原文：
The engine was chosen by Northrop to power their MQM-74C Chukar II aerial target drone (i.e. their only purpose was for being shot at...hard life ;-). Yet, this power plant was also used for testing of reconnaissance drones produced by other manufacturers, even if it later didn’t come to fruition. The engine that I got originated from one of the latter applications.
The leading particulars of the WR24-7-2 are:
Type of engine:
Turbojet
Layout:
Single shaft, 1 axial and 1 radial compressor, annular combustor with centrifugal injection, 1 axial turbine, convergent thrust nozzle
Rated thrust (ISA conditions):
800N (180lbs) minimum
Rated RPM:
60900+-250 / min
“Idle” RPM:
50000+-500 / min
Steady state EGT:
765°C maximum
Specific fuel consumption:
no more than 0.123 kg/Nh
Lubrication system:
Compressor-bleed operated oil mist system, total loss
Oil consumption:
125 g/h maximum
Oil type:
MPDS-3.26.020 (whatever that is)
Engine control system:
Electronically controlled fuel metering system using RPM as the only input signal
Ignition system:
High energy system, mounted remotely of engine
Fuel System:
Bleed air pressurized fuel cell, proportional valve integral to the electronic fuel control unit
Recommended fuel:
JP-5, Jet A-1
Accessories:
Integrated switched reluctance alternator on main shaft, 40-70VAC, 20A, doubles as RPM pickup
Calculations show that the engine will have a pressure ratio of around 5:1 and an air mass flow of 1.4 to 1.5kg/s. The gas temperature at the turbine stage entry will be around 980°C. Here’s a link to a short lineup of the WR24’s turbomachinery particulars.
This sketch shows the basic layout of the engine (click the image for a higher resolution TIFF version). The engineers at Williams Research found some very clever solutions for details to keep the engine as simple as possible while it requires minimum maintenance and can still be manufactured at reasonable expenses. There are three peculiarities that distinguish this engine from most other small turbojets.
Firstly, there’s the rotating atomizer that was introduced by Turbomeca of France but had been pioneered for very small engines by Williams with the WR2. The advantage of this fuel injection pinciple is that only a very low fuel pressure is required and that the atomisation over a wide RPM range stays close to perfect. The most serious disadvantage is that no shaft tunnel can be used for the bearing arrangement and hence the engine casing becomes a structural member of the rotor suspension, requiring high precision on that component.
Secondly, and closely related to the fuel injection system, is the way the fuel is delivered to the engine. The Williams engineers used compressor bleed air to slightly pressurize the fuel cell in order to feed the fuel to the engine-mounted fuel control unit. This completely eliminates the need for a pump.
And finally, the lubrication system is configured as a total loss system, running on a finely atomized oil mist. A small oil tank is attached to the engine, containing round about 0.4 liters, and compressor bleed air is used in some kind of venturi arrangement to atomize the oil. The oil flow is very low (specified to be less than 125 gramms per hour), and many “big” engines have higher losses through their labyrinths, even though they feature a return oil system.
The engine is specified to be recoverable after salt water immersion (of course only after shut- and cooldown...) and is only supposed to be flushed and cleaned with different liquids and then test run to dry out the cleaning agents. For such a simple motor, that’s a great advantage since it’s not automatically a write-off if the drone has to go down over the sea (i.e. it runs out of fuel while operating as a target over water).
On the particular engine that I got, this cleaning procedure probably has not been carried out completely or in a timely manner, the result was some minor corrosion on the aluminium parts, major corrosion on the magnesium alloy (probably...) axial compressor rotor and salty-oily deposits everywhere in the engine. Otherwise, the engine was in a very nice condition, yet I had to tear it down completely to clean everything. And I won’t do a job like this without my camera, so now some photos of the components are show (from intake to the exhaust...)..
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The intake with the fuel inlet and purge air fittings
The area that shrouds the axial compressor is internally lined with a (brownish) polymer, thus permitting an interference fit with the axial impeller for very small tip clearance.
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The center cone houses the carbon (graphite) seal that slides on a polished insert in the rotor shaft. Fuel enters through the annular orifice between the capillary tube and the graphite disc while the scavenge air is introduced through the tube. The engine manual didn’t mention it but I guess the air is required to scavenge all fuel from the shaft upon engine shutdown and thus preventing the four injection nozzles from coking when the heat soaks from the hot engine components to the shaft.
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The components of the rotating assembly from top left to lower right: The shaft nut, the axial compressor impeller, the alternator rotor, a spacer, the compressor bearing, the radial compressor impeller (with another spacer more or less permanently attached to the rear of its hub), and finally the turbine wheel/shaft assembly with the roller bearing inner race riding on the short shaft end.
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Here a close-up of the spare axial compressor impeller that shows minor traces of corrosion but is generally still in acceptable shape. It’s incredible how light-weight this component is, hence I assume it’s made of either magnesium alloy or even aluminium-lithium.
The alternator’s stator is located inside the axial compressor stator. The stator itself consists of a set of iron laminations that contain AlNiCo magnets to provide the bias field. the rotor only consists of laminations that close and interrupt the magnetic flow, thus producing an oscillating field that induces the voltage in the coils. This type of alternator is called “switched reluctance machine”. Since the AlNiCo magnets are susceptible to demagnetization if a too strong counter-field is applied, inside the stator a “magnetic keeper” has to be installed during disassembly.
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The wires of the alternator run through one of the three struts of the radial compressor cover. The other strut extracts first stage compressor bleed air (here in the 12 o’clock position) to cool the turbine bearing, the third one (in the 4 o’clock position) feeds the oil mist to the compressor bearing.
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The rear of the compressor cover. There’s slight surface corrosion present but it’s nothing to worry about. Also the starting air nozzle is visible that directs compressed air into the radial section of the second stage compressor to spin up the engine to ignition speed (air impingement starter).
The compressor bearing seat contains two O-rings that prevent the outer race of the front bearing from rotating. Moreover, they act as vibration dampers. The labyrinth area is lined with a polymer coating to permit minimum clearance.
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This is the starting air port that contains a check valve. To the right, some of the starting air is diverted to pressurize the oil mist generator and the fuel cell. Top left is the oil mist inlet for front bearing lubrication.
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This sketch shows the starting air scheme and how it is diverted to supply the oilmist generator and to pressurize the fuel cell.
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The front bearing is of the four-point contact type and is of the highest precision.
The split inner race has to be assembled in a certain orinetation. For this purpose, an arrow (V) has been etched over the two halves of the race to indicate the direction of the axial force. Upon assembly, the “V” has to be matched.
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A total of 34 blades are a lot for a radial compressor of that size (just short of 150mm).
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In contrary to the previous engine(s) - WR2-6 -, the hub of the compressor is MUCH stronger.
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The radial compressor placed inside the diffuser housing.
The radial compressor diffuser housing without the compressor. Once again, there’s slight surface corrosion present in this area, but ist’s basically just surface discoloration. No material loss can be found.
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A close-up of the radial diffuser vanes. Due to the shallow angle it’s hard to tell the shape of the vanes, but using a soft wire to “feel” the shape lets me assume that the vanes are of the wedge type, but I’m not 100% sure of this.
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Here the axial diffuser/flow straightener vanes are visible from the rear of the diffuser housing.
That’s the central labyrinth seal that is placed between the radial compressor rotor and the combustion chamber. It also ducts compressor air to the cooling channels inside the rotor shaft.
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The front of the combustor with the heat shield. It’s amazing how small the clearance is for the inrushing primary combustion air.
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The combustor front viewed from the compressor side.
And here the “flame side” of the combustor is shown. This part is of rather simple construction, just a pressed sheet metal part/welded cosntruction with the primary air slots stamped/pressed into. Yet I guess, this “simple” part was the result of many hours of testing and experimenting.
WR24-7-2型发动机
This photo is a close-up of the fuel injector section of the turbine shaft. The tangetial bores duct cooling air from the central labyrinth seal into the hollow shaft and to the aft combustor labyrinth seal.
WR24-7-2型发动机
This sketch displays how the fuel is ducted through the hollow shaft and into the combustor.
发表于 2018-1-5 20:59:19
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