Last Canard Pusher we
discussed again the effects of rain or surface contamination on the pitch flying qualities
of the Long-EZ. This subject his been addressed and discussed in the Owner’s Manual
since it was discovered in 1975 that our VariEze prototype experienced a nose up trim
change when encountering IFR conditions or flight in rain. This phenomenon had not been
encountered during our earlier experience with the VariViggen aircraft. At that time it
was recognized that assessing the trim change due to boundary layer trim transition (ie:
due to leading edge Insect accumulation or flight into rain conditions) would need to be
accomplished in order to verify that the effect on the pitch flying qualities would not be
adverse. Studies subsequently done using data from many different VariEzes did not reveal
consistent results, in that some of the airplanes would tend to trim nose up when entering
rain conditions and others would tend to trim nose down when entering flight into
moisture. Occasionally a VariEze was found to exhibit a relatively strong nose down trim
change which would require several pounds of stick force to maintain the same flight
condition and require a re-trimming when entering or leaving rain conditions. The
confusing result about the Investigation was that there was an apparent disagreement
between theory and flight test data. Theory would predict that if an airplane were
relatively rough to begin with the trim change should be less than that experienced than
on a very clean well built surface In which a larger extent of laminar flow is lost when
entering rain.
Experience with conventional airplanes and investigation of test
data for wing sections in general revealed that when an aircraft enters rain, its flying
surfaces produce less lift at a given angle of attack and also the maximum lift is
reduced, resulting in a higher stall speed. At the time NASA was testing a full scale
VariEze in the 30’ x 60’ wind tunnel at Langley, and we asked Joe Chambers,
director of those tests, to spray water on the aircraft and attempt to measure the change
in lift and to compare that change with that found when the laminar boundary layer is
transitioned by applying grit or tape near the leading edge. The results of those tests
were published in the last CP and show a definite loss of maximum lift. The NASA wind
tunnel tests indicated that a larger elevator deflection is required to fly in rain
conditions. This was an expected result for some of the aircraft which had reported a
definite aft stick requirement when entering rain.
We instrumented the VariEze prototye, N4EZ with an accurate
elevator position indicator and gathered the elevator position versus speed data shown in
the adjacent plot. Upon landing we applied grit and tape to the aircraft flying surfaces,
wing and canard to provide a positive transition of the boundry layer at 5% of chord. This
consisted of adding a "step to the otherwise smooth surface of the airfoil that was
sufficient to destroy all the laminar flow, a condition caused by either an accumaltion of
insects on the leading edge, or flight in rain. We then added the fuel used during the
first flight to bring the airplane back to the same exact gross weight and cg and flew
again gathering the same elevator position data. As shown in the adjacent plot the
elevator position required to achieve a given indicated speed was greater than with the
smooth surfaces. It should be emphasized though, that the trim change that the pilot feels
is not the same as the shifted elevator position since the transitioned boundary layer
alters the pressure distribution around the elevator. Even though the elevator is more
trailing edge down it does not necessarily result in an aft stick force, in the case of
the VariEze N4EZ, the trim change due to the trim change transition (the force
required to fly the airplane without adjusting the trim lever) is extremely small and is
for most of the flight regime not noticeable as a nose down trim change.
The NASA concern for a greatly increased stall speed, was not
achieved as you can see from the data, the minimum speed achieved with the transitioned
aircraft was higher but only by approximately 1 to 2 knots.
While we are discussing the VariEze elevator data, it is
interesting to note the shape of these curves and discuss why the VariEze was designed in
a way to provide natural stall limiting. Notice that as the pilot slows up, the normal
stability requires a greater elevator position. The shape of this elevator position versus
speed curve is similar to a conventional airplane at all speeds above approximately 55
knots. As the airplane slows to less than 55 knots however, the pilot notes that all of a
sudden he requires a large change in elevator position to achieve a small reduction in
speed. For example from the elevator position of 4º at 53 knots, the pilot can apply an
additional 8º elevator and only slow down to 48 knots. As he pulls the stick back further
the elevator itself and the canard begin to stall and the airplane "bobs"
noticeably up and down. If the pilot pulls the stick back an additional 6º or- more,
(greater than 18º elevator position) the airplane begins a very apparent pitch bucking
i.e: the nose bucks up and down a couple of degrees approximately once every two seconds.
This is a generally stable flight condition and the full use of yaw and roll control is
retained. Compare this to a conventional airplane: when the elevator is brought back. a
stall of the main wing and the airplane either drops or "departs" (rolls to one
side or yaws Into a spin).
Note that transitioning the boundary layer did not change the
highly desirable shape of these curves, it only resulted in a minor Increase in the
minimum speed. Looking at the high speed end of the same plot shows that tripping the
boundry layer did have a significant effect on the airplanes maximum speed. Reducing the
surface deterioration reduced the maximum speed by nearly 9 knots. This is a significent
increase in drag of approximately 20%.
Referring now to the data of Long-EZ N26MS, a definite shift
in elevator position is apparent at all normal speeds. After collecting the clean data the
aircraft was trimmed to 100 knots ‘hands off’. Then, without changing pitch
trim, it was landed, the tape applied, and the fuel burned was replaced to keep cg and
gross weight identical. It was then flown back to 100 knots. Data show a 2 1/2° shift in
elevator position and the pilot reported a 1 1/2 lb. pull force. Then, without changing
trim, the aircraft was flown to 110 knots where it was again ‘hands off’ i.e. no
stick force. Note that the force was the same (zero) even though the position was 2.2º
different.
The minimum speed at 53 knots was unaffected by transition. This
does not agree with earlier data from Long-EZ N79RA in which a 9 knot difference was
measured. This points up the importance of recognizing that relatively small changes in
contour (particularly with the GU canard airfoil) can adversely effect the transition
characteristics.
Turning now to the Solitaire data, the pilot of the Solitaire
could not feel any stick force trim change when operating between clean and flying through
rain showers. The transition elevator data, however, do show a minor trailing edge down
trim change at speeds below 63 knots and trailing-edge-up trim change when faster than 63
knots. Remember however that this is elevator position rather than stick force data and
the changes seen here were not significant enough to be noticed by the pilot. As in the
VariEze, the minimum speed achieved when the surfaces were deteriorated with grit and tape
were approximately 2 knots faster. The gliding performance was degraded considerably when
the boundary layer was transitioned. The data shown are for powered flight with the self
launch engine running at constant power. A similar change is experienced during gliding
flight except that the transition trim change "cross over" speed is reduced from
63 knots to 60 knots. With power off, the minimum speed achieved on the clean Solitaire is
within 1 knot of that achieved with fixed transition. Note that the Solitaire has a
relatively high amount of longitudinal stability in that the elevator position changes
rapidly with speed changes. This condition results in large elevator deflection
(approximately 6 to 8%) required for normal thermalling flight. This results in a trim
drag that reduces thermalling performance. Some fine tuning of the aerodynamics and cg
range is being considered in order to see if improved thermalling performance can be
achieved by reducing the large elevator deflection.
Referring to the Defiant data, tests show that with identical
trim settings there was no stick force change due to fixed transition. Interestingly the
minimum speed with tape applied was less, probably due to the fact that the wing was more
affected by the transition than the canard. This would result in a higher trim
angle-of-attack.
We recently read an unpublished article written by a retired
NASA engineer, which claims that all canard-type aircraft have a strong nose down
trim change when encountering rain and that this characteristic may generally be
dangerous. The article also interpreted the strong stable break in the pitching moment
characteristics of the tandem wing airplanes as a ‘undesirable deficiency in elevator
efectiveness at low speeds’ rather than the desired characteristic of natural stall
limiting that results in the safe flying qualities achieved by most of these airplanes.
Due to the large number of errors in this unpublished article, the editors did not publish
it. However, the author has succeeded in spreading rumors about these characteristics that
some have attributed to our homebuilts. The author of the article has not flown any of the
aircraft and had made some speculation based on reported results of other types that
apparently do have strong or possibly unsafe trim changes in rain conditions. In his
article he even goes on to caution a pilot from pulling back on the stick in rain for fear
that the nose will drop sharply. These characteristics, of course, are not seen in our
homebuilts. As you see from the adjacent plots, the nose up positive elevator required to
reduce speed is achieved at all conditions up through the flight conditions at which the
aircraft’s nose ‘bobs’ or ‘bucks’.
Rain or no rain, the VariEze, Long-EZ or Solitaire can be
maneuved at normal speeds from base to final turns without fear of insufficient control
power.
An analysis of the flying qualities resulting with fixed
transition should always be done during the flight test program of any new design, be it a
canard, tandem wing or a conventional tail aft configuration. This is a relatively simply
test to do. It is done by simply applying a strip of masking tape approximately 1/4"
to 1/2" wide down all the leading edges, (top and bottom) at approximately 5% of
chord. The effect on stability and maneuverability of the Long-EZ or VariEze due to this
transition will be noticeable but not serious. For example, Mike and Dick both do low
altitude aerobatic maneuvers with their Longs in driving rain conditions and notice only
that a higher force is required to complete a given high-g maneuver. The takeoff
performance in rain is degraded in rain conditions, particularly at forward cg, much as it
is on a conventional aircraft.
The following information is also interesting to note: the
airplanes which exhibit a stronger nose down trim change in rain are generally found to be
those that require too much trailing-edge-down elevator to trim in the clean (no rain)
condition. One Long-EZ who reported a strong nose down trim change in rain, corrected his
canard incidence by increasing it by 1° (whIch brought the elevator position back into
the proper trim range) and thereafter found that the rain induced trim change was greatly
reduced. You would think that if a very small contamination of the surface caused by a few
bugs or rain would cause a noticeable trim change, a large change would be experienced
when the aircraft accumulated large build ups of airframe ice in icing conditions. The
opposite is true, ice has been accumulated on the Defiant and Dick’s Long-EZ
airframes without producing trim changes. Stall speeds increase, of course, similar to
conventional aircraft.
The GU type airfoils used on the VariEze and Long-EZ are more
susceptible to a change of lift due to rain than are more conventional, lower lift
sections. The GU-type airfoils are not low drag sections, however and several attempts
have been made to increase the performance of the VariEze or Long-EZ by the use of
different airfoil sections. The original VariEze prototype N7EZ first flew with a NASA
GAW-1 (now designated the LS013 section which resulted in unacceptable stall
characteristics and a high stall speed. More recently same modern sections have been flown
both with slotted elevators and with plain elevators on three different Long-EZs. None of
those tests have indicated that a overall improvement could be achieved in the Long-EZ or
VariEze due to an airfoil modification. Note that this does not apply to all tandem-wing
types, it is quite probable that an airfoil improvement may be necessary or desirable on
other aircraft which do not have sufficient control power at low speeds due to the
transition of the boundary layer.
Click on images to enlarge
VariEze Characteristics
Solitaire Characteristics
Long-EZ Characteristics
Defiant Characteristics
VariEze Characteristics
Solitaire Characteristics
Long-EZ Characteristics
Defiant Characteristics