IN THE FIRST installment of this 3 part article, we discussed

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1 By Don Hewes (EAA 32101) 12 Meadow Drive Newport News, VA IN THE FIRST installment of this 3 part article, we discussed primarily aerodynamic characteristics that could influence the flight behavior of tail-first airplanes when they encounter some form of contamination; that is, rain, snow, frost, bugs, dirt or what have you. The term "Flight Behavior Change" or "FBC" was coined to identify this influence. This month we will look specifically at some pertinent wind tunnel and flight test data obtained by NASA using the VariEze airplane and relate these findings to our previous discussions of FBC and to the behavior of several other current tail-first designs. We will also discuss various design, construction and operational factors that can have some significant effect on FBC. Before proceeding, however, I would like to restate what I said previously: that the VariEze was selected by NASA because it incorporated a number of unique and interesting advanced design features and NOT because it has some particular problem. The data were used for this article because they were the only data available pertinent to the subject of this article. The specific data applies only to the VariEze design and should not be applied to the other designs without proper consideration being given to the physical differences. However, some of the trends and effects noted can be applied in a more or less straight forward manner as will be discussed in this installment. Wind Tunnel Data For VariEze Tail Surface In the first installment we focused our attention on the critical nature of the tail surface, so let's examine this further by looking at the NASA wind tunnel data obtained from Reference 1 for the VariEze's tail surface. Figure 5 shows how the lift of the tail and the complete airplane varies in the wind tunnel as the angle of attack of the airplane is varied with the elevator set at several deflections for the clean or uncontaminated condition. (These data for the tail are given in terms of the area of the main wing and, consequently, have smaller values than you would normally expect to see. The difference between the two curves represents the lift contribution of the wing and the fuselage combination.) Note that the increments of tail lift produced by deflection of the elevator (the spacing between the tail lift curves) decrease as the elevator is deflected further and further downward. This is especially evident for angles of attack above about 10. Also, note that the canard begins to stall at about 14 with zero deflection BUT it begins at about 8 with maximum down deflection. Thus, DOWNWARD deflection of the elevator REDUCES THE LIFT EFFECTIVENESS OF THE ELEVATOR AND CAUSES EAR- LIER STALL of the complete canard surface. Bear in mind that this surface uses a slotted flap as the elevator which generally is more effective in generating lift increments with deflection than 48 JUNE 1983 the simpler plain flap used in other tail-first designs. The general trends in the data as illustrated here should be fairly similar, however. In Figure 6, three sets of data are shown, one for the tail with a dry smooth surface (FREE), one with the leading edges artificially roughened with a small amount of coarse grit glued to both left and right tail panels (FIXED), and the last with both panels smooth but with one panel wetted with water sprayed from nozzles to simulate rain (FREE, WATER ON). The elevator deflection is zero for all cases. (The terms "free" and "fixed" refer to the types of air flow transition associated with the smooth and roughened leading edges.) There is a LOSS OF ABOUT 27% of the TAIL LIFT throughout the normal flight range of angles of attack for the second set of data compared with the first set showing that the application of the coarse grit, representing a "bad case of the bugs", had a very pronounced effect on the aerodynamic characteristics of the canard. The loss of lift produced by the WATER SPRAY IS ROUGHLY EQUIVALENT TO THAT OF THE GRIT when you consider that only half of the canard was sprayed with water. There are also corresponding increases in the drag of the canard associated with these losses of life. These are very large changes in the aerodynamic characteristics and certainly indicate a significant effect of contamination on the characteristics of the tail surface. (Remember, however, we are looking only at the tail contribution and not the total aero characteristics of the airplane.) Of course, the influence of surface contamination on the drag of sailplanes has been recognized for many years and is associated with the premature TRANSITION from laminar to turbulent flow which causes the drag to increase significantly but does not alter the lift to any extent at cruise conditions. The somewhat unusual aspect of these data is the relatively large loss of lift at the low angles of attack due to the roughness but this type of loss can be found with some other airfoils that are unusually thick. The loss is attributed to premature turbulent flow SEPARATION near the trailing edge occurring at the lower angles of attack. The trends noted in the data do correspond to those assumed in our earlier discussions, however, we are missing elevator hinge moment data which unfortunately were not available. Discussion of Tail Sensitivity to Contamination What is it that makes the tail aerodynamics so sensitive to the effect of rain and bugs? The quick answer to this question is that it is the particular type of airfoil that is being used for the tail surface and is not the location of the surface on the airplane. But this needs explanation. As air flows past an airfoil, the molecules of air next to the surface are slowed by the frictional forces acting between the molecules and the airfoil surface as well as the molecules themselves. This action forms what is referred to as a boundary layer on both the top and lower surfaces. We will concern ourselves with the layer over the upper surface. This boundary layer and what happens to it are the key factors to the FBC phenomenon.

2 FIG. 5 - LIFT CHARACTERISTICS OF 8 f.6 Q-«. VARIEZE (REF I.) m WING STALL CANARD STALL a, cleg SPORT AVIATION 49

3 CANARD TRANSITION O FREE D FIXED O FREE, WATER ON.1 0, a,deg FIG. 6 - EFFECTS OF CONTAMINATION ON CANARD. The boundary layer grows in thickness from just a few molecules at the nose to several tenths of an inch as the flow progresses toward the trailing edge. If we were to measure the velocity distribution across the boundary layer with a very small velocity probe and plot the variation of the velocity as a function of the distance from the airfoil surface, we would be looking at what is called a "velocity profile" as shown in Sketch A. As you can see, the actual outer edge of the layer where the velocity becomes constant is very difficult to locate, so for convenience we actually refer to the thickness as that point where the velocity has reached some arbitrary percentage of the free stream velocity above the surface. We are not really interested in the actual thickness for this discussion but rather in the manner in which it changes and the character of the flow within this layer. The flow in the boundary layer near the nose of the airfoil is very orderly as the molecules slide smoothly, one over the other, and we have what is called "laminar" flow as long as the surface itself is relatively smooth. This results in a low drag or shearing force on the airfoil. The velocity profile for laminar flow shows a very rapid increase in velocity in a distance of only a few molecules. The shape of the profile is depicted in Sketch B. Although the boundary layer gets progressively thicker as the flow moves aft from the nose, the shape of the profile does not change. The significance of this is that the molecules within the boundary layer maintain most of their energy and are able to continue moving in a uniform manner following the contour of the airfoil for some distance. The greater the distance the laminar flow continues, the lower the drag will be. At some point along the airfoil, however, the molecules are going to start loosing their uniform motion and begin to tumble over each other. This action is referred to as "transition" and the 50 JUNE 1983 point where this occurs is referred to as the "transition point". At this point, the layer thickens, as illustrated in Sketch C, and the flow from here on to the trailing edge and beyond is very random in nature and referred to as "turbulent". The velocity profile for this condition, also shown in Sketch B, indicates that not only has thickness increased but the velocity of the molecules at a given distance has decreased greatly. This condition creates much more drag on the airfoil and the flow finds it much more difficult to follow the contour of the airfoil exactly. Although the lift being generated by the airfoil is not altered to any significant degree by the transition to turbulent flow, the POTENTIAL FOR SEP- ARATION of the flow from the airfoil altogether is increased. If separation does occur, the result is then a marked loss of lift over those portions of the airfoil aft of the "separation point". In effect, the flow at the separation point starts to reverse direction and the air from the lower surface starts to spill over the trailing edge. Two of the key factors affecting the SEPARATION POINT are the THICKNESS of the boundary layer and the LOCATION OF THE TRANSITION POINT. If the transition point is located FORWARD on the airfoil where the boundary layer is quite thin, then the boundary layer becomes thicker immediately behind the transition point, as indicated in Sketch C, but the overall velocity or energy level within the turbulent boundary layer is relatively high. The potential for separation in this case is RELATIVELY LOW. If, on the other hand, the transition point is located much further AFT where the laminar boundary layer has thickened considerably and become "aged", that is, the molecules close to the surface have lost most of their energy, then the turbulent layer that now forms is very much thicker, as indicated in Sketch D, and its energy level is greatly reduced. The potential for separation is now RELATIVELY HIGH.

4 The expanding shape of the forward portions of all airfoils creates a positive pressure gradient which helps the air molecules move out of the way of the wing as it passes through the air. This favorable gradient also helps to damp or stabilize the small amounts of airflow turbulence that tend to build up in the boundary layer. This causes the TRANSITION POINT to be further aft on the airfoil than it would be if there were no favorable gradient. The tapering shape of the rear portion of the airfoil creates a negative or adverse pressure gradient which tends to cause the airflow to become turbulent sooner than it would if there were no adverse gradient. Thus the TRANSITION POINT tends to move forward in the presence of such a gradient. Not only this but also the negative gradient, if strong enough, will cause the SEPARATION TO OCCUR PREMATURELY. Increasing angle of attack to produce greater lift increases the pressure gradients over both portions of the airfoil. This increase in the negative gradients over the aft portion is what causes the airfoil to stall ultimately. If one airfoil at zero lift has a greater negative gradient than another airfoil due to greater trailing-edge taper, then that airfoil will stall at a lower angle of attack and develop less lift than the other. The first of the two airfoils shown in Figure 7 is known as the GU 25-5(11)8 and is the one used on the VariEze tail. The GU 25 and variations of it are used on most all of the other canard designs currently flying as well. It is a laminar-flow airfoil developed to give high lift at quite low Reynolds numbers (low airspeed) and also to have some unique stall characteristics which make it very useful for the highly loaded condition unique to the tail-first designs. The GU airfoil obtains its special characteristics by having a rather large thickness (20%) and having this thickness located quite far aft. These features cause the boundary layer to remain laminar over about 55% of the upper surface and much further aft on the lower surface under normal conditions thereby producing very low drag. Comparison with the modified NASA GAW-1 airfoil used for the less heavily loaded main wing of the VariEze, also given in Figure 7, shows that the maximum thickness of the GU airfoil is greater than that of the GAW airfoil. For the GU airfoil, as long as the flow is laminar before reaching the negative-gradient region and the angle of attack is not too large, the flow will continue to stay attached to the surface so that the full chord length of the airfoil can produce lift until the normal stall angle is reached. If, however, the flow has become turbulent due to roughness on the leading edge surface before reaching the region of adverse pressure gradient, then premature separation or stalling may occur even though the angle of attack is far below that for the normal stall, perhaps at zero angle of attack or less. Consequently, the GU airfoil will experience PRE- MATURE SEPARATION when there is sufficient accumulation of any type of material near the leading edge that will induce turbulence. A small amount of roughness or waviness existing in the surface may not, by itself, cause the separation because of the stabilizing effect of the positive pressure gradients along the forward portions of the airfoil. But this should tend to make the tail susceptible to much smaller amounts of contamination than otherwise. It should be noted that other types of airfoils, such as that used for the wing, are not as critical from the standpoint of premature separation because 1) the shape of the forward position tends to cause transition to occur further forward, 2) the adverse pressure gradients due to the aft shape are not as severe as that for the GU airfoil, or 3) there is a combination of both. However, these other airfoils are not necessarily immune to the problem. It is possible that there are a number of other airfoils, including modifications of the GU section less critical as far as contamination is concerned, that would be better suited for use on tail-first designs. the airplane flying in steady flight. The other points represent non-steady flight, but the distinction is usually not made in this type of figure by the engineer because of normal practice. As pointed out earlier, the "clean" canard stalled in the range of about 8 to 14 degrees angle of attack but it can be clearly seen in Figure 5 that the complete airplane is not fully stalled until about 24 is reached. Furthermore, the figure shows that the ELEVATOR CANNOT TRIM THE AIRPLANE TO ANGLES OF ATTACK HIGHER THAN ABOUT 17. There appears to be a wide safety margin for stall of the wing built into this design, however, the margin actually is not as wide as it appears because there are aerodynamic problems associated with a partially stalled wing which must be considered. Departure of the lift curves from a straight line at about 17 identifies the initiation of the partially stalled condition. Thus, the margin actually is very small. The loss in elevator effectiveness associated with the stallproofing feature discussed earlier is evidenced by the bunching of the curves for the higher elevator deflections which occurs in the range of 15 to 17 angle of attack. (Refer to the portions of the pitching moment curves close to the CM = 0 axis in the upper figure.) The significance of this loss can be seen with the aid of the solid curve of Figure 8 which shows the ability of the elevator to trim the airplane. This curve is a cross-plot of the data given in Figure 5 for the "clean" condition. It shows that the elevator cannot trim the airplane to a trimmed lift coefficient greater than about 1.5 which corresponds to the 17 angle of attack limit just mentioned. (An airspeed scale in knots has been added to the original figure for reference purposes. These airspeeds refer to non-maneuvering level flight for the aft CG position at gross weight at sea level and may not agree directly with the airspeeds indicated in flight.) This indicates that a speed lower than about 61 knots could not be reached even though the elevator was deflected to relatively large downward (positive) angles. Changes in weight and center of gravity position will alter the curve somewhat but the general trend will still be evident. This figure also shows a dashed curve for the "dirty" condition in which the flow was tripped by grit at the leading edges of the wing and tail. Comparison of the two curves shows that the effect of contamination was to cause a noticeable downward shift in the elevator deflection required to trim at a given lift coefficient or airspeed and that an increase of about 8 to 10 knots for the minimum airspeed due to contamination is indicated. This result can be traced directly to the loss of tail lift illustrated in Figure CANARD GU 25-5(11)8 WING Complete-Airplane Wind Tunnel Data The pitching-moment data for several elevator settings as measured for the "clean" condition of the complete airplane are also presented in Figure 5 along with the previously mentioned lift data. The pitch data are given in coefficient form based on the wing area and mean chord of the main wing with the center of gravity located at the design aft limit. For those of you who are not familiar with wind tunnel data, I'll point out that only the data points for lift and drag for which the pitching moment coefficient is zero, that is, trimmed lift and drag, correspond to MODIFIED NASA GAW-I FIG. 7-AIRFOIL COMPARISON. SPORT AVIATION 51

5 15 10 CANARD TRANSITION FREE FIXED max AIRSPEED, KNOTS L trim FIG. 8 - ELEVATOR TRIM CHARACTERISTICS FOR AFT C.G.CREF. 0 6 and its associated effect on the pitching moment produced by the tail relative to the CG of the airplane. The results of the wind tunnel tests are consistent with the trends that were discussed in the previous installment. Some of you who recall that an airfoil normally stalls at about 14 to 16 degrees may wonder about the rather high 24 angle of attack at which the VariEze main wing stalls as shown in Figure 5. Bear in mind that the VariEze main wing is quite highly swept and this has the effect of reducing the slope of the lift curve and extending the stall to the higher angles of attack. You will find that the straight or nearly straight wings used in the other tail-first designs will stall at the lower angles. Discussion On Use Of Excessive Elevator Deflections The marked upswing of the solid curve shown in Figure 8 is attributed partly to the significant stable break (nose down) in the pitching moment curve as angle of attack is increased beyond about 14 with zero elevator deflection. This break is associated with the initial stalling of the canard. However, the upswing of the curve is also attributed to the loss of effectiveness of the elevator as the elevator is deflected further and further from its zero position, noted earlier. It is especially important that sufficient elevator effectiveness be retained at speeds just above minimum so that the airplane can be safely maneuvered for take-off or landing. This being the case, then it is important to not "use up" the elevator for purposes of trimming the airplane in cruise and high speed flight. In other words, it is important to adjust the relative incidence of the tail and wing so that the elevator will be trimmed close to zero deflection or, perhaps, slightly up for cruise conditions. If flight tests reveal that excessive amounts of down elevator are being used to maintain steady conditions in the normal flight envelope, there are problems with the construction and/or rigging of the airplane. These problems can be traced to improper airfoil 52 JUNE 1983 shape, incidence or twist distribution of the lifting surfaces and should be corrected before considering the airplane to be fully airworthy. We will say more on these items a little bit later, but the reason for mentioning them here is to make the point that the amount of ELEVATOR DEFLECTION required for a given airspeed can be used as a PARAMETER in evaluating how closely a given copy of a particular design matches the original or other copies. If the deflections match within 2 or 3 degrees for the same airspeed, weight and CG location, then there is some assurance that the airplane is properly constructed and rigged. Comparison With Flight Data Now, let's look at some VariEze flight test data to see if they agree with the wind tunnel data. Figure 9 shows a plot of the variations of elevator deflection with airspeed for the conditions of level flight with both "clean" and "dirty" leading edges. These data were obtained from a series of unpublished NASA tests of the VariEze that were related to those reported in Reference 2 for the Long-EZ. The figure reveals that the airspeed was reduced, the minimum airspeed was increased and more down elevator deflection was required to maintain a given speed within the speed band when the leading edges were contaminated. Note that very little additional deflection was required at the high speed end whereas added deflections of 3 to 5 degrees were required at the low speed end. Aside from an offset of about 3 at the high speed end of the curves, there is reasonably good agreement between the curves given in Figure 8 for the wind tunnel tests and those of Figure 9 for the flight tests. Thus, it appears that there is good qualitative agreement between the wind tunnel results and those obtained from flight test. (There are a number of technical reasons for the data from these two different types of testing to differ to some extent, consequently engineers never expect to get exact agreement.)

6 V MIN. Transition o free fixed ELEVATOR DEFLECTION DE V MAX. I_ INDICATEDAIRSPEED, KNOTS FIG. 9 -FLIGHT TEST DATA FOR VARIEZE. A reasonable question to raise at this point is, "Well, did the flight tests reveal any significant FBC as far as the pilot was concerned?" The answer to this is a qualified "Perhaps". It was observed that the landing and take-off speeds with the "dirty" edges were noticeably higher than normal. This might present a problem to a pilot unfamiliar with the airplane, especially if he tried to operate from a short field. However, it should be noted that the tests really were inconclusive relative to FBC behavior because they were not tailored to study this phenomenon and the test pilot was not specifically asked to address this problem. The tests were made primarily to obtain the performance data for straight and level flight with power required to sustain constant altitude. No tests were made to determine effects on rate of climb and on the stick forces or trim. Also, banked and accelerated maneuvers at low airspeeds, such as might be performed in abnormal landings and take-offs, and forward CG loading conditions were not performed because they were not considered pertinent to the primary objective of these particular tests. Inasmuch as these maneuvers and conditions require additional down-elevator deflections from those required for normal level flight, they are pertinent to the FBC problem and need to be performed. More on this subject will be said later. An additional comparison of flight data can be made by examining the Long-EZ test data given in Reference 2. We should note that the Long-EZ appears to be similar to the VariEze but does differ in several respects. The wing is considerably larger and carries more of the total weight of the airplane. It also has less sweepback and uses a different airfoil section. The canard, however, appears to be quite similar. The data, shown in Figure 10, were obtained for the same conditions as those for the VariEze and indicate somewhat similar results. Some later data provided by Burt Rutan for the Long-EZ show essentially the same trends as in Figure 10 but indicate that the minimum airspeed was affected only slightly by loss of the laminar flow. Just as I was completing this article Burt invited me to come to Mojave and fly the Long-EZ to check on its flight behavior. As it turned out, I arrived there in the midst of the series of heavy rain storms that hit California at the end of January and had an excellent opportunity to fly in both rain and dry air. Dick Rutan provided me with a very effective demonstration of aerobatics with his plane in the rain and then I explored low speed behavior in both rain and dry air and found no significant deterioration in the flight characteristics. I essentially repeated the same thing with Mike Melvill in the factory Long-EZ because Dick's bird is not a stock Long-EZ, but there was very little difference between the two as far as rain effects were concerned. Inasmuch as I was riding in the back seat, I did not make the take-offs and landings but these did appear to be reasonable even though it was raining at the time. Although these flights do not represent exhaustive testing under all conditions they did serve to demonstrate to me that the Long-EZ design and these particular copies of it have no significant FBC piloting problems even though the flight tests do show some influence on the elevator characteristics. Comparison of FBC Characteristics For Copies of the Same Design There are no specific data available to me for comparing the FBC characteristics of various copies of the VariEze design. However, in recent discussions, Burt Rutan has stated that there have been noticeable differences in these characteristics observed for the large group of VariEze airplanes currently flying. In the case of a few of these airplanes, very definite pitchdown tendencies have been encountered while, for the majority, only relatively mild tendencies have occurred having no significant effects on flight behavior. Surprisingly, in a few instances, a very mild pitchup has been encountered. It would appear that the results obtained from the NASA tests were consistent with those obtained SPORT AVIATION 53

7 for the majority of these airplanes currently in use, but these tests do not account for the extremes noted in a limited number of cases. As I'm sure you're already aware, no two of these homebuilt copies can be built EXACTLY alike and none can be exactly like the original version because of the building tolerances involved and the small changes introduced by each builder to suit his individual requirements. It, therefore, should be no surprise to anyone in learning that the flight behavior of these do differ to some extent. The real problem is the large range of this variability. While some of these extreme cases may be attributed directly to rather extensive modifications to the original design or very poor workmanship, there apparently are some where there are no readily apparent differences to explain the behavior. In our earlier discussions, we spent much time showing that different forms of FBC could be produced by each of the basic aerodynamic forces and moments. Also we showed that the exact form would depend on the extent that the aerodynamic contributions of the components of the airplane were influenced by contamination. Discussions of the tail and its airfoil, which have been identified as being a primary source for the FBC phenomenon, revealed that the tail's contributions were very sensitive to small irregularities on the surface of the tail. It is only reasonable, therefore, to conclude that some cases of extreme behavior may have resulted from SMALL DIFFERENCES in various features of the airplane or other factors that are not necessarily readily apparent. If so, then, we need to evaluate the effects of small variations of a number of design, construction and operational factors. Unfortunately, there are no technical data to show us the relative effects of the numerous factors that might be involved. Consequently, we can only list those that we think might be involved and discuss their possible impact. Obviously, this involves a high degree of personal opinion and I freely admit that I am merely making educated guesses at this stage. The reader is invited to make his own assessment after reviewing this article, and the author will be glad to hear from anyone who has some constructive information to offer. Discussion of Possible Factors We will discuss the following: 1) airfoil shape and surface skin conditions, 2) elevator hinge gap and trailing edge shape, 3) incidence and twist, 4) center of gravity location, and 5) weight. First, we should consider how close the wing and tail surfaces can be duplicated by the many different homebuilders, a larger number of them building their very first airplane. Good workmanship is vital in building to a high degree of accuracy and an excellent finish is always a very good index of the quality of workmanship. But a smooth and glossy finish and good workmanship do not necessarily guarantee that the airfoil itself is made to the required accuracy. Much can depend on the construction techniques involved in addition to the skill and experience of the builder. Airfoil designers will tell you that departures of only a few thousandths to hundredths of an inch in critical areas of the airfoil can alter basic characteristics very markedly. The leading edge and upper surface are particularly critical. If the construction techniques do not permit the builder to work to required accuracies in these areas, then inconsistent results may be obtained even by very careful and experienced builders. Prior to working with the fiberglass-foam type of construction myself, I thought that it should be a very accurate way of reproducing a given airfoil surface. Now that I have been involved to some extent in building several of them, I have changed my opinion, at least concerning the current homebuilding techniques I have used or observed. There are several steps and working conditions in the process that can influence the final shape. Some of these are 1) copying template shape, 2) excessive melting due to cutting the cores too slowly or with the wire too hot, 3) under cutting or scalloping due to wire lag (wire too slack, wire too cold or cutting speed too fast), 4) wire bowing due to variable density at core joints, 5) foam core bowing (after wire cutting) due to internal and surface stresses, 6) assembling final structure from many core sections without accurate jigs, 7) surface irregularities caused by core defects and cloth texture, overlap and joints ELEVATOR DEFLECTION 10 6 e ' de V min trim TRANS ITION OFR D FIXED 4 V max INDICATED AIRSPEED, V (, knots FIG. 10 -FLIGHT TEST DATA FOR LONG-EZ. CFtEF. 2) 54 JUNE 1983

8 Some of these items are beyond the control of even a very careful builder. Furthermore, there are no procedures using templates for checking the final airfoil shape after completion. The builder has no way of knowing what the exact shape is unless he goes to a great deal of extra work. But if he does, he has no way of knowing that the final shape is acceptable. Based on my own experience and observations, I believe that deviations of 1/16 to 1/8 inch or more from the design shapes for both the tail and wing are possible and that deviations of this size are sufficient to account for some of the extremes in FBC that have been noted. There is little doubt in my mind that the CON- STRUCTION TECHNIQUE is responsible for these deviations to a large extent. Aside from producing significant differences in the shape of the airfoil, the fiberglass-foam construction methods also result in roughness and waviness of the surface skin which requires extensive filling and sanding to achieve an acceptable condition. Of the two, WAVINESS probably is the more critical factor from the aerodynamic standpoint and probably is the more difficult to detect and eliminate. I know that at least one of the designers has gone to great lengths to get the builders to pay attention to the waviness problem, but I expect that there is a wide variation in these two factors for the final surfaces from one plane to the next. Another surface factor to consider is that of the actual SUR- FACE FINISH. If the surface is covered with a very glossy and slick finish, the rain will tend to adhere in the form of raised drops which will cause significant turbulence. On the other hand, if the finish is dull or satiny, the rain may tend to spread out in a thin smooth layer with little influence on the airflow. Also, the quantity of bugs, ice or other materials that adhere to the surface may depend to a large extent on the type of surface treatment. Closely related to the airfoil "shape" problem are those of the ELEVATOR GAP and TRAILING EDGE. The shape and width of the gap between the elevator and the fixed portion of the tail has a very strong influence on the flow separation behavior over the elevator. Consequently, small differences here can cause two otherwise identical tails to have significant lift and elevator control power differences. The manner in which the elevator is mounted with the external hinges does lend itself to possible variations in the gap, but I really have no feel for how much of a problem this may be. The shape of the trailing edge has a very powerful effect on the hinge moments of the elevator although the lift is not influenced to any appreciable degree. Here again I do not have any information relative to the variability of these factors in the field, but I have a feeling that little attention is paid to the trailing edge shape. As a matter of fact, I believe that some of the early VariEze airplanes had a rather blunt trailing edge and that this was later changed to a sharp edge. This certainly could account for noticeable differences in the stick trim characteristics between some of the airplanes. As noted earlier, variations in INCIDENCE and spanwise TWIST in both the tail and the main wing can be a source of trim differences from one airplane to another. Furthermore, inasmuch as LOCATION of the CG has a direct effect on trim of the airplane, we can see that differences in CG location as well as lifting surface incidence will lead directly to differences in elevator deflection required to maintain a given airspeed. Also, differences in WEIGHT of the airplane will cause the elevator to be trimmed differently for a given airspeed. Discussions with two of the designers have indicated that there have been cases where the elevator deflections required for cruising flight have been down from the normal settings by several degrees, indicating some significant and unusual variations in the rigging and loading of the various airplanes. I strongly suspect that these particular airplanes have significantly different FBC than those that were rigged and loaded uniformly. Discussion of Other Current Tail-First Designs Now, let's try to relate our information on some of the other current tail-first designs to our discussions of the VariEze and Long-EZ. We are going to talk about the Quickie, Q2, Dragonfly and a new single-place design called the "Retro" by its designer and builder, Gion Bezzola of Estavazer Le Lac, Switzerland (see February 1982 SPORT AVIATION). Unfortunately, we have only limited and very sketchy information on these designs. While these designs differ considerably from each other and from the VariEze configuration, they all have heavily loaded front lifting surfaces which employ the GU 25 airfoil used by the VariEze or modified versions of it. The first three designs employ Eppler airfoils for the main wing which I believe is similar to that used with the Long-EZ. The Retro uses a NACA 74-series section for the wing. The first three designs also use a plain flap design for the elevator rather than the slotted flap design employed by the others. There are numerous copies of both the Quickie and Q2 airplanes and there are several reported encounters with significant pitchdown. As a matter of fact, most all of the reported cases covered in the previously mentioned magazines have involved one or the other of these two designs. In reviewing the literature that Gene Sheehan, current president of Quickie Aircraft Co.,, very kindly sent me, I found that he and the late Tom Jewett have devoted a significant amount of effort studying the pitchdown behavior experienced with their designs. Gene indicated that his company had provided the builders with considerable information on the subject warning against flying in rain or taking off with wet or dirty wing and tail surfaces. Also he mentioned that the design modification permitting the ailerons to be reflexed to provide additional pitch control power has proved to be very effective in minimizing or eliminating the pitchdown behavior. He also mentioned the design modification developed by Carry LeGare, a former associate, was also effective and was available to builders. This modification consists of a small horizontal trimming surface mounted on the vertical tail. In my recent visit to Mojave, I also visited Gene and saw the Q2 prototype which has been fitted with the NASA LS(1>- 0417MOD airfoil for the front lifting surface. He reported that rain has been found to have very little effect on the behavior of the modified Q2 and that the company plans to make the new surface available for retrofitting on current Q2 airplanes. He also said that they would be evaluating this airfoil for the Quickie also but he was not sure that it would be as effective because of the lower Reynolds number invovled. Bob Walters, designer of the Dragonfly, states that there is only a mild pitchdown with his airplane and claims that it is not a significant problem. He does caution builders about flying in rain and warns not to take-off with wet or dirty surfaces. It should be pointed out that, up until very recently, his airplane has been the only Dragonfly flying, consequently he does not know yet how typical the behavior is for the numerous copies that are now approaching flight status. One of the first builders to fly his Dragonfly, Terry Nichols, reported in the recent Dragonfly newsletter that he performed stalls in and out of rain and noted a 10 mph increase in stall speed and apparently a more pronounced stall break due to the rain. This result is consistent with Bob's recent comment that he found the stall speed of his prototype was increased about 8 to 10 mph. While on my recent trip, I was able to discuss this further with Terry as well as with Rex Taylor who has taken over the Dragonfly company. Rex has had similar experiences in rain but indicated he has not tested the airplane extensively in the rain. None of these people feel there is any significant problem with the Dragonfly. Although I was able to fly the prototype Dragonfly through some extreme maneuvers at low speed with no problem, I was not able to do this with contaminated surfaces and, therefore, cannot verify their opinions. Examination of the Dragonfly templates given in the plans revealed that the tail airfoil is different from the standard GU 25 airfoil in that it is slightly thinner and the maximum thickness appears to be moved slightly forward. When I asked Bob about this, he stated that he was concerned about the abrupt contour change of the GU 25 section and made the modification to help eliminate the separation tendencies encountered with the VariEze tail. (He is one of the early builders of a VariEze.) He did not explain whether this was merely to reduce drag for better cruise performance or to minimize the effects of rain, but regardless of which was his intention, it appears that his modification to the airfoil may have been effective to some degree in alleviating FBC characteristics which otherwise might have been more evident. It is pertinent to note here that an airfoil developed independently for another application by airfoil designer John Roncz, whom I have consulted, matches closely the contours of the Dragonfly airfoil. John's study shows that his airfoil is not as sensitive to flow separation problems as the original GU 25 section. Thus, since the two airfoils appear to be quite similar, it is probable that the Dragonfly airfoil truly is effective in minimizing the pitchdown problem as Bob's results have indicated. But until more Dragonflys have been test flown, this supposition will not have been proven. SPORT AVIATION 55

9 DISTANCE REFERENCE THICKNESS LAMINAR VELOCITY f SKETCH A SKETCH B TRANSITION SKETCH C SKETCH D SEPARATION 56 JUNE 1983

10 On a recent personal trip to Switzerland, I was fortunate to have the opportunity to talk with Gion Bezzola and observe him flying his prototype version of the Retro. He stated that it is subject to pitchdown much the same as the VariEze but, unfortunately, we did not have time to discuss the subject in depth. He is a pilot for the Swiss Air Force and has many hours in numerous airplanes of all types, including the VariEze. His views on the factors involved in the pitchdown behavior were consistent with those I have expressed in this article. Let's summarize our discussion at this point by observing that we have covered a total of six different designs and have found that each does demonstrate some form of FBC but the type and severity are not necessarily the same for all designs. Certainly, there are basic design differences between the various designs that are important and we should not expect that the behavior of all would be necessarily the same. I would like to point out that most of the FBC encounters that I have heard about for any of these designs have been isolated incidents with the airplane flying into rain while in NORMAL CRUISING flight and were NOT SPECIFIC TEST flights designed to isolate and identify the behavior. In some cases, these encounters have been fairly mild and the persons reporting these events seem to be left with the impression that the phenomenon is of little or no concern. Inasmuch as FBC may be much more critical for low speed flight conditions, there is the POSSIBILITY that some airplanes that have been judged as having no problem will, in fact, demonstrate undesirable or unacceptable behavior. The "bottom line" for this discussion is that, regardless of the specific design, the builder or pilot must be aware of the POSSI- BILITY that his particular airplane may have some unusual characteristics. Therefore, the only way to know for sure is to conduct proper flight test with his own airplane. Discussion On Potential Dangerous Behavior We have been discussing at great lengths "FBC", "potential problems", and "undesirable or unacceptable behavior". But what do we mean when we use these words when applied to airplanes? Do we mean that their behavior is dangerous, and what is it that actually makes it dangerous? To answer these questions, let's first define "dangerous behavior" as being "motions of the airplane, either controlled or uncontrolled, that expose the occupants to the threat of immediate injury or loss of life". Next, "unacceptable behavior" will be defined as "motions that require extraordinary or exceptional pilot skill for safe controlled flight". Finally, we will define "undesirable behavior" as "motions that require normal pilot skill but impose an extra or disconcerting workload". I prefer not to use the term "dangerous" to describe the behavior of an airplane because it is imprecise and can be misunderstood by many people. For instance, the act of getting out of bed is "dangerous" if it results in falling and breaking your leg. Of course, many people also consider that flying in any airplane is "dangerous" but many others do not. Actually, it is more appropriate to use "dangerous" to describe the total situation; that is, the airplane PLUS the specific flight conditions or environment. By these definitions, then, we can say that, if the behavior of an airplane is judged to be "unacceptable", it may or may not be dangerous because the skill level of the pilot in command must be taken into account. At this point, we must be a little bit careful about assuming what the term "pilot skill" actually means. Many pilots consider themselves highly skilled because they have several thousand hours in many different type airplanes, and they certainly are justified to do so. However, in the case of flying an airplane that is not familiar to them and that has an unusual behavior under some flight condition, some of these pilots may not be able to cope because they may not understand what is happening or the situation does not permit them the time to "feel out" the problem. If they were provided the information about the nature of the problem beforehand, then it is highly likely that all of them would be able to cope. Thus, we can see the importance of being ADEQUATELY INFORMED, as well as SKILLED, in cases of dealing with airplanes having UNUSUAL CHARAC- TERISTICS. There should be little confusion in dealing with the term "undesirable". You can consider it as referring to nuisance traits. There are a number of airplanes that have one or more characteristics that can be termed "undesirable" yet the airplanes are considered acceptable on the basis of their overall behavior. The process of evaluating the acceptable behavior of a homebuilt airplane is strictly a qualitative process utilizing the judgment of one or more knowledgeable and experienced pilots. Strictly speaking there are no firm quantitative parameters that must be met, however, the pilot should make his judgment of FBC based on the extent that the "numbers" are changed for the following items: 1) take-off distance and speed, 2) rate of climb following take-off, 3) landing speed and distance, 4) stick force and trim travel, and 5) maneuvers at take-off and approach speeds.. Closing Remarks The discussion covered in this article leads to the following statements: 1) FBC is a premature-stall phenomenon associated with tailfirst airplanes in which a significant portion of the weight is carried by the two lifting surfaces and the elevator is located on the forward surface. Because thick laminar-flow airfoil sections such as the GU 25 section, which are quite sensitive to surface roughness or contamination have been used for the forward tail surface, the tail can have a major influence on the behavior. However, there are a number of aerodynamic and physical factors which influence the nature of this phenomenon and these factors can vary significantly from one copy to the next of a given design. Use of other airfoils less sensitive to the effects of contamination should minimize or eliminate this behavior. 2) Builders should be particularly aware of the critical nature of the various factors involved and are cautioned to follow the designers instructions and design details as closely as possible. They should avoid making any changes in the airfoil shape or control surface configuration without thorough knowledge of the influence of these changes on the flight behavior of their airplane. Such changes may cause the airplane to fly completely different from the original design with severe degradation in performance, flight behavior and safety. 3) FBC appears to be more critical for low speed flight conditions than for cruise and high speed because of loss of pitch control power and lifting capability along with increased drag. Maneuvering at low speed may aggravate the behavior. Pilot control inputs in response to the FBC encounter will depend on the type of behavior. Pilots who are unfamiliar with this phenomenon may not be able to recognize aggravated behavior and may apply wrong recovery inputs. 4) Although this phenomenon is not necessarily a serious problem for a particular design, FBC of each copy should be evaluated completely with flight tests in much the same manner as stalls and other flight behavior. These tests should be made during the initial airworthiness fly-off period required for certification of the airplane in the experimental category. 5) Care should be exercised by the pilot when flying a tail-first airplane for the first time and he should check with others who may have flown it previously to learn about its specific behavior. He should be aware of the potential for encountering some unusual behavior and the proper technique for recovering. The statements made here and throughout this article have been based on an analysis of a number of basic aerodynamic facts and well established principles of flight dynamics and pilot behavior, as well as a very limited amount of experimental wind tunnel and flight test data. However, there are areas of personal judgment and speculation, and consequently, these statements should not be treated as firm conclusions until a much broader base of experimental data can be gathered and a more thorough analysis made. Next month we will conclude with a number of suggestions and recommendations pertaining to flight testing and operation of tail-first airplanes. References 1. Yip, Long P. and Coy, Paul F.: Wind-Tunnel Investigation of a Full-Scale Canard-Configured General Aviation Aircraft. 13th ICAS Congress; AIAA Aircraft Systems and Technology Conference, Seattle, Washington, Aug , ICAS Paper Number Holmes, Dr. B. J.., Obara, C. J.: Observations and Implications of Natural Flow on Practical Airplane Surfaces. 13th ICAS Congress AIAA Aircraft Systems and Technology Conference, Seattle, Washington. August 22-27, ICAS Paper Number SPORT AVIATION 57