Do Small Airplanes Fly Better on Cold Days Than on Hot Days?

To a non-pilot’s eye, on cold days and hot days, an airplane taking off, flying overhead, or landing looks pretty much the same.  Sure, if it’s icy or raining very hard, there is a chance that the airplane could slip or hydroplane, but other than that, there doesn’t seem to be much of a difference.

But there is a difference.

Stated simply:  Airplanes tend to perform better when it is colder.

Why should that be?

 

The “Small Airplane”

Even though all airplanes benefit from colder weather, we will focus on what are called small airplanes.  These are mostly the four-seater or perhaps six-seaters that are usually used for fun or very-small-business transportation.

We are focusing on these airplanes for a few reasons, but the most significant of these is that people first learning to fly almost always learn in small airplanes.  Also, small airplanes experience the greatest boost from cold weather; it is more obvious and generally more advantageous for small craft compared to the effects on the giant airliners.

If you learn to fly, these factors will become very important to you very quickly.  And if you’re a passenger on a small airplane, it will be a more enjoyable, informed flight for you if you know about cold-weather vs. warm-weather differences.

 

Millions of Molecules

Don’t worry – not a lot of chemistry here.

Airplanes depend on the air in a number of ways, and the characteristics of that air, or the air masses through the airplane files, affect every one of them.

 

You sure don’t have to memorize this, but you will probably find it interesting:  Air over the earth is comprised of several gases in the following percentages:

Nitrogen:  78.08%

Oxygen: 20.95%

Argon:  0.93%

Other stuff (carbon dioxide, neon, helium, methane, krypton, dinitrogen oxide, hydrogen, xenon, and ozone):  0.04%.

 

No matter how hot or cold, or how high or low, the percentages are the same.  However, as one gains altitude, the overall “amount” (partial pressure) of these gases gets lower.

It is the oxygen that affects airplane engine performance.  The entire airmass affects other parts of an airplane’s functioning.

What we need to focus on now is how close together or far apart the molecules may be at a given moment.  The closer together the molecules are, the more effective they are in effect being how an airplane flies.  For example, airplane engines that run on gasoline depend on a mixture of gasoline and air.  If an airplane takes in the equivalent of 1 cup of oxygen, and if the molecules are very close together, that’s a certain number of oxygen molecules.  But if the same airplane takes in the equivalent of 1 cup of oxygen, but the molecules are very far apart, that is a much smaller number of oxygen molecules even though they’re all in the same size cup.

You may remember that heat is related to how fast molecules are moving, and, in a gas, how far apart the molecules are.  In cold air, the molecules are not moving as fast as they are in hot air.  So one cup of cold air actually contains more air molecules than does one cup of warm air.  This is the crux of the whole issue here, as we will see.

 

How Airplanes Depend on Air

Airplanes depend on air in three major ways: engine combustion, the mechanical interface of the outside of the airplane in the air, and for certain sensors that drive instruments on the cockpit panel.  Oh, and the pilot and passengers have to breathe, too!

 

Air and Engine Combustion

To turn the propellers of a typical small airplane, the engine must have many thousands of small explosions in the engine’s cylinders every minute.  These explosions push the pistons, and the pistons help turn the propellers.

Each explosion depends on an aerosol made of air (containing oxygen, of course) and a mist of gasoline.  When the air/gasoline mixture is just right (this is called in aviation the stoichiometric mixture), and the glow plug (like a spark plug) fires, that explosion pushes down a piston and that helps turn the propellers.

At a given altitude, on a cold day, there will be more oxygen available for a cup of air than on a hot day.  That means that the engine will actually be able to develop more power to turn that propeller when the day is cooler compared to when the day is hotter.  So far, so good.

Things get a little complicated as an airplane climbs.  Standard temperature automatically decreases as one goes up in altitude.  You notice that if you drive up to a mountain resort and find that it’s many degrees cooler than it was at the foot of the mountain that same day.  When an airplane climbs, not only is the airmass it getting cooler, but the air pressure is also getting lower.  You’ve of course heard that “outer space” is a vacuum.  The density of air starts at the highest just above the surface of the earth and gets lower and lower until you get all the way up to space where the density of air is zero.

The small boost to engine operation because of cooler air up there is not enough to overcome the loss of engine performance due to reduced air density.

Some airplanes have turbochargers.  These devices shoot extra air into the engine’s ports so the air pressure is much more like it is near the earth’s surface even if the airplane is several thousands of feet above that surface.  This permits “turbo’d” engines to perform better rather than worse as they climb.

 

The Air and Mechanical Interfaces

Another way that air affects an airplane’s ability to fly is when those air molecules hit airfoils and other portions of the airplane that stick out into the wind.  The basic idea is that the more air molecules hit an airfoil each second, the more effective that airfoil is.

You have probably had the experience of sticking your hand out the window in a car as a drives on the street and comparing that feeling to the same car driving on a freeway.  You get much more pressure against your hand when it sticking out in a 50 or 60 or 70 mile an hour wind than you do when it’s only being hit by 25 or 30 mile an hour wind.  That’s the same principle: more air molecules per second have a greater effect.

Let’s consider an airplane’s ailerons.  Ailerons appear near the wingtips of both wings, at the rear of each wing.  They allow the airplane to roll.  A roll is a clockwise or counterclockwise motion of the airplane, as the pilot commands. A roll contributes to an airplane’s right or left turn.  (The vertical rudder is also involved in a turn.)

When an aileron is deflected up into the wind, the air molecules hitting that aileron will push against it.  Pushing against an aileron that is tilted up from the rear of the wing will cause that wing to get pushed down.  Think about it:  Air molecules are hitting an upward-tilted aileron at an angle, and some of the force of the wind tries to shove the wing downward.  Obviously, the aileron on the opposite wing that is canted downward into the airstream will tend to lift that wing.  So when ailerons are used, one wing is being pushed down and the other wing is being pushed up.  That initiates the airplane’s roll.

More air molecules hitting an aileron will push harder, causing the airplane to roll faster.  Easy to predict, right?

The same logic applies to all external control surfaces the airplane.  Colder (denser) air allows the airplane to be controlled more precisely because more molecules contact the surfaces each second.

 

The Air and Airplane Instruments

The third way that air effects and airplanes flight relates to an outside sensor called a pitot-static probe.

The pitot portion of the pitot-static probe is a tube with a hole at the end that faces into the airstream.  The faster an airplane goes, the faster air blows into that hole.

The airspeed indicator depends on the air flying into the pitot hole.  The instrument translates air pressure into what’s called indicated airspeed.  The more pressure that’s blowing into that hole, the faster the airplane is going, and the higher the indicated airspeed.  Very straightforward.

Here things get just a little complicated, though.  It is true that the pitot tube takes in air molecules and causes the airspeed indicator to show the airplane’s indicated airspeed.  But if the airplane is flying in a very hot environment, the airplane must move faster through the airmass to show the same speed.  If it takes a million air molecules per second to show an indicated airspeed of 75KT (knots, a measure of airspeed), it still takes a million molecules whether the air is cold outside our hot outside.

So if it’s hot outside (those air molecules are therefore further apart), the airplane must fly faster through the air mass to collect that same million molecules.  If the airplane is still to maintain 75 KT indicated, and if it’s nice and cool outside, the airplane can fly slower through the air mass to collect those million air molecules.

 

Implications for Flight

This has a lot of implications for flight.  Let’s just assume that there is no wind.  If the pilot wants to fly at 75 KT, and it’s hot outside, he/she must fly faster over the ground to collect those million air molecules (per second) down the pitot hole!

An airplane moving faster over the ground takes a longer runway for takeoff and for landing.  That’s why you may have noticed that runways in hot areas like deserts are much longer than they are in cooler areas such as in most parts of Alaska.

The technical term for what you have just learned about is density altitude.  The altitude at which an airplane appears to be flying, in terms of its performance, depends on air density.  Air density depends on the temperature of the airmass as well as the altitude.  An airplane operating in high-density altitude does not perform as well in terms of the operation of the engine, in terms of the mechanical interfaces, and in terms of the instrumentation.

So there you have it.  Airplanes flying in cooler air really do perform better, on the whole.

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