Speed Power
Background
One of the most critical characteristics of a new airplane is its range capability,
that is, the distance that it can fly before running out of fuel. Range is also
one of the most difficult features to predict before flight since it is affected
by many aspects of the airplane/engine combination. Some of the things that influence
range are very subtle, such as poor seals on cooling doors or small pockets of
disturbed air around the engine inlets.
The measurement of range capability for an actual airplane is therefore a critical
flight test goal and has led to the development of special instrumentation to
measure fuel flow, engine inlet and exit temperatures and pressures, etc. The
most useful method for determining the range capability of an airplane is a series
of tests to measure the power required and the fuel used during a short segment
of completely stabilized flight. These test maneuvers are called "speed powers".
- Specific Objective of the Test
Determine the power required and rate of fuel consumption for a particular
combination of Mach number and altitude.
- Critical Flight Conditions
There are several conditions that will influence the data collected for a speed
power. The important ones are:
- Mach Number
- Atmospheric turbulence
- Atmospheric temperature
- Atmospheric pressure
- Weight
- Configuration (flaps and landing gear position)
Mach number is the most influential parameter in the determination of range
for most jet-powered aircraft. The most efficient cruise conditions occur at a
high altitude and at a speed which is just below the start of the transonic drag
rise.
Since the drag (and thus the thrust required to maintain constant Mach number)
will change as the weight of the airplane changes, it is important that the measurements
be taken so as to account for the changing weight as fuel is burned during each
test. The method is based on the fact that the angle of attack (and thus the drag)
of an airplane will become slightly lower as fuel is used since the airplane is
becoming lighter and less lift is required to hold it up. To place the airplane
at a comparable angle of attack to data taken at a heavier weight, but still retain
the same Mach number, the altitude is increased slightly for the lighter weight
test point. This method is referred to as flying at constant 
(W-over-delta) and requires some preparation ahead of time as well as some in-flight
calculations to determine the altitude for each successive test point as the flight
progresses. In the term W-over-delta,  ,
 ,
 ,
and
 .
The parameters usually measured and recorded during speed powers are shown
in Table (1-1). The engine instruments shown are representative
but not complete. They will vary markedly depending on the type of engine. The
engine instrumentation will be used to correct the power and fuel flow measurements
to standard day pressures and temperatures by applying corrections for engine
thrust.
A continuous time history of these parameters is desirable but not essential
for these tests.
The flight test engineer will establish a table of altitudes (which are specified
as W-over-delta's), and Mach numbers where speed powers are to be performed. These
test points will be grouped around the Mach number and altitude that are expected
to produce the best range for the airplane. A typical sample table of flight conditions
for speed powers is shown in Table (1-2).
- Description of a Speed Power
A speed power test point is actually nothing more than a very carefully performed
"trim shot". The pilot may spend several minutes in order to achieve the necessary
conditions of absolutely stabilized engine power, Mach number and altitude. When
the pilot is satisfied that the airplane is stabilized, the data recorder will
be turned on and the stable condition recorded for one minute. At the completion
of each test point, the amount of fuel remaining in the aircraft will be calculated
by the test engineer, and the pilot will be advised of the proper altitude for
the next test point (at constant W-over-delta). The pilot will then climb slightly
to the new altitude and restabilize the aircraft at the next test Mach number.
A sample speed power is shown.
A successful series of speed powers will meet the following test criteria:
- All instrumented parameters recorded properly.
- Engine thrust, fuel flow, Mach number, and altitude were all stabilized during
the one minute recorded test point.
- Post-flight calculations of W-over-delta are consistent for each successive
test
point.
The fuel flow measurements at each stabilized point are combined with the true
speed calculations to produce the specific range parameter, NAMP, "nautical-air-miles-per-pound"
(of fuel used).
The specific range factor is plotted vs Mach number for each value of W/delta
(which corresponds to a single altitude for a constant weight). The maximum values
of the specific range factor, combined with the corresponding Mach numbers, identify
the range capabilities of the airplane at different altitudes.
Listing of Instrumentation Parameters
| Parameter |
Used For |
| Airspeed |
compute mach and dyn. pres. |
| Pressure Altitude |
| Outside Air Temperature |
| Altitude |
compute W/delta |
| Engine RPM |
Compute power req'd and thrust corrections tostandard-day
conditions |
| Engine tailpipe pres. & temp. |
| Engine inlet pres. & temp. |
| Total fuel flow |
compute weight and NAMP |
Table of Speed Power Conditions
| Config. |
Approx. Alt. |
W/delta |
Stabilized Mach number |
| CLEAN |
10,000 |
72700 |
.55, .6, .65, .7, .75, .8 |
| 20,000 |
108800 |
.6, .65, .7, .75, .8, .85, |
| 30,000 |
168350 |
.65, .7, .75, .8, .85, .9 |
| 35,000 |
212450 |
.7, .75, .8, .83, .86, .9 |
| 45,000 |
343550 |
.7, .75, .8, .83, .86, .9 |
| GEAR,FLAPS |
5,000 |
60090 |
140, 150, 160, 170, 180 (Knts) |
Author: Robert G. Hoey
|
Speed
Power
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