"Full powered" implies "irreversible", which implies that all stick-feel is artificial, and there is no elevator force feedback to the stick. In that case I'd expect the elevator to be only a function of stick position in a system fitted with a trim tab. A conventional lgiht aircraft (say PA28) system is reversible and thus elevator would be displaced.
But, for this very reason I'd not expect a trim tab in a system with irreversible controls. I'd expect the cockpit trim control to be deflecting the neutral elevator setting in some way, and would be very surprised to see a trim-tab.
Is there any actual aircraft out there with fully powered controls and a trim tab in the same circuit? If there is, it is likely to be low-powered, and operating in the opposite sense to the trim-tab in a reversible control system.
Logically with a powered control system and the aircraft in aerodynamic trim the elevator would be in line; however I suspect that the designer chappies may use a tad of elevator or non ideal tailplane angle to optimize drag or structure loading. But this does not fit in with flaps extended as the question asked. Again many engineers / designers do not like ‘no load’ conditions, particularly where systems vibrate, thus with additional flap downwash the elevator may be designed to carry a small ‘engineering’ load and thus be offset. If so, then the question is the usual JAR ‘aerodynamics / controls’ type of question that as no connection (pun intended) with the facts and little or no value in operation.
Don't think a trim tab is relevent. The question specified 'trimming tail' and 'elevators'. I take that to mean incident adjustable tailplane + elevators ie not an all moving tailplane, a trim tabbed elevator or power held elevator trim system.
What the question refers to is almost without doubt neutral shift.
With a 'flying' or trimming tailplane the aerodynamic neutral is not the same as mechanically neutral.
When the stab is leading edge up, the elevator tends to be trailing edge down. As the stab is moved through its range towards leading edge down, the elevator is repositioned to an aerodynamic neutral by a device known as a neutral shift mechanism so that when it is fully leading edge down, the elevators will now be slightly trailing edge up!
Here is an extract from the B737 (classic) MM which gives figures.
"Elevator neutral position relative to stabilizer is determined by stabilizer attitude. The range of elevator neutral shift varies with mach trim actuator position. With the stabilizer at 3 units of trim, the elevator neutral position is 4 degrees down from faired. As the stabilizer leading edge moves down the elevator neutral position moves up. When the stabilizer moves to 10 units of trim, the elevator neutral is at zero degrees, and with the stabilizer at 14 units of trim, the elevator neutral is 2-3/4 degrees up. Elevator maximum travel with the stabilizer at zero units is 20 degrees up and 22 degrees down."
The elevator movement can be clearly seen if with hyds powered the stab is moved. As the stab is trimmed a/c nose down so the control column moves forward as the feel and centering neutral point is moved by the neutral shift mechanism.
By the way for Genghis, the 737 has tabs. they operate as servo tabs (i.e. they deflect in the opposite sense to the elevator) to assist the elevator deflection. They are always operating but are primarily to assist the pilot with pitch control in the event of a total hydraulic failure or manual reversion.
I agree with what has been said above. Downwash from the flaps, or any other aerodynamic influence, cannot vary the angle of fully powered flying controls (unless PFCU jack stalling occurs). Normally, the controls would be designed such that the elevator is in line with the tailplane when trimmed as this is the minimum drag condition for balanced flight. It sounds like the B737 fuctioning that mono describes was a fix to increase pitch trim authority without increasing overall tailplane deflection (or to maintain trim authority whilst reducing tailplane deflection). It is feasible that an automatic trim input could be designed into an aircraft for when, say, the flaps were lowered, but I would imagine that this would deflect the tailplane and not the elevator.
The BAC 1-11 also has a tab on the elevator for trimming when in manual following a double hydraulic failure.
My post refering to the 737 was just an example. To my knowledge the 727/737/757 and 76 all have a neutral shift system of some sort. I don't know about he 74 and 777 as I don't work them or have any documentation for them.
Will look at the airbus now but the MM is not a good reference for this kind of detail.
The way in which the controls are moved is not relevant, IMO.
As Tinny says, "in trim" means there is no power required to hold the controls in neutral position.
I think it is relevant. I don't know anything about powered systems, but in an unpowered system stick-free does not necessarily correspond to elevator neutral (i.e. in trail). It corresponds to the elevator position with zero-hinge-moment.
In a powered system, I could envisage a situation where stick-free does not correspond to zero-hinge-moment, because the designer of the control system can arrange it that way. So it could be designed to be in trail when stick-free. Whether that is the way these things are in fact designed, I don't know.
I suspect we're just debating the definitions of neutral, whether mechanical or aerodynamic.
Further interrogation suggests that the question read "with a power assisted system", not fully powered. In this case I would expect the same reaction as with a manual system. You will be trimming for zero stick force, not zero elevator deflection, and will end up with some elevator deflection when, for trim reasons, the stabiliser is set markedly nose up or nose down. This, I think, would be elevator tail down if the stab is nose down, and vice versa. Have I got that right?
In a fully powered system I would expect zero elevator deflection when in trim, in the absence of any deliberate sytem adjustments as mentioned.
Fully powered controls = irreversible = neutral most likely defined by position. Therefore tab effectively acts as an elevator in it's own right - assuming nothing is done with the elevator.
Unpowered or power-assisted controls = reversible = neutral most likely defined as zero moment acting. Therefore tab acts to deflect the (much larger) elevator and thus works in the opposite sense to an elevator on it's own.
Which is what the F**** it has to do with the question.
Dick, I agree with the first part of your last statement, but not the second. If the elevator is irreversible, then most-likely it is used to set trim, so it is only in the neutral position by luck - it is the control used to trim the aircraft. Even in the case of the 737 as described by Mono, the trim tab is probably only being used to trim out actuator forces and thus reduce stresses on the system, rather than to trim the aircraft - I'm guessing that it's probably mechanically connected as a pro-balance tab?
Which, as bookworm suggests, seems again to come down to how the neutral position is defined. Is it a fixed angle relative to the fuselage or mainplane, or the position where pitching moments acting about the elevator hinge are neutral, or some moving position depending upon mode of flight / CG / whatever?
This does increasingly sound suspiciously like one of those irritating exam questions where an examiner is much more interested in the candidates grasp of his particular slant on terminology than a genuine understanding of the subject.
Dick, Not so sure about you last post – re a power assisted system. Building my technical argument starting from a fully powered all-flying slab tail (Lightning) and working back. The powered tail surface was positioned to achieve aerodynamic trim whilst stick force was reduced independently via a ‘spring feel’ system.
Splitting such a conceptual system into a trimming tail with a power assisted elevator (Hunter); the tail had to be irreversible thus no force feedback to the pilot, but the pilot probably had some elevator load fed back, thus a need to trim. The elevator maneuvered the aircraft and was positioned to achieve aerodynamic trim (balance); the power assister / pilot holding some residual load. Zero stick force was achieved by a combination of pilot ‘spring feel’ trim and flying tail adjustment. As the load held by the assister / pilot reduces the elevator moves, but there is a corresponding proportional (and opposite) movement of the tail, maintaining the aircraft in aerodynamic trim. A null position is achieved when the elevator and power assisted tail are in line, similar to an all flying tail. Furthermore, in the Hunter with auto ‘tail follow up’ engaged, this enabled the tail to move automatically to reduce the feel forces; thus during combat the crew did not have to trim with maneuver or speed change. Aahh Hawker, clever people; a bit like an A320 FBW??? Now with flaps down, it all depends on the authority (range of movement) of the tail; I suspect that even in a civil airliner a zero elevator (or optimum design position) can always be achieved. Sorry for the old aircraft examples, dates me a bit; but there again there were some good old ones.
Hey, I was there! I flew the Hunter 1, which had a 4.2:1 power assisted elevator, and an electric moveable horizontal stabilizer [to pin things down accurately]. I also flew the Hunter 4, with a fully powered elevator, I think [don't have the piklots notes handy] and later marks with the high speed electric "follow-up" horizontal stabilizer, which I shall now call the "tail" for simplicity.
The later follow-up tail effectively gave the Hunter an all-moving tail, as the later Sabres had, and as the Hunter should have had from the begining. Until that fit its longitudinal control at high Mach was rubbish.
But I digress. Having talked about this, and with the benefit of all your advice, I still thuink I have the right answer. In a fully powered elevator/electric tail trim system stick force must be measured from elevator deflection, relative to the tail. So, if you are out of trim you are holding on some elevator. As you move the tail to trim out the stick force the tail gradually takes over the trim task and the elevator gradually gives up and reverts to neutral - mechanical neutral, in line with the tail. So, elevator neutral, no stick force. Tail takes over trim, attitude held constant.
In a power assisted system you still get stick force from the aerodynamic load on the elevator - only a piece, depending on the degree of power assist. Now, as you move from out of trim to in trim the electric tail is taking over, and the elevator giving up, but you reach zero stick force when the aerodynamic load on the elevator reaches zero - not when it is at the mechanical neutral. This will happen when the elevator is in line with the resultant airflow off the back of the tail. This airflow will be a downwash/upwash if the tail is at a high alpha, and will be zero when the tail is at zero lift alpha. Since the tail position will depend on what trim output is required it will vary with configuration and speed.
Many of us, and I digress again, suffered from a. the Hunter's lack of longitudinal control authority at high M, and 2. the nose down change of trim when combat flap was extended. The pitching in the resultant dive made it almost impossible to reach the flap lever and knock it up. Some of us recovered at low altitude, some did not.
Dick, I still think you don't quite have the right answer.
The 727/737/757/767 all have fully powered flight control systems with trimmable tails (stabilisers). As the tail is moved to trim out any load on the elevators so the elevator neutral position is moved such that in a trimmed condition there is no aerodynamic load (aerodynamic moment about the elevator hinge point), this done by the neutral shift system. However, this aerodynamic neutral is NOT a mechanical neutral (see my previous post for figures a la B737)
Just because an a/c has a fully powered control surface does not mean that the designer will not engineer the system to remove any aerodynamic load on the system. A standing load on a flight control is not a desireable factor. It leads to un-necessary heating of hydraulic fluids (pressure required to maintain mechanical rather than aerodynamic equilibrium) and strain on pumps, stressing of mechanical load points, rod ends, bearings, etc. And finally, in the event of a loss of system pressure the attitude of the a/c would change (as the elevator took up the aerodynamic neutral from which it had been artificially displaced).
As an aside and to confuse the matter even more. Several fully autoland compatible a/c, during the final stages of approach deliberately trim the stabiliser to an a/c nose up position and offset this with elevator deflection. This is to assist the pilots in the event that a go around is necessary. As far as the crew are concerned the a/c is in trim as it is behaving normally.
The neutral position in a powered system, such as most of the Boeing aircraft types, uses a spring loaded Vee cam that would force the elevators to the neutral position should the pilot release the controls. Aerodynamic loads on the elevator cannot do this in a powered system. But what is the correct neutral position for a given stabilser position?
The Vee cam is mechanically repositioned according to a predetermined schedule dependent upon movement of the stabiliser, so that with no control input, the position adopted by the elevator relative to the stabiliser changes. The positioning schedule is set so that the elevator remains biased to the aerodynamic neutral applicable for whatever is the current position of the stabiliser. So mono, the neutral shift system eliminates the undesirable aerodynamic loads on the elevator and this aerodynamically neutral position is achieved by a mechanical system. Does that make it any clearer?
Then we could move onto the neutral shift sensing system that allows the autopilot to remain synchronised to the applicable neutral position while it is engaged. One wouldn't want to jump to a new neutral when disengaging the autopilot would one? Ah! Such are the complexities of the AFCS.
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I agree with what you say regarding fully powered systems, accepting design complexities such as those described for the B737 et al.
I think that the way to consider what is happening with power assisted elevators is to start off considering manual elevators on variable incidence tailplanes. The zero hinge moment elevator angle, for a given angle of attack, will be a function of the downwash angle of the airflow off the tailplane. This will be affected in particular by the camber of the tailplane section but also by tailplane aspect ratio, taper ratio etc. Therefore, even at the zero lift angle of attack of the tailplane, the elevator angle may not be zero. This will also be influenced by the reduction in the influence of the wing downwash as the airflow travels rearwards.
Now, if we go to a power assisted system, we are somewhere between the two cases above, depending on the amount of power assistance and the artificial feel system.
Out of interest, the elevators on all marks of Hunter were actually power assisted as the stick was permanently connected to the elevators (and ailerons). However, the hydroboosters effectively provided all of the force to overcome the hinge moments. The spring feel bias had to be adjusted post airtest to minimise the trim change following reversion to manual, thus showing that the artificial feel influenced the elevator angle when in trim.
PS. I remember some interesting discussions with you when I was doing my ATPL exams (Bristol, Dec '98)
I was leaving out the detailed type specific system adjustments to the elevator neutral position in fully powered systems to try and simplify the case. I sincerely hope that the JAA never try to examine students to this degree of technical knowledge. If the question/answer was "how does the elevator lie in relation to the trimable tail in power assisted systems when in trim?" and the book answer is "we don't know, could be a bit up or a bit down", then I am happy with that.
If a question turns up in relation to fully powered systems then that will open a whole new can of worms.