SECTION 15 FINAL INSPECTION AND FLIGHT TEST
RV AIRCRAFT
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SEC 15r8 12/23/10
how to immediately and subconsciously stop an incipient spin. Then, fully developed spins, and the need to recover
from them, will become less probable.
Spin testing, like other forms of limit testing, should only be attempted while wearing a parachute and after memoriz-
ing escape procedures. Memorize anticipated recovery techniques and act deliberately and calmly throughout the
entry and recovery from the spin. Perform intentional spins in progressive steps, starting with immediate recovery,
recovery after 1/2 turn, recovery after one turn, etc. Also, begin spin testing with forward C.G. loadings and proceed
to more aft loadings as satisfactory recoveries are experienced.
All homebuilt RVs should be individually tested because small variation in configuration can sometimes greatly affect
spin characteristics. This is particularly true for any variations in vertical surface areas forward of the aircraft center,
and for changes which may affect airflow over the forward surfaces and/or the tail surfaces. For example, spin test-
ing of prototype RVs has shown that spin characteristics differ noticeably with wheel and gear leg fairings installed or
removed. The vertical area of these components, located forward of the center of rotation of the airplane, causes a
destabilizing effect that degrades spin recovery. There are after-market gear leg fairings being marketed which are
wider than those tested and supplied by Van’s Aircraft. Because spin testing has shown that small changes such as
this can cause a noticeable change in spin recovery, builders are advised to use caution when making changes such
as this to their RVs.
One often cited example of how small alterations can affect spin characteristics is the Beechcraft Musketeer. The
early production airplanes had an engine cowling with a rather abrupt transition (squared off) from its top to side sur-
faces. A later version had a reshaped cowl that had a smoother transition between the top and side cowl surfaces.
The result was that while in a spin mode, the cross flow over the cowl now produced more lift and held the nose up,
inhibiting spin recovery. As with all other areas of testing; don't make any assumptions! Recommended spin test
altitude is between 6,000' and 8,000' AGL to allow plenty of altitude margin for recovery.
Inverted spins were not tested because the prototype test aircraft were not equipped for inverted flight.
Van's Aircraft Inc. does not consider spins to be a recreational aerobatic maneuver, and recom-
mends that they not be casually undertaken.
Propeller Evaluation:
Your propeller should load the engine sufficiently in level flight that the engine, at full throttle
will not exceed its redline limit. Nor should the engine exceed redline rpm during takeoff. Sometime these require-
ments are hard to meet with the same prop (see the discussion of fixed pitch props in Section 11.)
Airspeed Calibration:
Air speed indicator systems, particularly in homebuilt airplanes, are often inaccurate. Some-
times very inaccurate! Note that we refer to the air speed indicator system, not just the air speed indicator instrument
itself. The system comprises five components: Dynamic pressure source (pitot tube), instrument, static pressure
source, air lines, and an indicator.
The location of the pitot tube relative to the air pressure areas around the airframe is of great importance. The ideal
location is one where the true air velocity relative to the airframe can be measured. The pitot tube cannot be located
at any point on the fuselage because it is within the influence of the propeller disc. The only exception would be
mounting it above the tip of the vertical stabilizer. This location is fine except for high angle of attack flight, as in land-
ing attitude, where fuselage and propeller airflow disturbances cause significant inaccuracies.
The ideal pitot location would seem to be forward of the wing, in undisturbed air. But, within the first 6 to 12 inches
forward, the airflow is already affected by the approaching wing, and this location results in pressure errors as much
as 10% high. It is necessary to locate the pitot tube least 1/2 the wing chord length forward of the leading edge to
eliminate pressure errors. This is why we see the large pitot “stinger” on factory prototype and test airplanes.
Since long leading edge pitot tubes are impractical, a compromise position is sought. This usually becomes some
experimentally derived point under the wing. The pitot tube shown on the plans is located for easy manufacture and
maintenance, and has proven to be a quite accurate pressure source. Use of pitot tube designs or locations other
than this could result in less accurate airspeed readings.
The airspeed indicator itself could be out of calibration due to age or manufacturing inaccuracies. Any instrument
repair shop can check and re-calibrate air speed indicators. However, one primary object of this sub-chapter is to
alert pilot/builder that an accurate airspeed indicator does not in itself guarantee correct indicated airspeed readings.
The static source must be located in an area of neutral or ambient pressure; an area where the shape of the airframe
has caused the airflow to be neither above or below atmospheric pressure. Cabin air pressure is not neutral as might
be thought. Canopy and door air leaks, air vents, etc. cause cabin pressure to vary enough to result in errors of 5
mph or more if used as the air speed static source. Production aircraft often use an experimentally located static
source point on the aft portion of the fuselage where airflow pressure recovery provides atmospheric pressure. The
static opening at this location is also less prone to ice formation than elsewhere. The recommended RV-static source
point and system components is shown in an earlier chapter of this Construction Manual or on the drawings.
The fourth system component is the lines for both the pitot and static air. Pressure requirements for either are mini-
mal, so practically any aluminum, plastic, or rubber line can be used. Airtight sealing of the lines is important because
SECTION 15 FINAL INSPECTION AND FLIGHT TEST
RV AIRCRAFT
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SEC 15r8 12/23/10
any leakage can compromise an otherwise accurate system. One method of checking a pitot system for leaks is just
a clear plastic tube partially filled with water and slipped over the pitot tube. Elevating the open end of the tube will
cause the water to flow inward (but not into the pitot tube) and build a slight pressure in the system. If the lines are
airtight, the water level will remain the same. If the water level slowly returns to a balanced condition, then the system
has a leak.
Such an airspeed indicator system installed in a RV should provide reasonably accurate airspeed readings; certainly
accurate enough for initial test flying. Most pilots will want to calibrate their airspeed indicator readings for the pur-
pose of documenting performance data and performing limit testing. One simple method of doing so is to fly along-
side another airplane and compare airspeed readings. This would be fine IF the other airplane's airspeed system
was guaranteed to be accurate. But, it probably isn't, even though it may be an expensive, late model airplane.
We recommend performing the airspeed calibration through time/distance calculations. All that is needed is a ground
course of known distance, preferably about 5 miles in length, and a stopwatch. Fly both directions over the course at
a steady indicated speed, power setting, and altitude. Time each run with the stopwatch. Compute the speeds for
each run, add them together, and divide by two to get the average ground speed. Do not calculate the average
speed from the total distance divided by the overall time. The effect of any wind will result in an erroneously low
speed.
A sample calculation is shown at the end of this section. We have intentionally factored in a strong wind to illustrate
the effect of averaging individual speeds rather than computing speeds from the elapsed round trip times.
(Performing speed calibration testing during windy conditions is usually futile because the turbulence associated with
winds will make it impossible to maintain steady airspeed and get accurate results.)
Use a flight calculator to compute true indicated airspeed from the indicated airspeed reading (factored for tempera-
ture and altitude) and plot this speed against the calculated ground speed. Repeat this procedure for indicated air-
speeds vs. timed ground speeds at 10-20 mph intervals from near stall speeds to max.cruise speeds. From this, an
airspeed calibration curve can be drawn and corrections made for any indicated airspeed.
An Alternate Calibration Method:
Loran and GPS have given the test pilot another valuable tool in more ways
than intended. Nearly all lorans provide a ground speed readout. For rough speed checks, this groundspeed read-
out can be recorded for two way runs at given power conditions. However, the groundspeed readouts usually fluctu-
ate over a range of several mph, and are therefore not a precise calibration tool. However, lorans also provide con-
tinuous position reports in the form of Lat./Lon. coordinates. These coordinates can be used just like visible ground
markers for a speed check course. All that is required is that the speed calibration runs be made on North or South
headings. Each degree of latitude equals 60 nautical miles. Thus, every minute of latitude equals 1 nautical mile and
each 1/10 minute (finest reading on most lorans) equals 1/10 nautical mile. Runs can be of any length desired. 10
nautical miles is a convenient figure, corresponding to 10 minutes latitude. Runs of this length are more accurate
than short runs because any variation in time starting or stopping the watch is averaged over a longer time. For in-
stance, if the course were only a mile long, a 1/2 second error in timing a 200 mph run would cause an error of over 5
mph. The same 1/2 second error made in timing a 10 mile run would cause an error of only 0.5 mph.
Some of the advantages of using loran (GPS) for speed checks is that the altitude is not important. The invisible mile
posts are at 8,000’ altitude as well as at the surface. Thus, speed checks can be made at normal cruise altitudes
where full throttle can be maintained for extended time periods, and where smooth air is available at almost any time.
Indicated airspeeds can be checked against timed ground speeds and against loran ground speed readouts.
An actual sample of an RV-6A test flight and computations from is included at the end of this section.
GPS tests for airspeed calibration
GPS is a more valuable tool for use in calibrating airspeed systems than is loran, primarily because of its greater ac-
curacy and more consistent ground speed read outs. GPS ground position reports could be used for speed compu-
tations as described above for loran. However, GPS ground speed reading have been found to be so accurate that
they can be used interchangeably with zero wind true air speed. Thus, if the air mass was perfectly stable (no wind),
GPS ground speed and true airspeed would be the same. However, there is almost always some wind, particularly
at altitudes where convective turbulence is not a problem. Thus, flying a multiple heading pattern is an easy and ac-
curate means of canceling wind effect from ground speed read outs.
The commonly accepted procedure is to fly a box shaped pattern on the prime headings of 90, 180, 270, and 360
degrees. (fly heading rather than track) Record the ground speed readings for these heading and compute the aver-
age. While this would seem a simple procedure, carefully flying is necessary to arrive at accurate figures. The air-
plane must be flown precisely and the atmosphere must be very stable (no vertical movement). Even at higher alti-
tudes where the air is generally smoother, there is often minor turbulence, wind shear, or waviness which makes it
difficult to hold a constant altitude and indicated air speed. For example, it is common to experience smooth waves
in the atmosphere, with low vertical velocities—you can’t feel any bumpiness but you can see the altimeter (or VSI)
alternating, up and down. Under these conditions, constant trim changes, and thus airspeed changes, are necessary
to maintain level flight altitude. A simple calculation showed that a 100 fpm vertical component would cause a true
airspeed variation of about 2.5 mph in an RV. Thus, flying from the positive to the negative phase of the wave would
show a 5 mph variation.
