Instruction: How to measure energy loss in a tight turn with Tacview and compare it with flight manual

[karasawa]

PS = specific excess power which means how fast the jet gains/loses energy in maneuvers.

 

Q: Why is it insufficient to measure STR (sustained turn rate) alone to verify the energy maneuverability?

A: There is an illusion that STR alone stands for energy maneuverability. If the game adopts a wrong drag profile, and tries to compensate the higher induced drag with modified zero lift drag or engine thrust, we may see an accurate peak STR, but the energy bleed rate at higher turn rate will still be higher than the flight manual. That is determined by equations of flight dynamics.

 That's sometimes misleading. Some people claim the peak STR above mach 0.5 is good, while some other people cry for high energy loss and they struggle to recover energy. They are not contradictory. We need to check the PS loss.

 

Q: How to get the ps since Tacview does not show that?

A: PS = (thrust-drag)*speed/gravity = longitudinal acceleration * speed / g = longitudinal G * speed.

Just check the “longitudinal G” in Tacview, read it, and multiply it by the true air sped (TAS), you get the ps value.

 

Example: (these numbers are for sample only) make a level turn at true air speed of 300 knots (154.3m/s), the lateral G force is 7G, and the longitudinal G force is -1.5G, we have:

Ps = -1.5 * 154.3 = -231.45m/s = -795feet/s

Level turn rate = square root (7^2-1) * g / speed = 6.92 * 9.8 / 154.3 = 0.4395rad = 25.18deg/sec

Read the flight manual for “300knots, 25.18deg/sec” and check if the ps is -795feet/s

(We may double check if the Gs are in body frame, but that won’t affect much the result)

 

Q: Why using true air speed instead of mach number?

A: The speed of sound in flight manual and that in DCS are slightly different. Using mach number causes some error. The speed of sound in flight manual is about 333.5m/s. This can be proven by picking a point in the E-M chart and do the maths.

Edited April 19, 2021 by karasawa

[karasawa]

Just found out that you need to update to Tacview 1.8.3 beta 3 or newer to get the longitudinal G.

[Frederf]

Tacview has a way to view mechanical energy directly or kinetic or potential individually. It's one of the tooltip values. Chart the energy and Ps is the slope of the line (once you work out the units, probably Nm per kg or something).

[Spurts]

The problem is the mechanical energy is likely not specific, so you need to divide by the mass to get specific energy as used in the charts.  AFAIK, tacview does not give mass.  

[Frederf]

Ah, yeah I guess with potential the mass cancels out but with kinetic it doesn't. I wonder what mass tacview assumes for the F-16 to do that calculation. If you could access that then you could set DCS equal (and set infinite fuel) or set tacview equal to DCS.

[karasawa]

On 6/20/2021 at 8:00 PM, Frederf said:

Tacview has a way to view mechanical energy directly or kinetic or potential individually. It's one of the tooltip values. Chart the energy and Ps is the slope of the line (once you work out the units, probably Nm per kg or something).

 

Calculating the slope is much more difficult than the proposed method. In signal processing, calculating the time derivative is always prone to measurement noise.

 

You can do the math and you will see the proposed approach is equivalent to the time derivative of kinematic energy/mass, and is expressed in terms that can be directly measured.

 

specific energy = 0.5 * v^2

 

d/dt(specific energy) = v * a = v * longitudinal acceleration 

On 6/20/2021 at 9:32 PM, Frederf said:

Ah, yeah I guess with potential the mass cancels out but with kinetic it doesn't. I wonder what mass tacview assumes for the F-16 to do that calculation. If you could access that then you could set DCS equal (and set infinite fuel) or set tacview equal to DCS.

The mass does cancel out.

 

specific kinematic energy = kinematic energy / m = 0.5 * m * v^2 / m = 0.5 * v^2

 

time derivative of specific kinematic energy = d/dt (0.5 * v^2) = v * a = v * longitudinal acceleration

[Machalot]

6 hours ago, karasawa said:

 

Calculating the slope is much more difficult than the proposed method. In signal processing, calculating the time derivative is always prone to measurement noise.

 

You can do the math and you will see the proposed approach is equivalent to the time derivative of kinematic energy/mass, and is expressed in terms that can be directly measured.

 

specific energy = 0.5 * v^2

 

d/dt(specific energy) = v * a = v * longitudinal acceleration 

The mass does cancel out.

 

specific kinematic energy = kinematic energy / m = 0.5 * m * v^2 / m = 0.5 * v^2

 

time derivative of specific kinematic energy = d/dt (0.5 * v^2) = v * a = v * longitudinal acceleration

Sorry to be a nit pick, I love the technical depth you put into your posts. But you must mean specific kinetic energy, right? 

Edited June 25, 2021 by Machalot

[karasawa]

4 minutes ago, Machalot said:

Sorry to be a nit pick, I love the technical depth you put into your posts. But you must mean specific kinetic energy, right? 

 

Sure.