We left off last week with the conclusion that for accurate total energy compensation a known coefficient of pressure of -1 is required somewhere on the aircraft. This weeks post will cover how to utilize that conclusion to give accurate total energy variometer readings. Today's post relies heavily on information gathered from NASA Technical Memorandum 73928, which can be found here.
A topic often covered in introduction to aerodynamics courses is the study of the flow field around a cylinder Link 1, Link 2. The body of knowledge regarding the velocity and pressure fields that surround a cylinder in cross flow is quite extensive. This body of work can be exploited for our study of the total energy probe given that the coefficient of pressure on the aft side of a cylinder is -1 under specific conditions. Below is a graph taken from this resource. This information is telling us that only for sub critical Reynolds numbers [less than 1.86x10^5] is the coefficient of pressure -1 on the aft side of the cylinder. It was this basic conclusion that led to the development of the small diameter total energy probe that we are familiar with today, however the detailed design of the probe was only developed after significant wind tunnel and empirical research. The following conclusions, taken directly from NASA TH X-73928, describes the other subtleties associated with an accurate total energy probe. These should give the average reader the tools to check and make sure their total energy probes are set up correctly.
- Cylindrical tube, diameter o f 3/16- to 1/4-inch.
- Tube end squared off with very slight bevel of sharp edge.
- A f t facing pressure orifice , adrilled hole about 1/3 the tube diameter (1/16- to 3/32-inch).
- Center of hole located a t a distance two times the tube diameter from the end of the tube (3/8- t o 1/2-inch).
- Probe swept forward about 20 degrees with respect to flow direction.
- Probe mounted in free-stream air, extending a minimum of 5 to 6 inches from the aircraft.
- Vertical tail location good; aft fuselage acceptable.
Additional important is the rage of accurate operation:
- 40-150 mph
- sea level to 20,000'
- Normal yaw and pitch attitudes (+/- 10 deg yaw, -5 to -25 deg pitch, hence the forward bend in the TE Probe to get a net +/- 5 degree invariance to pitch)
Most importantly, the total drag of the TE probe was found to be around 1/10 of a pound at 100 mph.
As important as the TE probe, the capacity bottle that is on the other side of the variometer needs to meet the design assumption made in last weeks post. If you remember we needed to assume constant density to develop the equations that allow up to exploit the properties if a Cp = -1. For that assumption to hold true we need to ensure the volume of air contained in the capacity bottle changes temperature (density) as slowly as possible. This is achieved by putting chore girl steel wool pads in the capacity bottles. The steel provides a good heat capacity to supply and absorb temperature differences.
Hopefully these posts will help you better understand the mechanics underway in the instruments used in soaring flight and help trouble shoot any potential issues in the future. Next weeks post will cover the mechanics of the online contest World Champion contest and how it proves that the number of quality of X/C soaring pilots in the United States is increasing. Until then...