Propeller article

[elippse]

There is an article explaining propeller pitch, planform, and multi-blades in issue #101 of Contact! magazine. For a limited time you can go on the magazine's web site and view it for free. On the web site click on the cover picture of the Pober, and the cover picture will come up with miniature magazine pages below it. On the right edge of the right page is an arrow that takes you to the pages with the article.

[Kinnerhatz]

Minimum induced losses for a wing occur when downwash velocity is constant along the span. Are you saying that this is also true for a propeller? I believe minimum induced losses for a propeller occur when the slip velocity is constant along the blade. Constant slip is not the same as constant downwash.

[elippse]

Originally Posted by Kinnerhatz

Minimum induced losses for a wing occur when downwash velocity is constant along the span. Are you saying that this is also true for a propeller? I believe minimum induced losses for a propeller occur when the slip velocity is constant along the blade. Constant slip is not the same as constant downwash.

According to the output of my program, the thrust/HP ratio of each inch along the blade is constant except for a slight decrease at the tip due to higher Mach-related CD. That's why it gives over 90% efficiency in cruise at 200 mph TAS, and 82% efficiency in a climb at 105 mph IAS. I don't have anything in my program that addresses "slip"; I'm not really sure what "slip" even is. Does a wing have more slip at low speed than in cruise?

[Kinnerhatz]

What I call slip velocity is merely the rearward component of the downwash velocity vector. I think you are using a chord distribution that creates a constant magnitude of the downwash vector along the blade, whereas I believe it is the rearward component of the downwash vector that should be held constant. If you do this, you get a more conventional looking chord distribution rather than shape you are getting.

[elippse]

Originally Posted by Kinnerhatz

What I call slip velocity is merely the rearward component of the downwash velocity vector. I think you are using a chord distribution that creates a constant magnitude of the downwash vector along the blade, whereas I believe it is the rearward component of the downwash vector that should be held constant. If you do this, you get a more conventional looking chord distribution rather than shape you are getting.

I calculate the downwash velocity as the opposite to the lift, and it, combined with the forward plus induced velocity, define the induced angle-of-attack from which the next iteration of lift is calculated. The lift is then resolved into the axial (thrust) and tangential components.

[elippse]

Kinnerhatz has asked about the slip of my propeller design. The use of this term has become so common-place that it's accepted as a real operating condition relative to a propeller. Somehow or other, this term "slip" or "slippage" has been introduced into the propeller vocabulary as a word to explain the difference in the pitch stamped on a propeller and its actual helical path through the air. It's as if the propeller should always follow the exact same helical path regardless of load and any difference, such as the speed in a climb at a given rpm and the speed at the same rpm in straight and level flight, is attributed to this mythical "slip". To me, when something slips, such as a tire on a wet surface or a clutch, it's not a good thing, But a propeller is just a wing in rotary motion. So if you're flying at a low airspeed and your wing is at a high angle of attack, would you say that it is slipping? After all, it's not pointed in the same direction as when in cruise. Of course not! It's just that at a lower speed, with reduced dynamic pressure, the wing must be flown at a higher AOA in order to produce the same lift. So, too, a propeller. When you are climbing with reduced dynamic pressure and mass flow due to lower rotational and forward velocities, the propeller blade must also operate at a higher AOA to produce the increased thrust. It isn't slipping; it's just doing what it should do in order to produce the thrust at the higher loading. But some would like to be able to explain this difference by assigning some value of slippage between the two flight profiles. Maybe there is some empirical number that could be attached to a particular prop - "When I climb with my Q-root prop, it has 10% slippage." or something like that. I would rather stick with the prop's percentage of efficiency. If you know the efficiency vs rpm and power, you can estimate quite well how your plane will perform at any given load. Let's throw out meaningless terms like that and concentrate on real physical values.

[hevansrv7a]

The question first: Paul, are you defining prop efficiency as BHP vs. THP or some other measure?

Now, the ribbing. Paul, what would be the harm in using paragraph breaks now and then?

[elippse]

Originally Posted by hevansrv7a

The question first: Paul, are you defining prop efficiency as BHP vs. THP or some other measure?

Now, the ribbing. Paul, what would be the harm in using paragraph breaks now and then?

Propeller efficiency, to me, is thrust horsepower divided by engine power.

You're right, Howard, I should break it up, but in the past when I've hit Enter and Tab to start the next line, it goes off into never-never land!

[Ironflight]

Originally Posted by elippse

You're right, Howard, I should break it up, but in the past when I've hit Enter and Tab to start the next line, it goes off into never-never land!

You're right - "Tab" is your enemy! A simple carriage return will suffice to start a new paragraph - don't indent...

Paul

[elippse]

Originally Posted by Ironflight

You're right - "Tab" is your enemy! A simple carriage return will suffice to start a new paragraph - don't indent...

Paul

I'm learning. but old habits are hard to break (or brake!)!