QuickTalk 10 - T-TAIL REPORT
- Category: Q-Talk Articles
- Published: Thursday, 30 June 1983 07:11
- Written by Mike Huffman, Owasso, OK
- Hits: 1521
/Readers of QAC Newsletter #19 may remember the availability of a report written by Quickie distributor Mike Huffman (#2182) concerning the T-tail modification. QBA has obtained permission to print an edited version of that report, courtesy of Mr. Huffman. -Ed./
Late in 1982, while accumulating approximately fifty hours of flight time on my Q2, N22QS, I noticed that the airplane exhibited several oddities in its flight characteristics, including:
1. The oft-mentioned pitch down tendencies in rain or with other flow-disturbing medium on the canard leading edge.
2. A decided tendency of the aircraft to vary its pitch attitude (with no control inputs) at landing approach speeds in gusty or turbulent conditions. This made it difficult to fly approaches at the normal speed of 85 MPH whenever turbulence was present. It was only by maintaining 100 MPH IAS or above that the aircraft felt comfortable under such conditions.
3. Changes in the elevator hinge moment at landing approach speeds in turbulence. When I would hit the bottom of a "bump", the control stick would try to move forward of its own accord.
4. A definite lack of elevator effectiveness beyond approximately 10 degrees down elevator deflection. Additional deflection produced only more drag without an appreciable increase in angle of attack.
Quickie Aircraft Corporation had introduced the aileron reflexer (in part) to be able to offset the decrease in lift on the canard when flying in rain. This was done by "reflexing" both ailerons upward to provide an equivalent decrease in lift on the wing, thus balancing the aircraft. My Q2 had always been equipped with the reflexer and I had found it to be extremely handy. I found, for example, that more reflex was required to keep the tail down on the landing when there was an accumulation of bugs on the canard leading edge. However, the reflexer was not the cure for all the problems on my aircraft. In the two instances where I had encountered rain, the reflexer did not provide enough control to offset the pitch down tendency.
This report describes the results I obtained by installing the LeGare T-tail modification on my Q2. Comments concerning the plans and the installation are presented along with results from ground and flight-testing. It should be emphasized that these results are valid only for my specific aircraft and should not be generally assumed to apply to all Q2's.
Garry LeGare offers both a set of plans and a kit for the T-tail mod. At the time I started my T-tail installation, the prefabricated parts of the kit were not available, so I built my own from the plans. The text included is very detailed and provides a good step-be-step procedure. However, a few minor problems with the plans were noted and deserve comment. First, in making the cutout in the vertical stabilizer foam for the mechanism, Garry says to cut "approximately halfway through" the foam. Apparently, my vertical stabilizer is thinner than his, since my cutout had to be almost the full depth of the stabilizer thickness in order to clear all the mechanism. Second, the pulley guard for the AN 210-1A pulley shown on Appendix Sheet 6 need to have a third "prong" to prevent the cable from coming off the pulley between the two tangent points. Third, the description of the manner in which the T-tail cables connect to the aircraft elevator trim system is inadequate. For instance, most builders make the tailcone of the aircraft removable, yet no instructions are given to allow the cables to be easily disconnected and reconnected when removing the tail. I installed turnbuckle assemblies in the baggage compartment, which are accessible from the front through the seatback bulkhead. When these turnbuckles are disconnected, the cable slack allows the tail to be raised, forming a gap at the bottom through which I can disconnect a couple of links, similar to those on the rudder cables. Also, no instructions were given for providing a pitch trim indicator to let the pilot know what position the trim wheel is set for. (This particular point was the cause of a near-accident when takeoff was accidentally attempted at the wrong trim setting.) Fourth, in my opinion, pop rivets are inadequate for attaching the coverplate on the tail. Inspections of the mechanism would require drilling out the rivets and likely damage the thin four-ply vertical stabilizer laminate. Also, I do not trust the pop rivets to stay secure under the effect of airloads through the thin laminate. I installed #6-32 nutplates under the laminate and attached the cover plate with screws. Last, a few drawing errors exist on the Appendix sheets. On Sheet 5 and Sheet 7, the 0.62" diameter TM1 part is shown larger in diameter than the 0.75" aluminum tube, causing some possible confusion as to where one stops and the other starts. On Sheet 4, the AN 960-10 washers called out on each side of the cable thimbles should be AN 970-10 to keep the thimbles from sliding off the AN 3-10 bolt.
Regarding the installation of the mod on my Q2, everything proceeded according to the plans except that the job required about thirty hours to complete rather than the six hours advertised.
Before beginning the flight test program, several things were done to the aircraft. First, an accurate weight and balance was performed, going so far as to weigh the aircraft with known weights in the fuel tanks, pilots seat and baggage compartment so that accurate calculations of the moment arms could be determined. Second, wing and canard incidence angle measurements were made. It was found that the main wing incidence angle was correct, but that the canard incidence angle decreased as one moved from BL48.8 outboard. Third, the landing gear geometry was rechecked to look for tire toe-in or camber problems (none were found). Fourth, the left elevator was fitted with a deflection template visible from the cockpit in flight. Fifth, a fitting was mounted on the control stick for measuring stick forces. And sixth, a G-meter was installed.
Pitch Trim Authority: The T-tail was found to be quite powerful in providing pitch trim. For instance, at a middle CG condition and constant elevator position (9 degrees trailing edge up), a pitch trim setting of zero degree T-tail incidence angle produces a trim speed of approximately 160 MPH IAS, while a setting of only 3-5 degrees (airplane-nose-up) produces a trim speed of 105 MPH. As the airplane slows down below 100 MPH, correspondingly greater amount of trim are required; landing approaches at 85 MPH seem to work best with about 20 degrees airplane-nose-up trim. Trim settings beyond 25 degrees either way cause no further change in pitch attitude. In fact, in smooth air, I could sometimes detect a stall occurring in the T-tail at about 25 degrees. If I held a constant elevator position and began feeding in trim, the pitch attitude would increase up to about 25 degrees trim setting and then would suddenly decrease somewhat and then become relatively constant up to the 45-degree limits of travel. These same effects were noted in both trim directions.
The T-tail airfoil appears to be very similar to the NACA 0009 and these results are consistent with the lift curve of this airfoil. For low Reynolds numbers, it rises linearly from zero to about 12 degrees and then flattens out to about 18 degrees. The foil undergoes a rather mild decrease ("stall") from there on up to about 30 degrees. By adding the aircraft angle of attack (about 9 degrees at 90 MPH) to these figures, we come fairly close to the 25 degree trim position at which the tail stall was observed.
With this in mind, I can see no reason for allowing the T-tail travel in excess of 30 degrees either way. Between 30 and 45 degrees the lift force actually decreases.
Longitudinal Stability Tests: The T-tail on my aircraft is set up to be easily removed and reinstalled. Thus it was that I was able to conduct testing with and without the tail on the same day, under the same ambient conditions. Stick-free longitudinal stability tests were performed with the aircraft ballasted to the center of the CG envelope, both with and without the tail, and at both ends of the airspeed range. The tests were conducted by first establishing a trim speed (either 155 MPH or 100 MPH), then pushing or pulling the control stick to obtain a speed 15 MPH greater or less than the trim speed. The stick was released and the airspeed changes observed. Ideally, the airspeed should exhibit the co-called phugoid characteristic, oscillating above and below the trim speed, with each oscillation becoming smaller in amplitude so that, within three or four oscillations, the trim speed is reestablished. If this behavior occurs, the airplane is said to have good, positive static and dynamic stability in the long-period sense.
The results of the stability tests show that, in all cases (with and without the tail), positive static and dynamic stability does exist. In the case of the high speed run without the tail, the stability could best be described as sluggish, with a considerable increase in speed beyond the 170 MPH stick release speed and a very flat peak at 180+ MPH before the nose slowly began to rise back toward the trim speed. This behavior is probably produced by the "tuck" tendencies, even though in my airplane the tuck did not go so far as to result in an unstable condition statically. The condition of the tail markedly changes the character of the high-speed stability to a well-damped phugoid characterized by low accelerations and a period of about 35 seconds.
At low speed, without the tail, a definite phugoid was exhibited, but the damping was considerably less than at high speed. The addition of the tail decreased the damping even further, almost to a condition of neutral dynamic stability - which the texts say is often encountered when static stability is increased under conditions of low damping.
The above tests were repeated at gross weight and most-aft CG, and at 50 lbs over gross and most-aft CG. Although no detailed data was taken during those runs, the qualitative character of the results was identical to the tests at mid-CG. The only testing done outside the CG envelope was only one very cursory test conducted without the tail at a gross weight of 1096 lbs and a CG of 48.3 inches aft of datum. Under these conditions, the aircraft is definitely statically unstable at both ends of the airspeed range.
As mentioned earlier, my airspeed changes pitch attitude quite markedly during landing approaches in turbulence without the T-tail. With the T-tail, the pitch changes are very much diminished, to the point that an 85 MPH approach speed can be maintained comfortably, even in gusty conditions. If, for no other reason, this result makes the T-tail worthwhile.
Effect on Minimum Controllable and Maximum Airspeeds: To sum up the results of several tests, the addition of the T-tail produced no discernable differences in either the minimum controllable airspeed or the maximum straight-and-level airspeed.
Effect on Elevator Controllability: The tests have shown that controlled flight can be maintained by overpowering the T-tail with the elevator, regardless of airspeed or T-tail incidence up to 45 degrees travel either way.
Effect on Tailwheel Steering Authority: Without the T-tail, my airplane seems to lack tailwheel steering authority. On landing in crosswind conditions, gusts tend to weathervane the aircraft into the wind and upon making a rudder correction, the tailwheel can be heard and felt skidding on the concrete for perhaps a second or two until it "takes hold" and begins correcting the yawed condition. It was expected that the T-tail would markedly increase steering effectiveness by providing a down load on the tailwheel. However, such an effect has not been observed. The steering effectiveness after landing seems to be about the same, with or without the T-tail.
Effect on Taxiing: Because of the light tail weight while on the ground, care should be exercised while taxiing in the same direction as the wind. I had the misfortune of having my airplane flipped onto its nose when a gust of wind caught the T-tail from behind while the trailing edge was up. One prop gone and a lesson expensively learned.
Effect of Flying in Rain: I had one opportunity to make takeoffs and landings in the rain with the T-tail. The rain was what I would classify as "light to moderate"; the windshield was wet all over and new droplets were falling on any given square inch about once every second or so. The aircraft happened to be loaded about on the forward edge of the CG envelope. The takeoff was made with the trim in the 20 degree airplane-nose-up position. As takeoff speed was approaching, I began feeding nose-up elevator, but it was not until I reached approximately the 10-12 degree down position that I finally broke ground. At that point I was exerting a pull of perhaps 20 pounds on the stick. As I turned crosswind and downwind, I experimented with the stick position and with the trim position to see if I could slow the airplane down to a reasonable approach speed. Additional T-tail incidence did not help (as would be expected) but additional backpressure did produce a gliding speed of about 95 MPH. By the time I was ready to land, my arm was becoming tired of holding the steady backpressure. I flew the airplane onto the runway on the main wheels at about 85-90 MPH, but the tailwheel did not want to come down. I cannot be sure whether I had all the available elevator travel in or whether my arm was too tired to pull any further. Since there was a light gusty crosswind, I weathervaned and recovered twice before deciding to go around, narrowly missing a runway light. The second landing attempt was successful, though probably by accident. (I came to the decision it was time to put the airplane in the hangar and clean out my shorts.)
These results do not agree with LeGare's experience with the T-tail. He has flown demonstration flights in the rain without apparent difficulty. So where is the difference? One possibility is in the previously mentioned "washout" in incidence angle in the outboard portions of my canard. Another possibility is that, even though the T-tail plans specify to leave the pitch trim springs in place on the elevator control, I know that LeGare has replaced his springs with very light ones. At the large elevator deflections necessary to fly in the rain, he would not have to exert the same amount of arm-tiring back pressure and might be more successful at controlling the airplane.
Conclusions: Considering all the above results and the more subjective feelings I have developed during the testing, the bottom line is that the T-tail is a valuable addition to my aircraft. The T-tail gives the airplane better, more crisp feeling in pitch throughout the flight envelope. With more testing, the T-tail may allow the CG range to be widened. Also, with other changes to the elevator centering springs, the T-tail may allow more comfortable flight in rain.
One recommendation is in order. If a builder is planning to make changes to the installation, he should proceed very carefully. Small changes in trim system design have been known to cause flutter or less of control.
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