Cargo Door

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SheepdogRD
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Cargo Door

Post by SheepdogRD »

A while back there was a thread discussing the addition of a baggage door: http://wingsforum.com/viewtopic.php?f=218&t=21584. I’ve gotten ours to the point where it works well enough to talk about.

We don’t “need” a cargo door. But I wanted one for longer trips, for inspections (when I’d end up crawling into the cargo bay to get to the aft access panel), and as easy access to the towbar.

It's almost ready for cover:
Cargo Door 3s2.jpg
Here’s the system opened up:
Cargo Door 5s2.jpg
The opening extends from the top longeron down to just above the rudder cable. It’s about 25” wide at the top, 5” wide at the bottom, and a minimum of 19” high.

The structural integrity of the fuselage isn’t compromised. I did cut out a section of the (non-structural) side stringer between the spigots.

The doorframe is made from 1-½” x .060” 6061-T6 aluminum strips that I made into Z shapes with two 90-degree bends. These channels make a doorframe strong enough to lean on, and stiff enough to hold fabric through shrinking. They’re installed so the outer face is flush with the longerons and stringer. The inner blade of each channel holds the same seal material that I used on the cabin doors, McMaster-Carr part 12335A56, http://www.mcmaster.com/#bulb-seals/=mq36t8. Here’s how the seal looks installed on a piece of channel:
Cargo Door Z-angle and Seal 1s2.jpg
Upper channels run from the top longeron down to the stringer/spigot, and the lower channels run from the stringer/spigot to the bottom longeron. They form a V that meets at the junction of the vertical fuselage tubes.

A short piece of the Z-channel is fastened between the channel legs just above the rudder cable to add strength and to form the bottom of the doorframe. Just below the rudder cable, the lower legs are attached with Adel clamps to the vertical fuselage tubes. At the stringer/spigot joints, both top and bottom channels are epoxied and riveted in with small angle brackets of .030” aluminum.

Where the top channels meet the upper longeron, a ½” aluminum U-channel is installed, angled slightly so its outer edge parallels the slope of the fabric. It fits between the upper Z-channels and is epoxied to them and the longeron. The U-channel provides a place to rivet the upper hinge and the seal flange without drilling into the longeron.

Door skins are .030” aluminum, and the edges are reinforced with .030” aluminum Z-channel (¾”-½”-½”) riveted to them with 3/32” countersunk squeezed rivets (AN426A-3-4). The inner corners of the Z-angles are riveted together, which gives the doors the strength of three-dimension panels, but keeps them light. They’ll be covered with fabric.

The doors are hinged with the same aluminum piano hinge that’s used on the cabin doors. The door half of the hinge is flipped over so the door face is flush when closed, and will still open 180 degrees. It’s riveted between the door face and the Z-channel door reinforcement.

The entire opening is surrounded by the seal material, which keeps out the elements and cushions the opening when leaning into it. A strip of the seal material is also installed on a flange built into the top of the lower door, and the upper door closes against it. The ends of the seal flange also help guide the lower door to the proper position when closed, making up for the surprising amount of “wobble” allowed by the bottom piano hinge, which is less than 6” long.

The lower door is held closed by pivot latches that swing down behind the seal and keep ithe door flush with the fuselage. They’re made from 1/2” aluminum angle, 6-32 nutplates and 6-32 stainless steel machine screws with nylon washers to eliminate abrasion. Here’s one in the unlatched position:
Cargo Door Pivot Latch 1s2.jpg
… and here it is latched. The stop keeps it from vibrating open.
Cargo Door Pivot Latch 2s2.jpg
The upper door isn’t perfectly flat (because the top longeron is straight, and the stringer is curved), and the seal material pushes against the door, so it needs to be held flush with the fuselage at its bottom corners. The outward pressure is light enough that spring-loaded grab catches, McMaster-Carr 12855A12, can do the job well:
Cargo Door Catch 2s.jpg
One is mounted on a bracket at the rear of the upper door.
Cargo Door Catch 1s2.jpg
While another catch at the front of the door would work, catches aren’t positive locks, and there would be no way to ensure the door won’t pop open in flight. So, instead of that second latch, I installed a keyed cam lock at the front. It holds the door in and keeps opportunistic thieves out. The lock is weather-resistant, and uses the same key as the cabin door locks. To minimize the weight of the door itself, I mounted the lock in the aluminum window panel forward of the door.
Cargo Door Lock 1s2.jpg
The steel locking blade turns down into an aluminum angle strike plate mounted on the forward edge of the door:
Cargo Door Strike 1s2.jpg
The strike plate is angled so the blade draws the door in as it drops into place. The mounting holes are slots, and the 6-32 stainless steel screws are held by nutplates in the door reinforcement channel. I’ll mount an .060”-thick plastic rub strip to cushion the contact area. Here’s how it looks in action, from inside the plane:
Cargo Door Lock 3s2.jpg
I’ve sized the gaps around and between the doors for covering, but I’ll get the fuselage cover on, and then gauge how much -- if any -- I may need to trim the edges of the doors before covering them.

In the pictures, the upper door isn’t installed, yet -- it’s just clamped in place. The door and the top flange for the seal material will be riveted to the top channel after everything is covered. The seal material and the door portion of the grab catch will be installed after cover, too.

Yep, this system is heavier than just covering the area with fabric. I didn’t weigh the parts as I installed them, so I can’t provide a number. Being the first one I’ve built, I focused more on keeping it strong, rather than light. If I were to build it again, lighter gauge skin material would work, and I could reduce the dimensions on the door reinforcements. As long as there’s enough material to resist the forces of shrinking the cover material, I think the skin could be just a frame around the periphery of the door. I’m considering lightening holes to save a good bit of weight in these.
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Richard Holtz
Highlander N570L -- Ms. Tonka -- in gestation

If just enough is really good, then too much ought to be perfect.
cowboy
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Re: Cargo Door

Post by cowboy »

Great idea, going to build some to sell:)
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SheepdogRD
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Re: Cargo Door

Post by SheepdogRD »

Well, not yet, anyway. While I don't expect any problems, this is not a flight-tested system. I'll want to see this prototype covered and flight-tested first. Then I'd like to explore some ways to reduce the weight.
Richard Holtz
Highlander N570L -- Ms. Tonka -- in gestation

If just enough is really good, then too much ought to be perfect.
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SuperFly
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Re: Cargo Door

Post by SuperFly »

Looks great Dick!

Great workmanship. I like the positive key latch on the front end, I am curious though about the rear door catch. I wonder how it will behave with differing pressures along the fuselage. just concerned that perhaps it could pop the back half open in a turn or something as the pressure outside the fuselage skin drops and inside the cabin rises. I guess in my little pea brain, I would feel better about a positive latch fore and aft. But I like the simplicity of the latch...
Im slowly catching up with work, and am excited to get back to my airplane, who knows, I might even find some time to update my website too...
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Ben Schneider
Highlander #263 converted to SuperSTOL
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http://www.stolairplane.com
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SheepdogRD
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Re: Cargo Door

Post by SheepdogRD »

That little grab catch is pretty strong, but it's still a question mark. It should withstand the small pressure that will be exerted. Even if it pops, the back bottom of the upper door would stick out only a little, but that wouldn't be acceptable. Before covering, I plan to epoxy in a small triangle of aluminum skin aft of that area in case flight testing shows that I need to add a second cam lock in place of the grab catch.
Richard Holtz
Highlander N570L -- Ms. Tonka -- in gestation

If just enough is really good, then too much ought to be perfect.
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SheepdogRD
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Cargo Door Update

Post by SheepdogRD »

The door is done, though it isn't painted or covered, yet. Here's the final result:
2015-02-10 09.23.08 s.jpg
I decided a second lock was just too much extra weight, so I took the lock out from in front of the door and filled in the hole. I replaced the lock with a second plastic catch, matching the one at the rear. The catches are strong... they hold the door so tightly that there’s no way to open it without a handle. So I formed a little tab that can be pulled easily. It’s riveted to the back of the door face and sits in a notch so it’s flush with the bottom of the door.

I moved the lock into the lower door, and then made a catch on the upper door for a the lock arm to swing up into. The heavy steel arm that came with the lock is now a much shorter and lighter arm I made from .090” 6061 T6. The usual way to install this would be with the lock in the upper door, but I wanted to keep that door as light as possible in case I have it open and a gust blows it closed onto my head. I figure a lighter door will result in a smaller scar. :?

A few tasks remain: cut lightening holes in both door skins, paint the doors with zinc chromate, and cover them.

This cargo door would have been easier to install if I’d just put a single door in the area above the stringer, or a single door below the stringer in the bay in front of its current location. Making two doors fit and work together took a good bit of extra time. But, now that it’s done, I’m enjoying it… it’s sure a lot easier to install the header tanks and the cargo area floor panels.
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Richard Holtz
Highlander N570L -- Ms. Tonka -- in gestation

If just enough is really good, then too much ought to be perfect.
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Johnny C!
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Re: Cargo Door Update

Post by Johnny C! »

Great work, RD!

I love the cargo door!

Be safe!

John
There are many things that happen really fast when you are
flying an airplane. There is no sense in rushing any of the others.

I would much rather be looking down at the runway, than up at it.

Duane Sorenson & Rick Norton Gone West 6/8/09. Godspeed
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BDA
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Re: Cargo Door

Post by BDA »

Richard
Can you post some of these door pics in the new door thread so we can get them all in the same place

Please?
SuperStol XL Alaskan With Titan 340
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SheepdogRD
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Re: Cargo Door

Post by SheepdogRD »

Happy to oblige. They're posted.
Richard Holtz
Highlander N570L -- Ms. Tonka -- in gestation

If just enough is really good, then too much ought to be perfect.
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CanadianSheepHunter
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Re: Cargo Door

Post by CanadianSheepHunter »

How much weight does this add?

thx
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SheepdogRD
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Re: Cargo Door

Post by SheepdogRD »

CanadianSheepHunter wrote:How much weight does this add?
Realistically, mine adds too much. It's strong like an ox. I've already decided it needs a redesign, but that won't happen until after she's flying. The existing doors are far too heavy.

If I were to build this door system again, I'd use a lighter frame, lighter seal and lighter doors with a very thin skin or fabric cover. I'd keep the latches, though... they really work nicely.

Because the fuselage tubes in the area of the cargo door are placed by hand at the factory, not fitted in a jig, there's no way to make a cargo door kit that fits universally. If the door area was fixtured, we could produce a simple fiberglass framework that would be a quick-build solution: install the light fiberglass frame and a light aluminum door with a standard seal.

In retrospect, I wish I'd put a cargo door on both sides.
Richard Holtz
Highlander N570L -- Ms. Tonka -- in gestation

If just enough is really good, then too much ought to be perfect.
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CanadianSheepHunter
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Re: Cargo Door

Post by CanadianSheepHunter »

Thanks
keabro
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Re: Cargo Door

Post by keabro »

Out of curiosity. Why did you decided to have the door open in 2 sections rather than one big one? I think I'm going to try to adapt your design to use a gas strut and be a full door.
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SheepdogRD
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Re: Cargo Door

Post by SheepdogRD »

I figured a one-piece door that large would have to be heavier to maintain its shape, and it might be more difficult to get it to seal well. But it's certainly do-able, and I think it could be made lighter than mine. It would work much like the pilot's door. If you can keep it light, I'd recommend one on each side.

Figure out how you'll fabricate it, frame it, seal it, and latch it. I'll be interested...
Richard Holtz
Highlander N570L -- Ms. Tonka -- in gestation

If just enough is really good, then too much ought to be perfect.
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Re: Cargo Door

Post by keabro »

Haha as always it's easier said than done, but I'm going to attempt it.
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