Pattern Help

[Earl Haury]

Bob, Ken, Jim

Appreciate your comments and agree with the items mentioned regarding pitot placement and function. Static pressure to the sensor can / has been a problem, more with some fuselage designs - less with others. (The older unit I had provided a connection to allow placement of the static input source - new one doesn't.) 

Airspeed accuracy is an unknown with these units for the reasons that you mention - but time through a measured distance suggests that they're reasonably close in level flight. Of course, for pattern flight / equipment comparisons, the true speed isn't very important as long as the data are repeatable and reproducible - and they seem to be, the information is in the deltas. The Eagle Tree graphing software appears to drop outliers and smooth the data some. Smoothing algorithms are necessary to render the data useful to look at maneuver information in Excel as well. Overall, pretty useful info regarding how one flew the pattern.

It's when one begins to look closely at the raw data that the "noise"  anomaly is noticeably consistent during the snaps. I agree with everyone's assessment regarding the pitot and shortcomings. However observed airspeed measurements before and after provide a reasonable delta across the snap - so measurement through the snap isn't of much interest. What I'm interested in understanding is whether or not the drop in signal can be used to ascertain the snap stall. I appreciate the thoughts regarding angular flow to the pitot and pressure changes within / around the fuse, both require a look. I do know that very high roll rates, and high pitch rates without stall, don't produce the same signature. Ah well - more to do.

Earl
  ----- Original Message ----- 
  From: Bob Richards 
  To: discussion at nsrca.org 
  Sent: Friday, January 14, 2005 9:30 PM
  Subject: Re: Pattern Help


  Earl,

  The 3/8" distance may not be enough. I suspect that in a stall/snap situation that there is enough air "spilling" around the LE to disrupt the operation of the pitot tube. (That's an over simplification, maybe)  I would try sticking it out maybe 2" or more just to try it out. Also, have you tried snapping in both directions to see if you get the same readings?

  On most aircraft the pitot tube is fixed, but on some test/experimental aircraft the pitot is mounted on a vane so it always points directly into the airflow.

  I'm very interested in this. Keep us posted!!

  Bob R.


  Earl Haury <ehaury at houston.rr.com> wrote:
    Bob

    Correct on instrument methodology. You may be correct regarding observed readings also. 

    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.

    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.

    Earl
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