nuckolls.bob(at)aeroelect
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 Posted: Wed Feb 16, 2011 7:31 am Post subject: Heated Pitot Tubes - Some first hand history |
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At 02:09 AM 2/16/2011, you wrote:
Bob
I just noticed your comments about loss of IAS on both sides of the
Beechjet. l attended 2 of your Seminars and I know we talked about
both having worked for Cessna. When I left Cessna as a Flight Test
Engineer in 1968.
I'd forgotten that conversation. Were you working at the Pawnee
or Wallace plant? I was a tech writer at Cessna from about 1964 to
1969 . . . just a few months over 5 years.
I went to work for the Army Aviation Systems Cmd in St Louis as a
Dept of Army Civilian Senior Aeronautical Engineer and was there for
28+ years. Most of my time there was spent with the Army's Fixed Wing
Electronic Intelligence Gathering Aircraft. In the later years I was
the Army Technical Manager for 7 different versions of the RC-12
(Super Kingair 200), the last 4 versions comprised of 36 aircraft
with 1200 SHP engines and Gross Weights in the 16,000 to 16,500 lb
range. In the early '90s, we took the RC-12N configuration which had
the largest antenna array to Duluth, Mn in January for natural icing tests.
The Army test crew from EAFB had a specially instrumented U-21 to
search for icing conditions and would then call out the RC-12N. I
rode with the Army test crew on flights 2 and 3 as a FTE/ Observer.
On flight 2, we were in a low stratus layer around 3000 ft MSL, 140
KTS, about 45% cruise power. At 50 minutes we has collected 5 1/2
inches of ice with all of the deice going, at 90% cruise power for
the same air speed and had just lost the right pitot airspeed. About
3 minutes later we lost the left pitot airspeed. About 2 minutes
later, we climbed out of the low stratus layer so the U-21 could get
photos of the ice buildups. Fortunately the U-21 was able to get a
picture of the right pitot which showed that ice had grown forward on
the unheated pitot mast and then curved downward to block the pitot
inlet, at no time actually touching the pitot tube itself. Before the
U-21 could move to the left side, the left mast had lost enough ice
that the left airspeed had come back and a few minutes later the
right airspeed was back.
Wow! That would be some interesting pictures to have
in the library. Talk about tweaking the tail of the
tiger. I have 2 hours of dual in a King Air. I had
the electrical/avionics group at Lear for the GP-180
program and Collins came down in their company demonstrator
to show me some new features of their autopilot system.
The demonstrator pilot was also an instructor and
he invited me to take the left seat.
One of the most profound demonstrations of the
King Air (after we BACKED out of a parking
space) was the autopilot's ability to quickly
manage a loss of engine event. Climbing out we
were showing something like 2700 fpm and he
pulled one throttle to zero thrust. The airplane
barely yawed and the only thing that happened
was that noise level dropped and rate of climb
decreased to 1500 fpm. Now THAT'S what I call
single-engine performance.
If there was ever an airplane capable of staying
in the air under the most adverse of conditions,
it would have to be the twin turboprops. I had to
run home after that flight to get my log book. Didn't
want to miss getting the experience recorded. I also
have 2 hours of dual in a Baron and got a chance
to play with single-engine performance of that
airplane. The differences are profound.
Beech attempted to get some heat in the mast , but abbreviated tests
the following winter showed that it was insufficient. Bob, I don't
know whether the Beechjet used the same pitot units or a similar
design, but that was our findings on the Model 200.
Back in that era, pitot tubes had a LOT in common
including the ways they distributed heat. While working
the BE400 pitot heat issues in 2002 I discovered that the
heaters are long, tubular devices that snake
from each connector pin through various locations
and designed to create localized hot-spots. These
are not linear, uniform toaster heaters but specifically
tailored to distribute heat based on tests in icing tunnels.
In this case, a percentage of total heat was dumped
into the mast.
Further, the inside of a pitot tube can feature
a labyrinth of passages, chambers and 'melting
pots' tailored to stand off the various effects
of icing. See:
http://aeroelectric.com/Pictures/Pitot_Tube/
Another interesting feature of pitot tube
performance is illustrated by the plotted
flight test data (gathered with my po' boy's
laptop based instrumentation system). At 40KFt
-30C RAT and over 200Kts IAS, temperatures
at some locations on the tube were well over
100C. Now would like to believe there's no way
that icing could be a factor in the loss of
IAS . . . yeah . . . right.
After the studies were completed on the 400 I think
they re-distributed some heat and moved some drain
holes. Unfortunately, they could not increase total heat
without impacting the DC load studies for the
whole airplane. We had to live within the existing
power budget in spite of the fact that for 99.9%
of all operations in ice, there was power to burn.
Any heated pitot tube installation certified in
the last 30+ years is probably blessed with a heated
pitot tube that is "similar" but not identical
to other installations. Of course, ANY heated
pitot tube adapted to a single engine light
airplane can be expected to perform pretty much
as advertised. I don't think there are 14v
airplanes certified for flight in to known ice
that would also have finely tuned pitot tube
heater systems. Virtually all supper-whizzy
heated tubes will be off a pretty heavy duty
foul-weather capable airplane and will have
a 28v system.
Thanks for sharing your experience with us.
Bob . . .
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