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Sep 07, 2004, 06:33 PM
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Prop Selection and LiPo C Rating


Can those of you flying larger LiPo packs provide some guidance on selection of an appropriately sized prop for the first flight of an ExtremeFlight Yak 54 to be used for FAI style flying only (i.e., no 3D). In my case, I'm using a C50 12XL with a TP10S3P 6000 "Lite" pack. Is the objective to find a prop which, at WOT on the ground, will pull the max C rating of the pack? For example, the TP10S3P 6000 is a 10C pack. Should I select a prop which will draw 60 amps at full throttle on the ground? Or, as happens with glow engines, will the motor "unwind" in the air such that the current draw at WOT as seen on the ground will drop in the air and therefore, a larger diameter or higher pitch prop can be selected compared to what was viewed on the ground? Your input would be greatly appreciated.

Mike M
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Sep 08, 2004, 04:24 AM
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vintage1's Avatar
The 8nwindig depends a lot on tehmodel.

Large fine pitched props unload a lot. Small coarse pitched props may increase current as the blades come out of stall as airspeed picks up.

Best to prop at about 8C static IMHO.
Sep 08, 2004, 06:16 PM
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Thanks for the input. Those flying C50 14XL motors with 22 x 12 APCE props in pattern aircraft appear to be operating their LiPo packs at or slightly above 10C (static current draw on the ground using a TP5S3P 6000 pack). As I thought about it, I didn't remember seeing any charts or procedures mentioned on Internet forums for determining the best prop for the performance desired without subjecting the lithium battery to potential damage due to current levels which are too high....therefore, my question in this post. Since I'm currently using a C50 12XL, I figured I would follow the lead of those flying with the C50 14XL and select a prop which provides about a 10C current draw on the ground (with a TP5S3P 6000 pack)....but perhaps it would be beneficial to be a bit more conservative.
Sep 08, 2004, 10:10 PM
Michael, there are some data loggers on the market that plug into the wires in your plane and monitor things like voltage and amperage. Short of buying one of those, I don't see a way to figure out exactly happens with loading in the air. I know on the 140DZ in pattern planes they do rev higher in the air, and continue to rev up as the plane accelerates to top speed. With a glow motor we are looking at the RPMs going up under a (almost) constant power supply as prop load decreases with airspeed. With an electric system you have a (almost) constant RPM, and since the only important difference from the glow system is a lower pitch speed, I would be willing to bet your load decreases as the airspeed increases.

As for prop with that system, the 22x12 is probably too much unless you are careful to almost never use full throttle. You could try shaving down the diameter on the 22x12 if APC doesn't make a 21x12. Shulman was saying he shaves down a 24x12 to about 22" on his 14XL. It sounds like your pack also has a slightly lower C-rating than what everyone else with the 14XL + TP10s3p has been using.

A device to monitor current, and motor and battery temperature in a ground run should be sufficient to determine if you are damaging your components.

Just checked, looks like a 22 hour driver from Nebraska to our nearest pattern contest =<
Sep 09, 2004, 12:17 AM
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adamg: You must be on either the east or west coast. It took me around 21 hours to drive back to NJ when I was stationed in South Dakota years ago. In my roaming around various websites on the Internet, I had "Google" translate a German site which provided a chart from Hacker. The chart indicated various combinations of Hacker motors, gearing, props and LiPo packs for FAI competition. For the C50 motors (12XL through 14XL), the chart called for 10S LiPo batteries rated at 6000 mah. Gearing was 6.7:1 for all three motors and the Master 90 Acro controller was also recommended for all three. The recommended prop for the C50 12XL was an APCE 20 x 12 while the 13XL called for a 21 x 14 and the 14XL, a 22 x 12. My LiPo packs are the new TP5S3P 6000 "Lite" packs. They weigh around 21.5 oz and are supposedly rated at 10C with a higher burst capability. Thunderpower refers to them as "FAI packs" as I believe they are targeted at the pattern community. I do have an Astro watt meter and an infrared thermometer so I suppose the best way to proceed is as you state: monitor current, and motor and battery temperature. I would think I should be safe if I keep the static ground current draw at or below 10C.
Sep 09, 2004, 03:19 AM
The lipos tend to go off balance very quickly if u constantly use it at the max rating. To be lighter on the pocket, and do less balancing, I would prop it for 8c and enjoy a more constant flight performance.
Sep 09, 2004, 07:56 AM
Use the 4S Luke
feathermerchant's Avatar
Also don't run it on the ground for long at 8 or 10C
Could be expensive


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