As I was pulling the airplane apart, I noticed that a majority of the doublers that were installed on semi critical areas are just glued on. For example, my cowl steps and cowl handles are installed like this.


Any thoughts?

Interesting technique. How long ago were they installed?

Not really how its done... LOL

Bigrenna wrote:Not really how its done... LOL

Come on Greg, maybe it's some super structural glue... Haha.

A1Skinner wrote:

Bigrenna wrote:Not really how its done... LOL

Come on Greg, maybe it's some super structural glue... Haha.

LOL... Your right. Come to think about it, that may be DSTTSA Glue.


(Double Secret Tip Top Speedy Airplane Glue)

Doesn't look right to me.
The 180/185 step installations I looked at before installing mine all bolted through the floor flange.
In other words, just a whisker lower than yours.
It would be a lot less work to just do it like that,
but I don't know how beefy it would be.

Aerospace adhesives with properly designed joints are usually a lot stronger than the materials being bonded together. Adhesives perform well in shear, but do not do well in tension or "peel".

If the purpose of this doubler is limited to reacting the load of a person on the aluminum angle step on the aircraft exterior, the stress in the glue would be pure shear. If the surface area is estimated at about 4"x6", and a 200 pound human were to place the entirety of their weight on the step, the stress in the glue would be 8.3 psi. If the surfaces were prepared properly and the adhesive mixed and cured properly, you could bounce up and down on said step all day long while listening to the glue laughing at the resulting loads.

The Grumman Tiger is constructed from bonded aluminum, and is a great airplane with a respectful service history.

We use adhesives for joining all major structural components in composite aircraft. When you do force failure through overload, the failure is almost always in the laminate, not the glue if it is done properly. This is similar to a glued joint in wood, which will fail in the grain before the glue.

If you have a spar with 2 inch wide caps, and glue it to a wing skin across it's entire length, the stress in the continuous bond is minuscule and evenly distributed in comparison to the stresses concentrated at rivets. Plus you don't need to drill holes in your skins or spar caps, which is a BIG advantage.

More than you wanted to know, I'm guessing, but I would not question that installation for strength or other service issues.

^^ This guy engineers.

And, to add to what Scolopax wrote.....this is a doubler, folks, all it's doing is spreading the loads from the step.

It obviously hasn't moved since it was installed. I'd call that success and wouldn't worry about it at all.

MTV

Scolopax wrote:Aerospace adhesives with properly designed joints are usually a lot stronger than the materials being bonded together. Adhesives perform well in shear, but do not do well in tension or "peel".

If the purpose of this doubler is limited to reacting the load of a person on the aluminum angle step on the aircraft exterior, the stress in the glue would be pure shear. If the surface area is estimated at about 4"x6", and a 200 pound human were to place the entirety of their weight on the step, the stress in the glue would be 8.3 psi. If the surfaces were prepared properly and the adhesive mixed and cured properly, you could bounce up and down on said step all day long while listening to the glue laughing at the resulting loads.

The Grumman Tiger is constructed from bonded aluminum, and is a great airplane with a respectful service history.

We use adhesives for joining all major structural components in composite aircraft. When you do force failure through overload, the failure is almost always in the laminate, not the glue if it is done properly. This is similar to a glued joint in wood, which will fail in the grain before the glue.

If you have a spar with 2 inch wide caps, and glue it to a wing skin across it's entire length, the stress in the continuous bond is minuscule and evenly distributed in comparison to the stresses concentrated at rivets. Plus you don't need to drill holes in your skins or spar caps, which is a BIG advantage.

More than you wanted to know, I'm guessing, but I would not question that installation for strength or other service issues.

This is why I like to post here. When I go through this area I will make sure that the skin and the doubler get cleaned up and installed properly. What adhesive would you consider using on this?

I think what wasn't said by Ryan is that this install was just done by the folks up north whom he had "restore" the bird.

The step was installed upside down, it is in the wrong place (should be on the floor pan rivet line,) the substrate wasn't cleaned prior, the sheet is a wonky size, and there is no joggle on the doubler where it lays on top of the angle above and floor pan below.

Reading between the lines, I think he is asking for affirmation of his disgust.

On the two 180's I inspected before installing steps the install was either lower down on the foot pan rivet line or further up on the next rivet line, the higher one due to being a float plane I guess. 185 may be different but in either case for me were bolted right up against the door post on one side of the step and to the rivet line carrying the floor pan or, for the higher up one, the rib carrying load up to the panel on the other side of the step. So both cases bolted to something more or less structural which is pretty logical. It looks kind of like yours are just bolted to the skin with a doubler on the back, not sure but in which case I'd be surprised if you don't see any decent flex in the skin when standing on them. Not sure if its right or wrong that way but if there is no flex probably leave it alone now the holes are already drilled. It does seem like something that's easy to get right or correct now if you think it may cause a crinkle or something down the road though. I can take some pics of both installs if you need.

I'm betting it will be okay since it is glued and goes up to the next stiffener above. One good thing is none of the original rivets were messed with. I'm not a fan of the steps and usually use a step ladder and have on occasion just stood on the tire but I'm a little over 6 ft. This is a similar situation that Corey is in with his 205.

pilotryan wrote:

Scolopax wrote:Aerospace adhesives with properly designed joints are usually a lot stronger than the materials being bonded together. Adhesives perform well in shear, but do not do well in tension or "peel".

If the purpose of this doubler is limited to reacting the load of a person on the aluminum angle step on the aircraft exterior, the stress in the glue would be pure shear. If the surface area is estimated at about 4"x6", and a 200 pound human were to place the entirety of their weight on the step, the stress in the glue would be 8.3 psi. If the surfaces were prepared properly and the adhesive mixed and cured properly, you could bounce up and down on said step all day long while listening to the glue laughing at the resulting loads.

The Grumman Tiger is constructed from bonded aluminum, and is a great airplane with a respectful service history.

We use adhesives for joining all major structural components in composite aircraft. When you do force failure through overload, the failure is almost always in the laminate, not the glue if it is done properly. This is similar to a glued joint in wood, which will fail in the grain before the glue.

If you have a spar with 2 inch wide caps, and glue it to a wing skin across it's entire length, the stress in the continuous bond is minuscule and evenly distributed in comparison to the stresses concentrated at rivets. Plus you don't need to drill holes in your skins or spar caps, which is a BIG advantage.

More than you wanted to know, I'm guessing, but I would not question that installation for strength or other service issues.

This is why I like to post here. When I go through this area I will make sure that the skin and the doubler get cleaned up and installed properly. What adhesive would you consider using on this?

This stuff is the gold standard for joints designed for shear: https://tds.us.henkel.com/NA/UT/HNAUTTDS.nsf/web/B02C099805BD377385257BC60067B0E3/$File/LOCTITE%20EA%209394%20AERO-EN.pdf

It's actually not too different from JB weld.

As far as being legal, you would need to look in to the details of the STC for what is being installed.

With adhesives, surface preparation is very important. Try sticking some duct tape to a piece of clean glass, and then to another that has been smeared with grease. Same material, same adhesive, different surface condition drastically different results...

Plenty of what scolopax is saying is accurate. That said most significant joints, even in composite aircraft are fastened. Reason being is that there are applied moments on most joints reacted in a "heel toe." Those joints are generally why you don't ever realize the expected weight savings for a composite a/c vs Al design. Even single lap joints primarily loaded in shear develop out of plane loads (peel) due to eccentric nature of the joint. Bonded joints can be very difficult to properly analyze for these and other reasons.

Bottom line is that if it was bolted. Bolt it. If it was riveted, rivet it.

Chicken rivets are another whole can of worms.

If it's a step and it's holding just fine, well it's a step. So who cares. Now if there are any open holes under that doubler I'd remove and fix that. Open holes are great places for cracks to nuclate. Fine for little drain holes that are not highly loaded but open holes near landing gear structure that is subject to cylical loading, not as cool. Fill the holes with a swelling or interfence fit fastener. Rivet or hi lok.

Again bond prep is important but also needs to be a controlled process not just a do your best kind of approach. And that process is backed up by coupon testing.

I am of the opinion that while the more technical thoughts and concerns shared here are true and accurate, that doesn't make them right for in this application.
That installation shows either;
A) a shortage of pride in work performed , or
B) a shortage of skill in the work performed

This need not be a deal breaker or ruin a relationship with the party that performed the job, instead it should be an education as to what tasks you assign to that person in the future, as well as what your expectations will be.
I believe the installer of that step honestly thought you'd never see the back side.
I agree with bigrenna, poor prep, poor doubler construction, and questionable (possibly illegal) methods...
Sure it's just a step install, but I suspect you paid fair market value for this work. You did not receive a fair product in return.

Take care, Rob

Rob wrote:.....I agree with bigrenna, poor prep, poor doubler construction, and questionable (possibly illegal) methods...
Sure it's just a step install, but I suspect you paid fair market value for this work. You did not receive a fair product in return.

What does AC43-13 "acceptable methods techniques and practices" indicate about how this should be done?

Rogue wrote: On the two 180's I inspected before installing steps the install was either lower down on the foot pan rivet line or further up on the next rivet line, the higher one due to being a float plane I guess. 185 may be different but in either case for me were bolted right up against the door post on one side of the step and to the rivet line carrying the floor pan.....

The steps I installed on my 53 C180 bolt through the foot pan rivet line.

pilotryan wrote:Any thoughts?

Get a better mechanic who actually cares about the quality of their work....and can read the installing instructions.

Bigrenna wrote:....The step was installed upside down....

?? Looks right side up in Ryan's photos to me.