Did They Have WEP (War Emergency Power)?
A quick answer to this would be "No", as WEP depended upon a supercharged engine and/or water/methanol injection into the engine, and neither was available until WWII (the term itself dating from this time). But if the question was rephrased, "Could they exceed their engine's maximum rated power in flight?" then, in many cases, the answer would be "Yes".
Most WWI aircraft engines had three entirely different figures for 'maximum power': nominal rated power (the one that is often quoted in books); then normal full power for 'continuous running'; and then a higher output at an increased engine rpm for just 'a few minutes only'. The Hispano-Suiza Viper for example, was nominally rated at 200 hp/2000 rpm/msl, but had an actual normal full power output of about 212 bhp/2000 rpm/msl, and could be run at 2100 rpm for 'a few minutes only' with an output of about 224 bhp at msl - a 'WEP' of approximately 12 bhp at msl, which is an increase of just over 5% of normal full power (1).
The distinction between normal full power for 'continuos running' and maximum power for 'a few minutes only' is explained in a WWI instruction manual, "Hispano Suiza Engines: Notes for Squadrons in the Field", where it defines normal full speed rpm as 'the speeds that may be maintained continuously for periods of three hours or more at a time', and maximum speed rpm as 'the speeds at which it is permissible to run the engine for short periods only (say, five minutes)' (2).
This distinction between rated power, normal full speed, and maximum speed of WWI engines can be seen again for British and French engines in the Air Board's Data Sheets, where the first three columns for each engine are labeled as 'Rated H.P.', and then under 'R.P.M. of engines in flight' the two sub-divisions 'Normal Full Speed Maxm for Long Periods' and then 'Maximum Permissable Speed for Few Minutes Only' (3). In the case of rotary engines there was also a distinction between 'gross' and 'net' power, 'net' power being the 'real' power rating after adjustment for 'windage' (the amount of power required by the spinning engine to overcome air resistance) which could vary according to how effectively the rotary engine was cowled. Manufacturers generally preferred to quote the 'gross' power, so Gwynnes rated their Clerget 9BF at 150 hp (4) but the Air Board continued to rate it at the nominal 130 hp whilst noting that it had an actual (net) output of 148 hp at 1250 rpm (5). Heron, who had the job of determining the windage losses on the early Bentley BR1/AR1 rotary notes that, although nominally rated at 150 hp/1250 rpm, "gross horsepower... was about 142. The windage horspower was 24 with the engine in the open air and 16.5 when cowled" (6). This gave it a 'real' (net) hp of about 126 when cowled, more-or-less identical to that of the (nominally rated) 130 hp Clerget 9B, which also had a net output of around 126 hp (7).
There was, in nearly all cases, no 'gate' or 'wire' (as was common in WWII) to prevent the pilot from inadvertently over-revving the engine into the 'maximum' rpm 'for a few minutes only' range (the only exception to this that I have found being for the overcompressed Benz Bz.IVa engine, where the pilot had to press a button in before pushing the throttle forwards into the highest rpm range). It is clear, however, that use of 'maximum power' was frowned upon in all situations other than dire emergency - as it certainly stressed the aero engine, shortened engine life, and could lead in extreme cases to sudden engine failure. Christopher Draper, the CO of No.8 Squadron RNAS, included in his list of "Don'ts" for new pilots the instruction 'Don't exceed 1,250 rpm [normal full power of the Clerget rotary engine] at any time. It causes the ball-races to "creep" and other unpleasant things' (8). The Clerget 130 hp rotary engine's max rpm for a 'few minutes only' is noted as being 1300 rpm in the Air Board's Data Sheets, an increase of 50 rpm above the normal full power rpm of 1250.
In the absence of a physical 'gate', British and French pilots were expected to maintain a close eye on the RPM gauge at all times and listen to the sound of the engine, as normal full rpm could also be exceeded by the 'windmilling' of the prop in a shallow dive. R.T. Leighton notes that for the 110 hp Clerget 'Fine adjustment about 1/4 of quadrant. Throttle about 1/2 of quadrant' would give full power on the ground (9), and another pilot, Neil Williams, noted that for the 80 hp Le Rhone "Even at full power one cannot push the levers beyond half-way as the rpm will fall" (quoted in 10). The quadrant was marked from 1 to 10, with the idle position being about 3 and maximum power about 7, although the exact position could vary from one engine to another or with changing atmospheric pressure and temperature. With direct drive and fixed-pitch props the maximum power was generally obtained at maximum permissable engine speed - so opening the throttle beyond this point usually resulted in a loss of power and damage to the engine - and so, in the example of the 80 hp Le Rhone C 'somewhere around 1500 rpm the cylinders can begin to stretch, followed closely by departing' (11).
But it is possible that for most German pilots with the early or mid-war 'low altitude 'stationary engines, this might not have been a problem - as it appears that their engines may have been 'limited' to a maximum of 1400 rpm by either throttle stops/governors or by calibration of the throttle lever. It is notable that nearly all of the German stationary engines of this period had their power output specified at a standard 1400 rpm. If we look at the throttle curves for captured examples of these engines it is remarkable that in almost every case the throttle curve also stops at 1400 rpm, although the power curve continues to go up in most cases for at least another 100-200 rpm (indicating that these engines may have had more power than the pilot could access via the throttle controls). In the British tests on a captured Daimler Mercedes D.IIIa, for example, the normal full power is quoted as 179.5 bhp at 1400 rpm (the point at which the throttle curve stops), but the maximum power is quoted as being 188 bhp at 1500 rpm, with a peak of 197.5 bhp at 1700 rpm for a very short time (12). Similarly, the report for the Mercedes D.IVa indicates that the normal full power was 252 bhp at 1400 rpm, the point at which the throttle curve stops, but increasing to 260 bhp at 1500 rpm and finally around 268 bhp at 1600 rpm on the power curve (13); and on the Austro-Daimler a normal full power of 200 bhp at 1400 rpm, again at the point where the throttle curve stops, but an increase on the power curve to 212 bhp at 1500 rpm and 222 bhp at 1600 rpm (14).
If this is so, it certainly changes with the introduction of the new high or overcompressed 'altitude' engines in 1918. These engines had to remain throttled back at low altitudes, to prevent damage to the engine, but they were designed to take advantage of the higher engine speeds of up to 1600 rpm at altitudes of 2000-3000 m and upward. In nearly all examples this altitude control was integrated with the throttle control: in the case of the overcompressed Daimler Mercedes D.IIIau, for example, there was merely an admonition to the pilot above the throttle quadrant, warning the pilot not to push the throttle lever forward into the 'high altitude' section of the throttle range at low altitudes; but in the Maybach Mb IVa there was a clearly marked divide to separate the 'low' from the 'high' section of the quadrant (15). In the overcompressed Benz engines there was a physical 'gate' in the form of a button that had to be pressed in by the pilot before the throttle lever could be advanced into the high altitude section of the quadrant (16), and in the case of the BMW IIIa there was a secondary, and entirely separate throttle lever that had to be engaged to increase engine speed above 1400 rpm (17).
The pilots were instructed not to engage this 'over-gas' at low altitude, as this extract from a letter by Lothar von Richthofen illustrates: 'In order to not unnecessarily stress the motor, and maintain advantage, the "over" gas throttle position should be used only over 2000 meters with direct climbing or in aerial combat. It is absolutely necessary that each pilot is informed in the mode of operation of the BMW motor, (in order to avoid unnecessary motor failure)'. There is evidence, however, in particular that of a letter from Goering, that pilots could, and did, sometimes use the 'over-gas' at very low altitudes: he states that 'As a rule, the "over" gas throttle position is not used under 3000 meters", but he goes on to immediately qualify this by saying that "Not only have we been operating in the "over" gas throttle position almost constantly throughout aerial engagement, but also at low altitude, and without any damage to the engine' ending with an anecdote to show that this was so, and moreover a lifesaver (18). I am a little suspicious of this letter (from the text of the letter, Goering was clearly trying to get the authorities to give his unit first priority in the supply of the new engine, and his anecdote is remarkably free of any facts that could be checked), and in particular because there is no further evidence of this from either Lothar von Richthofen (see above) or Ernst Udet (19). But there is, apparently, an entry for the BMW IIIa engine in "Typenhandbuch der deutschen Luftfahrttechnik" by Bruno Lange, to say: "Notleistung in Bodennähe bis 200 kw (230 PS)..." or "Emergency performance near ground level 230 PS" (20, but there is some confusion over the actual figure as 200 kw does not eqaute to 230 PS). In contrast to this, the British report from tests on a captured BMW IIIa indicates that at 1400 rpm, the BMW IIIa was already producing 234 bhp at normal full power, before the 'over-gas' was engaged, and a msl equivalent of 254 bhp with the over-gas fully open at 1600 rpm (a 20/80 benzol/petrol mixture in use). The British tests with the 'over-gas' control fully open were, however, only made possible with use of a "blower" to simulate the conditions at altitude (21): and it remains unclear just how much 'over-gas' could be employed, and therefore just how much 'boost' a pilot could get, by engaging the secondary throttle at such low altitudes.
Summary: it is clear that most WWI aero engines had both a 'normal full power' and then a 'maximum' power for short-duration or emergency use only, although the difference between them appears to have been not much more than 5% of full engine power in most cases (or even less, probably, for rotary engines). It is possible, however, that this 'maximum' power 'for a few minutes only' might not have been available to the pilots of the many 'low altitude' German and Austrian aircraft, at least until the introduction of the new overcompressed high altitude engines in 1918. It is apparent that these altitude engines could be 'over-revved' at low altitude, but the resulting 'boost' in performance from this, although it might have been potentially significant, still remains uncertain - but did have the potential to wreck the engine and was therefore discouraged for anything other than emergency use.
Bletchley
1. "Viper" Hispano-Suiza power curve. 1917/18. PRO AVIA 6/25950
2. British Ministry of Munitions. Notes for squadrons in the field: Hispano-Suiza engines. 1918. PRO AIR 10/352.
3. British Air Board. Data for structure and stability calculations of aircraft. 1917. PRO DSIR 36/4828.
4. Gwynnes Ltd. Clerget patent aero engines: instructions and list of parts. c.1917 .Facsimile published by Camden Miniature Steam Services, 2001.
5. British Air Board (as above).
6. Heron, S.D. History of the aircraft piston engine: a brief outline. Ethyl Corp., 1961.
7. British Air Board (as above).
8. Draper, Christopher. The mad major: autobiography. Air Review, 1962.
9. Leighton, R.T. Pilots' notes for the handling of World War I warplanes and their rotary engines. The Shuttleworth Collection.
10. Coombs, L.F.E. Control in the sky: the evolution and history of the aircraft cockpit. Pen & Sword Aviation, 2005.
11. Shooting down the myths of the rotary engine, in: 'The Great Times', vol.6 no.4.
12. British Ministry of Munitions. Report on the 180 hp Mercedes aero engine, 1918. PRO AIR 10/268.
13. British Air Board. Report on the 260 hp Mercedes aero engine, 1917. PRO AIR 10/250.
14. British Ministry of Munitions. Report on the 200 hp Austro-Daimler aero engine, 1918. PRO AIR 10/355.
15. British Ministry of Munitions. Report on the 300 hp Maybach aero engine, 1918. PRO AIR 10/338.
16. LVG. Pilotweb.
http://www.pilotweb.aero/content/articles.
17. British Air Ministry. Report on the 230 hp Bayern aero engine, 1919. PRO AIR 10/397.
18. Extracts from translated letters by Goering and Lothar von Richthofen, posted by Dave Watts on The Aerodrome forum.
19. Udet, Ernst. My experiences with the BMW motor type IIIa, Cross & Cockade journal vol.2 no.2 Summer 1961 (originally published in German in 'Motor' May-June 1919, and translated by Alex Imrie).
20. Rammjaeger, posted on The Aerodrome forum.
21. (see above, 17)
