Prop question
07-27-2004 | 11:55 PM
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From: ashburn,
VA
Prop question
I almost completed my Sig 4 star 60 ARF. I got a Super tiger 75 for the Airplane. According to instruction of the engine, you should be using 12x8 to break in the engine. That said, can anyone explain what the difference between using difference prop sizes to power an airplane. All things being equal what if I go with a 12x5 or 10x8 or a 15x8 - how does this affect thrust or speed. My NextStar is is using an OS .46 and 11x6 prop...so would not a 12x6 or 12x8 be to small for Super Tiger 75. If anyone can shed some light on this would be great..or point any readings on prop theory or selection.
If you have a Sig 4 start 60, let me know what engine and prop you are using.
Thank you.
Qwazzy!!
If you have a Sig 4 start 60, let me know what engine and prop you are using.
Thank you.
Qwazzy!!
07-28-2004 | 12:31 AM
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From: Macho Grande, KS
RE: Prop question
Oops, missed part of the question in your thread in Beginners.
Too small dia and pitch, the engine will have to run higher rpm to give thrust as using correct prop. Too large or too much pitch, you notice the engine load up once you break ground with the engine rpm dropping off. This is much harder on the engine than being too little pitch.
Too small dia and pitch, the engine will have to run higher rpm to give thrust as using correct prop. Too large or too much pitch, you notice the engine load up once you break ground with the engine rpm dropping off. This is much harder on the engine than being too little pitch.
07-28-2004 | 01:01 AM
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From: Schuylkill Haven,
PA
RE: Prop question
Ok I will try to shed some light on this topic with what i have read in fly rc magazine to the best of how I remember. If you use a prop with less pitch you will get better Torque and better hovering performance but also will have a shorter takeoff roll and less high end speed. A huger pitch prop will give you a longer takeoff roll but will increase the top speed of your aircraft. The lower pitch prop will have less of a load on the motor, witch will let the motor rev up higher. You want to be careful not to over rev your engine. To do this as you go up in pitch you can decrease the diameter of the prop or vise versa, this is because a bigger diameter prop is more weight to spin so therefore a little more drag. finding the best prop is a trial and error thing. because you have so many different factors that affect it like different motors, different prop manufacturers, different planes, and etc. I hope this helps a little
07-28-2004 | 10:22 AM
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RE: Prop question
I think you will find top speed is more to do with the airframe than the prop. I say this from past expierience. I had a plane that was old faithfull. It was a SuperSportster .60. I flew it many, many years. I always hated trying a new engine/plane combo. So I would break in the engine and test fly it on the SS. That being said I broke in a ST.75 and several ST.90s on this plane. The bolt patter was the same as the ST.60 so why not. What I found was even with the .90 it wasn't much faster. The big difference was when you pointed it up. It didn't slow down with the .90 on it.
Pitch difference in props is another thing. I have a Saito .72 powered Pizzaz. I have run an APC13x6 and an APC13x4W. The 13X4W has instant throttle response. It is better for short takeoffs and hovering. The 13x6 tranfers a little slower but has better top speed. On this plane however the higher top speed causes flutter.
I hope I am not confusing the issue for you. Let me finish by saying I ran a ST.75 for three years on a Ultra Sport .60. The 13X6 Master Airscrew was the best prop for it. I flew it as a sport/patter type plane. If I had wanted more verticle I would have tried a 14X4w. I guess it depends on how you want to fly it.
David
Pitch difference in props is another thing. I have a Saito .72 powered Pizzaz. I have run an APC13x6 and an APC13x4W. The 13X4W has instant throttle response. It is better for short takeoffs and hovering. The 13x6 tranfers a little slower but has better top speed. On this plane however the higher top speed causes flutter.
I hope I am not confusing the issue for you. Let me finish by saying I ran a ST.75 for three years on a Ultra Sport .60. The 13X6 Master Airscrew was the best prop for it. I flew it as a sport/patter type plane. If I had wanted more verticle I would have tried a 14X4w. I guess it depends on how you want to fly it.
David
07-28-2004 | 10:47 AM
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From: Corona, CA,
RE: Prop question
Your engine has a rated maximum rpm. On the ground you lose about 15-20% of the rpm’s a prop will turn in the air due to reduced drag on the prop. In the case of the ST 75 red line is 16000 rpm. You want a prop that turns about 80% of that on the ground, or approximately 13,000 rpm after the engine is broken in.
Some engines produce maximum hp at much lower rpms than the maximum rpm. On those engines, this should also be taken into account. In the case of the ST 75 the max hp is at 15,600 and you can pretty much treat the hp and max rpms as the same.
The pitch on a prop is the number of inches it will move forward in one revolution. An 8 pitch prop will move forward 8 inches, in the air, not taking minor losses due to slippage into account. A 10 pitch prop would move forward 10 inches.
In order to keep the rpm where you want it, you adjust the diameter of the prop. A low pitch, larger diameter prop will produce more thrust (what you need for hovering or vertical performance). A higher pitch, with a smaller diameter will produce higher speed. More thrust also means a shorter takeoff run, while more speed means a longer takeoff run.
There are empirical formulas that predict which prop to use for what application you desire. One website that does this for you is located at http://pages.sbcglobal.net/limeybob/ . There is also an online manual that gives much more information and explanations of the formulas.
Some engines produce maximum hp at much lower rpms than the maximum rpm. On those engines, this should also be taken into account. In the case of the ST 75 the max hp is at 15,600 and you can pretty much treat the hp and max rpms as the same.
The pitch on a prop is the number of inches it will move forward in one revolution. An 8 pitch prop will move forward 8 inches, in the air, not taking minor losses due to slippage into account. A 10 pitch prop would move forward 10 inches.
In order to keep the rpm where you want it, you adjust the diameter of the prop. A low pitch, larger diameter prop will produce more thrust (what you need for hovering or vertical performance). A higher pitch, with a smaller diameter will produce higher speed. More thrust also means a shorter takeoff run, while more speed means a longer takeoff run.
There are empirical formulas that predict which prop to use for what application you desire. One website that does this for you is located at http://pages.sbcglobal.net/limeybob/ . There is also an online manual that gives much more information and explanations of the formulas.
07-28-2004 | 04:47 PM
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From: ashburn,
VA
RE: Prop question
ORIGINAL: J_R
Your engine has a rated maximum rpm. On the ground you lose about 15-20% of the rpm’s a prop will turn in the air due to reduced drag on the prop. In the case of the ST 75 red line is 16000 rpm. You want a prop that turns about 80% of that on the ground, or approximately 13,000 rpm after the engine is broken in.
Some engines produce maximum hp at much lower rpms than the maximum rpm. On those engines, this should also be taken into account. In the case of the ST 75 the max hp is at 15,600 and you can pretty much treat the hp and max rpms as the same.
The pitch on a prop is the number of inches it will move forward in one revolution. An 8 pitch prop will move forward 8 inches, in the air, not taking minor losses due to slippage into account. A 10 pitch prop would move forward 10 inches.
In order to keep the rpm where you want it, you adjust the diameter of the prop. A low pitch, larger diameter prop will produce more thrust (what you need for hovering or vertical performance). A higher pitch, with a smaller diameter will produce higher speed. More thrust also means a shorter takeoff run, while more speed means a longer takeoff run.
There are empirical formulas that predict which prop to use for what application you desire. One website that does this for you is located at http://pages.sbcglobal.net/limeybob/ . There is also an online manual that gives much more information and explanations of the formulas.
Your engine has a rated maximum rpm. On the ground you lose about 15-20% of the rpm’s a prop will turn in the air due to reduced drag on the prop. In the case of the ST 75 red line is 16000 rpm. You want a prop that turns about 80% of that on the ground, or approximately 13,000 rpm after the engine is broken in.
Some engines produce maximum hp at much lower rpms than the maximum rpm. On those engines, this should also be taken into account. In the case of the ST 75 the max hp is at 15,600 and you can pretty much treat the hp and max rpms as the same.
The pitch on a prop is the number of inches it will move forward in one revolution. An 8 pitch prop will move forward 8 inches, in the air, not taking minor losses due to slippage into account. A 10 pitch prop would move forward 10 inches.
In order to keep the rpm where you want it, you adjust the diameter of the prop. A low pitch, larger diameter prop will produce more thrust (what you need for hovering or vertical performance). A higher pitch, with a smaller diameter will produce higher speed. More thrust also means a shorter takeoff run, while more speed means a longer takeoff run.
There are empirical formulas that predict which prop to use for what application you desire. One website that does this for you is located at http://pages.sbcglobal.net/limeybob/ . There is also an online manual that gives much more information and explanations of the formulas.
J_R,
You basically answered my question. Excellent, thank you. So if on the intructions sheet it says to use a 12x5 to 12x8 prop for ST 75. Could I technically go to 15x4 for better vertical performace and hover?
07-28-2004 | 05:52 PM
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From: Corona, CA,
RE: Prop question
If you try to use a 15x4 it will probably not be able to turn enough rpms to keep the engine in the power band (15600 rpm). Try a 13x4, if that works OK then you might want to try a 14x4. All the forumulas will only provide a guideline so that you do not have to buy every possible prop size when you experiment.
Trying a 15x4 should not hurt anything... as long as you respect what your ears and eyes tell you when the plane is in flight. If it sounds like it is bogging down, or looks like it is barely flying, get it down and let it cool. Then change to a smaller diameter prop.
The formulas project 17.5 pounds of thrust with a 15x4 at 11400
16 pounds of thrust with a 14x4 at 12500
and 14 pounds of thrust with a 13x4 at 13800.
Trying a 15x4 should not hurt anything... as long as you respect what your ears and eyes tell you when the plane is in flight. If it sounds like it is bogging down, or looks like it is barely flying, get it down and let it cool. Then change to a smaller diameter prop.
The formulas project 17.5 pounds of thrust with a 15x4 at 11400
16 pounds of thrust with a 14x4 at 12500
and 14 pounds of thrust with a 13x4 at 13800.
