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Ken: Interesting about the static ports. I many times had occasion
to fly the B-727 that was Boeing's test ship. You could always tell
it was that one because it had several sets of static ports on the
lower fuselage side, just forward of the baggage pit door. As I
recall, there were two experimental sets that were used, and they
settled on a third location. The all were only a foot or so apart.<br>
Bill Glaze<br>
<br>
Ken Blackwell wrote:<br>
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<div><font face="Arial" size="2">Earl, Folks,</font></div>
<div> </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> </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> </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> </div>
<div><font face="Arial" size="2">Cheers,</font></div>
<div> </div>
<div><font face="Arial" size="2">Ken B.</font></div>
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Original Message ----- </div>
<div
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<a title="ehaury@houston.rr.com" href="mailto:ehaury@houston.rr.com">Earl
Haury</a> </div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><b>To:</b>
<a title="discussion@nsrca.org" href="mailto:discussion@nsrca.org">discussion@nsrca.org</a>
</div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><b>Sent:</b>
Friday, January 14, 2005 3:24 PM</div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><b>Subject:</b>
Re: Pattern Help</div>
<div><br>
</div>
<div><font face="Arial" size="2">Bob</font></div>
<div> </div>
<div><font face="Arial" size="2">Correct on instrument methodology.
You may be correct regarding observed readings also. </font></div>
<div> </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> </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> </div>
<div><font face="Arial" size="2">Earl</font></div>
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Original Message ----- </div>
<div
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<a title="bob@toprudder.com" href="mailto:bob@toprudder.com">Bob
Richards</a> </div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><b>To:</b>
<a title="discussion@nsrca.org" href="mailto:discussion@nsrca.org">discussion@nsrca.org</a>
</div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><b>Sent:</b>
Friday, January 14, 2005 2:38 PM</div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><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 <a class="moz-txt-link-rfc2396E" href="mailto:ehaury@houston.rr.com"><ehaury@houston.rr.com></a></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> </div>
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