## Friday, November 28, 2008

### 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.

Anonymous said...

75ish?

dpierce said...

64 knots? Is the blue bug in your airspeed indicator your approach speed?

Aviatrix said...

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.

E said...

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.

david said...

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).

Ramiel said...

58kts
tried reading TAS correction on ASI assuming you turned the correct temperature at your altitude.

Anonymous said...

Thought the blue line was the minimum safe single engine speed, below which you'd roll in the direction of the dead engine?

Sarah said...

I think the red line at the bottom of the green arc is Vmc.

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.

Anonymous said...

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.

Aviatrix said...

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.

Anonymous said...

Sarah, of course someone cares.
Your point has even been proved experimentally for cyclists!

Darkside

Worldpilot said...

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

Anonymous said...

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.

E said...

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!