Myth #1 - 3 bladed props give more thrust than 2 bladed ones - Page 11 - RC Groups
 Feb 11, 2013, 01:25 PM Wake up, feel pulse, be happy! The problem with catching snowflakes is that they always seem to slip through the holes in my butterfly net. Oh, well. I suppose some mysteries aren't meant to be solved by science.
Feb 11, 2013, 01:39 PM
Senile Member
Quote:
 Originally Posted by Ron H It rarely snows here, but I have noticed that when it does, some flakes fall right to the ground while others seem to stay aloft indefinitely. If someone could catch one of each and examine the airfoils we may all learn something.
Well, I'm certainly not going out in that cold stuff to experiment.

Larry
Feb 11, 2013, 03:52 PM
Registered User
Quote:
 Originally Posted by JetPlaneFlyer ALL airfoils make lift in EXACTLY the same way. There is not one theory for 'curved' airfoils and one for flat or symmetrical airfoils. Newton and Bernoulli are not competing theories, one describes the laws of motion the other the relationship between velocity and pressure in fluid flow. Both are true when it comes to describing how a wing makes lift, it's not a case of one or the other. Simply put; wings produce lift by turning airflow (regardless of what the shape of the airfoil is). http://www.grc.nasa.gov/WWW/k-12/airplane/right2.html Steve
With all control surfaces in a neutral position and symmetrical wings what makes the plane go up and not down?
Feb 11, 2013, 03:56 PM
Registered User
Quote:
 Originally Posted by rcmaverick With all control surfaces in a neutral position and symmetrical wings what makes the plane go up and not down?
Angle of attack combined with apparent wind speed.

Dennis
Feb 11, 2013, 04:02 PM
Grumpy old git.. Who me?
Quote:
 Originally Posted by rcmaverick With all control surfaces in a neutral position and symmetrical wings what makes the plane go up and not down?
It can go up, down or fly level. The 'thing' that decides which is the 'longitudinal dihedral' (the difference in incidence between wing and tail) combined with the CG position. Otherwise known as 'trim'.

Basically the force generated by the tail in balance with the moment produced by the CG hold the wing at an AoA to the airflow, which makes the wing lift. This is exactly the same as it works with any other airfoil, with the possible exception of some reflexed airfoils (and I guess swept and twisted wings) that don't need tails.

There is really nothing fundamentally different in how a symmetrical airfoil plane flies compared to any other, except it can fly upside down just as well as upright.
Feb 11, 2013, 04:14 PM
A disaster in the making.
Quote:
 Originally Posted by rcmaverick With all control surfaces in a neutral position and symmetrical wings what makes the plane go up and not down?
That is determined by the mainplanes centre of pressure at that particular trim.
Feb 11, 2013, 04:24 PM
Senile Member
Quote:
 Originally Posted by rcmaverick With all control surfaces in a neutral position and symmetrical wings what makes the plane go up and not down?
I believe you are working under a misconception. It sounds as if you think that because all control surfaces are neutal that the wing's symmetrical airfoil will be at 0 degrees angle of attack. That just ain't so. It is as JetPlaneFlyer said, the wing's angle of attack is determined by the incidence angle between the wing's chord line and the horizontal stab's chord line. Most planes are set up so that the wing's incident angle is from 1 to 2 degrees more positive than the horizontal stab's incidence angle with the elevator at neutral. Also, don't confuse incidence angle with angle of attack. They are not the same thing. The incidence angles of the wing and stab are constant, but the angle of attack the airplane assumes from that decalage is dependent upon the fore and aft position of the CG.

Larry
Feb 11, 2013, 04:30 PM
Senile Member
Quote:
Originally Posted by Damnandblastit
Quote:
 Originally Posted by rcmaverick With all control surfaces in a neutral position and symmetrical wings what makes the plane go up and not down?
That is determined by the mainplanes centre of pressure at that particular trim.
Oh goody, another myth.

Larry
Feb 11, 2013, 04:37 PM
Registered User
Quote:
 Originally Posted by JetPlaneFlyer 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.
can you elaborate on this one ? my power glider diamond 2500 was tail heavy and i had to put four 7G lead weights in the nose. Tail heavy when balancing from CG as per manual. and it was like flying a porpoise on crack
 Feb 11, 2013, 04:37 PM Night Flying Well, no rain after all. But I was able to experiment with a balloon. Believe it or don't, a balloon will fall to the ground every time if you hold it upside down the release it. It even appears to accelerate, so thick symmetrical wings may have an edge when it comes to speed.
Feb 11, 2013, 04:48 PM
A disaster in the making.
Quote:
 Originally Posted by Lnagel Oh goody, another myth. Larry
No myth. You yourself stated that AofA was the reason why planes climbed or descended. Since they are both irrevocably linked you cannot alter one without the other.

Just seeing if you are awake and being a little provocative myself.

Does anyone know how to turn the useless predictive text off on a kindle fire? It is really doing my head in.
Last edited by Damnandblastit; Feb 11, 2013 at 05:02 PM.
Feb 11, 2013, 04:58 PM
Senile Member
Quote:
 Originally Posted by Damnandblastit No myth.
'tis so.

Larry
Feb 11, 2013, 05:04 PM
Senile Member
Quote:
 Originally Posted by Ron H Well, no rain after all. But I was able to experiment with a balloon. Believe it or don't, a balloon will fall to the ground every time if you hold it upside down the release it. It even appears to accelerate, so thick symmetrical wings may have an edge when it comes to speed.
Well, I tried it and my balloon just turned rightside up and went up. I don't know if it has anything to do with it, but my lungs are too weak from years of smoking to inflate the balloon so I inflated it with my pressurized helium bottle. It did, however move in the direction of the more rounded side, so mayber there is something to this.

Larry
Feb 11, 2013, 05:05 PM
Wake up, feel pulse, be happy!
Quote:
 can you elaborate on this one ? my power glider diamond 2500 was tail heavy and i had to put four 7G lead weights in the nose. Tail heavy when balancing from CG as per manual. and it was like flying a porpoise on crack
If the plane is excessively nose-heavy, it'll take more up-elevator trim to fly level at a given "comfy" speed. When you start bumping the speed up past there, the CG position obviously remains the same but the force acting on the stab increases, which drives the tail down, points the nose up, and induces the excessive climbing condition. Moving the CG back will allow you to reduce that trim, which will keep the pitch changes less severe with increasing speed.
Feb 11, 2013, 05:06 PM
Grumpy old git.. Who me?
Quote:
 Originally Posted by Damnandblastit No myth.
If it's only the centre of pressure that determines if the wing lifts or not then take the tail off and see how it flies

FWIW the centre of pressure on a symmetrical airfoil is static at (near enough) 25% chord for all angles of attach below stall. (assuming sub sonic flight)