Prop Question- 3 blade vs 2- Catto vs Sensenich

[elippse]

Originally Posted by PerfTech

And if you think about it, what relationship exactly does hp have with blades? As long as the engine can turn the blades to the rpm you want, say 2700-2800, then what more can it do? The blade only knows it is going 2800 rpm, it doesn't know what hp is pushing it. So hp is more related to blade pitch, length, width and number (only so much as you want it to spin at a given rpm).

ACTUALLY IT HAS EVERYTHING TO DO WITH IT!

The true reason for efficiency loss as you add blades is this and this alone. You are adding more frontal area (leading edge of blade) and also increasing the total wetted surface area. These two items account for the only differences in efficiency. This extra drag consumes a considerable amount of power that could be used to create thrust and power the plane. For instance, were you to keep adding blades you would soon need to start taking out pitch in order to maintain the RPM. Eventually you would have a propeller with no pitch and many blades that would consume all available power just to spin it through the air and create zero thrust. Keep in mind the leading edge tip speeds are approaching 650 mph on each blade and that requires considerable power to push through the air. Regards All, Allan[/quote]

Sorry, but I have to disagree with you on all points. When you add more blades for a given speed and rpm, you do not decrease the pitch, you decrease to chord. There is only one loss on a propeller blade and that is to due to parasite drag.
Wide tips at high Mach have a higher parasite drag coefficient so not only do they have higher drag, due the the greater CD and area, that drag force is multiplied by the radius to give a higher loss-horsepower. Keep in mind, torque times rpm / 5252 = horsepower. A 3.5" wide tip on a two-blade prop turning 2700 rpm at sea level at 135 mph will have a 3.5 HP loss for just the last inch of span due to tip drag. If your prop has wide tips you're throwing away a lot of HP, especially if they have a round planform.
A loss drag half-way out on the blade will only consume 1/2 the horsepower as will tha same loss drag at the tip. Multiple blades that have streamlined shapes following the correct pitch angle all the way into and sealed to the spinner are every bit as efficient in cruise as a two-blade. If what you were saying was true, which it isn't, then Tom Aberle's Phantom biplane would have gone slower in going from a two-blade (221 mph) to a three-blade (241 mph) and even slower still when going from the three-blade to a four-blade (260.805 mph). Actually multiple blade props can be more efficient since they can be made smaller in diameter and have the same efficient mass flow which reduces tip Mach and loss drag.
Unfortunately, your thinking process has been poisoned by all of the nonsense that has been printed about propeller blades by people who didn't have the first idea of what they were talking about. They spoke of blade-to-blade interference when in reality each blade follows its own individual helical path through the air.
Just look at the pix of the C-130J that has eight-blade props with narrowed tips or the A-400M with similar blades. The old ideas are slowly melting into the past but it will take a long time for them to be purged entirely since they have been around and have appeared in books for so long. Where's Fahrenheit 451 when you need it?

[PerfTech]

I was assuming all dimensions remaining the same for comparison purposes only. This was an attempt to simplify the answer to the original question, not to try reinventing the propeller! I am a design engineer for Garrett with 46 years experience, am very capable of running the numbers as well as accessing the worlds largest database on the subject. The question was simple and only requires a simple answer. Over complicating or introducing complex mathematical equations that are Greek to most people on this forum serve no purpose and only adds to the confusion. Regards, Allan

[meloosifah]

Does anyone know if Paul ever made props? He last posted on here in 2011. There was some great ideas bouncing around in this thread and then it just stopped...

[Caveman]

Originally Posted by meloosifah

Does anyone know if Paul ever made props?

Yes he did make very innovative propellers...

Here's one link:

http://rexresearch.com/lippsprop/lipps.htm

[Kent Ashton]

After reading Paul's stuff, I trimmed down the tips on a 3-blade Performance Prop that did not give me enough RPM. Trimmed it from the tip to about 8" in, that is, I reduced the chord and thinned the tips. I got 125 more RPM and an increase in speed.

Earlier, I damaged a tip on this same prop and sawed off about 1.25" of the original blades. That had very little effect.

http://forum.canardaviation.com/showthread.php?t=5170

[pierre smith]

Craig Catto is trimming the tips too, down to almost a point..there's a brand new one on a -4 in my hangar...a work of art.

Best,

[brad walton]

Pierre, can we get a picture?

[RVbySDI]

Originally Posted by meloosifah

Does anyone know if Paul ever made props? He last posted on here in 2011. There was some great ideas bouncing around in this thread and then it just stopped...

Sadly, Mr. Lipps passed away a few years ago. His Ellipse props were very unique in design. There is a gentleman in Kansas who retains the rights to manufacture the Ellipse props designed by Paul Lipps. I cannot recall his name but he does fly an RV6 with an Ellipse prop. He has also designed his wingtips in the similar shape as the prop tips. It is a unique looking RV6 when you see it.

Here is a quick google search result of some discussions concerning Paul Lipps and his props:
https://www.google.com/search?q=elli...+ellipse+props
and some pics showing some of his props:
https://www.google.com/search?q=paul...bLBY44ChDsCQg_

[Raymo]

Not to over-simplify the answer but I am in the same situation and going with a 3 blade Catto simply for the ground clearance. Pulling the engine for an inspection will slow me down far more than the 3rd blade (if it slows me at all).

[RVbySDI]

Originally Posted by brad walton

Pierre, can we get a picture?

I am not Pierre but here is a side view shot of the newest 3-blade Catto design. I started with an earlier prop design from Catto, which you can see in the second picture. Perhaps this side view will help with showing a profile of the blade shape:


Here is a frontal view of my plane with the original 3-blade Catto. Although this picture is an older one in which I had Catto's earlier design, I thought it valuable to post in the event you were wishing to see the overall appearance of the 3-blade prop:


The appearance of the two generations of props is not initially noticeable until you look up close and study the details. The overall thickness of the blades is much thinner. The tips curve much more than the earlier generation blades. The tips come to more of a point as the curve reaches the end. Subtle differences that really do make a difference in performance.