Cruise
performance is a big issue. figuring out how the airplane is performing
encompasses a large number of items including rigging, prop selection,
instrumentation calibration and external factors such as ambient
temperature.
First thing first. One must have accurate instrumentation, or all efforts
are for naught...
Airspeed Indicator
Calibration
Obviously it's important to know how
fast you "think" you are going. Airplane speed is measured with a pressure
sensing instrument. It gets it pressure from a tube that protrudes into
the air stream. The airspeed indicator is nothing more that a sensitive
pressure gauge with some markings on it. Unfortunately, these markings
frequently have little bearing on the speed the airplane is actually
moving thru the air. By building a device that simulates the pressure the
instrument sees, and checking it against a known table, allows the pilots
to use a correction table. If for example the pilot knows his airspeed
indicator show 150 knots when 146 knots of pressure is applied to it, then
he can simply subtract 4 knots from all indicated speed around 150 knots
to give him a very accurate reading.
The time proven and accurate method of checking airspeed indicators is by
using a device called a manometer. The manometer relies on two very steady
references for its accuracy. These are water and gravity. Neither one
changes much and as such the instrument is
 inexpensive,
simple and deadly accurate!
In the left hand photo below, is a picture of my manometer. It's made from
a 2x4 and some home depot tubing. I added food coloring to the water to
make it easier to see. On the left side of the tube, above the water level
is a strip of white tape onto which I have written speeds in knots. The
location of these speeds comes from an table found in an aerodynamics book
. The height of water is the difference from the upper level to the lower
level on the opposite side, so when marking the tape, only half the
distance is used. You can download the table in JPG format by clicking
here. It's
definately a keeper!
The photo below shows a little jewel called a Mityvac. This makes enough
vacuum to drive an altimeter to 30,000+ feet. it also makes enough
pressure to calibrate airspeed indicators. Very easy to use and an
inexpensive addition to any aircraft builders toolbox.
My airspeed indicator is now dead on, within one knots accuracy throughout
its range. Unfortunately, now there is the errors that come from airframe
installation to deal with.
Calibration - Fuel Flow
Installed is the JPI FS-450. This device was selected because of it
fantastic capabilities and manufacturer reputation. It has proven to be a
trouble free performer. It's capabilities include, fuel flow, time
remaining, fuel used, fuel remaining, fuel to next way point and mileage
in nautical miles per gallon.
Calibration is easy, it goes like this...
The aircraft tanks are filled to the limit. The instrument is told one
time in the setup procedure that the aircraft tanks hold 52 gallons, the
the fill up button is press on the unit.
The aircraft is then flown and fuel is burned. The closer to empty I can
get the more accurate the calibration procedure will be. When it was to
fill up the airplane again, I went to an airport that is known to have a
new and accurate fuel counter on it's pump. The plane was refilled. It
took 40.3 gallons as metered by the pump. The instrument had a different
opinion as it was un-calibrated, but that was about to change. By
following a calibration procedure in the owners manual, the unit it easily
and automatically calibrated based on the quantity of fuel I told it we
burned (from he pumps accurate counter). Done!
All that remains is an accuracy check. Once again the plane was flown
around and more fuel burned away. Again back to the same pump and again
refilled. This time the little JPI claimed we had burned 38.0 gallons.
After refilling, the actual amount of fuel used was 37.93 gallons. This is
an error of about one cupful on 38 gallons or .18%. Fantastic! Now the
fuel flow, mileage, and fuel used can be used for meaningful performance
evaluation.
Engine
temperature instrument calibration
The engine has three temperature
instruments. These are oil, cylinder head, and exhaust gas. Accuracy of
these instruments must be verified early on in any flight test program, as
erroneous indications can cause unneeded modifications by the builder.
Most F1 Rockets are plagued by high oil and cylinder temperatures and as
such must be carefully monitored. My F1 experiences high CHT but normal
oil temps. In addition, it was necessary to replace the single channel EGT
with a six channel unit to aid in injector nozzle tuning. Early on I
bought a 2 channel handheld Thermocouple unit from Omega Research for
$120. It was a great investment. It comes with two thin wire type K
thermocouples that can installed almost anywhere. I used these mini-probes
to verify the accuracy of the oil temp and cylinder head temp instruments.
I also measured the temperature of the alternator in flight. It ran up to
140 deg F in flight, well within it's rating.
Along the way, the temperatures of the magnetos was measured. They ran at
132 deg F in flight with no blast tubes. After shutdown and heat soak, the
mags went up to 220 deg F. This was also within the 250 deg F limit
imposed by the manufacturer.
Correcting airspeed installation error
After getting the airspeed indicator to provide accurate information, it
comes time for the hard work. Because the location of the pitot tube and
static ports vary, every airplane has some errors that still show up at
the airspeed indicator. This is called installation error. When a pilot
flies his F1 at 180 knots indicated, he likely isn't going 180 knots due
to this additional error. For example at 180 knots indicated, my F1 is
going 188 knots for real, or as it's called "calibrated airspeed"
identifiying the error is not difficult, but it is time consuming as it
requires several hours of flight test time to document the
 errors
so another correction table can be made.
There are many methods of determining this. I chose a method where the
pilot flies a triangular shaped course and notes his GPS ground speed for
each leg. This data is fed into an Excel spreadsheet, which calculates the
zero wind average true airspeed (see the diagram to the right)
If you want to see this spreadsheet, you can download it by clicking
here. This program tells you the
TAS your airplane is really going as the average groundspeed always equals
the TAS. If you are into real pain, you can read for hours about this
stuff
here.
By noting the air temperature, you can figure out the CAS that was needed
to generate the TAS you see.
It sounds confusing, but here is an example:
Fly 3 legs average 197,knots groundspeed
Noted altitude 10500 feet
Noted IAS was 160 knots
True airspeed= 197 knots
Note air temp is 59 deg F.
Using an E6B or similar computer, we solve to find that 164 knots
indicated airspeed is needed to go 197 knots true airspeed.
Remember 160 knots indicated? Your airspeed indicator has a 4 knots
installation error. By flying the same profile for every 10 knots, it
gives you a nice correction chart from say 50 knots to 190 knots. Finally
it's done. You're flying around, the airspeed indicator shows 166 knots,
you look at the correction table which shows you need to add 5 knots and
voila...you know your speed within 1 knots. You buddy says "yeah man i was
going 232 knots this morning" and you think to yourself "another GPS
idiot".
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Flight test 1 
