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<DIV><FONT face=Arial size=2>Earl, Folks,</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Speed is calculated from a thing called
dynamic pressure. (dynamic pressure = total head pressure minus static
pressure). dynamic pressure = 0.5 x air density x velocity squared. So what we
have to do is measure both total (pitot) and static pressures to be able to
calculate airspeed....then there are a few more complications like factoring air
temperature into the density part of the equation.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>It's mostly the bad static pressure measurement
that causes the problem with our trying to measure airspeed with the little
model data recorders. Total head pressure...that which enters the nose of a
pitot tube is fairly insensitive to angle of attack, but static pressure is very
sensitive to angle of attack. I think that these devices that we are using in
models take the static pressure from the inside the fuselage....and is therefore
inaccurate at any speed/attitude...but better in level flight. In the end the
best you can get is a relative reading for fairly steady
conditions.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>For full scale aircraft the designers/testers spend
a lot of time finding a static port(s) location that will yield fairly accurate
airspeeds over as wide a range of attitudes and airspeeds....then there is still
a correction factor. If you notice test aircraft pictures, they usually have big
booms on the wing/nose of the airplane that extend way out in front of the
aircraft. The boom is instrumented in several ways including pitot and static
pressure ports, along with angle of attack vanes and temperature sensors...etc.
In flight test they usually calibrate airspeed by several methods....etc. etc.
etc.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Cheers,</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Ken B.</FONT></DIV>
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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=ehaury@houston.rr.com href="mailto:ehaury@houston.rr.com">Earl
Haury</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=discussion@nsrca.org
href="mailto:discussion@nsrca.org">discussion@nsrca.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Friday, January 14, 2005 3:24
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Pattern Help</DIV>
<DIV><BR></DIV>
<DIV><FONT face=Arial size=2>Bob</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Correct on instrument methodology. You may be
correct regarding observed readings also. </FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>However, very high roll rates seem not to
generate the same speed reading effect. The pitot extends about 3/8" forward
the center of the wing LE in my installation. I've not seen any difference in
normal speed data with it varied 1/4" or so from that position.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Pressure anomalies will definitely affect the
altitude sensor, however I don't see the same "signature" on downline snaps.
I've not been specifically looking at snaps to this point, the data were
generated flying the P-05 sequence. I plan to look at this further in a
different (more expendable) airplane with a G sensor also. Possibly that
sensor can be oriented to provide pitch load and thrust (longitudinal) accel /
decel info. (Should receive sensor in the next couple of days.) BTW, data rate
is 10x/sec.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Earl</FONT></DIV>
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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=bob@toprudder.com href="mailto:bob@toprudder.com">Bob Richards</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=discussion@nsrca.org
href="mailto:discussion@nsrca.org">discussion@nsrca.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Friday, January 14, 2005 2:38
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Pattern Help</DIV>
<DIV><BR></DIV>
<DIV>Earl,</DIV>
<DIV> </DIV>
<DIV>I'm not familiar with that device, but I will make two assumptions. 1)
The altitude transducer works on barometric pressure. 2) The pitot tube
works just like on a full scale plane.</DIV>
<DIV> </DIV>
<DIV>I suspect the unexpected zero airspeed has to do with the airflow
at the tip of the pitot tube not being parallel with the pitot tube due to
the high rotational speed of the snap. In an extreme case, the airflow
traveling perpendicular could actually cause a vacuum in the pitot
tube.</DIV>
<DIV> </DIV>
<DIV>If the altitude measurement works on barometric pressure, there must be
a static air source. I suspect the vertical speed anomaly is caused
by a change in the static air pressure due to the sudden change of
airflow around the airframe, including the static air source.</DIV>
<DIV> </DIV>
<DIV>Just guesses.</DIV>
<DIV> </DIV>
<DIV>Bob R.</DIV>
<DIV><BR><BR><B><I>Earl Haury <ehaury@houston.rr.com></I></B>
wrote:</DIV>
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<DIV><FONT face=Arial size=2>While recently evaluating engine /
prop combinations (very top secret), I noted something that looks useful
regarding snaps. During both up and downline snaps the speed signal drops
to zero for a period of time roughly equivalent to snap duration. It
appears that possibly airflow separation from the wing includes the pitot
tube. Upline snaps also show a marked decline in ascent rate during the
snap. </FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
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