Why are so many pilots wrong about Bernoulli’s Principle?

For decades new pilots been taught that lift is created because the air flowing over the wing travels a longer distance than the air flowing under the wing, and therefore, for the two airflows to meet at the end of the wing, the air flowing over the wing must move faster. This hypothesis is not correct and this video explains why.

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[jacobstump]

I’m an aerospace engineer who graduated from Embry Riddle, the top rated aviation school in the country. And even there, in our early aerodynamics lessons, the equal transit time fallacy was taught. I remember because I asked - Why does the air on the top HAVE TO reach the trailing edge at the same time as the air on the top? And my professor didn’t know… but to his credit, he came back the next week and taught everyone the fallacy of the equal transit time.

[petermortensen]

Being a carpenter I remember being taught about Bernoulli's principle and the reason why roof tiles come off a roof during high winds. I have never forgotten about this phenomenon. Love it.

[DD-gikx]

its was good to see that smoke demonstration clearly showing the air over and under do not get to back of the wing at same time...I've always wondered how people just seemed to conclude it does

[User-jrvf]

I have a private pilot license, but have not flown aircrafts or even being in contact with the world of aviation for about 7 years. Now YouTube recommends me this video and it reminded me of when I was taking classes, that one of our textbooks warned against these misconceptions regarding aircraft lift. I also remember the book saying that it is still not completely understood, what makes an aircraft fly. I still have the books with me as well as my (expired) private pilot license.

[christophertelford]

The fact that you can make a flat square fly if you put the centre of gravity in the right place and give it control surfaces and enough thrust and a positive angle of attack tells me that everything else including the aerofoil cross section and other twiddly bits like wingtip vortex generators etc are all about improving efficiency. All you need is enough surface area to direct some air downwards, get the basics right, and it'll fly.

[KenIsFlying]

I have been dying to find a good video that actually explains lift for pilots in a correct fashion. As an aerospace engineer it's really hard for me when my student pilots tell me about the Equal Transit Time theory for lift. Thank you for this video!

[boeingpilot]

Thank you. This was a good explanation and properly addressed the "equal transit time" assertion, which is STILL incorrectly taught in many flight manuals...

[jamesspash]

ERAU grad here also. I note in the smoke test demonstrating the angle of attack is at what would be a stall angle. Not a good representation.

[austinblake]

As an instructor, I've been teaching Bernoulli's for straight and level briefs. I tell them it's not a direct translation from venturi tube to an aero foil, but just explain that there is a similar effect of decreasing static pressure above the wing. Didn't realize people were trying to explain the "reason" for it as an equal transit time...

[kevinbarry]

You are 100% correct. When I hear people explain this wrong idea I ask them how is it that aerobatic planes can fly and their wings are symmetrical. And how can they fly upside down?

[Avianthro]

Two other phenomena need mention: Vortex around the wing caused by viscosity, and simple flat plate lift. The wing's lift is actually a sum of those two, and the latter is still active even when the wing is stalled as long as its AOA remains positive.

[aerospacedoctor]

Hi Magnar. Many have an issue with Bernoulli, and they are mistaken. As long as you are outside the boundary layer Bernoulli's principle applies. In fact, when most engineers use the pressure coefficient it is directly related to Bernoulli, as the ratio of the change in static pressure to the dynamic pressure. The ETT was initially a hypothesis of D'Alembert, and is a result of potential flow, the first real attempt to apply Newton's laws of motion to a fluid. In this situation the curvature of the streamlines at the training edge and leading edge are symmetric, and you get no resultant lift force. As such, saying that the curvature is responsible for the acceleration (while true), neglects the resolution to D'Alembert's paradox which resulted because viscosity was not understood until Navier and Stokes 100 years later. So, the asymmetric acceleration around an aerofoil is due to viscosity. There are two specific effects, the Kutta condition, which moves the rear stagnation point to the TE. and the induction of more flow upwards ahead of the wing. The end result is an asymmetric velocity of the flow (circulation) which at the surface of the wing is given as a pressure force, which will also be asymmetric, with lower pressure above and relatively speaking higher pressure below.

[skooterk]

When I used to fly RC models, we took a wing on a 3 channel trainer and put it on backwards. It flew just fine

[BuzzMoves]

I’ve had this argument with CFIs and FAA examiners more than once. Myths are hard to overcome.

[steffanjansenvanvuuren]

Nothing can be accelerated instantly. Because it has inertia.
That is why air molecules are literally forced apart at the top faster than it can accelerate toward the wing,
becoming less dense (lower pressure), by a wing at speed.
The air at the bottom of a wing is forced by the wing so fast it becomes compressed faster than it can accelerate away from the wing.
(Higher pressure)

[californiadreamin]

It’s more important for pilots to understand what causes a wing to stop producing lift, so the passengers don’t get upset.

[HH-mwsq]

As a CFII and someone with an undergraduate degree in Aerospace engineering, the reason it is taught the way it is, is because the students seeking their pilots license would neither understand nor are they interested in fully learning how a wing generates lift. I know, I have tried. What an airfoil really does is to rotate the air in a clockwise manner using the diagram of the airfoil used in the video. This rotation accelerates the air above the airfoil, and retards the air below the airfoil. As mentioned, the total pressure around the airfoil is constant and the same. But with the higher airspeed above the wing, it has a higher dynamic pressure than below the wing, and therefore has a lower static pressure. Lift is generated due to the differences between these static pressures, multiplied by the surface area of the wing. Anything that rotates air, will generate lift.

[adb]

Hi Magnar, I am a PPL but more important for this I am an Aeronautical Engineer and also was an Aerodynamics teacher at college.
While it is true that Bernoulli's principle applies only to one flow line, it can be also applied to two (or more) flow lines if there is a point where the energy state (speed and pressure) was the same in both flow lines. Sufficiently ahead of the wing, the parcels of air that are going to flow just above the wing and the ones that are going to go just below have the same pressure and speed. So you CAN apply Bernoulli's principle between a point above of the airfoil and another below. Still, transit time is wrong so you can't deduct the speed just by the differences in length. How lift is generated is at the same time more simple, more complicated, and more disappointing (or unsatisfactory explanation) than most people think. Let me know if you want me to expand.

[bernhardecklin]

I like Magnar's no bullshit approach to everything and his calm and easy-to-understand explanations.

[tomg]

The angle of attack has an impact on the lift as well. It correlates to the under the wing lift.