Especially for Wizofoz
If you want to make an estimate of wind effect on the choice of optimum Vx the first thing to do is to see what happens if you use the Vx from the manual.
The change in climb angle will then be proportional with the wind-component as a percentage of the TAS corresponding with Vx (IAS, EAS or CAS converted to TAS).
Then comes the question what will happen if I use another Vx. Changing Vx will change your drag and hence your excess thrust. It will also change your body attitude and hence the vertical component of your thrust and will also change your trust, this may be significant in the lower sped range.
If you decide to use another Vx speed then the one from the manual, you will have to accelerate or decelerate, this also will have an effect on your angle. So what might look, as an advantage in the long run may be a disadvantage on the short term.
In the end what counts is the true relation between TAS, Drag, Lift, both components of the Thrust. Just to calculate a correct TAS while taking into account all the possible errors is already a big problem and you have to do it since you need it to integrate the wind factor. The graph of Cpn Bloggs clearly shows the effect of increased TAS with altitude for a given CAS. It also reveals that the effect on optimum Vx is only significant at very low speeds.
Now Mr Wizofoz: AOA does affect IAS be it only to integrate the effects of position error and Mach and yes on a heavy jet when clean the Vx goes up to 280 kts. Some Airdata computers have software to deal with this and some do not.
So as a general rule one could say that increasing or decreasing your speed away from Vx by a value that results in minor changes in body attitude, will not significantly affect you angle once the new speed has been attained.
There are simply too many variables in the game to draw any conclusion. Those nerds that want to find out simply will have to try it out in practice, and their conclusion will only be valid for those condition.
On a high bypass engine you loose about 30 % of your thrust simply by accelerating down the runway. And to add to this I would like to mention that in our airline we even took into account the change of TAS with constant IAS while calculating the climb angle to cope with an N-1 take off.
So Wizofoz, before you go on rambling on another forum about what I wrote on this one you may want to think twice.
Last edited by Pitch Up Authority; 9th Mar 2011 at 14:35.