Pattern Help

[Earl Haury]

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
  ----- Original Message ----- 
  From: Bob Richards 
  To: discussion at nsrca.org 
  Sent: Friday, January 14, 2005 2:38 PM
  Subject: Re: Pattern Help


  Earl,

  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.

  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.

  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.

  Just guesses.

  Bob R.


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

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