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July 2, 2015

Kaplan: blades and BAR

How do we change the BAR for a Kaplan-style propeller? Some of our customers request less BAR that what the program offers (e.g., 0.65 DAR on 3 blades, but they want 0.55).

One inconvenience of the Kaplan propellers prediction methods is that the scope can be rather narrow. In some cases, you may only have one combination of blade count and expanded blade area ratio (BAR). For example, if you want to size for a Kaplan19A propeller with five blades, you are limited to 0.75 BAR. For three blades, your one choice is 0.65 BAR.

So how can you model a propeller with higher or lower BAR in PropExpert?

First, you’ll want to modify the T and P factors. For example, let’s consider modeling a propeller with 25% to 30% more BAR (such as 0.95 as compared to the standard 0.75). One evaluation that we conducted indicates that there is virtually no increase in thrust for this increase in BAR (less than one-half percent), but there will be a two- to four-percent increase of required power. Therefore, you might keep the T factor at 1.00, but increase the P factor to 1.03. Likewise, for a 15% reduction in BAR, such as from 0.65 to 0.55, you might lower the P factor to 0.99.

Second, you will need to appropriately use the “BAR criteria” in the Sizing Details. For example, if you have a 0.95 BAR propeller, but are limited in PropExpert to a figure of 0.75, then your “criteria” will be 127% (i.e., 100 x 0.95 / 0.75), not 100%. In other words, for the purposes of the “BAR criteria” PropExpert thinks that you have a 0.75 BAR propeller, not the 0.95 that you intend to install. For the three blade example, your “criteria” limit will be 100 x 0.55 / 0.65, or 85%.

Remember, you can always use the Propeller Library to pre-set a special model with your own combination of parameters. Then you can simply import the “model” that you defined, including your preferred propeller series, as well as T and P factors.

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Article details

Article ID: 73
Category: PropExpert
Date added: 2015-07-02 16:30:18
Views: 469
Rating (Votes): Article rated 3.2/5.0 (29)

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