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Fresh powder and prop size

Old 02-25-2008, 11:27 AM
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Rufcut
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Default Fresh powder and prop size

I finally got the chance to fly my trainer Saturday. Beautiful crystal clear skies and dead calm air. I got the Kadet Senior down to the pasture and when I set it down the skis sank about 2 inches into fresh powder. When I tried to take off the plane just didn't want to accelerate to speed due to the drag from the snow. Finally, I rocked the ailerons back and forth and I was able (barely) to get it into the air. I am using an O.S. 46fx with an 11x7 prop. What size prop will give me the maximum acceleration on take-off?
O.S. recommends: Sport- 10.5x6, 11x6-8, 12x6-7 Aerobatic- 11x8-10, 12x7-9 (with tuned silencer)

Thanks

Rufcut
Old 02-25-2008, 11:42 AM
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Deadeye
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Default RE: Fresh powder and prop size

You have discovered that in order for skis to work, you need just the right snow pack. Floats work way better in any snow condition than skis. As far as prop, you need maximum static thrust. I use an APC 14X4 on my Stick with an OS .46 FX, 15% nitro.
Old 02-25-2008, 04:10 PM
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Default RE: Fresh powder and prop size

What skis are you using and trike or taildragger? Try an APC 11-5 or 12-4 for that engine, a 14-4 is way to big. Once the plane gets some speed it should lift up on the snow and not dig in. An 46 is lots of power for a trainer.
Old 02-25-2008, 06:08 PM
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Default RE: Fresh powder and prop size


ORIGINAL: RVman

What skis are you using and trike or taildragger? Try an APC 11-5 or 12-4 for that engine, a 14-4 is way to big. Once the plane gets some speed it should lift up on the snow and not dig in. An 46 is lots of power for a trainer.
Could you clarify? Are you talking about ground clearance? If you think I'm propped too big, I'm not. I've got good revs, and am balistic at 1/2 throttle. 11X7 is recommended for this engine, and if you want to go up in length an inch, you need to drop your pitch 1 size too. 12X6, 13X5 14X4.
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Old 02-25-2008, 07:06 PM
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Default RE: Fresh powder and prop size

All these engine makers tell you big props as part of their advertising. They advise you to put heavy props on because they know people looking at two engines will compare prop sizes recommended, and people tend to buy the one that "turns" the bigger props. Sure, they will turn the prop, but not as well as a lighter size. Bigger props tend to make the engine run hotter, make the needle range limited, and produce much less power. That engine with a 11-6 will drag another engine with the 11-7 all over the field.
Old 02-25-2008, 07:26 PM
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Default RE: Fresh powder and prop size

Thanks for the input everyone.
I have tricycle gear on this plane RVman. I'm not trying to be dense here, but I'm still a little confused. Will the 11x6 give me the most thrust for takeoff or will the 12x6? I generally stay with the manufacturers recommended prop sizes because I feel they should have the R&D behind the engine to know what will work best although it is always fun to experiment. Is there a tutorial on propellers that someone could point me to?

Thanks,

Rufcut
Old 02-25-2008, 07:40 PM
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Default RE: Fresh powder and prop size

Bigger diameter, lower pitch will give you more static thrust. This is what you need for takeoff on deep snow and water. Smaller diameter and more pitch will get you around the air faster, but you won't have the power needed for takeoff. Think of the high pitch prop as third gear, and the low pitch as first gear in a vehicle. Which gear is going to get you to twenty MPH faster?

I generally ignore manufacturer rec's, and experiment on my own. 14X4 gives slightly less RPM's than a 11X7, and it spools up a little slower. But an 11X7 sure won't pull this plane out of a hover.
Old 02-25-2008, 09:07 PM
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Default RE: Fresh powder and prop size

Thanks Deadeye.......12x6 it is. I'm off to the prop balancer.

Rufcut
Old 02-25-2008, 09:23 PM
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Default RE: Fresh powder and prop size


ORIGINAL: Rufcut

Thanks for the input everyone.
I have tricycle gear on this plane RVman. I'm not trying to be dense here, but I'm still a little confused. Will the 11x6 give me the most thrust for takeoff or will the 12x6? I generally stay with the manufacturers recommended prop sizes because I feel they should have the R&D behind the engine to know what will work best although it is always fun to experiment. Is there a tutorial on propellers that someone could point me to?

Thanks,

Rufcut
Sorry my post was a little confusing. Although the engine will turn a 12-6 ok it is a lot of load for a 46. That's why I recommended the smaller props 11-5, 12-4. OS and others often recommend props for noise constraints as the larger prop turns slower therefore less noise, but also less performance.
I've never had good luck with trike gear in deep powder snow...

Deadeye, curious what the 46 turns for rpm on that 14-4?
Old 02-26-2008, 08:32 AM
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Default RE: Fresh powder and prop size

For the maximum thrust needed to get the plane moving in the powder with the OS 46 AX, definitely go with the APC 12x4. The 12x6 will run quieter, but you will sacrifice thrust as the prop overloads the engine and it will not get to the RPM necessary for equivalent thrust.

For example, I usually get about 12400 out of a 12x4 prop on an OS 46 AX. Not claiming that ThrustHP is perfect, but the numbers are close to my experience, so I'll work the example. The horsepower for this combination is approximately 1.13 and that generates approximately 9.5 lbs of static thrust. When I check with a 12x6 prop at the same horsepower rating, I get approximately 10800 rpm, and approximately 7.3 lbs of thrust. Checking the numbers with an 11x6, the static thrust drops to 6.9 lbs. In reality, since the rpms are lower, the actual HP will also be lower (further from the peak on the performance curve), and the thrust will actually be even lower.

These numbers are consistent with my real world experience as well. If noise is not as big an issue, go with the 12x4. It will be plenty fast enough to pull the plane through the air, as well as overcome the additional drag of the skis in the air.

Brad
Old 02-26-2008, 05:47 PM
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Default RE: Fresh powder and prop size

If the 12x6 is a little too much for your .46AX, APC makes a 12x5 which should work. Not normally my favorite choice of prop ----- sand off those sharp edges before you use it!
Old 03-05-2008, 12:06 PM
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Default RE: Fresh powder and prop size

Hi: I love to fly off the snow. My plane is also a kadat Senior with skis. My plane has a OS 61 and has plenty of power to lift off most snow conditions. I do something that is seldom see in the winter. Get a can of Pam and spray a good coat on your skis or floats. Pam acts like a lubracant and does not freeze on the gear. Try it you may like it, great funnnn.

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