduced by the ducting and should a blade become detached the duct would contain the damage. However the duct adds weight, cost, complexity and (to a certain degree) drag.

Mesin turboprop adalah jenis pesawat pembangkit yang menggunakan turbin gas untuk menggerakkan baling-baling. Turbin gas yang dirancang khusus untuk aplikasi ini, dengan hampir semua output yang digunakan untuk menggerakkan baling-baling. Mesin gas buang mengandung energi sedikit dibandingkan dengan mesin jet dan memainkan peran kecil dalam penggerak pesawat.

Baling-baling ini digabungkan ke turbin melalui gigi reduksi yang mengubah RPM tinggi, torsi output yang rendah untuk RPM rendah, torsi tinggi. Baling-baling itu sendiri biasanya dengan kecepatan konstan (pitch variabel) tipe serupa dengan yang digunakan dengan mesin pesawat yang lebih besar reciprocating.

Mesin turboprop umumnya digunakan pada pesawat subsonic kecil, namun beberapa pesawat dilengkapi dengan pesawat turboprop memiliki daya kecepatan melebihi 500 kt (926 km / h, 575 mph). pesawat militer dan sipil besar, seperti Lockheed L-188 Electra dan Tupolev Tu-95, juga telah menggunakan kekuatan turboprop. The Airbus A400M ini didukung oleh empat TP400 mesin Europrop, yang kedua mesin turboprop paling kuat yang pernah dihasilkan, setelah Kuznetsov NK-12.

Dalam bentuk yang paling sederhana turboprop terdiri dari intake, kompresor, ruang bakar, turbin, dan mendorong nozzle. Udara ditarik ke dalam intake dan dikompresi oleh kompresor. Bahan bakar ini kemudian ditambahkan ke udara dikompresi dalam ruang bakar, di mana campuran bahan bakar-udara kemudian combusts. Pembakaran gas panas memperluas melalui turbin. Beberapa kekuatan yang dihasilkan oleh turbin digunakan untuk menggerakkan kompresor. Sisanya ditularkan melalui pengurangan gearing untuk baling-baling. perluasan lebih lanjut dari gas terjadi di nozel mendorong, dimana gas buang dengan tekanan atmosfer. Nozel mendorong menyediakan proporsi yang relatif kecil dari dorongan yang dihasilkan oleh sebuah turboprop.

Pesawat turboprop sangat efisien dengan kecepatan pesawat sederhana (di bawah 450 mph) karena kecepatan pesawat jet dari baling-baling (dan buang) relatif rendah. Karena tingginya harga mesin turboprop, mereka sebagian besar digunakan di mana kinerja tinggi pendek lepas landas dan mendarat (STOL) kemampuan dan efisiensi pada kecepatan penerbangan sederhana diperlukan. Aplikasi paling umum mesin turboprop dalam penerbangan sipil di dalam pesawat komuter kecil, di mana keandalan mereka lebih besar dari mesin offset reciprocating biaya tinggi awal mereka.

Mesin Turboprop adalah mesin turbojet dengan turbin tambahan yang dirancang sedemikian rupa untuk menyerap semburan sisa bahan bakar yang sebelumnya menggerakkan kompresor. Pada prakteknya selalu ada sisa semburan gas dan sisa inilah yang dipakai untuk mengerakkan turbin yang dihubungkan ke reduction gear, biasanya terletak di bagian mesin, memutar baling-baling.

Jenis mesin ini irit bahan bakar untuk pesawat berkecepatan rendah/sedang dan terbang rendah (400 mil per jam/30.000 kaki). Melalui teknologi maju, selain irit juga menghasilkan tingkat kebisingan yang rendah dan mampu meluncurkan pesawat degnan kecepatan 400 mil per jam.

Contoh mesin turboprop yang populer adalah mesin Rolls-Royce Dart yang dipakai pada pesawat Britih Aerospace atau BAe (dulu Hawker Siddeley) HS-748 dan Fokker F-27. Kemudian mesin Rolls-Royce Tyne yang digunakan pada pesawat jenis Transall C-160 dan BAe Vanguard.

Mesin jenis ini tenaganya diukur dengan total equivalent horsepower (tehp) atau kilowatt(kW)-shaft horsepower (shp) plus sisa daya dorong. Sebagai contoh, mesin Tyne dengan take-off power 4.985 tehp (3.720 kW) sampai 6.100 tehp (4.550 kW) merupakan mesin turpboprop yang paling kuat dan irit bahan bakar.

Turbo propeller Engine atau bisa juga di sebut turbo prop adalah salah satu jenis mesin pesawat terbang, tergolong dalam jenis Gas turbine Engine.Memanfaatkan baling-baling (propeller)untuk menghasilkan gaya dorong pada pesawat.Mesin ini digunakan pada pesawat Casa-212, Hercules C-130, Fokker 27 dan pesawat berbaling-baling lainnya, merupakan pengembangan dari turbojet engine.Prinsip kerja mesin ini sama dengan mesin turbojet, hanya bila pada turbo jet hasil pembakaran yang menghasilkan tekanan langsung di manfaatkan sebagai daya dorong dengan langsung mengeluarkan nya melalui exhaust (saluran buang mesin) yang di buat kerucut (konvergent)sehingga membentuk nozzle yang menyemburkan tekanan tinggi dari gas buang.Pada turbo prop engine gas buang tersebut hanya digunakan untuk memutar turbin yang kemudian untuk memutar propeller, putaran propeller itulah yang menghasilkan daya dorong sehingga pesawat dapat bergerak maju. Kelebihan pesawat yang menggunakan mesin ini adalah dapat takeoff dan landing pada landasan yang pendek dan pesawat dapat bermanuver baik pada berbagai tingkat ketinggian terbang. Cara kerja secara umum mesin ini adalah, Udara luar dihisap masuk melalui saluran masuk(intake) oleh Compressor yang memiliki beberapa tingkatan untuk menghasilkan tekanan udara yang lebih besar, untuk kemudian tekanan tinggi dari udara tersebut di alirkan ke dalam ruang bakar (combustion Chamber),bersamaan dengan masuknya udara bertekanan tinggi di kabutkan pula sejumlah bahan bakar (fuel) dan dibakar dengan penyalaan pertama dengan sparkplug (busi). Hasil pembakaran yang berupa panas dan tekanan kemudian mendorong sudu-sudu (blade) turbine yang terdiri dari beberapa tingkatan sehingga turbine berputar untuk kemudian keluar melalui saluran buang (exhaust), hasil putaran turbine itulah yang digunakan untuk memutarkan propeller dan poros mesin.Proses tersebut terus berlangsung selama mesin hidup. Bahan bakar mesin ini menggunakan Avtur (aviation turbine) yang merupakan bahan bakar utama mesin turbine.Demikian sekilas mengenai Turbo Prop Engine semoga dapat bermanfaat menambah

wawasan kita...wasalam...Bekti83.