Originally Posted by
nciflyer
...Vg is the speed that will give you the best L/D aerodynamically in a given aircraft configuration and does not include environmental factors. L/D is simply the aircrafts (coefficient of lift / coefficient of drag), and is only airfoil related...
Not sure what you mean exactly, unless you are saying that L/D max is independent of environmental factors due to being based solely on angle of attack, and that Vg
does vary with environmental factors such as density, wind, and weight.
...The best glide angle is not necessarily best L/D because now we are speaking in terms of distance over the ground vs altitude...
No! The best glide angle for range purposes is based on L/D max, period. It does not vary with anything else. It is a known value for a particular airplane design, and it does not have anything to do with weight, density, or wind conditions at the time.
... The best glide speed & angle is all inclusive and factors in wind, lift/sink, L/D, & weight...
Ok, except pull the " & angle" out of this sentence lest it be incorrect. The angle is exclusive to any factor except L/D and in the case of maximum range, L/D max. Best glide speed Vg varies (only), and here's the equation:
The overall best glide speed is not necessarily Vg.....
Yes it is. Perhaps you mean the "published" Vg is not always correct, and the true Vg may differ due to circumstances- and I agree with that. The airplane will not go as far through the air without being at L/D max. Ground location is another issue altogether.
...Actually, I should correct myself... L/D can be purely dealing with airfoils, but in the terms we are speaking about it it is really calculated from flight test data. The flight [data] test accounts for the aircraft's overall L/D which also includes lift and drag produced by the fuselage, etc. The test involves doing sawtooth climbs/descents and power off descents...
Yes, drag polars are made from flight test data usually because when the aircraft exists it is easy to obtain them. Preliminary polars are made using hand calcs and CFD. Actual drag polars are very hard to get hold of because they tell you so much about an airplane that manufacturers won't release them if they can avoid it.
As far as the wind speed issue goes, it is the sum of the speeds that counts in the above equation. So if the airplane is going 150 ft/s in still air and a 20 ft/s headwind bumps this figure up to 170 ft/s, then the airplane needs to slow down by that amount to remain at L/D max.