Northrop XB-35 hampered by electric power supply

[Supercharged]

When is was reading from Northrop's XB-35 Flying Wing I became quite confused about the electric power supply on this ship. As you might already know, it consits of the following major parts:
- 2x Franklin 4 cyl. APU's 113HP each, located in bomb bay #3 & #6
- each APU is driving one 208v, 400hz alternator (AC POWER)
- 2x motor generators driven by ac motors to generate 28v, 200amps each (DC POWER)

Regarding the flight hand book, the APU is limited to max. 20.000ft altitude. During flight testing both XB-35's suffered heavily from constant APU failures, even at much lower altitudes(Garry R. Pape & John M. Campbell "Northrop Flying Wings" Page 131,264-265).

And now to my questions....
#1 - Why do they, even for flight testing, design an electric power supply system which must fail at higher altitudes (FL200+) for an future strategic bomber?
#2 - What prevent them to use the existing generator drive pads on the R-4360 main engines accessory section?

 

[Sabrejet]

APU's are not designed to be the primary electrical source for the aircraft in flight; in fact on many aircraft the APU cannot even be operated in flight. Thus having an APU that operates up to 20,000 ft would be a benefit, not a hindrance. I'd also expect the motor-driven generators to be "emergency power". Both systems make sense.

Not answered is where the main electrical power generation for the aircraft comes from. I'm no XB-35 expert but I'd be very surprised if at least two of the main engines (and possibly all) did not have electrical power generation.

 

[Supercharged]

You're right, usually APU's are not running continuously and in this case they should named Primary Electric Power Unit.........(PEPU... i'am yoking)
Because this aircraft doesn't have engine (main) driven generators at all, as i suggest with question #2.

From :

Avialogs: Aviation Library - 4085 Handbook of Erection and Maintenance Instructions for the XB-35 Airplane

{phocadownload view=file|id=4085|text=Download PDF}

www.avialogs.com

 

[Sabrejet]

"auxiliary a.c. power". No mention of main power; that's an omission from the manual, not the aircraft.

 

[Supercharged]

"auxiliary a.c. power". No mention of main power; that's an omission from the manual, not the aircraft.

You mean the manuals are wrong?

From pilots flight manual (see link above)

In my opinion it doesn't make sense that all the documents are wrong.

 

[ThomasP]

Depending on what type of flight control actuation systems and how much total power was used by the (intended) avionics at any one point in time, it may have been significantly easier to run the main electrical system off an APU instead of splitting the power generation up amongst the engines. The manual mentions that the flight controls were powered by electricity in case of emergency (no idea how many kW this would take but it could be substantial). Whatever the electrical load would have been for the planned radars and jammers would also have been substantial - the area noise jammers in particular would have needed a lot of power.

 

[Sabrejet]

The manuals are not fantastically lucid. The key being engine-driven "alternators" (not "APU-driven"; alternator is not a precise term either).

So the manuals are ambiguous, not 'wrong'. You need to look at it from the other end: electrical power is required at operating ceiling so how is it done? It's not that it cannot or isn't.

 

[cvairwerks]

I did note in a short amount of chasing info, that one source stated that the AAF did not deliver the alternators that were to be used on the 4360's and directed Northrop to come up with the APU's to provide power for flight testing. As to how accurate this is, would require more data chasing.

 

[swampyankee]

Relying on APUs as the sole source of electric power would not be unprecedented. See, for example, the Bell FM-1. One hopes that the use of APUs as the main source of electrical power (I presume there would also be some batteries) was only to be an expedient during flight testing. Alas, temporary expedients frequently last far longer than they should.

 

[Supercharged]

More than likely i found an explanation for XB-35's strange elec power design.

Summary:

All piston driven XB & YB-35 as well as jet driven YB-49's which entered flight testing flew with APU driven AC & DC power supply as mentioned above. This means flight testing was limited to altitudes of 15000 to max. 20000 feet. According to which all the max speed and max range figures are computed and estimated, because they could never be proven in real flight condition. As written by Pape & Campbell Page 273, YB-49 SER# 42-102368 was forced to abort range test flights due to APU misfiring at optimum cruise altitude (just an example for numerous APU troubles).

The only Type of the "big flying wings" which received an suitable elec power supply was the sole YRB-49 (42-102369) in form of:
2 x 40 KVA CSD driven AC generators on the J-35 pod engines
6x 400 AMP DC starter motors on each J-35
(refer AN01-15EBB-1 flight instructions)

So far so good, but back to XB/YB-35, piston powered flying wings. Finally i found a quite interesting illustration in YB-35 parts catalog(AN01-15EAA-4)

Avialogs: Aviation Library - 4087 AN 01-15EAA-4 Parts Catalog YB-35

{phocadownload view=file|id=4087|text=Download PDF}

www.avialogs.com

This picture shows the final design resolution planned to be installed(my opinion) on the YB-35, a CSD driven AC alternator connected to each of the inboard prop gear boxes! As stated by Pape & Campbell Page 130&131, apu, prop and gearbox as well as armament were government supply. Furthermore at Page 121 they mentioned design troubles "redesign of prop gearbox to incorporate drive for alternators", which in turn led to more cost and time overruns. That means the equipment required to install a suitable electrical power supply was not available from governmant and northrop was, unfortunately forced to continue with the unreliable APU solution.

 

[SaparotRob]

More than likely i found an explanation for XB-35's strange elec power design.

Summary:

All piston driven XB & YB-35 as well as jet driven YB-49's which entered flight testing flew with APU driven AC & DC power supply as mentioned above. This means flight testing was limited to altitudes of 15000 to max. 20000 feet. According to which all the max speed and max range figures are computed and estimated, because they could never be proven in real flight condition. As written by Pape & Campbell Page 273, YB-49 SER# 42-102368 was forced to abort range test flights due to APU misfiring at optimum cruise altitude (just an example for numerous APU troubles).

The only Type of the "big flying wings" which received an suitable elec power supply was the sole YRB-49 (42-102369) in form of:
2 x 40 KVA CSD driven AC generators on the J-35 pod engines
6x 400 AMP DC starter motors on each J-35
(refer AN01-15EBB-1 flight instructions)

So far so good, but back to XB/YB-35, piston powered flying wings. Finally i found a quite interesting illustration in YB-35 parts catalog(AN01-15EAA-4)

Avialogs: Aviation Library - 4087 AN 01-15EAA-4 Parts Catalog YB-35

{phocadownload view=file|id=4087|text=Download PDF}

www.avialogs.com

View attachment 821336
This picture shows the final design resolution planned to be installed(my opinion) on the YB-35, a CSD driven AC alternator connected to each of the inboard prop gear boxes! As stated by Pape & Campbell Page 130&131, apu, prop and gearbox as well as armament were government supply. Furthermore at Page 121 they mentioned design troubles "redesign of prop gearbox to incorporate drive for alternators", which in turn led to more cost and time overruns. That means the equipment required to install a suitable electrical power supply was not available from governmant and northrop was, unfortunately forced to continue with the unreliable APU solution.

Impressive bit of "source-ery".

 

[Sabrejet]

Yee-ha! Seems that the Flight Manual reflected the state of the flight test aircraft but not the definitive configuration. Makes sense.

 

[Supercharged]

Impressive bit of "source-ery".

What do you mean? You think i made something wrong in my conclusion? i always try to avoid the history wizard.....

Yee-ha! Seems that the Flight Manual reflected the state of the flight test aircraft but not the definitive configuration. Makes sense.

Iam just saying what is written in the sources i have, do you have other sources or documents?

 

[ThomasP]

These 3 pages from the XB-35 Pilot's Handbook manual talk about the flight controls and the use of the APU vs the normal hydraulic, electrical, and battery system.

If I am understanding what is said correctly, 1 or both of the APU would be needed in order to power the emergency backup control systems (which use electric motors in emergency mode) for the elevons, rudders, slots, and flaps. I think it is saying that at least 1 of the APU would need to be operating in emergencies to generate enough power to operate the elevons over prolonged periods of time.