BY BRIEN A. SEELEY
John Willard Thorp (1912-1992) designed and built the first Thorp T-18 in 1961 as an all-metal, low wing monoplane. He employed the "matched hole" riveting technique to meet his priorities of a low cost and quick to build aircraft. Using the Lycoming O-290 engine, it became one of the first high-performance homebuilt aircraft. Thorp published a series of articles on how to build the T-18 in Sport Aviation in 1962-63.
Over the last 33 years, the T-18 has been one of the more popular designs among EAA builders. The ruggedness of the design was showcased in 1976 by Don Taylor's flight around the world in his T-18, N455DT, which now resides in EAA's Air Adventure Museum in Oshkosh. This was the first homebuilt ever to circumnavigate the globe.
Thorp was inducted into EAA's Hall of Fame in November 1995. Well known as an aeronautical designer, he received his formal training at the Boeing School of Aeronautics in Oakland, California, and later worked at Boeing, Vega, Lockheed and Fletcher Aircraft.
Richard C. Eklund, a former Piper Aircraft engineer who received much of his education from his long association with Thorp, now manages the T-18 plans and product support from the historic Locke Family Home in Lockeford, California. John Thorp was the grandson of Dr. Dean Jewett Locke, the founder of Lockeford, and was raised from age 4 in that same home.
THE TEST AIRCRAFT
N42KB was built by Ken Brock in 1982 with a 180 hp Lycoming engine and Hartzell constant speed prop. When asked to provide the aircraft for this Aircraft Performance Report, Ken enthusiastically agreed. He did an excellent job of making the camcorder mount, sensor and wiring installations necessary for these tests. Mike Melvill and the team at Scaled Composites made the barograph wing cuff mounts exceptionally strong and light.
Both Ken and Richard Eklund attended the CAFE testing sessions and were very helpful in making flight test preparations.
N42KB differs from "stock" in having gear legs that are 3" longer and in having no wing leading edge stall strips. Its gross weight is 1600 lbs. while the original Thorp used a 1500 lb. gross weight. Otherwise, it was built according to plans.
The setting for the propeller flat pitch stops on N42KB limited it to 2600 RPM instead of the allowable maximum 2700 RPM. This produced lower speeds than would have been obtained if the propeller RPM had been set for its maximum.
The testing consisted of 7 flights, the last 2 of which were the flying qualities assessments at forward and aft CG's, respectively. Flight #5 was used to determine the level speed and power performance. Flights #1 through #4 were devoted to zero thrust glides. Otis Holt served as flight engineer for all of these flights, with C.J. Stephens as PIC on flights #2-7.
This was the first zero thrust glide test performed by the CAFE Foundation on an aircraft with a constant speed prop. A consistent 6.8 thousandths of an inch dynamically measured thrust bearing clearance was observed, with 3.4 thousandths thus chosen as the zero thrust position of the crankshaft.
Due to the excellent climb capability of this T-18, the zero thrust glides were of long duration from 11,000' down to 5,500' over the ocean at Pt. Reyes National Seashore.
Glide data analysis in this report included new software incorporating correction for induced drag artifacts. An additional program to extract the lapse rate during the several climb segments conducted at different times of day was written by Steve Williams to graphically analyze the atmosphere during these tests. The lapse rates so obtained appear consistent with acceptable flight test conditions and this is supported by the smoothness felt in the cockpit. Air mass lift or sink was not measured.
Cruise speeds shown in this report are those measured with the CAFE barograph, corrected for the wing cuff drag penalty, and are felt to be accurate to 0.1 mph.
The crankshaft incidence was 6° nose down. A pitot-static system check showed no leaks. N42KB's pitot-static probe location gave large errors both with change of power setting and yaw angle.
T-18 FLYING QUALITIES
By C. J. Stephens
Sample C.G. Calculations, T-18 N42KB
|Aft sample item||Weight||Arm||Moment||Forward||Weight||Arm||Moment|
|Main gear||996||55.312||55090.752||Main gear||996||55.312||55090.752|
|Nose gear||33||215||7095||Nose gear||33||215||7095|
|Fuel, lb||24||50||1200||Fuel, lb||170||50||8500|
|Oil, included||0||28||0||Oil, included||0||28||0|
|Gross Weight||1600||Gross Weight||1600|
|Actual Weight||1534||Actual Weight||1349|
|c.g. range, in||62.5"-71.0"||c.g. range, in||62.5"-71.0"|
|c.g. range, %||15%-32%||c.g. range, %||15%-32%|
|c.g. in inches||69.13||c.g. in inches||61.91|
|c.g. in % MAC||28%||c.g. in % MAC||14%|
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.
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
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 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 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.
Classic maneuvering stability at maximum gross weight was examined using the hand held stick force gauge and G meter. The results were as follows:
Cruise configuration and 150 mph IAS, with the CG 21% MAC: 2 g = 10 lbs. 2.5 g = 17 lbs. 3 g = 24 lbs. Flaps full down configuration, 93 mph IAS, CG 21% MAC: 1 g = 0 lbs. 1.5 g = 3 lbs. 2 g = 6 lbs. Cruise configuration, 150 mph IAS, 27% MAC: 2 g = 10 lbs. 2.5 g = 12 lbs. 3 g = 14 lbs. Flaps full down configuration, 93 mph, 27% MAC: 1 g = 0 lbs. 1.5 g = 3 lbs. 2 g = 7 lbs.
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.
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.
|Calibration of Instrument Panel to CAFE Sensors|
|Panel M.P.||CAFE M.P.||Panel CHT||CAFE CHT|
|90||85 Power on||4000||3994||2100||2003||18||17.5||245||361|
|96||71 Power + rudder||5000||4981||2200||2130||20||19.6|
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:
93 mph rolling right 10° per second 93 mph rolling left 15° per second 150 mph rolling right 7.5° per second 150 mph rolling left 4° per second
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.
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
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
CAFE MEASURED PERFORMANCE
|Propeller static RPM, full throttle||2605 RPM|
|Takeoff distance, ft, 120” MSL, no wind, 1583 lbs., 80°F.||600ft.|
|Liftoff speed, per barograph data, CAS, 1586 lbs., 80°F||70.2 mph|
|Touchdown speed, barograph, CAS, 1500 lbs., 68°F.||78.2 mph|
|Max. rate of climb, 2560 RPM, 2500-3500 ft., Std, 105 CAS,|
|1585 lbs., 27.4” M.P., 15.4 gph||1539 fpm|
Max. rate of climb, 2592 RPM, 10,000 ft., Std., 101 CAS
1567 lbs., 21.7” M.P., 13.9 gph
|Noise level, full power climb/75% cruise||97 dBA/98.2 dBA|
|Cruise Speeds: bar04.pnt file source|
|8124’ density, 75%,22.2”/2500 RPM/10.8 gph/1513 lbs.||201.7 mph|
|8062’ density, 65%,20.5”/2499 RPM/9.8 gph/1511 lbs.||193.6 mph|
|8025’ density, 55%,18.5”/2500 RPM/8.8 gph/1510 lbs.||181.8 mph|
|10,137’ density, 70%,20.5”/2600 RPM/10.0 gph/1557 lbs.||200.0 mph|
|6,026’ density, 84%,24.2”/2605 RPM/14.5 gph/1575 lbs.||202.2 mph|
|Stall speed, Vso, CAS, 1 g, level, 1572 lbs., 1978 RPM|
|22% MAC||59.3 mph|
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.
Eklund Engineering, Inc. Ken Brock
19960 Elliott Road/P.O. Box 1510
Lockeford, CA. 95237.
209/727-0318 FAX: 727-0873
Ken Brock Manufacturing, Inc.
11852 Western Ave, Stanton, CA. 90680.
714/898-4366 FAX: 894-0811
|Cost of plans||$250|
|Plans sold to date||1600|
|Estimated hours to build, basic||2000|
|Prototype first flew, date||1962|
|Normal empty weight, with O-290 Lyc.||900 lbs.|
|Design gross weight, with O-290 Lyc.||1500 lbs.|
|Recommended engine(s)||Lyc. O-290, O-320, O-360|
|Advice to builders:||Keep it light, stick to the plans, approved for aerobatics at 1250 lb. or less, however, requires proficiency in aerobatics due to tendency for rapid speed build-up.|
CAFE FOUNDATION DATA, N42KB
|Wingspan||20 ft. 10 in.|
|Wing chord||50 in.|
|Wing area||86 sq. ft.|
|Wing loading, 1600 lbs./86 sq. ft.||18.6 lbs./sq. ft.|
|Power loading, 1600 lbs./180 hp||8.9 lbs./hp|
|Span loading, 1600 lbs./20 ft. 10 in||76.8 lbs./ft|
|Airfoil, main wing||631412, modified with no bottom T.E. cusp|
|Airfoil, design lift coefficient||4|
|Airfoil, thickness to chord ratio||12|
|Aspect ratio, span2/86 sq. ft.||5.05|
|Thrust line incidence, crankshaft||6° down and 3° to the right|
|Wing dihedral||8° at midspan|
|Wing taper ratio, root/tip||1.0|
|Wing twist or washout||0°|
|Steering||Differential braking, swiveling tail wheel|
|Landing gear||Tailwheel, tubular spring steel, wheel pants|
|Horizontal stabilator: span/area||83 in./14.4 sq. ft.|
|Horizontal stabilator chord||25 in.|
|Elevator: total span/area||NA|
|Elevator chord: root/tip||NA|
|Vertical stabilizer: span/area incl. rudder||32 in./6.2 sq. ft.|
|Vertical stabilizer chord: root/tip||22 in./12 in.|
|Rudder: average span/area||44 in./1.8 sq. ft.|
|Rudder chord: top/bottom||8 in./16 in.|
|Ailerons: span/chord, each||49.25 in./10 in|
|Flaps: span/chord, each 4||7.6 in./10.7 in.|
|Total length||18 ft. 11 in.|
|Height, static with full fuel||5 ft. 1 in.|
|Minimum turning circle||13 ft. 3 in.|
|Main gear track||62.8 in.|
|Wheelbase, tailwheel to main gear||13.3 ft.|
|Acceleration Limits at 1250 lbs.||limit load +6/-3 G’s and ultimate load +9 G’s|
|AIRSPEEDS PER OWNER’S P.O.H., IAS|
|Never exceed, Vne||182 kt/210 mph|
|Maneuvering, Va||138 kt/159 mph|
|Best rate of climb, Vy||NA|
|Best angle of climb, Vx||NA|
|Stall, clean at 1500 lbs. GW, Vs*||*56 kt/65 mph|
|Stall, landing, 1500 lbs. GW, Vso*||*50 kt/58 mph|
|Flap Speed, Vf||95 kt/110 mph|
|* Compare to CAFE measured performance.|
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.
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
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
|Empty weight, with oil/gross wt||1029.7 lbs./1600 lbs.|
|Payload with full fuel||399.9 lbs.|
|Useful load||570.3 lbs.|
|Engine make, model||Lycoming, O-360 A2D|
|Engine horsepower||180 BHP|
|Engine TBO||2000 hrs|
|Engine RPM, maximum||2700 RPM|
|Man. Pressure, maximum||29 in Hg|
|Turbine Inlet, maximum||NA|
|Cyl head temp., maximum||500°F|
|Oil pressure range||25-100 psi|
|Oil temp., maximum||245° F|
|Fuel pressure, range .||5-8.0 psi|
|Weight of prop/spinner/crank||120 lbs.|
|Induction system||MA4-5 carb, bottom mount|
|Induction inlet area||8.25 sq in|
|Exhaust system||2 into 1 crossover, stainless, 1.75” O.D.|
|Oil capacity, type||8 qt., 15W-50|
|Ignition system||Bendix magneto S4LN200/S4LN-204 on right|
|Cooling system||Pitot inlets, downdraft|
|Cooling inlet area||51.8 sq. in.|
|Cooling outlet area||80.5 sq. in.|
|Prop extension, length||7 in. from crankface to blade axis|
|Prop ground clearance, full fuel||4.625 in. with level fuselage|
|Spinner diameter||12.75 i.n.|
|Electrical system||Prestolite: P/N ALY8403L5 alternator|
|Fuel system||1 tank in forward fuselage, mechanical fuel pump|
|Fuel type||91 octane|
|Fuel capacity, by CAFE scales||170.4 lbs./28.4 US gal|
|Fuel unusable||1 oz.|
|Braking system||Cleveland discs, single caliper|
|Flight control system||Dual center sticks, push-pull tubes, rudder cables|
|Tire size, main/tail||5:00 x 5/ 8” Maule tailwheel|
|Cabin entry||sliding canopy|
|Width at hips||NA|
|Width at shoulders||36 in.|
|Height, seat to headliner||37 in.|
|Baggage capacity/size||65 lbs./ 19.5L x 35.8W x 26H|
|Baggage door size||oval opening behind folding seatback = 15” x 32”|
|Approved maneuvers:||No snap maneuvers Roll, Chandelles, lazy 8’s can be done.“It spins well and recovers well but has never been through a full spin program” -Ken Brock|
|CENTER OF GRAVITY:||15% to 32% MAC|
|Range, % MAC||62.5 in. to 71 in.|
|Range, in. from datum||60.5 in.|
|Empty weight CG, by CAFE||Wing L.E. = 55”|
|From datum location||55.3 in.|
|Main landing gear moment arm||215 in.|
|Tailwheel moment arm||50 in.|
|Fuel tank moment arm||85.5 in.|
|Crew moment arm|
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