As I write this on the Sunday after Thanksgiving, the ground is lightly covered with the first snowfall of the 2005/2006 winter flying season. I am sure many of you have already prepped your airplanes for hibernation and put them away on storage racks for the winter. I have been told that this involves running out any fuel remaining in the engine and replacing it with a water displacing lubricant such as air-tool oil, Marvel Magic Mystery Oil, or special purpose model airplane lubricants such as my favorite ... “Snake Oil” by Robart, (insert WD-40 argument here!), then carefully cleaning the airframe of any residues, and finally storing the fuselage in a nose down attitude to make sure the main engine bearing is bathed in oil for the winter. I say “told” because I have never done it. Instead, I simply fly my aircraft year-round, regardless of the weather.
If you have not flown in the winter months in Chicagoland, you have been missing out on some of the best flying conditions your airplane will ever see. Let me repeat that ... the best flying conditions your airplane will ever see are found during the winter. Your fingers might disagree, but your plane will be happier than any other time of year. This happiness occurs through the magic of “Density Altitude”, which I will now attempt to explain.
First, the anecdotal evidence given entirely in other pilots words. Last Sunday morning I had the Turtle out for a few flights. Yes, that's the same overweight slightly-bent under-powered 1990 vintage Midwest Sweet Stick .40 that during the summer months normally suffers from a stunning lack of performance. But the temperature was about 40 degrees Fahrenheit with 10 mph winds. The 30 minute flight included the entire flight envelope with high-speed low-altitude flybys, unlimited vertical climbs followed by powered descending flat spins, a few slow speed inverted laps of the field, and a few dozen touch and gos inside the center circle on the tarmac. The 12 ounce fuel tank does not help with the weight, but it does eliminate refueling.
The comments from the assembled pilots watching the flight included “What did heck you do to the Turtle?”, “She screams like a stuck pig”, “I'm glad today isn't a contest day”, and “Boy, you were really throwing the plane around up there.” Last winter I was asked more than once “Did you put a new engine in the plane?” Clearly, there is a noticeable performance difference during the winter months, and that performance advantage was noticed by spectators and is not just a figment of the imagination of the pilot with his fingers on transmitter sticks.
So, what is to account for this Jeckle and Hyde performance difference between Summer and Winter?
The answer is buried in the concept of “Density Altitude”.
Everybody should be fairly comfortable with the idea that the density of the Earth's atmosphere is inversely proportional to altitude. As altitude increases, air pressure decreases and the air gets thinner or less dense. At about 5,000ft, 20% of the Earth's atmosphere is below you, at 10,000ft, 30% is below your, and by 15,000ft, fully 50% of the Earth's atmosphere is below you. The human brain starts to suffer if deprived of 40% of its normal oxygen supply, which why full scale pilots require supplemental oxygen above 10,000ft and why the cabins on commercial airliners are pressurized to 6,000ft.
Airplane performance suffers with altitude due to reduced air pressure for three major reasons – reduced engine output, reduced propeller efficiency, and reduced wing efficiency. We will examine each of these effects on a model airplane.
First is reduced engine output. All internal combustion engines are essentially air-pumps. Air is drawn in through a carburetor and mixed with a fixed percentage of fuel which is then ignited and burned. The excess heat from the burning fuel charge induces expansion of the nitrogen in the air to drive the piston and produce the power required to both suck more air into the carburetor and to drive the airplane forward. Since the oxygen to fuel ratio needed to ignite the fuel charge inside the cylinder is a “fixed” proportion, less oxygen at altitude translates directly to less fuel at altitude. Less fuel means less flame and heat. There is also less nitrogen in the cylinder to heat. Less nitrogen means less expansion, which means less power. A double whammy.
Secondly, this smaller amount of power produced at altitude is harnessed to spin the same propeller. As the propeller blades spin, the blades “bite” into the air and push the air backwards producing thrust. Issac Newton said for every action there is an equal and opposite reaction and that Force = Mass times acceleration. So as the propeller throws the air mass backwards, the plane is thrown forwards by an equal amount. The problem is that at altitude, there are fewer air molecules to throw around. Fewer air molecules equals less mass, and less mass equals less force. The propeller gets less efficient at producing thrust at a given rpm. Bummer.
But it gets worse. An airplane wing generates lift by deflecting air downwards. Forget all of that crap you have been told about Bernoulli 4 and the supposition that individual air molecules travel further over the top of the wing than it does over the bottom, and that further travel means faster travel, and faster flow means lower pressure on top of the wing, which means lift is generated ... It's all crap that is repeated by ignorant people in an attempt to explain aerodynamics to even more ignorant people. So I will repeat. An airplane wing generates lift by deflecting (accelerating) air downwards.
If you still do not believe me, then consider the case of a hovering aerobatic airplane performing a torque roll. The entire weight of the plane is hanging on the propeller. The propeller is accelerating a column of air directly downwards. The balancing reaction to this force is to accelerate the plane upwards. When the forces are equal, the plane hovers.
Back to the wing. One more time. An airplane wing generates lift by deflecting (accelerating) air downwards. The deflected air has mass. Hmm. Mass times acceleration equals Force. So in a given unit of time, we have to push the wing forward far enough to encounter a large enough mass of air to accelerate downwards fast enough to produce a force large enough to overcome the downwards acceleration of gravity, and poof, the airplane flies! The difficulty is that at elevated altitudes, the air is less dense and has less mass per cubic foot, so we have to fly faster per unit of time to encounter the same mass of air that would counter gravity at lower altitudes.
Finally, all three of these effects viciously reinforce one another. Lower density air means a faster wing speed is required to encounter enough mass of air to deflect and overcome gravity. But that faster wing speed must be obtained by a less efficient propeller as there is less air to “bite” into. So the propeller must be spun even faster than dictated by just the increase in wing speed alone. And now we have the poor engine which must produce more power by using less fuel (to keep the mixture the ignitable range). Ugly, ugly, ugly.
The astute reader who is still reading this article will ask ... “But what do the effects of increased Altitude on airplane performance have to do with airplanes that are flown at the CRCM field in Busse Woods where the altitude at the field is always exactly 715 ft above sea level ???”
The short answer is that the apparent density of the Earth's atmosphere at sea level routinely varies by as much as 20% depending on weather conditions. The Standard Atmosphere (59oF and 29.92 in/hg) was devised to represent the average atmospheric conditions at sea level.
Higher temperatures, higher humidity levels, and low pressure storm systems all lower atmospheric pressure with measurable effects on airplane performance indistinguishable from high altitude effects. Conversely, lower temperatures, lower humidity levels, and high pressure systems effectively increase atmospheric pressure with effects on airplane performance that are indistinguishable from low altitude effects.
From classical physics, the ideal gas law ( P*V = n*R*T ) states that pressure multiplied by volume is equal to some constants multiplied by temperature. Realizing that over the short term in the real atmosphere, pressure remains constant as it is determined mostly be the height of the column of air above, we can reduce the pressure term to a constant and then reduce all the constants to a single constant and were are left with the equation ( V = kT ). So as atmospheric temperature increases, the volume of the air must expand and the air becomes less dense. But everybody already knows that hot air rises.
That higher humidity levels also reduce the mass of a cubic foot of air is not so obvious. The obvious question is how can adding water (which as a liquid is heavier than air) to the atmosphere make it less dense? Recall that air is roughly 78 percent molecular nitrogen, 21 percent molecular oxygen, and 1% trace gasses (mostly argon) which can be ignored. Counting the number of neutrons and protons in each atom, we find that a nitrogen atom has an atomic weight of 14 and an oxygen atom has an atomic weight of 16. Hydrogen has an atomic weight of 1. Ignoring the E=MC2 relativistic effects of weight gain in binding energy of atomic nuclei, a nitrogen molecule (N2) weighs roughly 28, while an oxygen molecule (O2) weighs roughly 32. So the average air molecule weighs about 31 (78% at 32 and 21% at 28). However, a water molecule is made of a single oxygen atom and two hydrogen atoms (H2O) for a total atomic weight of only 18. So every time we increase the water vapor content of the atmosphere by 1.0%, (replace some of the heavier nitrogen and oxygen molecules with lighter water molecules) we decrease the mass of the air by 0.5%, which makes it less dense. Notice that a 1% increase in water vapor also decreases the oxygen content by 0.21%, which works directly against engine power through the constant ratio fuel mixture effects. Again, a double whammy.
Finally, large scale weather systems have a direct effect on air density. The low pressure systems associated with stormy skies typically have pressures below 29.7 inches of mercury. The high pressure systems associated with clear skies typically have pressures above 30.3 in/hg.
So putting all of this together, the difference in air pressure between a typical humid summer day with the temperatures in the 90s and the pressure falling before a summer storm, and a typically drier and much colder winter day in the center of a high pressure system, will be the same pressure difference as several thousands of feet of altitude.
A real world example will help illustrate the magnitudes of the changes in density altitude The hottest most miserable day of the 2005 fun fly season occurred on July 17th, with a temperature of 95oF, a barometric pressure of 29.92, and a dew point of 61oF, which was not that bad for relative humidity of 32% at 11:00am. I will not bore you with the details of solving Van der Waals equations for partial vapor pressures in real gasses, but these numbers correspond to a density altitude of 2,535ft. Last Sunday, November 20th, the temperature was 42oF, the barometric pressure was 30.17, and the dew point was 36oF, which oddly enough had a higher relative humidity of 76%. These conditions produced a density altitude of -1,318ft, which is effectively a pressure greater than the standard atmosphere at sea level. The total difference in atmospheric pressure between the two days was equivalent to an altitude difference of 3,853ft.
Almost 4000ft worth of altitude is nothing to sneeze at. Differences of 6000 or 7000ft of density altitude between the hottest most miserable summer days and the coldest “flyable” winter days are not uncommon. Even at only 4000ft difference, the engine will produce about 20% more horsepower, the propeller will be about 17% more efficient, and the wing will be able to fly about 15% slower and still support the plane. And remember, the savings are cumulative for strait and level flight, so the plane appears to fly about 50% better, hence the “what did you do to the Turtle?” comments from the peanut gallery.
If you want to wish to experiment for yourself, detailed past weather data for every hour during the day are available at ... http://weather.chicagotribune.com/auto/chicagotribune/history/airport/K ORD/2005/11/20/DailyHistory.html ... and a very nifty on-line density altitude calculator can be found at ... http://www.srh.noaa.gov/elp/wxcalc/densityaltitude.shtml
To recap, on cold dry winter days with thicker air ... The engine produces more power, The propeller is more efficient, The stall speed is much lower.