From the Air and Space Museum website:
"Operation Alert" began on July 12, 1957, and by mid-afternoon,
Eisenhower had become the first U.S. President to fly on board a
helicopter. Maj. Barrett flew the H-13J bearing serial number 57-2729.
He carried President Eisenhower and a Secret Service agent. The
President sat in the right rear seat but leaned against a special
armrest installed for him on the center seat. He also used a special
footrest. The second Ranger, serial number 57-2728, carried the
President's personal physician and another Secret Service agent. Both
helicopters were based at National Airport, along with the other
presidential aircraft."