### Post by Tom Goodrick on Sept 13, 2008 10:20:34 GMT -5

This is a new topic on flying jets. It is basewd on a new equation I worked up as a minor revision of an equation that has appeared in aeronautical engineering texts for many decades.

The significance of the new performance equation is that it gives thrust/weight ratio required to fly a particular indicated airspeed and is independent of altitude. Previous published versions had always show T/W versus true airspeed althoung this was unnecessary and brings in altitude dependence where there is none! Of course the amount of thrust you can get is a function of speed and altitude. To make it easy to use this equation, I have developed a gauge that shows the thrust/weight ratio on the panel at all times. You simply adjust the throttles until you see the desired thrust/weight ratio. Bear in mind that in a jet the weight varies continuously during flight. In real life the gauge would work with current engine management computers to present instantaneous information.

Here is the equation:

T/W = (CD0 * Vk*Vk) / (194.8 * (W/S)) + (93.84 * (W/S)) / (e * AR * Vk * Vk)

where Cd0 is zero lift drag coefficient from part 1100 of the air file,

Vk is the indicated airspeed in KIAS,

(W/S) is the wing loading or weight over wing area using data from aircraft.cfg file as are e and AR,

e is Oswald's Efficiency factor (0.6 to 0.9)

AR is wing aspect ratio (b*b / S).

I will post a table for 25 aircraft of these parameters.

This equation can be put into a spreadsheet showing a graph of T/W versus KIAS for the range from 100 to 380 KIAS. That graph shows a curve that starts moderately high, decreases to a minimum, and then generally increases toward the high speed end. You have to be careful with the wing loading because that varies considerably as fuel is burned. I present a minimum value based on empty weight and a max value based on max gross weight. You can look at values within this range.

A value of T/W near, or just faster than the minimum, can be held constant as you climb for best results. At high altitude, your choice of speed and T/W is very limited because you must have more than the minimum amount of thrust/weight and yet stay below the max indicated speed (VMO) or the max Mach number (MMO).

I'll send an Excel file and/or the T/W gauge to anyone who requests it by email.

The significance of the new performance equation is that it gives thrust/weight ratio required to fly a particular indicated airspeed and is independent of altitude. Previous published versions had always show T/W versus true airspeed althoung this was unnecessary and brings in altitude dependence where there is none! Of course the amount of thrust you can get is a function of speed and altitude. To make it easy to use this equation, I have developed a gauge that shows the thrust/weight ratio on the panel at all times. You simply adjust the throttles until you see the desired thrust/weight ratio. Bear in mind that in a jet the weight varies continuously during flight. In real life the gauge would work with current engine management computers to present instantaneous information.

Here is the equation:

T/W = (CD0 * Vk*Vk) / (194.8 * (W/S)) + (93.84 * (W/S)) / (e * AR * Vk * Vk)

where Cd0 is zero lift drag coefficient from part 1100 of the air file,

Vk is the indicated airspeed in KIAS,

(W/S) is the wing loading or weight over wing area using data from aircraft.cfg file as are e and AR,

e is Oswald's Efficiency factor (0.6 to 0.9)

AR is wing aspect ratio (b*b / S).

I will post a table for 25 aircraft of these parameters.

This equation can be put into a spreadsheet showing a graph of T/W versus KIAS for the range from 100 to 380 KIAS. That graph shows a curve that starts moderately high, decreases to a minimum, and then generally increases toward the high speed end. You have to be careful with the wing loading because that varies considerably as fuel is burned. I present a minimum value based on empty weight and a max value based on max gross weight. You can look at values within this range.

A value of T/W near, or just faster than the minimum, can be held constant as you climb for best results. At high altitude, your choice of speed and T/W is very limited because you must have more than the minimum amount of thrust/weight and yet stay below the max indicated speed (VMO) or the max Mach number (MMO).

I'll send an Excel file and/or the T/W gauge to anyone who requests it by email.