Methods of ASI / PEC calibration
 

Just an idle question really, but I've been doing a lot lately on ASI calibration methods.

Worldwide, what are the preferred methods for PEC determination of the various test centres these days?

And is anybody actually still using the formation method?, or has that died out completely.

G

Re: Formation method for ASI calibration
 

Difficult to use any method other than fly by of another aircraft for supersonic PEs?

Re: Formation method for ASI calibration
 

Groundspeed method would work with supersonic PEs, but I imagine an INS or differential GPS may be required for sufficient accuracy.

Re: Formation method for ASI calibration
 

Most of the work I've done in recent years has admittedly been firmly subsonic, but has relied upon GPS groundspeed truth data. If you've enough sky, I can't see that the method is invalid for supersonics - how optimal it is will presumably depend upon other factors (particularly the availability of a calibrated chase).

But anyhow, what is everybody using these days?

G

In the last 10 years I have used both the chase and trailing bomb methods up here (rented the bomb from Boscombe). That one was to get accurate (ish) PE data for a BO105 with radar on the nose and it worked very well.

Speed course, tower fly by, GPS groundspeed, trailing static and trailing pitot/static methods. Never found anything to beat tower fly by for static pressure errors. Bear in mind that you can only use groundspeed methods on it's own to calibrate an ASI if you already know the static errors, or if you know the pitot errors are negligible.

Tower fly-by in the Bell 47 at EPNER was good fun....and accurate.

From a rotary wing perspective, it really depends to what equipment you have access .... ie a range, tower/kinetheodolite, trailing static bomb, trailing pitot/static bomb, keil tube for acurate pitot measurement, gps or another 'calibrated' aircraft on which to formate.
Before starting, calibration of the ASI or Altimeter is required to establish instrument errors.
The course - ground speed method requires an airfield, a simple stop watch and accurate flying into and downwind. Light and constant winds are essential. It only measures ASI PEs.
The GPS triangulation method is less dependent on wind strength although steady conditions are important. Again only ASI PEs are measured.
The trailing bomb (pitot/static or just static) methods do provide good data, both for altimeter and ASI PEs by comparing the calibrated ASI or Altimeter reading from the bomb to those on the aircraft. The PEs can also be measured in the descent and climb - particularly important when establishing the altimeter errors on an instrument approach. However, carriage of a bomb on a long line (30 - 50 metres below the aircraft) needs careful consideration. How is it attached ?(by a weak link and/or a crew-controlled hook), built-up areas must be avoided, where does it fly at high rates of descent ? (emergency entry into autorotation - contact with tail rotor?), HV considerations when picking up and reposing the bomb.
The KEIL tube and trailing static is a method preferred by Westlands. However the KEIL tube - an instrumentation boom attached to the nose with a pitot (often swivelling) - requires some careful engineering (stress and resonance considerations). The correct distance in front of the nose of the aircraft is essential to place the pitot well ahead of the stagnation area. I am unsure of the system's efficacy for a helicopter since downwash and angle of attack considerations in high rates of climb/descent may be suspect.... better for fixed wing aircraft with a more limited AOA range. In reality the system itself should be calibrated on the aircraft by another method. If this is done, then it is a useful piece of equipment for flying very accurate airspeeds for further flight tests and can be moved from aircraft to aircraft of the same type for PE measurements after modifications.
For a quick and dirty (?!?) sanity check, the formation method is simple and quite accurate. Precise formation flying is required in non-turbulent air.... national airworthiness agency approval may be required for this activity. The errors of the lead aircraft must be known (do you trust the manufacturer's manual data?). The ASI and altimeter readings are thus noted at each test condition and compared.
With a few calculations to be made - that's about the sum of it!