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  #1  
Old 04-04-2017, 12:08 AM
Flyin'Bryan's Avatar
Flyin'Bryan Flyin'Bryan is offline
 
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Default Is it OK to shorten the Pitot Mast a little?

I searched the forums and found one post that did not quite give me the answer I was looking for, so I am putting this out the group for some help. I am finally getting around to re-mounting my replacement Dynon Heated Pitot Tube. To make a long story short, my first attempt at marking and drilling the mounting holes in the pitot tube and the mast did not go as well as I had planned, and this time I want to try to re-drill new mounting holes in the same mast.

Since the mast is a pretty expensive part, I wanted to try to re-use the same pitot mast by cutting off a little less than .5 inches to remove the old holes, and then mark and drill the new ones. This would put the pitot tube about 6 inches from the bottom wing skin, instead of the typical 6.5 inches from the bottom skin.

Has anyone else ever done this? If so, have you flown with that configuration without any issues? I will put a call in to Vans tomorrow to get their advice, but I was curious if others have faced this same issue and made a similar modification to the pitot mast.

Instructions from Vans on DWG 15A seem a bit vague about the actual distance from the bottom of the wing for the pitot tube by using phrases like "approximately 6-7 inches" from the bottom of the wing. Based on this information I think I am OK if I do this as long as I do not go lower than 6 inches, but I would like to know if anyone is flying with a SafeAir mast and Dynon Heated Pitot in this "slightly shortened" configuration.

Here is a pic with my current mast and the markings I made for trimming it down beyond the existing holes:

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Building RV8
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  #2  
Old 04-04-2017, 12:25 AM
scsmith scsmith is offline
 
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Default No problem shortening the pitot tube

As long as the probe is outside the boundary layer (probably about a 0.1" thick) the pitot pressure will be the same no matter where you put the probe. There can be a small angle error depending on angle of attack, but the wing acts as a flow straightener to reduce that error, and moving it a little closer to the wing may even reduce the error more.

Note that if you also have an AOA probe on the pitot tube, then all bets are off -- you should mount it where they say to mount it.
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  #3  
Old 04-04-2017, 07:46 AM
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RV6_flyer RV6_flyer is offline
 
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Default Garmin AOA Installation says:

Garmin put the mounting instructions for their AOA in the G3X manual. Following is a COPY / PASTE from section 5.5.1 of the G3X Installation Manual 190-01115-01_V.pdf.

"The tip of the GAP 26 should be located at least 4” from, but not more than 10” below the bottom surface of the wing. The tip should be located 2” to 18” behind the leading edge of the wing.

Use the designer/manufacturer’s recommended pitot mounting location (if specified).

If the static port is located under the wing, it is recommended that the port be moved to an alternate location as part of the GAP 26 installation."

If you search the web, you may be able to find more info on the probe you are going to use.

I am using the same "Blue Mast" with the Garmin GAP 26 AOA on my RV-8.
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  #4  
Old 04-04-2017, 08:58 AM
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Toobuilder Toobuilder is offline
 
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Default

When I replaced my Dynon AOA unit for the heated version on my Rocket, I trimmed the mast to get rid of the old holes. I also shortened it up well beyond that because it just looked too long. The pitot opening is now at 4.5 inches (was 6, IIRC) and has not shown me any difference in performance. I also got rid of the stock Vans bent tube on the -8 and replaced it with the standard AN type heated probe salvaged from my Mooney. At 5.5 inches It is also shorter than the one it replaced and performs the same.

Not that it makes a difference, but both tubes were moved to the aileron bellcrank access panel.
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  #5  
Old 04-04-2017, 10:13 AM
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Flyin'Bryan Flyin'Bryan is offline
 
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Default Thanks all for the responses

I also understand that this will need to be flight tested and instruments will need to be calibrated, etc. Steve, as of right now I don't intend to use the Dynon EFIS for my avionics, so the AOA line on the Dynon Pitot tube will be capped off and not connected to anything.

You raised an interesting point about the position of the AOA, and I guess I am now wondering if instrument calibration procedures, i.e with the Dynon AOA, would account for any variance in the actual location of the AOA port.

Also interesting to see the Garmin instructions for the position of the tube, as well as the mod performed on the rocket. I do plan on putting my tube under the next outboard bay from where the aileron control horn resides, as many other builders have done in the past. I'll be mounting the heat control unit on my inspection plate to make it easier to service/inspect, so that will take up the space in the bay with the control horn.

I tried to call Vans this morning but as my luck would have it nobody is there to man the phones, so I will need to wait till this afternoon to discuss it further with them. Sounds like I should be OK trimming the mast a bit.
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  #6  
Old 04-04-2017, 12:21 PM
Robert Anglin Robert Anglin is offline
 
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Default

Quote:
Originally Posted by RV6_flyer View Post
Garmin put the mounting instructions for their AOA in the G3X manual. Following is a COPY / PASTE from section 5.5.1 of the G3X Installation Manual 190-01115-01_V.pdf.

"The tip of the GAP 26 should be located at least 4” from, but not more than 10” below the bottom surface of the wing. The tip should be located 2” to 18” behind the leading edge of the wing.

Use the designer/manufacturer’s recommended pitot mounting location (if specified).

If the static port is located under the wing, it is recommended that the port be moved to an alternate location as part of the GAP 26 installation."

If you search the web, you may be able to find more info on the probe you are going to use.

I am using the same "Blue Mast" with the Garmin GAP 26 AOA on my RV-8.
We took this from the manual and because we had a Garmin heated Pitot with AOA in it did the math and cut the mast to be in the medal of 4 and 10.
Seems to work very well there and have had not problems with it.
Yours, R.E. A. III #80888
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  #7  
Old 04-04-2017, 06:36 PM
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Toobuilder Toobuilder is offline
 
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Default

I moved mine from the bay just outside the aileron bellcrank back to the inspection panel. It resides there along with the heater control box. If you do go outboard of the rib "like everyone does", make sure you have some doublers and/or stiffeners under it. That skin is pretty floppy and there is a bunch of leverage with that mast.
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  #8  
Old 04-05-2017, 10:08 AM
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Default

I have a Garmin Heated Pitot and followed their instructions for install. As you can see, from the posts above, there is quite a bit of leeway. I would think trimming a tad off your mast would be okay. As long as your cut is square to the tube, the angle of the pitot shouldn't change.

I'm glad to see you're back at it, Bryan. I'm in a bit of a mental rut right now, but I'm also trying to keep plugging away at my project, a few hours when I can.
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  #9  
Old 04-05-2017, 12:38 PM
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Flyin'Bryan Flyin'Bryan is offline
 
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Default Thanks again for the additional responses...

Most appreciated. Vans also said no problem with the planned mast reduction, and we also discussed a number of folks that have mounted a Piper-style pitot tube that only extends down about 3-4 inches or so. I do remember accessing someone's build log where they had reinforced the mast mount just a bit after moving it to the outer bay. I'll have to try to remember where I saw that so I can follow suit.

I think the problem with keeping it in the original bay is that when you add a roll servo for the auto pilot, in addition to mounting the heating unit on the access cover, it starts to get a bit cramped in there, and running the lines without interfering with the aileron belcrank also becomes a bit of a chore. Thus the reason for moving it over to the next bay. That decision also comes with some other concerns regarding tie down clearance under certain situations which I discussed in a separate thread long ago. The give and take never ends I guess.

Now if I can just get past drilling and tapping the mounting holes properly - I can move on to the next challenge....
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Building RV8
EAA Chapter 301
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SportAir Workshop Graduate x 4: Sheet Metal, RV Building, Electrical, and Composite classes.
Tail Wheel Endorsement Completed
Empennage done, Wings in progress, N462AK reserved.

Last edited by Flyin'Bryan : 04-05-2017 at 12:42 PM.
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  #10  
Old 04-05-2017, 02:00 PM
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Flyin'Bryan Flyin'Bryan is offline
 
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Default Fixing that mental rut......

Mike, all that means is that you have spent way too much time on the ground, and you just need to get airborne for a while! "Clearing the mind" is right up there in my top 10 list of reasons why I am building this thing.

Lets go flying some time.
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Building RV8
EAA Chapter 301
CFII/MEI, ATP
SportAir Workshop Graduate x 4: Sheet Metal, RV Building, Electrical, and Composite classes.
Tail Wheel Endorsement Completed
Empennage done, Wings in progress, N462AK reserved.
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