B52-yasim readme

2003/05/11 Lee Elliott.

See the notes in the YASim config file

  Aircraft-yasim/b52-yasim.xml

for some info and limitations about the fdm.

First of all, this shouldn't be regarded as an accurate represntation
of a B-52 - it's based on real numbers etc, where I could find them but
there's a lot of guess work and compromise incorprated.

This model tries to emulate a B52-F but there's currently no support
for water injection in the YASim solver.  To get around this I've
used the reheat/afterburner option.  This mimics the thrust levels well
but the fuel-flow rate is increased by far too much - approx eight
times the normal flow rate.

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.

I wasn't able to get a working YASim solution when using the 46500ft
and 46700ft altitudes so I've used the 554kt @ 21000ft for the cruise
settings.  I've also had to set the throttle settings to 0.6 at cruise,
even though YASim would normally expect the max speed and alt here,
because it was difficult to get the a/c much above 25000ft.  This fdm
will climb to 30000ft, albeit with a rest as 20000ft to re-gain some
speed, when taking off with a full fuel load.  With 0.46 fuel load
altitudes greater than 40000ft can be attained.

Using a value of < 1 for the cruise throttle setting may have a degree
of validity here as there are some indications that the a/c speed was
limited more by flutter concerns than raw power - the NB-52E model was
able to fly "ten knots faster than the speed at which flutter normally
would have disintegrated the aircraft".  The NB-52E did not have more
powerful engines but additional control surfaces (winglets at the nose
and extra control surfaces on the wings), which were controlled by
electronic sensors and actuators.

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.

The 'set' file adjusts some of the keyboard bindings as follows:

'[' & ']' (Flaps) have been adjusted so that the step is -1 and 1
respecively.  This is because the B-52 flaps have only two positions:
fully extended or fully retracted.

'b' (Brakes) have been re-bound to operate all four brakes on each
of the main gear units.  To be able to animate the independent
suspension (at a future date) I need to specify each of the six gear
units individually and this does not fit well with the current default
of assigning multiple wheels and brakes to a single gear unit, which
results in only three brakes operating.

I've made no attempt at trying to create a realistic panel for this a/c
and both the VFR and mini panels are more for monitoring and AP control
rather than 'proper' flying.  I've experimented with using 'digital'
text only instruments in the mini panel - this was to reduce the
texture requirements and also because I wanted more accurate figures
while monitoring stuff during flight testing.

The water injection/reheat can be controlled by the red-handled slider
on the mini panel - I've not yet added this control to the VFR panel.
The water injection/reheat can also be controlled by the
/controls/engines/engine[0]/afterburner property.  This entry controls
reheat on all eight engines.
---

Taking off (from sea-level).

Don't try taking off with any obstacles in front of you i.e. west at
KSFO - I recommend 10L or 10R.

I recommend using the autopilot for take-offs and it has been configured
to try to make a reasonable job of it.  The autopilot cruise speed is
set to 310 kts, which is the maximum speed for low-level flight and
it should not be exceeded.

Select the autopilot altitude hold (ctrl-a) - the default value of
3000ft is fine for sea-level take-offs.

Select autopilot throttle control (ctrl-s)

While the a/c is accelerating, and assuming you haven't already done so,
display the mini panel.

When the speed gets up to about 120kias click the top quarter of the
reheat slider control ten times to set afterburner to 1.0.

With a full fuel load the a/c should left off at around 135kias and
start climbing.

When you've got to about 160kias and 200+ ft alt start the flap retraction.
This will take 45 seconds.

Once the a/c has accellerated enough to start regaining altitude (you'll
loose some after the flaps have retracted) you should cut off the
afterburner by clicking ten times in the bottom quarter of the reheat
slider.

I've not been able to land this a/c using the glideslope function.  I think
the normal 3 deg slope is too steep for the B-52 and they appear to use a
much shallower slope, if any at all.  In effect the a/c has to be flown
onto the runway.

The best way I've found to land so far is as follows:

I recommend using the ILS nav approach for steering but manual control
over speed - the AP auto-throttle is excellent at holding
speed, once it's attained but it tends to react too slowly when the drag
changes while manuevouring in approach.  Reduce the AP target-descent-rate
to 500 fpm or less and set the max roll to 20 deg or less.

The most important point of the approach is extending the flaps.  These
produce a lot of lift and a lot of drag, and as mentioned earlier, they
only have two positions.  As the flaps extend, the a/c will both rise and
slow down quite quickly.  It takes 45 seconds for the flaps to extend and
while that's happening you must try to ensure that you don't slow down
too much.  With a nearly full fuel load you should start extending the
flaps at about 160-180 kias but shouldn't let the speed drop below 130kias.

You should use the AoA indicator as a guide to the required approach speed -
the AoA shouldn't be much more than 1 deg either way.

You should aim to be at about 200ft agl and still far enough out to have
time for a slow descent to about 50ft agl before crossing the beginning
of the runway.  As you descend through about 75ft agl the lift will
increase and it's helpful to get through this stage before you get to the
runway.  Once the AP has got the a/c flying level at 50ft agl you should
disengage the AP height hold and control the alt using the the elevator
trim control.  Once you're in this state it is fairly easy to fly the a/c
down onto the runway.

It is important throughout that the a/c speed is monitored and controlled
carefully as small changes in speed produce relatively large changes in
attitude and drag.

It's important to note that I'm not a pilot, have never flown a B-52 and
don't know the actual procedures for getting one down - what I've said
above has just worked for me when trying to land this particular fdm:)

LeeE 2003/08/13
