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Old Feb 08, 2013, 04:48 AM
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Myth #1 - 3 bladed props give more thrust than 2 bladed ones

I am going to start this mini series of myths - TV series mythbuster style. We find a myth - Confirm the myth then verify/ bust/ plausible it.

This is an easy one - but anyone who has verified it through practical tests using any means is more than welcome to post their findings. You can even propose an accepted engineering formulae from a scientific journal or wiki (?)

Feel free to add your own myth but for the sake of continuity please number them sequentially.

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Old Feb 08, 2013, 08:09 AM
jrb
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Busted!
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Old Feb 08, 2013, 08:25 AM
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MYTH 2 - The tip of a wing can stall and not the whole wing.
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Old Feb 08, 2013, 08:27 AM
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Quote:
Originally Posted by jrb View Post
Busted!
I'm not so sure. It all depends what other variables you want to hold constant.

For instance if you take an electric set up with a two blade prop and you swap the prop for a three blade (same diameter, pitch and blade shape) thrust will indeed increase (along with watts). So in that respect three blades do give more thrust than two, so ‘myth’ is verified.

If the question was re-phrased to say:
“'Watt for Watt' do three blades make more thrust” the answer would be “No”.. In fact the opposite is true, as your thrust/power graphs show.

Morale of the story is:
Ask woolly ambiguous questions - get woolly ambiguous answers
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Old Feb 08, 2013, 08:29 AM
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I've always heard that multi-bladed props give less thrust than 2-blade props. That has also been my findings as well with some models. I had a 3-blade prop on a .60-sized P-40 and a 4-blade prop on a .60-sized Spitfire. Both looked awesome on the ground and in still photos while in flight but they were definitely slower than their 2-bladed glow-powered counterparts (even though mine were electric and superior in every other way ) I kid... I kid... don't anyone get their knickers in a twist.

Performance went up significantly in both cases after switching to a 2-blade prop.

So is that to say that if WWII fighters such as the F-4U, P-51. etc., had 2-bladed props they would have been even faster? Assuming ground clearance for the prop wasn't an issue of course.
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Old Feb 08, 2013, 08:30 AM
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Quote:
Originally Posted by 1320fastback View Post
MYTH 2 - The tip of a wing can stall and not the whole wing.
Thats perfecty possible and in fact occurs very often on wings with high taper.

http://naca.central.cranfield.ac.uk/...aca-tn-713.pdf
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Old Feb 08, 2013, 08:39 AM
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The only thing that needs to be "busted" are blanket statements. Some 3 bladed props give more thrust than a two blade, others less. Some wings can tip stall, some won't. There are very few absolutes in this world, it's all shades of grey.
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Old Feb 08, 2013, 08:44 AM
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Quote:
Originally Posted by Mike_Then View Post
So is that to say that if WWII fighters such as the F-4U, P-51. etc., had 2-bladed props they would have been even faster? Assuming ground clearance for the prop wasn't an issue of course.
No, because RPM and diameter are limited due to need to prevent the prop tips going supersonic. So the only option once you get to a certain power level is more blades, even though less blades are more efficient in terms of basic momentum theory.
This is not an issue on models due to much smaller props.
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Old Feb 08, 2013, 08:45 AM
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Originally Posted by mark wolf View Post
the only thing that needs to be "busted" are blanket statements.
+1 ...
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Old Feb 08, 2013, 09:02 AM
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Here's one that I think is a real 'Myth' (Myth 3 on the list)

"If your plane climbs too much when you open the throttle, it needs more nose weight"

I've lost track of how often i've seen people advise to do this when it's possibly the very opposite of what's needed.
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Old Feb 08, 2013, 09:03 AM
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Quote:
Originally Posted by JetPlaneFlyer View Post
Here's one that I think is a real 'Myth' (Myth 3 on the list)

"If your plane climbs too much when you open the throttle, it needs more nose weight"

I've lost track of how often i've seen people advise to do this when it's possibly the very opposite of what's needed.
If a plane climbs when power is applied, I would think it's a thrust angle issue, not a balance issue.
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Old Feb 08, 2013, 09:05 AM
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And another (Myth 4):

"The optimum CG position for any plane is 25-30% chord" (insert whatever % range you like, the myth has variations)
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Old Feb 08, 2013, 09:06 AM
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Quote:
Originally Posted by Mike_Then View Post
If a plane climbs when power is applied, I would think it's a thrust angle issue, not a balance issue.
Yes, possibly, but counter intuitivly it could also mean that it's nose heavy (i.e. you need to remove nose weight)
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Old Feb 08, 2013, 09:24 AM
jrb
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Out runners are similar to IC in that you simply can't blame the prop when it's simply swapped!

Will performance be increased going to 2 blades from the stock 3 of the micro Corsair?

Back in the day when we used gear boxes you could actually get equal performance at the same watts from the motor with various props by changing the gear ratio.

Though they key point here is w/too many parameters it's just babble.
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Old Feb 08, 2013, 10:34 AM
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The Myth of Sisyphus is one of my favorites.
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