WWII MISTERIES: What happened with the JU390?

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So that when the Japanese floated balloon bombs across the Pacific they had no idea where they were going? Surely they knew of the streams of wind at altitude and the rough direction or they would have had no idea where they would end up?

Wind currents can change 100knts over the first 5000-20,000 feet that were common flight routes . Heres a web site where you can track wind currents at altitude and predict flight routes. After a short while you notice predictable general paths . Not saying it would work every time but could explain ideas behind experimental flights.

Aviation Weather Center (AWC) - IFFDP Text Products--Winds and Temperatures - ICAO Area H

Click on the FL number [39 = 39,000 feet] and it gives you a north Atlantic polar view map of wind currents. The blue dot on each selection gives a closer-bigger map to view. The directional arrow indicates the wind direction and the figure beside it is temperature [I believe they are below zero unless otherwise mentioned]. The symboles indicate the air speed. As I recall, each flag is 50knts and each long dash is 10knts while each short dash is 5knts.

The wind channels are all over and couldn't go unnoticed. All prop flight is controlled by these winds so any range figures you see quoted are purely theoretical limits and not practical figures to debate about.


PS the wind stream was first reported in December 1934 , according to this web link....so they should have known about it.

http://www.todayinsci.com/12/12_07.htm


Germans had artic weather out posts for a good part of the war that gave them an ability to predict weather patterns effecting the North Atlantic and Europe.
 
Actually during 1944, the U-boat rusting at Liverpool U-534 was posted to perform weather reporting patrols south of Greenland, not to mention other weather bases in the high Arctic.



When first certified in late 1943, the Ju-390's Maximum Take Off Weight (MTOW) was cited as 75,500kg with an 8,000km range. According to pilot Hans Joachim Pancherz, the aircraft was put through a weight reduction program in May 1944 and re-certified with an MTOW of 80,500kg with a range of 11,000km.

Crittics however remain unconvinced so I made an effort to track down an April 1945 report on bench testing of the BMW 801D engines by NACA, forerunner of the agency now known as NASA. Early flights by the Ju-390 used BMW 801D engines. The BMW 801E engine was an experimental engine often mentioned as being fitted to the Ju-390. It was in fact intended for the Export aircraft planned for production by Japan. The BMW 801E had modified gearing on the supercharger to give better fuel consumption and an extra 100hp at altitude.

Page 29 of that NACA report discloses an individual engine on lean settings below 21,000 feet altitude consumes either:

* 250 L/hr (40.83 US Gal/hr) at 1600rpm (1469.9lb/hr for six engines), or
* 275 L/hr (45.83 US Gal/hr) at 1800rpm (1649.9lb/hr for six engines)

Above 21,000 feet the supercharger cut in automatically and almost doubled fuel consumption.

Reichsluftministerium (RLM) performance specification for the Ju-390 were to carry 10,000kg (22,000lb) over 8,000 kilometres. Fuel capacity with such a payload when the aircraft's empty weight is subtracted from MTOW results in 57,268lb.

Interestingly 32 hours worth of fuel consumption at 1800 rpm equates 52,797lb of fuel. Thus a 32 hour endurance attributed to the Ju-390 is well within known take off weight and fuel consumption.

A Ju-390 therefore could carry 10,000kg from France to New York and back with a 4,471lb weight buffer, though some of this spare weight capacity may have been consumed in take off power at the start of such a flight. Fuel consumed by a Focke Wulf 190A fighter powered with a BMW 801D engine to reach 19,000 feet, equates 1152lb for six engines.

Remember that this was calculated keeping in mind a 10,000kg payload so the Ju-390 was not anywhere near the limits of it's performance had the flight been performed without a payload.

What is clear is that the flight would have been below 21,000 feet most of the way and early in the flight unlikely to have been above 12,000 feet.

Jetstream headwinds apply to jet aircraft above 30,000 feet. Not unpressurised aircraft below 21,000 feet.

Sources:

* UK Air Ministry report PRO AIR40/53 (for BMW 801D)
* UK Air Ministry report PRO AIR40/55 (for BMW 801E)
* NACA Technical Memorandum Report MR E5D19 (archived by Nasa as PDF #19930093290_1993093290)
 
A question has been raised how the Ju-390 compared with the B-29 ?

Ju-390:

10,380 hp Installed horsepower
75,500 kg (166,400 lb) MTOW weight
254 m² (2,730 ft²) Wing area
0.17 kW/kg (0.10 hp/lb) Power to weight ratio

B-29:

8,800hp Installed horsepower
60,560 kg (133,800lb) MTOW weight
161.3 m² (1,736 ft²) Wing area
(0.6577hp/ib) Power to weight ratio

B-36:

21,000hp Installed horsepower
190,000kg (410,00 lb) MTOW weight
443.3 m.2 (4,772 sq.ft) Wing area
120 kW/kg (0.086 hp/lb) Power to weight ratio


The Ju-390 had 17.95% more power than a B-29 and 79.7% more wing area. The Convair B-36 with less power to weight ratio but similar large wings managed to take off at maximum weight in just 1500 metres.

The B-29 with less power and smaller wings could manage such ranges so why not the Ju-390 ?
 
The point about the B-36 was that in terms of characteristics the Ju-390 was more like the B-36 than the B-29.

The B-36 was twice as big again and twice as powerful as the Ju-390 but if you scaled the ratios of a B-36 down you would get an aircraft like the Ju-390 and thus the runway performance would be similar too.

The B-29 had less power than the Ju-390 and more heavily loaded wings. At 89% of MTOW the B-29 required 5,100 feet, or 1500 metres, so the point is this... If the relatively less powerful B-29 with heavier wing loading and the relatively similar albiet scaled up B-36 could both take off with 1500 metres of runway then the Ju-290 whose performance figures fall between the two American aircraft, must have been able to use a 1500m runway and possibly somewhat less.

I do know for a fact that the Me-264 had inferior runway performance to the Ju-390 and required 2,400 metres.
 

As a pilot you know the following things.

The B-29 had an aspect ratio of 11.5 and the Ju 390 is 9.9. To give you an idea of implications;

If the wings of the two ships had the same CL at the same speed the Induced drag of the B-29 would be about 14% less. I don't have the actual wing CL nor do I have the Cd-zero lift for the 390. I suspect that it wasn't as good as the B-29 because of the ceiling limitations despite the better wing loading for the 390 - but I don't know the answer regarding the Ju 390. Do you?

The B-29 wing had a Cd-zero lift of .0241 and an L/D of 16.8 which is VERY good. A P-38 had .0268 Cd-zero lift wing and an aspect ratio of 8.26.

So the CD for the B-29 was very low and the CL/CD was 16.8 - a very nice number explaining long range capability. What is the corrsponding set of figures for the Ju 390? Do you have them?

You have discussed minumum fuel consumtion rates for the 801D and 801E engines at different rpms and boost. Let's assume they are valid.

What we don't know yet due to lack of flight test data or other dependable sources is what was the best cruise speed to fuel consumption settings to give max range for the Ju 390. We do not know what the specific fuel consumption data is for optimizing range - which is gallons per mile with enough Hp to attain a speed with that load, at that altitude to go the longest distance possible. (independent of winds aloft, temperature, etc)

In other words to get a true estimate for a Ju 390 we have to have the entire System data in context of :

For Max Takeoff Weight A (divided into fuel, bomb load to be dropped, and fixed load of crew and eqipment) what is the optimal flight speed/engine setting which will derive greatest mile/gallon of fuel at what altitude... not number of hours in the air.

In addition to climb to cruise altitude and descent at return, The flight plan considers the steady rate of weight loss due to fuel consumption, the major step function loss at the mid point of the bomb drop and the steady fuel loss on the way back.

You haven't noted what the Ju 390 flight speed is for any engine setting/fuel consumption rate you have shown. As a pilot how would you do a flight plan on the outer edge of your aircraft performance if you didn't have those basic numbers? How do you evaluate a claim of performance for one you might buy?

Is 10,000 gallons the right maximum internal fuel capacity number for the Ju 390? Is the maximum internal fuel capacity one that was used for New York Bomber claim? Where is a definitive source to confirm or deny? Is 45 gallons per hour the lowest possible fuel consumption rating? what is Hp output for that setting and how does that translate to true airspeed at 5800m?

Is 5800m optimal cruise altitude based on L/D and SFC? How do you know?

Optimal cruise is directly proportional to L/D and inversely proportional to SFC so you have to know what those data are also, to plot a potential max range.

How do you know?
 
Optimal cruise is also effected by the thrust curves for the engines at different speeds and altitudes. (obviously in the economical cruise power range)
 
kool kitty89 said:
Optimal cruise is also effected by the thrust curves for the engines at different speeds and altitudes. (obviously in the economical cruise power range)
Click to expand...

"We do not know what the specific fuel consumption data is for optimizing range - which is gallons per mile with enough Hp to attain a speed with that load, at that altitude to go the longest distance possible. (independent of winds aloft, temperature, etc)


This is what Specific Fuel Consumption is all about - with a jet engine it would be in terms of Thrust instead of Hp.

If you look at Drag polars for different payloads and speeds and altitudes, the bottom of the curve where CDp and CDi cross is a good place to start to match up engine thrust or Hp.
 
But with propeller engines the thrust output will deminish as speed increases. (above a critical speed, which is at the low end of the speed range, but varies with prop design)
So, depending on the case, optimum cruise speed (best MPG) may be at a significantly lower speed than the minimum drag speed. (the better thrust/power making up for the higher drag, and spicific power fuel consumption remains about the same, while specific thrust fuel consumption is much less)
And in the entire economic cruise power setting range for the engine fuel/power/hr will be about the same. However there will be a specific speed (on the low end, varying by propeller characteristics) where fuel/thrust/hr will be highest. This speed will also vary with altitude.

The best MPG speed (max range) should be somewhere between the max thrust/power speed and the minimum drag speed. (as well as an optimum cruise altitude, again determined by both engine/prop and airframe performance characteristics))


With jet engines this is all a lot simpler as thrust is pretty much linear across the speed range, and SFC is fairly constant throut the power range. (though thrust will vary with altitude, iirc the 004B's max thrust was acheived at ~20,000 ft, and the engine will have an optimum cruise power RPM range as well)
But in any case, for jets, the optimum cruise speed will be controlled almost entirely by the aircraft's minimum drag speed.
 

What calculator do you use?
1hp = 745.6999 W = 746 W

10,380 hp / 166,400 lb = 0,0624 hp/lb Ju-390
(10380 x 746) / 75500kg = 102,56 W/kg

8,000 hp / 111,522 lb = 0,0717 hp/lb Ju-290B-2 (with 2000hp BMW 801E with C3 fuel)
(8000 x 746) / 50600kg = 117,94 W/kg
takeoff distance 4537ft or 1375m
Service altitude 26,235ft or 7950m
Climb to 6,336ft or 2 km = 13,1 minutes
(ju-290b-2 info from Monogram close-up 3)

8,800 hp / 133,800 lb = 0,0658 hp/lb B-29 less power ?
(8800 x 746) / 60560kg = 108,4 W/kg

21,000 hp / 410,000 lb = 0,0512 hp/lb B-36
(21000 x 746) / 190000 = 82,45 W/kg
 

True for best range - With a jet L/D will decrease with velocity but the sfc (c) may not increase significantly as velocity increases ----> optimal cruise may be at a higher speed for the same aerodynamics and wing - depending on V/sfc

The formula is slightly simpler because you strip out complexities of propeller efficiencies and efficiencies as they vary with altitude - both affecting Thrust.

But if you wish to get the same graphical relationship between Power Required and Velocity - simply multiply the Drag x the Velocity from the L/D Polar. Pick the lowest point on the TV vs V polar for best Speed for range.

The key and slightly more complex question for jet engines is examining the sfc at say 1.5 M versus .8 M as the conversion from Propeller to jet thrust substitution in the Brequet equation brings V/sfc.

So the aerodynamics of the a/c become more complicated. The Concorde had better range at 1.5M than it had at .8M as an example, whereas no prop job I am aware of maintained a positive L/D enough to offset the sfc as the velocity continued to increase past optimal cruise speed.
 

On the minimum drag speed issue: (hypothetically) If minimum drag is reached at such a power setting that the engine has to be run at or near max continuous (at rich mixture and higher rpm), this will obviuosly have a drastic effect on SFC, and enough to make cruise at this speed impractical.

Of course, in such a situation, the engine would not be properly matched with the aircraft, and this would be an extreme example. (the a/c obviously being underpowered, or at least an odd engine selection was made)

The proper propeller would have to be chosen as well, to have a high efficiency at the optimum drag speed.
 

Independent of the propeller - if it is the SAME prop, max range occurs for that propeller at the max L/D - and requires less power (and fuel flow) at max L/D than it takes to get to that point in your flight profile.

I found something for you that might help. take a look

Maximum Endurance, Maximum Range, and Optimum Cruise Speeds
 
Thanks, I was already looking up that http://me.nmsu.edu/~aseemath/Schulz_96.PDF, but thanks for the refrence.

You possibly could have an oddball engine that had max economic cruise power at 1/3 of its max power. (say a 1,200 hp take-off with water injection, but max economical cruise of only ~400 hp, but now i'm just going into odd hypothetical examples, which isn't very useful...)
 

That hypothetical could work for most of the Drag polar curves for a powerful engine. The Mustang was in a lower ratio than that.

From memory the cruise setting for TO weight of 8500 pounds (why load it up in civilian world) it used about 45-48 gph depending on rpm at 27" hg. From the charts that is around 450-500 Hp at ~ 16,000 feet for max range. Didn't have an egt/cht set up in those days. So for take off power at ~ 1650Hp that is less than 1/3.
 
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