Delusions about Bernoulli's Principle
Illustrate Radial Momentum Instead
© by Ed Seykota, 1999
Textbooks and Internet Bernoulli sites present many
experiments to demonstrate Bernoulli's Principle. These experiments rarely
demonstrate Bernoulli's Principle at all. They demonstrate Radial Momentum.
As high school students must learn, the standard explanation
for lift is Bernoulli's Principle; high velocity means low pressure. While
Bernoulli's Principle is fundamentally correct, the application of it to explain
lift is fundamentally incorrect. The real principle behind lift is Radial
Momentum. Ironically, Bernoulli's principle does not even explain the devices
commonly used to demonstrate the Bernoulli Principle itself.
Momentum theory explains all of the experimental observations of pressure
decrease in open systems with flow motivation. It explains atomizers, airfoils,
airplanes, curve balls, lift, cavitation, eddy currents and a full range of
behavior involving pressure decrease.
Bernoulli's Principle does not explain the experimental
observations, particularly those differentiating between radial flow and
non-radial flow in open systems with flow motivation. Bernoulli's Principle is
widely misapplied. Ironically, Bernoulli's Principle does not explain the
experiments commonly used to demonstrate the Bernoulli Principle.
This experiment comes, complete with Bernoulli misapplication, from NASA.
Materials: 2" x 8" strip of paper
& paper clips
Experiment: Place the strip of paper so
that one end is resting just under your lower lip and the other end is
hanging down. Holding it in place, blow over the top of the paper. The end
of the strip of paper hanging down will lift up so that it is
"flying." Try placing paper clips on the hanging end of the
paper and see if it has enough lift to overcome their weight.
Explanation: With the increased air speed
over the top of the paper, the air pressure over the top of the paper is
decreased (Bernoulli's Principle). This makes it so that the paper has
higher air pressure underneath it, lifting it into the air.
flowing air over the top of the strip of paper causes lift and the paper
hold a strip of paper against my bottom lip and blow air across the top.
Radial Momentum induces a region of low pressure above the strip and the
higher pressure under the strip elevates it.
air inside the cone keeps the ball inside the cone
effluent stream from a vacuum cleaner is seen to hold a ball aloft, even
if the air stream is tilted somewhat off true vertical, in which case
the ball may spin rapidly. Radial Momentum induces a region of low
pressure inside the cone that keeps the ball inside the cone. Higher air
velocity at the center of the cone spins the ball.
air over the nozzle sucks the perfume up the tube.
a perfume atomizer, the air supply from a squeeze ball, flows over a
delivery tube. Radial Momentum induces a cone of low pressure that acts
to elevate the perfume into the flow. Atomizers work with an air supply
(that expands radially) and do not work with a liquid supply (that does
not). An atomizer nozzle that entrains flat and expanding flow is more
This wing cross-section diagram appears in numerous
textbooks and on countless web sites. It accompanies the claim that
Bernoulli's Principle explains lift, in terms of higher velocity over
the top of the wing causing lower pressure.
Does Bernoulli's Principle explain lift? Not at all!
The real reason for lift (aside from angle of attack) is Radial
Momentum. The classic drawing to the left is inapplicable to lift and is
Faster air over the top of the wing induces lift. The
airplane wing receives extensive misapplication of Bernoulli's Principle
to explain lift as a function of high air velocity. This is simply not
the case; Radial Momentum, not velocity, induces low pressure. To try to
make Bernoulli's Principle work, the literature is replete with a wide
range of rather inventive add-ons, patches and upgrades such as
circulation and induced lift. These rely on unsupported hypotheses about
the behavior of air such as (1) air circulates around a wing against the
wind and (2) air speeds up to reunify with partner molecules separated
by the leading edge of the wing.
airplane wing may develop a cone of Radial Momentum over the shoulder of
the wing. The lower pressure may induce eddy currents, cavitation,
separation and turbulence. Lift is primarily a function of angle attack,
not of so-called Bernoulli lift or synthetic circulation. The main
effect of curvature is to entrain laminar flow, reduce turbulence, and
conserve energy which indirectly contributes to lift.
Radial Momentum ... The Real
Air, striking the front of the wing,
deflects upward, actually pushing the front of the wing down. It then
continues upward by momentum. When it encounters the curvature on the
top of the wing, it fans out. This results in lower air density, and
lower pressure ... however, this effect occurs only behind the crest of
the wing ... and it has very little to do with lift.
In actual practice, the net effect of
wing curvature on lift is small; most of the lift comes from angle of
attack; the curvature mostly helps train the air off the back edge of
the wing ... so that less energy disappears into turbulence and more
energy goes to propulsion.
Airplane wings do not need to be curved on top to
create lift. Lift has to do with the angle of attack as you can easily
verify with your hand out a car window. The curvature of a wing is
useful, however, in training the flow of air around the wing so it
remains laminar and does not become turbulent at the trailing edge.
Laminar flow is more efficient and conserves power. In this way, it
provides more net power to the wing and hence, more power to lift the
plane. It also provides more net power to the bathroom lights. So in
that regard, curved wings also cause more photons.
Balsa wood gliders work very well with flat, warped
and twisty wings. Some helicopter blades have equal curvature on both
sides. Some planes have flat wings. Some planes fly upside-down. This
all indicates that wings do not need humpy curvature to have lift.
To the extent that a wing has a curvature, the leading
edge has a downward angle of attack and the trailing edge has an upward
angle of attack. This creates a net forward rolling torque. In effect,
an airplane with a curved wing would like to roll forwards and fly
forward with the nose down, with the curved part of the wing into the
Still, some scientists hold on to the notion of
curvature = lift. Some even add curious theoretical patches to, well, to
try to make it fly better. These include the principle of equal transit
times, starting vortex, induction, circulation, and induced angle of
M. Craig has a nice web site documenting induced induction and
circulation of crazy theories.
Turbulence, an effect observed at high Reynolds
numbers, may be induced by Radial Momentum. For example, the cone of
vacuum induced at the shoulder of an airplane wing can induce eddy
currents and provoke turbulence.
spin of the ball creates a relatively faster air flow over the top.
Therefore the ball is supposed to rise ... actually it falls.
spin of the ball sets up a small region of Radial Momentum at about 7:00
on the diagram ... this pulls the ball downward. The spin also creates a
pressure front at about 1:00 and this also acts to push the ball