Wind Tunnel Models

In selecting the model size to obtain the desired Reynolds number, several tradeoffs were considered. To achieve a given test Reynolds number, the mea­sured forces increase with decreasing chord. While large forces are desirable, models with small chords are difficult to build accurately. For this work, a model shop was not used; rather, experienced model sailplane enthusiasts were solicited to build the models. Consequently, construction tolerances were on the order of that found on model sailplanes. For these reasons a 12 in chord was selected as a compromise between the two competing effects. The model span was 33 I in. Construction techniques ranged from all-balsa with ribs, spars and open bays, to fiberglass-covered foam. All models were fully-sheeted except one, which had open-bay construction (see NACA 6409).

2.2.1 Digitized Profiles

As a check for model accuracy and for later airfoil performance computa­tions, every model was profiled using a digitizing Coordinate Measuring Ma­chine (CMM) to obtain the actual airfoil shape. A comparison was then made with the desired airfoil shape to determine the accuracy of the model. Profiling was performed at Fraser-Volpe Corporation in Warminster Pennsylvania using a Helmel Checkmaster CMM with full computer software for measurement pro­cessing. This machine and software made it possible to determine the location of a point in space within 0.0005 in absolute and 0.0003 in typical for all three axes.

A drawing of the CMM is shown in Fig. 2.3. The machine itself consists of a marble slab which is finished to a high surface flatness. On the slab is a gantry which can traverse the length of the table. Mounted on this gantry is a second gantry which in turn holds a vertical column to which the probe is attached. This arrangement allows the probe to be positioned anywhere within a large volume beginning at the surface of the slab. The probe consists of a hard plastic ball mounted to the end of a steel shaft which is screwed into a precision motion detector. The equivalent measuring diameter of a probe is found (the probe is said to be “qualified”) by touching it to a sphere of precisely known diameter. The computer then calculates the measuring diameter using basic geometry. Coordinates are read from the sensors when the probe is deflected in any direction by approximately 0.0003 in from its rest position. (This deflection is automatically accounted for by the CMM software.)

Outputs from the three sensors are routed to a computer which runs com­mercial software allowing one to reduce the raw information from the three axes to determine diameters, lengths, angles, differential locations, rotations, many different kinds of deviations from standard shapes, and so on. It will refer all measurements either to the table itself or to an arbitrary coordinate system based on the part or its fixtures. Indeed, setting up this coordinate system, or reference frame as it is called, is a major part of any measurement.

In addition to simply making a measurement, the computer can be “taught” a program of steps which represent a specific series of measurements. The com­puter will then prompt the operator for what point to measure next, and “knows” what to do with the measurement once it is taken. This capability was used here, allowing the entire process of measuring a model to be completed in about 25 minutes, including fixturing and post-measurement data reduction.

The machine is routinely calibrated to standards traceable to the National Bureau of Standards.