[REPORTED]F-16 pitches up when accelerating

[deadpool]

The plane is supposed to be trimmed for 1g, yet it noticably pitches up when accelerating.

[Bouli306]

The plane is supposed to be trimmed for 1g, yet it noticably pitches up when accelerating.

 

level flight...?

[deadpool]

level flight...?

 

Apologies, yes. Level flight, looking straight ahead of course

[BIGNEWY]

If you think there is a problem please attach a short track replay showing the problem and we will all take a look.

 

thanks

[Bouli306]

Although very benign (2-3 degrees) it pitches up on accell. Now it must be said that the accelerometer placement might have something to do with this.

Edited April 25, 2020 by Bouli306

[BIGNEWY]

I mean no disrespect ...

 

But how hard is it to spawn an air start F-16 in level flight flying 200 knots and then push throttle towards?

 

Example:

 

[url1.15/url]

 

 

 

We ask for a track for all issues, it is important to know what the user has used as a base line, we can see input, loadout, weather in the track which can all affect the potential issue.

 

At this stage I have no idea if this is a bug, I am not a pilot, so I will have to investigate and ask the team on Monday.

 

My own thoughts are if you have trimmed for a speed and then change the speed you will need to re-trim.

Edited May 17, 2020 by NineLine

[canadianbaken]

Basic aerodynamics: speed up = trim for new speed, slow down= trim for new speed. Any change will require an adjustment.

[razo+r]

Basic aerodynamics: speed up = trim for new speed, slow down= trim for new speed. Any change will require an adjustment.

 

Basic aerodynamics for basic planes. F-16 isn't really basic anymore.

[Tholozor]

Here's my understanding. Under normal flight, the aircraft always wants to maintain 1G. When you increase the throttle, the extra thrust causes the aircraft's vector to shift closer to the boresight, thereby decreasing the AoA. Since the boresight is elevated above the horizon line, the decrease in AoA causes the aircraft to pitch up in the direction of the vector, yet the amount of G has not changed, so the FLCS decides that everything is nominal.

Edited April 25, 2020 by Tholozor

[Deano87]

Here's my understanding. Under normal flight, the aircraft always wants to maintain 1G. When you increase the throttle, the extra thrust causes the aircraft's vector to shift closer to the boresight, thereby decreasing the AoA. Since the boresight is elevated above the horizon line, the decrease in AoA causes the aircraft to pitch up in the direction of the vector, yet the amount of G has not changed, so the FLCS decides that everything is nominal.

 

Exactly.

[Jester2138]

Basic aerodynamics: speed up = trim for new speed, slow down= trim for new speed. Any change will require an adjustment.

 

"Basic aerodynamics" for your Cessna 172, sure, not necessarily for a relaxed stability fighter flown by a digital FBW system.

[Frederf]

Here's my understanding. Under normal flight, the aircraft always wants to maintain 1G. When you increase the throttle, the extra thrust causes the aircraft's vector to shift closer to the boresight, thereby decreasing the AoA. Since the boresight is elevated above the horizon line, the decrease in AoA causes the aircraft to pitch up in the direction of the vector, yet the amount of G has not changed, so the FLCS decides that everything is nominal.

 

I don't understand, if the airplane accelerates upward then by definition vertical G cannot be 1.0. FLCS might be reactionary but all accelerations >1g would cause airplane to react by adjusting pitch to return to 1g and very very quickly.

[mvsgas]

This versions of the F-16C should maintain 1g with zero trim, so if you trim you will change that.

 

Also keep in mind you can be at 1g inverted. If the aircraft back then would determine it is not at set g (depending on trim), it ill try to change pitch. So less that 1g it will increase pitch to get 1g. More that 1g it will decrease pitch to get 1g. It does not mean it will maintain level zero pitch.

 

I say back then because obviously flight control computer and it software has change several time from 2007. The most obvious change is the integration of the Automatic Grown Collision Avoidance System (AGCAS).

[deadpool]

The FLCS is damn fast and it should stay exactly at level.

Yes the tendency would be to pitch when thrust changes, BUT the FLCS would work that out quicker than later.

[Dee-Jay]

We ask for a track for all issues, it is important to

My own thoughts are if you have trimmed for a speed and then change the speed you will need to re-trim.

 

True, on "classic" airplanes. F-16 is different in ptich axis due to FLCS laws.

[mvsgas]

Obviously, youtube video lack a lot of things. We can not tell the year this was film, this is not the version of the F-16 in DCS. We can not tell the pilots inputs nor the inputs from the guys in the back. This does not proof anything is wrong or right in DCS.

 

However, I do believe it shows the difference between a trained pilot and somebody without training. You can tell the pilot flying is smooth and pitch and altitude are very consistent.

 

At 13:58, the guy in the back seat take over controls. You can see right away how the aircraft starts changing altitude depending on speed, at or close to trim g.

 

The video is set to the time just pres play

 

Edited May 20, 2021 by mvsgas
updating link and spelling

[Deano87]

The FLCS is damn fast and it should stay exactly at level.

Yes the tendency would be to pitch when thrust changes, BUT the FLCS would work that out quicker than later.

 

Are you sure the FLCS would notice that amount of vertical acceleration? I mean a change in 1000fpm accrued over 30 seconds is 0.017G, I don't know if that would be enough to elicit a response from the FLCS?

[Rlaxoxo]

We ask for a track for all issues, it is important to know what the user has used as a base line, we can see input, loadout, weather in the track which can all affect the potential issue.

 

At this stage I have no idea if this is a bug, I am not a pilot, so I will have to investigate and ask the team on Monday.

 

My own thoughts are if you have trimmed for a speed and then change the speed you will need to re-trim.

 

...

 

https://www.dropbox.com/s/kipgtbkulqwxyab/F-16_Pitch.trk?dl=0

 

(Had to do it via dropbox because forum attachment limit is 5 MB ...)

 

Okay, here you go, Instant Free flight F-16 mission.

Only thing I had to do was move the throttle from idle to max and vice versa.

 

Problem: F-16 does not adjust pitch to maintain level flight during acceleration or de-acceleration.

 

I remember Mirage having this issue during early development and was quickly fixed after they learned what FBW is supposed to do.

Edited May 10, 2020 by Rlaxoxo

[Sharpe_95]

I would like to add to this thread.

 

I have noticed recently that the F16 seems to want to settle slightly above straight and level and regardless of acceleration will tend to pitch up a few degrees over fairly short periods of time for no apparent reason.

 

PSA track that hopefully highlights this issue (possibly linked to the OP?).

 

In the track you will note that I set my thrust to 90% RPM, I never touch it for the rest of the track.

 

I then set the autopilot for alt hold and steerpoint so as to ensure a straight and level flight.

 

Note when I come out of autopilot the aircraft starts nicely straight and level and with no input of stick or thrust, begins to pitch up.

 

I then deliberately set the aircraft in a slightly nose down attitude and despite this the F16 begins to rise again, beyond the horizon line (I rather run out of room slightly earlier than I would have liked but I think the track should highlight the issue well enough.

 

-S

Pitch Up.trk

[Bouli306]

ED, i just tested this in the RNLAF sim and when straight and level and accelerating the jet does NOT go up. Also spoken to the pilots and they confirm it does not go up.

 

Also spoken to FLCS specs and they confirm this systemwise also. However, rolling inverted and releasing the stick the nose goes down because the jet is trimmed for 1G. Als going nose up (starting around 30 degrees pitch) the jet wants to go slightly nose up for the same reason.

Edited May 16, 2020 by Bouli306

[deadpool]

ED, i just tested this in the RNLAF sim and when straight and level and accelerating the jet does NOT go up. Also spoken to the pilots and they confirm it does not go up.

 

Also spoken to FLCS specs and they confirm this systemwise also. However, rolling inverted and releasing the stick the nose goes down because the jet is trimmed for 1G. Als going nose up (starting around 30 degrees pitch) the jet wants to go slightly nose up for the same reason.

 

Thank you for the effort of confirming this. I want to buy you a beer now, please PM if you accept :-)

[NineLine]

ED, i just tested this in the RNLAF sim and when straight and level and accelerating the jet does NOT go up. Also spoken to the pilots and they confirm it does not go up.

 

Also spoken to FLCS specs and they confirm this systemwise also. However, rolling inverted and releasing the stick the nose goes down because the jet is trimmed for 1G. Als going nose up (starting around 30 degrees pitch) the jet wants to go slightly nose up for the same reason.

 

Thanks, I will pass this along.

[NineLine]

Reported, thanks all.

[mvsgas]

https://forums.eagle.ru/showpost.php?p=4350294&postcount=6

[deadpool]

https://forums.eagle.ru/showpost.php?p=4350294&postcount=6

 

Different effect I'd say.

 

This defect is not about transitioning to supersonic or vice versa, this is about the FPM moving up and down during speed changes and staying there for too long.

 

But your video was awesome and it would make sense to have that in a different thread as something to implement!