Longitudinal speed affects climb/descent. Is this right?

[escaner]

I have noticed that when I increase the speed, the airplane starts climbing and when decelerating it does the opposite. Is this the correct behavior? Looks odd when it should maintain 1G.

 

Look at the velocity vector and acceleration chevrons in this video:

 

[shagrat]

Absolutely normal. Speed increases lift.

That is how airfoils work.

 

EDIT let's elaborate a bit. If a plane starts from the runway it increases speed until the airfoils generate enough lift to let you take off. The AoA (when you pull) simply increases the effect, but it is the air flowing over the airfoils that generates lift.

 

Now, when you are airborne this effect is still present. That is why whenever you increase/decrease airspeed you increase/decrease lift and need to trim.

 

The Mirage has an FCS, but that only keeps a stable attitude when letting go of the controls. That does only affect attitude.

If you want to hold an altitude/heading/course you need to use the autopilot.

Edited May 1, 2017 by shagrat

[nrgized]

It's a common reason people have a hard time mid air refueling and or landing.

 

 

Sent from my iPhone using Tapatalk

[escaner]

Absolutely normal. Speed increases lift.

That is how airfoils work.

 

The Mirage has an FCS, but that only keeps a stable attitude when letting go of the controls. That does only affect attitude.

If you want to hold an altitude/heading/course you need to use the autopilot.

 

In a normal stable plane without FBW, increasing speed effectively increases lift. By design, this produces a pitch up, and the lift vector tilts back until its vertical component equalizes the weight again and the resulting stable situation is a 1G climb.

 

But, the Mirage FBW maintains 1G in pitch with the stick in neutral (IIRC it was above 300 KIAS), it does not keep attitude. If you increase speed, lift is increased, which for the same weight induces acceleration. In order to maintain 1G, the FBW system should decrease the AoA which would reduce the lift to its previous value and maintain the velocity vector where it was.

 

In fact, in the video, you can see the ladder slightly moving too, which implies small changes in attitude. The aircraft clearly pitches down with higher speed and up with slower speeds, which is what I would expect from its FBW. But the velocity vector still moves up and down, which means that 1G is not maintained.

 

As far as I can see, it could be that the FBW systems is a bit slow reacting to changes (maybe even in the real plane is so), but what seems clear for me is that 1G is not kept.

[shagrat]

As far as I understood the FBW interprets the inputs of the pilot and makes an unstable aircraft design react like a "normal"/stable plane.

Its purpose is to keep the plane controllable.

What you describe is what a ALT HOLD autopilot is supposed to do. Trim the plane to keep the vector stable and level, while speed (and thus lift) changes.

Think about a controlled descent for landing. You decrease speed and let the plane lose altitude while slowing down. If the FBW would require to drop the nose, it would also pick up speed or at least reduce decelaration on landing approach, which is undesirable.

Edited May 1, 2017 by shagrat
Corrected auto-correction

[kobeshow]

In a normal stable plane without FBW, increasing speed effectively increases lift. By design, this produces a pitch up, and the lift vector tilts back until its vertical component equalizes the weight again and the resulting stable situation is a 1G climb.

 

But, the Mirage FBW maintains 1G in pitch with the stick in neutral (IIRC it was above 300 KIAS), it does not keep attitude. If you increase speed, lift is increased, which for the same weight induces acceleration. In order to maintain 1G, the FBW system should decrease the AoA which would reduce the lift to its previous value and maintain the velocity vector where it was.

 

In fact, in the video, you can see the ladder slightly moving too, which implies small changes in attitude. The aircraft clearly pitches down with higher speed and up with slower speeds, which is what I would expect from its FBW. But the velocity vector still moves up and down, which means that 1G is not maintained.

 

As far as I can see, it could be that the FBW systems is a bit slow reacting to changes (maybe even in the real plane is so), but what seems clear for me is that 1G is not kept.

 

You seem to be very focused on the "keeps 1G" part.

 

If speed increases, the FPM goes up (closer to the nose axis), which essentially means that AoA is decreased. This does not increase G significantly, since the nose does not rotate. G measures the acceleration in the vertical plane, which in case of change in AoA is practically non existant.

I would bet that if you show the video from the first post again, but this time with an angle that allows us to read the G that acceleration/deceleration cause, we would see that the difference in G from this smooth AoA adjustment by the FBW is negligible.

Edited May 1, 2017 by kobeshow

[Pocket Sized]

The FBW maintains 1G, but it's not instant. The FBW will slowly & smoothly "trim" itself to 1G as flight conditions change.

[jaguara5]

https://forums.eagle.ru/showthread.php?p=2902866#post2902866

[jojo]

The FBW maintain 1G in aircraft's referential Z axis.

 

It means that when you dive or climb steeply you no longer have 1G on aircraft's Z axis, so the plane react to keep back 1 G.

 

When you dive steeply you must push the stick forward to keep constant dive angle.

[Pocket Sized]

The FBW maintain 1G in aircraft's referential Z axis.

 

It means that when you dive or climb steeply you no longer have 1G on aircraft's Z axis, so the plane react to keep back 1 G.

 

When you dive steeply you must push the stick forward to keep constant dive angle.

 

I believe OP's tests were done in level flight.

[jojo]

Ok, but at least he should select the gun so we could read the G in the HUD.