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Old Feb 08, 2013, 02:00 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
12,009 Posts
Quote:
Originally Posted by jrb View Post
....also, more baldes give more watts out per watts in -- better efficiency.
Nope, it's the opposite. The general rule is more blades reduces propulsive efficiency. this is because you want to have the largest 'disk' size as possible and fewer blades means you can have larger diameter while absorbing the same input power.

quote Prof Martin Hepperle:
Quote:
For a given power P, it is always desirable to use the largest possible propeller diameter D, which may be limited by mechanical restrictions (landing gear height) or aerodynamic constraints (tip Mach number). That's why most man or solar powered airplanes use large, slowly turning props. These catch a large volume of air and accelerate it only slightly to achieve the maximum efficiency.
http://www.mh-aerotools.de/airfoils/

Another good read: http://www.nar-associates.com/techni...ncy_screen.pdf
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Last edited by JetPlaneFlyer; Feb 08, 2013 at 02:06 PM.
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Old Feb 08, 2013, 02:20 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,639 Posts
The prop thing has been raised so many times, (plenty of thread if searched for), often the conclusion is the counter-balanced single blade prop is the most efficient.
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Old Feb 08, 2013, 02:23 PM
I hate waiting for parts
Mike_Then's Avatar
United States, NC, Garner
Joined Apr 2001
6,966 Posts
Quote:
Originally Posted by Lnagel View Post
That would be myth #5.

Larry
Quote:
Originally Posted by Sparky Paul View Post
.
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..
I understand that some high-lift airplanes will climb when power is applied but most airplanes are neutral in that they go straight and level whether they are at 50% power or 100% power. My experience has been that a neutral airplane which climbs severely under full power has a thrust angle issue. Just like a plane that dips severely when the throttle is released has too much up thrust (needs too much trim to maintain level flight under power).
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Old Feb 08, 2013, 02:33 PM
jrb
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Edina, MN, USA
Joined Oct 1999
11,433 Posts
Actually Prof Martin Hepperle are in I agreement; have been since he editted his original document about multblade props!

Also his point about diameter is exactly why 1/2 CL speed uses a single blade prop!

They must ROG and need all the prop disk diameter possible.

If they could bungee or catapult launch they'd go back to two!
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Old Feb 08, 2013, 02:51 PM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,499 Posts
Quote:
Originally Posted by Mike_Then View Post
I understand that some high-lift airplanes will climb when power is applied but most airplanes are neutral in that they go straight and level whether they are at 50% power or 100% power. My experience has been that a neutral airplane which climbs severely under full power has a thrust angle issue. Just like a plane that dips severely when the throttle is released has too much up thrust (needs too much trim to maintain level flight under power).
.
At a constant airspeed where the airplane is not descending or climbing, the trim is set to maintain that airspeed.
Adding or reducing power makes the trim setting incorrect, and the airplane will climb or descend. The pilot will adjust the trim to suit the condition.
Disturb the airplane from trimmed flight with an elevator pulse, and the airplane will oscillate but return to the initial trimmed condition.
Thrust offsets are usually only effective at low speeds.
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Old Feb 08, 2013, 03:04 PM
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Ron H's Avatar
Bishopville S.C.
Joined May 2003
4,057 Posts
Quote:
Originally Posted by eflightray View Post
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 ).
I disagree, I think you are right.
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Old Feb 08, 2013, 03:08 PM
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Joined Jan 2009
7,643 Posts
[re: foamies having or not having spars...]

Quote:
Originally Posted by Stick_01 View Post
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.
I just checked -- my micro 4-Site doesn't have spars. As far as I know, every other foamie in my fleet has 'em, including a 24" Depron fella with about 6 oz. AUW.
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Old Feb 08, 2013, 03:26 PM
Wake up, feel pulse, be happy!
C₄H₁₀'s Avatar
United States, AK, Fairbanks
Joined Aug 2009
12,441 Posts
Myth #8: Pusher props are needed for pusher planes

Myth #8.1: The numbers on a prop are always on the front face

Myth #9: A brownout crashed your plane

Myth #10: Four-channel planes are always harder to fly

Myth #11: Models can't exceed calculated pitchspeed

Myth #12: Leaving lipos fully charged won't hurt them

Myth #13: 13 is an unlucky number

Myth #14: Balsa is better than foam

Myth #15: You can use throttle endpoints to limit ESC current and heat

Myth #16: A lipo will explode if you look at it funny

Myth #17: Foam is better than balsa

Myth #18: You can fly that jet since you play a lot of videogames

Myth #19: You can acquire leptospirosis from a Coke can

Myth #20: A brushless motor will burn out if you exceed its voltage rating

Myth #21: It can't happen to me

Myth #22: A brushless motor will burn out if you exceed its watt rating

Myth #23: Adding weight to your electric plane will make it go faster

Myth #24: Trims are a set-n-forget affair

Myth #25: I don't need mechanical setup. My radio does all that.

... I think this'll hold us for awhile
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Old Feb 08, 2013, 03:49 PM
CAP smasher
compressor man's Avatar
GA,USA
Joined Jul 2010
215 Posts
Quote:
Originally Posted by C₄H₁₀ View Post

Myth #18: You can fly that jet since you play a lot of videogames

HaHaHaaa
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Old Feb 08, 2013, 03:55 PM
ARFs make me BARF
mrittinger's Avatar
United States, MI, Roseville
Joined Dec 2000
9,076 Posts
Efficiency and thrust are not the same thing

Many full scale birds added blades to absorb the ever-increasing HP while maintaing ground clearances.....doesn't mean they were more efficient, they just absorbed more power while allowing for practical use on an aircraft...you can only fit so large a prop before hitting the ground

Google the Thunderscreech for supersonic prop tips.....

REPUBLIC XF-84H "Thunderscreech" (sound recording) (0 min 31 sec)
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Old Feb 08, 2013, 03:57 PM
Registered User
scirocco's Avatar
Australia, ACT, Kambah
Joined Feb 2001
3,227 Posts
Quote:
Originally Posted by JetPlaneFlyer View Post
Nope, it's the opposite. The general rule is more blades reduces propulsive efficiency. this is because you want to have the largest 'disk' size as possible and fewer blades means you can have larger diameter while absorbing the same input power.

quote Prof Martin Hepperle:

http://www.mh-aerotools.de/airfoils/

Another good read: http://www.nar-associates.com/techni...ncy_screen.pdf
Funny the things that stick in memory - my aerody prof put it this way: for perfect propulsive efficiency, accelerate an infinite amount of air an infinitesimal amount. Very much in the context of bigger disk area and less acceleration.
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Old Feb 08, 2013, 04:05 PM
Registered User
Liverpool, UK
Joined Aug 2004
129 Posts
"Myth #19: You can acquire leptospirosis from a Coke can"

I'd never heard that one before but I guess it's safe calling it a myth unless it's a coke can full of rat pee?

Kev
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Old Feb 08, 2013, 04:11 PM
Y=C+I+G
cmdl's Avatar
United States, CA, Rosemead
Joined Jan 2012
8,822 Posts
Quote:
Originally Posted by C₄H₁₀ View Post
Myth #23: Adding weight to your electric plane will make it go faster
does 4s make you faster vs. 3s? allowing for prop changes of course.
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Old Feb 08, 2013, 04:25 PM
Fueled by Arabica Beans
ChillPhatCat's Avatar
United States, NY, Syracuse
Joined Oct 2008
4,038 Posts
Quote:
Originally Posted by C₄H₁₀ View Post

Myth #11: Models can't exceed calculated pitchspeed
OK you got me interested... how's this work?

Obviously since a prop unloads in the air you can get more than the pitch speed calculated from static RPM on the ground...
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Old Feb 08, 2013, 04:35 PM
Wake up, feel pulse, be happy!
C₄H₁₀'s Avatar
United States, AK, Fairbanks
Joined Aug 2009
12,441 Posts
Quote:
does 4s make you faster vs. 3s? allowing for prop changes of course.
If speed is the only goal and the battery and prop are the only changes then yes, switching to 4S from 3S with appropriate prop changes will normally make a plane faster... But you'll always be sacrificing something, and I think the negatives will tend to outweigh the positives.
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