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Old Feb 08, 2013, 10:35 AM
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Florida
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In order to prove whether 2 bladed or 3 bladed props are best, you would have to make sure the power from the motor was held the same in both cases; i.e. have the same wattage reading for each prop tested. Back in the 40's during WWII, I read a report that the Canadian version of the FAA did test on single blade versus 2 bladed props and proved that the single blade gave more thrust per HP than the 2 bladed did but other problems with balance and side thrust were quite difficult to deal with on the single bladed props.
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Old Feb 08, 2013, 11:31 AM
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Quote:
Originally Posted by JetPlaneFlyer View Post
And another (Myth 4):

"The optimum CG position for any plane is 25-30% chord" (insert whatever % range you like, the myth has variations)
.
"Balance on the spar, I do that with all my planes" is another good one.
Whereat's the "spar" in an all-foam wing?
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Old Feb 08, 2013, 11:34 AM
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Quote:
Originally Posted by Mike_Then View Post
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.
.
Ground clearance for the prop WAS the issue! The correct prop for those very powerful motors in 2-bladed form would be many feet longer than the 4-blades they use. It's why the wing was cranked in the Corsair, to clear the prop and keep the landing gear a reasonable length.
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Old Feb 08, 2013, 11:35 AM
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Originally Posted by 1320fastback View Post
MYTH 2 - The tip of a wing can stall and not the whole wing.
.
The wing section ahead of the down-going aileron can stall. The aileron is at the tip of the wing... 1+1=
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Old Feb 08, 2013, 11:51 AM
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Quote:
Originally Posted by Sparky Paul View Post
.
"Balance on the spar, I do that with all my planes" is another good one.
Whereat's the "spar" in an all-foam wing?
Your foamie wings don't have spars?
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Old Feb 08, 2013, 12:04 PM
Stick, roger ball.
United States, TX, Rockwall
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Quote:
Originally Posted by Sparky Paul View Post
.
Ground clearance for the prop WAS the issue! The correct prop for those very powerful motors in 2-bladed form would be many feet longer than the 4-blades they use. It's why the wing was cranked in the Corsair, to clear the prop and keep the landing gear a reasonable length.
Not exactly, Ground clearance was a problem, but tip speed (as stated in response #8) was the problem. Due to air compressibility (or lack of it at supersonic speeds), bad things happen to props when they go supersonic.

The logic for the corsair went something like this: Here's the motor we have. How big a prop can we put on it? Ok, now how do we get that to fit on our airplane since we're limited on the length of our landing gear?
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Old Feb 08, 2013, 12:06 PM
Stick, roger ball.
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Originally Posted by rafe_b View Post
Your foamie wings don't have spars?
I've built some that don't. Some planes have low enough wing loading and gentle enough flight characteristics that a spar is only extra weight.
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Old Feb 08, 2013, 12:08 PM
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Many don't...
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Old Feb 08, 2013, 12:10 PM
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Moab, Utah, USA
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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...
That would be myth #5.

Larry
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Old Feb 08, 2013, 12:11 PM
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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.
.
ALL planes climb when power is applied. It's what makes them go up.
Some planes pitch up badly with power because there's a problem with the wing incidence-tail incidence, or the c.g..
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Old Feb 08, 2013, 12:16 PM
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Myth # 6 Planes without ailerons can't do rolls.
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Old Feb 08, 2013, 12:17 PM
aka: A.Roger Wilfong
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Novi, Michigan, United States
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Quote:
Originally Posted by Sparky Paul View Post
.
Ground clearance for the prop WAS the issue! The correct prop for those very powerful motors in 2-bladed form would be many feet longer than the 4-blades they use. It's why the wing was cranked in the Corsair, to clear the prop and keep the landing gear a reasonable length.
And even with the bent wings, the Corsair needed a 3 or 4 blade prop to get the full power out of the engine.

For the same diameter, same RPM, a three blade prop will draw more current (and should have more thrust) than a two blade - not 50% more, but more. The three blade will be less efficient than a two blade, but if you need more power and can't afford a larger diameter or make use of a higher pitch, adding a blade will give you more thrust.

- Roger
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Old Feb 08, 2013, 12:21 PM
Grumpy old git.. Who me?
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Quote:
Originally Posted by Sparky Paul View Post
.
"Balance on the spar, I do that with all my planes" is another good one.
Whereat's the "spar" in an all-foam wing?
Yep , yet another one I've heard quire a few times is "balance at the thickest point of the wing"
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Old Feb 08, 2013, 12:30 PM
jrb
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Edina, MN, USA
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Per my chart more blades get's co=loser to calculated pitch speed; also, more baldes give more watts out per watts in -- better efficiency.

That's not to say more of less when just swapping a prop on a given power systems/motor.
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Old Feb 08, 2013, 12:55 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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Myth #7 Modelers always agree with each other as there is really only one correct answer.

(Problem seems to be, everyone has there own correct answer ).
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