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

[iChris]

For the Venturi theorists only:

 

So if there is higher pressure below the wing than above the wing, why is there not an overall UPwash behind a wing instead of downwash? Hmmm? Doesn't air go from high to low?

 

Because, the total pressure behind an airfoil must return to the free stream pressure that was ahead of the airfoil. Otherwise, a vacuum would be created behind the airfoil as it moved through the air. The earth’s atmosphere doesn't allow for that. The deflection is made at the airfoil, thereafter the pressure equalizes and the flow returns to free stream. (See free stream pressure graph below)

 

The rotor downwash does not just move downward, some of the air makes its way over and around the blade tips as a rotational component (due to the pressure differential) and creates the swirls or blade tip vortex that accounts for a large part of the induced drag. In theory, if a rotor blade or wing had infinite length (no tips) there would be little if any induced drag.

 

That being said, the basic principle of wings and rotors is that they accelerate a mass of air and the resultant lift or thrust is Newton’s law reaction to that acceleration. Producing sufficient lift to permit a helicopter to hover is only a matter of having enough power to accelerate a mass of air equal to the mass of the helicopter each and every second. Accelerating air downward constantly provides lift. This concept is known as the Momentum Theory or Disk Actuator Theory. In simple terms it looks at the overall big picture of how lift is produced.

 

The other concept is known as the Blade Element Theory. In simple terms it is a more detailed look at the physics, the how and why, of what’s actually happening to the airflow at the blade itself. You can think of it as a dissection of the Momentum Theory.

 

The Venturi concept is just an over simplified explanation into the Blade Element Theory that’s looking at airflow along the blade. However, there are similarities that allow for it to be a good approximation of the truth.

 

It’s not unlike this sentence from a helicopter textbook, Understanding Gyroscopic Precession and the Helicopter Rotor as a Gyroscope, as discussions in past post showed, the helicopter’s rotor system is not a gyroscope, but only has some similar characteristics. REF: Post Dec 2010 - Gyroscopic-Precession

 

 

"It is the mark of an educated mind to rest satisfied with the degree of precision which the nature of the subject admits and not to seek exactness where only an approximation is possible."

Aristotle,

 

 

 

FoilSim III Student Version 1.4d

Edited July 13, 2013 by iChris