Jun 12, 2013, 10:38 AM
Registered User
Joined Jun 2012
804 Posts
Quote:
 Originally Posted by Brandano Hmm, if I have a cylinder and place a tight fitting piston on its top, I can let the piston drop until the increase in air pressure in the cylinder supports its weight. If then I apply more weight to the cylinder the piston will again drop a little, until pressure balances its weight again. There is a small change in momentum in the volume of air supporting the piston, but that change in momentum alone isn't enough to explain the newly reached equilibrium, especially as the rate of change reaches again zero in the equilibrium state.
Haha yep, I was sitting on that analogy a week or so ago as an example system where air supports weight without net momentum flux (the final equilibrium state has no momentum flux, that little momentum flux you see is a transient state). I also had a fairly similar one for air applying an upwards force while having a net upwards momentum flux.

Was waiting for FunFlys arguments to go down a particular road for those but it didn't happen. Makes me sad.

Quote:
 Originally Posted by funfly2 In that scenario you have the air “trapped” inside the cylinder and when you reach equilibrium you have just the action/reaction such as when you’re pushing on a wall or when your're standing on a floor. Regarding the helicopter however, the air is never “trapped” in the same way as it is within your cylinder. As the helicopter gets close to the ground the air pressure increases just between the heli and the ground, this causes the air density to increase under the blades, which in turn requires less AoA in order to get the same lift force. So, the change momentum is still there despite ground effect.
Viscocity and inertia of surrounding air - the mechanism by which air is contained just like the cylinder, or more reasonably, like a cylinder with a small hole in it, but to a lesser magnitude.

Most of the rest of your post falls apart once that is noted so I don't really need to deal with it.

As for the last bit, not sure why a density increase or less AoA being required shows that the momentum flux is still there. Pretty sure that is just a non sequitur. Explain it better.
 Jun 14, 2013, 07:11 AM Registered User Zurich Joined Apr 2006 3,272 Posts thought re: ground effect I suggest that the ground affects the spanwise flows associated with the tip and related vortices of fixed-wing aircraft. That is, if the ground effectively widens the tip-vortex, for ex., that will have a significant affect on the induced drag, which in turn is a very significant part of the overall drag. This should somehow also apply to helis, and that may also explain the reduced power-req. when in ground effect. I don't have time [or interest: sorry] to explore wind-tunnel studies, etc., but thought I'd just pass on this idea .... Keep at it, guys [your brains must be getting very strong from all of this exercise]. Lee
 Jun 15, 2013, 07:57 AM Registered User Germany, BW, Stuttgart Joined Mar 2012 675 Posts Compressing air in a cylinder with a piston provides a way to explore an idea that has been discussed here. It has been suggested that only the helicopter's rotor blades influence the motion of the "nearby" air. In other words, only air that is outside of a certain distance from the rotor blades will be influenced by the ground. A piston that is being pushed down into a cylinder will exert a downward force on the air immediately below it. When equilibrium is reached, this air will still experience a downward force from the piston, but it will not be accelerating downward. According to Newton's Second Law, the sum of the forces on this region of the air must be zero. For a vertical cylinder, there are only two surfaces that can exert a vertical force on the air in equilibrium. The piston can push down on the air and the "floor" of the cylinder can push up on the air. In order for the air immediately below the piston to be in equilibrium, the floor of the cylinder must influence the air below the piston such that it experiences an upward force that is equal to the downward force exerted by the piston. This remains true no matter how close you get to the piston. No matter how close you get to the surface of the piston, the air is never influenced exclusively by the piston. It always experiences a simultaneous upward force created by the influence of the floor. You could modulate the downward force on the piston so that it moves downward at a constant rate. In this case, the air immediately below the piston would be in motion, but it would not be accelerating. Again, the influence of the floor would create an equal and opposite upward force. So even when the piston is in motion, its influence on the air immediately below it is not exclusive. The idea that there is a region of air around a helicopter's rotor blades influenced exclusively by the rotor blades is a construction that is at odds with the air's observed behavior. There is nothing about the behavior of subsonic air that would lead you to conclude that the ground would not be simultaneously influencing the air near the rotor blades. How does this apply to the bigger discussion about the air's downward momentum? It suggests all of the air is under the simultaneous influence of the helicopter and the ground. This means that you cannot point to a region of the air and say: this air must be experiencing an unbalanced force exactly equal and opposite the aerodynamic force acting on the helicopter, and is therefore experiencing a rate of change of its momentum that is equal and opposite the aerodynamic force. Last edited by ShoeDLG; Jun 15, 2013 at 08:03 AM.
 Jun 15, 2013, 11:52 AM Registered User Joined Oct 2007 5,178 Posts obvious - try this : cut your cylinder vertically in half now fly the helicopter up n down with 1/2 the blade disc nestled in the 1/2 cylinder . is the copter controllable?
Jun 15, 2013, 12:43 PM
Ascended Master
Palmdale, CA
Joined Oct 2000
13,125 Posts
Helicopters.. videos... saving 1000s of words.

 A wider box, to fly out of or not (2 min 9 sec)

 2 Helis over a tall open box (1 min 46 sec)

 eflight mQX in and out of the box! (0 min 59 sec)

 Helicopter over a large plate resting on a scale. (1 min 37 sec)

 Bernoulli vs Newton (1 min 5 sec)
Latest blog entry: Small helis in the street