Takeoff behaviour and performance

[Fox One]

I was comparing takeoff in simulator with the well known video:

 

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In the video, the longitudinal G during takeoff is 0.66-0.69 and when the speed is 138, the inverted T symbol is overimposed on the -5deg pitch line.

 

Trying to match this in simulator, with 100% fuel - longitudinal G during takeoff was 0.80-0.85 and when the speed was 60-70 I pulled the stick fully aft and kept it there until the aircraft was airborne, even though that's an unrealistic technique and an over rotate was guaranteed. Just trying to rotate the aircraft as soon as possible with max pitch rate. By the time the inverted T was overimposed on the -5deg pitch line the speed was about 150.

 

I think in simulator the aircraft starts to rotate at a speed that's too high and when it finally rotates the pitch rate available is too low, especially considering that during all the rotation I was with full aft stick, something I'm reasonably sure is not done IRL.

 

In order to have the the inverted T on the -5deg pitch line when the speed is 138 you need 25% fuel or less. I'm sure the aircraft in video wasn't fuelled 25%, the longitudinal G wold have been higher.

 

About longitudinal G - in simulator the aircraft has a longitudinal G comparable with the one in video with 100% fuel, 4 missile and one external tank. The aircraft in video probably had only two smoke generators. So currently in simulator the engine installed thrust at low speed is probably overestimated.

 

One thing I noticed - in external view during takeoff when the speed is about 150kts the elevons would quickly deflect up at a moderate angle. Is that behaviour correct? At 150kts you're usually pretty much in the middle of rotate with stick pulled a more or less amount, and if right then the FCS suddenly "gives you" this upwards elevons deflection that adds to what you're demanding, you would surely have to make a very quick and well coordinated stick forward move, or you will over rotate.

 

As it currently is, I find the aircraft handling during rotate to be quite severe and not exactly intuitive. Smoothly putting that inverted T on the horizon line like you're supposed to do is close to impossible.

[spiddx]

I don't know if 25% is realistic, but air show demonstrations are usually flown with very low amount of fuel (definitely not 100%) to make them more exciting. That could be a factor...

[Paganus]

I'm not saying this to discount the OP's concern, because I'm not sure myself, but...

 

Also consider altitude of the test vs the video.

 

Demonstration aircraft are often lightened in other ways beyond just the fuel load.

[Fox One]

I don't know if 25% is realistic, but air show demonstrations are usually flown with very low amount of fuel (definitely not 100%) to make them more exciting. That could be a factor...

 

Let's suppose the plane in video had 50% fuel. Longitudinal G on takeoff was 0.69

 

In simulator, with 50% fuel I have a longitudinal G of about 1. The difference is too big to be put down on different air temperature, airfield elevation, etc. Such a huge difference would mean the simulator is completely off, which I don't think it's the case. It just accelerates a bit too quickly on takeoff.

 

Longitudinal G for plane in the video above is not particularly impressive. IMO the plane was fully fuelled, this was just a normal airshow training flight.

[Azrayen]

Demonstration aircraft are often lightened in other ways beyond just the fuel load.

I can't tell if this is true as a "general" rule, but those AdlA demonstrations take place with regular, operational aircraft. So low fuel, no loadout/pylons but that's about it.