HondaJet's Over-the-Wing Engine Mount Cuts Drag by Placing Nacelles at 75-80% Chord, Honda Engineers Found

The HondaJet carries its two engines on short pylons above the wings, a layout Honda calls OTWEM (over-the-wing engine mount) and has patented. Designer Michimasa Fujino, who began sketching the aircraft in 1997 and settled the concept by 1999, wrote in 2004 that engine location was the single biggest design decision of the programme. Most light jets mount engines on the rear fuselage, which requires a heavy carry-through structure that takes up cabin space; moving the engines to the wing removes that structure and enlarges the cabin.

Over-wing nacelles were conventionally expected to strengthen shock waves and raise drag at a lower Mach number, and the VFW-Fokker 614 of the 1970s, which used over-wing engines, was limited in speed by that interference. Honda's team used a three-dimensional Euler code and then tested a scale model in the Boeing Transonic Wind Tunnel, sweeping Mach numbers from 0.70 to 0.84. On the clean wing the shock sat at about 70 percent of the chord. With the nacelle's front face placed at roughly 75 to 80 percent chord, the shock weakened and drag divergence moved to a higher Mach number than the wing achieved without nacelles, while a mid-chord placement instead produced a strong shock near the trailing edge. Fujino and colleague Yuichi Kawamura called this favourable interference and published it in the Journal of Aircraft.

The aircraft also uses natural laminar flow. The wing uses the SHM-1 airfoil, with a favourable pressure gradient to about 42 percent chord on the upper surface and about 63 percent on the lower, validated with a test glove flown on a Lockheed T-33. The upper wing skin is a single machine-milled integral panel. According to Fujino's 2004 paper, the laminar-flow nose cut fuselage drag by about 10 percent versus a turbulent-flow nose.

Honda's small-jet work dates to the late 1980s with the all-composite MH02, flown with Mississippi State University's Raspet Flight Research Laboratory. Honda R&D Americas opened a facility at Piedmont Triad International Airport in Greensboro, North Carolina, in October 2000, and the proof-of-concept HondaJet first flew there on 3 December 2003. It was shown publicly at EAA AirVenture Oshkosh on 28 July 2005, and commercialisation was announced at the 2006 show. The prototype used two Honda HF118 turbofans of 1,670 lb thrust each; production aircraft use the GE Honda HF120.