Myth #1 - 3 bladed props give more thrust than 2 bladed ones - Page 5 - RC Groups
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Feb 08, 2013, 07:35 PM
Registered User
rcmaverick's Avatar
Quote:
Originally Posted by C₄H₁₀
Myth #8: Pusher props are needed for pusher planes


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


... I think this'll hold us for awhile
Well I did a search for pusher prop and lipo and this is what I found:

http://ecx.images-amazon.com/images/I/51HDkgRebQL.jpg

And I was looking at it funny..

so I will say it is CONFIRMED

But You can Also call it BUSTED?


Sorry Ladies.. (Its just a joke)

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Feb 08, 2013, 08:00 PM
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scirocco's Avatar
Are we up to #19?

forward cg helps wind penetration, whatever that is.
Feb 08, 2013, 11:19 PM
Closed Account
Quote:
Originally Posted by JetPlaneFlyer
And another (Myth 4):

"The optimum CG position for any plane is 25-30% chord" (insert whatever % range you like, the myth has variations)
It's a decent rule, and has been for many decades for a maiden. I assume you are saying it's incorrect with unconventional aircraft.
Feb 08, 2013, 11:21 PM
A disaster in the making.
Quote:
Originally Posted by JetPlaneFlyer
Yep, they were wrong..

ALL AIRFOILS produce more lift as speed increases, cambered, symmetrical, flat plate.. it makes no difference. Lift increases in proportion to the square of airspeed, for any and all airfoils.
Think about it.... If symmetrical airfoils made the same lift regardless of airspeed then how come they need adequate airspeed to fly?

the only time lift would not increase is if the airfoil was at an angle of attack at which it made no lift in the firstplace, in that case lift is zero for all airspeeds (again this applies to all airfoils)
A symmetrical airfoil produces no lift at zero angle of attack whatever it's airspeed. It is neutral.

By your thinking a highly cambered wing should float off the ground from rest.

Naughty boy, I think you knew exactly what I meant all along and are being deliberately provocative in order for me to have to do battle again with the bloody awful predictive text on me kindle.
Feb 08, 2013, 11:25 PM
Closed Account
Heh, #20 - 100W/Lb is enough for scale flight (my idea of it anyway). Google says: 100 (watts per pound) = 220.462262 m^2 / s^3
Last edited by eagle777; Feb 08, 2013 at 11:59 PM.
Feb 09, 2013, 02:38 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Quote:
Originally Posted by Damnandblastit
A symmetrical airfoil produces no lift at zero angle of attack whatever it's airspeed. It is neutral.

By your thinking a highly cambered wing should float off the ground from rest.

Naughty boy, I think you knew exactly what I meant all along and are being deliberately provocative in order for me to have to do battle again with the bloody awful predictive text on me kindle.
Guilty to being provocative but i'd like to point out:

No.. by my thinking ALL airfoils have lift proportional to airspeed squared, therefore no airfoil will float off the ground at rest because the square of zero is zero. It's actually the thinking that symmetrical airfoil lift is not proportional to airspeed squared that leads you to thinking that a symmetrical airfoil will fly just as well at zero airspeed as it would at 100mph.

Also ALL airfoild have a zero lift angle of attack. For symmetrical airfoils it's zero degrees, for cembered airfoils its a negative value. This means nothing anyway because in flight airfoils all make lift, so the symmetrical airfoil must have some positive AoA while it's flying... Therefore it's lift WILL increase with speed exactly like any other airfoil.

Which leads me to myth #21

Flat bottom airfoils 'balloon' at high speed while symmetrical do not.
Last edited by JetPlaneFlyer; Feb 09, 2013 at 02:44 AM.
Feb 09, 2013, 02:42 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Quote:
Originally Posted by eagle777
It's a decent rule, and has been for many decades for a maiden. I assume you are saying it's incorrect with unconventional aircraft.
Yes, but it also produces a sub-optimal CG for many aircraft that are not unconventional. The eFlite Apprentice for instance has a CG at 36% chord as per manual, and it's very stable, the 'rule' says it should be un-flyable.
Feb 09, 2013, 02:45 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Quote:
Originally Posted by scirocco
Are we up to #19?

forward cg helps wind penetration, whatever that is.
yeah good one.. I've never quite figured the difference between 'wind penetration' and airspeed But either way a forward CG doesnt help.
Feb 09, 2013, 03:03 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
I cant believe that no one has mentioned the best myth of all.

Myth #22 - Downwind turns are dangerous because the plane loses airspeed due to the wind blowing from behind.

This myth may even be supported by (flawed) math.
  1. Wind in from the North at 15mph. Plane is flying East at 25mph.
  2. Plane turns south, wind is now from behind so planes airspeed = 25mph - 15mph = 10mph.
  3. Plane stall and falls out of sky due to loss of airspeed.
Feb 09, 2013, 03:12 AM
A disaster in the making.
Quote:
Originally Posted by JetPlaneFlyer
Guilty to being provocative but i'd like to point out:

No.. by my thinking ALL airfoils have lift proportional to airspeed squared, therefore no airfoil will float off the ground at rest because the square of zero is zero. It's actually the thinking that symmetrical airfoil lift is not proportional to airspeed squared that leads you to thinking that a symmetrical airfoil will fly just as well at zero airspeed as it would at 100mph.

Also ALL airfoild have a zero lift angle of attack. For symmetrical airfoils it's zero degrees, for cembered airfoils its a negative value. This means nothing anyway because in flight airfoils all make lift, so the symmetrical airfoil must have some positive AoA while it's flying... Therefore it's lift WILL increase with speed exactly like any other airfoil.


Which leads me to myth #21

Flat bottom airfoils 'balloon' at high speed while symmetrical do not.
I think we are at cross purposes. Of course all airfoils produce lift as a product of half rho v squared x s.

The symmetrical foil however produces it at top and bottom and they cancel each each other out therefore no overall lift. This is why they are used in all flying tails as they produce up or down force as a result of the angle of attack: only.

Damn this bloody kindle
Feb 09, 2013, 03:31 AM
Registered User
I was just checking in to see if we'd got to the downwind turn yet.

Of course as all sensible people know that isn't really a myth. I once had a crash and I'd just turned downwind so that's proof positive that downwind turns are dangerous. Plus it's simply obvious that if the wind is blowing the wrong way across the airfoil the plane is bound to fall out of the sky.

, just in case.

Steve
Feb 09, 2013, 03:47 AM
A disaster in the making.
Quote:
Originally Posted by slipstick
I was just checking in to see if we'd got to the downwind turn yet.

Of course as all sensible people know that isn't really a myth. I once had a crash and I'd just turned downwind so that's proof positive that downwind turns are dangerous. Plus it's simply obvious that if the wind is blowing the wrong way across the airfoil the plane is bound to fall out of the sky.

, just in case.

Steve

Sorry this is a myth. Your relationship to a steady airstream does not alter, only your relationship to the ground.
Wind shear however is a different thing altogether and you may come a cropper if you are low and slow on a gusty day.
Feb 09, 2013, 03:53 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Quote:
Originally Posted by Damnandblastit
Sorry this is a myth. Your relationship to a steady airstream does not alter, only your relationship to the ground.
Wind shear however is a different thing altogether and you may come a cropper if you are low and slow on a gusty day.
I think I detected a tiny hint of sarcasm in Slipstick's post

PS.. Steve, check post # 69
Feb 09, 2013, 08:45 AM
Registered User
It's amazing how people will reply in deadly seriousness, even to posts which carefully include the "wink, wink, I'm only joking" smiley .

Still I suppose if you're new round here and don't know anyone you can always convince yourself you're the only person in the world who understands anything , blast it .

Steve
Feb 09, 2013, 08:52 AM
AndyKunz's Avatar
Myth 23: The noisier they are, the faster they are!

Myth 24: Jets are the fastest model planes.

Andy


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