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This subject have been talked about before, but I would like to re-open the discussion if anyone is still interested.
Back in the day I used to tow with a G-62 powered tug. The standard prop for me was a Menz 24x10 which enabled me to tow most 1:3 scale single seat gliders without much trouble.
Doing a bit of thinking led me to conclude, that using a 26x8 prop instead would make it an even more powerful tug, but to my surprise I found that the power available was even lower than before. I had to climb at a much shallower angle to prevent going way too slow and stalling out.
After this experiment I went back to the 24x10 prop.
Years later, last year to be specific, I designed and built a new towplane. This was powered by a King 140 boxer known for it´s low rpm torque .
Most people use a 32x10 prop on this engine, and when installed in a Frisch 1:3 scale Wilga, a SEP 32x10 WIIIIIIDE blade "Wilga" prop is more or less standard.
I also own a Moki 250 radial, and on this engine most people recommend props with 16-22" pitch to prevent overspeeding the engine, and I decided to try the same approach with the King 140.
The first flights was performed with either a 3W 32x12x2 or the previously mentioned SEP Wilga 32x10x2. These props would spin 5.000 and 4.600 rpm respectively, and I found the performance to be average at best. A lot of noise and no real work being done.
I then tried a Fiala 30x16x2 prop, spinning about 5.300 rpm, and the tug was transformed to a very powerful towplane.
With this prop I could tow even very heavy and draggy gliders at a steep angle without running out of airspeed, and when plugging the numbers into the late Pé Reivers prop calculator I noticed that, in theory, I would have about 4 kg (+33%) more thrust at 70 km/h, which is a good towing speed in my experience.
The theroretical static thrust of the high pitch Fiala prop is lower than the 3W 32x12x2, but as soon as the airspeed reaches 20 km/h the Fiala starts to outpull the 3W. This was also very noticeable in real life.
This was a very long intro, but I would be interested in hearing if anyone else have been doing similar experiments, or if most people just stick to the "standard" size props.
My own experiments have been very convincing and I have just ordered a Fiala 28x16x3 in an attempt to lower the noise a bit. The Fiala 2-blade is quite "rippy", and at most of the fields I fly at, I´m not able to use full power without busting the noise limits. With the 3-blade I hope to get below the normal limit of 94 dB measured at 3 meters.
Back in the day I used to tow with a G-62 powered tug. The standard prop for me was a Menz 24x10 which enabled me to tow most 1:3 scale single seat gliders without much trouble.
Doing a bit of thinking led me to conclude, that using a 26x8 prop instead would make it an even more powerful tug, but to my surprise I found that the power available was even lower than before. I had to climb at a much shallower angle to prevent going way too slow and stalling out.
After this experiment I went back to the 24x10 prop.
Years later, last year to be specific, I designed and built a new towplane. This was powered by a King 140 boxer known for it´s low rpm torque .
Most people use a 32x10 prop on this engine, and when installed in a Frisch 1:3 scale Wilga, a SEP 32x10 WIIIIIIDE blade "Wilga" prop is more or less standard.
I also own a Moki 250 radial, and on this engine most people recommend props with 16-22" pitch to prevent overspeeding the engine, and I decided to try the same approach with the King 140.
The first flights was performed with either a 3W 32x12x2 or the previously mentioned SEP Wilga 32x10x2. These props would spin 5.000 and 4.600 rpm respectively, and I found the performance to be average at best. A lot of noise and no real work being done.
I then tried a Fiala 30x16x2 prop, spinning about 5.300 rpm, and the tug was transformed to a very powerful towplane.
With this prop I could tow even very heavy and draggy gliders at a steep angle without running out of airspeed, and when plugging the numbers into the late Pé Reivers prop calculator I noticed that, in theory, I would have about 4 kg (+33%) more thrust at 70 km/h, which is a good towing speed in my experience.
The theroretical static thrust of the high pitch Fiala prop is lower than the 3W 32x12x2, but as soon as the airspeed reaches 20 km/h the Fiala starts to outpull the 3W. This was also very noticeable in real life.
This was a very long intro, but I would be interested in hearing if anyone else have been doing similar experiments, or if most people just stick to the "standard" size props.
My own experiments have been very convincing and I have just ordered a Fiala 28x16x3 in an attempt to lower the noise a bit. The Fiala 2-blade is quite "rippy", and at most of the fields I fly at, I´m not able to use full power without busting the noise limits. With the 3-blade I hope to get below the normal limit of 94 dB measured at 3 meters.
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