This is what the prop tip vortex looks like more or less.
Only one is shown but two are generated.
Much, if not the majority, of the prop HP consumption is
in the tips. Drag on the tips is tip velocity squared.
The tip velocity is the vector sum of the forward speed
and the tangential speed of the tip. This is why prop driven
air craft are pretty much limited to 500 MPH.

HP consumption is drag times speed so it takes HP cubed
to drive the tip.

The air flows off the bottom of the
blade and around to the top of the blade creating the vortex.
The bottom of the blade is high pressure and the top of the
blade is low pressure. Just like a wing. It takes HP to generate
this vortex.

The energy in this tip vortex is known as induced drag.
The formula is: induced drag  is equal to the lift coef. squared
divided by pi times the aspect ratio of the blade.
To reduce it you increase the diameter of the prop and
reduce the blade width to keep the blade area the same
increasing the aspect ratio. The same reason sailplanes
have long skinny wings. The problem is the tip velocity
increases for a given RPM. So too the vector sum of
the velocity. That is why lower diameter props do
well at higher speeds.

In addition to this induced drag the form drag on the tip
is directly proportional to the section drag coefficient.
All aerodynamic bodies have section drag coefficients. If the
angle of attack of the tip is reduced to zero with no lift
generated then the tip will still have drag.
Therefor it pays to generate most of the thrust near the hub and
the middle of the blade. That is why the root area and angle
of attack is very important. See Paul Lipp's and Perry Mick's
props.

One of the  ways to reduce the drag coefficient of a wing
approaching supersonic or supersonic is make it thinner.
The F104 is a good example. The wings are so thin and sharp
they can cut you.

Paul Lamar ...No rotor no motor.

The Prince "Ptip" put an end plate at the tip of the prop, Virg

Virgil N. Salisbury

Yes they do but apparently it is a wash as they don't appear
to be overwhelmingly superior. Perhaps the additional intersection,
surface area and form drag out weighs the reduction in tip vortex.

Q tips are similar and they have been around for years on certified
airplanes.

Paul Lamar ...No rotor no motor.

Here is the corrected helix.

-- 
Paul Lamar ...No rotor no motor.

Rats... here  I am messing with you again!   Take a look at the bottom
propeller... the twist of the bottom prop is still not there yet bud!!
(man, do I feel like a heel sending this to you!!)

Gary Buster
Tyler, Texas

Don't worry about that. That is what I am here for.

But I think the lower prop is OK. If you stand to the left and
face the prop it is rotating counter clockwise.
The twist is correct.