Saturday, December 20, 2014

High Angle of Attack Total Pressure Measurement

Measurement of airspeed at high angles of attack requires specially designed devices. In this article we introduce the Kiel total pressure probe. I'll introduce a BasicAirData design that will be tested during the 2015 year.

In a conventional Pitot tube the total pressure measurement is sensible to the angle of attack. For conventional accuracy and operating ranges, let's say for angle of attack under 10°, the impact of such a non ideal behavior is commonly acceptable. However as the angle of attack is increased beyond 20 degrees we register a remarkable loss of accuracy.

Figure 1. Kiel probe mounted on a NASA X31 Experimental plane. Source NASA Book page 7. That Kiel probe was the cause of X31 fly accident. 

To reduce probe sensitivity to the operating angle of attack and side slip a sleeve is put over a conventional total pressure tube. Have a look to the following figure
Figure 2. Kiel probe design extracted from NACA-tn-2530

The figure is a Kiel probe with a Venturi shaped shield. Nacatn-2530 reports the performances of such shield on table 1, first row. That probe can handle about 40°, appreciate the measure error vs angle of attack on page 15 Figure 5. In this Nacareport the probe is considered to operate correctly if there is a total pressure error under 1%.

Alternative designs are available, there are many industrial fields of application. For a commercial probe example have a look to figure 3.

The Flowkinetics probe is rated up to 49°, of course compared to a standard Pitot solution is remarkable; probes with similar design are used in HVAC and turbo machinery applications.

In the next period of time at BasicAirData we will test a Kiel like probe. The probe head will be as per the next figure. The inlet of the shield will have an elliptical shape.

Figure 4. BasicAirData Pitot head shield draft model

You find here below a video that includes the total pressure pipe.

Video 1. Preliminary High Angle of attack Pitot Head

Target performances requirements are, operating airspeed from 3m/s to 55m/s. Measurement error should be under 1% for angle of attack under 50°.

Follow up in this article.
Post a Comment