Oxygen for the Falco
by Stephan Wilkinson
This appeared in the June 1989 issue of the Falco Builders Letter.
How silly that I, of all Falco builders one of the slowest, most pedestrian and most process-rather-than-product-oriented, should be the one to describe as flighty a system as an oxygen-tank installation. Of course, it's through the use of such dodges-and the construction of myriad ornate fairings, the constant puttying and reputtying of real and imagined surface faults, the endless pondering of such non-crises as upholstering problems-that I continue to avoid completion of the airplane. (Hey, you finish it you gotta fly it, right? Am I ready for this? Where does it say I signed on for this???)
In any case, one of my favorite forms of flight is trundling up into the narrow band of uncluttered airspace from 10,000 feet up to 18,000 (where, of course, positive control begins). Everybody else is either higher-the jets, many of the turboprop commuters, the pressurized guys and the turbo freaks-or lower. A lot of lightplane pilots apparently get nosebleeds any higher than exactly 7,500 feet.
You're above the summer haze layer, you're on top of a lot of weather, and you often can even get a good look at thunderstorm activity-nothing like being in the clear at FL 410, but a lot better than soldiering on through the summer crud at 7.5 wondering where the embedded cells are.
I used to fly up there as often as possible with my old Comanche 250, a light and overpowered little machine that buzzed up to five-digit altitudes quite nicely, and more recently with the Mooney 231 that I flew thanks to the generosity of a prosperous friend. The 231 was, of course, turbocharged, so it was quite at home at altitude.
Taking the Comanche to 16,000 feet could be a form of free turbocharging if I was going in the right direction and got one of the not-infrequent strong tailwinds. (My Comanche was an early-1958-model that had small fuel tanks, only four seats and a spare interior, so when I flew it alone, as I usually did, it had a rather remarkable power-to-weight ratio.)
More accurately, it was a form of flat-rating an engine, a common procedure with turboprops: since turboprops lose efficiency relatively fast with altitude, a manufacturer will install an engine that's much more powerful than necessary at sea level but that therefore ends up with adequate power at altitude. Put in a 1,000-hp engine but restrict it to a maximum of 750 hp, and you'll be able to pull that 750 hp all the way to 10,000 feet, say.
So since the little Comanche was originally designed to fly on 180 hp, let's say a Comanche 180's maximum output at 7,000 feet is 75-percent power, or 135 hp. My identically airframed Comanche 250 was still able to put out 135 hp (for it, 55-percent power) at about 15,000 feet. (I once wrote about this for the British magazine Pilot and got letters from horrified engine mavens who claimed that flying an engine steadily at such low power settings was a dreadful thing to do, but what do they know? I don't think you can kill a Lycoming O-540 with anything short of a sledgehammer, and in any case, the percentage of the engine's lifetime that it actually spent running at 15,000 feet or higher was pretty small. Some people take life so seriously.)
Anyway, this is the long answer to the question, "Why are you putting a 180-hp engine in your Falco, since that engine is virtually unobtainable and it won't make the airplane go that much faster anyway, huh?" If your 160-hp Falco is down to 120 hp at about 7,500 feet, I'll still be able to pull that power out of my "flat-rated" Falco at a somewhat higher altitude, and I'll be able to climb to altitudes you wouldn't bother with. (I used to also argue that a strong climb rate could get you through icing more quickly, but I've grown cowardly enough that I hope to never again experience the relevance of that.)
Hence my oxygen system, since the FAA says you need it above 12,500 and might well want it above 10.
Oxygen used to be a big pain in the ass. You snorted it through nosebags that recycled every belch, interfered with microphone use and were so leaky and profligate they'd empty a big oxygen tank in two or three hours. That's a thing of the past, though, once you discover the nasal cannula-a maskless oxygen-delivery system that pipes the gas directly into your nostrils through tiny, soft-neoprene hoses so light they rest on your upper lip. Sounds gross, I know-and I remember aviation writer Peter Garrison saying in Flying that his initial concern was that one would get what he uncharacteristically delicately referred to as "cooties" on the pipettes-but in practice the nasal cannula remains unsullied.
Even better, without needing to take off a mask, you can talk to ATC or passengers, eat, rip off enormous burps or do whatever else you normally do to pass the time in a cockpit.
Best of all, a nasal cannula is so stingy and direct in its delivery of oxygen that an ordinary tank lasts not three hours but more like 10. That's a big factor when you discover that the only thing harder than wearing an oxygen mask is getting an oxygen tank filled, at least here in the Northeast. One helpful local FBO said, "Go to Kennedy." Another had a set of big fill tanks but reserved them for his corporate tenants. A third mechanic elsewhere filled my little portable tank largely because it seemed like a good excuse to stop feeding wire bundles into the darkest recesses of a Cheyenne. It cost me $41, about half a mile of walking and the filling out of a variety of liability-mandated work orders, however.
The tank I use is a lightweight, uncharacteristically narrow bottle borrowed directly from modern medical technology. Your typical aviator's tank is borrowed directly from Curtiss P-40 technology, and I doubt the Scott Co. has done anything new for general aviation since they perhaps simultaneously discovered the wheel and fire.
Both the tank and my two nasal cannulae (pilot and passenger) were bought from Aerox Aviation Oxygen, 215 Masarik Avenue, Stratford, Connecticut 06497. As I remember, the set cost something under $400. Each cannula comes with its own flowmeter/indicator, which is nothing but a graduated plastic tube with a ball bearing trapped inside it: hold the tube vertical and adjust the needle valve on the tank until the ball bearing is floating on the stream of incoming oxygen at a level commensurate with your altitude. No messy regulator or complex valve. (At 15,000 feet, as an example, that level will mark a flow of just under 0.5 liters per minute of oxygen; at the same altitude, an ordinary mask will be using more than three times as much oxygen-1.5 liters per minute.)
One caveat about nasal cannulae: they're only good up to about 18,000 feet. Some salesmen will try to tell you that this is only because "they didn't bother to certificate them for higher altitudes-too much paperwork-but they'll work fine." Don't believe 'em. I took my system up to the low 20s in the Mooney one night and found myself coming unwrapped-the old feeling, "two plus two equals... lessee, carry the two... plus the, uh, what was the question?" A brisk descent to 17,000 solved the problem.
My first thought was to mount the oxygen tank semi-permanantly in the Falco under the baggage compartment floor, hidden away inside the tailcone with only the regulator end poking into the cockpit through a round cutout in the bulkhead of frame six. Dr. Ing. Alfredo Scoti pointed out that (1) there would be no way to secure the tank sturdily enough to prevent it turning into a 155mm artillery shell in any major deceleration, and (2) the bottle would be in unheated airframe space, making it absolutely necessary to use expensive and hard-to-obtain "aviator's oxygen."
If I ever plummet to my doom from 18,000 feet, what I'm about to write will certainly free anybody involved (other than myself) from liability, but since I think that's how it should be anyway, here goes: You can recharge your bottle with much more easily obtainable medical oxygen (have a doctor friend give you the necessary prescription). Medical oxygen differs from aviator's oxygen only in that it isn't moisture-free. Oxygen with any moisture content could freeze at altitude in a bottle or flow lines mounted in any unheated portion of the airframe, thus blocking the flow.
Tell you the truth, I know people who use even cheaper, vastly more available industrial oxygen-you get it at any welding-supply house-with no problem. I won't propound doing that without a little further research, but I did go chat with the guys at the local welding-supply house, and they said they'd be delighted to lease me a set of big (welder-size) tanks that could be refilled at $25 or so a pop, and what I did with the oxygen was my business. And yes, they'd be happy to fabricate the appropriate connector, as long as I did the actual connecting. After payment of an initial deposit, you deal with the bottles just like household propane: they replace the empty with a full one at the cost of the refill alone.
With such a rig, I'd be able to charge my little 2,000-pound bottle fully once, then be able to fill it the next time to maybe 1,500 pounds, then perhaps 900 pounds, as the main bottle depleted. (You need to gang a whole set of high-pressure feed bottles to consistently get full pressure out of the fill pipe.) Yet even two shots per bottle would be a lot cheaper than FBO prices, and the benefits of only having to walk out to the barn to do it are enormous.
It should be pointed out, however, that amateur oxygen-handling is nothing to be done lightly. The fire risks are tremendous if it's done improperly, and these tanks are potentially enormous projectiles that frequently kill and maim when mishandled. Just walk into any industrial-gases supply house, and you'll see a pretty sobering assortment of OSHA posters, handouts and instruction booklets.
If you aren't discouraged yet, my bottle is now mounted, at Alfred Scott's suggestion, on the forward face of the bulkhead of frame six, just high enough to clear the aft end of the center console, cradled in semicircular spruce standoffs and secured by stainless-steel straps bolted through solid structure in that frame.
The straps are straight out of the Aircraft Spruce catalogue, where they're listed as "fuel-tank hold-down straps"-0.50" stainless steel an inch wide and 36 inches long. (All it takes is one, obviously, cut into four appropriate sections.) Along with the straps are listed a pre-formed rubber anti-chafe channel that slips over the strapping to prevent metal-to-metal contact between tank and strap, and the necessary T-bolts to complete the installation (see diagram). Fifteen dollars buys you all the hardware you need.
Each pair of straps has a quick-release wingnut on its T-bolt so the bottle can easily be removed for refilling. The regulator end of the bottle extends toward the passenger's side of the cockpit, with the bottom of the bottle at the extreme left side of the cockpit, which puts the regulators and cannulae quick-disconnects in a position accessible to the pilot if he or she leans back slightly to reach behind the passenger's seat.