ELIPPSE propeller, Contact! magaine article

Paul Lipps, an aerospace engineer, has developed a new prop for
experimentals. He's taken a new look at a lot of issues with props
and has come up with something that looks really strange, but works
extremely well.

Among the many things he attempted to incorporate in his prop design
was to utilize elliptical lift distribution, which many experts
claimed could not be done, but he's managed to do it.

He also wondered why folks were using flat bottomed turbulent airfoils
for high speeds. The flat bottomed airfoil coupled with a sharp
leading edge makes for a prop that has a narrow range of peak
efficiency, and also creates a lot of drag and noise. After all, the
tip approaches the speed of sound, what airfoil do you know for near
supersonic flight has a flat bottom?

Paul decided to use laminar flow symetrical airfoils.

Lift force on a wing is proportional to the square of the velocity.
That means if you double the speed, the lift force is quadrupled.
That being the case, you don't need a wide cord at the tip of the
propeller because it's moving at near subsonic speed. Going that
fast, it does not have to have a wide cord to produce lift.

Also, because the tip is moving at such high speeds, you need to
minimize it's size to reduce drag and noise.

So this prop looks like nothing you've ever seen before. It's
relatively wide at the spinner and expands gradually from there till
it reached max cord at about the end of the cowling (Lancair 235),
then suddenly and dramatically narrows from both the leading and
trailing edges. The narrowing gradually straightens out until both
edges reach the tip, which is squared off. By the time the you get to
the tip, the planform has narrowed so much, you'd think that very
little lift could be developed out there. That's sort of true, the
majority of the lift is developed at about half the radius of the prop
although it's distributed all along the radius. The point in
narrowing down the tip so dramatically is to reduce drag at the tip so
as to increase efficiency and reduce noise.

Lipp said in the article that those props that have wide tips or those
techy looking turned up tips are actually pretty inefficient because
no matter what the shape of the turned up tip (Q Tip comes to mind),
it's still more area for the shockwaves to form on than just narrowing
the tip and shearing it off.

Another interesting point he makes is that the prop definately IS
capable of creating lift even right next to the hub as long as it has
been given the proper angle of attack. Because the airspeed right
next to the hub is so low and he's using a symetrical laminar flow
airfoil which demands a high angle of attack to produce lift, the
pitch at the hub looks so steep as to be almost flat to the rotation
of the prop.

But the prop is designed for a fast airplane and when it's moving
forward, that rate of advancement guarantees that that portion of the
prop does produce lift.

If I were to attempt to describe the planform so that folks could
imagine what this look like, think of the glass that fits over the
wick of an old kerosine lantern. Holding that glass up and looking
directly at it from the side is a fair approximation of the planform.
Starting at the base, it widens out, reaches it's maximum diameter,
then narrows appreciably and somewhat abruptly as it extends to the
top. The ELIPPSE has that planform, but where it reaches it's widest
cord (at a little less than half the radius) the shape is more pointed
there where the prop begins to narrow as you move out towards the tip.

What's intriguing is how narrow the prop eventually gets out at the
tip. It actually looks ridiculous, at first glance.

But it works. Or at least it works for his lancair 235.

Corky Scott