Silly Question on Prop Load: APC17x8 = APC18x6W
 

Hi all

I know there are many factors on prop selection but I wonder if any one have experienced similar.

Currently I have APC18x6W on my G26 and it give good thrust but the spool up is slower than I want it to be. When I tested it with Menz16x8 it is so responsive and instantaneous.

APC18x6W give me about 7500rpm where the Menz16x8 for 8500. I wanted to use biggest possible prop with still good respond. The LHS only have APC17x8 and Menz 17x8.

I wonder if this 17x8 have lower or similar load to the engine? If there are lower than it will be perfect so I won't lose much thrust while getting good spool up!

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

Typically reducing the diameter by 1 and increasing the pitch by 2 will keep your rpms in the same ball park. You reduced the diameter by 2 and increased the pitch by 2, especially from a wide prop to a standard width. That's why you got another 1000 rpm. A wide prop is a little like having more pitch. It is has more surface area to grab air. It just doesn't pull it through any faster like a higher pitch would, but does increase thrust without lowering the pitch. Also there is a difference in the weight of the props which also effects acceleration.

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

Thanks Alan. I will try out the 17x8APC and Menz and shall see how the response and max rpm like.

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

Hi!
I flew my SIG CAP 231 using a Menz 18x6 (MVVS 25cc glowengine)and that prop was much better than the 18x6W APC . The APC was to heavy.

Regards!
Jan K
Sweden

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

The APC 18x6W is really heavy compare to Menz. The Menz spool up is fantastic!

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

I think the 17x8 prop will present a much heaver load than the 16x8 and in a ratio of 1815/1608. I don’t know how to correlate a wide APC to the Metz prop.

If the Metz 16x8 turns 8500 then the Metz 17x8 should turn about 7600.

Bill

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

Hi Bill

Here a bit more info from my earlier post:

G26 with APC 18x6W: 7500 rpm on stock muffler. With Abell, I suspect easily 7800-7900.(not measured yet)

With Menz 16x8, I get 8500 on stock muffler and 8800 with Abell muffler.(measured)

So if I were to use Menz 17x8, you predicted it would drop to 7600(stock muffler)? That means the load is similar to APC 18x6W then, or at least close enough??

I also bought a Menz 18x6 what its the likely rpm?

If the Menz 18x6 will hit 7600 on stock muffler (expecting similar load as 17x8), I will run it with abell muffler to hit 7800-7900.

This will go into my GP Super Stearman. I need more thrust than speed! It should improve as I have only run in like 4-5 tank only.

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

If the Metz 16x8 turns 8500 then the Metz 18x6 should turn about 9000. The Metz 18x6 presents a lighter load than the Metz 16x8 and the ratio is 1526/1608.

Depending upon the engine you may be approaching the top of the power curve at 9000. When you get outside the sweet spot in the torque/horsepower curves all changes will be disappointing. Glow engines will usually have a higher sweet spot than gas engines. 8500 is probably close to the top of the power curve.

I will be interested in knowing how close the guess is to measured RPM numbers for the Metz props.

Good luck with your experimentation.

Bill

The props mentioned indicate that you are probably running a 25cc or so engine.

If it happens to be a ST you are running there are 13 pages of information at:
http://www.rcuniverse.com/forum/m_3836695/tm.htm

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

Hi Bill

You are spot on the engine. I'm running it on Zenoah G26 cc engine. BTW, what is the nos 1526/1608? Is there any where I can do more read up on?

If the Menz 18x6 can spin anything at 8500, I'm a very very very happy man!

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

No magic and not complicated. The numbers are simply a way to compare relative prop loads. The numbers are the volume that a prop will sweep in a single revolution. Slip and efficiency are ignored. I was simply looking for a relative number to compare same brand name props. The numbers are surprisingly good at predicting RPM as long as you stay within the power band.

Bill

Here are the volume numbers sorted by load.
16x6 1206
17x6 1361
18x6 1526
16x8 1608
17x8 1815
16x10 2010
18x8 2035
I was surprised that changing from 16x6 to 18x8 represented a 70% increase in load. When an engine turns 7500 and you are attempting to prop for 8000 the selection is too course.

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

Found two more addresses with G-26 information.

http://www.rcuniverse.com/forum/m_413509/tm.htm
http://www.rcuniverse.com/forum/m_2428494/tm.htm

Hope they help.

Bill

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

There is no universally accepted formula for calculating prop load. Over the years, I have seen diameter times pitch, diameter squared times pitch, diameter cubed times pitch and diameter to the 4th power times the pitch. It is generally believed that a change in diameter has a larger effect than a change in pitch. For example, changing from a 16-8 to a 17-8 puts a larger load on the engine than changing from a 16-8 to a 16-9 would.

I have been testing and comparing for years and I have not been able to verify one method or the other. I generally use the diameter cubed times the pitch, but like all formulas, you have to use them intelligently. You may have a prop that puts the rpm out of your engine's "sweet spot." When you change props, the new one might be the prop that turns where the engine likes it so it may blow the formula out of the water. Any formula you use will tell you something, just don't believe the answers as absolute because they aren't. Power and torque aren't linear functions in any engine.

Going up an inch in diameter and down an inch in pitch does work within a narrow range, and I use it. You can't extrapolate this very far or it loses its validity.

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

Ed,

Without a doubt the diameter has more effect than the pitch. The sweep area goes up as the square of the prop radius.

I am not quite with you on the above statement but believe you may have an engineering basis for cubing the diameter.

And someone used to pay a kings ransom for this drivel.;)

Bill

RE: Silly Question on Prop Load: APC17x8 = APC18x6W
 

Magna,

Had an opportunity to test a 25cc engine with several props this past week. Sometimes predictions do not turn out as expected. I do not understand the data.

The test numbers were:
15x8 MA 7500
16x6 MA 7300 Over 8000 was expected.
16x8 MA 6700/6800

Bill