A horse track was also established to the rear of the cottages. Macquarie Grove was to have a number of short-term owners after the Dangar period (from James White MLC MLA to Henry Lamont Mackellar in 1896; to David James Maxwell in 1900; to Percy Crossing in 1907; and to James Pritchard in 1913). Finally, Macquarie Grove was purchased by Arthur Macarthur-Onslow (great grandson of the district's pioneer John Macarthur) in 1916 who used the property as a sheep farm and as part of the Camden Park Co-operative Dairy scheme. Arthur's second son, Edward, attempted but struggled to maintain the farm as his father had and, in 1937, spurred on by his keen interest in aviation, sold the entire sheep flock and converted the former horse racetrack into a private aerodrome. Edward established the Macquarie Grove Flying School which was used for aviation teaching as well as for recreation and public displays of flying, gliding and parachuting. At the time, Edward had restored The Groom's Cottage, in which he lived with his wife Winifred, using bricks from the derelict stables and had renamed the property "April Cottage" after the month they had moved in. However, due to an expanding family, Edward and Winifred "swapped" residences with Edward's mother Sylvia (widow to Arthur) and moved into Macquarie Grove. Upon settling into her new home, Sylvia renamed the house "Hassall Cottage". Upon completion of a period of initial training, the Macquarie Grove Flying and Glider School was officially opened by the Commonwealth Minister for Civil Aviation, James Fairbairn, on 30 September 1939. As Edward was enlisted for World War II and was due to leave Macquarie Grove, he decided that, rather than close the aerodrome, he would sign a Gentleman's Agreement with Minister Fairbairn (also a personal friend) in 1939 for the government's occupation and use of the property for the duration of the war. To achieve this, a portion of the neighbouring "Wivenhoe" property was acquired and the runway extended to a length necessary for use by the Commonwealth Government. In 1940, the Crown acquired 468 acres of Macquarie Grove for the purposes of defence and aviation during wartime for wartime activities. The Macarthur-Onslow family was to retain only 15 acres (including Hassall Collage but not Macquarie Grove which was reused as officers' quarters). In late 1942 when the Macarthur-Onslow family returned to the property, No 13 OBU (Operational Base Unit)