Modern Transport Aircraft Stability Question

[VinRouge]

Simple one.

With the advent of modern FBW control systems, together with the (significant) fuel saving advantage of flying with an aft CofG, why are modern FBW transport aircraft not designed to be longitudinally unstable in the cruise with an aft co of G? I hear some of the modern airbus aircraft schedule fuel to push the CofG rearwards, but are they fully longitudinally unstable? As I understand it, efficiency savings from tailplane up force are similar to the savings from winglets (on my type, we are talking 4% of cruise burn for most rearward C of G).


Would FARs/JARs prevent it from happening?

Or are there concerns with fallback modes (in particular 4 eng flame out/complete electrical failure case, although FADEC engines typically hold independent PMAs for this case)?

I would imagine there would be a maximum limit to the benefit from this, when the alpha at the tailplane reaches a stage that it starts generating more drag.

Any insight from People in the industry?

[Dit]

Simple, no they're not unstable in pitch. This is easily proved: the a/c is flyable by pitch trim alone if necessary.

[john_tullamarine]

I can't speak to specific aircraft without numbers .. however, generically

(a) computers can fly an unstable aircraft due to the far greater sampling rate and control input discrimination

(b) for the MIL FJ fraternity Martin Baker provides a level of redundancy

(c) for the civil world one could reliably not see other than acceptably longitudinally stable aircraft being certificated

(d) I suspect that the emphasis is on keeping the CG towards the aft region of the certificated envelope for the fuel savings

(e) the real question of interest is the pilot workload level if the automatics fail and the human pilot is required to save the show on the day


the a/c is flyable by pitch trim alone if necessary

Keep in mind that long stab is to do with the pitch stick forces and stick force gradients which the pilot perceives rather than what the controls are doing physically.

[hawk37]

So...Capt Tullamraine, if you could comment, what would be the problems a pilot would face flying a "conventional" aircraft with a near zero pitch force gradient? If that was the situation for a particular C of G, mach range, for example. And for simplicity, assuming of course no auto trim, no mach trim etc.

Seems to work ok for airbus pilots.

What if the 737 auto trimmed for zero stick force?

[Dit]

Keep in mind that long stab is to do with the pitch stick forces and stick force gradients which the pilot perceives rather than what the controls are doing physically.

Really? I thought it was the aircrafts tendancy to return to its current state if disturbed.

[john_tullamarine]

Hhhmmm ... that got some reactions ...

So...Capt Tullamraine,

.. actually wearing my Consulting Engineer's hat at the moment.

what would be the problems a pilot would face flying a "conventional" aircraft with a near zero pitch force gradient?

A very difficult aircraft to fly. Unacceptably low stick force/g. Result is that the pilot is set up to overstress the aircraft .. and maybe rip the wings off. Not a good life strategy.

What if the 737 auto trimmed for zero stick force?

You are confusing two things here, unfortunately. Long stab is about STARTING from a trimmed on-speed condition and then looking at the stick forces needed to maintain off-trim speeds.

(a) if you use the stick to fly slower, you MUST have a pull force. Conversely, if you fly faster, you MUST have a push force.

(b) if you release the stick force steadily the aircraft MUST resume a speed somewhere near the original trim speed

(c) the gradient has to be acceptable and MUST NOT reverse. Otherwise one needs to look at SAS kit such as you see on some light aircraft turboprop conversions. Generally, this is to address a stick force problem during the missed approach.

I thought it was the aircrafts tendancy to return to its current state if disturbed.

Common misconception associated with poorly executed basic theory training using the "ball-in-a-teacup" analogy. Nothing wrong with the analogy .. just that it is explained incorrectly most times. Your statement is a part of the story only .. see (b), above.

If you are interested, the current words for FAR are at

(a) FAR 25.173 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=67240b78b1d644a171cbdcc7b988c200&rgn=div8&view=text&node=14:1.0.1.3.11.2.158.27&idno=14) for the basic reg,

(b) FAR 25.175 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=67240b78b1d644a171cbdcc7b988c200&rgn=div8&view=text&node=14:1.0.1.3.11.2.158.28&idno=14) for some specifics, and

(c) AC 25-7A (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/c2614e27b49bf38686256ba300696689) for the testing.

There are a few matters where the certification bits get a little smudged by the time the pilot training system gets to it ... leading to the odd misconception amongst the piloting fraternity.

[ALK A343]

Getting back to the A330/340 fuel transfer. I haven't flown them for some time now, but back then I noticed that with fuel in the trim tank the horizontal stabilizer used to stay at around 2 degrees up with the C.G. maintained between 38-40 % MAC through forward fuel transfer, well within the normal C.G. range. So there was never a situation when you would have zero down-force on the stabilizer and thus the aircraft was always stable in the longitudinal axis. All the aft transfer would do is reduce the down-force to a certain extent and the associated induced drag. It is similar to loading an aircraft without trim tank to achieve an aft C.G., the difference is that the trim tank allows you to maintain the optimum C.G. location for a longer time as fuel is burned in flight.
If I remember right the only problem would be a failure of the forward fuel transfer. In that case the fuel in the trim tank would shift the C.G. further aft as fuel is burned in the main tanks and you would leave the normal C.G. envelope after some time, I think the time limit was 4 hours and there is a QRH checklist for that. I guess if you exceed this time yo u might end up with reduced longitudinal stability.

[VinRouge]

Thanks all, answered the question.

[hawk37]

Mr Tullarmarine, when I mentioned "near zero pitch force gradient" I was referring to the change in stick force with a change in airspeed. Perhaps an unfortunate choice of words on my part. I was not referring to the "stick force/g" you referred to.
Perhaps "stick force gradient" is the proper terminology, while implying that the gradient is respect to a changing airspeed.
I realize you have said that these stick forces must meet certain conditions with changing airspeed, but I was wondering why? What would be the problems a pilot would face flying a "conventional" aircraft with zero change in stick force with changing airspeed. For example, a 737 with an auto trim system.
Thanks, Hawk

[poina]

Red Wine, I'm a little confused by your stating the manu. to make their planes unstable by aft c/g. All a/c are flt tested to an acceptable c/g range. The more forward the a/c is loaded the more stable it is. Futher back in the c/g range decreases stability but increases fuel economy.
The MD-11 works like this, after t/o and clean the fuel system controller starts to transfer fuel to the tail tank to control to an aft c/g to reduce the need for nose up stab which is a source of drag, doing this nose up input by weight of fuel instead. Of course, all this time the auto-trim is taking out all steady state elevator loads. Don't kid yourself, an aft c/g can ruin your day in a hurry, especially if combined with an altitude selection above optimum. Do a search on MD-11 high alt upsets of which there are many, all caused by aft c/g, above opt, with turb.

[john_tullamarine]

I was not referring to the "stick force/g" you referred to.

Then, perhaps, my explanation was inadequate as well, confusing the two considerations .. the concern with static stability is for the pilot to perceive a force tending to return the aircraft to its trim speed condition and is directly related to stick force against speed delta from the trim speed condition. Keep in mind we are talking about stick forces not stick displacement or position. The basic considerations will be the same, whether for a C172 or a B737.

Unfortunately, I have not had the opportunity to fly a variable stability research aircraft so I am unable to provide a detailed "pilot's eye view" of how the experience unfolds.

Perhaps Genghis, or one of the TP fraternity within our group who has done so can offer comment ?

The matter comes down to experimental flying handling qualities assessment, which is a little out of my direct ambit. I did have an incident in my early flying where, due to carelessness on my part, I misloaded an Apache somewhat outside the forward limit. On that occasion, the aircraft was quite flyable but unpleasantly so. The experience, however, caused me to be far more attentive to loading thereafter... certainly, I am only too happy to stay well away from a significant aft CG misloading configuration.

Consider variation from an acceptably stable aircraft, and what that might mean to the pilot's problems, to gain an initial insight into the answer you seek.

We all have a small understanding from our normal flying in that stick force gradient varies within a small range from higher (at forward CG) to lower (at aft CG). This is more so with larger transports having a larger approved CG range and, for these, the effect should be easily observable by the pilot

If the stick force gradient increases signficantly, the aircraft becomes physically tiring and difficult to manoeuvre with an associated concern regarding detail design loads on tail and control run assemblies. A bit like flying a truck, as it were.

If the stick force gradient reduces towards zero, the aircraft becomes progressively more imprecise, tiring, and anxiety producing for the pilot to fly. A bit like trying to ride a big beach ball in a disturbed sea, I should imagine ..

If the stick force reverses (static instability), then the workload to fly is increased dramatically. Recall this means that, from the trim speed condition, a decrease/increase in speed (due to whatever initiator - not necessarily pilot input) then requires a varying push/pull load to maintain below/above trim speed speeds.

Think carefully about that for a moment and the hand-brain stress associated with keeping such an aircraft under control given that all our experience, intuition and exposure is for the reverse loading pattern. Think, also, about the immediate adverse consequences if the pilot gets it wrong .. which is the usual outcome.

I can relate the description given by one TP who was caught in such a predicament during a flight test as a consequence of inadvertent gross misloading for the test point. His observations were that

(a) unless one recognised (immediately) what the problem was .. and the technique needed to control it, the outcome would be quick and inevitable. Certainly, the accident records have many examples of loss of control (with a subsequent hull loss) following significant loading excursions outside the aft CG - a commonplace initiator for an unacceptable static stability condition.

(b) flying technique involved numerous short, sharp control inputs to put the nose where it needed to be (ie stick movement remains normal) and then freezing the control position to avoid the variable loading perceptual problem.

He managed to continue this long enough to drag the aircraft around the circuit and throw it back onto the ground. His commentary suggested that it would not be desirable to have continued for much longer in such a high workload situation. If one notes that this particular TP is a very experienced trainer of TPs .. puts the observation in some sort of perspective, I think ...

Although not specific in his comments .. he made some side references to a (presumably) quite robust technical discussion immediately following with his FTE .. the latter had been responsible for the loading calculation error. One certainly gained the impression that the particular TP was less than happy about the hazard to which the program and crew had been exposed.

Those of us who don't venture outside acceptable handling qualities in our flying are protected by the collective and historical wisdom gained by those who have done so in the past and continue to do so now.

[Genghis the Engineer]

Telepathic hint to join in received from JT!

To get a handle on this discussion, I think that it's important to start by appreciating that longitudinal static stability has a large number of flavours, the main five being:

Aerodynamic stick free
Aerodynamic stick fixed
Apparent stick free
Apparent stick fixed
Manoeuvre stability

These are all linked, but separate.

The first two are what the aerodynamics textbooks talk about when they consider LSS but from pilots perspective they aren't all that important because designers have lots of mechanisms to transform those aerodynamic characteristics into different apparent characteristics

The characteristic which (rightly in my opinion) is the main concern of all of the certification codes is apparent stick free longitudinal static stability. This is measured by stick force per airspeed change. It's important for the simple reason that pilots perceive control forces much more than they do control positions, and really couldn't care less about aerodynamics so long as it's all sorted out by the time it reaches the cockpit.

There are various bits of guidance out there as to what is an acceptable gradient for aLSS; for example a well known textbook by a chap called Jan Roskam recommends a minimum of 6kn/lb; and when I have in the past been involved in certifying light training aeroplanes I had a personal working minimum of 0.1 daN/kn, which comes out pretty much at Roskam's number if you convert it. Most transport aeroplanes are likely to want a higher gradient than that but, it's important to remember, this is only what is perceived at the cockpit.


However, the designer and airline accountant have other views on aerodynamic stability. In simplistic terms, at forward CG the tailplane is providing a downforce and the further aft you move CG, the less that force is. That force pushes performance down and fuel burn up, so it's a bad thing (to airline accountants) so there will always be pressure to move it back. A CG of around 40%MAC (as mentioned by ALK A343) will give a neutral to slightly negative aerodynamic LSS; such an aircraft is flyable aerodynamically even without stability augmentation (not a heavy, but most of us have flown a C150 at some point - at mid-aft or aft CG, 30+ flaps, 65%+ power, that is statically unstable, but some very low ability pilots manage to fly go-arounds in it safely). However, the modern designer will provide a Stability Augmentation System (SAS) to modify that in a large aircraft so that by the time the pilot sees the longstab, an unstable aerodynamic longstab which would be unpleasant and push workload up, has become "just another airbus".

All this is done in the name of efficiency - you could push the CG forward, not bother with the SAS, and have a flyable, but less efficient aircraft.


Having said that, part 25 contains requirements that in the event of loss of a single axis of primary aerodynamic control, the aircraft must still be controllable. For most airliners, the design philosophy this forces is that in the event of an elevator / stabilator control failure, the aircraft is designed to be flyable on the pitch trimmer - not a lot of fun, but doable. This is doable by ensuring that the aircraft remains flyable (just!), by not allowing aerodynamic stability to become too negative. - hence an aft limit of around 40%, rather than (say) 50).


Stick force per g (what I've referred to above as manoeuvre stability) is a bit of a red-herring here. It's a related but separate entity. Essentially it's a function of how much pull is needed to pull how much g - that little bit of back pressure that we all know we need to apply in a level turn for example. Airworthiness standards since the 1950s have pretty much all required the same - that you have to pull at-least 15lbf to reach the positive g limit. This requirement is generally a trivial one since most aircraft in any class, which have acceptable LSS, will comfortably exceed this requirement. We were entertained the other day by discovering a research aircraft flying at a weight of about 37 tonnes flying 2g turns, started to display neutralish manoeuvre stability, as result of which 12 scientists up the back all got treated to 2.4g - but that's my problem!).

G

[john_tullamarine]

As Genghis observes, stick force/g is not directly tied to long stab certification concerns. Perhaps I should have been a little more expansive in my earlier post.

However, it is important to note that the factors which affect long stab likewise affect stick force/g .. the concern for the pilot being that, as well as having stick force problems, per se, the increasing manoeuvre capability might well see an inadvertent overstess in the pitching plane .. even a structural failure. Genghis' boffin colleagues are a point to observe ..

[VinRouge]

poina, my original idea related to modern FBW systems that can easily handle the unflyable condition created by a C of G well aft of the Aerodynamic centre.

[hawk37]

Mr Tullamarine and Genghis have provided a great deal of info. But I must be missing something here....

Academically speaking, if an aircraft is flying level at 250 kias and is trimmed, then there is no force required on the stick to keep it level. With slight changes to the flight path due to a variable air mass, the pilot can gently push and/or pull on the stick to maintain the altitude. The pilot can fly the aircraft without a problem.

Now the thrust is reduced slightly and the aircraft decelerates to 240 kias. Now there is a pull force required by the pilot to maintain straight and level flight. This pull force will likely vary slightly due to a variable air mass, but likely to remain a pull force (vice a push). Again, the pilot can fly the aircraft without a problem, although there is a trim system to help him which he would normally use.

So if I was flying this aircraft manually, and it had a zero stick force gradient, then there would be no trim required as the speed was reduced from 250 to 240. But it would surely be easily controllable.

So why is there a need for a (minimum?) positive stick force gradient of XXX lbs per knot?

VinRouge,

I'm assuming you mean A320/330 etc. Do you really think the aft CG condition allowable by these aircraft would be "unflyable" with conventional hydraulic/mechanical rods control? I can see that the the aircraft may not pass present certification criteria, but you said "unflyable".

F16 FBW not considered here, of course.

[Port Strobe]

hawk37
So if I was flying this aircraft manually, and it had a zero stick force gradient, then there would be no trim required as the speed was reduced from 250 to 240. But it would surely be easily controllable.

JT says
If the stick force gradient reduces towards zero, the aircraft becomes progressively more imprecise, tiring, and anxiety producing for the pilot to fly...

I can relate the description given by one TP who was caught in such a predicament during a flight test as a consequence of inadvertent gross misloading for the test point. His observations were that

(a) unless one recognised (immediately) what the problem was .. and the technique needed to control it, the outcome would be quick and inevitable. Certainly, the accident records have many examples of loss of control (with a subsequent hull loss) following significant loading excursions outside the aft CG - a commonplace initiator for an unacceptable static stability condition.

(b) flying technique involved numerous short, sharp control inputs to put the nose where it needed to be (ie stick movement remains normal) and then freezing the control position to avoid the variable loading perceptual problem.

He managed to continue this long enough to drag the aircraft around the circuit and throw it back onto the ground. His commentary suggested that it would not be desirable to have continued for much longer in such a high workload situation. If one notes that this particular TP is a very experienced trainer of TPs .. puts the observation in some sort of perspective

Apparently it's not easily controllable. In reality how many lots of 4% of trip fuel saved (it was likened to winglets earlier so I'll take that at face value, I don't know enough to say otherwise) is an aircraft hull and all those aboard worth when it all turnd turtle ? I'm all in favour of pushing things forward but a sensible balance must be maintained I'd think. Although JT's TP friend story involves negative stability, then neutral stability must be around halfway bewteen where we're at and that episode described. I'd rather keep as far away as possible from handling characteristics like those for as long as possible thanks. Just my 2c worth, more than willing to be educated if I've missed something big though.

[john_tullamarine]

So if I was flying this aircraft manually, and it had a zero stick force gradient, then there would be no trim required as the speed was reduced from 250 to 240. But it would surely be easily controllable.

Don't feel too bad about this stuff confusing you a tad .. as it did me as an undergraduate for some time. (It's OK for bright lads like Genghis who pick up this stuff more easily than do we mere mortals).

Consider, with a zero stick force gradient, that the airspeed, once varying, will continue to vary hither and thither depending on the aircraft's characteristics. All very well to think about slowing down .. 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 ?

The workload on the pilot TO MAINTAIN a speed (which is pivotally important to flying) increases significantly and, in the limit, becomes impossible for the human pilot. The aircraft is, indeed, quite easily controllable (which means, in flying, that you can manoeuvre with relative ease) .. however, have a think about how you might intend to fly with any chance of doing anything else other than trying to get the aircraft to do something along the lines of whatever you might intend it to do. That is to say, stability and control go together as antagonistic bedfellows.

The TP story involved a VERY experienced and competent TP .. one with whom I have flown in the past during a training course and could only hope to emulate in some small manner. If HE finds the thought daunting then you and I would find the actual experience more along the line of frightening I suspect.

F16 FBW not considered here, of course

Therein lies the problem .. the MIL FJ driver has an in-aircraft alternate available if it gets sticky. The question is how far do you push it in an airliner, and how much testing validation is appropriate, before you get to a new limit for the electronics which you deem to be OK from a risk viewpoint ?

In reality how many lots of 4% of trip fuel saved

You pick up the bulk of the dollars within the approved CG limits and that is a sensible operating technique.

With some fancy electronic footwork and deft certification negotiation you might get a little bit further than that. No problem on a risk basis .. but, if the software proves to be less capable than at first thought .. it might provide some unpleasant surprises.

[Mad (Flt) Scientist]

Here's one example of why positive longitudinal stability is pretty important
:
Falcon 900B accident (http://aviation-safety.net/database/record.php?id=19990914-2)

Crew: Fatalities: 0 / Occupants: 3
Passengers: Fatalities: 7 / Occupants: 10
Total: Fatalities: 7 / Occupants: 13
....
Between FL150 and FL140, for approximately 24 seconds, the aircraft experienced 10 oscillations in pitch axis which exceeded the limit manoeuvring load factor. Maximum recorded values were: +4.7 g and -3:26 g.
....
CAUSAL FACTORS:
1. Inadequate risk assessments of the PITCH FEEL malfunctions.
2. Overriding of the A/P on the pitch channel by the crew.
3. Inappropriate inputs on the control column at high speed and with Arthur unit failed in 'low-speed' mode leading to Pilot Induced Oscillations.
4. Seat-belts not fastened during descent flight phase.

They didn't even have no feel; they just had reduced feel, and it rendered the aircraft all-but uncontrollable in pitch.

[Genghis the Engineer]

Mr Tullamarine and Genghis have provided a great deal of info. But I must be missing something here....

Academically speaking, if an aircraft is flying level at 250 kias and is trimmed, then there is no force required on the stick to keep it level. With slight changes to the flight path due to a variable air mass, the pilot can gently push and/or pull on the stick to maintain the altitude. The pilot can fly the aircraft without a problem.

Now the thrust is reduced slightly and the aircraft decelerates to 240 kias. Now there is a pull force required by the pilot to maintain straight and level flight. This pull force will likely vary slightly due to a variable air mass, but likely to remain a pull force (vice a push). Again, the pilot can fly the aircraft without a problem, although there is a trim system to help him which he would normally use.

Well taking your last paragraph first - if thrust is reduced slightly, any effects of change in speed are not a function of change of thrust as such - if thrust acts through the vertical CofG then there will be no change in speed. In fact, in most airliners, the thrustline lies below the CG, so reducing thrust will cause a slight nose-down pitching moment, and thus a small increase in speed.

However, for the sake of argument, you've changed thrust in an aircraft with a low thrustline and wish to adjust flightpath, either to maintain speed (pull), or to maintain level flight (more pull). Imagine that there is no stickforce gradient, then how can you correctly judge and hold the pull to maintain speed, or a bit slower to maintain level flight, and not to pull too much and bring the aircraft back to a thoroughly unwanted 150 knots? You can do it, but only by a high workload task with constant reference to instruments - whereas with a stick force you can feel where that is, maintain it, then trim out in good time. Also it's that stickforce / LSS gradient which stops the aircraft constantly drifting off the speed you've trimmed the aircraft to - otherwise speed will start drifting around whenever the pilot is not actively monitoring and controlling it.

G

[hawk37]

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. 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.
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

[Genghis the Engineer]

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.

That was my understanding of your post, yes.

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.

Again, yes, my understanding also.

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.

In order to give the pilot a resolution of changes in speed with stick force which is reasonably perceptible. An aeroplane without that sort of gradient is still flyable, but pilot workload to maintain fine speed control goes up, and ability to fly by attitude alone goes down.

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).

The odds are that without a stick force gradient, you'd lose the trimmed airspeed with change in attitude and pilot workload to re-establish this would be significant. In "steady" flight the relationship between attitude and airspeed is fairly direct - so a lb/kn problem would also be a lb/deg problem.

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,

I've seen several (prototype light) aircraft over my career with a stick force per inch problem - my personal working rule has become to look for a minimum gradient of 0.1mm/kn. I'm also aware that this was a problem with the prototype F-16 which had no movement at-all. However, in each case it's been the lack of enough movement to allow the pilot, through tactile feedback, to accurately resolve forces - not the shallow movement gradient in itself.

or stick force per G. Not a changing of stick force per knot.

The odds are that these are closely related. Whilst I've said that the two characteristics are unrelated, this isn't strictly true. In *most* cases, an aircraft with poor lb/kn will also have poor lb/g, and vice versa.

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.

No, it seems to have been the problem in maintaining fixed pitch attitude - the relationship between that and lb/kn is a strong one, if not linear.

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.

What is this business about adjusting throttle to change airspeed? Whilst there will always be a secondary pitching moment related effect of power on speed, it's pitch attitude / elevator angle / elevator trim tab angle which affect(s) airspeed, whilst power affects rates of climb and/or descent.

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.)

I'm unfamiliar with the specific accident but if pitch control is problematic at constant speed, that indicates probably something like a poorly damped longitudinal SPO? That is consistent with being out of aft CG, but so is poor lb/kn- so it all ties together without the lb/kn being the specific problem.

"Don't feel too bad about this stuff confusing you a tad". Yep, I'm confused

Contrary to what Mr Tullamarine may assert, despite 20ish years of learning and using this stuff, I still struggle regularly and get the textbooks out on a regular basis. It ain't simple, and don't let anybody pretend to you that it is.

G

[john_tullamarine]

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.

[MarkMcC]

Fabulous discussion...and worth reading twice:ok:

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':confused: 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.

[john_tullamarine]

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.

[hawk37]

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.

[MarkMcC]

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...

[Genghis the Engineer]

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.

As has been mentioned, it's a light aircraft with reversible controls so not particularly representative of a modern heavy aircraft - but a C150L or C150M, ballast on the back bulkhead to about 90% of the available CG range, full flaps, power for level flight or a gentle climb - that'll give you about neutral apparent LSS. (Or, although I don't know the type so well, most variants of Zenair CH601 at mid to aft CG trimmed for a fast cruise will show you approximately neutral apparent LSS.)

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

[safetypee]

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.

[MarkMcC]

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!! :):):)

[john_tullamarine]

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.