Prop size and load

I picked this off a RC forum: "As for the difference in load between a 7x3 and a 7x4 propeller, if I understand pitch correctly, the 3 indicates that the prop will move forward 3 inches for each turn of the prop whereas the 4 indicates that the prop will advance 4 inches for each turn of the prop. Since the 7x4 has the potential to move 25% faster through the air when running at the same rpm as the 7x3 it appears to me that it does represent a significantly larger load on the engine"

I think I understand this, makes sense to you guys?

Thanks,

>I picked this off a RC forum: "As for the difference in load
>between a 7x3 and a 7x4 propeller, if I understand pitch
>correctly, the 3 indicates that the prop will move forward 3
>inches for each turn of the prop whereas the 4 indicates
>that the prop will advance 4 inches for each turn of the
>prop. Since the 7x4 has the potential to move 25% faster
>through the air when running at the same rpm as the 7x3 it
>appears to me that it does represent a significantly larger
>load on the engine"
>
>I think I understand this, makes sense to you guys?
>
>Thanks,
>
>Paul Heimgaertner

Paul,

If you take the numbers literally, which is what the author appears to be doing, to get from 3 to 4 requires an increase of 33 1/3%, not 25%.

On the other hand, if your question relates to the basic goodness inherent in swiping a posting from an RC-related site, plopping it down here where we take modeling seriously, no, what you have done does not make sense.

>
>On the other hand, if your question relates to the basic
>goodness inherent in swiping a posting from an RC-related
>site, plopping it down here where we take modeling
>seriously, no, what you have done does not make sense.
>
>Dan

Sir Dirt Dan,

I brought it here to get clarification, and you did answer the question with insight
and the correct numbers, my friend
you've been sucked into the r/c world of ignorance and made all of us control liners more proud and our egos bigger!

Love

Paul H.

>
>>
>>On the other hand, if your question relates to the basic
>>goodness inherent in swiping a posting from an RC-related
>>site, plopping it down here where we take modeling
>>seriously, no, what you have done does not make sense.
>>
>>Dan
>
>Sir Dirt Dan,
>
>I brought it here to get clarification, and you did answer
>the question with insight
>and the correct numbers, my friend
>you've been sucked into the r/c world of ignorance and made
>all of us control liners more proud and our egos bigger!
>
>Love
>
>Paul H.

Paul,

I do what I can, but it's a big job and help is needed.

LAST EDITED ON Dec-10-03 AT 07:05 PM (CST)
 
OK, I'll bite, but I suspect you are poking fun...just the same.

I tend to use the old multiply the diameter of the prop by itself, then that number by the pitch...D X D x P = Load factor. This really doesn't work, but on the .40 size engines it puts you in the ball park. So a 10 X 6 = 600, and 11 X 5 = 605, and a 12 X 4 = 576, in reality on my .40 fp the 12 X 4 does turn the most rpm's static, but the 11 X 5 turns more than the 10 X 6, which, according to this formula, it probably shouldn't.(all APC props) When I use this formula on my 25, it doesn't work so well, with a 9 X 6(Lf=486), my (RC on this one) P-51 unloads nicely, and flies fast, and carries speed through the maneuvers, but an 8 X 7(Lf=448) loads the engine up too much and never gets up enough speed to unload, maybe on a lighter plane? I suspect as you add pitch, the drag on the prop increases in a hurry. What does all of this tell me? Not a lot, but, when I'm looking at trying a new prop, I feel a little more comfortable about the cash I'm about to lay out. I don't remember where I got this formula, but there it is.

If you are *truly* interested, go to the AMA Model Aviation online archives, and do a search on propellers. There are at least 3 articles on carving your own wooden propellers, the most detailed being one by Hal Debolt. There are several other articles on modifying props, and checking pitch, and balancing...etc. But, there is one with a nice and complex formula that you can imput several factors like barometric pressure, temp, elevation...et al, and it's supposed to give you *real* answers. The author says that all of the formulas we use for model aviation are wrong, and this formula is for predicting prop performance on full scale planes, and he actually built a test stand with a scale that is calibrated to measure thrust, and drove to different elevations to prove the formula works on model propellers also...I dunno, I read the article a while back, and most of it went over my head, but it will all give you something to chew on. Most of these articles seem to be written along the line of racing props, or electric props, I'm still looking for good info on props for our purposes. I actually got interested in all of this because I can't find "good" props for my .10 and .15 size engines at my LHS, not a very wide selection in this size, and if a quater inch can make a difference on an 11 or 12 inch prop, then it has to make a difference on a 7 inch prop, right?

It depends if you mean static ground load or in-flight load. On ground certainly, but if you let the model with higher pitch flight at higher speed or – what we does on stunt models – at the same speed but lower rpm, then not necessary.

Here I attach prop power absorption ON GROUND.

igor

If I read the chart correctly, Igor, when the rpm doubles the horsepower consumbed by the prop goes up ten times. Is that what you (or this) is saying? (Check the rpm points at 9 and 18 thousand.)

Actually, the power required for a given prop goes up as the cube (3rd power) of the rpm. If you scale a prop up (6x3 becomes a 7x3.5 becomes an 8x4 etc. with the scaled up additional blade area) the power required for a given rpm goes up by the 5th power of the diameter, if I recall correctly.

C*x props in the gray series were all scale ups from the same master, so the above formula applies. Makes it easy!

Unless I am reading the chart wrong (bad translation or something) I look at 9 and 18 at the bottom or 10 and 20 at the bottom and then see where it intersects the same prop and it looks to me as if it is reading ten times the power consumed (left side). Is this correct?

>>>If I read the chart correctly, Igor, when the rpm doubles the horsepower consumbed by the prop goes up ten times. Is that what you (or this) is saying? (Check the rpm points at 9 and 18 thousand.)<<<

Larry says it is power of three. It is very close to true. The derivation is simple. If you turn it twice as much, then drag (means torque) is power of two, and we speak about power so it is +1 – means power of three. So it would be 0,2kw at 18000 if it is 0,025kw at 9000.

But problem is, that it is STATIC absorption, so with higher rpm you have also higher AoA and thus also drag coefficient and also tip induced drag is higher, so in reality the load is little more than expected. It will depend on prop and so 0,25 instead of 0,2 look OK for me.

Somewhere on my page http://www.netax.sk/hexoft/stunt/notes.htm are several other props. You can see it depends on prop, but it is every time close to power of three.

Hey, guys...

Note in Larry's (Designman's) post that pitch is not an engineering factor for prop performance or load. Near as I can figure, engineers using the cube/fifth power relationsships assume pitch and RPM will pretty closely match the plane's airspeed...

Also, remember that we run our props from 3 to 10 times the RPM of anything that flies with the people inside. All kinds of funny flows become more important, plus, that old Reynolds' Number is a few galaxies away from "full-scale, people lifter" charts and equations.

And, even the finest CF props from Kiwis or Oz are only approximately precise for the various work we put them to. It's a secret art. We've learned a lot about how to select and tune props, but precise predictions may still be beyond our hopes.