Drilling into spar

[Sam D]

As I turn to messing with the wings in my project, I'm faced with potentially putting some holes in the spar (e.g. pitot tube bracket secured to rear spar and maybe magnetometer mount). This makes me a little nervous!

The best discussion I found in this area was a series of posts (beginning here) in Bill Rusk's Javron build thread.

It sounds like I'm ok with generally drilling into spar in the center of the web (equidistant from top and bottom)

Is there anything else I should be concerned about here?

Thanks!
Sam

 

[stewartb]

I appreciate your caution and don’t have any “rules” to offer but I drilled my spars in a few places. Every Cub with Atlee Hurricane ties has, too. A few holes won’t threaten the integrity.

 

[Litecub]

Make sure to deburr any holes made.
LiteCub

 

[CharlieN]

I might be a bit different but prefer my pitot tube up front in the jury strut.

 

[skywagon8a]

This is mounted on a rib centered between the two spars. I have flight tested it throughout the speed range from 35 up to 155 and it is 100% accurate. Notice the yarn tied to the pitot tube at the high angle of attack. It is straight back at all Angles of attack. remaining within the recommended +/- 10 degrees to the relative wind. I recommend this location for accuracy.

Edit: It is also midway between the jury struts and the lift strut wing attachments.

 

[astjp2]

Make sure to deburr any holes made.
LiteCub

Ream them, most drill bits leave ragged holes, I used to work in a fabrication shop. That built parts for boeing. Tim

 

[Steve Pierce]

Outboard of the lift strut is not a high stress area.

 

[CharlieN]

Outboard of the lift strut is not a high stress area.

How far outboard, anywhere near the strut attachment, inboard or outboard is the highest loaded area on the wing.

 

[Steve Pierce]

Piper's own documentation says you can only splice outboard of the struts. Legend and Dakota Cub wings were tested in DC's wiffle tree with lightening holes outboard the fittings but I don't know how far. Experimental folks should do some testing.

 

[skywagon8a]

Piper's own documentation says you can only splice outboard of the struts. .

This only means you and I are only allowed to repair outboard of the struts using Piper's instructions. Piper Service Memo 3a is supposed to cover this.
A DER would be able to develop repairs in other locations.

 

[Steve Pierce]

Correct, as an A&P/IA I am not going to go drilling inboard from what I see Piper published and what I have learned from talking to Mark at Dakota Cub on the testing of the Super 18 wing and the Legend Cub wing. If I was building an experimental Cub and wanted to go drilling holes I would do some testing. I don't want to be that guy that broke a spar and spiraled down. That sounded very scary to me and I have already survived a midair fall straight down from 100 feet.

 

[stewartb]

I remember buying my Cessna’s safety net from Atlee and balking at the idea of drilling the spar carry-through for the Ancra hook bases. He told me not to worry about it, the carry-through was in compression. The same is true of the wing spar. I also remember pics of the lightened early Carbon Cub spars. Do they still do the lightening holes?

Steve Kracke told me about a wing test for Atlee tanks and how -12 wings flexed up and down inboard of the strut. Not being an engineer type I found that surprising. Further proof that what I don’t know is far greater than what I do know.

 

[skywagon8a]

True stewart, but the wing spars also have bending loads which the carry-through does not. When the spar is both flexed in bending and loaded in compression at the same time it becomes weaker because it is not as strong in compression while it is bending. I know it doesn't look like a lot, but if it is not perfectly straight the column strength is less.
What Steve Kracke was referring to was the flexing taking place about the hinge where the struts attach to the wing.

 

[frequent_flyer]

I also remember pics of the lightened early Carbon Cub spars. Do they still do the lightening holes?

None in my FX-3 main spar. I don't know what was done in the lower gross weight Carbon Cubs.

 

[Steve Pierce]

None in my FX-3 main spar. I don't know what was done in the lower gross weight Carbon Cubs.

I can't remember on the SS either. Here is an FX3 wing.

 

[stewartb]

Steve described the flex as a wave that moved full length of the wing. I thought the strut would reduce it. Apparently not.

 

[Gordon Misch]

See figure 18. And superimpose the compressive load inboard of the strut attachment onto this. From https://engineering.purdue.edu/~ce474/Docs/DA6-BeamFormulas.pdf

 

[CharlieN]

Outboard of the lift strut is not a high stress area.

Are we talking tiny holes for mounting a pitot tube as the OP was asking that are no more than the rib mounting or are you talking about BIG lightening holes through the web?

 

[skywagon8a]

As I recall from an engineer I used to work for, small holes in and near the center of the web are not critical. Big ones like the lightning holes are a different story.

 

[Sam D]

Are we talking tiny holes for mounting a pitot tube as the OP was asking that are no more than the rib mounting or are you talking about BIG lightening holes through the web?

Good question. Also, I *think* I am ok with the pitot bracket because it appears that it is installed inside the lift strut in the parts manual as shown in the attachment (figure 5 from the parts manual)

 

[Gordon Misch]

As I recall from an engineer I used to work for, small holes in and near the center of the web are not critical. Big ones like the lightning holes are a different story.

Yes. Note the transverse shear graph referenced above, then add to that the internal shear imputed from the moment diagram. For me, I wouldn't be making big holes without careful analysis or proven experience.

 

[RVBottomly]

See figure 18. And superimpose the compressive load inboard of the strut attachment onto this. From https://engineering.purdue.edu/~ce474/Docs/DA6-BeamFormulas.pdf

Thanks, Gordon. Something added to my library!

 

[Steve Pierce]

The screw holes for the pitot bracket are in the middle of the web and should be no issue. I tend toi look at how Piper did things and figure it was for a reason and stick to their engineering unless a reputable source proves otherwise. I am not smart enough to engineer on my own

 

[RaisedByWolves]

I can't remember on the SS either. Here is an FX3 wing.
View attachment 57701

Sport cubs had lightning holes everywhere. Front and rear spar. Carbon cubs do not.


Sent from my iPhone using Tapatalk

 

[KeithO]

This is mounted on a rib centered between the two spars. I have flight tested it throughout the speed range from 35 up to 155 and it is 100% accurate. Notice the yarn tied to the pitot tube at the high angle of attack. It is straight back at all Angles of attack. remaining within the recommended +/- 10 degrees to the relative wind. I recommend this location for accuracy.

Edit: It is also midway between the jury struts and the lift strut wing attachments.

My thoughts are turning toward installing a heated pitot tube to replace the stock Piper tube that is now loose on it's mount.
Can you give some details on how you mounted the pitot tube to the rib?

 

[skywagon8a]

My thoughts are turning toward installing a heated pitot tube to replace the stock Piper tube that is now loose on it's mount.
Can you give some details on how you mounted the pitot tube to the rib?

These are not Piper ribs, they are TCOW/Smith ribs. This is a pitot mast from a Cessna T-50. If you have Piper ribs, you could rivet a flat plate similar to this on which to attach your pitot mast.

 

[KeithO]

CubCrafters has a STC for a heated pitot tube that mounts to the rear spar near the stock location.
On the Carbon Cub, they use a Garmin GAP 26 heated pitot mounted to the back side of the front spar to get AOA as well.

I would like to install a thermostatically controlled GAP 26 on the front spar.

Does anyone have any experience with field approval of the GAP 26 on the front spar and static port in the location used on the Top Cub??

 

[KeithO]

I'm getting ready to install a heated pitot. CubCrafters sells this bracket which installs on the rear face of the PA-18 front spar between the second and third ribs outboard of the jury strut attachment.

Looking for opinions on drilling before I proceed.

 

[Steve Pierce]

What does the STC say?

 

[KeithO]

What does the STC say?

The STC that covers the installation is the G3X for certified aircraft. The pitot installation instructions don't go into detail regarding the specific attachment method.

I've spoken to Cub Crafters and have been told that mounting the bracket to the rear face of the front spar about 70" from the wing root is fine.

Another option would be to put a doubler on the lower leading edge skin forward of the front spar and attach the pitot mount there.