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Aviation-TAS(true airspeed) question

Posted on 2013-06-19
Medium Priority
Last Modified: 2013-06-20

I hope that someone can help with the following question. I read the following sentence in an aviation book but need a further explanation why this is:

"In comparison to the true airspeed in still air conditions, the TAS for the best range in a strong tailwind will be slightly lower"

My initial thought was if the wind is behind you would your TAS not be greater? Your groundspeed would be higher as you would have the TAS + windspeed.


Question by:ross13
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LVL 84

Assisted Solution

by:Dave Baldwin
Dave Baldwin earned 336 total points
ID: 39258656
The statement says 'best range'.  Maybe that's the key.
LVL 37

Assisted Solution

TommySzalapski earned 664 total points
ID: 39259349
"true air speed" is the speed of the craft relative to the air around it. So if you have a tailwind, the wind is moving with you relative to the ground, so your speed relative to the air is going to be slower.

Yes, your ground speed would be TAS + wind speed. So your ground speed would be higher than in still air.
LVL 37

Assisted Solution

TommySzalapski earned 664 total points
ID: 39259395
Also, you are reading an aviation book not a physics textbook so the tailwind most likely isn't pushing you perfectly straight. It would generally be at some angle, so you wouldn't get the full thrust of the wind in the forward direction.

Technically, velocity relative to the ground will be a vector sum of the true air velocity and the wind velocity. Only if the tailwind is 0 degrees relative to the airplane's flight path will a simple TAS + wind speed really work.
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LVL 27

Assisted Solution

aburr earned 332 total points
ID: 39260271
As baldwin says
the TAS for the best range in a strong tailwind will be slightly lower"
is the key.
"My initial thought was if the wind is behind you would your TAS not be greater?"

Not TAS but ground speed
LVL 27

Assisted Solution

d-glitch earned 332 total points
ID: 39260965
The design of an airplane will be optimized for a particular set of conditions:  load, range, air speed, rate of climb, altitude, ...

There will certainly be a speed that optimizes range (miles per tank of fuel).
If you idle on the runway, you can use up all your fuel in several hours and go nowhere.
Or you can run at full throttle and waste fuel due to nonlinear air resistance.

But if you don't know the function of range to TAS for still air in some detail, I don't see how you can make a such a general statement.

They are making claims about the derivative of a function without knowing anything about the function.  I actually believe they are mistaken.  I would expect fuel consumption and time aloft (and hence range) to depend only on TAS.
LVL 32

Accepted Solution

aleghart earned 336 total points
ID: 39261474
They're not discussing the effect of the wind on your IAS or TAS.  It's how you fly to best effect.

In a headwind, you fly faster to reduce the amount of time the headwind negatively affects your range.

In a tailwind, you fly slower to increase the amount of time the tailwind positively affects your range.

There are rules of thumb saying 1/2 headwind speed added to your V(br) in your POH, adjusted for weight.  And 1/4 tailwind speed subtracted from your V(br).

Somebody did calculation here, showing that rule was close for certain speed ranges.

Author Closing Comment

ID: 39261715
Thanks for the help.

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