Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Exactly what roll does diameter play in speed?

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Old 01-24-2008, 10:38 AM
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Zman39
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Default Exactly what roll does diameter play in speed?

I been testing a lot of props on a Jett and I keep hearing that pitch is where you will get the speed and I understand the logic behind that statement. But I know diameter has to play a role in the configuration somewhere. For instance I got a better GPS speed reading going to 10x6 from 9x7 back in the early stages of testing. Is the answer use as much pitch as possible while still maintaining the rpms? or does diameter play another role as well?

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Old 01-24-2008, 11:17 AM
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Mike Connor
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Default RE: Exactly what roll does diameter play in speed?

More prop diameter equals more thrust at a given rpm. Thrust must exceed drag to increase speed.
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Old 01-24-2008, 11:25 AM
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Default RE: Exactly what roll does diameter play in speed?

Reduce drag and the 9x7 will be faster.
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Old 01-24-2008, 11:37 AM
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Default RE: Exactly what roll does diameter play in speed?

It's all a balancing act. The pitch moves the air at a certain speed but think of diameter being part of the area of a circle, the prop area. You have to have enough prop are to move a volume of air sufficient to move the plane. Secondary is how much of that are does the spinner or fuselage block.

After all this then you can take into effect other things like blade cord, tip shape, root to tip pitch change, etc.
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Old 01-24-2008, 11:46 AM
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Default RE: Exactly what roll does diameter play in speed?

Think of diameter as load. It is a balancing act because as diameter increases so does drag and load, thus decreasing RPMs. A 10x6 at 20,000 rpm will usually fly a plane faster than a 9x6 at 20,000 rpm because you have more effective pitch moving air (more oar in the water). Unfortunately, the reality is as you increase diameter you increase aerodynamic drag and load on the engine so you give up some RPMs and thus some speed (you still have to lift and move that big oar). The key is to find the prop that has the right combination of diameter and pitch to get your engine in it's peak power producing range. Generally speaking use the largest diameter prop that allows your engine to unwind at a given pitch.

I am not an aerodynamacist, but I did stay at a Holiday Inn express last night.

Anthony
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Old 01-24-2008, 02:32 PM
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Default RE: Exactly what roll does diameter play in speed?

Interesting, thanks guys. I guess it is also a balancing act to find a diameter and pitch that the pipe likes. Much to play with, thanks again


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