Introduction

At the time of these experiments there was no accurate way to find the minimum drag on air foils with high Reynolds numbers. It was decided that tests should be performed in a 7 Ft. wide wind tunnel. The thickness of the sections should not exceed 6 inches and the width would be 6 ft. They decided to use Joukowski air foils because the shape and flow pattern could be figured out analytically

The sections were created out of a wood frame with a ply wood surface. The trailing end was rounded of instead of tapering to an infinitesimal thickness. In the center of the sections an air tight channel made of copper was fitted. The channel was split into sections with one hole drilled into it through the plywood. These were used to read local pressures on the surface of the air foil.

Experimental Procedure

Seven of these sections were created with different dimensions except that each was 6 Ft. wide. Each of the sections was placed into the tunnel with dummy ends on each end fixed to the walls. The sections were tied to a balance that read the drag and to gages that read the local pressures over the surface. They performed these tests at 5 different wind speeds of 21.6, 35.3, 50.3, and 80.2 Ft/Sec.

Experimental Data
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References

Case Study developed by Dr. Laura Pauley. Data from Experiments on a Series of Symmetrical Joukowski Sections by A. Fage, A.R.C.Sc., V.M. Falkner, B.Sc., and W. S. Walker. ARC Report 1241. April 1929.

Analysis

Using

  • Density = .0739 lbm/Ft^3
  • Dynamic Viscosity = 1.24 * 10^-5 lbm/(Sec*Ft)
  • Other values at STP

Perform the following on one air foil section:

Part 1

  • Use the delta-P in Table 1 to find Cp
  • Plot Cp vs x/c for each wind speed.
  • On the same graph plot the Theoretical Pressure Coefficient from Table 2
  • Do the Theoretical solutions fit the measured results?
  • If not what are the possible reasons?

Part 2

  • Numerically integrate P across Y to find Pressure CD using the following equation.
    Pressure Drag Force=F
    delta-P=P
    Y=delta-Y, vertical height of the surface segment, calculated from Table 1
    w=width of airfoil (6 ft)
    F=2w∑(PY)
    (This is multiplied by 2 for two sides of the airfoil.)
  • Plot Pressure CD vs Re and on the same graph plot the Total CD from Table 3
  • Calculate the Frictional CD and plot on the same graph
  • Which is higher, pressure or frictional drag?
  • Is the air foil streamlined body or a blunt body?