Tailwind
I took some pictures on the way south, although as usual the instrument shots didn't work out well. I'll leave it as an exercise for you to calculate how much tailwind I had.
It's the sort of thing you can do in flight to keep your mind awake.
posted by Aviatrix at 03:55
14 Comments:
75ish?
Fri Nov 28, 04:07:00 AM UTC
64 knots? Is the blue bug in your airspeed indicator your approach speed?
Fri Nov 28, 05:10:00 AM UTC
The blue line is not a bug, but rather a line painted on the airspeed indicator showing the single engine best rate of climb speed. I fly an approach at 110 to 120 kts.
Fri Nov 28, 05:43:00 AM UTC
23ish... but thats just using a rule of thumb from low altitude flying... for every 1000 feet above sea level your TAS is about 5 knots faster than IAS.
Fri Nov 28, 02:06:00 PM UTC
Assuming you're at 9,000 ft indicated with a standard atmosphere, and that CAS is the same as IAS:
IAS: 149 kt
TAS: 171 kt
GS: 223 kt
So let's say a 52 kt tailwind component (or "about 50 kt", given the approximations in my calculations).
Fri Nov 28, 02:42:00 PM UTC
58kts
tried reading TAS correction on ASI assuming you turned the correct temperature at your altitude.
Fri Nov 28, 04:39:00 PM UTC
Thought the blue line was the minimum safe single engine speed, below which you'd roll in the direction of the dead engine?
Fri Nov 28, 05:42:00 PM UTC
I think the red line at the bottom of the green arc is Vmc.
Tailwinds are never as good as the headwinds are bad.
As nerdy mathematical aside, I used to think you'd make up time going downwind on a roundtrip you lost going upwind. Turns out not to be the case, especially if your airspeed is close to windspeed, too often the case for me.
If V = airspeed, W = windspeed component on course, Td = Time downwind, Tu = Time upwind, and k = V/W, the fraction wind is of airspeed...
Td/Tu = (1-1/k) / (1+1/k)
k=1 (V=W) = 0 you never get upwind
k=2 Td/Tu = 0.333
k=3, Td/Tu = 0.5, about the case here.
Fri Nov 28, 06:28:00 PM UTC
More to the point about elapsed round-trip time above, still assuming k=W/V and a round-trip with wind W and airspeed V...
total time = 2D/V * 1 / (1+1/k)*(1-1/k)
so again if k=1 you never get there,
k=2 1.33 times calm-wind time
k=3 1.25
k=4 1.07
... I need to get out more.
I know, no one cares, but I thought it was interesting.
Fri Nov 28, 08:41:00 PM UTC
Ha ha ha, keep 'em coming, even you Sarah.
I haven't written the follow up post to this yet and it will probably be a week before I have the time, but I'll come back to it.
Yes, red is Vmc, blue is Vyse, and no, the ASI calculator is not set.
Fri Nov 28, 09:02:00 PM UTC
Sarah, of course someone cares.
Your point has even been proved experimentally for cyclists!
Darkside
Fri Nov 28, 10:52:00 PM UTC
Nice task. Really enjoy reading your blog!!!
Using a quick, probably not to exact "in-head" formula I came up with 50kts, assuming ISA conditions.
110-120kts seems fast, are those your final approach speeds?
Keep on blogging,
FE
Sat Nov 29, 03:10:00 PM UTC
Reading on a dark & snowy day, about a/c instruments. Maybe you all know this, but I just found out.
The Laser Ring 'Gyro' ( w/no moving parts) works on the same principle as the upwind/downwind round-trip elapsed time blather I went on about. By measuring elapsed time "stretch" in laser beams bouncing opposite directions in a ring - or triangle, usually, you can detect rotation about that ring axis. With two .. voila
Cool.
Sun Nov 30, 10:30:00 PM UTC
Ok, I think my earlier ROT must have been based on a specific a/s... Anyway, crossing the North Atlantic earlier this week I saw 150 kt tailwind... made for a bumpy ride but at least it was a quick crossing!
Thu Dec 04, 08:20:00 PM UTC
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