Cape Town, South Africa - WINDS

[MongooseMike]

I recently flew from Cape Town Intl. to Port Elizabeth east along the coast at 12K feet with the Airbus. I had a constant tailwind of between 45 and 50 knts. Enough to put me in overspeed until I backed the throttle down a bit. I'm not a weather expert. Are those winds normal for that area?

[alis14]

Not to be difficult but a tailwind would result in a lower Indicated Air Speed (i.e. more throttle would be required)
A nose wind would result in a higher IAS and so an overspeed.
Certainly in the UK along the coastlines due to the differences in temp of land/sea you can get some pretty strong blows but not sure about specific areas, sorry.

[MongooseMike]

>Not to be difficult but a tailwind would result in a lower
>Indicated Air Speed (i.e. more throttle would be required)
>A nose wind would result in a higher IAS and so an overspeed.
>Certainly in the UK along the coastlines due to the
>differences in temp of land/sea you can get some pretty strong
>blows but not sure about specific areas, sorry.

I'm not a pilot but....
My indicated airspeed was well within range, I think around 340 knts, as well as my TAS but I was getting the overspeed warning until I dropped down around 330. Wouldn't a tailwind effectively "push" your aircraft thus creating the overspeed?

I don't know, but I'm certainly going back for another roller coaster ride. LOL

[wdscobie]

FS deals with wind changes incorrectly.

in general (and simplified) in the real world a change in the wind speed will not result in the same immediate change in aircraft IAS.

example:

aircraft is in calm air going 300kts IAS. head wind of 10 kts comes up causing FS to make the aircraft immediately go 310kts. in reverse is true if a tailwind occurs (aircraft going 290kts).

real world the aircraft stay, most, a constant speed as it is going through the air mass regardless of the air mass speed (this is the idea of relative motion).

so ... FS don't do it right.


--

[MongooseMike]

Sorta like the balloonist drifting in relative calm...at 100knts

[lnuss]

A wind, no matter head, tail or cross, will not affect the airspeed of an aircraft, but it WILL affect the ground speed, which you might read on your GPS. But the overspeed warning is based on airspeed (or on mach number, at higher altitudes), so a 5 knot or a 100 knot tailwind won't have any effect on that.

That being said, a large CHANGE in wind speed will momentarily affect an aircraft's airspeed (due to inertia) but, assuming you don't change anything, the airspeed will shortly return to its previous value.


[HR]
http://home.comcast.net/~lfn3/Cub_Pix/LarryCub03_s.jpg http://home.comcast.net/~lfn3/Peg_Ab...590Mklfn_s.jpg
Larry N.

[RFields]

Flight Simulator crash thresholds are about

60 seconds of 1% overspeed

or
3 seconds of 10% overspeed

or
0.1 second of 15% overspeed

Despite how realistically or unrealistically FS might portray changes in wind speed and direction.

[lnuss]

I'm not a pilot but....
My indicated airspeed was well within range, I think around 340 knts, as well as my TAS but I was getting the overspeed warning until I dropped down around 330. Wouldn't a tailwind effectively "push" your aircraft thus creating the overspeed?

No (see my other post here). Dropped down? Apparently you descended (not mentioned in your previous post). A lower altitude would likely change the speed of sound (it's mostly temperature related -- temps tend to decrease with increasing altitude), or mach 1, thus you were likely at a lower mach number after descending and leveling off, if you maintained the same or less TRUE airspeed. And your TRUE airspeed would be reduced (thus your mach number, also) during your descent if you maintained your same indicated airspeed.

Thus it was your descent that got rid of the overspeed warning. You could have also eliminated it by reducing power to slow down, while maintaining the same altitude.

I hope this post didn't come out too combobulated, but I've been interrupted several times by the needs of my new granddaughter, and occasionally also by my two year old grandson.

Oops -- here we go again...

[HR]
http://home.comcast.net/~lfn3/Cub_Pix/LarryCub03_s.jpg http://home.comcast.net/~lfn3/Peg_Ab...590Mklfn_s.jpg
Larry N.

[MongooseMike]

To all of the above threads;
I think what I'm dealing with here is an FSX problem (as mentioned) not a aerodynamic problem. I was at constant speed and level and changed neither one until the overspeed warning. At which time I reduced speed but not altitude.
As I understand the real world implications of the flight, everything would be relative. IAS is your speed relative to the air around you and how fast your pushing through it?? Ground speed is your relation to the earth? Thus a tailwind would actually reduce your indicated speed but increase your ground speed. The reason I think it is a FSX problem is because my IAS and TAS did not change! I have been on flights where my TAS was well over the aircraft limit but had no overspeed warning.
Congratulations on the new grandaughter Larry. I can hear my week old grandson right now. What a blessing grandchildren are!! :P

[lnuss]

A tailwind, a headwind or a crosswind will not affect either indicated OR true airspeed -- by definition they are strictly in relation to the air (a moving mass of which is perceived by those on the ground as wind). Only groundspeed is affected by the wind.

The key to understanding this is that at zero airspeed (if it were possible in an airplane), the aircraft would be moving exactly the same speed and direction as the air, when compared to the ground. In relation to the air, though, it is motionless. Visualize a balloon. It has no airspeed -- zero, but moves with the mass of the air. So, once airborne, the only speed an aircraft "knows" about is its speed in relation to the air -- it doesn't "know" that the ground even exists.

So picture a balloon and a Piper Cub both flying in the same air mass at the same altitude. Lets assume that the Cub is on a 90º heading at 70 miles per hour IAS (Indicated Air Speed). Let's further stipulate that they are both at an altitude of 2,000 feet MSL, with a temperature of the air around them at 59º F. (15º C.) with 0% humidity (to keep it simple), and that the current altimeter setting is 29.92" Hg. In still air, the balloon would stand still in relation to the ground, while the Cub would move in the 90º direction at 72 mph over the ground and at 72 mph through the air, while indicating 70 mph. With no wind (the air mass isn't moving in relation to the ground), true airspeed (TAS) and ground speed will be the same.

Now let's get that air mass moving. Let's say that a ground observer saw a 30 mph wind from the west -- a direct tailwind for the Cub. Now, the balloon has 0 mph airspeed, and 30 mph ground speed, and is moving directly east. The Cub is flying at an IAS of 70 mph, a TAS of 72 mph, and it has a ground speed of 102 mph, moving still at 090º.

Now let's move that air mass in the opposite direction, also at 30 mph, from the east. Now the balloon is moving west at 30 mph, and the Cub is is still moving 090º with a 70 mph IAS and a 72 mph TAS, but the ground speed is now 42 mph.

If we now made that air mass come out of the north, the balloon would be moving south at 30 mph over the ground (still 0 mph air speed, both IAS and TAS). The Cub would be moving at an IAS of 70 mph and a TAS of 72 mph but would need to fly a heading of 066º to maintain the 090º course over the ground, and would have a ground speed of 66 mph (just coincidence -- I didn't plan it that way).

I hope this helps.

[HR]
http://home.comcast.net/~lfn3/Cub_Pix/LarryCub03_s.jpg http://home.comcast.net/~lfn3/Peg_Ab...590Mklfn_s.jpg
Larry N.