Purpose
The scope of this mod is twofold and, likewise, will have two phases:
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Fix various bugs/issues and implement base systems in the Boeing 307 Stratoliner
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Bake in some state saving and realism for an enhanced simulation experience (This is looking more and more like I will do this for MSFS 2024 given the added systems depth of the 2024 SDK.)
Note that Phase 1 will be followed up with additional releases to address and tune any related issues/oversights. Phase 2 will be a follow-up release once its ready.
The reason for splitting the development into two phases is primarily to give owners (and prospective owners) of the Stratoliner something to be excited about and to get them flying and enjoying the aircraft now without having wait on an extended development period while I attempt to implement all my desired features.
Introduction
Readily available technical documentation for the Boeing 307 is scarce and not very detailed. Only ten aircraft were constructed and only one is still in existence. Several of the aircraft went through significant modifications due to their WWII service and subsequent post-WWII refitting history. All of which adds to the problem of gathering accurate and detailed information. That said, vintage documentation, technical articles and drawings were sourced to direct all modifications. Additionally, a 2012 paper presented to the American Institute of Aeronautics and Astronautics regarding the history, development and implementation of the Boeing 307’s pressurization system was also referenced. I placed requests with both Boeing Historical Services and the Smithsonian National Air and Space Museum for detailed technical documentation. Only Boeing responded and wouldn’t allow access to documentation due to National Security(!) reasons, however they did offer to answer as many questions as they could, which was greatly appreciated.
Some creative license was taken in terms of direction where no information has been available to detail how it “ought” to be. As such, much “reading between the lines” has been done to make sense of various aspects of the aircraft systems. Furthermore, what iteration of the aircraft should be modeled? The 307 as it was originally built for Pan Am, the 307 as it was originally built for TWA, the 307 as it was refitted, or some combination of the three?
This is the first time I have ever attempted developing a mod of this magnitude and the learning curve has been exceptionally steep. This project has been a labor of love and is open to constructive criticism and suggestions for improvement and future features. A huge thank you to those in the community who offered assistance!
Understanding the 307’s Pressurization System
When Pan Am caught wind that Boeing was developing the 307, they were keen to invest in the aircraft for use on their South American routes. Due to the terrain, they were having to fly around mountains. Pan Am saw the 307 as a means to shave days off of travel time by being able to fly over the mountains and take more direct routes between destinations. Boeing asked Pan Am what altitude the highest airport they flew out of was, which was 7000’. Boeing, therefore, set the initial cabin pressurization altitude at 8000’. This means that the cabin altitude in the 307 is non-pressurized up to 8000’. Once the aircraft passes 8000’ the system begins pressurizing the cabin and maintains a 2.5 psi (2.65 psi max) cabin pressure differential. Interestingly, early aviation high altitude flight experimentation determined that supplemental oxygen was not required up to 12000’. The 307’s cabin altitude, therefore, maintains 12000’ when the aircraft is flying at 20000’. The 307 was the first aircraft to employ a Flight Engineer, whose job it was to primarily monitor and manage the pressurization system.
The gauges and levers on the Flight Engineer’s panel are not quite right for truly simulating the 307’s pressurization system. The Cabin Differential Pressure gauge was actually a Cabin Blower Pressure gauge, which was used to monitor the output pressure of the individual cabin superchargers. The Flight Engineer would use the three-position (modeled as a two-position) Cabin Differential Pressure lever and Suction gauge to monitor atmospheric pressure and the duct flow of each cabin supercharger. From those values he could determine what the pressure differential was. The Cabin Supercharger Outlet valve was a five-position lever that allowed the system to be (from left to right): pressurized from the #2 cabin supercharger, open to atmosphere (no pressurization), pressurized from both cabin super chargers, open to atmosphere, pressurized from the #3 cabin super charger. Since the lever is modeled as two-position, fully left is open to atmosphere and fully up is pressurized by both cabin superchargers when altitude is >8000’.
Understanding the 307’s Supercharger
Pan Am stated that the 307 was designed for full throttle takeoff at sea level. As such, their takeoff performance at higher altitudes was compromised. The takeoff performance in this mod has been modeled upon this, so full throttle take off is necessary and available takeoff power will be lower with altitude. Since there is no available supercharger shift lever, the second-stage shift will occur automatically and is set to not over boost the manifold pressure at the shift point. Shoot for a 1200’/min climb rate noting this will falter somewhat as you approach the supercharger shift point. This will, again, falter as you approach the cruise ceiling of 20000’.
Understanding the 307 Cross Feed System
This was tricky. After reading and re-reading numerous references to the cross feed, it was determined that the system was designed to allow one engine’s mechanical fuel pump to feed pressurized fuel to both engines in the event of a mechanical fuel pump failure. In fact, the SOP for starting the engines without ground power was to start engine #1, shut off #1 boost pump, turn on 1/2 cross feed and start engine #2. The same would be done on the opposite engine pairing – start #3, shut off #3 boost pump, turn on 3/4 cross feed and start #4.
NOTE: This mod is meant to fully replace the Microsoft/Aeroplane Heaven Boeing 307 Stratoliner Fixes mod. As such, it has been renamed, so care should be taken to remove the previous release from the Community folder.
Engines:
- Reworked engine performance to match the GR-1820-97 as installed in #2003 from 1954
- Added a two-stage supercharger (as installed in TWA’s 307B variants)
– Automatically shifts at ~10000’ (precise shift depends on current barometric pressure)
– Pan Am pilots stated the 307 was designed to take off with full throttle at sea level. They noted takeoff performance was a challenge at high altitude airports. - Increased carburetor icing effect
- Revised Carb Heat lever interaction method to now behave as other levers do via dragging (Thanks, Duckworks!)
- Implemented engine prime
- Altered feathering system to place prop levers into a negative value (feathering) position allowing SDK feathering code to function correctly
- Altered feathering system to maintain high pitch propeller lever position when unfeathering to allow for following realistic in-air engine restart procedure
- Increased stopped engine friction to prevent feathered props from very slowly windmilling engines
- Reworked engine start code to fix oddities, add a bit of cranking-to-startup time realism and implemented engine “momentum” to allow for engine in-air restarts via windmilling
- Limited vacuum pumps to engines 2&3
- Refined engine start process to allow engine to fire briefly when Start, Boost & Prime switches are engaged when fuel pressure is present, but will stall if mixture is not set immediately after engine fires
- Adjusted RPM threshold for engine shutdown sounds so they can be heard
Flight Model:
- Adjusted htail_incidence to reduce need to overuse nose-down trim
Cabin Pressurization System:
- Enabled pneumatic system on Engines 2 & 3 and set to 18 PSI to simulate cabin superchargers
- Set PRESSURIZATION CABIN ALTITUDE GOAL to 8000’ to achieve the original Boeing implementation to provide a cabin altitude of 12000’ at 20000’ cruise altitude
- Set Cabin max_pressure_differential to 2.65 PSI the max cabin differential pressure set by Boeing
- Implemented pressurization system to be inoperable below 8000’ feet as originally implemented by Boeing
- Edited .gLTF files to uncouple Flight Engineer’s VSI needle from Pilot & First Officer’s VSIs to facilitate Flight Engineer’s VSI showing cabin’s rate of change rather than aircraft’s vertical speed
- Altered Suction gauge to reflect suction pressure of cabin superchargers rather than aircraft vacuum system pressure
- Implemented Blower Discharge Pressure lever to display Eng #2 or Eng #3 Cabin Supercharger on Suction gauge
- Altered Cabin Pressure Altitude gauge face texture and code to recalibrate for 0-20000’
- Added Cabin Pressure logic preventing windows and door from being opened when cabin is pressurized due to internal pressure
- Added Cabin Pressure logic to recognize opened/closed pilot/F.O. windows and cabin door
Electrical System:
- Reworked electrical system busses
- Upgraded generators from 50A to 200A
- Tuned generator curves to prevent significant charging below ~1200 RPM
- Separated Generator busses
- Created an AC bus and connected AC instrumentation, directional gyros and fluorescent cockpit/cabin lighting and avionics bus
- Enabled Inverters switch to power AC Bus
- Altered Flight Engineer’s AC voltmeter to indicate inverter AC voltage
- Recoded Landing Gear Control Lock warning lamp to trigger when extended on ground rather than when retracted in air
- Fixed Panel & Compass lighting to eliminate various errant lit gauge faces/needles when switches were on, and power was off
- Rewired Compass light switch to illuminate Whiskey compasses rather than gyro compasses
- Fixed transponder display to illuminate only when Avionics bus is powered (Thanks, Duckworks!)
- Fixed F.E. Low Oil Pressure annunciators
- Altered Boost switches to be Ignition Boost switches
- “Fuel Booster Pump” switches must be used to provide starting fuel pressure
- Added starter high amperage electrical load when engines are cranking
- Starters draw less when being overrun as the engine fires
- Added electrical draws for ignition booster coils and engine primer solenoids
Fuel System:
- Redesigned fuel system
- Added fuel pressure bleed down effect
- Renamed fuel tank references so W&B sheet labeling matches cockpit fuel selector valve placard labeling
- Set fuel tanks to #2003 capacities – Main 425 Gal. Extra 215 Gal. (note faces still indicate 307-G values, if this causes much gnashing of teeth it can be reverted, however see notes below)
- Set mechanical fuel pump pressure to 17 PSI, added pressure curve that alters with engine RPM
- Reworked Cross Feed to allow a shared wing pair of engines to feed from one engine’s mechanical fuel pump (e.g. Eng1 mechanical fuel pump feeds Eng1 and Eng2)
- Redesigned fuel dump system to flow at specified dump rate and limited fuel dump to the main tanks only as implemented by Boeing
- Enabled mixture control
- Enabled mixture density modeling to take advantage of MSFS fuel system improvements
- Fixed Inboard/Outboard fuel gauges indicating each other’s tanks
- Altered the Right Outboard fuel gauge such that it displays the correct level of fuel
- Altered the fuel selector valves such that Main feeds from the Main tanks and Inboard Extra feeds from the Inboard Extra fuel tanks
- Altered the fuel selector valves such that they are in the OFF position by default
- Implemented mixture lever detents for Cut-Off, Auto-Lean, Auto-Rich and Full-Rich (Thanks, Duckworks!)
- Implemented Auto-Lean and Auto-Rich mixture modes
Hydraulic System:
- Implemented hydraulically boosted elevator and rudder
– Note these are currently being treated as fully hydraulic, as modeling boosted will require further specialized coding - Implemented toe brakes, prop feathering and fuel dump dependent on online hydraulic system
- Limited hydraulic pumps to engines 2&3
- Removed hydraulic pressure gauge needles for engines 1&4
Deicing System:
- Enabled structural deicing system
Sperry Gyropilot:
- Removed Sperry Gyropilot power knob from face of Sperry
- Reworked Autopilot pedestal lever to power Sperry Gyropilot
- Cleaned up Sperry Gyropilot face texture to accomodate removed power knob
- Reworked Sperry Gyropilot code to resolve inconsistent behaviors
- Required presence of vacuum to operate Sperry directional gyro
Other:
- Altered the altimeter(s) faceplate to replace the erroneous 6 with a correct 8
- Altered all .flt files to ensure electrical service and fuel flow when starting a flight in any scenario with engines running
- Reworked camera view “8” to facilitate easier management of mixture and carb heat
- Added Instrument View 9: Full Flight Engineer’s station for easy access to all fuel selectors
- Added Instrument View 10: Cabin window view
- Hide yokes for Instrument Views 1 & 2
- Hid throttle, propeller and tailwheel lock levers for Instrument View 8
- Adjusted Instrument View 4 to afford easier access to Master Battery switch
- Adjusted Instrument View 6 to afford full view of F.E.'s panel
- Add “lag” to gauge needle animations to give needles a “sweeping” motion rather than an instant “jumping” to a position
- Fix loading screen tips formatting and other errors in en-US.locPak (English localization) file
- Added code for ATC to state type and model
- Note: Azure is pronouncing Stratoliner “StratoLEEner”
- Added checks to prevent the “Re-Fuel” and “Services” switches from being operated when aircraft is in the air
- Added a full aircraft default Payload
- Included an alternate layout.json file for FTsim+ users (replace the layout.json file in mod’s directory with the alternate in the "For FTsim+ Users directory)
- Be Advised: for the ctrl-E or spawn on Runway/in air types, you MUST immediately press shift-F4 (or whatever control(s) you have set) to place the mixtures in full rich. From there set them by hand to Auto-Rich or Auto-Lean as necessary.
Notes:
- As stated, this is very much a work in progress and constructive criticism regarding engine and flight model performance is welcome and desired
- Attempts to dial-out slight tendency to roll left were unsuccessful. Seeking input from someone more familiar with flight model editing to assist with this.
- A number of gauge needle engine number tags are setup such that when the gauge needle is in its normal indicating position, the engine number is upside down. (Some of the needles are even flipped front to back.) I’m seeking input on how to correct this via texture editing.
- I’m seeking input on how to alter the text labels on switch panels
- There is no oil level gauge present (with 4-way selector switch), as would be on the Flight Engineer’s panel, which is frustrating
- There is no switch setup to place the hydraulic system in “automatic” (presumably to enable an electric hydraulic pump as needed to boost hydraulic pressure at low RPM and in case of mechanical pump failure)
Some of Phase 2 plans:
- Further refine flight model
- Implement realism with regards to engine start and climate conditions
- Further tune electrical loads
- Implement pedestal lighting
- Add gauge needle vibration “bounce” when engines are running
- Monitor engine vitals (temperatures, pressures, advised RPM ranges to avoid, etc. ) and allow for failures from abuse
- Require electrical load shedding to prevent generator failure when operating high electrical loads
- Eliminate Battery/Ground power switch from Flight Engineer’s station (this redundant switch makes no sense given the heavy power switch on the pedestal) and give it an alternate function (the AC gauges were originally “Autosyn” gauges and were powered via a switch on the F.E.'s pane, perhaps those?)
- Alter “Landing Light Test” switch to be the correct “Warning Light Test/Bright/Dim” switch
- Implement missing audio such as booster pumps and engine starter cranking sounds
Note: This post will be updated to incorporate key changes as mod evolves. Release Notes will be posted in this thread as separate posts and will also be incorporated into the included README file.
Find it here:
Redgrave Redux — Boeing 307 Stratoliner Improvement Mod for Microsoft Flight Simulator | MSFS
