# How Do Aircraft Fly?

| April 15, 2014 | (10) Posted in Tips

If you're new to the hobby or new to flying you'll be wondering, "How do Airplanes Fly?" Well I hear you!

### 1. The Forces On An Aircraft

Every aircraft no matter shape, size, wing area or pilot for that matter all experience the same forces; Lift, Thrust, Gravity and Drag. They are crucial in deciding wether an aircraft flies, or plummets into the ground like a rock.

Before looking into that we must first understand that the air we breath is filled with small molecules. The movement of these small molecules is what we feel as wind. The amount of molecules in an area is measured as density, it becomes less the higher we go up. The higher the molecules, the higher the pressure.

### a. Lift

$L = \tfrac12\rho v^2 A C_L$   it's that simple! OK, no it isn't.

ρ or Rho is the density of the air. (at sea level it is 1.2754 Kg/m3)

V is for velocity. (is true airspeed)

A is for the plan of the wing. (swept, delta etc. will visit this later)

Cl if for the Co-efficient of lift at the desired angle of attack, Mach number, and Reynolds number. (Getting a bit too complicated now!)

So what is lift? Lift is generated when the airfoil directs the air on either side. The air on oneside travels faster to catch up with the air on the other side creating an area of low pressure compared to the area on the other side. The pressure difference makes the area of high pressure want to push towards the area of low pressure which also causes it to push up against the airfoil producing lift! See it really is that simple, until we dig a bit deeper...

α is the greek letter alpha and we use it to show 'angle of attack'. The angle that is made form the line through the chord of the airfoil and the direction of the relative wind.

From the graph above we can see that lift increases with 'angle of attack' until a point where it suddenly stops. This is because the airflow over the wings changes with 'angle of attack' shown below.

Wind is lazy. When an airfoil is flat it will happily change direction and flow over it because it doesn't require much energy to do so. It will also flow over a wing at a high angle of attack, but not as easily as this requires more energy. At a stalling angle of attack, the wind will not flow over the airfoil completely because it hasn't got enough energy to be directed around the curvature of the surface causing it to break away and spool of the back of the airfoil.

The stall of a wing can be manipulated by shaping the wing in certain ways such as; sweeping them, adding flaps or by adding slats.

For example a rectangular wing will produce alot of lift and not much drag at low speeds but will produce too much lift and drag at high speeds, instead a simple delta wing would be used because it does not produce much lift at low speeds and it doesn't produce much drag. Also, at high speeds it will produce good amounts of lift and not much drag meaning that it has to land at much higher speeds than a rectangular wing.

Flaps push the airflow down producing high amounts of lift and slats divert airflow over the wing at high angles of attack.

But how does my simple airfoil poduce lift?

Sir Isaac Newton's third law states that all forces have an equal and opposite force. This happens on all airfoils. When high pressure air pushes on the surface of the airfoil, if the airfoil is stong enough to with stand the push of the air it will move in the direction of the force form the wind like a kite. The middle diagram shows the airflow being diverted downwards, producing an upforce against the wing pushing it forwards and upwards.

The diagram above show that the air being diverted downwards will give the wing lift and drag (blue and red). These two change with angle of attack. The green line shows another force which is when the airflow is being diverted downwards it produces an opposite and equal force back onto the wing propelling it upwards.

However, The lift coming from the shape of the wing alone is not enough to make lift nor is the opposite and equal forces alone enough to make flight. They actually work together.

### b. Drag

Drag is simmilar to lift in it's equation.

$F_D\, =\, \tfrac12\, \rho\, v^2\, C_D\, A$  is it a bit more simple yet?

FD is the drag force,

ρ is the density of the air,

v is the speed of the object relative to the air,

A is the cross-sectional area,

and CD is the drag coefficient.

As the angle of attack increases so does the drag because a larger surface area is being subjected to the airflow and the airflow is using up more energy whilst being diverted around the wing. (shown below)

### c. Thrust

Thrust can be from a propeller, a jet engine or from gravity.

#### Propellers

Propellers are essentially radially mounted airfoils. They work in the same way by producing lift using the pressure difference on either side of the airfoil.

#### Jet Engines

It's best to show you a diagram..

### d. Gravity

Not all gliders have engines. Instead they are towed up and slowly soar down to the ground using gravity.

As objects on Earth increase in height they gain Gravitational Potential Energy (GPE).

The equation is GPE = M x G X H

G is the gravitational field strength. The G on Earth is 1; on Jupiter it is 2.6 and on The Moon it is 0.6.

1G is 10N/kg

M is the Mass normally measured in kg.

H is the Height normally measured in metres.

**Fact of the day: Since 1983, a metre has been defined as "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.**

Test Question: On Earth, a ball of mass 0.5kg is kicked straight up. How much GPE does it have at its highest point 6m off the ground?

Answer: GPE = 0.5 x 10 x 6 = 30J

As an object falls it converts its GPE into kinetic energy or speed to you and me.

This is how gliders work.

Modern gliders have a glide ratio of 1:50 meaning that for every 1 unit down they go they go 50 units forwards. E.g. 1 metre lost in height equates to 50 metres forwards.

Understood? No? Oh well, I tried my best!

petedotnl on May 1, 2014
interesting stuff. thank you.
Ultimate_Red on May 1, 2014
You're welcome! This is my first article so I hope people like it.. :D
FinalGlideAus on May 2, 2014
Actually I think the explanation in this video is the most logical :). It also explains why you can't use phones on planes...
ChrisCal on May 1, 2014
When interviewing for my first pilot job I was asked by the interviewer/owner of the small charter airline" What make an airplane fly?" After babbling about all the stuff above he stopped me and said "NO, you're all wrong. You know what makes an airplane fly? MONEY! Lots and Lots of money!" :) Great article.
RaptorTech on May 3, 2014
Article looks good except for on thing: why does the air flowing over the top of the airfoil meet up with the air flowing under the airfoil in the same place? This is a misconception. Nothing says they have to meet up. In fact, air flowing over the top of the airfoil moves even farther than air flowing underneath. If you were to release to air molecules at the same time, the one going over the top would get the the trailing edge first! Even though it had more distance to travel. I don't understand the property myself, and this is something that I have been wondering about for a fair amount of time, and I haven't found the answer online yet. If you know anything about why this occurs please reply!

Pic of what I mean: http://flyinglines.org/odds.0607.airflowintunnel.gif
Ultimate_Red on May 12, 2014
Yes that is correct. Scientists have found that the airflow does not meet up with the airflow at the trailing edge.
I will input this then.