|WITTMAN TAILWIND W10 AIRCRAFT
PERFORMANCE REPORT OVERVIEW
BY BRIEN A SEELEY
|The Wittman Tailwind is an historic
aircraft design. It first flew in 1953, a few weeks before the birth of the Experimental
Aircraft Association. It demonstrated exceptional flight efficiency, incorporating a
number of aerodynamic design features which Steve Wittman had gleaned from his extensive
air racing experience. Prospective homebuilders at that time were both incredulous and
inspired by the Tailwind. It was the stuff of which dreams were made and can be credited
with helping the fledgling EAA to grow. Jack Cox's excellent history of the Tailwind was
published in the September 1993 issue of Sport Aviation.1
Steve Wittman and his original Tailwind were called upon by the CAA to serve as the testbed for establishing G load limits for homebuilts. Steve, with parachute, performed the high speed dives and pullups with a Polaroid camera aimed at the G meter. The Tailwind was also the first homebuilt certified by the CAA for carrying non-revenue passengers.
Dr. August Raspet, a professor of aeronautics at Mississippi State University, conducted an elaborate drag polar evaluation of the Tailwind by towing a propeller-less example to 10,000' altitude with a 450 BHP Stearman, releasing it as a glider, and measuring its gliding sink rate at known weights but differing airspeeds. This work, published in 19562, confirmed the Tailwind's remarkably low drag coefficient.
The CAFE Foundation, 1993 recipient of the Thirty Third August Raspet Memorial Award, felt it was particularly appropriate that the latest version of the Tailwind, the W10, be the subject of this Aircraft Performance Report, wherein a new zero thrust glide testing method is used to evaluate its drag characteristics.
Direct comparisons of the drag characteristics of this Tailwind with the one tested by Professor Raspet, unfortunately, are not pure due to the evolution of the design since 1956. The earlier version had no wheel pants, a shorter fuselage, stabilizer end plates, no spinner, a shorter span, shorter landing gear, a different cooling and exhaust system, different wing tips, 350 lb less gross weight and a different airfoil.
The W10 version was longer than the W8 in having 5.5" longer chord in its tail surfaces. It also had slightly taller landing gear to accomodate the larger engines.
As is our practice in selecting aircraft for testing, we consulted the designer, Steve Wittman, for his reccomendations as to the current best representative of the W-10 Tailwind. He offered a list of those who had purchased W10 plans, and several were contacted. The most outstanding candidate was Jim Clement of Merrimac, Wisconsin. Jim used a week's vacation and flew to the CAFE Aircraft Performance Evaluation Center in Santa Rosa via Albuquerque, arriving on 3-3-94.
C.J. Stephens flew his subjective flight test evaluation the following afternoon, on 3-4-94, after the aircraft had been drained of all fuel and an empty weight c.g. had been obtained.
The next 36 hours were spent by a crew of 7 CAFE Board Members installing the DAD, CAFE Barograph, camcorder and all the attendant sensors.
Five performance evaluation flights were conducted on 3-6-94, beginning at 5:40 AM. Multiple attempts were required to obtain usable zero thrust data. That evening, the test equipment was removed and the aircraft was returned to original condition. The following morning, Jim departed in his Tailwind homeward to Wisconsin.
Tailwind N6168X is not "stock". It has 1 ft less span, 4 sq ft less wing area and the firewall was moved 2 inches forward from the cabin to provide greater leg room. The door openings are 2" wider on their aft edges than the standard plans. This aircraft has a custom modified Sterba propeller on a 4" prop extension. The wingtips are of the latest design, which Steve Wittman claims improves the performance significantly. The wingtip lights are concealed in custom-built flush lens covers. The wheel pants are also customized to reduce drag.
FLIGHT TEST METHODS
This flight test was conducted using equipment and techniques as described in the May 1994 issue of Sport Aviation3.
Takeoff distance was measured at a weight of 1431.9 lb by observers stationed at 100 ft intervals along a 1* downhill runway into an 17 kt headwind.
Maximum level speeds at altitude were obtained in smooth air with the CAFE Barograph using full throttle with mixture leaned for best power, and are compensated for the known flat plate drag due to the barograph wing cuffs with 4' boom (.09 sq ft).
Rates of climb are computed based upon the calculated geometric altitude change which would obtain on a standard day at the recorded aircraft weights.
A 1 G clean stall was performed from level flight with less than 18" of manifold pressure and less than 1750 RPM, using a 1 kt per second deceleration. The stall was then repeated using full flaps.
The zero thrust glide information is considered an approximation on this aircraft due to post-frontal atmospheric disturbances and technical problems in detecting the zero thrust crank position amidst the .011" endplay of this engine. The flat plate drag equivalent for this aircraft is deemed accurate to plus or minus .1 sq ft .
Confidence values were applied to the data points before curve fitting the drag polar to the glide data. Consideration was given to the flat plate drag value implied by the low altitude Vmax demonstrated by this aircraft, 211.7 mph TAS. During that speed run, the aircraft was still accelerating strongly when it reached its 200 mph redline IAS. At that point the pilot terminated the run because the CAFE Foundation test program is confined to the normal operational envelope of the aircraft.
The high altitude cruise speeds of this Tailwind would imply that it is capable of 220 mph at sea level. The owner has reported near 220 mph IAS in level flight at 2900 RPM at low altitude. With 16.2 gph at 2828 rpm, our test implies 180 BHP at .54 bsfc. This stock Lycoming O-320 B1B (nominally 160 BHP) had accumulated 80 hours since overhaul. It had a crossover type exhaust system and showed extremely stiff compression when hand turning the propeller. A "dipstick" tool was used to check this engine's piston height at TDC. The height was identical to the known stock piston height value on Steve Barnes' O-320 B1B, confirming that normal compression pistons were in use.
The Vetter Digital Acquisition Device (DAD) was used to record engine parameters. PropTach rpm's are plus or minus 1 RPM. Fuel flows were calibrated to better than .5% accuracy. Noise levels were measured on a TES1350 Digital Sound Meter placed adjacent to the pilot's right ear with a forward facing microphone.
All altitudes are accurate to plus or minus 1 ft . CAFE Barograph airspeeds are CAS, obtained with the pitot-static source positioned 51.4" forward of wing L.E. and 72.5" outboard of the propeller diameter. A chart comparing CAS to the aircraft's airspeed indicator readings is provided at the end of this report. The IAS errors at low speeds are presumed to be due to the placement of N6168X's static port on the midline of the fuselage belly 4' forward of the rudder trailing edge.
Test equipment totaled 57 lb including barograph #2 and pitot missile #2, computer, camcorder, DAD, fuel pump and batteries. The 1 amp barograph heater was powered from the wingtip light wire, while barograph data reached the cockpit via an .5" x .003" copper foil adhesive applied at the 60% chord on the bottom wingskin.
The CAFE Scale was used to determine all aircraft weights. The takeoff weight and c.g. were determined for each of the 7 flights. Practical loading considerations precluded flights at extreme forward c.g.'s. Weighing after each flight allowed an accurate calibration of the DAD's fuel totalizer and gph.
The DAD, camcorder and barograph clocks were all synchronized just prior to each flight.
The graph above was obtained in accordance with the zero thrust glide method developed by CAFE Board Member Jack Norris working with his partner, Dr. Andrew B. Bauer. A zero thrust sensor, installed on the engine crankcase so as to detect fore-aft movement of the crankshaft during flight, senses the transition point from tractoring to windmilling, i.e., the zero thrust condition. At zero thrust, the propeller effectively becomes "invisible" and the aircraft becomes a "pure" glider. The wing cuff-mounted CAFE Barograph accurately records time, airspeed and sink rate while gliding in the zero thrust condition. Synchronized recording of fuel flow, computed instantaneous aircraft weight, RPM, and incline angle of the crankshaft at each different airspeed, yields data, which, when corrected for crankshaft incline vector, can be entered into the following formula:
W x Sink Rate = Drag x TAS Where W = instantaneous aircraft weight, lbs. TAS = true airspeed in feet per second Sink Rate is in feet per second and Drag is in pounds.
The "J" shaped curve is a plot of calibrated indicated airspeed (CAS) at gross weight versus drag and is called the aircraft's "drag polar". The drag polar, wingspan, wing area, gross weight and r, the air density at sea level, provide the information needed for the calculated results above. The term Carson's speed refers to the excellent paper, "Fuel Efficiency of Small Aircraft", (AIAA-80-1847, 1980) by Professor Bud Carson of the U.S. Naval Academy, which, using prior work by Gabrielli and von Karman, defines this speed, as the maximum speed per unit of fuel burned. Carson's speed can be calculated as 1.316 times the speed for maximum lift to drag ratio, which, in turn, is 1.316 times the speed for minimum power and minimum sink rate. Carson's speed is also defined as the tangent point on a line which is tangent to the drag polar and passes through the origin.
The lowest point on the drag polar is the point of minimum drag and this occurs at 104 mph CAS, which is the speed for maximum lift to drag ratio. . The value of 2.03 sq ft, the drag area from the parasite drag equation in the legend above, is here deemed accurate to plus or minus .1 sq ft .
EVALUATION BY C.J. STEPHENS
During the period March 3rd through 7th, 1994 the CAFE Foundation completed a thorough evaluation of Jim Clement's Tailwind, N6168X. The first flight of the series was my subjective evaluation of the stability and handling qualities in addition to the airplane's general accommodations.
I had not flown a Tailwind prior to this evaluation. At first look it was a very impressive airplane. The wings had an extremely smooth, clean appearance with no bumps, antennae or other objects to interrupt the airflow. The entire wing surface, with as nice a finish as I have ever seen, was hindered only by the single wing strut attach point. Even the wingtip lights were faired in with smooth precision. It was obvious that the builder was extremely conscientious during its construction. The aircraft was only recently completed and had logged only 80 hours of flying time.
The aircraft was fueled and ballasted to 18.2% MAC c.g. at the maximum allowable gross weight. The CAFE doctrine of not exceeding any specified limit or previously demonstrated capability was followed throughout the series of test flights.
Like many pilots, I have seen this square-looking plane over the years and given it little attention since it lacked the rounded lines which one associates with modern high performance aircraft. The outwardly boxlike appearance of the design belied its actual performance. The preflight inspection quickly showed that Jim Clement had done an excellent job of keeping the plane simple, just as intended by the designer. He had carefully avoided the installation of unnecessary equipment.
The instrument panel contained a basic set of instruments plus a turn coordinator that could be switched on if needed. The radios were limited to an intercom, VHF comm and a loran. All were quality equipment and worked perfectly throughout the period of the evaluation.
The fuel filler spout was located externally in the forward right lower corner of the windshield. The fuel quantity could be easily checked by dipstick and the cap security could be seen even from the cockpit. All 33 gallons of fuel were in one tank located forward of and below the instrument panel. A short fuel line and one on/off valve controlled the fuel flow. Big tank, short line, and an on/off... now that is a simple fuel system. One could argue against the safety aspect of having a large fuel tank in the cockpit, however, it is difficult to dispute the principle that simplicity, when dealing with fuel management, is a major design priority.
I am 5'-10" weighing 170 lb and I found the cockpit to have adequate room. During some of the test flights I was accompanied by an engineer of about my size. It was 'snug' but not uncomfortable. Another CAFE test pilot who is 6'-3" found his head just in contact with the overhead structure. His leg room was also at a minimum even though the seat did allow for some adjustment fore and aft.
A large cabin door, located on each side, opened widely. No boarding steps seemed necessary and the wing strut attachment was well forward and out of the way. Entrance to and egress from the cockpit were unhampered, requiring only one large step to slide into the cockpit seats. The seats were comfortable, providing good support in the proper places. Even the longer flights produced no discomfort. Very nice shoulder harnesses were provided for both the pilot and the passenger.
The O-320 started quickly on every start using only the accelerator pump for priming. The field of view while on the ground is somewhat limited with the high nose position typical in tailwheel aircraft. There is a need to stretch to see over the nose, but depending on your sitting height, full view of the taxiway is available to within 150' in front of the plane. Field of view up and to the left or right (as in clearing prior to takeoff at an uncontrolled airport) is restricted and less than desirable. By raising slightly in my seat, my field of view was good enough so there was no need to use S-turns to taxi.
The short wings made taxiing in tight places quite easy. The tailwheel was steerable, but not full swiveling, and very effective for ground operations. The brakes were excellent and were used to assist during the tight turns on the ground. The plane could pivot at about the wing tip by using rudder, brake and some power.
Ground handling without the engine running was easily accomplished by manually picking up the 50 lb. tail and pivoting the plane to the desired position. This was even done several times with two people in the cockpit when moving it on and off the scales, although it required two people to raise the tail with a full payload aboard.
In keeping with the simplicity theme, no parking brake was installed, nor were any cowl flaps. The magneto switch was located on the far left of the instrument panel. This was inconvenient. On tailwheeled airplanes in which the throttle is in the right hand and the stick must be held back during the run-up, the magneto switch should be accessible to the right hand.
The pitch trim, located under the seat, was very nice. It had friction washers to hold the setting and it loaded a tension spring against the elevators by use of a small lever. I used the setting recommended by the builder for takeoff, which was done by feel, and was easy to operate.
The roll trim annoyed me at first. It involved a sliding washer fit on a tube which loaded a spring against the right aileron rod behind the passenger seat. It took some practice to fully understand and operate this system. The initial tendency was to work it backwards. It was, however, a simple device and light in weight. With enough practice one could adequately trim the plane in roll.
A conventional vernier throttle was installed. This is not my preference of throttle types especially if the flying includes a lot of power changes or formation flying. Vernier throttles, however, are very nice on cross country flights.
FIRST FLIGHT IMPRESSIONS
As I taxied the Tailwind onto the runway for my first flight I was eager to see what it held in store. There was a 7 knot direct headwind.
The control stick was floor-mounted just forward of the seats in the center of the cockpit. The top of the center stick curled to the left over the pilots right thigh and downward so as to create the conventional feel of holding a stick that was directly between your legs. It worked very well except that it took a little practice to find a neutral aileron position.
The radio transmit button was on the end of the stick, pointed downward at the floor. It presented no problem as long as you knew where to find it. Since it was not visible from the normal sitting position, you could look in all normal places and never find it.
The aircraft accelerated rapidly due to the high power to weight ratio. Directional control was very quick initially during the takeoff roll, but once the tailwheel came off the ground, it was less sensitive. Very light stick forces were obvious right from liftoff. These were more noticeable in pitch than roll.
Liftoff occurred naturally at an indicated 65 mph. Initially with 2400 RPM and 28.3" manifold pressure, it was climbing at an impressive indicated 1600 fpm. Even though stick forces were light, it was easy to hold a constant 120 mph IAS.
The owner had recommended leaning the mixture during the climb. This was done, although, with no CHT installed, it was only "best guess" and experience to achieve a workable mixture setting.
With the small size of the plane and the relatively high power, P-factor was noticeable but was easy to control with a light application of rudder. During the climb it was necessary to briefly level off at 4500' to fly out from under a cloud shelf. At 2550 rpm at 4500' the cockpit airspeed indicator went right to 180 mph. The noise level in the aircraft at this point was substantial, and demanded the 20 dB noise protection provided by my headset.
The location of the wing root leading edge is well forward and slightly above the pilot's visual line of sight from a normal sitting position. During turns this obstructed the pilot's view. It was more noticeable in a left turn than a right turn. As the bank is increased the large window above the pilot can be used to see what is ahead in the turn, so that with greater than 40 degrees of bank, a full field of view is again available. During shallow bank turns I felt a little uncomfortable with the limited view and would compensate by occasionally raising the wing to look under, or, increase the bank to look out the top window. Due to the limited amount of horizon in view, there may be an increased possibility of spatial disorientation while flying in reduced visibility conditions.
During several subsequent flights the humidity was high and windshield fogging occurred. The cabin was very well sealed and afforded little natural airflow, which kept it nice and warm but allowed for the accumulation of the condensation. With a handkerchief, some of the accumulation could be removed, but without unstrapping, most of the windshield was just too far away to reach. Two small vents from the engine compartment had been installed to help the fog problem, but had been capped off for the trip to Santa Rosa. The cabin heater worked very well. There was a very simple cuff around the exhaust manifold which could be controlled with an on/off valve on the instrument panel. Turning the heater up to full volume helped some with defogging the windshield.
The only gyro was a turn coordinator that was switched so it could be left off when it was unneeded. No yaw trim system was installed.
A small flap was installed on the aircraft with a three position manual extension system. The first two notches of flaps were easy to use, however quite a twist of the body was required to get the handle far enough aft to catch the last notch. The forces of flap extension/retraction were light.
STATIC LONGITUDINAL STABILITY
The aircraft was trimmed for 120 mph at 8500' to evaluate the speed stability. A hand-held stick force gauge was used to measure the elevator stick force. Without re-trimming, the stick force was measured every 10 mph over the entire range from 80 mph to 180 mph. The resulting stick force gradient is plotted on the graph in Figure 1. The results show a change of only 1.45 lb stick force over the entire speed range. This amount of stick force is considered extremely light. An inexperienced pilot may find it difficult to fly with so little feedback. The pilot must rely on other inputs such as the indicators to control pitch accurately. A temporary lack of attention, even by a more experienced pilot, could result in a dangerous loss of airspeed control.
Pitch doublets, first down then up were introduced to evaluate the natural damping qualities of the airplane. Both stick-free and stick-fixed methods across the full speed range were evaluated. The results showed deadbeat response; that is no overshoot or oscillatory tendency was observed. Displacing the airplane in yaw and roll to explore the Dutch roll tendencies also showed quick damping with no tendency to persist. Thus, even though the stick forces are very light, the plane exhibits excellent natural dynamic stability qualities.
The aircraft was trimmed for level flight at 130 mph and bank was established at 15 degrees, first right then left, to determine if it would over bank or level out on its own. The aircraft held the bank angle exactly during these maneuvers. It seemed as if it were connected to an automatic pilot. After completing nearly 360 degree turns the test was ended, noting the absolute neutrality of the spiral stability.
ROLL DUE TO YAW
With the aircraft in trim at 100 mph, stick forces to maintain level flight were measured in roll with first 1/2, then with full rudder deflection. Approximately 1.5 lb of force was required in each direction with 1/2 rudder displacement. With full right rudder a 5 lb left aileron force was required and with full left rudder a 4.5 lb right aileron force was required to keep the plane in level flight. Considering the otherwise very light stick forces of this plane, these values show a very strong dihedral effect. To further explore the dihedral effect, a 45 degree bank was established. Then, with rudder alone, the wings were leveled keeping the ailerons neutral. This airplane exhibited, without a doubt, the fastest rate of roll that I have seen in a straight winged airplane using rudder only. This tendency was consistent in both directions at all airspeeds explored. This strong roll due to yaw may be caused by the tapered wing tip design since the wings have no geometric dihedral.
Full deflection aileron maneuvers were examined to measure both the roll rate and stick force. In one G flight, the time required to change bank angle from 45 degrees in one direction to 45 degrees in the other, including the acceleration, was measured. Roll rates of 47 degrees per second at 120 mph, and 45 degrees per second at 100 mph were observed in both directions. The stick forces steadily increased with greater deflection up to 9 lb at full displacement. This amount of natural feedback, though light, blends well with the very light elevator force. It would prove undesirable to fly if the ailerons were heavy and the elevators very light.
Adverse yaw was evaluated by using aileron only to establish a bank, then observing the yaw displacement/hesitation. The Tailwind showed mild adverse yaw in that it would only yaw about 5 degree and hesitate slightly before starting the turn.
Maneuvering performance was evaluated at 120 mph at 2 and 3 G's. The results were 4.5 lb and 7 lb of elevator stick force, respectively. Full flap maneuvering at 87 mph produced a stick force of 4.0 lb No overshooting tendencies or stick force lightening were observed during any of the maneuvers. These stick forces were consistent with the very light stick forces noted during other phases of the evaluation. Though enjoyable to fly the Tailwind requires a gentle hand.
It was fascinating to perform the stall evaluation in this airplane. The stall test flight had been loaded to maximum allowable gross weight. The actual stall would occur with the airspeed indicator's needle dropping to below 41 mph. Later flights with the CAFE Barograph showed a large error in the low speed accuracy of the cockpit airspeed indicator.
There was a very pleasant and mild aerodynamic buffet with onset 4-5 mph above stall, and it increased to the point of stall. Power setting was not a factor in the stalls since low power settings were used to decelerate at about 1 mph per second. All stalls broke straight ahead with neither wing wanting to stall ahead of the other. Recovery occured with the slightest bit of power or relaxation of stick back-pressure. All recoveries resulted in less than 100 feet of altitude loss.
The aircraft could be trimmed to level flight at all airspeeds from Vne down to 86 mph. I would consider this to be good trim authority. Roll trim was adequate.
APPROACH AND LANDING
During the flight it became evident that careful planning was required to set up a proper approach to the airfield. The plane was clean, fast and did not give up airspeed easily.
My first arrival on the base leg position was about where I thought it should be but as I got closer it became evident that a slip would be necessary. A moderate slip was called for to correct for my slight miscalculation of glide angle. By holding 100 mph, an excellent glide angle for a power off approach was established.
The light wooden propeller allowed quick response of the engine to all power applications. With even the smallest of amount of power applied, the glide range became deceptively long. My first landings were wheel landings and caused no appreciable problems as long as the flare speed was about 80 mph. Any excess speed would set up conditions likely to cause porpoising in a normal wheel landing.
On subsequent flights, three-point landings were explored. The plane handles very nicely in these provided the tail wheel is the first to contact the runway. The positive steering of the tail wheel helps with the directional control immediately upon touchdown. Braking and post flight operations were straight forward.
This Tailwind, by keeping the 'extras' to a minimum and doing quality construction, is a simple, inexpensive plane with excellent performance. N6168X, as we evaluated it, contained only equipment essential for safe, efficient flight. The flying qualities were brisk and light.
Inexperienced pilots should be cautioned about the light stick force gradient of the Tailwind. As with most high wings, the restricted field of view due to the wing roots is a negative factor when considering this design. However, the plane exhibits brisk control, rapid climb rate and high speed. It can carry two average- sized people a long distance quickly and in good comfort using very little fuel. This makes it well-suited as a personal VFR cross-country aircraft.
After my first flight, it is my responsibility to decide if this airplane is an acceptable candidate to proceed with a full CAFE evaluation. It seemed like an outstanding choice.
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