Drilling into spar

[stoldriver]

The reality is that every hole in the spar weakens it to some degree. Holes concentrate the stress and cracks often initiate from a hole. Is a specific hole in a spar a bad thing? Maybe, maybe not. Do you feel lucky? Personally, I'm avoiding putting holes in the spar. To date, I'm up to one hole each wing about a foot from the tip for wire pass-thru. Loads are very low there and I am comfortable that the risk is low. My magnetometer is on a bracket mounted between two ribs and my Pitot will be pretty much what Pete suggested above. If I was putting a hole inboard, I would probably do some kind of analysis. Just my opinion, of course. Yours may vary.

Wayne

 

[KeithO]

The reality is that every hole in the spar weakens it to some degree. Holes concentrate the stress and cracks often initiate from a hole. Is a specific hole in a spar a bad thing? Maybe, maybe not. Do you feel lucky? Personally, I'm avoiding putting holes in the spar. To date, I'm up to one hole each wing about a foot from the tip for wire pass-thru. Loads are very low there and I am comfortable that the risk is low. My magnetometer is on a bracket mounted between two ribs and my Pitot will be pretty much what Pete suggested above. If I was putting one inboard, I would probably do some kind of analysis. Just my opinion, of course. Yours may vary.

Wayne

I'd like to avoid holes as well. Once the holes are drilled you can't undo that. I'm looking at three alternatives for hanging the pitot:

1) Use clickbond studs which are bonded to the spar to mount the pitot bracket. This was suggested by another forum member.
2) Hang the pitot mount from the lower leading edge skin forward of the spar using a doubler to strengthen the skin around the mount. I have pictures of this type of installation on another aircraft.
3) Attach a bracket between two ribs per post post #5 by @skywagon8a

I'm leaning towards 1). The G3X STC is not specific on the pitot installation method for the PA-18 other than general guidelines on probe location, so I think it's a conversation with my IA on whether the clickbond approach is acceptable. I like 3) except for the complexity and the need to attach to ribs while working through access holes.
My fallback plan is 2)

 

[Steve Pierce]

So is this on a certified or experimental airplane? If certified then what is the approved data to install the heated pitot?

 

[KeithO]

So is this on a certified or experimental airplane? If certified then what is the approved data to install the heated pitot?

Certified. The Garmin GAP 26 heated (unregulated) pitot installation is an option under the G3X STC. The PA-18 is on the G3X AML

 

[skywagon8a]

Certified. The Garmin GAP 26 heated (unregulated) pitot installation is an option under the G3X STC. The PA-18 is on the G3X AML

Those are your instructions. Generally small (fastener) holes in a spar web are of no concern. Look at how your compression ribs are attached.

 

[astjp2]

Drill or pilot drill and ream?