Kevin Brooks wrote:

"John Penta" wrote in message
...
On Sat, 07 Aug 2004 18:51:31 GMT, Guy Alcala
wrote:

If
you're serious about this, you're going to get real familiar with the use
of
nomograms;-)

Nomo-whats?

Interesting little charts where you have to construct a line beween two
input values to read the value of the unknown off a third parrallel axis.
Common in hydraulics--the Army also used them in things like pavement
design, IIRC. A Google of the term will give you some examples.

The a/c ones tend to have multiple interrelated ones. You start with the first
one, reading across, up or down until you find the intersecting values you're
looking for, then typically go across, down or up to a second one to cross
reference some other value(s), and in some case then move across, up or down to
yet a _third_ one. And then sometimes, once you get that answer, you have to
work some of the previous ones again, because in the course of doing all the
above the a/c's weight has changed or some other value has to be allowed for
(see para. g. through i. below for an example).

Just to give an idea of how tedious this can be, I'm going to give one of the
sample takeoff problems from the F-104A/B (w/J79-3B engine) -1-1. My
explanations or comments are in [ ]:
---------------------------------------------------------------

Takeoff Planning Problem At Light Weight.

The procedures for accurate takeoff planning are illustrated by two sample
problems. The first problem contains the minimum amount of planning necessary,
and the procedures shown are adequate for normal operation at light weights.
The second problem [the one I'm going to give in full] illustrates the
procedures for a complete solution when operating at heavy weights or under
adverse conditions. The method used for solution of the examples apply to the
actual performance charts discussed. Sample charts are provided for
illustration purposes. . . . [skip simple problem]

Takeoff Planning Problem At Heavy Weight.

Determine takeoff performance for the following conditions:

Configuration -- Pylon and tip tanks.

Takeoff Gross weight: 25,390 lb. - 150 lb. [Note, allowance for fuel burned in
start and taxi in this particular case] = 25,240 lb.

Maximum thrust.

Field pressure altitude -- 2000 feet.

Field ambient air temperature -- 100 deg. F.

Wind -- 20 knots from 060 deg.

Runway -- 10,250 feet long, 035 heading, dry, hard surfaced, 1% downward slope.

Lineup allowance -- 250 feet from end of runway.

a. From Figure A9-1, at a reported wind velocity of 20 knots and an angle
between wind and runway of 25 degrees (60 deg. - 35 deg.), the runway component
of the wind is 18 knots headwind.

b. From Figure A2-4 (Takeoff Ground Run Distance), the zero-wind, zero-slope
ground run distance, for use in determining the go, no-go requirements, is 8700
feet [using a nomogram which you enter along the bottom X-axis at temperature,
go up to intersect the curved lines which represent pressure altitude, across to
the right to intersect the curved lines which represent the gross weight, then
straight down to a graph appended below the one above, to read the ground run
distance in feet along the x-axis, ignoring for the moment the use of the
slanted lines that take account of the slope and wind velocity]. Predicted
distance with wind and slope is 7250 feet [using those slanted lines you'd
previously ignored, you enter the appended graph at the same spot you did for
the no wind/slope combo and drop straight down until you intersect the
appropriate head/tailwind component slanted line, often needing to interpolate
between the index lines, then you move parallel to those lines until you
intersect the appropriate slope line, at which point you go straight down to the
bottom of the graph to read the takeoff distance on the x-axis].

c. The groundspeed at liftoff should be obtained in order to determine whether
or not a maximum performance takeoff technique is advisable. From figure A2-3,
the takeoff groundspeed for the existing conditions will be 216 knots. Normal
performance technique will be used [because you're below the max. tire speed of
240 knots. Otherwise you'd want to use max. performance takeoff technique].
The airspeed at takeoff (also from Figure A2-3) is 212 knots IAS.

d. The distance from the starting end of the runway to the No. 10 marker is 125
feet (see runway marking text for explanation). Brakes will be released 125
feet beyond the first marker for a lineup allowance of 250 feet. The remaining
runway length available is10,000 feet.

e. The refusal speed, from Figure A2-5 (Refusal Speed) is164.5 knots [enter the
nomogram along the left y-axis at gross weight, read across until intersecting
the appropriate real or interpolated curved line showing pressure altitude, read
down to intersect the real or interp. line representing temp., then read across
to the right to intersect the R. or Int. curved line representing runway length
available, in this case 10,000 feet, and finally down to the appended part of
the graph representing braking effectiveness, which allows for corrections for
pavement type and wetness, either going straight down to the x-axis along the
bottom if dry pavement, or following the appropriate braking curve for the
conditions if not, to find the refusal speed].

f. Enter figure A2-6 (Velocity during Takeoff Ground Run) at the takeoff gross
weightof 25,240 lb., and zero-wind, zero-slope takeoff ground run distance (8700
feet) to establish the normal acceleration guideline [I'll skip giving an
explanation of the values in the nomogram - you should have an idea of how this
works by now].

g. Still using Figure A2-6, enter (from the bottom), at the refusal speed
(164.5 knots) At the intersection with the normal acceleration guide line read
the refusal distance on the ground run distance scale -- 5200 feet.

h. The refusal point is reached (5200 ft. + 125 ft.) 5325 feet beyond the No.
10 marker, 325 feet beyond the No. 5 marker. The No. 5 marker becomes the go,
no go marker. Ground run distance to the go, no-go marker is 325 feet less than
to the refusal point 5200 ft. - 325 ft., or 4875 ft.

i. Using figure A2-6 (Velocity During Takeoff Ground Run), enter the ground run
distance scale at the go, no-go distance (4875 ft.) At the intersection with
the normal acceleration guide line, read the normal speed at the go, no-go
distance 159 knots.

j. From figure A2-7, the allowable speed tolerance is 3 knots IAS. The
zero-wind, go, no-go speed is 159 knots - 3 knots, or 156 knots IAS.

k. Adding the headwind component to the go, no-go speed found above provides
the final go, no-go speed of 18 knots + 156 knots, or174 knots IAS. The takeoff
should be aborted if airspeed is below 174 knots IAS at the No. 5 marker. Drag
chute and maximum wheel braking should be used.

l. The acceleration check marker is reached 2000 feet before the go, no-go
marker. Ground run distance to this marker (No. 7) is the go, no-go distance of
4875 ft. - 2000 ft., or 2875 ft. from the brake release point.

m. Enter figure A2-6 (Velocity During Takeoff Ground Run) with the acceleration
check distance (2875 ft.). At the intersection with the normal acceleration
guideline read the normal speed at the acceleration check marker (zero wind) 120
knots.

n. The runway wind component (18 knots headwind) added to the normal speed at
the acceleration check distance provides the expected speed at the acceleration
check distance 120 knots + 18 knots, or 138 knots. When the acceleration check
speed is marginal, the speed at the go, no-go marker should be given close
attention.

o. Referring to figure A2-8, the total distance to takeoff and clear a 50 foot
obstacle, based on normal acceleration, zero wind, and zero slope is 14,500
feet. Predicted distance with wind and slope is 12,500 feet.

Assume that conditions exist as in the above problem, but the temperature is 120
deg. F with a two-knot tailwind. Figure A2-3 showsthe ground speed at liftoff
to be 240 knots for a normal performance takeoff. Under these conditions, a
maximum performance takeoff would be advisable in order to avoid high tire
speeds. The ground run distance will then be the normal performance takeoff
ground run distance (from figure A2-4) multiplied by 0.91. The total distance
to reach a 50-foot height will be approximately the same as for a normal
performance takeoff if the aircraft accelerates to normal speed schedule as the
50-foot elevation is reached. The go, no-go and acceleration check analysis may
be made in the same manner as the foregoing problem.
-----------------------------------------------------------------------

I don't mean to imply that a pilot needs to go through this rigamarole for every
takeoff. When operating out of a particular base with standard loads, the ops
people will have developed charts for the local conditions (runway length,
pressure altitude, slope) that simplify things a great deal, and there's more
or less a standard technique, with the speeds at the various points (check
speed, refusal speed, rotation distance, nosewheel lift off (NWLO) speed, t/o
speed) being given to the pilot on his flight lineup card, and with experience
of the conditions and loads the pilots themselves have a pretty good idea of
what to expect. If you ask Walt or Ed, they can tell you within a few knots
either way of what a particular a/c should do under the conditions in a
particular area/base with a typical load.

Nowadays with electronic flight computers it's a relatively trivial exercise. I
don't know if the USAF/USN/USMC has a way to dowload the formulas into pilot
flight planning computers or not, or even allows this (Bufdrvr would know), but
that makes the whole process simple even for the pilot who has to roll his/her
own, as the computer walks you through the steps and just asks you to input the
appropriate variable values. As long as they're all displayed so they can be
checked for errors, because Murphy's Law still applies, and you still need to
have a good idea of what the answer _should be_ as a cross check.

Guy