How do airplanes fly?

From time to time, passengers ask me what it is that keeps an airplane in the air. After all, we’re talking about a metal cylinder weighing some 400,000 kilos. It’s something of a miracle for many people. In fact, though, it’s not rocket science. It has everything to do with aerodynamics and propulsion. So how does an aircraft fly?

Aerodynamics

Aerodynamics is the name for the interplay of forces that drive an object through the air. If we look at a cross section of an aircraft wing, we can see that the upper edge is rounder on the lower edge. Let’s have a look at two air particles. One passes over the wing’s upper edge and the other goes past the lower edge. What we then see is that the air traveling around the upper edge has to travel a greater distance to get around the wing. In other words, the speed of the air around the upper edge is greater than that of the air travelling below the wing.

How does an aircraft fly

Some of you may have learned about Bernoulli’s principle in physics at school. It states that, the faster a gas (in this case, air) travels, the lower the pressure it exerts. That’s exactly what’s happening around an aircraft wing. The pressure against the wing’s upper edge is considerably lower than the pressure against the lower edge.

Air Flow

So now we can see the reason why an aircraft flies. We are actually hanging on the air because the pressure against the upper edge of the wing is lower than that against the lower edge. However, the wing does have to be passing through the air. In other words, there must be air flow around the wing in order for the pressure differential to arise.

How does an aircraft fly 2

Then we need engines to create propulsion. They cause the wings to pass through the surrounding air. It creates a balance in which we can see two forces horizontally. First, there is the forward propulsion created by the engines. Opposite that, there is the aircraft’s wind resistance. Vertically, we see the upward lift. And the aircraft weight, pulling it downward.

Flying Without Engines

Even if the engines stopped working—which happens very rarely—aircraft are still capable of gliding great distances. On every flight, as we make our approach for landing, we reduce the engine’s propulsion. Passengers in the cabin can hear this happening. We don’t turn the engine off completely, but they turn because of the air passing through them. So they are not creating any propulsion. In effect, it’s comparable to an engine that has been turned off.

An aircraft’s descent can often take about twenty minutes. During that time—and depending on the aircraft type—we can glide about 120 miles (about 220 kilometres). So, during that time, we’re sitting in a great big glider.

Wing Components

If you happen to be sitting at a window near the wing, you’ll have some interesting things to see. This is where it’s all happening—on the wing’s leading edge, at the rear, and above the wing, you can see all kinds of moving parts.

The wing is designed to provide optimum lift at normal speed, which we call cruising speed. For a jet, that’s about 850 kilometres per hour. Of course we can’t take off or land at that speed. We would need a much longer runway to do that.

RamonKok_flapsPhoto credit: Ramon Kok

Flaps

Lift depends on two things—the aircraft’s speed and the curvature of the wing. We adjust the wing’s curvature on takeoff and landing so that the wings provide greater lift. We do that with the help of the flaps. Flaps are long panels on the trailing edge of the wing that we can extend bit-by-bit. Some aircraft also have flaps on the wings’ leading edge—sometimes called slats. After takeoff, as the aircraft increases speed, we retract the flaps.

FLAPfoto

Ailerons

But you can also see moving parts on the wing’s trailing edge during the flight. These are called ailerons. We use these to make the aircraft turn or to make it bank. By varying the wing’s curvature, we cause one wing to give it more lift than the other. For instance, when we make a left turn, we push the right aileron down which gives it greater curvature and more lift, thereby pushing the left wing down.

RamonKok_speedbrakesPhoto credit: Ramon Kok

Speed Brakes

Finally, the wings have long panels along the upper edge. We call these speed brakes. It’s not a true brake. Rather, it “disrupts” the air around the wing. It reduces lift and allows us to descend more quickly, if necessary.

The more I think about it, the more fantastic it is that such an enormous aircraft can fly so easily. It might not be rocket science, but that Bernouilli was certainly a clever guy! Fortunately, steering an aircraft is a lot easier.

Posted by:   Menno Kroon  | 
Join the conversation Show comments

JM

“For instance, when we make a left turn, we push the right aileron down which gives it greater curvature and more lift, thereby pushing the left wing upward.”
Shouldn’t it be instead “thereby pushing the RIGHT wing upward” ?

Menno Kroon

You are absolutely correct. When the right aileron is pushed down, the right wing will create more lift, resulting in the right wing going up and the left wing going down.
Thanks for reading so carefully. The good thing is that you totally understand how it works !

Dimitris

Very nice article! The only thing that is slightly bothering me is the “it is not rocket science” comment. Aerodynamics is as challenging a topic as rocket science and in fact both these fields are governed by pretty much the same equations, the Navier-Stokes. It is also the reason why many jet engine manufacturers also have divisions for rocket engines.

Menno Kroon

You are absolutely right. I’m sorry if I gave you the impression that designing an aircraft is any way less challenging than building a rocket.

Alireza Jahanjou, Atlas Copco

Very nice articel, cool !!

Adrian Cholico

This a simple way for understanding how this amazing thing fly! Thank you.

Lolita M. balboa

Very nice blog. Educational indeed!

John

I agree with both JM’s comment about it being the right wing being pushed up and Dimitris’ sentiment regarding your wording about it not being “rocket science.” Well, it’s certainly in the same field!

Thank you though for a very informational post, and the photo is quite charming, too!

Owen Guerrini

Does the same principle apply to a helicopter?
Do the 4 rotors also provide lift by their curvature.

Thank you.

By the way, your explanation was excellent.

Adham

Yes. Air flow -hence lift- is created by rotation, not linear motion as in airplanes. This is why helicopters do not care about forward speed (or rather air speed), which is created by slightly tilting the rotor plane to get some of the lift produce forward motion.

Wenche Braaten Lien

I often as I could try to sit near the wings because I like to see the wings and the flaps. Because it is exciting. and I like to see the flaps. It is not long time ago I flew with you to Amsterdam. Also for some years ago I flew with you but then I flew further to Prague. I love to fly with you. The take off and landing was very perfect.
It was very interesting to read about how the Aircraft is flying.

Menno Kroon

Hello Wenche,
I’m glad you enjoyed it. I hope that next time your sitting near the wing, it all makes sense. We hope to see you soon

Chris

What’s it like piloting a Catolina Flying boat? I imagine it’s much more labour intensive than the newer aircraft, but more rewarding. Do you fly the Catalina for relaxation or the challenge?

Menno Kroon

Hello Chris,
Flying a PBY Catalina is a lot of fun. I really think I have the best job in the world, but flying a Catalina is special. Landing and take off in the water can be quite challenging, but above all, it is a fantastic experience. Although not commercially, we do fly passengers and it is fantastic to be able to share this experience with others. It is an amazing sound, when those radial engines come to life.so for me I guess it means relaxation as well.

Mike de Souza

Hoi Menno, prachtige informatie. Je moet dan eens naar Suriname komen en natuurlijk voordat jij komt kontakt maken met Peter Relyveld die bij de Surinaamse zweefvlief club zit. Marcel Nijdam een gepensioneerde captain van de KLM heeft de zweefvlieg club in het begin veel geholpen. Dus als je ooit eens naar Suriname komt, maak dan kontakt met Peter Relyveld en zijn email adres is pre@sr.net en hij zal je zeker een antwoordje sturen. Fijne blog en geniet van het vliegen.

Menno Kroon

Beste Mike,
Dat zal ik zeker doen. Helaas vliegt de A330 bij KLM op dit moment niet naar Suriname. Dat wordt wellicht de hoogste tijd (-:

penelope burreci

Hi Menno, thank you for such a detailed and informative article. It constantly amazes me how
these gigantic flying machines take off, fly and land again! I have worked with a European
airline and so have a pretty good insight into aircraft flight but still, its fantastic to actually
get a comprehensive and educational guide to the workings of an aircraft. Great to have
such good photos too and I love the close-up of KLM taking off with Air Canada on the
ground. Thanks again. More articles like this please!!!!!!11

Menno Kroon

Thank you Penelope.
I’m glad you enjoyed it.

Nicki

As sombody who is scared of flying reading an article like this really helps. I do believe that knowledge is power in this circumstance.

Darryl Snook

Even with understanding why it works, I am happy that even an airline pilot continues to be amazed that we are capable of doing this. Another amazing wonder that when we just stop to think about it, its really cool! Thanks for sharing :-)

Menno Kroon

Hi Darryl,
Even when you understand, it can still be an amazing thing. It makes me realise that I certainly have the most fantastic job in the world

A.W.M. Munas

Thanks for information. May I get to know how it creates necessary thrust against the grab.

Gerrit Grol

How much lift effect of the wings is actually lost underneath and near the engines? The air velocity will be much higher in that region because of the engine exhaust. For lift effect you want the opposite or not? Would it not be better to have the engine above the wing instead of below?

Gerard Willemsen

This explanation of lift, while the one taught in high schools, is wrong. Lift is generated because the wing diverts massive quantities of air towards the ground (something like 50,000 cubic metres per second for a 737 wing). The “opposite reaction”, is lift. It has far more to do with Newton than Bernouilli. The pressure differential between the wing surfaces contributes very little to lift.

Richi Jennings

Exactly right. It’s a commonly-repeated myth. But it’s scientifically illiterate — a moment’s thought should tell you it makes no sense! How does the air on top “know” it has to go faster to meet the air on the bottom?

For more info, see Cambridge University’s explanation: cam.ac.uk/research/news/how-wings-really-work

Richard F. Stripe

Bernouilli’s explanation only applies to airfoils and doesn’t apply to flat wings like on a paper airplane, which is another reason why it’s scientifically illiterate.

Brito

Correct, the explanation given for lift in this article is mostly wrong. Otherwise, how could certain aircraft fly inverted (upside down)? https://www.youtube.com/watch?v=BImR4wUgrNo

liz

Thank you for the explanation! I have never been on an airplane because I’m terrified. I’d really like to fly, but need to trust pilots and planes more first :). So understanding it seems a first step in the right direction. Though, a big plane that is ‘only’ ‘hanging’ on air doesn’t really comfort me.. I guess thats why I do love flying with an hot air balloon, but thinking I’ll die when I’ll get on an airplane..

Richard F. Stripe

I realize your comment is two years old so maybe you’ve since overcome your flying fear, but I’d personally much rather be in an airplane than a hot air balloon or helicopter. Think of it as floating on air. when you fly, you’re virtually weightless, and even if you lose your powerplant you can still glide to safety. You must remember that much like shipbuiling, we’ve been building planes for a long long time (well over a hundred years) and we pretty much know exactly how things must be for a plane to be safe and efficient. Big airlines like KLM don’t operate on a shoestring operation, they’re very experienced and have strict safety guidelines that airplane companies share to ensure you don’t fall out of the sky

Mercedes Elsa Lòpez

Thank you Menno, you reply my questions before I even have asked them. Many thanks.

Kipchumba Boniface

Great thanks, Menno, for such a detailed,elaborate and a comprehensive explanations of the aerodynamic airplane movement which integrates us to fantasy-like realities .I look forward to piloting an aircraft in future like you.Thank again.

Rupinder

What imparts the tremendous strength to the wings, and the flaps and the ailerons? How are control cables running to them are ensured to be foolproof? Can we have a spare set of retractable wings to take over if the main ones fail?

Cole

Love your articles! My father was a naval aviator and he flew the PBY and PBY2 in the WWII.

Robert Dupuis

Thank you for this great article. What are the typical take-off speeds of modern aircraft?

Gaffy

Lovely article

Thomas

This article was amazing! Last thursday I was out flying for the first time in 15 years. I was pretty nervous about it. The day before my flight I was at the airport watching the planes take off and land, thinking to myself; “How on earth is it possible for those tin cans to fly. It should not be possible.”
Turns out I had nothing to be nervous about. Flying was actually really fun and interesting. Too bad I’m to old to make a career of it, but I’m thinking of getting my PPL at least. The freedom you must feel when you’re up there all alone must be amazing.

So when I got home i watched alot of videos on Youtube about flying and found this article, which I should have read BEFORE my flight. Reading this, it actually made sense how those “tin cans” can stay in the air. Thanks fora great explanation! :)

aditya kaushish

but sir, I think the lift an aircraft gets works on Newton’s Third Law Of Motion instead of Bernoulli’s theorem.

eren yildiz

When we are asked how does an aircraft fly, is it enough if we only explain how the lift is generated on the wings? Because thrust is required to make an aircraft move forwards as far as I know.

Andreas Nilsson

Hi, Menno. You really should update your explanation of how lift is achieved. Your explanation is all wrong. It’s about the wings deflecting air and not about air pressure.

You can find a correct explanation here:
http://www.explainthatstuff.com/howplaneswork.html

(You would imagine that as a pilot you would know, or at least be interested in, how planes actually fly!)

Best regards,
Andreas Nilsson

Satyam sharma

Hello sir, the first part u mentioned about creating lift is wrong. As air flows over the curved upper surface, its natural inclination is to move in a straight line, but the curve of the wing pulls it around and back down. For this reason, the air is effectively stretched out into a bigger volume—the same number of air molecules forced to occupy more space—and this is what lowers its pressure. For exactly the opposite reason, the pressure of the air under the wing increases: the advancing wing squashes the air molecules in front of it into a smaller space. The difference in air pressure between the upper and lower surfaces causes a big difference in air speed. Lift has nothing to do with air above wind covering long distance

Aly Bayoumy

The Lift theory explanation in the beginning of the article is totally wrong!

I wonder so much how a big airline like KLM produce such an article with misleading and wrong information.

PatrickMuP

Hello

redjdakota

Hello Menno,
Fabulous article! I see that you have given credit where it is due with regards to Ramon Kok as a photographer. I do believe, unless my eyes and memory deceive me, that the main picture used for the article was shot by none other than René Paquette. It would be lovely if you could credit him for his talented work.
Best regards

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