Australian Ultralight Federation

Constructor's Corner

Peter Franks'
Curtiss JN-4D 'Jenny' project

Jenny sketch

The Curtiss JN-4D 'Jenny' was probably, after the Avro 504, the most successful trainer of the first World War. Fitted with the 90 hp Curtiss OX-5, an 8 cylinder water cooled vee engine, the biplane had a top speed of 61 knots at its ceiling of 6500 feet, weighed 970 kg and took 10 minutes to climb to 3000 feet.

Some 2800 of the JN-4D model were manufactured, about half the total JN-4 production. After the war large numbers were sold on the U.S. civilian market at give-away prices, which contributed greatly to the enormous boom in civilian flying. The Jennys were the mainstay of the "barnstormer" era and some were still being flown in the 1930s. There were at least six Curtiss Jennys in Australia during and after WW1. The NSW School of Aviation at Richmond had two JN3s and two JN4Bs received in 1916 and 1917 and there were two JN4Ds in private hands after the war.

Peter Franks started building his 2/3 scale model in December, 1999 with plans from the Earlybird Aircraft Co. in the USA, but Peter says "the plans leave a lot to be desired, so most of my plane has been 'make it up as you go' type building".

Peter is a boilermaker-welder by craft who has built high performance motorcycles and who then became interested in building and flying radio controlled model aircraft, subsequently progressing to ultralights. The Jenny is his third ultralight building project, the first was a CAO95.10 Mustang followed by a Nieuport 11, built from the same materials as the Jenny - you can read further details in the Photo Gallery.

Two thirds scale Curtiss JN-4D 'Jenny'
  • Wing span: 8.4 m
  • Length: 5.6 m
  • Height: 2.13 m
  • Wing area: 16.2 sq.m
  • Fuel capacity: 35 litre fuselage tank & 25 litre top center section tank
  • Seats: 2
  • Engine: modified Suzuki Swift
  • Power: 65-70 HP @ 6000 rpm
  • Empty weight: 240 kg
  • MTOW: 490 kg
  • Vne: 85 kts
  • Cruise speed: 55 kts
  • Climb (MTOW): 680 fpm
  • Stall speed: 35 kts
  • Aerofoil: modified " Easy Riser " [ The "Easy Riser" was an early foot launched biplane type glider ]

Jenny fuselage

The fuselage covered and painted, ready for engine fit-out. The fuselage and empennage are of MIG-welded ERW SAE 1020 steel tube with a wall thickness of 1.2 mm. It was oxy-welded in some areas. The covering system is Stits polyester fabric, glued to the airframe and ironed with the household clothes iron to tension it, then painted with Stits paint.

Lower mainplane

Peter with the starboard lower mainplane. Ribs are steel die formed from 5005 H34 0.5 mm aluminum sheet. The wing spars are 44 mm diameter x 1.6 mm wall thickness 6106 T83 aluminum tube

Suzuki Swift 1000cc 3 cylinder engine

The Suzuki Swift 1000cc 3 cylinder engine, fitted with a Rotax "C" gearbox. Rotax 503 on the left for comparison. Peter estimates 65-70 hp at 6,000 rpm.

Jenny fuselage and engine

The fuselage with top centre section and engine test fitted. Upper mainplanes on the wall.

Cowling trial fitment

The engine mounted and the trial fitment of the cowling.

Jenny with Bonnie

January 2002 – Bonnie and Jenny. "The fuselage is ready and waiting, it just needs a battery and fuel. The tail feathers are covered and painted. The only problem I am having, is a corrosion problem in the cockpit area from all the spit. [ From me sitting in it and making engine noises ]"

Lower wing  covered and painted

"One lower wing is covered and painted and I have started covering the other bottom mainplane."

Completed Jenny

March 2002. "It is all together now and is waiting at Lethbridge Airpark for the registration number and the final inspection etc."

 Completed Jenny

"I have done some engine runs and all seems OK. So no more sitting in the cockpit and making engine noises now that it can make its own [and they sound great]! Weight and balance check out OK and it should be in the air soon."

 Jenny's first flight

April 2002. "The Jenny flew for the first time on April 20 and all went well. I had to make some small changes to the engine thrust line but other than that it flew beautifully. It is a big puppy after my Nieuport, stalling ever so gently at about 30 knots, cruise 50 knots at 4300 rpm and maximum speed 65 knots at 5500 rpm."

May 2002. "A word of warning to anyone intending to use a Suzuki 3 cylinder 1000cc engine. I have had one BIG problem, at about 5 hours flight time the engine started losing power and throwing coolant out of the recovery bottle. On stripping the engine I found that it had 9.5:1 pistons and had been detonating very badly, destroying the pistons and the head gasket was leaking! All the information received indicated that the 3 cylinder engines were all 8.8:1 and were fine on standard or unleaded mogas, so you can guess what I had in it ! The engine is now back in with the 8.8:1 pistons, but I haven't flown it yet due to bad weather."

July 2002. "The aircraft has now completed 15 hours flight time and the Susuki has been running beautifully with the 8.8:1 pistons."

October 2004. "The Jenny has just ticked over 150 hours flight time and it's all going great. I said previously that the Suzuki engine might make 65 to 70 hp — well its making about 60 hp and using about 10 litres per hour."

..... Peter Franks