[NSRCA-discussion] Weather Vane
James Oddino
joddino at socal.rr.com
Wed Oct 10 19:45:51 AKDT 2007
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
>>
>>
>> _______________________________________________
>> NSRCA-discussion mailing list
>> NSRCA-discussion at lists.nsrca.org
>> http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
>> _______________________________________________
>> NSRCA-discussion mailing list
>> NSRCA-discussion at lists.nsrca.org
>> http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
>
>
>
> _______________________________________________
> NSRCA-discussion mailing list
> NSRCA-discussion at lists.nsrca.org
> http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
> _______________________________________________
> NSRCA-discussion mailing list
> NSRCA-discussion at lists.nsrca.org
> http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.nsrca.org/pipermail/nsrca-discussion/attachments/20071011/662bf111/attachment.html
More information about the NSRCA-discussion
mailing list