Pattern Help

[Bill Glaze]

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.
Bill Glaze

Ken Blackwell wrote:

> 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 <mailto:ehaury at houston.rr.com>
>     To: discussion at nsrca.org <mailto: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 <mailto:bob at toprudder.com>
>         To: discussion at nsrca.org <mailto: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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