Su-27 Flight Model Discussion

[archer86]

That friction you talk about pilot doesn't feel as it doesn't transferr to stick. The air friction will only transferr into higher drag, and puts more pressure force on the wing, but doesn't block it from moving, as both part of the elevator are working oposite each other (as whole wing is elevator and rotates pretty close to its center where its hinged to the fuselage)

 

I am not talking about pilot or his feelings and I know it is not related with stick.

Sure there is nothing to feel with flybywire controls.

my problem is all about movable parts of controllers and the force above their surfaces at high speeds.

you can move the stick at higer speeds(1000kmh or over) but should have latency on your commands.

it is missing on dcs su-27 from the day it has released.

and there is similar problem with f-15 rudders and ailerons over 700knots.

Edited January 29, 2015 by archer86

[GGTharos]

If you mean aerodynamic force vs. hydraulic system force simulation, I'm not sure it's there either.

[theropod]

and there is similar problem with f-15 rudders over 700knots.

i ve just tested:)

at 800knots i moved the rudder left to right and movement gone wild.

it like saying big no no no no:)

yes there is no friction on rudder but f-15 has mass friction force on elevator and aileron like it has to be.

lets hope a fix for flanker

[Tombstone]

my problem is all about movable parts of controllers and the force above their surfaces at high speeds.

you can move the stick at higer speeds(1000kmh or over) but should have latency on your commands.

 

At zero speed, control surfaces wont move at infinite rate either. That means that there are other factors, than aerodynamic forces acting upon surface, that limit actuator+surface assembly speed. Like inertia, but I think that flow constriction imposed by control valves are more significant. Now, why are you so sure, that throughout flight envelope aerodynamic forces should become dominant? Sure, control surfaces wont move any faster with load on them, but exactly how significant this change would be. Have you got any relevant information on this? Like rate of movement vs. force diagrams? I doubt that they would be linear.

[theropod]

At zero speed, control surfaces wont move at infinite rate either. That means that there are other factors, than aerodynamic forces acting upon surface, that limit actuator+surface assembly speed. Like inertia, but I think that flow constriction imposed by control valves are more significant. Now, why are you so sure, that throughout flight envelope aerodynamic forces should become dominant? Sure, control surfaces wont move any faster with load on them, but exactly how significant this change would be. Have you got any relevant information on this? Like rate of movement vs. force diagrams? I doubt that they would be linear.

 

take off and close asc, gain speed to 1000kmh or higher. you'll feel that aircraft is still sensitive like it is on low speeds.

the force that make the wing flex is only a visiual on dcs flanker,it is not exists

i guess this is his problem

 

''Sure, control surfaces wont move any faster with load on them'

with this reason, plane turns or moves at more wider angles but our flanker is nearly same at all speeds.

Edited January 29, 2015 by theropod

[archer86]

If you mean aerodynamic force vs. hydraulic system force simulation, I'm not sure it's there either.

 

for flanker,it is not there

but DCS F-15 has that force on elevator(ailerons and rudder has not)

Edited January 29, 2015 by archer86

[theropod]

i made an example track

it looks weird even at lower speeds.

where is the air friction?

test friction.trk

Edited January 29, 2015 by theropod

[Weta43]

where is the air friction?

Hiding behind a hydraulic accumulator ?

[Yo-Yo]

At zero speed, control surfaces wont move at infinite rate either. That means that there are other factors, than aerodynamic forces acting upon surface, that limit actuator+surface assembly speed. Like inertia, but I think that flow constriction imposed by control valves are more significant. Now, why are you so sure, that throughout flight envelope aerodynamic forces should become dominant? Sure, control surfaces wont move any faster with load on them, but exactly how significant this change would be. Have you got any relevant information on this? Like rate of movement vs. force diagrams? I doubt that they would be linear.

 

Generally, this dependace is close to parabolic.

[Yo-Yo]

take off and close asc, gain speed to 1000kmh or higher. you'll feel that aircraft is still sensitive like it is on low speeds.

the force that make the wing flex is only a visiual on dcs flanker,it is not exists

i guess this is his problem

 

''Sure, control surfaces wont move any faster with load on them'

with this reason, plane turns or moves at more wider angles but our flanker is nearly same at all speeds.

 

There are two counteracting factors: higher dynamic pressure makes the plane more agile (increased moments vs constant MOI, increased spiral rate for sustained roll) vs higher hinge moments that slow controls movement. Moreover, the available actuators velocity at lower speed can be more than required, and FBW system uses only part of it.

[Witchking]

As I am way out of my league in these detailed conversations, all I want to say is ...nice to see you here Dima. And fantastic job on the flanker. Its the most dynamic representation of this jet ever and I love the FM. Keep up the realism. Looking forward for more updates to flanker and perhaps the fulcrum some day. ;)

 

Having seen only documentaries..no first hand experience, I recall Russian designers actually did design in some cases the FBW system to add in flaperon inputs to roll inputs in order to stabilize aircraft based on test pilot inputs...especially during combat when missiles are shot. Check the Mig 25 documentary on youtube and you will see why.

[theropod]

There are two counteracting factors: higher dynamic pressure makes the plane more agile (increased moments vs constant MOI, increased spiral rate for sustained roll) vs higher hinge moments that slow controls movement. Moreover, the available actuators velocity at lower speed can be more than required, and FBW system uses only part of it.

 

thank you for your interest yo-yo:)

 

at higher speeds ,is the agility same for elevator?

i specially wonder about it.

can a real flanker perform cobra at all speeds without any friction force limit?

is increasing speed doesnt have any pressure on elevator movement?

Edited January 29, 2015 by theropod

[Ironhand]

I tried to balance the aircraft at 400, 600, and in supersonic 1300 kph IAS below 2000m. The third case is against all existing rules :) but the plane is stable there so it's not so dangerous if you are fast and coordinated enough.

 

Ufortunately... the stab position was exactly the same with and w/o FBW as it should.

Just reviewed my tracks and, indeed, they do not show the difference I saw which caused me to post them in the first place. The horizontal stabilizers are in the same position. Sorry for throwing up a flag. Something weird must have been happening on my end.

[Yo-Yo]

thank you for your interest yo-yo:)

 

at higher speeds ,is the agility same for elevator?

i specially wonder about it.

can a real flanker perform cobra at all speeds without any friction force limit?

is increasing speed doesnt have any pressure on elevator movement?

 

This is very long and specific talk about so called frequency response of the aircraft. The aircraft itself (if you have ideal actuators for controls) becomes more agile with IAS growing. It's trivial, because its angular accelerations depend on IAS^2 and MOIs (inertia) are constant. So, the frequency responce or agility of the plane itself grows with speed.

In its turn, hinge moment of a control (elevator, for example) also depends on IAS^2 but not linear. Anyway, this factor reduces increased agility but can not decrease it with speed and even maintain it constant.

For correctly designed actuators having enough power, for sure...

 

Yes, the aircraft can perform cobra within Mach numbers range where it has required pitch moment characteristics to perform it. The difference will be in the g-load and altitude gain.

Edited January 29, 2015 by Yo-Yo

[archer86]

i made an example track

it looks weird even at lower speeds.

where is the air friction?

 

your track shows the abnormal pitch move for that speed

[theropod]

Yes, the aircraft can perform cobra within Mach numbers range where it has required pitch moment characteristics to perform it. The difference will be in the g-load and altitude gain.

 

thank you yo-yo but could g load cause any wing damage ?

Edited January 29, 2015 by theropod

[OnlyforDCS]

thank you yo-yo but could g load cause any wing damage ?

 

At high enough speeds you can even achieve a complete breakup of the airframe due to the extreme g loads. I think the damage modeling is still missing from the FM of the Su27 though. (Please correct me if Im wrong, I have never tried shutting of the FBW at high mach numbers, but many have reported 20-30g or more manouvers, enough to kill the pilot and break the plane)

[theropod]

At high enough speeds you can even achieve a complete breakup of the airframe due to the extreme g loads. I think the damage modeling is still missing from the FM of the Su27 though. (Please correct me if Im wrong, I have never tried shutting of the FBW at high mach numbers, but many have reported 20-30g or more manouvers, enough to kill the pilot and break the plane)

 

totally agree with you, there should be structural damage limit for flanker.

 

here is a record 55g :) https://www.youtube.com/watch?v=22QYzTlMoyE&feature=youtu.be

 

i am corious about the real limit but i guess ,after 700kmh, performing cobra may cause damage to wings

[Yo-Yo]

Guys, what is the point to discuss the matter that obviously is not completed? I mean this 55g, 155g, etc...

[OnlyforDCS]

No point, you are aware of it. We are aware that you are aware of it. :) Im not sure theropod was, but he most definitely is now.

[theropod]

i am definitely aware of that:) i have just little exaggerated my curiosity.

anyway, thank you for that perfect bird and thank for all your answers yo-yo

su-27 has the best FM i have ever seen on sim and sometimes that makes me to forget its beta stage situation.

Edited January 29, 2015 by theropod

[NineLine]

http://en.wikipedia.org/wiki/Constructive_criticism

 

Please use your head before posting, I have cleaned a couple posts that dont get this concept.

[Hummingbird]

I really don't get the constant need for heavy trimming, seems unlikely that the real aircraft suffers so badly from this. I mean the thing has got a FBW stability system.

 

Never heard a Russian pilot say he constantly needs to trim the aircraft.

[GGTharos]

And won't hear it, because pilots always constantly trimmed their aircraft, and they still do.

[NineLine]

I really don't get the constant need for heavy trimming, seems unlikely that the real aircraft suffers so badly from this. I mean the thing has got a FBW stability system.

 

Never heard a Russian pilot say he constantly needs to trim the aircraft.

 

 

I never heard a Russian pilot say he doesnt need to constantly trim his aircraft ;)