The Amazing 120AX
08-25-2008, 06:00 PM
#1
The Amazing 120AX
Hey, Bax:
I'm putting my new 120AX in a TopFlite Gold Edition P-51 (1/7 scale). Thought you might like to know what I've found so far.
I need to go inverted with the engine so it will fit in the cowl, (I'm using a Macs muffler) and as I'm painting my spinner, it has to start reliably by hand. I built a test stand to evaluate it against my two "conditions".
I have to say I have never had an engine that starts this easily. At WOT I give it two flips with my thumb over the air horn, set it to just above idle, hookup up the igniter, and the most I've had to flip it is three times (using an old heavy leather Army issue glove with the wool liner inside).
It seems rather insensitive to fuel type- I've used some 15% that I added a lot of oil to, bringing it to maybe 25% lube content (30oz run through it); some 10% (20oz) and some 15%.(10oz) I settled on using the 10% after reading what you had to say about nitro content.
I tried a 15x10 MAS K prop, as I want a lot of speed (close to 100 mph) AND a lot of thrust, but it was too much, in my estimation. The engine turns it at about 8000 when peaked, and considerably slower when off peak. The 16x8 is right out- not enough airspeed, as it will only turn about 8300 rpm.
Today I tried an APC 14x10, and she was a bit fast- I could have probably peaked it at 9500 or over, which you said previously is not a good idea. Then I installed an APC 14x12, and it gives the combination I've been looking for- it peaks at around 8900, and I've backed the needle out to give about 8600. This yields nice, cool running and a real good "pinch" test. Thrust/HP says if it unloads to 9500 in the air, I'll get 108 mph, around 11# static thrust, and output will be 2.8HP.
The engine idles reliably at 1700 or less, and will transition clean to WOT after over a minute at that rpm, but I think I'll set it at around 1800 for flight. I found I had to turn the low speed needle in about 1/2 turn to get a good idle mixture. (I'm a big believer in the pinch test- if I squeeze the fuel line at idle and it speeds up right away, it's too rich. I lean until it just speeds up a little after pinching for a second or two.)
Think it'll be OK, or should I go ahead and try the APC 15x10? Keep in mind that raw speed is a scale Mustang's only friend, and that we suffer from unusually high density altitudes here- we're only 1200 feet above sea level, but have mostly sunny and somewhat humid days during the summer. For example, Thursday evening it was only about 71 degrees F, but the density altitude program at the airport showed almost 3000 feet, so we give up a lot of power and lift.
Great engine, as are my two 55AX's and my 46AX. Maybe even better. Can't wait to fly it.
I'm putting my new 120AX in a TopFlite Gold Edition P-51 (1/7 scale). Thought you might like to know what I've found so far.
I need to go inverted with the engine so it will fit in the cowl, (I'm using a Macs muffler) and as I'm painting my spinner, it has to start reliably by hand. I built a test stand to evaluate it against my two "conditions".
I have to say I have never had an engine that starts this easily. At WOT I give it two flips with my thumb over the air horn, set it to just above idle, hookup up the igniter, and the most I've had to flip it is three times (using an old heavy leather Army issue glove with the wool liner inside).
It seems rather insensitive to fuel type- I've used some 15% that I added a lot of oil to, bringing it to maybe 25% lube content (30oz run through it); some 10% (20oz) and some 15%.(10oz) I settled on using the 10% after reading what you had to say about nitro content.
I tried a 15x10 MAS K prop, as I want a lot of speed (close to 100 mph) AND a lot of thrust, but it was too much, in my estimation. The engine turns it at about 8000 when peaked, and considerably slower when off peak. The 16x8 is right out- not enough airspeed, as it will only turn about 8300 rpm.
Today I tried an APC 14x10, and she was a bit fast- I could have probably peaked it at 9500 or over, which you said previously is not a good idea. Then I installed an APC 14x12, and it gives the combination I've been looking for- it peaks at around 8900, and I've backed the needle out to give about 8600. This yields nice, cool running and a real good "pinch" test. Thrust/HP says if it unloads to 9500 in the air, I'll get 108 mph, around 11# static thrust, and output will be 2.8HP.
The engine idles reliably at 1700 or less, and will transition clean to WOT after over a minute at that rpm, but I think I'll set it at around 1800 for flight. I found I had to turn the low speed needle in about 1/2 turn to get a good idle mixture. (I'm a big believer in the pinch test- if I squeeze the fuel line at idle and it speeds up right away, it's too rich. I lean until it just speeds up a little after pinching for a second or two.)
Think it'll be OK, or should I go ahead and try the APC 15x10? Keep in mind that raw speed is a scale Mustang's only friend, and that we suffer from unusually high density altitudes here- we're only 1200 feet above sea level, but have mostly sunny and somewhat humid days during the summer. For example, Thursday evening it was only about 71 degrees F, but the density altitude program at the airport showed almost 3000 feet, so we give up a lot of power and lift.
Great engine, as are my two 55AX's and my 46AX. Maybe even better. Can't wait to fly it.
08-26-2008, 01:22 PM
#2
My Feedback: (11)
RE: The Amazing 120AX
If you want static thrust, you can't have speed. The two are not compatible. To get static thrust, you want larger-diameter propellers with lower pitch. The low pitch number prevents speed. To get speed, you want smaller-diameter propellers with higher pitch. Higher pitch is like having a higher gear in your car...you have good top speed, but lousy low-end pull.
If you prop the engine for proper RPM, then see how the model flies. You will normally want an 8-pitch or higher propeller. Once you've selected a prop that has the diameter to give you the proper RPM level, you have to accept the model's performance that you get. If the model is within the manufacturer's weight recommendation, it should fly nicely.
If you prop the engine for proper RPM, then see how the model flies. You will normally want an 8-pitch or higher propeller. Once you've selected a prop that has the diameter to give you the proper RPM level, you have to accept the model's performance that you get. If the model is within the manufacturer's weight recommendation, it should fly nicely.
08-26-2008, 04:53 PM
#3
RE: The Amazing 120AX
Yeah, I'm lookin real close at the APC 15x10. I think it represents considerably less load than the MAS K series. (If you cube the diameter and then multiply that by the pitch, a 15x10 and a 14x12 represent almost the same loading, assuming the same design. (33750 for the 15x10 and 32928 for the 14x12) I'm thinking that if I can get sufficient rpms, the 15x10 is a better choice.
