How well does the airspeed trimming law work in a SU-27?

[LJQCN101]

The airspeed trimming law seems quite charming in the manual which is said to be responsible for adjusting the stick longitudinal deflection according to ram air pressure in order to provide a neutral speed stability. (Most of the FBW aircrafts provide apparent neutral-speed stability.)

 

However in game I can still notice a high positive-speed stability of the aircraft. I don't know if the Su-27 still retains some kind of conventional trim "feel", but when I manually trim the aircraft to level flight and gradually increase speed, it feels like the airspeed trimming doesn't work at all and the aircraft pitches up from level flight and keeps pitching up.

 

Therefore I wonder how well does the airspeed trimming law work in RL. To what extent will it hold the set flight path for the pilot if the stick remains in the same position? ;)

Edited November 10, 2014 by LJQCN101

[Yo-Yo]

The airspeed trimming law seems quite charming in the manual which is said to be responsible for adjusting the stick longitudinal deflection according to ram air pressure in order to provide a neutral speed stability. (Most of the FBW aircrafts provide apparent neutral-speed stability.)

 

However in game I can still notice a high positive-speed stability of the aircraft. I don't know if the Su-27 still retains some kind of conventional trim "feel", but when I manually trim the aircraft to level flight and gradually increase speed, it feels like the airspeed trimming doesn't work at all and the aircraft pitches up from level flight and keeps pitching up.

 

Therefore I wonder how well does the airspeed trimming law work in RL. To what extent will it hold the set flight path for the pilot if the stick remains in the same position? ;)

 

You slightly misunderstood FBW conception. THe FBW operating with Ny parameter does not require ram pressure info for maintaining neutral speed stability because the main effect has the negative feedback from normal (Y- in Russian notation) acceleration. Ram pressure is useful to maintain almost constant quality of controlling as ram pressure affects automatic control loop gain, frequencies, etc.

 

Su-27 artificially adds a signal to the FBW to imitate conventional plane stability.

 

So, it needs constant trim. It's a cost for pilot's awareness in curent IAS especially in subsonic range.

[LJQCN101]

You slightly misunderstood FBW conception. THe FBW operating with Ny parameter does not require ram pressure info for maintaining neutral speed stability because the main effect has the negative feedback from normal (Y- in Russian notation) acceleration. Ram pressure is useful to maintain almost constant quality of controlling as ram pressure affects automatic control loop gain, frequencies, etc.

 

Su-27 artificially adds a signal to the FBW to imitate conventional plane stability.

 

So, it needs constant trim. It's a cost for pilot's awareness in curent IAS especially in subsonic range.

 

Wow thanks for clarifying. So the Longitudinal FCS operates as a Ny-command system and uses normal acceleration feedback to maintain neutral speed stability while the airspeed trimming law only imitates conventional plane stability (which is positive speed stability). Is that correct?

[Yo-Yo]

Wow thanks for clarifying. So the Longitudinal FCS operates as a Ny-command system and uses normal acceleration feedback to maintain neutral speed stability while the airspeed trimming law only imitates conventional plane stability (which is positive speed stability). Is that correct?

 

 

Yes, right.

[LJQCN101]

Yes, right.

 

And here's another question if you don't mind.;)

 

Let consider the longitudinal FCS in TAKEOFF-LANDING mode. There's not much mentioned in the manual and I maybe wrong but does it suppose to use pitch-rate feedback to hold a zero pitch rate if the stick is in neutral position? If that so, a change in CG position or the extend of flaps or speed brakes won't change the pitch attitude. If not, then I'm glad to accept any info from you guys.

 

I made a guess like this because it reminds me of F-16 whose TAKEOFF & LANDING gains of the longitudinal FLCS operates as a pitch-rate command system which means when no input is requested, the system commands zero pitch-rate (until 10° AOA). Just trying to find the similarities.:book:

Edited November 10, 2014 by LJQCN101

[Yo-Yo]

And here's another question if you don't mind.;)

 

Let consider the longitudinal FCS in TAKEOFF-LANDING mode. There's not much mentioned in the manual and I maybe wrong but does it suppose to use pitch-rate feedback to hold a zero pitch rate if the stick is in neutral position? If that so, a change in CG position or the extend of flaps or speed brakes won't change the pitch attitude. If not, then I'm glad to accept any info from you guys.

 

I made a guess like this because it reminds me of F-16 whose TAKEOFF & LANDING gains of the longitudinal FLCS operates as a pitch-rate command system which means when no input is requested, the system commands zero pitch-rate (until 10° AOA). Just trying to find the similarities.:book:

 

No, just direct link plus massive damper. As the plane is near neutral (+ - depends on actual CoG position - fuel and payloads) the stick balance position is constant at speed.

[aaron886]

So basically... not g-command, just a series of feedbacks to give somewhat consistent stick-force-per-g? That would explain both the conventional trim and the consistent maneuvering potential.

[Yo-Yo]

So basically... not g-command, just a series of feedbacks to give somewhat consistent stick-force-per-g? That would explain both the conventional trim and the consistent maneuvering potential.

 

If you scribe the equation for the direct link minus g-force you will see that it is almost the same as to set required g. And, negative, there is special artificial ram pressure-dependant addition to the FBW to have this conventional trim necessity. :)

Edited November 10, 2014 by Yo-Yo

[LJQCN101]

Well then, the manual seems quite confusing when it comes to the airspeed trimming law part.

 

First it said:

When the indicated airspeed is increased, the FCS causes the stabilizers deflection (up to 5 degrees) nose down. When decelerating, it causes the nose to come up. This imitates the airspeed stability of the aircraft, which is neutral in the presence of a g loading feedback signal.

 

My comprehension is that, according to the control logic diagram given below, the aircraft do change pitch attitude when accelerating or decelerating. Without this control law, the Su-27 would be an aircraft with neutral speed stability.

 

 

But I don't think it is imitating the airspeed stability of a conventional aircraft for the reason that an aircraft with positive speed stability will go pitch-up when accelerating and pitch-down when decelerating. Totally the opposite.

 

 

EDIT:

1. "Stabilizers deflection nose down" doesn't actually mean the nose will go down. It means "stabilizer's leading edge down", hence nose up. See post #13.

2. There's also a mistake in the diagram. "leading edge up" is wrong, should be "leading edge down".

Edited November 13, 2014 by LJQCN101

[LJQCN101]

Secondly, the manual also states that:

The law is also intended to increase the flight safety when decelerating. As the aircraft is neutrally stable by airspeed, it should increase the angle of attack in decelerated flight. The aforementioned control law prevents from this by deflecting stabilizers to decrease the AoA.

 

According to what a neutral speed stable aircraft behaves, the angle of attack does increase when decelerating. But with the airspeed trimming law incorporated into the FCS, the nose will go up when decelerating hence more angle of attack?:huh:

 

EDIT: Now I got it right. The diagram actually means that the nose will go down when decelerating. No mistake in this quote.

Edited November 13, 2014 by LJQCN101

[LJQCN101]

And the last sentence of the Longitudinal Channel section:

 

When accelerating, to keep the aircraft level, it should be trimmed "nose down" as the airspeed increases.

 

When accelerating, the FCS commands the aircraft to go pitch-down according to the airspeed trimming law. So in order to keep the aircraft level, shouldn't the pilot trim nose up instead of nose down as the airspeed increases?

 

EDIT: When accelerating, the FCS commands the aircraft to go pitch-up (not pitch-down). No problem in this quote either.

Edited November 13, 2014 by LJQCN101

[Yo-Yo]

Secondly, the manual also states that:

 

 

According to what a neutral speed stable aircraft behaves, the angle of attack does increase when decelerating. But with the airspeed trimming law incorporated into the FCS, the nose will go up when decelerating hence more angle of attack?:huh:

 

I guess, you mixed your thoughts yourself... :) THe stable conventional airplane goes pitch-up with IAS increased and FIXED stick. THe same thing Su-27 does with artificial stability.

How to achieve it? Just add to the stick position the signal you see on the chart - nose up as IAS increases (imagine the second pilot who pulls the stick with the dynamic pressure :) )/ The first pilot have to PULL the stick to counteract his fellow virtual pilot) exactly as he does in conventional plane.

[LJQCN101]

THe stable conventional airplane goes pitch-up with IAS increased and FIXED stick. THe same thing Su-27 does with artificial stability.

How to achieve it? Just add to the stick position the signal you see on the chart - nose up as IAS increases (imagine the second pilot who pulls the stick with the dynamic pressure :) )/ The first pilot have to PULL the stick to counteract his fellow virtual pilot) exactly as he does in conventional plane.

 

Yeah I thought the same as you. An aircraft with positive speed stability will go pitch-up when accelerating and pitch-down when decelerating (with FIXED stick) as I stated before.

 

Maybe my problem is the comprehension of the phrase "leading edge up" and "stabilizers deflection nose down".

 

I was thinking:

Hence nose down.:doh:

Edited November 12, 2014 by LJQCN101

[Yo-Yo]

Yeah I thought the same as you. An aircraft with positive speed stability will go pitch-up when accelerating and pitch-down when decelerating (with FIXED stick) as I stated before.

 

Maybe my problem is the comprehension of the phrase "leading edge up" and "stabilizers deflection nose down".

 

I was thinking:

Hence nose down.:doh:

 

Maybe wrong translation?

 

What is the source of this quotation?

[karambiatos]

im not understanding a thing, the manual says

The fly-by-wire system is a trajectory control system. In other words, if the control stick remains in the same position, the system will hold the set flight path. This is why the reduction of airspeed (for example) and resulting decreased lift and increasing angle of attack, will lead to the aircraft attempting to hold the initial flight path and prevent the aircraft from descending. This will result in the airspeed stability degradation up to neutral stability.

 

in game it wants to gain altitude even when its about stall, it still keeps increasing pitch so much that its still gaining altitude.

 

or is this quote referring to the altitude hold autopilots?

[LJQCN101]

Maybe wrong translation?

 

What is the source of this quotation?

 

From the beta manual: http://forums.eagle.ru/showthread.php?t=133669

[Esac_mirmidon]

So the Su-27 has statical and dynamical near zero stability by design?

[Yo-Yo]

So the Su-27 has statical and dynamical near zero stability by design?

 

What kind of stability do you mean - its own stability, stability with FBW system without adding artificial "counter trim", full functional FBW?

[Yo-Yo]

im not understanding a thing, the manual says

 

 

in game it wants to gain altitude even when its about stall, it still keeps increasing pitch so much that its still gaining altitude.

 

or is this quote referring to the altitude hold autopilots?

 

Maybe it's the first part of the statement? Describing, for example, the reason why this artificial stability was added.

[Yo-Yo]

For better understanding what is the natural speed stability try to fly the TF-51 Mustang from 110 to 400 mph in level flight or shallow diving watching the stick position.

[Esac_mirmidon]

I´m asking about a "naked" Su-27 without anything, pure aerodynamical factor ( No FBW, no FBW + Counter Trim ).

 

His own, by design, static and dynamic stability only because his shape, CG, lift, etc.

[LJQCN101]

im not understanding a thing, the manual says

 

 

in game it wants to gain altitude even when its about stall, it still keeps increasing pitch so much that its still gaining altitude.

 

or is this quote referring to the altitude hold autopilots?

 

IMO this quote is referring to the FBW system without airspeed trimming law which uses normal acceleration feedback (and g command signal) to provide the aircraft with neutral speed stability. The neutral speed stability is characterized by no pitch changes with speed increase or decrease (stick fixed).

 

An airspeed trimming law is added to Su-27 to imitate how a conventional speed stable aircraft will behave with speed changes. That control law changes the speed stability of the aircraft from neutral to positive, which means it has a pitch-up tendency at high speeds and a pitch-down tendency at low speeds. All that pitch up/down characteristics is artificially made by the airspeed trimming law of FCS.

Edited November 13, 2014 by LJQCN101

[Yo-Yo]

I´m asking about a "naked" Su-27 without anything, pure aerodynamical factor ( No FBW, no FBW + Counter Trim ).

 

His own, by design, static and dynamic stability only because his shape, CG, lift, etc.

 

Static is close to neutral (from slightly negative to slightly positive depending on weight and balance), dynamically stable (if you operate it as it is static stable) because of natural damping. I mean subsonic area, of course. In transsonic and supersonic areas the plane is stable as usual, excessive stable.

[Esac_mirmidon]

Thanks Yo-Yo

 

This helps me a lot to understand better the Flanker.