OTTAWA—The Department of National Defence is blaming an apparent design flaw for the cracks recently discovered in the tails of most of the military’s new Cyclone helicopters. In a statement Monday, the department said it was believed the weight and pressure of equipment installed on the tail are likely responsible for the cracks, which have been found on 21 of the military’s 23 Cyclones. Nearly half of them remain grounded. That conclusion was based on an “initial determination” from U.S. manufacturer Sikorsky Aircraft, which has been working with the military to identify the cause and find both a short-term fix and a long-term solution to the problem. It wasn’t immediately clear who is to blame for the problem. The department said it was due to “structural loads” caused by the helicopters’ electronic warfare suite and satellite communications antenna while in flight. The Defence Department referred questions about the cause of …