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EFFECTS OF RAIN OR SURFACE CONTAMINATION ON PITCH STABILITY AND CONTROL
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
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
aircrafts 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 Dicks Long-EZ
airframes without producing trim changes. Stall speeds increase, of course, similar to
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.
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