As long as the damage is in the stagnation area
probably won't make a lot of difference but side
gusts and sideslip will take it out of that area.
Probes work by laminar flow separation. If you
trip the flow to turbulent the flow changes and
the pressure coefficient will change.
A perfect probe has a pressure coefficient Cp of
-1.00 and doesn't vary much from this as normal
values of sideslip and pitch change occur.
A pressure coefficient of -1.00 means that the
pressure is below static pressure by the same
amount that pitot pressure is above it. A pitot
tube head on to the flow has a Cp = +1.00
At 100 KIAS the pitot pressure above static is
close to 16.2 hPA so if your probe Cp changes due
to sideslip or side gusts by 2% this is is 0.02 x
16.2 or 0.324hPa or at sea level nearly nine feet
so if you boot the rudder or get a side gust and this happens over one
second the vario will see a pressure change of
nearly 9 feet of atmosphere in one second or in excess of 5 knots.
Probe design has not changed much in 40 years
since Frank Irving designed the Irving probe. It
is pretty good. The two whisker probe with slots
is harder and more expensive to make but also
good. Interesting that I have a paer describing it dated 1976. It is called a
Bardowicks probe. The Irving probe came along
just afterwards and is a lot easier to manufacture.
The drag of these probes is an issue as a round
tube at nearly right angles to the airflow has a
drag coefficient Cd of 1.0 to 1.3, resulting in a
drag at 100 KIAS that is of the 0.5 to 1.0% order.
At Benalla I talked to Johann Bosmann, the
Jonkers aerodynamicist and designer and he was
appalled at one of the Diana 2's that had a two
foot long 2.5 mm diameter radio antenna sticking
out of the turtledeck. He'd calculated the drag. We also agreed the usual
Irving/Bardowicks designs were high drag and he
was minded to do something about that but the
student he had working on that (she did contact
me for some guidance) is now pursuing another
project. I have a couple of ideas that might make a lower drag probe that is
still insensitive to yaw and sideslip.
Unfortunately all these types of probes will help
your vario tell lies much of the time as they
system is sensitive to horizontal gusts. The
problem depends on the square of the True
airspeed at which you penetrate the gust but I
was surprised at how much the vario was
effected by these even in thermals. I mentioned
this to Johann Bosmann and said that they must
see that their varios basically don't work in
high speed cruise in South Africa where they
cruise at 110KIAS between thermals at 15000 to 18000 feet. He thought about it
for a couple of seconds and agreed.
This was revealed by a recent Dynamis flight
test where we got good video of a Dynamis driven
B800 alongside the normal pressure B800. Movies released soon!
Mike
10:03 PM 2/18/2017, you wrote:
Hi all,
My TE probe has some wear/corrosion on the
leading edge so is not perfectly round (quite
un-perfect actually). How will this impact its performance?
Cheers Ben
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