B-52F (YASim) readme.

This is not an authentic representation of a B-52F but it's based on
the right numbers, where I could find them.  There is a lot of
guesswork in the fdm.  It should also be noted that this a/c is still
very much under development, in nearly every respect and there are
several aspects which need fixing/improving.

History
-------
The development of the B-52 can be traced back to a specification
issued on the 23rd of November, 1945 calling for a bomber with an
operating radius of 5000 miles and a speed of 300 mph at 34000ft
altitude with a 10000lb bomb load.  This specification was seen as a
replacement for the Convair B-36, which at that time had still not
made it's maiden flight.

The Boeing proposal was the Model 462, which looked very much like a
scaled-up B-29 powered by six Wright XT35 Typhoon turboprop engines.
Although the Model 462 fell far short of meeting the range
requirement Boeing was informed on the 5th of June, 1946 that it had
won the competition (against proposals from the Glenn L. Martin Co.
and the Consolidated Vultee Aircraft Corp.) and by mid-June the Model
462 was assigned the designation XB-52.

However, by October, 1946 the USAAF had started to have misgivings
about the XB-52, concluding that it offered few performance
advantages over the B-36, that it was too large and expensive while
having little growth potential and that it simply did not meet the
range requirement.

Boeing responded by proposing a scaled-down version of the Model 462,
the Model 464, powered by just four of the Wright XT35 turboprops but
this was rejected by Gen. LeMay, then deputy Chief of Air Staff, who
thought that it was still not good enough, deciding that the future
B-52 should have a higher cruising speed as well as a longer range.

In December, 1946 the USAAF requested that a study be undertaken for
a four-engined bomber with a range of 12000 miles, a crusing speed of
400 mph and the ability to drop the atomic bomb.  Boeing's response
to this were the scaled-up Model 464-16 and Model 464-17, both still
powered by four Wright XT35 turboprops, which by now promised
significantly higher power and efficiency.  The -16 version was a
specialised aircraft, intended to carry a 10000lb bomb load (nuclear
only) over a long range, whereas the -17 version could carry a 
90000lb conventional bomb load over a reduced range.

The USAAF initially accepted the Model 464-17 but Gen. LeMay was
still unhappy with the size and cost of the aircraft and there were
still concerns that it offered little advance over the B-36 and so
the Model 464-17 too was eventually shelved.

During a six month grace period before the B-52 project would be
finally dropped Boeing came up with the Model 464-29.  This still
retained the four XT35 turboprops but now featured a sharply tapered
wing with 20 degrees of sweep back and a design speed of 445 mph.

By mid 1947 the USAAF were still looking for an effective means of
delivering nuclear weapons and a Heavy Bombardment Committee set up
to investigate the requirements decided that speed and altitude were
the primary qualities needed in a aircraft to carry the nuclear bomb.
This resulted in a specification issued on the 8th of December, 1947
calling for a cruising speed of 500 mph (dropped from 550 mph) and a
range of 8000 miles.

The Model 464-29 would clearly be inadequate under the new
requirements and the entire B-52 project was nearly cancelled at this
point but during this period of uncertainty Boeing had been trying to
improve the performance of their design.  The result, in January,
1948 was the Model 464-35, which was still powered by the four XT35s.
This design was similar to the Model 464-29 but had been down-sized,
with considerable weight savings.  The estimated performance was 500
mph at 41000ft with a range of 11635 miles.

The USAAF, while still not entirely happy with this design, and under
pressure due to the Berlin blockade gave the go-ahead for phase 2 of
the project, including the development and testing of two prototype
XB-52s based on the Model 464-35 design.

In May, 1948, While work on the XB-52 mock-up was under way the USAAF
asked Boeing to explore the possibility of switching to jet engines
for the B-52 (jet engines having previously been excluded on the
grounds of their high fuel consumption).  Boeing responded, in late
July with the Model 464-40, which was to be powered by eight
Westinghouse XJ40-13-12 turbojets, in underwing podded pairs.  This
model offered an improved performance over the -35 version,
especially at high altitudes where the max speed was estimated at 507
mph at 47000ft.

Although the USAAF were favourably impressed by the -40 model, they
were still concerned about the fuel consumption of the early jet
engines and instructed Boeing to continue working on the -35 model
while also encouraging them to continue exploring the possibility of
a jet powered version.

However, on the 21st of October, 1948, when Boeing engineers arrived
at Wright Field to confer with USAAF officials about the future of
the turboprop powered B-52 they were told to drop the turboprop
design and that turbojets should power the B-52, specifying the Pratt
& Witney JT3 (J57), a turbojet adaption of the 10000hp T45 turboprop
as the power plant.  The Boeing engineers, after some thought about
this, announced on the following day that they would have a new
proposal by the following Monday (in four days time) and the result
was the Model 464-49.  This was broadly similar to the -40 version
but with an increased sweep of 35 degrees and an increased wing area
- up by 1400ft2 to 4000ft2.  Estimated performance was now 565 mph
at an altitude of 46500ft.

After a final evaluation earlier in January, 1949, Boeing were
finally given the go-ahead to proceed with the Model 464-49 on the
26th of January, 1949, under the terms of the original -35 phase 2
contract.

By November, 1949, concerns about the limited range of the -49
version were being expressed that threatened the project, once again,
so Boeing had to once more re-design the Model 464.  The result was
the Model 464-67.  This had the same wing as the -49 version but the
fuselage was extended by nearly 22ft to 152ft 8in, to enable more
fuel to be carried.

On the 24th or March, 1950, the Model 464-67 was approved to replace
the Model 464-49 but there was no definite commitment to manufacture
until the 14th of February, 1951 when an initial batch of 13 B-52As
was authorised.

The last major change, at this stage, was a switch from tandem
seating for the pilot and co-pilot to a side-by-side seating
arrangement, however, the first two prototypes - the XB-52 & YB-52
were built with the original tandem seating.

The first operational B-52 version was the 'B' model, the last one
being delivered in August, 1956.

Over the following years, a series of updated versions were built,
culminating in the B-52-H, which is the only version remaining in
service (excepting NASAs two NB-52Bs, which are still used for air
launches of experimental vehicles).

The B-52 has seen combat several in several theatres since it's
introduction, most notably in the Vietnam war between 1965-1972 but
also in the more recent Iraq conflicts of 1991 and 2003/4  

Model
-----
The model was originally constructed in Realsoft3D (linux beta V4.5),
exported as a .OBJ format file and imported into AC3D where it was
converted into .ac format and textured.

The accuracy of the model is heavily dependent on the data and
drawings available for it, and in most cases, the side, front and top
views in a typical 3-view drawing rarely align correctly or measure
consistently.  For example, when the model is scaled to the correct
length, the wing-span is likely to be a little out.

Flight Data Model
-----------------
The Flight Data Model uses the FlightGear YASim fdm solver, which
uses a combination of aircraft geometry and performance data to
generate the flight model.

Apart from the basic length, span and height of the aircraft, most of
the measurements needed for YASim are not generally available so
after uniformly scaling the 3d model to one of the basic measurements
i.e. length, the geometry data was taken from the model.

While this may not give the most accurate numbers, with respect
to the original aircraft, it does mean that what you fly matches
pretty closely to what you see, at least as far as the geometry
is concerned.

Information on the B-52Fs performance is fairly abundant but
achieving the full performance has proved difficult so the current
fdm should be regarded as developmental and still incorporating a lot
of guesswork.

While the low altitude performance seems more or less acceptable, it
cant reach it's service ceiling of 46,700ft with a useful fuel load.

The fdm is configured for a full fuel load - this just about
represents the heaviest weight at which the B-52F operated.

I was able to obtain some good performance and approach data from an
ex-flyer of B-52s, including the 'F' model, and while the fdm doesn't
exactly match the numbers I was given they are only about 5kts out.

Currently, YASim does not have support for water injection so this
has been 'spoofed' by using the reheat feature.  The 'dry' thrust
rating is correct for the engine model but the 'wet' thrust rating
has been used for the reheat setting.

Only a limited amount of water was carried for injection so the
reheat on this B-52F should only be used for a couple of minutes -
after that, the water would have run out.

I found several performance figures that could be used for the cruise
parameters:  554kt @ 21000ft, 495kt @ 46500ft, service ceiling at
combat weight = 46700ft, cruise speed = 454kt

The range of weights for the B-52 vary quite widely:
empty = 173599lbs, combat weight = 291570lbs, max take off = 
450000lbs.

With the model fully fueled, it comes in around 422908lbs and I'm
assuming this is the ferry configuration.

I've had to work on the basis of the ferry configuration, so that
less fuel can be specified - an initial fuel load of 0.46 will give a
take off weight about equivilent to a combat load/configuration.

The range of the a/c was too low as well so I've reduced the tsfc
value from the default 0.8 to 0.7.  I don't have a proper number
for this value yet but as it's quite an old engine I don't think it
would be very low.

Keyboard mapping
----------------
The following additional/non-standard keyboard mappings are set up
for this aircraft

Key               Action
'K' (Shift-k)     Toggle trajactory markers on & off
'U' (Shift-u)     Update the drop-view location.

Panels
------
Currently, there are two simple 2D panels for the model, neither of
which are in any way accurate - they are simply holders for the
instruments.  The 'vfr' panel includes the basic instruments needed
for 'vfr' and calls the 'standard' FlightGear instruments from the
FlightGear installation.  The 'mini' panel includes a subset of the
instruments on the 'vfr' panel, with a transparent background.

In addition to the standard FG instruments, both panels also
incorporate a number of custom instruments.  These are mostly
informational but two of them can be used to control some of the
Autopilot functions - see below.

Custom Controller Instruments
-----------------------------
There are two custom instruments on both the 'vfr' and 'mini' panels
that can be used to control some of the autopilot functions.  These
are the speed controller and the altitude mode controller.

AP Speed Controller
-------------------
The speed controller can be used to hold the aircraft speed by
throttle, either to a set KIAS, or to a set mach value.  Clicking
with the mouse on the yellow 'K' will set the AP speed controller
into KIAS hold, while clicking on the blue 'M' will set Mach hold.
The numeric value displayed in either yellow or blue indicates the
set speed, in either kias or mach, relatively.  There is a small
array of '+' and '-' characters to the left of the instrument and
these can be used to increment or decrement the speed setting, in
either 10kt or 1kt steps for kias or 0.1 and 0.01 steps for mach.

AP Altitude Mode Controller
---------------------------
The altitude mode controller appears as a strip reading

    AP Mode: AH TF TO IL MC

The meaning of the different modes are:

	AH = Altitude Hold
	TF = Terrain Following
	TO = Automatic Take-Off
	IL = Automatic Instrument Landing
	MC = Mach Climb

AH Mode
-------
The AH (Altitude Hold) function is intended to hold the aircraft at
the altitude set in /autopilot/settings/target-altitude-ft.  When
engaged, the set altitude can be changed by using the standard FG
keystrokes.

TF Mode
-------
The TF (Terrain Following) function is intended to hold the aircraft
at a constant distance above ground level (agl).  The separation
distance is set in /autopilot/settings/target-agl-ft.  It is not
currently possible to change this setting from either of the panels -
it must be changed via the property browser.

It should also be noted that FG does not currently provide a
look-ahead function that could be used for a proper terrain following
system so the current terrain following function works by simply
checking the agl directly below the a/c.  This means that the TF
function can only react after the separation has increased or
decreased and will not stop you from flying into steep sided ground
elevations i.e. cliffs.

TO Mode
-------
The TO (automatic take-off mode) function is intended to be used to
automate the take-off process.  It should be noted that the a/c has
the parking-brake engaged when FG starts and this should be released
before trying to take-off.  In addition, because the flaps take a
very long time to extend/retract, they need to be extended manually
before engaging TO mode.  If TO mode is engaged before the flaps are
fully extended nothing will happen, however, once the flaps have
completed extending the TO sequence will start.

When TO mode is engaged, the following sequence of actions occur:

  The current heading of the a/c on the runway is set for both
  the ground-roll and in-air heading.
  
  Hold speed-with-throttle is engaged (KIAS mode)
  
  The wing-leveller is engaged
  
  Rudder/nose wheel steering is engaged.

As soon as speed-with-throttle is engaged, the a/c will start
accelerating down the runway and once it has sufficient speed it will
lift off from the ground.  Note that during the ground roll there is
no specific means of keeping the a/c on the runway centre-line so
while the a/c will hold the heading, there may be some drift across
the runway in cross-winds.

Once the a/c has climbed above 50ft agl, a climb-out pitch-hold
controller is engaged, to hold the a/c at a constant pitch, the
under-carriage is retracted, the rudder control is reset and the
rudder re-centred.

As the aircraft continues accelerating, the flaps are retracted and
when the a/c exceeds 260 kias the heading hold is switched to
true-heading-hold, speed control is set to mach-with-throttle and
Mach-Climb mode (see below) is engaged.  The final action is to
disable the AP TO mode so that it cannot be engaged in flight.

It is possible to set a number of way points before engaging the TO
function but it is then necessary to hit Ctrl-h a couple of times to
dis-engage true-heading-hold, which is set whenever a way point is
entered, and re-centre the ailerons before TO is engaged.  What will
happen in this case is that once the take-off sequence has finished
and true-heading-hold is engaged, the a/c will turn to the
appropriate heading and follow the way points.  If no way points have
been set the take-off heading will be followed.

IL Mode
-------
The IL (automatic instrument landing) function is intended to land
the aircraft automatically, provided that the runway you wish to land
on has an instrument landing system and that the radio nav equipment
is already correctly tuned for the intended landing runway.

You should be in Altitude Hold mode and well below the glide-slope
when IL mode is engaged.  This is because extending the flaps has a
considerable effect on the drag and trim and should be done while the
a/c is in level flight.  Even so, at heavy landing weights you are
likely to gain up to 200 ft altitude as the flaps extend and at light
weights you will gain nearly 500ft.  While the flaps are extending,
which takes a full 60 seconds (or 120 seconds if one of the two flap
control motors has failed, although this failure mode has not yet
been modelled), and although the height gain is unavoidable, the AP
AH controller should be able to maintain control of the a/c and will
bring it back down to the set AH height once the flaps are fully
extended, provided that the glide-slope has not been intercepted due
the the altitude gain as the flaps deployed.

The a/c should then fly level until it intercepts the glide-slope, at
which point it will start following it down, maintaining 1 deg AoA.

The one deg AoA actually results in a two deg nose down attitude on
a 3 deg glide-slope and allows for 2 deg in which to flare and land
level.

The a/c should follow the glide-slope down until it is 100ft agl, at
which point it switches to Touch-down mode and tries to land the a/c
at < 1 fps.

In an attempt to cater for cross-winds, and the final nav1 correction
that kicks in shortly before reaching the runway, the AP switches to
a modified nav1-hold controller which uses the nav1 needle deflection
for guidence when the a/c is a few degrees off the nav1 course and
which, in my testing has proved better at putting the a/c down in the
right place.  However, this modified mode can also temporarily kick
in if the nav1 course is crossed so it's a good idea to be clearly
heading towards the intended runway before engaging IL mode.

From my testing, I'd recommend an AH height setting of between 2000
3000ft above the target runway, dependent on the distance from the
target runway - this should give enough time for the AP to get the 
a/c stable on the glide-slope.  If it isn't, any oscillations will
only get worse as you get closer to the runway and will result in a
crash.  You also need to be travelling at more than 230 kts before
engaging IL mode as this is one of the internal thresholds that
trigger some of the internal IL functions.  As the B-52 should not
exceed about 350 kt at low level you should also be at or below this
speed when engaging IL mode - around 220-310 kts, depending upon the
weight seems to be about the right speed range.

Who said landing an aircraft is easy?  ;)

MC Mode
-------
The MC (Mach Climb Mode) function is designed to command the highest
climb rate that can be sustained for a given mach setting and is only
enabled when mach-hold-by-throttle is selected on the AP Speed
Controller.  This function has some limitations, one being that it
works best when the aircraft is travelling below the set mach number
and is accelerating to achieve it.  If the aircraft is already
travelling at the set mach number the climb rate is likely to be very
low and it may be necessary to temporarily reduce speed, and then
increase it again (using the AP Speed Controller) or force a climb by
pulling back on the stick.


Lee Elliott.     2004/06/07
