SECTION 15 FINAL INSPECTION AND FLIGHT TEST
RV AIRCRAFT
15-17
SEC 15r8 12/23/10
Stick free, aileron snatch will result in a rolling tendency. A fixed trim tab will not correct this as it would just push the
ailerons over center to one side, rather than returning them to center as desired. Correcting aileron snatch can usu-
ally be accomplished by reducing the trailing edge shape and radii to that shown on the plans.
After the lateral control trim has been completed, another test can be made. Establish a medium bank of 20-30 de-
grees and release the stick. If the wings return to a level attitude, the airplane has exhibited positive lateral stability. If
the angle of bank remains the same stick free, the aircraft has neutral lateral stability. If the bank angle continues to
increase when the stick is released, the lateral stability is divergent; a potentially dangerous condition.
Neutral lateral stability is common for RVs because of their short span and low dihedral angle. Negative or divergent
lateral stability is more likely to occur in aircraft with long wings and/or insufficient vertical control surface area.
Sometimes stability investigations can be confusing; situations where unlikely or unexpected factors cause seeming
unrelated symptoms. One instance comes to mind where a certain RV exhibited asymmetric roll rates and control
force. Naturally, the investigation centered on possible wing twist or wing rigging (we checked for unequal incidence
angles, or aileron abnormalities.) The cause was eventually found to be a rather severe twist in the horizontal stabi-
lizer which imparted a constant rolling moment. So, a lot of aileron trim was needed just to maintain wings level, and
the stabilizer induced roll force either added to or subtracted from the rolling input of the ailerons. Corrective action in
that case was construction of a new stabilizer.
This ends our presentation on stability and control. However, it by no means is a complete thesis on the subject.
Rather, it is deemed sufficient to help an RV pilot evaluate his airplane and make corrections to minor abnormalities.
A more authoritative and thorough dissertation can be found in the book FLIGHT TESTING HOMEBUILT AIR-
CRAFT by Vaughn Askew. This text is highly recommended and is available from various sources including the
Iowa State University Press, and Van’s Aircraft, Inc.
STALL TESTING:
We mentioned testing of mild, power off stalls during the initial test flight. After more confidence in the aircraft is
gained, the pilot should proceed to perform stalls entered from all anticipated flight conditions. All types of stalls
should be practiced; departure (climbing) stalls, approach (gliding) stalls, stalls with varying degrees of bank, stalls at
minimum and maximum weights, cross-control stalls, and accelerated stalls. Stalls at every imaginable attitude and
from every imaginable entry condition. The object is not only to gain familiarity with stalls from every conceivable
flight condition, but to become comfortable with recognition of and recovery from these stalls. Not comfortable in the
sense of being careless, but comfortable in the sense of being confident in your ability to control any situation. Prac-
ticing many and varied stalls will heighten your awareness of attitudes and flight conditions to be avoided because of
the severity of the stalls which might result from them.
Except for accelerated stalls and secondary stalls, approach each slowly (a deceleration rate of 1 mph per second is
recommended) while correcting for P-Factor (for power stalls) with the rudder. Allow the speed to bleed off until you
feel a slight buffet. Note the airspeed and recover with a smooth forward movement of the stick as power is added.
Maybe simply relieving back pressure on the stick when the stall occurs would be sufficient for your airplane. Stalls
entered from steep bank or climb attitudes will require more aggressive recovery control application. But remember,
at some loading conditions, an RV has light elevator forces, and over controlling can easily occur, and secondary
stalls can be encountered.
After gaining familiarity with stalls with instant recovery, delayed recovery can be practiced. Starting with wings level,
1 G stalls, delay the recovery by a count of 1,2,3, etc. seconds. The only purpose of this is to gain further experience
with handling qualities in extreme conditions and to determine your ability to control the aircraft in a prolonged stalled
mode. While one should always recover immediately at the first warning of an accidental stall, intentionally holding
the airplane in a stall will provide the pilot with a greater experience base.
Another bit of wisdom to remember is that the airspeed systems can be inaccurate at the high angles of attack ex-
perienced at stall speeds. Indicated stall speeds can be in error by 5 mph, possibly even more. However, the read-
ings are relative and you can believe that your gauge will indicate the same stall speed consistently, if the stall is ap-
proached at the same rate and G-load every time.
While practicing stalls, the pilot is not only gaining familiarity with that specific airplane for his piloting benefit, but is
also evaluating that airplane’s stall characteristics against an ideal. The ideal is that when a stall is encountered, the
nose tends to lower, or can easily be lowered by an easing of stick back pressure or by a forward stick pressure. In
most RVs, there is little advance stall warning in the form of pre-stall buffet. The buffet which does occur does so
within just a mph or two of the fully developed stall. The other characteristic being evaluated is a laterally uniform
stall—or what is often called a straight forward stall. Airfoil irregularities, wing incidence misalignment, and wing twist
can cause one wing to stall at a higher speed than the other. This obviously will cause one wing to drop when the
stall occurs. This is not uncommon for RVs, and if the extent of wing drop is slight, no more than 10-15 degrees, it is
of little consequence. Sometimes an asymmetric stall can be corrected by altering the angle of incidence of one wing
by re-drilling off-center an oversize rear spar attach hole. This method will have limited success because structural
constraints limit the extent of oversize hole which is acceptable. Consult with service personnel at Van’s Aircraft be-
fore attempting this.
SECTION 15 FINAL INSPECTION AND FLIGHT TEST
RV AIRCRAFT
15-18
SEC 15r8 12/23/10
Another cause of asymmetric stall is airfoil irregularity caused by landing lights in the wing leading edge. The “lip”
which usually occurs between the wing skin and the plexiglass lenses causes a disrupted airflow which acts as a
spoiler, reduced lift, and causes that wing to stall prematurely. If this is suspected, smooth tape can be placed over
the offending edges before re-testing. If this is found to be a factor, a re-work of the landing light installation could
minimize the misfit and thus the stall asymmetry. A small stall strip on the opposite wing can also be used to achieve
a balanced stall. Very few RV pilots have added stall strips to their wings. Whether this is because there is no need
or because of lack of knowledge about the potential benefit, we do not know.
SPIN TESTING:
“A spin is a condition in which an airplane rotates because one wing is deeper in stall than the other. A spin is a
highly complex dynamic maneuver that is still not fully understood, even by the experts.”
Flight Testing Homebuilt Aircraft,
by Vaughn Askew.
Accidental spins can result from a variety of conditions in which asymmetric wing lift is induced. Spins normally are
caused by improper rudder usage coupled with a stall (including accelerated stalls) Out-of-coordination rudder pro-
duces a yaw which in turn causes asymmetric wing lift which drives the rotation. Avoid these conditions, and acci-
dental spins won’t happen. Since this utopian condition cannot be guaranteed, a degree of spin investigation training
is suggested.
Intentional spin entry should be initiated from a power off stall with full rudder in one direction and full elevator follow-
ing the initial stall break. Typical spin behavior for an RV is that if control pressures are released immediately follow-
ing spin entry, recovery will be automatic and almost immediate—no more than 1/2 spin revolution. If spin rotation is
held for approximately one full revolution, recovery can be accomplished quickly through application of anti-spin con-
trol (opposite rudder, stick centered). If pro-spin controls are held until two full revolutions have been completed, the
spin will be fully developed. Recovery techniques will vary.
For RV-3s, 4s, and 8s, the most effective recovery technique is as follows:
x
Power off.
x
Elevator centered. (or stick free)
x
Full opposite rudder.
x
Recover from dive as soon as rotation stops.
Recovery time (time to stop rotation) will vary depending on C.G. position and other factors. Step #2 is best accom-
plished “hands-on stick” rather than stick-free because while in spin rotation, the outside aileron will sometimes float
up, thus driving the stick out of center.
(As an example, here is what we found when spin testing the prototype RV-6. Remember, this is one individual airplane!
Our results and yours may vary significantly.)
Testing was performed up to the limit load (1375 lb. aerobatic gross) and C.G. (25% aft of leading edge) with satisfactory
recoveries being easily affected.
For prototype RV-6 and RV-6A aircraft, spin characteristics and recovery procedures were found to be as follows:)
The prototype RV-6 & RV-6A aircraft exhibited good spin resistance. Forceful pro-spin (full up elevator and full rud-
der) control pressures were necessary to induce a fully established spin. Good spin recovery was evident during the
first two rotations. Simply releasing the controls during the 1st rotation stopped the spin, and opposite rudder and
forward stick caused a quick recovery during the second rotation. After two turns, the rotation rate increased and
stabilized between 3 and 4 turns with a high rate of rotation of about 180 degrees/second. Once past approximately
2 spin rotations, the spin had stabilized and if the controls were freed, the RV-6 would continue spinning until anti-
rotation control inputs were applied. One reason for this is that in a fully developed spin, the elevators float up and
remain there hands-off. Recovery procedure consists of the following:
x
Power to idle.
x
Apply full opposite rudder, (opposite the direction of rotation)
x
Center the ailerons and elevator. (Because of the up elevator float, forward stick pressure is needed to center the
elevators.
x
Hold the above control positions until rotation stops, then use the elevator to recover to level flight. 1 1/4 to 1 3/4
rotations are usually required for rotation to stop.
Because of the high rotation rate and the positive (rather than automatic) spin recovery technique required, Van's
Aircraft Inc. recommends that pilots of RV-6/6A and RV-7/7A aircraft limit their intentional spins to two turns or less,
and that recovery from incipient accidental spins be initiated immediately upon recognition.
The RV-9/9A is not in-
tended for spins at all.
Learn the conditions that lead to accidental spins, how to recognize the onset of a spin, and
