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  #1  
Old 01-08-2007, 12:20 PM
prkaye prkaye is offline
 
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Location: Ottawa, Ontario, Canada
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Default pitot tubes?

Now that I'm getting into my wings I'm realizing I have to start thinking seriously about all kinds of things... now wondering about Pitot tubes.
I want to have some kind of redundancy... at least a pitot tube plus AOA probe. Possibly two pitot tubes... I want to have the option of doing IFR.

I intend to go with Dynon avionics. Will these systems require a special pitot tube, or can they work with Vans standard pitot tubes? One idea is to have two separate Pitot tubes feeding two separate Dynon EFIS units. Perhaps put one pitot tube in each wing? How much of this work can be done at a later stage, and how much has to be done while I build the wings?
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  #2  
Old 01-08-2007, 12:27 PM
zsadecki zsadecki is offline
 
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Van's standard pitot tube is just a tube. For IFR you need a heated pitot, so the super-simple van's tube is out.
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  #3  
Old 01-08-2007, 01:52 PM
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briand briand is offline
 
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Location: Grand Rapids MI
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You could leave the bottom outbord skins off until you decide. That what I'm doing.
I'll probably put some kind of heated pitot and a servo but don't want to tie up the money in that stuff right now. The outboard skin should only take a couple hours to rivet on. You can get it all dimpled and deburred ready to go. maybe cleco it in place while the wings store in their cradle.
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  #4  
Old 01-08-2007, 02:04 PM
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Kevin Horton Kevin Horton is offline
 
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You want redundancy, with pitot + AOA. Which failure modes are you trying to protect against?
  • Blocked pitot (icing, mud daubers, etc)
  • Bird strike
  • What else?

Possible architectures with both pitot and AOA:
  1. Dynon pitot/AOA tube
  2. Heated pitot + separate AOA system (e.g. Proprietary Systems)
  3. Dynon pitot/AOA tube + second heated pitot

Option 1, while somewhat popular, doesn't protect against either pitot icing or bird strike. The Dynon AOA needs the pitot pressure to work, so pitot icing will kill the AOA and airspeed. It also does not protect against bird strike, as both functions are on the pitot tube. Also note that the currently available Dynon pitot is not heated, although they are working on a heated version.

Option 2 does provide protection against both pitot icing and bird strike, but it means yet another indicator to mount somewhere.

Option 3 does provide protection against bird strike, but still doesn't cover you for pitot icing. If the heated pitot ices up, the non-heated Dynon pitot would also be iced up. This option also forces either two ASIs, or a valve to select between pitot sources. Complicated. Heavy. Expensive.

Realistically , what are the odds of pilot icing, if you have a heated pitot tube? You would have to have pitot tube heater element failure, and be in icing conditions. You should be trying to avoid icing conditions, and it is possible to check the pitot heat before each flight. Don't worry about this one.

A bird strike on the pitot tube is possible, but that is a fairly small area to hit. Not too likely. Birds that see you are coming dive, so they wouldn't hit the pitot tube unless you were upside down. And, even if an unlucky bird knocks off your pitot tube, I'm betting you will still be able to safely land the aircraft without the ASI, unless this happens before you get to know the aircraft.
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  #5  
Old 01-08-2007, 02:50 PM
prkaye prkaye is offline
 
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Default Dynon Pitot

is there anything special about the Dynon pitot tube? Can any old pitot tube (Vans?) work equally well with the dynon EFIS?
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  #6  
Old 01-08-2007, 03:21 PM
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Kevin Horton Kevin Horton is offline
 
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Quote:
Originally Posted by prkaye
is there anything special about the Dynon pitot tube? Can any old pitot tube (Vans?) work equally well with the dynon EFIS?
A pitot tube, is a pitot tube, is a pitot tube. They are just a source of total pressure. No magic at all. The contour of the tube does affect how much the airflow can be misaligned and still have the tube be accurate. But, if the airflow is reasonably well aligned with the tube, then any tube will work. NASA Reference Publication 1046 Measurement of Aircraft Speed and Altitude (14 MB PDF) has test results showing pitot tube accuracy vs airflow misalignment - see the section starting on page 25 of the document (page 39 in the pdf). Van's plain old piece of tube should be accurate as long as the airflow is within about 20 degrees of the tube axis. This is actually better than the typical pitot tube shape, as far as this characteristic goes. And no, I don't know why most pitot tubes have an ogival profile. Maybe it lowers the drag, for a tube of given diameter.

Of course, if you want the Dynon AOA function, then you need a Dynon pitot tube.
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  #7  
Old 01-08-2007, 03:23 PM
sglaeser sglaeser is offline
 
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Location: Pittsburgh, PA
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Quote:
Originally Posted by prkaye
is there anything special about the Dynon pitot tube?
The Dynon pitot tube has the AOA functionality built in. It accomplishes this through a second port located on an angled face of the pitot itself. I thought Dynon offered a heated version of their pitot tube, but I'm not finding it on their website... maybe I dreamed that one up.
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  #8  
Old 01-08-2007, 03:26 PM
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briand briand is offline
 
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Quote:
Originally Posted by prkaye
is there anything special about the Dynon pitot tube? Can any old pitot tube (Vans?) work equally well with the dynon EFIS?

Looks and.......AOA

If your going with the dynon the dynon pitot with AOA wouldn't be a bad idea.
The Dynon has two ports, one for air speed and the other for aoa. I have the dynon with mntg. brkt. and the safeair AOA plumbing kit. I will sell it for $240 with everything so if you really know you are going with Dynon heres your chance to save $50. I changed my mind and decided to go with GRT's dual system so the Dynon with aoa is just extra weight and I now want heat.

I'm not in a hurry to sell it but I might as well get rid of it since I know I'm not going to use it.
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