Ju 88 endurance

[TigerTimon]

Very interesting, thank you all for your contributions. How an airframe behaves with different speeds is interesting.

The reason I asked this is to confirm the following story, of a Ju 88 C-6 that flew into Slovakia. It's in the following link:

https://aviation-safety.net/wikibase/231136

Kind regards,

Timon

[Jukka Juutinen]

Originally Posted by Graham Boak

Sorry Jukka, but I sincerely hope that no-one relies on this advice. All aircraft have a minimum drag speed. This is also the optimum speed for maximum endurance, because more power is needed to fly either faster (because of an increase in zero-lift drag) or slower (because of an increase in lift-induced drag). I suggest looking up the term "drag polar".

I'm afraid my official technical education in performance was restricted to jet engines. In this case the optimum cruise speed is indeed faster than the optimum endurance speed, because of the behaviour of the turbine engine with velocity. (As I understand it.) Many years after first being gainfully employed in the business, I had the opportunity to study the notes of a fellow engineer from another college, and was surprised to learn that for a piston engined aircraft the speed for best endurance and the speed for best range are identical - the minimum drag speed.

So once at the optimum cruise speed, any Ju.88 pilot could only fly at a lower speed by increasing power. This is because of the need to fly at a greater angle of attack to obtain enough lift, hence increasing the lift-induced drag, which would increase the total drag more than the reduction in zero-lift drag gained by reducing speed. More power = higher fuel flow = less endurance. And less range.

Graham, you are totally missing the point. Endurance is time, therefore the greatest endurance is obtained at the lowest power that maintains altitude and that is much less that for maximum air range. For example, the PBY-5 Catalina's Pilot's Notes states that maximum air range is obtained at 90 - 92 kts. IAS whereas when flying for maximum endurance the IAS may be reduced to as low as 80 kts. comfortably. I challenge you to find a pilot manual for a WW2 piston aeroplane stating that max air range and max. endurance speeds are exactly the same.

[Jukka Juutinen]

Just take a look at e.g. Mosquito FB.6 Pilot's Notes. E.g. at sea level the best range TAS at 17,000 lbs. is about 190 kts. At that setting the fuel flow is about 73 Imp. gallons/h. At 160 knots (lowest speed given in the curve) the fuel flow is about 63 gallons/h (extrapolated from the curves). What is more, this data is at 2000 r.p.m. which is not ideal.

[Graham Boak]

Fascinating. This is very much the characteristics I was familiar with from jet engined-aircraft, which is perhaps why I remember the contradiction so clearly. I presume that you can confirm that these numbers were obtained at the same conditions of weight and drag. The statement "extrapolated from the curves" has me a bit worried, as such extrapolations have been known to introduce considerable error. (From practical experience...)

Can you explain further the comment about 2000rpm being not ideal? It is certainly true that longer ranges (and endurances) can be obtained by different aircraft settings: low rpm and high boost being optimal. This is behind the tale of Lindberg's advice to P-38 pilots in the South Pacific, although the tale does make me wonder about the attention these pilots paid to ground lectures, or the quality of those lectures. Similarly the light lead-laden streaks behind the exhausts of, for example, RAAF Spitfires in the Pacific.

However, the comments in the PBY manual appear quite definitive.

[Snautzer]

Perhaps of intrest. Notice consumption engine oil at 12 l/hour . This will have a limiting factor on long flights.

[Jukka Juutinen]

Graham: The extrapolation was very slight as the Mosquito FB.6 Pilot's Notes has its curves (at SL) cut off at 170 kts. TAS. At 170 kts. the fuel flow is about 66 g.p.h. Mosquito PN can be found here: https://ww2aircraft.net/forum/thread...to-fb-6.22836/


As for the 2000 r.p.m. not being ideal, the same set of curves suggest that up to 220 knots is obtainable at 2000 r.p.m., thus at e.g. 160 kts. the power reduction is obtained through reducing the r.p.m. and this obviously violates the high boost/low r.p.m. rule. With standard Merlins the highish minimum recommended r.p.m. was the generator not charging properly at low r.p.m. According to David Birch of R-R Heritage Trust, with certain modifications, the Merlin could be run as slow as some 1200 r.p.m. The Allison could not this routinely. And in fact, e.g. the Bf 109 F-4 got its best air mileage at lower altitudes at about 1350 r.p.m.


BTW, e.g. the F6F Hellcat pilot manual recommends r.p.m. down to about 1300 for cruising for max. range and endurance (ma. range I.A.S. is given as 135 kts.).

[Jukka Juutinen]

Ther above document is interesting for it shows a lack of flexilibity in the settings provided. The setting 2250 r.p.m./1.15 ata is the maximum continuous ("Dauerleistung") setting for the Jumo 211 J and gives about 960 hp (TO power being 1420 hp). For example, a Martin B-26 cruises for long range with about 600 b.h.p. per engine.

[Denniss]

The Jumo 211F/J had similar lower power states for maximum range, should be 2000 rpm and 1.0 ata for ~800PS (data varies by alt).

[BABIN]

Hello,


On 10th and 12th of November 1942, two Wekusta 2's Ju 88D-1, each equiped with two tanks of 800 litres, flew long range missions from Nantes in West of France to Trondheim in Norway, bypassing Ireland from the west and passing between the Ferroe and the Shetland Islands. Both times the distance traveled was about 3200km (precisely 3178 and 3191 km) and the duration of the missions of 8h08 and 8h11. But they were only tests that remained without result.


Pierre

[TigerTimon]

Amazing! :-O