Perry just as a matter of interest please try a forward slip at high
altitude and
tell us what happens. For example is the canard stable or
does it buck in yaw or pitch? I am curious. The C 172 with full
flaps pitch bucks a little bit in a forward slip.
Paul Lamar
Paul,
Here's what Burt said about VariEze slips:
VARIEZE LOSS-OF-CONTROL
We have just completed another series of flight tests on N4EZ to test
its departure (loss-of-control) resistance. What prompted this is
reports from two VariEze pilots in Texas that they experienced a partial
or full snap roll at about 80 knots. These occurred below pattern
altitude and fortunately the pilots managed to recover in time to avoid
an accident. The maneuver was described as follows: Full aileron and
partial rudder steady sideslip, then full rudder was applied which
caused the airplane to yaw excessively and abruptly roll, experiencing
negative g. Recovery with neutral control was prompt, but several
hundred feet of altitude was lost.
The most surprising thing about these incidents to us was that control
was lost at such a high speed - 30 knots above stall. Reinspection of
our stall/spin test data and the NASA tests indicated no susceptibility
to departure. We then initiated a new test program to investigate
this. Dick performed full rudder sideslip with N4EZ at all speeds and
experienced no departures. Concentrating on the 80-knot speed range he
then aggravated the yaw with abrupt rudder inputs while in a rolled
attitude. On one of these he experienced a departure - a roll off in
the direction of the slip. He then tried to repeat the maneuver and
could not get a departure in
over 20 attempts. We then adjusted the aileron and rudder rigging,
moved the cg aft, and repeated the tests. Dick found that by learning a
specific technique he could cause a departure nearly every time, if
speed were above 75 knots and an excessive sideslip angle were
generated. The departure generally consisted of an uncontrolled roll
away from the rudder input direction. Recovery with neutral controls
was prompt. However, on several of the maneuvers the yaw angle was
extreme at departure causing a massive stall of
the winglets and blanking of the upstream wing. The airplane then yawed
past 45 degrees, abruptly rolled, and entered a 1 to 2 turn inverted
incipient spin. The airplane always promptly recovered with neutral
controls. If aileron or rudder were applied for recovery it could cause
a further "snap" departure and delay recovery. Altitude loss on the
worst of these maneuvers was as much as 1500 ft.
Why did we not find this departure when we did the original tests and
the tests with cuffs in 1978 (CP #19)? The most probable reason is that
most of those test were done at high angle of attack (full aft stick)
which was thought to be the worst case. However, we have found that at
lower angle-of-attack ie, 80 knots, the rudders can generate more
sideslip than at high angle-of-attack, and thus can be powerful enough
to stall the winglets in an accelerated yaw maneuver. We were then faced
with a decision on what to do: (1) caution pilots that the airplane can
be departed when using excessive yaw inputs or (2) fix the airplane to
improve its departure resistance. Since we feel strongly that good
departure resistance is an important asset and design goal for the
VariEze, we set out to attack # (2).
We have always known that the EZ has more rudder power than needed for
normal maneuvers - a full aileron steady sideslip at low speeds requires
only 60% of the available rudder to hold heading. The available travel
is 3 1/2 inches, measured at the top of the rudders trailing edge. We
then limited the rudders travel on N4EZ in various increments, 3", 2.6",
2.3" and 1.8". At each increment we flew tests to determine departure
susceptibility and the necessary rudder authority for crosswind landings.
As expected, the departure susceptibility reduced as rudder travel was
limited. After extensive testing and evaluation by three pilots we have
N4EZ's rudder now rigged for a two-inch full travel. With this rigging,
the following characteristics exist: crosswind landings up to a
component of 24 knots are possible without tire scrub. The aircraft is
not susceptible to departure during any normal maneuver. Thus, we are
now recommending a mandatory rigging change to limit the rudders to
two-inches of travel.
It must be noted that this may not guarantee total departure
resistance. This may vary from one airplane to another, due to expected
tolerances in things like winglet leading edge finish and shape,
fuselage shape etc. Also, even at 1.8 inch rudder travel, Dick was able
to induce a departure by learning an unusual and aggressive combination
of control inputs: full left aileron, full left rudder at 30 degrees
bank, then at 100 degrees bank abrupt full right rudder. When this was
done a departure was possible (not probable) even though the rudder was
limited to 1.8 inches.
The important thing to note is that, even though this design is not as
susceptible to loss-of-control as a conventional aircraft it
should not result in over-confidence on the part of the operator.
Assume your aircraft is susceptible to departure until proven resistant
during your stall tests with lots of altitude and a parachute. Refer to
the plans-changes section of this newsletter for a caution note to be
added to your owners manual and for instructions on rigging rudders to
two inches.
Sorry for the length.. I thought everyone should see what the whole
story was..
This issue never resurfaced on the VariEze. I think this was the only
loss of control problem found on the airplane. I know people that
fairly regularly do all sorts of crazy aerobatics in them.. Except of
course snap roll/spin maneuvers.
I don't think the Long-EZ based airframes including Berkut, Cozy, Eracer
had the same issue.. Much less wing sweep and much larger winglets than
the VariEze probably help.
Here's what Burt later said about the Long-EZ:
Also of interest to NASA was the departure immune stall characteristic
we have noted during our tests. NASA wanted to test the spin
susceptibility to supplement the extensive data they have gathered on
all types of general aviation aircraft. Phil put the Long-EZ through
all types of extreme stall entry conditions: accelerated entries,
vertical entries, etc. with all combinations of control
inputs. He also alternated left and right rudder inputs at the dutch
roll natural frequency, combining opposite aileron to add adverse yaw
effects, at the maximum attainable angle of attack. Despite all
combinations of gross misuse of flight controls, and attaining over 45
degrees sideslip, he was unable to obtain a departure from controlled
flight. Phil left with the comment that he could
find no way of inducing loss of control in the Long-EZ.
This is a stark contrast to the general aviation aircraft configurations
he has testing at Langley, all of them being relatively susceptible to
loss of control or spin entry from any aggravated stall entry. Many of
them have unrecoverable spin modes that require
wingtip rockets or anti-spin parachutes to effect recovery.
Regards,
Matt-
It is not exactly departure but if you pull high positive G's
and the canard stalls first (as expected) the airplane
will turn sharply towards the ground.
This IMHO was one of the factors in Paul Conner's fatal crash.
I also suspect this happened in a fatal Berkut crash
at an air show in SZP a few years ago. Both done at low
altitude. Of course if you try this in a tractor airplane
and you stall the main wing you will die as well.
Just don't get the idea that you cannot make a canard fall out
of the sky.
Paul Lamar
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