On August 18, 2021, about 1850 central daylight time, an experimental, amateur-built Hummer 260L helicopter, N777TC, was substantially damaged when it was involved in an accident near Livingston, Tennessee. The private pilot and the pilot-rated passenger were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot/owner stated that the flight was initiated from Livingston Municipal Airport (8A3) with an adequate supply of fuel. After warming the engine, he lifted the helicopter to a hover, hover taxied, ground taxied, and practiced setting down and lifting-up to a hover for about 10 to 15 minutes with no engine issues noted. He then intended to takeoff and fly in the airport traffic pattern. The pilot reported that the auxiliary fuel pump was on for the takeoff. He started to takeoff, and between 100 to 150 ft above ground level (agl) while passing through effective translational lift at an airspeed between 40 and 50 knots, the helicopter experienced a sudden complete loss of engine power. The pilot lowered the collective, made a partial throttle reduction, and entered an autorotation. When the helicopter was at 70 to 75 ft agl and 50 knots, the engine power increased. The pilot increased the throttle, and the engine power increased to full rpm; but as he increased collective, the engine again lost total power. According to the pilot, the helicopter was operating in the shaded area of the helicopter’s height/velocity diagram, and although he rapidly lowered the collective to gain energy in the main rotor blades, there was not enough inertia in them for a normal autorotative landing. The helicopter impacted hard on soft, wet, and slightly sloped terrain, and the landing gear wheels became buried and separated. The helicopter then rolled onto its right side, and the main rotor blades contacted the ground. The pilot stated that during recovery of the helicopter, fuel drained from a depressed fuel tank sump drain valve. After recovering the helicopter and elevating it, an additional 4 to 5 gallons of fuel were drained, which emptied the fuel tank; no fuel contamination was reported. One of four 4130 steel tubes that structurally secured the tailboom was cracked and deformed. The fuel shut-off valve was in the open position. Examination of the engine following recovery revealed that all fuel hoses were installed correctly and tight. Residual fuel was present in each hose. Residual fuel and some debris was present in the airframe fuel strainer; the debris was sand-like in particle size. The inlet screen of the controller, or servo fuel injector, was clean. There were no discrepancies found with the engine controls, the air induction, exhaust, and ignition systems, or the auxiliary fuel pump, which was electrically tested. Crankshaft, camshaft, and valve train continuity were confirmed. Borescope inspection of each cylinder revealed no discrepancies with the valves or pistons. Operational testing of the fuel injector nozzles revealed an equal amount flowed from five of the six fuel injector nozzles; the No. 3 nozzle’s flow was about two-thirds that of the others. The controller was retained and sent to the manufacturer’s facility for operational testing, which revealed no evidence of preimpact failure or malfunction. The unit was operationally tested with airflow and found to flow nearly identical to the values that were recorded when it was manufactured in 2012. According to the Federal Aviation Administration’s Helicopter Flying Handbook, the height/velocity (H/V) diagram shows the combinations of airspeed and height above the ground that will allow an average pilot to successfully complete a landing after an engine failure. The handbook states that operation of the helicopter in the shaded area of the diagram may not allow sufficient time or altitude to enter a stabilized autorotative descent. It further states that “in the simplest explanation, the H/V diagram is a diagram in which the shaded areas should be avoided, as the pilot may be unable to complete an autorotation landing without damage.”