Oberursel rotaries

[Bletchley]

Somewhat puzzled by the performance of the Oberursel U-type rotaries (U.0, U.III in particular), and I am hoping that someone can help:

My understanding is that the U.0 was based on the 80 hp Gnome Lambda, U.I on the 100 hp Gnome Monosoupape, and the U.III on the 160 hp Gnome Double Lambda (14 cylinder two-row). My understanding of the Gnome Lambda and Double Lambda is that, unlike the Monosoupapes, they had no pilot-controlled fuel regulator and could not be 'leaned' as the aircraft gained altitude, which appears to limit the aircraft powered by them to an effective ceiling of about 5000 feet or so (so that, for example, the Sopwith two seater scout 'Spinning Jenny' powered by the 80 hp Gnome Lambda had a quoted ceiling of 3000 ft, and the Breguet AG4 powered by the 160 hp Gnome Double Lambda had a quoted ceiling of 4290 ft - from J M Bruce 'Fighters') . But the frequently quoted ceiling for the Fokker E types powered by the Oberursel U.I or U.III is 10,000 feet or more...so did these engines have a fuel regulator, or some other means of 'leaning' the engine as the aircraft gained altitude, or could they simply reach this altitude without leaning the engine??

Also, was there a U.II? (I know there was a UR.II)

Thanks

Bletchley

[Dan_San_Abbott]

Bletchley:
There were two engine designations, 1. Those derived from the 7 cylinder Gnome Lamda, the U.O, 80 Ps, The U.III the 160 Ps twin row Lamda-Lamda. The 100 Ps U.1 is a licensed built 9 cylinder Gnome Delta. 2. The Oberursel 110 Ps 9 cylinder URII derived from the LeRhone 9j 110 hp rotary engine and the URIII, 145 Ps 11 cylinder which I believe originated at Oberursel.
Blue skies,
Dan-San

[Bletchley]

Thanks Dan, so there was no U.II - just U.0, U.I, U.III, UR.II and UR.III. That clears up one question

I would guess they all had the 'coupe' button or blip-switch, and an On/Off fuel cock, but what other engine controls did the pilot have, if any? Most importantly, how did the pilot 'lean' the engine to reach the (relatively) high altitude, for early 1915, of 3000 m/10,000 ft plus? My understanding is that these normally aspirated engines required a fuel regulator for 'leaning' from between 1000m-1500m (3000 ft-5000 ft) or the fuel mixture would become too rich as the air became less dense, the revs. would fall and the aircraft would gradually loose engine power and would not be able to continue climbing any further. And if these engines didn't have a fuel regulator for the pilot, were the engines tuned by the fitters on the lean side for take-off so that they would reach their maximum rated power at a higher altitude, and therefore boost their maximum ceiling (although this seems unlikely to me, as maximum power is usually required for take-off)?

Bletchley

[StephenLawson]

Correct there was never an Oberursel U.II. Rotary engine aircraft were usually most effective at lower altitudes and their ceilings had to be accepted. Josef Jacobs commented specifically that rotaries had to be flown full out at all times.

[franzkait]

all Oberursels had a fuel regulater , and the Ur2a and Ur3a are high altitude engines

[Bletchley]

Thank you Stephen, Franzkait

This prompts another, but related question: the Gnome Lambda had a throttle and a fuel regulator, but no controls for the pilot to work them in flight; the Gnome Delta monosoupape had no throttle, but it had a fuel regulator that could be worked by the pilot in flight; the Le Rhone (also Clerget and Bentley) had both a fuel regulator and a throttle, of sorts, that could be worked by the pilot in flight.

So, did the Oberursel U.0 and U.III, derived from the Lamda and Double Lambda respectively, have a throttle as well as a fuel regulator that could be worked by the pilot in flight?

And did the Oberursel Ur.II and Ur.III (and the high compression IIa and IIIa), derived from the Le Rhone, have a throttle as well as a fuel regulator that could be worked by the pilot in flight?

Bletchley

[franzkait]

Oberursels had a throttle and a fuel regulator the pilot can use at flight. The Ur2a and 3a are not high compression , but high altitude engines with a differend air intake.

[Bletchley]

Thanks again, Franzkait

Bletchley

Just two more questions, and then I will stop bothering you

The Le Rhone, Clerget and Bentley rotary engines also had a throttle that could be worked by the pilot, but the fuel mixture was not automatically adjusted to the throttle setting (except in the case of the Le Rhone, but in this case it is reported to have been somewhat ineffective) so the pilot had to manually re-adjust the fuel regulator (the 'fine adjustment') for each and every change in the throttle setting. Is this also the case with Oberursels? Or was the fuel regulator, like that of a stationary engine, only used to lean the engine as altitude increased?

And how far could these Oberursels be throttled back? Although the French and the British rotaries varied in this respect none of them, it seems, could be throttled back much more than 50% of engine power (between approx.600-800 rpm) in the air before cutting out, which is why the pilot still needed the blip-switch for landing and for taxiing on the ground.

Thank you. I do appreciate these answers, as I cannot find this information anywhere else. Reliable information on this engine series is very thin on the ground, as most publications simply dismiss them as 'clones' or copies of the Gnome and Le Rhone engines, which they clearly were not (at least, not in terms of pilot control).

[Greybeard]

Nice thread.

I would only add that 600 RPM aren't for sure 50% of power if max rated one is developed at 1200 RPM: power curve is just... curve, not a straight line. That's to say 600 RPM in the above example are probably more than 50% of max power output.

GB

[Bletchley]

Yes, I was being very approximate Greybeard

It does seem to vary quite a lot, from one rotary engine to another, and it appears that the figures derived from running an engine on the ground are going to be less than those derived from an aircraft in flight because the windmilling of the prop is said to add another 100-150 rpm.

The lowest quoted 'idle' rpm I have come across is for the 160 hp SH.III, which was "governed by a proper throttle control, sensitive down to about 350 rpm" (Profile Publications no.86, 1966).

The lowest quoted rpm I have for a Le Rhone is 500 rpm, for the 80 hp Le Rhone, quoted in GWAA Great Times vol.6 issue 4: 'The rpm range of these engines is about 500-600 rpm at idle, static, and 1200-1300 rpm at full throttle in flight... [and] ...that 500 rpm you had on the ground at idle is over 650 rpm in a glide.' But Neil Williams, quoted in L F E Coombs book 'Control in the sky: the evolution and history of the aircraft cockpit' (Pen & Sword Aviation, 2005), quotes a narrower range for the 80 hp Le Rhone, saying "The engine has an rpm range from 800 to 1,150 rpm". Various rpm figures have been quoted in forum threads here, also: one from Pilot Magazine [October 2001?] for the 80 hp Le Rhone "700 to 1,150 rpm on the ground, noted on the cockpit rpm gauge, which equates to about 50 to 100 per cent of available power respectively" (Drew Ames 13/8/2002); another one from Air Publication 129 'Flying Instruction' for the Le Rhone, where "Pilots are cautioned... to use fine adjustment to keep engine running smooth at speeds of 600 rpm-1200 rpm" (William 23/8/2002). Robert W Bradford, of the [Canadian?] National Aviation Museum, also quotes an 'idle' figure of 500 rpm for the 110 hp Le Rhone, commenting that "They simply do not 'tick over' as a radial or in-line engine would do - in fact, with the fixed pitch wooden propellor, they idle at about 45% of full engine speed (500 rpm as against 1150 rpm for take-off at full power)" (www.billybishop.net/bishopF.html).

Bletchley