At 11:14 AM 28/08/04 +1000, you wrote:
On a different note.
Has any Libelle owner experienced ASI error due to the position of the
pitot tube. GBX has a small flattened tube at the top of the aero tow
release. My suspicion is that it is under-reading. This follows checking
against GPS on opposite headings where it seems to indicate a permanent
tail wind.
ASI has been checked OK during form 2 inspections.
I have not checked plumbing at this stage. It is difficult to connect to
the end of the flattened tube.
I am aware that other Libelles only have a connection to the chamber in
front of the bulkhead.
Any comments. (apart from may the wind always be on your tail).
Colin,
Do you have a flight manual?
First check the position error curve and see what the relationship is
between calibrated and indicated airspeed. This may explain part of your
problem.
The next thing is to realise that at 5000 feet for example the True
airspeed(the speed at which the air is moving past your glider) is about
10% higher than at sea level. So even if your glider doesn't have any
significant position error (think of it as static ports that suck or blow
slightly due to their location on the glider and/or a pitot that isn't
aligned straight on to the airflow due to location or angle) and you fly at
say 70 knots indicated and there is no wind your GPS will show you are
flying at about 77 knots. (no wind so groundspeed = TAS). This will look
like tailwind. Wind doesn't change this except that your GPS will read a
lower speed into wind and higher downwind.
This once had an interesting consequence for a couple of well known
Australian glider pilots.
About 20 years ago Paul Mander and the late Klaus Breitkreutz were doing a
500Km our and return out of Narromine just before a NSW State contest in
there ASW 20's. They had the then new Cambridge CNAV glide computers and
the old style flow sensor Cambridge varios. It was a good day with max
altitudes in the region of 10000 feet. There wasn't much wind but heard on
the radio after the turn from Klaus was "there CAN'T be a tailwind on both
legs". There wasn't of course but I found out then that there wasn't an
altitude sensor in the CNAV and it assumed you were at about 3000 feet all
the time. At 10000 feet compared to 3000 feet the difference in TAS is
about 10% so if you are cruising at 100KIAS(110KTAS) in your ASW 20 at
10000 feet or so as far as the glide computer is concerned it says you have
10 Knots tailwind. And it doesn't care which leg of the O/R you are on.
Hence the puzzled comment.(this was all pre GPS of course the glide
computers worked by dead reckoning. Distance travelled is airspeed x time
plus wind x time.)
Shameless plug: we did it right in the B20 system and since.
If any of this IAS/CAS/TAS stuff is news to any of you, you need to do a
websearch. Start at the EAA site.
Mike
Borgelt Instruments - manufacturers of quality soaring instruments
phone Int'l + 61 746 355784
fax Int'l + 61 746 358796
cellphone Int'l + 61 428 355784
Int'l + 61 429 355784
email: [EMAIL PROTECTED]
website: www.borgeltinstruments.com
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