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echristley(at)nc.rr.com
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 Posted: Thu Aug 31, 2006 5:39 am Post subject: Static port on Dynon AoA probe |
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This only has a cursory connection to electronics in airplanes, but I
still thought it would be the best place to ask.
The Dynon AoA probe doesn't have provisions for a static port. It was
designed with the expectation that the ports would be located on the
sides of the fuselage. That's not such a good location in a Delta. The
only flat space to put a port would be along the turtleback, aft of the
cockpit, which would put them deep inside of the wing's wake. The
readings would be all over the place. The plans location is for the
static probe to be low on the front leading edge, along with the pitot.
I have to build a mounting post for the Dynon probe (again, the
available mounting kits are designed for RVs, and don't match up to the
Delta's curves and angles) which is basically a streamline tube flaring
into the wing. I can see two options:
1) build another tube into the streamlined mounting post pointing forward
2) design ports into the side of the streamlined tube mounting post
The second option would look nicer and be aerodynamically cleaner (for
a sufficiently small value of aerodynamically clean). The question is,
where along the streamline crossection would I put the port in order to
reliably pick up static pressure? Would it be better to have a length
of flat section to put the ports in, and if so, how big would a flat
spot need to be? I'm making this mounting post from composite, so
complex shape isn't an issue.
--
,|"|"|, Ernest Christley |
----===<{{(oQo)}}>===---- Dyke Delta Builder |
o| d |o http://ernest.isa-geek.org |
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brian-av(at)lloyd.com
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| Posted: Thu Aug 31, 2006 12:07 pm Post subject: Static port on Dynon AoA probe |
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Ernest Christley wrote:
| Quote: | 1) build another tube into the streamlined mounting post pointing forward
2) design ports into the side of the streamlined tube mounting post
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If you examine the pitot tube on a Yak-52 or CJ6A you will find that the
static ports are in a ring around the pitot tube about 1" back from the
front. The whole assembly sticks out of the leading edge of the wing by
about 16". It works just peachy and the errors are minimal.
Brian
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khorton01(at)rogers.com
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| Posted: Thu Aug 31, 2006 12:44 pm Post subject: Static port on Dynon AoA probe |
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On 31 Aug 2006, at 09:38, Ernest Christley wrote:
| Quote: |
<echristley(at)nc.rr.com>
This only has a cursory connection to electronics in airplanes, but
I still thought it would be the best place to ask.
The Dynon AoA probe doesn't have provisions for a static port. It
was designed with the expectation that the ports would be located
on the sides of the fuselage. That's not such a good location in a
Delta. The only flat space to put a port would be along the
turtleback, aft of the cockpit, which would put them deep inside of
the wing's wake. The readings would be all over the place. The
plans location is for the static probe to be low on the front
leading edge, along with the pitot.
I have to build a mounting post for the Dynon probe (again, the
available mounting kits are designed for RVs, and don't match up to
the Delta's curves and angles) which is basically a streamline tube
flaring into the wing. I can see two options:
1) build another tube into the streamlined mounting post pointing
forward
2) design ports into the side of the streamlined tube mounting post
The second option would look nicer and be aerodynamically cleaner
(for a sufficiently small value of aerodynamically clean). The
question is, where along the streamline crossection would I put the
port in order to reliably pick up static pressure? Would it be
better to have a length of flat section to put the ports in, and if
so, how big would a flat spot need to be? I'm making this mounting
post from composite, so complex shape isn't an issue.
|
There is no reliable way to predict an exact probe profile and
location that will give an acceptable static source. I have been
involved with two projects that tried to chose locations for static
pressure ports. One project, on a type-certificated light aircraft,
used engineering analysis and experience to chose a static port
location. The chosen location had large errors, and the easiest
solution was to launch into a fly, fix, fly approach, using a
modified Piper pitot-static port. They kept on changing the angle on
the bottom of the Piper port until they found an acceptable
compromise. I recall that there were at least four configurations
tested, maybe more.
The other project, on a large business jet, had a much larger
budget. They used computational flow dynamics (CFD) computer
predictions to chose a static port location. The chosen location
produced such large errors at high angle of attack that the indicated
airspeed would stagnate as you approached the stall, then it would
start increasing as you got closer to the stall. It was fixed by
putting a very complicated correction curve in an air data computer.
Bottom line - the best bet is to copy exactly what someone else has
done, assuming they have a successful installation with acceptable
errors. If you start experimenting, the best chance of success is to
get the static port as far ahead of the wing as possible. Any
location near the wing is a recipe for trouble, as the wing's purpose
is to modify the local pressures to create lift, and a static port
needs to see the far-field ambient pressure. If you put a "roll your
own" system near the wing, expect to find large errors. If you enjoy
flight testing, this is a good thing, as it will keep you busy for
months doing fly, fix, fly.
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://www.kilohotel.com/rv8
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echristley(at)nc.rr.com
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| Posted: Fri Sep 01, 2006 8:58 am Post subject: Static port on Dynon AoA probe |
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rparigor(at)suffolk.lib.ny.us wrote:
| Quote: | Hello Ernest
"The Dynon probe does nothing more than implement what this gentleman
describes. As I understand it, the exact dimensions aren't critical, as
it has to be calibrated for each airplane anyway. Look on the Dynon site.
They give the calibration procedures, and it is exactly what Mark and
David describe. As for exact dimensions:
3/4" aluminum tube that tapers down to 3/8" for the pitot, which is pushed
up to the top.
The bottom of the head has a triangular slice cut off at a 30 degree angle
from the bottom. The AoA sense hole is 5/8" from the tip. A little less
than 8" long."
I am not certain that the Zodiac home made probe is doing the exact same
thing as the Dynon.
http://www.ch601.org/resources/aoa/aoa.htm
What size is the size of the port hole of the Dynon AOA hole?
What angle is the port hole drilled in relation to the sliced face?
Is the sliced face flat or has a slight radius?
Thx.
Sincerely
Ron Parigoris
It's about a 1/16th hole drilled tangentially to a flat face. The hole
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size and location aren't critical. The critical part is that you need
two 'pitot' ports pointing forward at different angles. As you change
angle of attack, the relative pressures on these two ports will change.
It's a relative pressure between just those two ports, so static
pressure or absolute dynamic pressure (from airspeed) are unimportant
and ignored. As you slow down to where the wing can't maintain
altitude, the wind is not coming straight at you, even if the nose is
still on the horizon. Riding the edge of stall with a power out, you
could be headed down at a 45degree angle even though the plane is in a
level attitude. As you're sinking, it is coming from below. The lower
pitot will 'feel' more of the rising air than the forward pointing one.
How much more it 'feels' will rely completely on AoA. Now all that's
necessary is a sensitive differential gauge to measure both pressures,
the calibration procedure will set a couple of points that correspond to
"doin' fine" and "lift all gone". Just throw in a semi-arbitrary red,
yellow and green ranges, and you're good to go.
It really is a simple yet profound concept.
--
,|"|"|, Ernest Christley |
----===<{{(oQo)}}>===---- Dyke Delta Builder |
o| d |o http://ernest.isa-geek.org |
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livingjw(at)earthlink.net
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| Posted: Sat Sep 02, 2006 10:38 am Post subject: Static port on Dynon AoA probe |
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The pressure differential, and hence needle position, will be proportional
to angle of attack but the amount of needle movement for a given change in
AoA will change with speed (i.e. dynamic pressure). You need to divide the
differential signal by the dynamic pressure to obtain a signal that does not
vary with speed. Then you can correlate the signal to AoA independent of
speed. For an aircraft with a limited upper speed range the instrument as
described might be useful, but it isn't quite an AoA indicator.
John
--
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