jets. The jets strike the ground and spread out. Where they meet under the fuselage they mix and can only move upwards striking the underside of the fuselage. How well their upward momentum is diverted sideways or downward determines the lift. Fountain flow follows a curved fuselage underbody and retains some momentum in an upward direction so less than full fountain lift is captured unless lift improvement devices are fitted. HGI reduces engine thrust because the air entering the engine is hotter and less dense than cold air. Early VTOL experimental aircraft operated from open grids to channel away the engine exhaust and prevent thrust loss from HGI. The Bell X-14, built to research early VTOL technology, was unable to hover until suckdown effects were reduced by raising the aircraft with longer landing gear legs. It also had to operate from an elevated platform of perforated steel to reduce HGI. The Dassault Mirage IIIV VTOL research aircraft only ever operated vertically from a grid which allowed engine exhaust to be channeled away from the aircraft to avoid suckdown and HGI effects. Ventral strakes retroactively fitted to the P.1127 improved flow and increased pressure under the belly in low altitude hovering. Gun pods fitted in the same position on the production Harrier GR.1/GR.3 and the AV-8A Harrier did the same thing. Further lift improvement devices (LIDS) were developed for the AV-8B and Harrier II. To box in the belly region where the lift-enhancing fountains strike the aircraft, strakes were added to the underside of the gun pods and a hinged dam could be lowered to block the gap between the front ends of the strakes. This gave a 1200 lb lift gain. Lockheed Martin F-35 Lightning II weapons-bay inboard doors on the F-35B open to capture fountain flow created by the engine and fan lift jets and counter suckdown IGE. Wing stall in ground effect The stalling angle of attack is less in ground effect, by approximately 2–4 degrees, than in free air. When the flow separates there is a large increase in drag. If the aircraft overrotates on take-off at too low a speed the increased drag can prevent the aircraft from leaving the ground. Two de Havilland Comets overran the end of the runway after overrotating. Loss of control may occur if one wing tip stalls in ground effect. During certification testing of the Gulfstream G650 business jet the