The instrument panel was beautiful in the way it was laid out and manufactured. All of the lettering was hand painted and looked exceptionally nice. With the side-by-side cockpit design, the panel size was excellent, an advantage enjoyed over tandem designs. A neat row of electrical switches and circuit breakers were located to the lower left of the instrument panel. Just above that were the magneto and master switches. This seemed to me to be a very functional and logical layout. Flight control was through dual center sticks with the radio transmit buttons on top of each stick. The manual flap lever was between the seats and had a 20° and 30° setting. Earlier T-18 models had a 40° setting but that was deleted due to a tail blanking and pitch down problem.

There were no entry steps installed. However, the wings are low enough to the ground that it is not difficult to step up on the wing. Entry was the same from either side of the plane. Caution needed to be exercised not to step on the flap since it is not de-signed to support that type of load. This "No Step" was well marked and visible wing walk strips helped remind the unsuspecting where to step.

Getting into the cockpit could be accomplished, without stepping on the seats, by stepping on the center structure that was between the seats. Once seated I found the cockpit to be very comfortable.

The rudder pedals were fixed in position as were the seat backs leaving little adjustment for taller occupants. I believe that persons taller than 6 feet might find insufficient headroom. The shoulder width was adequate for myself (180 lbs) and Otis Holt (150 lbs). However, during flight our shoulders were just touching. A nice sized storage compartment was available under each seat. The main baggage compartment, located behind both seats, was roomy and accessible by leaning either seat forward. The baggage maximum capacity was 40 lbs.

The canopy, which seemed large by comparison to the rest of the plane, slid straight back on excellent rails and rollers. The single canopy locking mechanism located in the center top of the canopy was positive and effective. Air loads during flight tend to push this canopy closed, so there is little danger of the canopy inadvertently opening in flight if the locking mechanism were to become disengaged.

The engine controls were in line left to right along the bottom of the instrument panel and each was color coded with a vernier knob. I liked the layout, but I personally prefer non-vernier throttles, which I find to be a more useful control in formation, traffic patterns and taxiing. Constantly having to deal with the push release of the vernier is distracting and rotating the knob is too slow to be useful. 

START/TAXI/RUNUP

Moving the plane around on the ground by hand is easily done, even loaded to its maximum weight. During the CAFE testing the plane is set on the scales in a fuselage level attitude fully loaded with fuel, test gear, and pilots and is then ballasted to the maximum allowable weight. The CAFE digital electronic scales, with their separate flush-to-the-floor platforms for main gear and tailwheel, permit quick computation and adjustment of the CG to the desired position. Even fully loaded it was no problem for two people to set the tail on the ground and push the plane into the starting ramp position.

Starting of the O-360 Lycoming was straight forward. Priming was accomplished through a couple of pumps of the throttle which worked very effectively. No electric fuel pump was installed. Run up and pre-takeoff checks were normal, however, it was noted that no checklist was available for use.

Taxiing gave me a chance to get the feel of the tailwheel and the response delays that would be present during takeoff. The plane responded quickly and positively to all rudder pedal inputs, no matter how slight. With a little practice on the taxiway I was satisfied with the directional control. My main tendency was to use too much rudder input so conscious effort to make very small rudder inputs helped to track straight at varying ground speeds. The toebrakes were positive yet not overly sensitive. There were no toebrakes installed on the rudder pedals for the right seat.

Field of view was better than the average of most tailwheeled aircraft. The taxiway could be seen at about 150 ft. directly in front, or even closer by moving my head to the left. Of course, with the large bubble canopy there was a clear view in all other directions. No canopy defog vent was installed. However, the mild fogging that occurred prior to our pre-dawn departure was quickly dissipated by opening the canopy about 3 or 4 inches. The moving air from the propeller wash rapidly cleared all windshield fogging. Diagram

TAKE OFF AND CLIMB 

The Thorp's takeoff acceleration really impressed me. The constant speed propeller and 0-360 combination produce the feeling of being shot out of a cannon. Using slight stick forward during the short takeoff roll helps raise the tail improving the view over nose and helps prevent early fly off. Directional control was no problem though I was aware of the sensitive rudder input which on later flights became quite comfortable. The plane literally jumped off of the ground when it was ready to fly. With the trim set the plane climbed easily and quickly to our test altitude of 8,000'. Field of view during climb was adequate over the nose and excellent in all other directions. Shallow turns during climbs ensured good clearing for other airborne traffic.

DYNAMIC STABILITY

Dynamic stability was explored with the input of pitch doublets then observing the natural damping tendencies. Both stick-free and stick-fixed situations were explored at 93 mph (1.3Vs) and 150 mph (Va). The T-18 displayed excellent dynamic stability with every sample.

STATIC LONGITUDINAL STABILITY

Static longitudinal stability was measured by trimming the airplane to 150 mph then measuring the amount of stick force required to fly at other airspeeds across the range. A hand held stick force guage was used to measure the force and a cockpit in-stalled camcorder recorded the information.

MANEUVERING STABILITY

Classic maneuvering stability at maximum gross weight was examined using the hand held stick force gauge and G meter. The results were as follows:

Response to control inputs was smooth and the T-18 showed no tendency to overshoot the intended G-loading, even in the aft-loaded configurations.

SPIRAL STABILITY

Stick-free spiral stability was examined by trimming the airplane for a level 15° bank turn then releasing the controls and recording the tendency to either increase bank or roll out. The test was repeated at both 93 and 150 mph IAS. The results were inconclusive due to a slight right wing heaviness caused by the ballast used to load the plane being located on the right side of the baggage compartment. With no aileron trim installed I was unable to fully trim the ailerons to neutral. My opinion is that the T-18 has neutral spiral stability. The left turns would generally roll level in 12 seconds and turns to the right would increase to 30° of bank in 16 seconds. Entry speed did not seem to affect the results of this test.

Load in G's Graph  |  IAS, mph Graph

ROLL DUE TO YAW

I found that roll-due-to-yaw could control the bank angle very nicely using rudder alone. The dihedral effect caused by the prominent dihedral change of the wing at mid span produces very effective control. I was so impressed with the rate of roll that could be generated with rudder alone (ailerons neutral) that I took some measurements:

The lower airspeeds and resulting higher angle of attack obviously produced better roll-due-to-yaw. It was a brisk roll rate compared to that of most straight winged planes that I have flown.

Adverse yaw was observed by slowing to minimum airspeed and introducing high aileron input without any rudder input. The resulting yaw excursion was minimal. The heading would only yaw opposite about one degree before turning in the proper direction. This airplane incorporates differential ailerons that deflect farther up than down in their movement. This, and the short wing span, seem to be contributing factors in demonstrating very little adverse yaw tendency.

ROLL RATES

Clean configuration, 93 mph IAS, full stick deflection, 120° bank change, time includes the time to establish rate. Rolling right 54° per second; stick force 15 lbs.
Rolling left 67° per second; stick force 13 lbs.

Repeating the test, all elements the same except flaps at 30°. Rolling right 52° per second.
Rolling left 60° per second.

The pitch and roll stick forces, being about equal, were well blended and they produced an excellent control feel.  Roll Rate Chart

STALLS

This Thorp T-18, as tested, has a very interesting and predictable stall. The stick force build-up is adequate to prevent an inadvertent stall (as indicated in the static margin results shown above). At about 15 mph before stall there is a mild buffet that can be felt through the stick and in the airframe. This buffet builds up to an easily noticeable level before stall occurs. The stall is instantaneous and crisp. In every case N42KB's left wing dropped about 30° in association with the stall. Recovery was as quick as the stall itself. By simply repositioning the stick about an inch forward, the angle of attack was reduced and the wing started to fly as rapidly as it stopped flying. I enjoyed doing the stalls and felt very comfortable throughout each of them. For beginning pilots, I feel stalls in the T-18 would require good training and regular practice.

This nimble little plane has such nice flying qualities that it is a pleasure making mild turns during de-scent. The T-18 is clearly designed for the enjoyment of flying. With the constant speed propeller the speed in-creases nicely on the descent, carrying the speed into the traffic pattern. The flaps worked nicely for glidepath control, as did slips.

Wheel landings were my choice to minimize the tailwheel problems that can occur with an unfamiliar and sensitive airplane. Each of the six landings was pleasant and comfortable. The main landing gear struts were stiff enough that there was no tendency to bounce back into the air upon touch down. The pitch control was positive and it was easy to judge the height during the touch down. The plane flew onto the runway very nicely at about 10 mph above stall speed. Directional control was very good as the speed diminished and the tail settled to the runway. As the tailwheel contacted the runway it showed an increased sensitivity to rudder inputs. However, that too could be controlled with care and practice.

CONCLUSION

The Thorp T-18 that Ken Brock presented to the CAFE Foundation for evaluation was an excellent example. Its fine handling qualities, good maneuvering and strong stability qualities show it to be a very well thought out design. It has simple systems that should be easy to maintain and operate. In my opinion it is a "pilot's" airplane. It has quick responses which make it fun to fly but which also re-quire paying attention as pilot. There can be no snoozing during landings or at high angle-of-attack maneuvering flight. If one observes these precautions, it is an airplane that can produce many years of pleasure and satisfaction.

Thorp T-18 N42KB Graph | T-18, Lapse Rates, 11-4-95 Graph

THORP T-18, N42KB
Estimated Cost: $14,000 materials,
$25,000 engine, $7,500 prop, $5,000 instruments and radios
Estimated hours to build: 1950 hours in 18 months
Completion date: July 12, 1982

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