Viper Trim Use

[DEX]

I've stumbled across something a bit puzzling. I was browsing "Haole's Q and A" on Frugal's Falcon4 forum when I stumbled across a question regarding trim use in the Viper. The general consensus among those using Falcon was that the trim would never be used becasue of FLCS. However, it was indicated that trim is used extensively in RL, though not as extensive as in traditional stick/rudder aircraft.

Now, I've always thought FLCS:

1) Handled the basic stick/rudder work to make the aircraft flyable (keeping it from flipping backwards end over end and disintigrating).

2) Changed various flight parameter goals (AOA/G, Coordination [rudder], Roll Rate, Pitch Rate, etc.) based on pilot sidestick and rudder pedal inputs. So, for example, pulling aft on the sidestick would say, "I would like to fly at 2.0 Gs instead of 1.0," and FLCS would move control surfaces to do that. Once the sidestick was released, FLCS would then maintain, say, twenty-five degrees nose up, one G flight, assuming constant thrust.

What I've just said must be incorrect. If the above were true, why would you ever need to use trim?

Would FLCS NOT maintain the pitch attitude? I'm a little confused about that. I've never touched the trim in Falcon.

[Hippy16th]

well, not being an real F16 pilot myself...i can only offer an explanation that may also be incorect, but it could be as well. Trim in the viper is used when damage is sustained, and that is about the only time that i know of....maybe also in adverse weather conditions too....but still just a guess.

[Charlie_VFP]

The Hornet's FCS automatically adjusts for damage and system failures. I am sure you have all seen pictures of the two hornets that had a mid-air and still landed.....

The pilot of the aircraft that was missing 12 feet off his wing said that it 'pulled a little to the right'. :bigsmile:

We have something called "Take off trim" That is set by holding down the FCS button the FCS Control Panel for 3 to 5 seconds. It sets the flaps & ailerons down to a certain degree (not sure of the exact # without looking it up) and sets the leading edge of the horizontal stabs down to a certain degree.

Note: It is the same TOT (Take Off Trim) for both ground and carrier operations.

[DEX]

hm.... I beleive the Eagles flight control system also automatically adjusts for damage, which would be why that one pilot landed with one wing in that well known incident. I think it would be safe to assume that FLCS also does this. This rules out the damage theory.

-DEX (still in a puzzled state) :confused:

[Charlie_VFP]

Here is an excerpt frrom the NFM for the Hornet....


2.8.2.8 Control Augmentation System (CAS). The longitudinal control system uses air data scheduled pitch rate, normal acceleration (NZ) and angle-of-attack (AOA) to compute stabilator actuator commands. The aircraft response is compared to the pilot command and the stabilator servoactuator is driven to reduce the difference to zero. Since, in the auto flaps up mode, any uncommanded pitch rate or g is reduced to zero, the aircraft is constantly trimmed to steady state hands-off 1 g flight and there is little or no occasion for manual trim. Pitch rate and g (NZ) feedbacks improve pitch characteristics and g control at medium to high airspeeds. Air data scheduled pitch rate feedback improves maneuvering characteristics and provides increasing stick-force-per-g at low to medium airspeeds. AOA feedback provides increasing stick force with increasing AOA above 22°. In the takeoff and land modes, AOA and pitch rate feedbacks are used to augment inherent airframe pitch damping and stability. The computer nulls the difference between the trim AOA and actual AOA. In turns, pitch rate feedback maintains tight pitch attitude control.

[DEX]

Cool beans.

See now that's more like how I THOUGHT things were, never having to touch the trim.

hm...

[Charlie_VFP]

I wouldnt say never.

Wait till you have an FCS degrade and your not in CAS mode anymore....

Think of trim as a much needed backup system.

[Obi Offiah]

Hi all! :smile:

This trim stuff is a little confusing. The Super Hornet pilot is required to apply manual lateral trim, in the event of lateral weight asymmetries from stores or fuel imbalances. I would have imagined the CAS was capable of automatically correcting for such circumstances?.

Thanks
Obi

[Sehm]

hm.... I beleive the Eagles flight control system also automatically adjusts for damage, which would be why that one pilot landed with one wing in that well known incident. I think it would be safe to assume that FLCS also does this. This rules out the damage theory.

-DEX (still in a puzzled state) :confused:The F-15 A/C do not have a fly-by-wire system, which means that the pilot has to trim it manually (please correct me if I'm wrong :smile: )

[Charlie_VFP]

Fly-By-Wire aircraft means that there is no mechanical linkage from the stick to the surface.

So, the F-15 IS a fly by wire aircraft as all modern day aircraft are.

Maybe the Eagle doesnt have the same FCS system (computer wise) as the F-16, F-18, ect.

[Obi Offiah]

The F-15 A/C do not have a fly-by-wire system, which means that the pilot has to trim it manually (please correct me if I'm wrong :smile: )

Hi Sehm

Although not fully a FBW aircraft (more pseudo FBW or 50/50) the F-15's CAS does endow it with auto-trim functionality.
I think FBW usually discribes the interface from the pilots controls to the aircrafts control surfaces i.e electrical. CAS/FCS is what provides the logic and flight characteristics?.

Obi

[Haole]

Trim is used in the Viper in RL. Every time that I take off I have to do some form of trimming. After that I dont have to touch it too much. Rudder trim is a big one that almost always has to be adjusted. We do a trim check on the ground after which we manually put the trim to a neutral position. It is very sensitive and if it is slightly off to one side or the other then it makes a big difference on the flight controls. So after takeoff I have to manually trim the jet out and then it is usually good to go after that. Strong winds, fuel imbalances, etc DOES change the characteristics and I have to trim again at times.
Yes the FLCS does a good job of helping you fly, but it doesnt do everything:bigsmile:
CHEERS

[Obi Offiah]

Hello Haole! :bigsmile:

Thanks for the reply. I've really been enjoying your posts, and have been following them closely at FrugalsWorld.

Whats confusing, is that the FLCS in the F-16 is capable enough to checking the aircraft in roll. E.g when you make a rapid 90 degree roll, as soon as you release the controls at 90, the FLCS will automatically apply opposite-flaperon briefly, to arrest/damp the roll, keeping the aircraft at 90 degrees, because any addition roll after 90 degrees is uncommanded.

It's just strange that the FLCS doesn't damp other unwanted tendencies, like weight asymmetries. The FLCS should recognise that the slow roll for example is unwanted, as there has been no input from the controls and thus should damp them out.

I wonder what design philosophy behind this is?. :smile:

Cheers
Obi

[DEX]

Yeah I'm wondering the same thing Obi. It's almost as if there is a discontinuity in the logic with which the system works. It will recognize this uncommanded roll (your 90 degree bank situation), but not the other (weight imbalance). Strange stuff.

[Marvin157th]

Obi,

This is probably going to sound stupid as my expertese is not engineering.. With regards to your question, maybe it has something to do with the FLCS will recognize a pilot made adjustment, and will compensate with data entered. Maybe it cannot recognize winds and weights, or the computer is designed to let the pilot adjust those in case of miscalculations, or the pilot intended to yaw the plane etc?

Marvin


Hello Haole! :bigsmile:

Thanks for the reply. I've really been enjoying your posts, and have been following them closely at FrugalsWorld.

Whats confusing, is that the FLCS in the F-16 is capable enough to checking the aircraft in roll. E.g when you make a rapid 90 degree roll, as soon as you release the controls at 90, the FLCS will automatically apply opposite-flaperon briefly, to arrest/damp the roll, keeping the aircraft at 90 degrees, because any addition roll after 90 degrees is uncommanded.

It's just strange that the FLCS doesn't damp other unwanted tendencies, like weight asymmetries. The FLCS should recognise that the slow roll for example is unwanted, as there has been no input from the controls and thus should damp them out.

I wonder what design philosophy behind this is?. :smile:

Cheers
Obi

[DEX]

Marvin, that crossed my mind.

This seems to be the answer I've received (f-16.net) -
FLCS does in fact attempt to correct, but it will only correct for such tendencies to a certain point. Once that point is reached, it is up to the pilot to add trim as necessary to further compensate for unwanted attitude changes.

I imagine this is for safety reasons. In the event FLCS' calculations lead it to try to do something potentially hazardous to one's health, it won't try too hard to do it. This may not be the reason (perhaps it's some other technical stuff), but it seems to make sense.

[Marvin157th]

DEX,

Yes, that does make sense in my mind. At least is didn't sound that stupid after all! ;)

Marvin


Marvin, that crossed my mind.

This seems to be the answer I've received (f-16.net) -
FLCS does in fact attempt to correct, but it will only correct for such tendencies to a certain point. Once that point is reached, it is up to the pilot to add trim as necessary to further compensate for unwanted attitude changes.

I imagine this is for safety reasons. In the event FLCS' calculations lead it to try to do something potentially hazardous to one's health, it won't try too hard to do it. This may not be the reason (perhaps it's some other technical stuff), but it seems to make sense.