Too Much Of A Good Thing
True or False: Oxygen can actually be harmful to breathe.
You might be able to see a curve ball coming from a mile away, but since when could it be dangerous to inhale…when it’s life-giving oxygen? With no intentions to mislead, nor to make inferences about the tragic fire inside the Apollo I command module in 1967, it isn’t the fact that 100% oxygen is highly supportive of combustion. It also has nothing to do with the dangers of an embolism when one goes SCUBA diving and then goes flying just a few hours later. It has nothing to do with taking a high-pressure oxygen tank up into a low pressure environment. And it isn’t because of the dangers of breathing industrial oxygen (which may contain impurities) or medical oxygen (which has water vapor that can subsequently condense and freeze, cutting off the supply). Mother Nature doesn’t have a Surgeon General, but the fact is, breathing even pure, 100% aviation oxygen can be harmful to your health. Yes, it’s true, as in: toxic symptoms.
And here’s how. The catch is, that you’d have to breathe it for quite a long time…like a few dozen hours at a stretch. In controlled experiments, using just 90% oxygen, observed symptoms included nervousness, lowered energy, irregular heart beat, as well as bronchial cough, fever, infection, and vomiting. True, these didn’t appear until the second day, and probably no pilot would ever have to be force-fed oxygen for that long a period at high altitude. (The maximum mission duration for even the few elite high-altitude reconnaissance pilots has been somewhere around 14 hours, which would still not be long enough.) Interestingly, switching to pure oxygen after an incident of decompression can actually aggravate the symptoms of hypoxia, which isn’t exactly intuitive. (The best remedy is to gradually increase the oxygen supply.) This phenomenon is actually known as oxygen poisoning.
Not Enough Of A Good Thing
Unpressurized flight above 14,000 feet MSL creates obvious problems for pilots. Aside from reduced aircraft performance and other commonly known problems experienced by non-turbocharged aircraft at higher altitudes, an unpressurized normally-aspirated Cessna 172 that could manage to claw its way up that high would suffer from what other problem?
- a rough-running engine if the pilot doesn’t lean sufficiently
- a non-running engine, if the pilot leans excessively
- a rough-running engine due to timing problems with the magneto
- loss of control from only slight increases in load factor or turbulence
Answer: The cold, thin air can also contribute to arcing and cross-firing in unpressurized magnetos, due to the fact that the insulation value of air is reduced along with its density. Air is a pretty good electrical insulator. Take it away, and the sparks won’t fly where and when we want them to. (Pressurized magnetos have sealed caps and plugs and obtain the needed extra pressure from a turbodischarger device.) So aside from mixture issues and mushy controls, the answer is C: you would also get caught in the crossfire from your own engine.
Mach 1, Standing Still
When traveling at the speed of sound at sea level (assuming we use a “standard” atmosphere at 15 degrees C), the dynamic pressure (the same pressure sensed by a pitot tube) is very roughly equivalent to…
- one fifth normal atmospheric pressure at sea level
- normal atmospheric pressure at sea level
- five times normal atmospheric pressure at sea level
- fifteen times normal atmospheric pressure at sea level
Answer: As anyone who has stuck their arm out the window of a moving car knows, moving air exerts a force. For those of us (the majority, I suspect) who have at least once in our youth stuck our outstretched palm out of a car window broadside at highway speeds, we have discovered that it exerts quite a lot. This force, when expressed per unit area, equates to what we know as pressure. In the case of moving air, or moving through the air, this is called dynamic pressure. Often symbolized by a variable using the letter “q”, dynamic pressure varies according to the density of the fluid in question as well as the square of an object’s speed through it. An equation quantifying this can be written as
q = ½rV2
where:
r = air density (in Kg/cubic meter, or slugs per cubic foot in our own quaint system), and
V = air velocity (in meters per second, or feet per second)
(As an aside, note that the units of dynamic pressure resulting from the product of these terms might not be what you would expect to see, such as pounds per square inch (or foot, in this case); or in the MKS system, newtons per square meter, but rather (using the metric system as an example) kilograms divided by meters and then seconds squared. However, these are the same units (or dimensionality, in terms of where the units of mass, distance, and time are located) as there are for a force per unit area: Pressure is a force per unit area. The unit of force is the newton; for the unit of area we’d choose the square meter. A newton is also a kilogram accelerated at a meter per second, per second, or Kg-m/sec2. Dividing that by one square meter in area gives Kg/m-sec2, or the exact same thing. QED)
So…if you were surprised by how your arm suddenly got shoved back at 60 miles per hour (or however fast one of your parents happened to be driving at the time) when you rotated your palm ninety degrees from edge-on “flight” to broadside, imagine what it must be like for a space vehicle accelerating to orbital velocity. This “max q” is why the Space Shuttle usually throttles back to as low as 65% power below 35,000 feet (after which point it can then throttle back up). So let’s plug some numbers in there. At sea level, the density of air is roughly 1.225 kilogram per cubic meter, or in the more clumsy English units, 0.002378 slug/ft3. (A slug is actually the amusingly proper term for a unit of mass in the British system, one slug “weighing” 32 pounds, and being the amount of mass that one pound of force would accelerate at one foot per second, per second.) The speed of sound at sea level is about 340.4 meters/second, or about 1116.9 feet per second. When you use these two quantities (regardless of the chosen units), it works out that an object traveling at Mach 1 would feel a dynamic pressure roughly equivalent to 70% of normal everyday atmospheric sea level pressure, which is closest by far to choice B (and not exactly equal to it, as Peter Garrison had said in FLYING magazine a few months ago). In the metric system, it would be about 70,972 Kg/m-sec2. Since sea level pressure is about 101,325 newtons per square meter (or 2116.2 lb per square foot, or the more familiar 14.7 pounds per square inch), that’s close. Dynamic pressure would about equal sea level atmospheric pressure at around Mach 1.2.) Still, it gives you an appreciation for how much power it takes to overcome air resistance at high speed…
