[NSRCA-discussion] Weather Vane
Nat Penton
natpenton at centurytel.net
Wed Oct 10 21:14:49 AKDT 2007
Jim
I'm waiting for the real aero guy to jump in, LOL
I think in some cases here we have nomenclature problems and in some cases misconceptions. If the airplane did not have directional stability we could not fly precision aerobatics. In all honesty I have not read a lot of the comments, too busy repairing crash damage due to a faulty receiver pack connector. Nat
----- Original Message -----
From: James Oddino
To: NSRCA Mailing List
Sent: Wednesday, October 10, 2007 10:45 PM
Subject: Re: [NSRCA-discussion] Weather Vane
My thinking is expressed in black below:
On Oct 10, 2007, at 5:51 PM, Ron Lockhart wrote:
I'm not Nat, but will throw a response anyway-
Assumption - the arrow is released with zero ground speed, eg from balcony of a TALL building.
There is zero wind.
As it is released it will weathervane into vertical down attitude flight. and within not too long (scientific term? <G>)
it will be falling perpendicular to the earth.
The reason it weathervaned into nose down attitude is because the cg is forward of the center of pressure. The cg tries to fall straight down but the air pressure on the tail feathers pushes the tail up.
Now, change the wind from zero to some amount, say 10 knots.
As arrow is released it will weathervane into vertical down flight, and within not too long (scientific term? <G>)
it will be falling with perpendicular attitude to the earth. (Arrow attitude will be perpendicular to earth, it's track will not be perpendicular to the earth) And, it will be drifting in downwind direction, at 10 knots.
Before it picks up much speed it will weathervane in yaw as well as pitch for the same reason. The cg wants to stay in the same vertical plane but the force of the crosswind on the tail creates a yaw moment. As it picks up speed, the force due to the forward speed on the tail feathers will tend to straighten it out until the force is equal on both sides of the tail.
In the case of dropping an arrow from zero airspeed, there is a temporary situation while arrow weathervanes to straight down attitude. It is similiar to a situation where an airplane in flight encounters a very sharp cross wind, windshear. Also a temporary situation. The airplane might weathervane into the wind for a short time, but will pretty quickly be flying on a heading only a few degree into the wind, and also drifting in downwind direction at the speed of the wind.
I agree with the last statement. If I'm landing from right to left with a strong wind at my back the plane will weathervane to its left and drift out. The amount is a function of relative speeds/vectors. If you come in too slow it can get pretty bad.
Ron Lockhart
Wish we had a real aero guy jump in here. Maybe you are one and I should shut up and listen? I've been reading Einstein and he questioned everything and everyone, so I think these discussions are good no matter who has the right slant on it.
Best Regards, Jim
----- Original Message -----
From: James Oddino
To: NSRCA Mailing List
Sent: Wednesday, October 10, 2007 7:20 PM
Subject: Re: [NSRCA-discussion] Weather Vane
Nat,
If you hold an arrow (or a bomb) horizontal and drop it, what happens to it's attitude on the way down and why? Then drop it with a crosswind of 32 ft/sec. What happens to its attitude?
Seems pretty obvious to me.
Jim
On Oct 10, 2007, at 2:21 PM, Nat Penton wrote:
Ron
You are correct - an airplane does not weathervane or weathercock - it fliies straight into the freestream unless given rudder. BUT, as Kennie says, he hopes to be here next year to reargue the point !! Nat
----- Original Message -----
From: ronlock at comcast.net
To: NSRCA Mailing List ; NSRCA Mailing List
Sent: Wednesday, October 10, 2007 11:07 AM
Subject: Re: [NSRCA-discussion] Weather Vane
I thought I'd throw in my 2 cents worth, but maybe it turned into too much. Ya been warned <G>.
There is a strong, almost overwhelming, visual perception that airplanes weathervane into the wind as seen by an observer on the ground.
If the airplane is partly connected to the ground (as in takeoff or landing roll) it will likely weathervane into the wind. (Like a real weathervane on the barn). Putting more side area behind the CG probably increases the weathervane effect.
But once airborne, an airplane no longer feels wind on side of the airplane and does not weathervane in the sense that the weathervane on the barn does. Nor does the hot air balloon mentioned in an earlier post.
Just after take off, we likely see the nose is pointed some amount into the wind. This visual perception is true. But is it proof of weathervaning? It probably weathervaned into the wind during take off roll. Once airborne the weathervane is now what we call a crab angle into the wind.
We all know stall turns are easier to do "into the wind". Is it because they weathervane? On the upline to a stall turn in a cross wind, does the pilot wind correct the airplanes track? Most of us do, consciously or not. If we are having any success at all with the wind correction, the fuselage is "leaning" into the wind a bit It's certainly easier to get a stall turn by continuing into the established lean, than it is to go against the lean to the downwind direction.
Pilots try to make heading adjustments to hold track, or hold distance in lines and maneuvers. Those adjustments (crab angles) can appear to be a result of weathervaning, but are often pilot inputs, consciously or not.
Related things to consider-
Airplanes do get "hit" in the side, top and bottom when in turbulent wind, and wind shear situations. We fly low, and are often in turbulence. There may be short, nearly random, turbulent air effects that result in a weather vane type effect.
A free flight glider does not weather vane into the wind. The glider may fly in circles, but it will drift downwind at the overall average of the wind speed. No matter how much side area is put behind the CG, it will not find and maintain a heading into the wind.
Given flight in no wind situation, a string tied on nose of an airplane blows exactly backwards and parallel to the fuselage. (unless the airplane is out of rig, or has control surface inputs). If flown in a cross wind direction, the airplanes track across the ground changes, but the string stays straight. (not counting any effect of propeller induced spiral airflow)
The old full scale stories about loosing airspeed and stalling while doing a "downwind turn" fit into this discussion to a degree. Does wind hit the tail of an airplane as it turns downwind thereby reducing airspeed? In the "old" days, pilots often flew low and partly judged airplane speed by visual observation of ground speed. (similiar to our situation of observing from the ground) This could lead to the perception of plenty of ground speed being plenty
Sorting out the visual perception of the "obvious" weather vane effect is tough. Lots of things complicate the observation - pilot inputs, turbulence, paralax, and more.
IMHO, bottom line, it's not wind hitting side of airplane, no matter how much side area is behind the CG.
Later, Ron
In summary, I agree that as observers on the ground, we "see" effects that appear to be weathervaning. But the "real" cause
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