War emergency power
War Emergency Power (WEP) is an American term for a throttle setting on some World War II military aircraft engines. For use in emergency situations, it produced more than 100% of the engine's normal rated power for a limited amount of time, often about five minutes. Similar systems used by non-US forces are now often referred to as WEP as well, although they may not have been at the time, as with the German Luftwaffe's Notleistung systems.
WEP in WWII aircraft
Maximum normal power would be limited by a mechanical stop, for instance a wire across the throttle lever slot, but a more forceful push would break the wire, allowing extra power. In normal service, the P-51H Mustang was rated at 1,380 hp, but WEP would deliver up to 2,218 hp. In the P-51D Mustang, the model most produced and used during WWII, the WEP increased the HP from 1490 to 1720. The Vought F4U Corsair, not originally equipped for WEP, later boasted a power increase of up to 410 hp (17%) when WEP was engaged. Several methods were used to boost engine power by manufacturers, including water injection and methanol-water injection. Some earlier engines simply allowed the throttle to open wider than normal, allowing more air to flow through the intake. All WEP methods result in greater-than-usual stresses on the engine, and correspond to a reduced engine lifetime. For some airplanes, such as the P-51D, use of WEP required that the engine be inspected for damage before returning to the air. 5 hours use of WEP on the P51D required a complete tear down inspection.
British and Commonwealth aircraft could increase power by increasing the supercharger boost pressure. This modification was common by the summer of 1940, with the widespread availability of 100 octane fuel. Raising supercharger boost pressure from 6 lb to 12 lb increased the Merlin III engine rating to 1310 hp, an increase of over 250 hp. Pilots had to log the use of emergency boost and were advised not to use it for more than 5 minutes continuously.
The German MW50 system methanol-water injection system required additional piping, as well as a storage tank, increasing the aircraft's overall weight. Like other boost techniques, MW50 was restricted by capacity and engine temperatures and could only be used for a limited time. The GM 1 nitrous oxide injection system, also used by the Luftwaffe, provided extreme power benefits of 25 to 30 percent at high altitude by adding oxidizer gases but required cooling on the ground and added significant weight. One of the few German aircraft that could be equipped with both Notleistung systems, the late war Focke-Wulf Ta 152H high-altitude fighter, could attain a velocity of some 756 km/h (470 mph) with both systems used together, reportedly as Kurt Tank once did in using both boost systems simultaneously when he was flying a Junkers Jumo 213E-powered Ta 152H prototype fitted with both MW 50 and GM-1, to escape a flight of P-51D Mustangs in April 1945.
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Perhaps the most dramatic WEP feature was found in the MiG-21bis fighter jet. This late variant of the standard Soviet light fighter plane was built as a stopgap measure to counter the newer and more powerful American F-16 and F/A-18 fighters until the next-generation MiG-29 could be introduced to service.
The MiG-21bis received the upgraded Tumanski R-25 engine, which retained the standard 42 / 65 kN normal and forsazh power settings of earlier R-13 powerplants, but added a new super-afterburning system. Use of this "diamond regime" provided a massive 97.4 kN of thrust for no more than 3 minutes in actual wartime use. Use of this temporary power gave the MiG-21bis slightly better than 1:1 thrust-to-weight ratio and a climbing rate of 254 meters/second, equalling the F-16's nominal capabilities in close-quarters dogfight.
In air combat practice with the MiG-21bis, use of WEP thrust was limited to 1 minute, to spare on the engines' 800 flight hours lifetime, since every second of super-afterburner use counted as several minutes of regular power run due to extreme thermal stress. When WEP was on, the MiG-21bis's R-25 engine produced a huge 5 meter long blowtorch exhaust - the six or seven brightly glowing rhomboid "shock diamonds" visible inside the flames gave the emergency-power setting its "diamond regime" name.
WEP in surface vehicles
Some modern military surface vehicles also employ WEP features. The EFV (next generation US Marine Corps Expeditionary Fighting Vehicle) sports a 12-cylinder 1200 bhp diesel engine developed by the German MTU company. When the EFV is swimming the powerplant can be boosted to 2700 hp via the use of open circuit seawater-cooling. Such extreme war power setting allows the MTU engine to drive four massive water-jet exhausts which propel the surface-effect riding EFV vehicle at sea speeds reaching 35 knots.
Although the EFV prototypes have demonstrated revolutionary performance on water and land, the reliability of their extremely boosted powerplants has yet to meet stringent military standards and the vehicle failed to enter Marine Corps service as of autumn 2007.
- Water injection
- MW50 (German, methanol/water mixture)
- GM 1 (German, nitrous oxide injection)
- Forsazh (Russian)
- Propane injection
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- AAF Manual 51-127-3, Pilot Training Manual for the P-51 Mustang, USAAF, August 1945, p.14.
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