Drilling holes in the main spar cap

[Capt. Geoffrey Thorpe]

My fuel tanks are in the wings behind the main spar and, as built, are covered by the wing fabric which means that any fuel tank maintenance involves dropping the wing (no big deal) and fabric repair (something of a pita). So, I figure that installing an aluminum panel over the tank would make my life easier. Of course, to mount the panel, I would be attaching about 9 nut plates to the main spar web for #8 screws in the most inboard bay of the wing.

Considering that installing a fire extinguisher bracket is a big deal, I thought I would give the experts a chance to explain how I am going to die when I do this - but just as a reminder, the aircraft is E-AB so the laws of physics are different than a type certificated aircraft.

[Strut braced wing, single bolt at wing root for main spar attach - much like yea olde Cessna 150. Spar has a constant cross section except for a doubler at the strut attach. ]

Fuel tank bay and main spar with the tank sitting on top. Wing root to the left.

Da ride:

 

[PaulS]

A picture is worth a thousand words, a sketch of what you want to is worth about five thousand words. Not enough info.

 

[Stewartb]

Your primary attach points will be ribs and whatever you fab up for an aft angle. For the front you can attach to nose ribs or add an angle to tbe spar web or the vertical leg of the spar cap.

Here's the leading edge metal peeled back on a Cub wing. Ribs are above spars. Nose ribs in front of the tank. The cover is rolled for shape, cut for the filler, and gets stiffeners added.

 

[Gary Austin]

The '46 Taylorcraft I have has the 12 gallon nose tank with two 6 gallon wing tanks, the wing was covered with fabric, so any leaks called for cutting the fabric, so when I recovered the wings, I put an aluminum cover over the tanks, from the butt rib to the next rib outboard, braced the rib outboard so it wouldn't bow when the fabric was tightened, the spars are wood, so i stayed away from the spar and put an aluminum angle under the aluminum leading edge and behind the tank, beings its a certified aircraft, it took some back and forth with the PMI, but got it all taken care of, it sure makes inspected the spar attach easy

 

[Briar Rabbit]

My fuel tanks are in the wings behind the main spar and, as built, are covered by the wing fabric which means that any fuel tank maintenance involves dropping the wing (no big deal) and fabric repair (something of a pita). So, I figure that installing an aluminum panel over the tank would make my life easier. Of course, to mount the panel, I would be attaching about 9 nut plates to the main spar web for #8 screws in the most inboard bay of the wing.

Considering that installing a fire extinguisher bracket is a big deal, I thought I would give the experts a chance to explain how I am going to die when I do this - but just as a reminder, the aircraft is E-AB so the laws of physics are different than a type certificated aircraft.

[Strut braced wing, single bolt at wing root for main spar attach - much like yea olde Cessna 150. Spar has a constant cross section except for a doubler at the strut attach. ]

Fuel tank bay and main spar with the tank sitting on top. Wing root to the left.

Da ride:

Can you install your nut plates in the middle of the web (halfway between the upper and lower spar caps) which then should be close to your neutral axis if the spar caps are the same dimensions?
Can you use existing holes by drilling out rivets and replacing them with the nut plate utilizing the same holes and new rivet going through the original hole and the nut plate?
Solid rivets?

 

[Capt. Geoffrey Thorpe]

The spar has a sheet metal web with aluminum angles riveted top and bottom for the caps. There is an aluminum leading edge riveted to the spar caps to form a D cell (that, near as I can tell, is mostly filled with foam ribs). The spar cross section is pretty much constant except near the strut attach where there is a doubler on the web.

The plan, in more detail, is to remove the pop rivets that hold the fabric on the ribs at each side of the bay and replace with sheet metal screws to hold the cover and fabric. In the rear, there is an existing angle that I can drill and install nut plates. And, in the front, I am using the space between the rivets that hold the D cell and the trailing edge of the top angle to drill and install nut plates to screw down the cover. Tight, but I can get the edge clearance. If you look at the bottom of the spar in the second picture you can see the rivets that hold the D cell skin to the lower spar cap - the top is the same.

Since the wing is strut braced with only one bolt at the wing root there is nearly no bending moment applied to the spar near the root - mostly the compression reaction to the strut and a bit of shear so I don't feel bad punching holes in the main spar cap for the nutplates.

BTW - I appreciate the pictures of other wing structures - not what you usually get to see when people show pictures of their airplanes.

 

[PaulS]

I don't think what you are planning is that big of a deal. The only part I would rethink is using sheet metal screws versus machine screws with some type of threaded nut, like a pem nut or even a clip on nut.

 

[Capt. Geoffrey Thorpe]

I don't think what you are planning is that big of a deal. The only part I would rethink is using sheet metal screws versus machine screws with some type of threaded nut, like a pem nut or even a clip on nut.

I would prefer machine screws, or at even tinnerman nuts, but the ribs are aluminum U caps over foam with an aluminum "web" on the fuel tank side - no access for a nut plate unless I get medieval on it. And, PK screws are not uncommon for attaching fabric, so...
It will get nutplates at the fore / aft ends.

 

[PaulS]

I would prefer machine screws, or at even tinnerman nuts, but the ribs are aluminum U caps over foam with an aluminum "web" on the fuel tank side - no access for a nut plate unless I get medieval on it. And, PK screws are not uncommon for attaching fabric, so...
It will get nutplates at the fore / aft ends.

Something like this might work, you drill the hole then put this in. The problem I have with sheet metal screws is holding power after they have been removed and reinstalled a few times.

 

[unsafervguy]

I would think as long as you can maintain proper edge distance on the holes you should not have a problem. but without more info, it just a guess.

Bob

 

[Tom-D]

but without more info, it just a guess.

Bob

That's really where ya have to be.

I've always been taught you don't weaken structure with out proper engineering data, but would not know how to do that with a E/AB unless you have contact with the kit manufacturer.

 

[Tarheelpilot]

Personally I would be very hesitant to drill holes in the spar. If there was no other way to get the job done I would reach out to the kit manufacturer to see what they thought about your plan. Hopefully the would actually engage you in the conversation and not just say no to avoid liability. If they won’t help you I would find a good structure DER to consult with prior to busting out the drill.

 

[Briar Rabbit]

I have an engineering degree and worked at Cessna in the early 1970’s doing fatigue analysis on A37B’s, also an A&P. I would never modify a spar cap with additional holes irregardless of where it was in relation to a strut without consulting the design engineer. The center of the spar web - maybe, but not the spar cap! How about putting a doubler under the “D” as you have referred to it right in front of the spar and attaching your nut plate to this skin? Just extend your new sheet forward over the top of the spar an inch or two. It will be just as strong and you have not put your spar cap in any jeopardy.

 

[DaleB]

Just a thought here. Instead of attaching the leading edge of your cover panel with screws, could you put a joggle in it to slip under the spar cap angle, then screw down the sides and training edge? Again, just an idea. I too would be very leery of adding any holes in a spar cap without REALLY knowing for sure it's safe and properly engineered.

 

[Stewartb]

Not any Cub guys here I guess. A Cub’s leading edge metal has 90* returns for wrapping around the upper and lower spar caps where you screw it to the vertical edges of the spar caps. You can see the drilled screw holes in the picture in post #3. I assume the leading edge of this plane is metal? Adding tinnermans or nut plates there should be fairly straight forward. How does the leading edge attach to structure on this plane?

If you look up PK screws in a catalog you ‘ll find two types of points, type A sharp and type B blunt. The blunts are for screwing to the spar. Sharps are for screwing to ribs.

 

[Checkout_my_Six]

well....they don't call it experimental for nutin.

I prolly would find another way....vs. drilling holes in your spar. And, FWIW the highest tensile loading is on the "bottom" of the spar....the top is in compression.

 

[Capt. Geoffrey Thorpe]

FWIW the highest tensile loading is on the "bottom" of the spar....the top is in compression.

Unless you are flying inverted.

Oh, and the kit manufacture went out of business years ago. The design was sold at least twice, but the last place to make it appears to be shut down as well.
I did find the following in the assembly manual:

 

[Dan Thomas]

And, FWIW the highest tensile loading is on the "bottom" of the spar....the top is in compression.

The top cap, which is in compression, is normally much heavier than the bottom. This stuff fails in compression much sooner than in tension. It buckles. And drilling holes anywhere in a spar cap removes metal, reducing both tensile and compressive strength. Doing that to what could be a marginal design might be fatal.

The OP mentioned clamping the fabric under the aluminum cover using the screws. That fabric needs to be glued securely to that rib, and not just on the tip of the rib. The finish needs to come off and the fabric wrapped around the cemented along the rib's sides as well. It would rip out from under screws; there's a lot of tension on it in flight.

 

[Briar Rabbit]

I would prefer machine screws, or at even tinnerman nuts, but the ribs are aluminum U caps over foam with an aluminum "web" on the fuel tank side - no access for a nut plate unless I get medieval on it. And, PK screws are not uncommon for attaching fabric, so...
It will get nutplates at the fore / aft ends.

I had some time today to ponder your idea of using machine screws to hold down this access plate on top of the wings and how you could calculate the load on each screw. Here is a potential quick formula for a rough estimate:
Gross weight of the airplane times the maximum G load for the total lift vector on both wings. Now divide that by the surface area on both wings and multiply this figure by the surface area of each access panel screwed to the top of the wing. That will give you a rough estimate of the vertical lift vector trying to suck your panel upwards. Take this figure and divide it by the number of screws you will be using. The load on each screw will in all likelyhood not be uniform, some will have more load than others so apply a factor in addition to a safety factor for each machine screw not being perfect. Additionally you will have forces trying to deform your access panel since you only plan to attach it on the front and aft. As this happens it will be applying shear loads on your machine screws and try to rip the holes out on your access panel.

Given this very rough calculation how much load without a safety factor will be required of each machine screw and nut? And what safety factor do you feel comfortable with?

Have you thought about trying to put your access panel on the bottom of the wing instead of the top?

 

[Dan Thomas]

I had some time today to ponder your idea of using machine screws to hold down this access plate on top of the wings and how you could calculate the load on each screw. Here is a potential quick formula for a rough estimate:
Gross weight of the airplane times the maximum G load for the total lift vector on both wings. Now divide that by the surface area on both wings and multiply this figure by the surface area of each access panel screwed to the top of the wing. That will give you a rough estimate of the vertical lift vector trying to suck your panel upwards. Take this figure and divide it by the number of screws you will be using. The load on each screw will in all likelyhood not be uniform, some will have more load than others so apply a factor in addition to a safety factor for each machine screw not being perfect. Additionally you will have forces trying to deform your access panel since you only plan to attach it on the front and aft. As this happens it will be applying shear loads on your machine screws and try to rip the holes out on your access panel.

Given this very rough calculation how much load without a safety factor will be required of each machine screw and nut? And what safety factor do you feel comfortable with?

Have you thought about trying to put your access panel on the bottom of the wing instead of the top?

Except that the lift is not distributed evenly at all. Not even close to evenly. See this:

 

[Briar Rabbit]

Except that the lift is not distributed evenly at all. Not even close to evenly. See this:

Good diagram! front screws will have 4 to 5 times or more the load of the rear ones IF the same number are used front and aft. I was thinking that the rough formula I suggested might discourage the use of machine screws? I am sure you and I agree this is not a good idea?

 

[Dan Thomas]

Good diagram! front screws will have 4 to 5 times or more the load of the rear ones IF the same number are used front and aft. I was thinking that the rough formula I suggested might discourage the use of machine screws? I am sure you and I agree this is not a good idea?

Cessna uses an aluminum tank cover over their tanks, held by #8 structural machine screws into anchor nuts in the spars and ribs. LOTS of screws; there are several dozen of them in each panel, and that cover is of heavier sheet reinforced by hat sections on its underside. There is serious lift being generated there, trying to pull that sheet off. It's in the prop blast, which makes it lift even more. Get a look at one someday, and see how many screws are used along the leading edge of the sheet.

But the wing needs to be designed for this sort of thing. The ribs need to be heavier to carry those nutplates, and the spars as well need to be strong enough to tolerate a bunch of holes drilled in them.

Someone mentioned sheet-metal screws. NO WAY. Not unless they are into Tinnerman anchor nuts, and I regularly see those broken and don't trust them for any serious loading. They're intended for fairings and such.

 

[Stewartb]

Probably more applicable to look at a Cub. Tank covers aren’t a big deal.

 

[Capt. Geoffrey Thorpe]

The cover has to be at least as strong as the fabric. And, Fabric is often fastened to the ribs with PK screws or wire clips. (Currently, my ride uses 1/8 aluminum Pop rivets)

Back of the envelope in round numbers... 1300 pounds * 3.5G / 26' span (not including fuselage) * 5' chord = 34 pounds per square foot (.2361 psi). So for a 2 foot x 3 foot panel that's a total of 204 pounds not accounting for the lift provided by the bottom of the wing.

 

[Stewartb]

If you ever get an opportunity to see a fabric wing in high AOA slow flight you may be surprised to see how the fabric lifts up between ribs. Many Cub builders extend the leading edge metal aft to provide a surface to bond the fabric to. Whether it helps or not is a point of disagreement among builders. In all the Cub hours and Cub discussions I’ve never heard of a tank cover separating from an airplane. Even old ones with well worn screw attachments. The point of stiffeners on the cover is to maintain the contour. With that accomplished there isn’t much potential for the screws to fail. If you’re working on a 1320# fabric covered plane? Your best examples for how to modify yours is to look at other light grossing fabric covered planes.

 

[Capt. Geoffrey Thorpe]

If you ever get an opportunity to see a fabric wing in high AOA slow flight you may be surprised to see how the fabric lifts up between ribs.

The top of the wing is one place I can't see.
I can see the landing gear with some effort (and sticking a camera out the door), but not the top of the wing.

 

[Stewartb]

Here’s a top view of a Backcountry Supercub right at the moment of touchdown. This is probably the highest lift wing most of us have access to. The pillowing you see is the fabric lifting between ribs. As we expect the lift is greatest toward the tips. I have the same airplane about to roll out. My leading edges have been extended about 15” aft of the spar, where most Cub leading edges end. It’ll be interesting to get some pics of my wing to compare to this one. Not that the fabric lift is a problem. I don’t think it is. My leading edge extensions have more to do with protecting ribs while sweeping snow than enhancing lift. I just think the dynamics of a fabric covered wing are interesting.

 

[Dan Thomas]

The cover has to be at least as strong as the fabric. And, Fabric is often fastened to the ribs with PK screws or wire clips. (Currently, my ride uses 1/8 aluminum Pop rivets)

Yes, screws, rivets, clips, and lacing are all used to hold the fabric down. But the fabric, when it lifts, also pulls sideways a lot, and as a unified sheet it transfers those loads to the root, tip, and leading and trailing edges. All the fasteners do is keep it in contact with the ribs and so on. If you have a single panel of fabric held only by screws, or one edge of a panel held by screws, and the lift is applied, it will pull sideways and rip out from under the screws. And that is deadly.

I have re-covered several airplanes.

 

[Capt. Geoffrey Thorpe]

Yes, screws, rivets, clips, and lacing are all used to hold the fabric down. But the fabric, when it lifts, also pulls sideways a lot, and as a unified sheet it transfers those loads to the root, tip, and leading and trailing edges. All the fasteners do is keep it in contact with the ribs and so on. If you have a single panel of fabric held only by screws, or one edge of a panel held by screws, and the lift is applied, it will pull sideways and rip out from under the screws. And that is deadly.

I have re-covered several airplanes.

yup!
Glue, reinforcing tape, overlay tapes, etc.