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Pitot Tube

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hgmckay

Joined: 23 May 2006
Posts: 397

Posted: Mon Nov 10, 2008 1:20 pm Post subject: Pitot Tube

I have an Allegro 2000 (S/N 713-04) which was manufactured in 2004. The

pitot tube, which is about 5 inches long, is located about half way up the

right hand wing strut. In my opinion Fantasy Air could not have picked a

worse location for the pitot tube. Here is why. As you know, right side

entry into the cockpit is just in front of the right strut, and when the

door is lifted and you are doing something inside the cockpit while standing

on the ground slightly bent over it is very easy to forget about the pitot

tube protruding out of the strut just behind you. One must be always

conscious of its location being very careful to avoid backing into it. Since

it is my airplane, I have become very conscious of it and am always careful

when working around the plane. 



 



On two separate occasions I have had the pitot tube broken loose from the

small aluminum fairing that covers the main strut fairing. Both of these

incidents have happened at airfields where I had to leave my plane

unattended and tied down with warning flagging/cover on the pitot tube. One

was at a fly-in, the other at a major airport. I am sure I know what

happened in each case. Someone liking the appearance of the Allegro 2000 was

walking around it and looking through the side doors, and when they were on

the right side either backed into the protruding pitot tube or walked into

it while they were bent over.



 



I am planning to relocate the pitot tube on my plane. I notice that the new

2007 Allegros have the pitot tube located on the leading edge of the

vertical stabilizer (fin) near the horizontal stabilizer. I suspect that one

of the reasons it has been relocated is the problem I have just described.

My plan is to relocate my pitot tube to the same location on my plane. I

would put it on the leading edge of the wing, but I fear the same problem

because the wing is so low to the ground. My question is has this ever

happened to anyone else owning an Allegro 2000? If so, how have you

corrected the problem? Are there any inherent problems in relocating the

pitot tube to the vertical stabilizer (fin)? 



 



Hugh McKay



Allegro 2000

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Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Tue Nov 11, 2008 5:04 am Post subject: Pitot Tube

Hugh,



Yes it happened to ours pretty much as you described. Our solution was

to cut off the pitot tube about 3/4" forward of its intersection with

the strut fairing, and connect the cut-off piece to the 3/4" stub with

a flexible hose of the proper size. This way, when someone hits it the

hose flexes and no damage results to either the pitot or the klutz

that ran into it.



Thom in Buffalo

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_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

hgmckay

Joined: 23 May 2006
Posts: 397

Posted: Tue Nov 11, 2008 5:41 am Post subject: Pitot Tube

Thom:



What about the "flexibility of the connection? Would not the velocity of the

air stream tend to bend the tube one way or the other at the "flexible"

point causing it to deflect out of being parallel to the airflow causing an

erroneous air speed reading on the ASI?



Hugh



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hgmckay

Joined: 23 May 2006
Posts: 397

Posted: Tue Nov 11, 2008 5:47 am Post subject: Pitot Tube

Thom: As a follow up to my question, why not just leave the 3/4" projection

and forget about attaching the piece you cut off? 



Hugh



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Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Wed Nov 12, 2008 4:04 am Post subject: Re: Pitot Tube

Hugh,



The pitot system does not rely on velocity inside the system only the PRESSURE due to the ram air from the velocity of the relative wind at the pitot tube entrance. The flexible tube we used (clear vinyl) is stiff enough to prevent it from mis-aligning in the relative wind but flexible enough to bend at the joint with a side load from an impact of a body. 



To receive undisturbed air, the pitot inlet must be far enough ahead of intersection to preclude any local air disturbances from affecting the perceived ram pressure. 3/4" is probably close enough to affect the airspeed reading. That is why you never see such a short pitot tube. They are always several inches long to get the entrance out of any locally disturbed airflow.



I did airspeed calibration tests both before and after the hose fix and detected no change whatsoever.

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_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

hgmckay

Joined: 23 May 2006
Posts: 397

Posted: Wed Nov 12, 2008 6:19 am Post subject: Pitot Tube

Thom:



Thanks again for the clarification. I've got it!



Hugh



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