L-NAV
The
airspeed displayed on my L-NAV does not match my
mechanical ASI, what is wrong?
Please explain the real working of the
"total energy final glide" feature.
How
do I go about wiring the external warning switches for my
L-NAV?
Why
doesn't L-NAV V5.x contain the ability to input the
start gate height?
What is the difference between an L-NAV and
an S-NAV?
How do I go about having my L-NAV upgraded
to the latest version?
How do I go about having my S-NAV upgraded?
The airspeed on my L-NAV is wrong, how can I
check my plumbing?
Why are the airport elevations missing from
my L-NAV when it's connected to a Filser GPS?
How do I install my GPS-NAV and L-NAV in a
two place ship?
The airspeed displayed on my
L-NAV does not match my mechanical ASI, what is wrong?
I begin by making a
summary of the problem as you have described it. The True
Airspeed (TAS) indication of the L-NAV is not accurate.
Your latest description is that at an airspeed of 90
lm/hr, the L-NAV Indicated Airspeed (IAS) varies rapidly
from 0 to 60 km/hr and keeps changing within these values
all the time. The earlier description is that at an
airspeed of 120 km/hr, the L-NAV TAS is between 40 and 60
km/hr. Also the L-NAV TAS changes a lot, and therefore
the L-NAV Wind indication changes a lot.
Your L-NAV is new. The
problems you describe are EXTREMELY unlikely to come
from the L-NAV itself. The problem is most likely in the
source of Pitot and Static pressures that the L-NAV is
using to measure IAS. In DG gliders, the L-NAV Pitot and
Static pressures come from the "Triple Probe"
mounted on the glider's vertical fin. Pitot pressure
comes from a small hole in the front of the probe. Static
pressure comes from the ring of small holes about 30 mm
back from the probe tip. The Total Energy (TE) probe is
also part of the "Triple Probe".
A properly functioning
"Triple Probe" will show very large errors in
Pitot - Static pressures when the DG-808b engine is
running. This is because of strong turbulence from the
propeller. Therefore, an L-NAV cannot be expected to make
accurate IAS -> TAS -> Wind measurement when the
engine is running.
DG has problems with
"Triple Probes" supplied with their gliders. I
have personal experience with this. A Cambridge customer
was extremely angry with us because his L-NAV was
"not working right". The pilot sent his L-NAV
back several times and we could find no problem. I flew
from Vermont to Florida for a weekend to find and fix the
problem. After many hours of troubleshooting, I found the
problem was in the DG-500 "Triple Probe". The
probe did not fit properly into the socket. This caused
air to leak between the ports of the probe. The result
was bad IAS measurement and bad Total Energy
compensation.
Unfortunately, DG does not
mark the "Triple Probe" to show when it is
fully inserted. The DG-500 probe went in a
"reasonable" distance and then stopped. After
several hours of careful measurement, I discovered that
the probe needed to be inserted ~ 1 cm more before the
o-ring seals would work properly. Either the probe
diameter was too large, or the o-ring seat within the
probe socket was too small. After carefully sanding the
end of the 8 mm diameter probe tube, we removed the
chrome plating. and a little of the brass metal. Finally,
we were able to push the probe fully in. After that,
there were no more air leaks and the L-NAV worked
perfectly.
Here are some hints for
finding air leaks in your "Triple Probe"
system.
1. Check the probe
diameter near the end where it goes into the socket. It
should be less than 8 mm. Mine is 7.95 mm.
2. My "Triple
Probe" goes in 102 mm from the small tip. It is not
a DG probe, so I am not sure what the proper insertion
depth is for your glider.
3. There are 3 internal
O-rings in the socket. One is ~ 5 mm from the outer end.
There is a small O-ring for Pitot port at the back of the
socket. The third O-ring is about 20 cm from the back of
the socket. This O-ring makes a seal between the Static
Port and the TE Port of the probe. The problem with the
DG-500 probe was that the 8 mm diameter outer tube would
not pass through this O-ring. However, when the probe is
held firmly in place, the third O-ring does make a seal
against the tapered end of the probe's 8 mm diameter
tube. This makes it very difficult to discover leaks when
you are working on the ground.
A properly functioning
"Triple Probe" can be pulled back ~ 1 mm from
full insertion without causing a leak. During leak
testing, pull the probe back ~ 1 mm from full insertion.
This will keep the third O-ring from making a
"false" seal against the tapered end of the 8
mm diameter tube.
I presume you will find a
defective "Triple Probe" system in your DG-800.
The next task is to make the L-NAV work properly with a
defective "Triple Probe". Here are my
suggestions:
1. Use the Glider's Nose
Pitot and Fuselage Static ports for L-NAV airspeed
measurement. The simplest way to do this is to find the
Pitot and Static sources for your mechanical airspeed
indicator. Connect the L-NAV Pitot and Static ports to
the same places. This has the additional advantage that
you will get good Airspeed and Wind indications while
your DG-808b engine is running. Cambridge recommends this
as a permanent solution to airspeed measurement during
engine run.
2. For the "Triple
Probe" TE port to work properly, you must seal off
the Pitot and Static ports of the "Triple
Probe" system. This is easy to do at the instrument
end of the system. Just connect the Pitot and Static
tubing together with a straight tubing coupler. However,
you must also seal off the probe end of the ports. Put a
piece of tape over the "Triple Probe" Pitot
port. Also, put a piece of tape around the Static ports.
Then the TE probe will work properly.
Please explain the real working
of the "total energy final glide" feature.
If you are flying at 100
knots on your final glide, your glider will gain more
than 300 feet when you pull up and start your landing
pattern. The kinetic energy (speed) of your glider can be
converted into potential energy (altitude). The L-NAV
adds this altitude to the gliders altitude in the
computation of altitude required for the final glide.
When flying with this feature, you will notice that the
computed differential altitude required will be
independent of the glider's airspeed. This was not the
case with our earlier design that did not take the
glider's kinetic energy into account.
When you increase the
MacCready setting, the L-NAV tells you to fly faster. If
you actually fly faster, you are moving to a part of the
polar curve that makes the glide less efficient. You will
get there faster, but you will be lower when you arrive.
This is why the L-NAV shows an increased Altitude
Required when you increase the MacCready setting.
In the latest version of
the Pocket-NAV, you can make the L-NAV and Pocket-NAV
talk to each other. This means you can change the
MacCready setting on the L-NAV, and the change will
appear on the Pocket-NAV.
How do I go about wiring the
external warning switches for my L-NAV?
This is covered in the
L-NAV Manual but a bit of clarification is in order.
Click here
to view a schematic diagram of the wiring.
All switches are
"normally open" type. Hold switch should be
open in cruise mode. Gear switch should be open when gear
is down. Spoiler switch should be open when spoilers are
retracted.
The choices of which
switch to use and how to mount it is left up to the
pilot.
Why doesn't L-NAV
V5.x contain the ability to input the start gate height?
With Version 5 L-NAV, we
did eliminate the 5000 foot "Start Height" for
points with the S attribute. This was done after
extensive discussions with pilots here and overseas. The
decision was not an easy one. Here are the reasons:
1. We found mistaken use
of the S attribute in many Turnpoint Databases. The Home
field was typically given attributes STFHA. The S game
requires a separate point (perhaps with the same
coordinates as the finish line). So pilots got goofy
Required altitudes (using Start Height) when gliding to
the finish line.
2. New rules for
international and US competitions using GPS specify a
start as an exit from the top or the side of a cylinder 5
km in radius around the start point. The concept of a
glide to a "Start point" has no meaning for
this type of start.
Please note that the
Version 5 L-NAV User's guide does note the removal of
special treatment for S attributes.
What are the differences between
the S-NAV and the L-NAV?
The S-NAV was designed in
1986 to do everything a pilot might want in a gliding
instrument. Its hardware and software have been upgraded
many times as we and our customers learned what is
useful. Building on 5 years of experience, the L-NAV was
designed in 1991 with a simplified user interface to do
what 90% of pilots want 90% of the time.
The S-NAV supports two
variometer displays. The second display can be configured
for 7 different functions. The L-NAV has one variometer
display.
The S-NAV has a high
resolution, 12 bit A/D converter; the L-NAV has a 10 bit
A/D converter. This gives the S-NAV the following extra
capabilities:
An aerobatic g-Meter with ±10g
range
Altimeter range of 0 - 36,000
ft. (L-NAV is 0 - 25,000 ft)
Electronic fine tuning of Total
Energy Compensation
How do I go about having my
L-NAV upgraded to the latest version?
You may return your
instrument to the factory or any of our Service Dealers
for the upgrade. You also have the option of installing
the firmware ROM yourself.
The cost of the upgrade is
$250.00 (U.S.). If you purchased your L-NAV after October
1, 1997, there is no charge for the upgrade.
If you are interested in
doing the upgrade yourself please see the upgrade section
of our website.
If you have a two seat
installation you should return your L-NAV and the
repeater to the factory or Cambridge Service Dealer as
the firmware in the repeater must be upgraded as well.
There is no additional charge for the repeater upgrade.
How do I go about having my
S-NAV upgraded to the latest version?
The S-NAV may also be
upgraded by the pilot. For further details see the
upgrade section of our website. The upgrade cost of the
S-NAV is $250.00 (U.S.). If the S-NAV is used in
conjunction with a repeater, the repeater must also be
returned. The cost of the upgrade for the S-NAV repeater
is $100.00 due to the replacement cost of the
microprocessor.
Depending on which
firmware version your instrument is presently at, there
may be additional charges for hardware necessary to
support the upgrade. Please call the service department
for specific details.
The airspeed indicator on my
L-NAV seems to be off, how can I check my plumbing?
When checking for leaks,
connect a "Y" tube to the pitot and static
(airspeed). In the calibrate screen you should see
no variation in flow when you suck on the "y "
tubing, (watch the numbers-pitot 1012, vario 511) . Do
the same for vario-between The T.E. port and the capacity
port.
I have connected my L-NAV to a
Filser LX400 Flight Data Recorder and do not see the
airport elevation on the L-NAV screen, why?
This problem (no airport
elevations reported) occurs because the Filser does not
supply the proper sentence for the L-NAV. In order for
the L-NAV (or S-NAV) to receive airport elevations from a
GPS, the data from the GPS must contain the sentences
PCAIB and PCAIC. The Cambridge GPS-NAV is the only GPS
Flight Data Recorder that transmits these sentences.
How do I install my GPS-NAV and
L-NAV in a two place ship?
Click here to view written instructions.
Click here
to view wiring diagram.
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