Effects of Rain or Surface Contamination on Pitch Stability and Control
January 1983
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 4� at 53 knots, the pilot can apply an additional 8� 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 6� or- more, (greater than 18� 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 2.2� 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