Been talking with a friend lately about the feasibility of a full greenhouse-style skylight in a Cessna-style semi-monocoque construction aircraft. Aside from the fact that a comprehensive STC would need to be granted for such a thing for a certified aircraft, and it would take 5 years of taking your DER to lunch, what are the engineering issues behind this?

What's up there in a Cessna? 2 carrythrough spars, some fuel lines, some control cables, maybe some wiring. Full greenhouse skylights are common in rag and tube construction, but in a monocoque construction the aluminum skin can be critical to the stress-bearing structure of the wing, which is why the modern Cessna wing only requires 1 strut. But what about that region between the wings directly above the cockpit? How much stress does that bear, and is there a good reason (Feds aside) that that skin could not be replaced with acrylic/Lexan/Plexiglas?

I use the Cessna example because it is ubiquitous and well known. There are a few monocoque kits available (Rebel, Tundra) that I would more heavily consider if I could open up that cockpit visibility and make it less cave-like. The old Cessna 120-style porthole skylights are a poor substitute for a full greenhouse.

I used to fly a Beechcraft Muscateer. Low wing with a big bubble windshield. Like flying a solarium. Hotter than a two pecker billy goat in the summer. I would like small skylights in the 182 but nothing bigger.

Tim

qmdv wrote:I used to fly a Beechcraft Muscateer. Low wing with a big bubble windshield. Like flying a solarium. Hotter than a two pecker billy goat in the summer. I would like small skylights in the 182 but nothing bigger.

Tim

I didn't ask if you liked the idea!

I'm thinking the skin takes the place of the cross bracing you find in a tube airframe. The overhead structure may start to parallelogram with gusts, landings ect. with a skylight on top.

I seem to remember the C-150 aerobat having 2 large 12 x 4 skylights.

While its not a full greenhouse roof it's close. The O-1 Bird Dog has several skylight windows above the cabin. I think that would be pretty cool to have.

For the 206 http://www.atlantic-aero.com/main/docs/cessna_observation_mods.pdf

robw56 wrote:While its not a full greenhouse roof it's close. The O-1 Bird Dog has several skylight windows above the cabin. I think that would be pretty cool to have.

It is very nice. Also the rear seat back is reversible, the rear pax can face aft & look out over the tail with great visibility.

I love the Bird Dog and all of it's windows. It's the only Cessna that I can think of that has that many windows. I like those 206 mods by the way.

maulewaco wrote: I have a 150 hanging from a tree for my kids to play in. I give spin training to my son who is six and all his buddies that come over.

Picture?

My C182P has the factory 'skylights'. If they were further forward they would be more useful to the pilot (I have my seat most of the way forward for over the cowling visibility) and if they were further back they would be better for the backseaters. I do like them for starboard steep banks-particularly in close quarters (Mtns. etc) and checking for a 'greasy airplane belly' a couple times on final.

lc

The two standard Cessna skylights could be installed anytime using Cessna parts, as they where a factory options for most models.
Most the windshield suppliers have the plastic, the rest are just doublers. The headliner is a trick.Have to move the speaker.

I saw an STC for two to seven or eight skylights in 180/185s. I recall by a speed-brake developer, but Precise Flight website does not show it. I saw one (with speed-brakes) at Bend. I have pictures in the hangar, will look later. Front row just like the L-19: the spars setst the front/back limits.

--Bill
Tracked it to Cubcrafters, too, but found good info:
viewtopic.php?f=7&t=3454
says five panes. looked like a lot of glass: 3 front, two back

SHORT REPLY
Zane, the only realistic way to do it with a chance of success is to use the Bird Dog in those photos as the engineering model from a structure point of view. Measure the thickness of the skins, doublers, frames, stiffeners, bulkheads etc. that allow the Bird Dog to have all those windows, and then your proposal for a 170 or 180 would be based on bringing the structure up to that level. Your DER (and the FAA ACO he answers to) will at least have a starting point that is known to be safe.

LONG REPLY
Having been through the approved modification game once or twice, I can verify it would of course be a lot of work, but it is clearly do-able IMHO. The reason it is (even remotely) do-able is because the FAA knows that the Bird Dog was indeed based on the 170 (and I believe some 195 thrown in). A good DER can easily run the load paths and stresses, it's pretty standard stuff for them. If this ever got to the 'serious' stage, I would say you'd have to plan on getting a junk fuselage section and rigging up a test fixture that applied different loads (via cheap hydraulic bottle jacks) to prove out the engineer's calculations. Your "sales proposal" to the FAA (PSCP, or Project Specific Certification Plan) would have to include the idea that the Model 305/L-19 has an excpetional record of structural strength that has been FAA certified for civil and military use, then proven out in combat, with no known structure failures. Then it would go on to paint a picture of increased safety, visibility, and situational awareness. Then it would go on to show how it is a pretty straightforward engineering task, based on long-understood principles of semi-monocoque structure. Then it would "close the deal" by showing you as being so conservative that you are going to eliminate the FAA's risk by performing NDT AND destructive testing. It would be easy enough to sell the general idea to the feds on a one-shot basis, but to get approved for an STC-PMA for production., it would have to be backed up by numbers from a DER who is well-known to the FAA, and then by not breaking and breaking the test fuselage in the fixture. Icing on the cake would be to test and break a stock fuselage and then another one with your mods.

I only use the Cessna as a well known point of reference. This would be on an experimental aircraft like the Dream Tundra or Murphy Rebel.

Ahhh... OK... no problem. Disregard the rant about an STC.

IMHO put .040" thick doublers (window "frames") that are at least 2 inches wide around your skylights, which have no sharp corners, and are at least 3 inches away from the nearest other window. or skylight. Use the photos of the Bird Dog as a guide... there is a structural reason that Cessna used several smaller windows instead of a few large ones.

This is the cub crafters STC. Pretty sweet!

since you used the word "greenhouse", I would politly suggest you spend some time in an Ercoupe in hot weather and direct sunshine before You start adding large windows to the roof of any plane. Greenhouse is an understatement.

I've got both a Skywagon and a L-19. If you look at the skin attach to the two carry throughs, you notice that they do not have full rivet patterns to support a skin stressed structure. The load is nearly all carried by the carry throughs. So in their case, punching through the skylites does not affect the strength. You do need to evaluate the actual way the plane you expect to mod carries the load. Unless they do it like the cessnas, you could indeed compromise your structure.

Skylites have two issues, they nearly always leak, or will eventually. In the dog, with all that glass and my Nordic pink scalp, nearly devoid of hair, a helmet is a good hedge against scalp burn.

I do credit my skylites to saving my life. ATC descended a Malibu above me, wich I noticed seconds before he hit me from above and managed to do a quick evasion. Never figured out why I happened to glance up just at the right moment.

dogpilot wrote:....I do credit my skylites to saving my life. ATC descended a Malibu above me, wich I noticed seconds before he hit me from above and managed to do a quick evasion. Never figured out why I happened to glance up just at the right moment.

Other than a fluke like this (happening to look up at the exact minute that an aircraft was descending on you), what is the real advantage to skylights? I don't have them in the 150XP & I didn't have them in the 170, and except for not being able to take celestial readings on my night cross-countries, never missed them. The C152 I trained in years ago a a small skylight (or pair of them, can't remember), and the only time I ever tried to use them to look for traffic I got a crick in my neck.

I think there should be a distinction between skylights and a greenhouse, because I have to agree-- skylights don't really add that much value in my opinion due to the limited field of view. They seem like more of a porthole. A greenhouse, however, meaning that the entire ceiling skin is actually plexi, provides an incredible field of view overhead, and in general is just pleasant, at least in the aircraft I've flown in that have them.

A for getting cooked, I think the smokey tint helps that quite a bit. I haven't really felt unduly baked in the Sport Cub flying in the summer.