May 11, 1970: A C-5A Began Bare Soil Landing Tests at Harpers Dry Lake Published May 11, 2021 Air Force Flight Test Center EDWARDS AIR FORCE BASE, Calif -- A C-5A began bare soil landing tests at Harpers Dry Lake. The AFFTC had leased the dry lake for the test program. At a weight of 470,000 pounds the transport left ruts only two inches deep on the lake surface. A mathematical model to predict sinkage and the resulting loads for aircraft wheels operating on bare soil surfaces is presented together with experimental results for a 29 x 11-10 8PR Type III tire. Four primary factors which determine soil rutting and drag have been identified. They consist of the tire spring rate, the soil load deflection relation, a drag inertia force, and a lift inertia force. Soil load deflections are based on the mobility number concept developed by the U. S. Army Corps of Engineers Waterways Experiment Station. Empirical constants obtained from tests conducted at the NASA Langley Landing Loads Track were used to compute the inertia forces. Comparisons of predicted and measured rut depths and drag loads cre made for a clay soil with CBR's ranging from 1.5 to 2.3 and speeds from 0 to 90 knots for tire inflation pressures of 30, 45, and 70 psi. Similar comparisons are made for sand having a surface strength of CBR 1.5. The experimental program included 173 tests with a single wheel and 39 tests with two wheels in tandem on buckshot clay and 24 single wheel tests on sand. Overall average differences in predictions and test data for rut depths were the following: 11% on CBR 1.5, less than 1% on CBR 213, and 1.5% on sand. Overall average differences for drag loads were the following: 6 percent on CBR 1.5, 9% on CBR 2.3, and 12% on sand. Average positive and negative differences were somewhat higher and were between 11% and 36%. An alternate computation using a spring-mass-damper model as used in vibrating foundation studies is also included. This alternate model is not recommended as it does not account for drag load interaction urid thus is not representative of the physical system. Methods for improvement of the alternate model are discussed. A computer program is described which incorporates the soil/wheel interaction model with a simulation of the C-130 aircraft during taxi and take-off. Analyses with this program show that moderate roughness has negligible effect on take-off distance for either soft fields or hard surfaces.