Power Required

[Asleep Helmsman]

So that prop is an E192 all the way from the hub to the tip?

But it is thicker towards the hub, meaning the chord ratio would change towards the hub?

As far as the edge shape is concerned is it mostly an ellipse with the leading edge a little exaggerated?

[Guest625101138]

Originally Posted by Asleep Helmsman

So that prop is an E192 all the way from the hub to the tip?
It is not an E192 (or E193) foil. I use a wide variety of foils. I think that one was based on the MA409 section or a slightly modified NACA 4-series similar to the MA409. The blade has a thickness of 7% for most of its length.
But it is thicker towards the hub, meaning the chord ratio would change towards the hub?
The blade ends up at 20% thickness right at the hub.
As far as the edge shape is concerned is it mostly an ellipse with the leading edge a little exaggerated?
It is not deliberately an ellipse that is the result of optimisation within certain constraints.

One of the reasons I use my own prop model is that JavaProp has set Re#. I get a little more precision by using the exact Re# for the actual blade section at my specified design conditions. Using JavaProp at a fixed Re# introduces an error of the order of 1 to 2% if you use the closest available Re#. It will be worse if you do not use one that is close.

The Single Analysis page of JavaProp displays the Re# at various radial positions if you want to check your selection.

My automatic optimisation will usually give an efficiency 1 to 2% higher than a first pass at JavaProp but you can also play with the values in JavaProp to gradually improve the result. The problem is that the end result with JavaProp will have an error of the order of 1 to 2% due to the variation in Re# so you really cannot be certain that the improvements shown will be actually achieved.

As you can see the potential for error from JavaProp is quite small unless you use Re# that are way out but I am chasing every little bit I can get so am interested in narrowing small errors.

Rick W

[Asleep Helmsman]

Originally Posted by Rick Willoughby

One of the reasons I use my own prop model is that JavaProp has set Re#. I get a little more precision by using the exact Re# for the actual blade section at my specified design conditions. Using JavaProp at a fixed Re# introduces an error of the order of 1 to 2% if you use the closest available Re#. It will be worse if you do not use one that is close.

The Single Analysis page of JavaProp displays the Re# at various radial positions if you want to check your selection.

My automatic optimisation will usually give an efficiency 1 to 2% higher than a first pass at JavaProp but you can also play with the values in JavaProp to gradually improve the result. The problem is that the end result with JavaProp will have an error of the order of 1 to 2% due to the variation in Re# so you really cannot be certain that the improvements shown will be actually achieved.

As you can see the potential for error from JavaProp is quite small unless you use Re# that are way out but I am chasing every little bit I can get so am interested in narrowing small errors.

Rick W

So you let Java Prop determine the edge?

[Guest625101138]

Originally Posted by Asleep Helmsman

So you let Java Prop determine the edge?

I do not use JavaProp for my own designs. I want greater precision and more control on wider range of parameters than it can give. I also check for and allow for cavitation but am not interested in compressibility issues.

Rick W

[Asleep Helmsman]

Originally Posted by Rick Willoughby

My automatic optimisation will usually give an efficiency 1 to 2% higher than a first pass at JavaProp but you can also play with the values in JavaProp to gradually improve the result. The problem is that the end result with JavaProp will have an error of the order of 1 to 2% due to the variation in Re# so you really cannot be certain that the improvements shown will be actually achieved.

Rick W

That’s an interesting point.

In all design work it is imperative to clearly define objectives, and to know when something is within the tolerances.

As we learned earlier, at 2 knots, the power consumption should be less than 30 watts; or about 25% of the total energy requirements, including on board computers and other equipment, making a 1 to 2% savings in propulsion is only a .25 to .5% savings on the total system.

How much work should be done to try to save that much power?
Something else has occurred to me as well.

What do you think about starting a tread (or maybe there is one already that could be revisited) about all of the mathematical terms used in fluid dynamics and an explanation of them that lay folks, like me, could understand and even some calculations that would work in spread sheets.

[daiquiri]

Originally Posted by Asleep Helmsman

In all design work it is imperative to clearly define objectives, and to know when something is within the tolerances.

Great observation. Though it might sound obvious to some, it is too often overlooked - particularily the part about tollerances and sensitivity analysis.

Originally Posted by Asleep Helmsman

What do you think about starting a tread (or maybe there is one already that could be revisited) about all of the mathematical terms used in fluid dynamics and an explanation of them that lay folks, like me, could understand and even some calculations that would work in spread sheets.

It is very broad question, AH. People spend years in universities just to learn basics about fluid dynamics.
How about you putting down questions about specific things that puzzle you and the rest of us trying to give simple answers?

[Guest625101138]

Originally Posted by Asleep Helmsman

That’s an interesting point.

In all design work it is imperative to clearly define objectives, and to know when something is within the tolerances.

As we learned earlier, at 2 knots, the power consumption should be less than 30 watts; or about 25% of the total energy requirements, including on board computers and other equipment, making a 1 to 2% savings in propulsion is only a .25 to .5% savings on the total system.
That was my point in the post above - no point trying to get better than the error band of the model. JavaProp will get you near enough. There is no point in polishing a turd. You accept it for what it is if it does the job. The difference between a flat plate foil and a perfectly profiled foil is probably around 10% anyhow.
How much work should be done to try to save that much power?
For me 1 to 2% is worth chasing down. At one stage I spent about 20 email exchanges chasing down the significance of 0.23W in 150W. We could not actually measure to that tolerance so ut was my estimate but he was not happy until he could confirm by testing that it was negligible as I had already determined mathematically. So it depends on the application.
Something else has occurred to me as well.

What do you think about starting a tread (or maybe there is one already that could be revisited) about all of the mathematical terms used in fluid dynamics and an explanation of them that lay folks, like me, could understand and even some calculations that would work in spread sheets.
I think there is a terminology thread already - Ihave certainly seen one somewhere. If you want to get a real good dose of jargon try applying the empirical formulas related to propeller curves like the Wageningen series.

See inserted comments.

Rick W

[Asleep Helmsman]

I totally get it when you are talking people power and world records.

One of our local museums has one of the early people powered airplanes, it actually might be the first one. With a 90 foot or so wingspan made out of stretched plastic bag material.

Every ounce accounted for, as much lift and as little drag as possible.

[Asleep Helmsman]

Well I think I’m going to purchase a plastic model airplane prop, $24.95; and if it’s not strong enough or thick enough.

I’ll take some measurements, add some West System Epoxy 105 filled with 410 Microlight, sand it off with a random orbital sander, throw it in some sand, and make an aluminum casting.

Next I’ll break out the random orbital sander again and make it smooth.
Hopefully that solves that problem.

Once again thanks for all your help guys and especially Rick.

Other harder questions are coming soon.

[Asleep Helmsman]

Any idea about how much % increase in power requirement between the first nacelle and the second?

[Guest625101138]

Proportion drag to the ratio of Volume to ^ (2/3).

Rick W

[Asleep Helmsman]

Thanks Rick,

It looks like we might be in one of those fuzzy areas where linear functions fall a little short.

I must assume that the formula ^(2/3) works for scaling object of identical shapes, and does not account for hydrodynamic forces due to variations in flow.

For example the second nacelle has an L/W ratio of roughly 4 to 1 and the first is 6 to 1. What effect will that have?

It would seem (intuition again?) that the actual increase in drag would be more than is indicated by, ^(2/3).

How do I account for that part?

[Guest625101138]

Go back a few posts and see what you said there about precision with estimates.

Until you get below fineness ratio of 3 I doubt that you are doing more than "polishing the turd" to use anything but the drag ratio based on volume as I suggested.

Rick W

[Joakim]

Originally Posted by Asleep Helmsman

For example the second nacelle has an L/W ratio of roughly 4 to 1 and the first is 6 to 1. What effect will that have?

It will change the form drag and also the surface area may increase. Look at post #16 for the formula. When the size changes, Reynolds number and thus friction coefficient changes as well.

Joakim