Differences in Static Pressure

[NinjaBill]

Hi there all

Just a quick question, im currently studying for my PPL, and am currently reading vol4 of the Thom series - Technical, and I have a couple of questions.

Firstly regarding the positioning of the static vent.


quote.. "The alternate source (in an unpressurised cockpit) often taps static pressure from within the cockpit, which is slightly less than outside pressure." Can anyone axplain why this is.

And secondly there is another parograph on why a IAS/TAS indicator using a temp/alt correction scale is only accurate upto apporx 220kts due to the compressibility of air, can anyone explain why this is in simple terms. I understand that this may be a case of RTFM, in which case could someone point me to a link which would explain this. Or if this is not available on the web, which would be a good book to read up on such a subject?


Thanks

NB

[BOAC]

Some would say it is due to the rarified atmosphere we pilots create, but more likely due to the pressure drop as the air flows over the canopy (cf wing upper surfaces)

Regarding second part - do you understand compressibility? If not, I suggest you try a search on Google for starters? In simple terms, 220 is the speed at which compressibility effects are considered to become significant and therefore distort the accuracy of pure pressure readings.

[john_tullamarine]

(a) will vary from aircraft to aircraft. Static port location is a major part of the certification test program - the alternate is permitted considerably more latitude in maximum error. Normally the alternate error corrections, while considerably more than for the primary system, are not overly great. There are some design standard requirements which require that the primary errors are tightly constrained.

(b) the ASI is a differential pressure gauge - not a speed gauge. Rather than calibrate the dial in pressure units which might confuse people, we use a standard equation to relate pressures to an equivalent speed value. However, this equivalence is accurate only at sea level for standard conditions. Elsewhere, the ASI tells fibs. One of the fibs is covered by the normal density correction using either a prayerwheel or the instrument equivalent adjustment ring. Another is due to errors associated with the compressible nature of air - if you don't allow for this error at higher speeds, then the instrument reading will include a not insignificant error. What speed you might get concerned about compressibility corrections is not all that critical but around 200-250 kt IAS at low level is probably a reasonable point to become interested.

If you want to check out the background, have a look at any standard undergrad aerodynamics textbook or else at some of the relevant sites listed in the Tech Log URL sticky. However, unless your maths is up to speed, you might find the details more confusing than illuminating. From a practical point of view, it is more important to be aware of the error's existence and correction, than the derivation from first principles ...