|May 25, 2001, 09:45 PM|
When does more pitch mean LESS amps?
What am I missing guys?
CAM 8x4.5 folder with MM pulls 27 amps static.
Robbe 8x5 folder pulls only 25 amps!
Motorcalc indicates (as expected) higher pitch = more current draw.
I don't get it. In flight that .5" pitch (or is it the reduced RPM maybe?) is enough to knock my 30 deg. climb to 20.
|May 25, 2001, 10:10 PM|
Believe the watt meter...and in my limited experience, the way the plane flies is the final say on what prop is best. I've run static thrust and watt draw tests to evalutate props and found the best "flying" prop was not the winner in the thrust/watt tests. Too bad we can't measure performance in the air other then subjectively.
|May 25, 2001, 10:15 PM|
Austin, Texas USA
Joined Mar 2001
I've run "smaller" props that pulled more amps and "bigger" props that pull less amps.
Different props by different companies vary widely. Only props within the same "line" can be used in this way and assumed to follow such a pattern. The #X# scheme of labelling is helpful, but not conclusive.
The shape can matter, the surface area, the weight of the prop, etc.
Such as in clothes that vary sizes from one manufacturer to another.
|May 25, 2001, 10:16 PM|
Dallas, Texas, USA
Joined Feb 2000
The above answer is correct. It's kind of like a 12x8 standard APC pulls more amps than a 12x8E APC on the same setup.
However, here is a bit more on the extremes to your question:
That "less draw" could also be due to your test being conducted with the plane held motionless where the prop is loaded up without forward movement. Large pitch props can be stalled out if held still.
OK, hmm... best way I can think of to put this is that the prop blades are at too high an angle of attack to actually produce lift (props are rotating wings by the way) at static loads. It's kind of like when you pull back to hard on the stick at low speeds and the wing stalls out instead of producing lift. The prop is spinning to fast for it's forward velocity for the air to move over it and out of the way at that angle. So the prop basically beats the air around instead of producing a lot of forward thrust.
Think about a prop with a 90 degree pitch (the blades like flat water paddles, not angled like "screws"). It would just flap around, no forward movement (and be pretty noisy).
A stalled prop is not putting as much of a load on the motor at static levels. Now, if that prop is still producing enough thrust to move the plane forward, then the faster the plane moves, the more the prop "loads up" and begins to push more. This means that your plane (once going 10-15, or more MPH) could actually pull more power. It's the reverse of what you would normally see in a prop - less amps (than the static level) ad the plane is flying in the air as the prop unloads. The higher the pitch, the higher the forward velocity the prop wants to travel per revolution. Obviously, there are limits to both end of the spectrum. Hope that helps some, but maybe not...
Read Keith Shaw's article on "flying the prop" here:
|May 26, 2001, 01:26 AM|
Joined Sep 1999
I tried the very same props you mentioned, and found the same result. But I also found that in the sky the Robbe prop gave worse performance than the CAM prop.
The amazing fact was that not only it gave less thrust but also LESS SPEED, regardless of the higher pitch.
I think that the variables are:
- Stiffness (the weak point of Robbe props!)
- Real measures (are you sure that the stated pitch is really 5in ?)
IMO the first is the most important. In fact, centrifugal force tends to reduce the pitch!
That's why (fixed) wooden props are better than nylon ones.
|May 26, 2001, 03:36 PM|
Don't believe why you read on the prop. Some props are wrongly labeled. The Graupner 14x9.5 prop is actually 14 x 6. And the shape of the prop, airfoil and manufacturing process varies greatly. Try an MA8x5 you will see vast difference between that and the, say, Graupner super nylon. Not all props perform the same.
|May 26, 2001, 05:33 PM|
And there is no substitute for real flight. Boeing still has test pilots, even with all the modern design goo-gaws available to the tall forehead crowd. I fly 15 oz all the way to 13 lb planes on E-power and have yet to find a system which will predict performance with any acceptable degree of accuracy. A starting place, yes, but don't hang your hat on the numbers these "programs" give you. Tex.
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