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7 Answers

Helicopter Lift Theory – Equal Transit, Skipping Stone & 1/2 a Venturi – all wrong…

Asked by: 3548 views , , , ,
Aerodynamics, Helicopter

OK so here is a frustrating situation for a new Helicopter CFI working on how to teach lift theory:

The 2012 FAA Helicopter Flying Handbook explains the creation of lift as a combination of half a venturi (the bottom half is the airfoil) and air pushing up on the bottom of the airfoil (like a water skier).

The 2012 FAA Helicopter Instructor's Handbook explains lift as the upper surface having a longer distance for the air molecules to travel than the lower surface and so causing the airflow above to speed up to make sure the molecules meet at the trailing edge.

Two different theories in books released within a month or two of each other!!! To make things even more frustrating, both explanations are completely wrong according to NASA: 




So apart from a moan at the FAA, the question is what do you teach a new helicopter student:

  1. What the Helicopter Instructor's Handbook says you should teach - 'Equal Transit Time' theory
  2. What the Helicopter Flying Handbook teaches - a combination of 'Skipping Stone' & '1/2 a Venturi' theories
  3. What NASA (who put a man on the moon using a pocket calculator) says - ' Turning of a fluid'

I WANT to teach option 3, but will they suffer in written tests and at the hands of DPEs because of it?

Note to the FAA - the NASA theory has been established for a long time - even longer than the amount of time ADFs have NOT been used in the cockpit... ;-)

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7 Answers

  1. Sam Dawson on Nov 16, 2012

    My theory on helicopter flight? PFM.

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  2. Sophia on Nov 17, 2012

    Sam…for certain squirly heads, I agree, but for those with innate ability it’s actually more than “pure magic.” The NATURAL, GOD-GIVEN ABILITY to fly helis is “PFM”!!!

    In response to the question, the most astute aspect of being “an instructor”, is to TEACH knowledge…Give the student ALL of the information you have regarding these theories. The more info you give your students, the better off they are, but don’t confuse them too much…Explain the word “theory”, first and foremost…that should clear things up!

    Do whatever it takes to keep the blue side up…unless of course, a pilot can handle a bad attitude.

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  3. Brian on Nov 17, 2012

    I don’t fly helicopters, nor am I read up on the lifting dynamics. That said, my advice for studying aerodynamics is first and foremost: Take anything the FAA published that contains the word aerodynamics, or any other synonymous word or phrase, and burn it.

    FAA texts provide great information about flight and how to fly. However, they are notorious for providing piss poor, and often completely incorrect, explanations of flight theory.

    NASA is a great source to start. I wish I could give you a good rotor wing text, but alas I cannot. I’d look around for something written specifically for rotor wing theory if you wish to delve beyond what NASA offers.

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  4. Timothy Broadwater on Nov 19, 2012

    Another reference that the FAA has is “Aerodynamics for Naval Aviators”


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  5. Nathan Parker on Nov 19, 2012

    How a wing generates lift has been known for almost a century, and there is no controversy in the scientific community. There is, however, much controversy in the laymen community, which includes the FAA and, most likely, the NASA websites. Seriously, do you really think the hotshot aerodynamicists at NASA are working on the websites? They used to published the equal transit theory years ago.

    Personally, for a non-CFI student, I leave the explanation at “a pressure differential is created around the wing”, which is something absolutely true and fairly theory-independent, because there simply is no possible mechanism for the air to apply a force to the wing that will not be associated with a pressure change on the wing itself.

    There is no PTS requirement for any checkride candidate to be able to explain lift.

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  6. Koehn on Nov 19, 2012

    The GRC website is correct. However, Wolfgang Langeweische was correct when he said that lift is created by pushing the air down. The mechanics of how it happens are really not all that important.

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  7. rotorguy on Sep 05, 2013

    Easiest way to explain lift is Bernoulli’s Principle, as velocity increases pressure decreases. Less pressure on the surface of the airfoil with more “camber” creates a “vacuum” so to say or low pressure, pulling the airfoil up ergo LIFT. A rotor is just an airfoil just like the wing of an airplane, the only functionality difference is that wings gain airflow via the plane moving forward where as helicopters create that forward airflow via rotation.

    Another thing a lot of people tend to forget is the LIFT formula. When I was going for my initial CFI (airplane) I got an odd look from my instructor when I brought it up, said he never saw it before.
    LIFT = coefficient of lift (angle of attack) x (1/2)Pressure (density alt) x Velocity^2 x surface area. Makes perfect sense when using Bernoulli’s principle.

    Where you really need to understand lift and where the lift formula REALLY comes in handy is when explaining dissymmetry of lift. That is where I have heard of a lot of people running into trouble trying to explain. its tough unless you break it down then it becomes quite simple.

    But most importantly just pick a method you most understand and stick with it. KISS model. don’t bore and confuse your student to death with 5 different theories. Or just say what my buddy said on his initial CFI ride (AS A JOKE) “Its all magic. That’s my theory, and that’s how anyone explains lift – its a theory, there’s no solid proof of who’s theory has more validity. So my theory is just as valid”

    anyone with questions just shoot a PM

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