Cabin Door Upgrade

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Cabin Door Upgrade

Postby SheepdogRD » Sat Feb 28, 2015 9:58 pm

Our standard cabin doors are light, functional, and easy to install. But they don’t seal very well, and they’re certainly not the airplane’s best feature. I wanted doors that seal out the elements. I posted earlier that we were working on a system, but suggested it was too early to declare victory. Now, though, we have a flight-proven system.

I tried plenty of approaches that led to dead ends or were impractical to install before I found a solution: an “edge-grip” seal. It mounts on a flange epoxied to the doorframe tubing. Here’s the seal and one version of the seal mounts we built:

Extrusions Mount.jpg

The system can be installed as the plane is built, or it can be fitted after covering and painted or powder coated before installation. It does slightly reduce the size of the door opening, so existing doors need modification.

Frame Components - Version 1
I’d been talking with Scot Blankenship about ways to build the support system for the seal, and he’d contributed some really good ideas. His Highlander was already covered and painted, so we decided we should install the system on his plane and get him flying.

Scot’s system is based on aluminum extrusions with 1/16” wall, like the picture above. The 3/4” (inside dimension) channel fits down tightly over three sides of the 3/4” steel fuselage tubes that form the door opening, and squares off the surface of the doorframe. A 1/2” aluminum angle is solid-riveted to the channel, and that assembly is epoxied to the fuselage rails. Once the frame is installed, the edge-grip seal goes on the angle, all around the frame (except at the top).

Here’s a cross-section sketch of his system, with the door just barely open:

Door Cross-Section 2.png
The C-channel is labeled 7/8", which is its outside dimension, but it's sold as 3/4" C-channel, referring to its inside dimension

The Lexan door material doesn’t lay against the fuselage fabric. Instead, it’s sized to fit just inside the channel so it rests only against the seal. The Lexan closes against the leaf of foam and presses it in. This seal is particularly good for this application, because air that might get under the door is trapped and simply pushes the seal tighter against the door, keeping out wind and weather. The base of the seal is hard rubber, and it protects humans from the “blade” it grips, so getting in and out is still comfortable. The seal also protects the bottom of the doorframe from feet.

On Scot’s plane, N982LT, we installed the C-channel first, and got all those parts to fit. Then we added the angle, and then made the door parts. We held the parts on with painter’s tape. In this picture, all the frame and door parts are installed (except the middle bar of the door), but no epoxying is done, yet:


The outer ring of silver is the seal mount; the inner ring is the square tubing door. The door is shimmed ⅜” away from the blade, resulting in about 1/4” gap between the door and the installed seal.

After Scot’s parts were fitted, he had them powder coated, and then epoxied the seal mounts in place. Here’s the finished product:


Scot has well over 300 hours on this door system, and he’s very happy with the result. He flies comfortably in winter weather.
Richard Holtz
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If just enough is really good, then too much ought to be perfect.
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Cabin Door Upgrade, Part 2

Postby SheepdogRD » Sat Feb 28, 2015 9:59 pm

Frame Components - Version 2
After we got Scot’s parts finished, I kept refining and testing new designs to make the system lighter. For N7340Z, I built seal mounts from .032” 6061-T6 aluminum sheet, bent to form inner and outer components. The two parts are riveted together with countersunk 3/32” solid rivets (AN426A-3-3 or -4) to form a mount that clips down over the door tubes. The mount sections are epoxied in place, and they’re absolutely solid. Here’s a sketch of that system:

Doors - Round Tube.png

The outer component is a 1-1/2” strip bent 90 degrees twice to form two 1/2” flanges. The inner component has a 1/2” upper flange, and the other segments are 1/4” and 5/16” wide. It uses two 45-degree angles to reduce weight, and to make it easier to install interior materials.

Around the door, each mount section is separate. Here’s the lower part of the right side door opening with the mount sections in place, but not epoxied:

Door Frame without Seal - Closeup.jpg

I put large rear windows in N7340Z, and built the window panels to be epoxied in place. I bent the forward edge of each window panel so it became the outer component of the seal mount:

Door Frame + Window without Seal.jpg

I made a new Windshield Angle Bracket that incorporates the seal mount, but it would have been easier to simply rivet aluminum angle to the standard part. Either way, this section of the system doesn’t get epoxied in place:

Door Frame with Seal - Front.jpg

On the SuperSTOL, the lower front door frame would need the added angle, too.

Here’s the back of the doorframe with the bead seal temporarily mounted:

Door Frame with Seal Pre-cover - Rear.jpg

You may notice that the doorstop blade between the two back tubes is gone. Troy confirmed that it’s non-structural, so I cut it out. On Scot’s already-covered plane, we didn’t cut it completely out, but we reduced the height so it wouldn’t interfere with the seal mount installation.

When covering, the fabric wraps over the outer component and can be stopped at the blade or it can run up onto the blade. Here’s N7340Z, covered, with the seal installed:

Door Frame Covered with Seal - Rear.jpg

I’m still working on these… the seal mounts were completed, but I’m revising the cowl side panels, so I’ll be rebuilding those parts.

The flange and seal shrink the door opening about 5/8”. It’s not a noticeable change in use, but it means that most of the door tubes must be slightly shorter. We both built doors from scratch, but existing doors could be made to fit by cutting a section out of each tube. The joints could be welded or patched with epoxied and riveted inserts. New Lexan would be necessary.

On N982LT, Scot’s doors are welded 3/4” square 6061 aluminum tubing. The center bar is ¾” round tube, and is bowed like the center bars in standard doors. The doors are strong, and a single latch pin at the front of the bottom bar easily holds them closed and sealed. Scot didn’t build separate windows in his doors.

For N7340Z, we built doors from ½” 6061-T6 tubing with welded joints, but I haven’t installed them. The SuperSTOL door “kits” would work well, too, and they eliminate welding. They consist of bent and welded steel fittings for the tube junctions, and aluminum tubes that are riveted (and optionally epoxied) onto them. Troy suggested gusseting the corners, and that’s a particularly good idea if epoxy isn’t used in the joints.

What About Windows?
Neither Scot nor I put windows in our doors. I have a couple of ideas for adding windows, but that’s another development project -- a good winter project after we’re flying.

It’s Do-It-Yourself
Either version will work on covered or not-yet-covered aircraft. We haven’t set up a kit system, because neither of us wants to be a manufacturer. If you want to build a system like this, I’d be happy to answer questions. I tried a whole lot of stuff, and I might be able to save you some trouble.

Scot’s system requires fewer tools, because we simply cut the extrusions to size and riveted the angle to the channel. The formed aluminum parts I used are thinner and lighter, but they take more fabrication time.

Neither of these systems is a weekend project. The major investment of time is in the details: getting the parts to fit so they meet perfectly at the corners. Some of the junctions are at odd angles, so there’s a lot of hand-filing to make them fit. We made several parts more than once. Doing a sample corner with some short pieces is instructional. Once you “get” the process, start with the long parts. If you goof, you can cut them down to make the shorter parts.

If you’re retrofitting, you can do all the fitting without taking your plane out of service. In fact, you could probably fly right up to the point where you revise the doors.

If your plane isn’t covered yet, perfect fit is less important because fabric and paint can cover a lot of sins. You still want to be sure to get a smooth top line so the seal can go on straight.

Alternative Systems
The primary advantage of the systems shown here is that we know they work. Finding the functional seal was the real key, and knowing it works opens up a range of possibilities.

The seal specs call for a blade between 3/64" and 9/64" thick. Ours were built with processes I know, but other blade designs are certainly possible, and you may come up with something lighter, or easier, or less expensive.

A middle ground between these techniques might be to form an .032 C-channel that just fits on the fuselage tubes, and rivet a formed blade to it.

If welding is in your skillset, a blade of .032” 4130 steel could be welded to the doorframe tubes, and just the outer component of formed aluminum or fiberglass could be attached to it. I’d start with some tabs and see if that’s all that would be necessary.

If fiberglass is in your comfort zone, perhaps the outer component could be made of preformed fiberglass, and that shape could simply be glassed onto the tubes.

Come and Visit
We’re both happy with the systems we have. If you want to see them up close, check with Scot at the factory and arrange to see N982LT. If you get to Atlanta, N7340Z and I are less than an hour drive west of Atlanta International Airport.

The two-for-one option is to be at the the Just Plane Fun Fly-In in Brasstown, NC. I may not have N7340Z flying yet, but I do expect to have her up there, and Scot’s there every year with N982LT.

Raw Materials
The seal we used is McMaster-Carr part 12335A56, We needed about 17 feet of it to go around both doors. I’m exploring adding the seal across the top of the door, and that will make the total requirement 22 feet.

The aluminum seal mount parts aren’t structural, so hardware store aluminum extrusions work. The C-channel is 3/4” x 9/16” x 1/16” (Home Depot Model# 802667, SKU 797138, The angle is standard 1/2” x 1/2” x 1/16” aluminum (Home Depot #800047, These extrusions come in 8-foot lengths. They’re currently shipping free with a $45 order, and three of each should give you enough material with some extra for mistakes, and will make it over the free shipping mark.

I used AN426-3-4 countersunk head solid rivets on the aluminum extrusions, and AN426-3-3 on the .032 parts. If you order the rivets from Aircraft Spruce, an ounce -- 250 to 300 rivets -- is plenty.
Richard Holtz
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Re: Cabin Door Upgrade

Postby Wmilbauer » Sun Mar 01, 2015 9:10 am

I used Richards second method and it works very well although I haven't flown with it. The only problem I had with it is the aft upper part of the door the clearance will not allow the thicker seal but the seal supplied with the super stol for the slats seems to work well for aft upper section and the recommeced seal for the rest of the door.
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Re: Cabin Door Upgrade

Postby Tralika » Sun Mar 01, 2015 10:36 am

Thanks for the very detailed post. It obviously took a great deal of time to put together the photos and diagrams as well as the clearly written text. Much appreciated.
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Re: Cabin Door Upgrade

Postby moving2time » Mon Mar 02, 2015 3:53 pm

Sheepdog, that is one heck of a post. When can we expect to see the fully detailed and illustrated build manual with tricks and tips? It would be a huge hit! Very nice work. I consider the door assembly the weakest part of the Highlander and I have though quite a bit about how to improve it. Your details are quite simple excluding the modifications that would be necessary to the door itself. Looks like your modification improves the seal as well as improving the drag. Did you happen to weigh the parts before assembling them for a total weight increase on both aircraft? Great post. Joe B
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Re: Cabin Door Upgrade

Postby SheepdogRD » Mon Mar 02, 2015 6:13 pm

Thanks, Joe.
moving2time wrote:Did you happen to weigh the parts before assembling them for a total weight increase on both aircraft?

We didn't think to weigh the parts as we built them, but I went back and dug up the weights of the raw materials. The seal material is reinforced with a steel spring to make it grip, so it weighs 0.106 lb/ft, or about 1-3/4 pounds for both doors. The C-channel adds about the same amount, and the angle adds just under a pound total. Throw in the weight of rivets and epoxy, and the extrusion system should still be well under 5 pounds. Using the formed .032 aluminum probably cuts that back to under 4 pounds.

I remind myself that those big fat tires we've put on add 4 or 5 times as much weight as better doors.

Scot says it's well worth it...
Richard Holtz
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