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Thorp T-18


The T-18 design was originally developed with the Lycoming 0-290G (ground auxiliary power unit) in mind, by John Thorp. Which was easily converted to a Lycoming 0-290D engine producing 125 hp. The T-18 is an all metal 2 place side-by-side high performance aircraft. Control response is exceptional and control forces are well harmonized. The T-18 is a very capable cross-country airplane. It will carry 2 people, 80 lbs. of baggage, and depending on engine and prop, can cruise around 180 mph. Powerplants range from Lycoming O-235 (115hp) to IO-360 (180 hp).

The T-18 Tiger is a side by side, two seat, all metal, semi-acrobatic, low wing monoplane, which was de-signed by John Thorp in 1961. The basic materials used are 2024-T3 aluminium alloy sheet for the fuselage frames, spar webs and outer skins. Where more severe forming is required, such as the flanged ribs, 6061-T4 aluminium alloy is used. The design utilises the matched hole method, i.e. rivet holes are drilled in the detailed parts which are then matched up during assembly. The wing consists of a centre section, and two up-swept outer panels. The complete wing may be removed from the aircraft by five quick release “pip” pins. The empennage incorporates an “all flying” tail, i.e. the entire tailplane rotates about its pivot on the fuselage, and an anti-servo tab supplies the necessary elevator feel. The main landing gear consists of a heavy gauge, heat treated chrome molybdenum steel tube “A” frame, which is attached to the fuselage by three bolts. A 23 Imp. gallon fuel tank is installed behind the firewall.

In 1962, John Thorp was toying with a friend’s suggestion that he develop a sport plane for homebuilders.He conceived the T- 18 as a "modern antique": a simple, sturdy little air-plane that would take advantage of 1960s hindsight upon 1930s technology. He concentrated upon simplicity first. A polygonal fuselage, its curves optimized longitudinally and its other lines filled in with a ruler, facilitated layout and made jigging unnecessary; the airplane would jig itself once a few holes had been drilled. By depending on the tires and heavy, heat-treated tubular steel legs for shock ab-sorption, Thorp reduced the landing gear to an A-shaped weldment. The engine mount was bolted to one side of it; to the other, the fuselage.




The T-18 was designed to be built from twelve 4' x 12' sheets of aluminum ranging in thickness from 0.016 to 0.040. Ribs and fuselage frames were designed to be hand formed with a mallet over plywood form blocks. Simple aluminum angle extrusions are used for stringers and longerons. The main wing spar caps are made from 1/4 "U" channel on top and angle on bottom with flat sheet for spar webs.

John Thorp designed the aircraft to be small, yet strong with excellent take-off and climb performance. The landing gear is a simple "A" frame of heavy wall 4130 steel tubing, heat treated to 180,000 lbs./sq. in. and bolted to the firewall with (3) 3/8" bolts. The engine mount is also attached to the "A" frame allowing landing stresses not to be transmitted directly to the fuselage. This landing gear system has proven to be rugged and durable. The T-18 incorporates the "flying tail" which John Thorp held the patent for. This design also incorporates a sliding bubble canopy, like a fighter style.

The original design had no canopy, no flaps, no wheel pants and no pressure cowl-ing. Fuel was gravity-fed to the 125-hp GPU from a 28-gallon fuselage tank. That the back of the tank was only inches from the instru-ment panel didn't matter, because this was to be a minimally instrumented, VFR-only air-plane. A narrow console between the seats housed the stabilator pushrod and trim actu-ator and provided a small boarding step. A folding side panel flipped down and you stepped over the low sill, put your leading foot on the center console, swung the other into the tunnel under the panel, and then low-ered yourself into the seat while holding on to the heavy tubular frame that supported the canopy and served as a rollover structure.




The first T-18, which had been built by Bill Warwick as the plans were being drawn, flew in 1964. It was immediately apparent that the open cockpit would not do, since it was filled with an intolerable storm of whirling air. A sliding canopy was designed, and the turtledeck modified to accommodate it.

Warwick had used a 180-hp engine and a hybrid constant-speed prop. With 180 horsepower and a fixed -pitch prop, theT-18 could cruise at 200 mph and reach a sea-level top speed of about 220.

The T-18 suffered from a lack of preliminary design. Hence the large role modifications and afterthoughts have played in the develop-ment of the airplane-canopy, flaps, folding wings-and the persistence of certain flaws, like the cramped cockpit, about which, noth-ing can be done, and the nose-heaviness and consequently marginal tail power that result from Thorp's having failed to foresee the lengths to which dozens of builders would go in modifying and adding power to the design.





The original design by John Thorp in about 1963

The original T-18 fuselage design, but incorporating Lu Sunderlands new Convertible or folding wing. Plans were available through John Thorp. The folding wing was a supplement to the original T-18 drawings. The plans were taken off the market in 1984 when John Thorp retired.

Not many of these exist, but is the wide body version of the fuselage as designed by Lu Sunderland, using the standard T-18 wing platform. Plans were available through John Thorp. The wide body modification was a supplement to the T-18 drawings. The plans were taken off the market in 1984 when John Thorp Retired.

This is the predecessor to the S-18. It is the wide body version of the fuselage, and also the Convertible or folding wing as designed by Lu Sunderland. Drawings were available from John Thorp and they contained supplements for the fuselage and wing modifications. The plans were taken off the market in 1984 when John Thorp retired.

Unofficial designation for a model using both the standard fuselage and the standard wing plan form, but incorporates the LDS airfoil designed by Lu Sunderland and used on the S-18. There are no actual drawings for the "L" version, as builders have taken it upon themselves to make the airfoil change.




1964 Article

Lu Sunderland S-18

Engine: Lycoming O-290-G, 125 hp
Wing span: 20 ft. 10 in
Airfoil: NACA 63-215
Length: 18 ft. 11 in
Height: 5 ft. 1 in
Wing area: 86 sq.ft
Wing loading: 16.5 lb/sq.ft
Seats: 2
Empty weight: 844 lb
Useful load: 569 lb
Gross weight: 1,413 lb
Power loading: 10.1 lb/hp
Fuel capacity: 28.5 USG
Baggage capacity: 40 lb
Rate of climb: 1,100 fpm
Max speed: 170 mph
Cruise speed (75% power): 152 mph
Range (at max cruise, no res): 560 sm
Stall speed (flaps down): 63 mph.

Engine: Lycoming O-320, 150 hp
HP range: 150-180
Speed max: 183 mph
Cruise: 180 mph
Range: 530 sm
Stall: 63 mph
ROC: 1200 fpm
Take-off dist: 1900 ft
Landing dist: 1900 ft
Service ceiling: 18,000+ ft
Fuel cap: 29 USG
Weight empty: 923 lb
Gross: 1500 lbs
Height: 5.1 ft
Length: 18.9 ft
Wing span: 20.8 ft
Wing area: 86 sq.ft
Seats: 2
Landing gear: tail wheel

Wingspan: 20' 10"
Wing chord: 50"
Wing Area: 86 sq. ft.
Wing loading, 1600 lbs: 18.6 lbs/sq. ft.
Power Loading, 1066 lbs: 8.9 lbs/hp
Length: 18' 11"
Height: 5' 1"
Wheelbase: 13.3 ft.
Main gear track: 62.8"
Normal empty weight (0-290 engine): 900 lbs.
Design gross weight (0-290 engine): 1500 lbs.
Acceleration limits at 1250 lbs: +6 / -3 G's
Ultimate G-Load at 1250 lbs: 9G's
Recommended engines: Lyc. 0-290, 0-320, 0-360
Never exceed, Vne: 182 kts/210 mph
Maneuvering, VA: 138 kts/ 159 mph
Stall, clean at 1500 lbs: 56kts/65 mph
Stall, landing at 1500 lbs: 50 kts/58 mph
Flap speed, Vf: 95 kts/110 mph
Best glide: aprox 87kts/100 mph

Engine: Lycoming O-360-A1A, 180 hp
TBO: 2000 hr
Prop: Hartzell 72in 2 blade CS
Length: 18 ft 10 in
Wing span: 20 ft 10 in
Wing area: 86 sq.ft
Height: 5 ft 1 in
Cockpit width: 37 in
Empty weight: 1050 lb
MTOW: 1600 lb
Useful load: 550 lb
Max fuel: 47.5 USG
Max wing loading: 18.6 lb/sq.ft
Max pwr loading: 8.89 lb/hp
Max speed: 182 kt
Cruise: 175 kt
Econ cruise; 152 kt
Stall: 48 kt
Max range: 850 nm
Rate of climb: 2000 fpm
Service ceiling; 20,000 ft
Takeoff dist: 1000 ft
Landing dist: 1800 ft
Seats: 2


Engine: Lycoming O360 A1A
Propeller: Catto 3 blade composite 66” Dia
Wing: Folding
Fuel, 3 tanks: 208 lt
Cruise 1000 ft, 2200rpm: 130 kt
Fuel at cruise: 28 lt/hr
ROC: 1500+ fpm








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