Modern Transport Aircraft Stability Question
Are we perhaps talking different definitions of stick force gradients? In post 4 I referred to "pitch force gradient" after which I apologized and in post 9 suggested I should use the term "stick force gradient", which I then clarified to mean "change in stick force with a change in airspeed". Other than that, I've been very careful to indicate the gradient is with respect to speed.
I see this from FAR 25..
"(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots"
Does my post 9 make sense now?
If not, then I offer the following:
Genghis, when I said thrust is reduced, I meant that the aircraft is to maintain level flight. I said this in para 1, but did not repeat it in para 2.
I see this from FAR 25..
"(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots"
Does my post 9 make sense now?
If not, then I offer the following:
Genghis, when I said thrust is reduced, I meant that the aircraft is to maintain level flight. I said this in para 1, but did not repeat it in para 2.
By level I meant maintain altitude, though I realize that I did not state that. And hence airspeed must decrease. I was trying to take a simple case.
But as to why it is necessary (other than the certification requirements) to have a 1 lb per 6 knot stick force change I can't understand.
Surely the aircraft would be very easy to fly without this stick force gradient with respect to speed (maintain altitude, very slowly increase pitch as the speed drops, then maintain this new attitude for the assigned altitude, as the speed settles in at about 240 kias).
MFS mentions the Greek Falcon 900 accident, and though I haven't read it again recently, my memory tells me this was more of a stick force per inch of travel type of problem,
or stick force per G. Not a changing of stick force per knot.
I believe the falcon was at something like 330 kias, and if the speed was decreasing at the time, it was not the decreasing speed that gave the pilot difficulty.
Mr Tullarmarine, you ask "but how are you intending to exercise any sort of control over what the aircraft is doing .. ie other than just being along on a roller coaster ride ?"
I'd say simple. I adjust pitch up or down to maintain my assigned altitude, and adjust the throttle to maintain my assigned speed. If I'm asked to slow from 250 to 240 kias, I pull the levers back a tad and crosscheck. I don't see why I can't do this very easily with a zero stick force per knot gradient. Sounds sort of nice, like an auto trim system engaged.
I'd say simple. I adjust pitch up or down to maintain my assigned altitude, and adjust the throttle to maintain my assigned speed. If I'm asked to slow from 250 to 240 kias, I pull the levers back a tad and crosscheck. I don't see why I can't do this very easily with a zero stick force per knot gradient. Sounds sort of nice, like an auto trim system engaged.
Are you possibly flying an aircraft in altitude hold engaged - so when you change the power, the AFCS modifies pitch to maintain altitude - creating a reduction in airspeed.
You also mention the TP incident, but I do not see how stick force per knot is a player here. Sounds like control was difficult even while at a constant speed, so a changing stick force per knot should not have been the root of his problem (which was admittedly that the loading was outside the envelope.)
"Don't feel too bad about this stuff confusing you a tad". Yep, I'm confused
G
Moderator
I'd say simple. I adjust pitch up or down to maintain my assigned altitude, and adjust the throttle to maintain my assigned speed.
If we introduce thrust changes, we start to complicate the basics a little due to pitching moments as indicated by Genghis. However, again I would ask you to have a coffee and contemplate just how you intend to exercise any sort of control with a very low gradient. From an on speed position, any inadvertent control input (or turbulence, whatever) will send you off on a speed variation. With little or no tactile feedback (the human system is not terribly good at figuring displacement but is reasonably fine with force) the aircraft speed and flightpath will wander hither and thither unless you are constantly directing cognitive effort to the flying task. If the gradient reverses, the workload goes up dramatically.
I go back to my analogy of trying to sit on a big beach ball out in a moderate sea ... a bit beyond my meagre gymnastic capabilities, I fear. With a decent stick force gradient, it becomes a bit like putting down a keel on a yacht.
I'm also aware that this was a problem with the prototype F-16 which had no movement at-all.
If I recall correctly from a course many years ago, the F16 initial problem was put down to having too low a stick break out force. Once this was addressed the twitchiness resolved itself. A bit like trying to hold your hand out steadily with or without some external support.
I'm unfamiliar with the specific accident
They got back on the ground fine as far as I am aware .. just a bit less than impressed with the situation. My understanding was that the main problem was associated with the reversed force gradient and conditioned responses. Genghis almost certainly would know the TP concerned.
Contrary to what Mr Tullamarine may assert
My comment was intended to be tongue in cheek .. knowing Genghis' background, I defer to his far greater knowledge and experience in this matter.
If we introduce thrust changes, we start to complicate the basics a little due to pitching moments as indicated by Genghis. However, again I would ask you to have a coffee and contemplate just how you intend to exercise any sort of control with a very low gradient. From an on speed position, any inadvertent control input (or turbulence, whatever) will send you off on a speed variation. With little or no tactile feedback (the human system is not terribly good at figuring displacement but is reasonably fine with force) the aircraft speed and flightpath will wander hither and thither unless you are constantly directing cognitive effort to the flying task. If the gradient reverses, the workload goes up dramatically.
I go back to my analogy of trying to sit on a big beach ball out in a moderate sea ... a bit beyond my meagre gymnastic capabilities, I fear. With a decent stick force gradient, it becomes a bit like putting down a keel on a yacht.
I'm also aware that this was a problem with the prototype F-16 which had no movement at-all.
If I recall correctly from a course many years ago, the F16 initial problem was put down to having too low a stick break out force. Once this was addressed the twitchiness resolved itself. A bit like trying to hold your hand out steadily with or without some external support.
I'm unfamiliar with the specific accident
They got back on the ground fine as far as I am aware .. just a bit less than impressed with the situation. My understanding was that the main problem was associated with the reversed force gradient and conditioned responses. Genghis almost certainly would know the TP concerned.
Contrary to what Mr Tullamarine may assert
My comment was intended to be tongue in cheek .. knowing Genghis' background, I defer to his far greater knowledge and experience in this matter.
Join Date: Jun 2005
Location: Mobile, AL
Posts: 50
Likes: 0
Received 0 Likes
on
0 Posts
Fabulous discussion...and worth reading twice
During my TP training we were given the opportunity to fly three sorties in a variable stability Lear Jet while we tried to wrap our heads around the concepts discussed above. We were given a number of exercises where the student was given a 'mystery' aircraft - i.e. the fly by wire system of the Lear was programmed to mimic a certain set of flight control laws - and he/she had to puzzle out the handling qualities of the aircraft, deliver a verdict on problems observed, and then suggest a fix.
One of the areas that came up frequently (and that I have also dealt with several times in my career as a TP to date) was that of relaxed or reduced longitudinal stability. We were given several examples of both neutral and negative apparent static longitudinal stability to fly, as well as several with negative longitudinal stability under manoevure. The description posted earlier of the technique required to fly these 'simulated' aircraft was spot on - sharp, direct, and continuous inputs in an effort to keep up with the aircraft. Needless to say, we were most often unsuccessful and the safety systems of the variable stability Lear disconnected our controls to prevent us from overstressing the aircraft. With a bit of practice it was certainly possible to fly these aircraft, but the workload was high and if one did not have a clear understanding of what was happening at the time the end result was inevitable.... Flight in cruise is one matter; flight in the slow or high speed regimes is quite another and it is often during an 'upset' that relaxed stability renders a crew's trained reactions ineffective.
Not to muddy the waters further, but it is important in these discussions to have an accurate appreciation of the control system of the aircraft. The C150, with its convention/reversible control system where one sets and commands an angle of attack (alpha stable) is a very different beast from the A340 with a fly-by-wire system and flight path stability, or the C17 which uses its FBW system to command a pitch attitude (theta stable). I don't throw this out to confuse the issue, but rather to point out that there can be a myriad of reasons why a 'simple'
subject like stability can mean so many different things in different situations.
Pilots expect their aircraft to behave in a safe and predictable manner - it is the purpose of the certification guidelines (such as minimum stick force gradients) to help ensure that they do! Force gradients are one way that an aircraft provides the pilot feedback - without him or her ever having to look at an instrument of crosscheck.
During my TP training we were given the opportunity to fly three sorties in a variable stability Lear Jet while we tried to wrap our heads around the concepts discussed above. We were given a number of exercises where the student was given a 'mystery' aircraft - i.e. the fly by wire system of the Lear was programmed to mimic a certain set of flight control laws - and he/she had to puzzle out the handling qualities of the aircraft, deliver a verdict on problems observed, and then suggest a fix.
One of the areas that came up frequently (and that I have also dealt with several times in my career as a TP to date) was that of relaxed or reduced longitudinal stability. We were given several examples of both neutral and negative apparent static longitudinal stability to fly, as well as several with negative longitudinal stability under manoevure. The description posted earlier of the technique required to fly these 'simulated' aircraft was spot on - sharp, direct, and continuous inputs in an effort to keep up with the aircraft. Needless to say, we were most often unsuccessful and the safety systems of the variable stability Lear disconnected our controls to prevent us from overstressing the aircraft. With a bit of practice it was certainly possible to fly these aircraft, but the workload was high and if one did not have a clear understanding of what was happening at the time the end result was inevitable.... Flight in cruise is one matter; flight in the slow or high speed regimes is quite another and it is often during an 'upset' that relaxed stability renders a crew's trained reactions ineffective.
Not to muddy the waters further, but it is important in these discussions to have an accurate appreciation of the control system of the aircraft. The C150, with its convention/reversible control system where one sets and commands an angle of attack (alpha stable) is a very different beast from the A340 with a fly-by-wire system and flight path stability, or the C17 which uses its FBW system to command a pitch attitude (theta stable). I don't throw this out to confuse the issue, but rather to point out that there can be a myriad of reasons why a 'simple'
subject like stability can mean so many different things in different situations.Pilots expect their aircraft to behave in a safe and predictable manner - it is the purpose of the certification guidelines (such as minimum stick force gradients) to help ensure that they do! Force gradients are one way that an aircraft provides the pilot feedback - without him or her ever having to look at an instrument of crosscheck.
Moderator
it is often during an 'upset' that relaxed stability renders a crew's trained reactions ineffective
.. which possibly may have some pertinence to a recent hull loss ? No knowledge but it is an interesting speculation.
.. which possibly may have some pertinence to a recent hull loss ? No knowledge but it is an interesting speculation.
Join Date: May 2003
Posts: 409
Likes: 0
Received 0 Likes
on
0 Posts
Well thanks for the inputs, Mr Tullamarine and Genghis. Some times you just have to believe what others in the industry tell you. This seems to be one of them. Guess I'll never experience what it's actually like, unless I magically get beamed into a cockpit that has a zero stick force gradient with respect to speed.
Join Date: Jun 2005
Location: Mobile, AL
Posts: 50
Likes: 0
Received 0 Likes
on
0 Posts
Actually...you can experience something very similar in your car (this is why test pilots should have their driving licenses removed upon qualification 
).
Compare, for example, a smaller import vehicle with one of the big American sedans with power steering that lets you turn the wheel with one finger.
Find a stretch of road with a long curve, and then put the driver's side wheels onto the white line and keep them there. Be as accurate as you can, and accept very little error. You will find that the task requires much less conscious thought, fewer wheel inputs, and is essentially 'easier' in the vehicle with the 'heavier' steering (to a point). Now, move the wheels off the white line and attempt to recapture it fairly quickly - same result. This illustrates very nicely the concept of force gradient compared with task workload.
Disclaimer: You do not need to be going fast or on sharp curves to try this out. If you are going to try anything like this please be prudent - I know it should go without saying...but this is the internet...I post this here strictly to illustrate a concept under discussion in a situation that most pilots can relate to. I accept no responsibility for cars flying off cliffs because someone wanted to try a test method...
).Compare, for example, a smaller import vehicle with one of the big American sedans with power steering that lets you turn the wheel with one finger.
Find a stretch of road with a long curve, and then put the driver's side wheels onto the white line and keep them there. Be as accurate as you can, and accept very little error. You will find that the task requires much less conscious thought, fewer wheel inputs, and is essentially 'easier' in the vehicle with the 'heavier' steering (to a point). Now, move the wheels off the white line and attempt to recapture it fairly quickly - same result. This illustrates very nicely the concept of force gradient compared with task workload.
Disclaimer: You do not need to be going fast or on sharp curves to try this out. If you are going to try anything like this please be prudent - I know it should go without saying...but this is the internet...I post this here strictly to illustrate a concept under discussion in a situation that most pilots can relate to. I accept no responsibility for cars flying off cliffs because someone wanted to try a test method...
Well thanks for the inputs, Mr Tullamarine and Genghis. Some times you just have to believe what others in the industry tell you. This seems to be one of them. Guess I'll never experience what it's actually like, unless I magically get beamed into a cockpit that has a zero stick force gradient with respect to speed.
Or if you're near LHR in the next few months, a colleague of mine is looking at risks of light aircraft departures from controlled flight as a function of apparent LSS gradients using a simulator near there. He's rotating pilots of variable experience levels through it, and I'm sure would be delighted to have another volunteer - particularly since it's much easier to get PPL volunteers than high hour ATPLs and for the research to work he needs a good spread of ability levels.
G
RAE Bedford investigated ‘relaxed’ longitudinal stability in their 1-11 (XX 105) – late 70’s. The aft cg was part simulated and part real. For real, it was quite a long way back – the tech log was annotated “do not use aft stairs or doors” !!
The next disconcerting point was that the simulation used an experimental auto control system (autopilot servos?) which had to be programmed by a ‘boffin’ using a computer patch-board (the size of a large Lego board) and covered in wires as if arranged in a cat’s cradle.
Anyway, the system was a delight to fly. The pitch control was attitude demand with auto trim. There was no attitude change with speed change, yet speed control was not onerous.
I can’t recall the stick forces, probably no change from the basic aircraft feel system, but overall the modified ‘speed’ stability did not cause any problems. The auto throttle was used fairly extensively, but manual flight was typically ‘fighter like’; point the aircraft with one hand, adjust the speed with the other. AFAIK, the control laws were similar to those used by Airbus in their tests / validation work pre A320.
I have a faint recollection that a flight-path demand control law was also flown, where again speed stability was not an issue. Also, (from a weak memory) there were tests of direct lift control where pitch demand resulted in lift change without attitude change – the stick moved the spoilers in / out, the auto system compensated with a pitch-maintaining elevator movement. The majority of this test work was flown independently of the aft cg work and the system was assessed during steep approaches – flight path accuracy and low flare ht.
The pitch demand system was evaluated throughout out the flight envelope (0-2g), excepting stalls (for obvious reason in the 1-11). The only notable problem was during landing where the pitching axis changed from the cg to the main wheels at touch down. The control system adjusted for the resulting nose down pitch with a back stick input which ‘skipped’ the aircraft back into the air. A quick patch-board change fixed that. IIRC one of the FBW fighter projects ‘discovered’ a similar problem.
The BAe owned 1-11 (G AY.. ) flew an experimental fly-by-light control system (alongside the conventional controls) using same/similar control laws as did RAE.
Tests on this aircraft were at ‘real’ aft cg, achieved by moving lead wts in flight. The emergency recovery procedure was for the FTE to pick up two 50 lb wts at the rear of the aircraft and run forward! My recollection from only one flight was that the handling was similar to the RAE tests; low altitude / landing tests were not flown – for other obvious reasons.
With hindsight, … … we did some very serious ‘silly’ things, but safely, and they were great fun.
Many of these tests were addressing the type of question starting this thread; those which were answered positively eventually have been used in aircraft. The process is slow and cautious, with small, safe steps, even if some appear giant leaps.
The next disconcerting point was that the simulation used an experimental auto control system (autopilot servos?) which had to be programmed by a ‘boffin’ using a computer patch-board (the size of a large Lego board) and covered in wires as if arranged in a cat’s cradle.
Anyway, the system was a delight to fly. The pitch control was attitude demand with auto trim. There was no attitude change with speed change, yet speed control was not onerous.
I can’t recall the stick forces, probably no change from the basic aircraft feel system, but overall the modified ‘speed’ stability did not cause any problems. The auto throttle was used fairly extensively, but manual flight was typically ‘fighter like’; point the aircraft with one hand, adjust the speed with the other. AFAIK, the control laws were similar to those used by Airbus in their tests / validation work pre A320.
I have a faint recollection that a flight-path demand control law was also flown, where again speed stability was not an issue. Also, (from a weak memory) there were tests of direct lift control where pitch demand resulted in lift change without attitude change – the stick moved the spoilers in / out, the auto system compensated with a pitch-maintaining elevator movement. The majority of this test work was flown independently of the aft cg work and the system was assessed during steep approaches – flight path accuracy and low flare ht.
The pitch demand system was evaluated throughout out the flight envelope (0-2g), excepting stalls (for obvious reason in the 1-11). The only notable problem was during landing where the pitching axis changed from the cg to the main wheels at touch down. The control system adjusted for the resulting nose down pitch with a back stick input which ‘skipped’ the aircraft back into the air. A quick patch-board change fixed that. IIRC one of the FBW fighter projects ‘discovered’ a similar problem.
The BAe owned 1-11 (G AY.. ) flew an experimental fly-by-light control system (alongside the conventional controls) using same/similar control laws as did RAE.
Tests on this aircraft were at ‘real’ aft cg, achieved by moving lead wts in flight. The emergency recovery procedure was for the FTE to pick up two 50 lb wts at the rear of the aircraft and run forward! My recollection from only one flight was that the handling was similar to the RAE tests; low altitude / landing tests were not flown – for other obvious reasons.
With hindsight, … … we did some very serious ‘silly’ things, but safely, and they were great fun.
Many of these tests were addressing the type of question starting this thread; those which were answered positively eventually have been used in aircraft. The process is slow and cautious, with small, safe steps, even if some appear giant leaps.
Join Date: Jun 2005
Location: Mobile, AL
Posts: 50
Likes: 0
Received 0 Likes
on
0 Posts
I did some variable CG work at EPNER in their Nord 262. There were two large water tanks in the aircraft and the test engineer pumped water back and forth. I have rarely laughed so hard as on the day when he came into the cockpit soaked from head to toe to announce a malfunction of the system!!
Moderator
However .. one would highlight the use of electronics/variable ballast to make the above work or provide a get-out-of-jail-free card ...
... the problem becomes the pilot's when the electronics/ballast systems fail... and, from Mark's tale .. on occasion, the FTE's.
... the problem becomes the pilot's when the electronics/ballast systems fail... and, from Mark's tale .. on occasion, the FTE's.
