• Lift Principle on aircraft:

    Where in the solar system would an aeroplane fly best?

Many people believe that planes fly by magic (well, not so many people actually), but the reality is that it’s all based on physics and we’re sure that you, as a loyal reader of our blog, already know this.

In some of our recent posts, we have already talked about key concepts to understand why planes fly: the Venturi effect, Bernoulli’s Principle and even the effect of the g-force when flying a plane.

In today’s post, we are going to talk about lift principle on aircraft and, to do so, it is essential to talk about density: what it is, how it influences lift and what its variability is when planning a flight. Ready? So read on!

What is density and what does it have to do with lift on aircraft

Before we start talking about density in aviation, let’s look at the generic concept.

We call density the ratio of the mass to the volume of an object, and we refer to it with the Greek letter rho ρ.

In the International System, the unit of measurement for density is the kilogram per cubic metre, kg/m³.

The easiest way to understand the concept of density is with an example:

Imagine two suitcases equal in size and completely full; one contains straw and the other contains lead. Which one weighs more? Clearly, the one with lead.

This is because the density value of lead is much higher than that of straw and, even with the same volume, it weighs a lot more.

Lift equation

Lift on aircraft is calculated with the following equation:

For an aircraft to fly, the lift must be equal to or greater than its weight. At equal values of speed, size, shape and position of the wing, the only value that is independent of the aircraft itself is the density of the atmosphere. The higher the density, the greater the lift.

This is why an aircraft flies more easily at low altitude and can only climb to a certain height (service ceiling) due to the reduction in the density of the atmosphere.

But is the density always the same? Not at all! Density changes continuously, either because of temperature variations or because of altitude.

The higher an aircraft flies, and also the higher the temperature, the lower the density. For this reason, the worst conditions for aircraft lift is flying at high altitude on a very hot day.

Believe it or not, variations in density are very noticeable when flying. For example, our DA20s, during the winter, allow a climb rate of about 1000 feet per minute; while, during the summer, it is reduced to 750 feet per minute… That’s 25% less!

What is density and what does it have to do with lift on aircraft

In aviation, we work with density altitude, a term that relates the actual density to the altitude in the standard atmosphere.

For example, if you take off from an airfield with an elevation of 2000 feet and at about 30ºC, it is normal that, shortly after take-off, the aircraft will have a density altitude of about 6000 feet. This means that the aircraft will behave as if it were flying much higher than it really is.

And if this is noticeable in general aircraft, imagine in commercial aircraft. For this reason, it is essential that pilots ensure that the atmospheric conditions respect the aircraft’s performance limits at all times.

Quito, a very unusual airport

If you ask any commercial pilot about density altitude, Quito airport will probably come to the fore.

Mariscal Sucre International Airport in Quito, Ecuador, is located at almost 9200 feet altitude (2800 metres above sea level). This, coupled with its almost 30ºC in summer, means that commercial aircraft have to adjust their load sheets to the millimetre.

What is the place with the highest lift in the universe?

And back to the ‘crazy’ question we posed at the beginning of this post about lift on aircraft. Where in the entire solar system would an aeroplane fly best?

On Saturn’s moon ‘Titan’, we find only 1/7th of the Earth’s gravity while supporting a much denser atmosphere, in which an aeroplane would fly without any difficulty.

In fact, a person with suitable wings could easily take flight just by moving the arms. We can say, then, that this is the place with the most lift in the entire known Universe.

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