Pattern Help

[Ken Blackwell]

Earl, Folks,

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.

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.

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.

Cheers,

Ken B.
  ----- Original Message ----- 
  From: Earl Haury 
  To: discussion at nsrca.org 
  Sent: Friday, January 14, 2005 3:24 PM
  Subject: Re: Pattern Help


  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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