RatherbeFlying said:
The fulcrum on the ground is the maingear axle. Backstick produces down force at the tail which:
Adds to the total load on all gears
Produces a nose-up moment which relieves a portion of the load on the nose-gear such that the maingear supports a larger portion of the total weight plus any aerodynamic downforce.
Can't disagree with that. It's manifestly the distilled essence of the underlying theory. I might have said: "plus
the aerodynamic downforce." Seeing it as the resultant of two moments or a type 2 lever acting upon the nose-gear as a dynamic fulcrum is also valid. But what is obvious (and missed by most) is that there is an "
effective" weight shift
towards the nose. i.e. We are getting side-tracked by static Center of Gravity computations saying that only about 5% of an aircraft's mass acts through the nosewheel. May be true but it's a red herring in this dynamic flight context. RBF's succinct statement also doesn't address (but obviously accepts) the reason why the nose stays down (a nose-down pitch is induced by reverse and whatever braking you are achieving courtesy of spoiler lift-dump when the progressive backstick input commences). As the backstick increases, wheel-braking becomes more effective, thus allowing more backstick etc etc. Eventually you do end up on the backstops, but by that time you will have killed that speed increment above the min aquaplaning speed -as well as any directional control or cross-wind induced problems that you may have otherwise had.
This may result in a change in AOA and lift....
The process
is dynamic and what is being advocated
is an early accelerated rate of deceleration. That dynamic process compares with the Air France, QF1 and SWA pregnant pause hiatus where there were interlock problems entering reverse and indecision/mind-changing. The latter process eats up landing real-estate at a great rate of knots and kills your options - whereas the backstick braking
uses those knots to achieve very early efficient deceleration
before the aircraft reaches that last 2000 odd feet of rubberised runway remaining. Chalk and cheese. Success and failure/disaster. I suspect that, in the fullness of time, Airbus (at least) will be automating the advocated process.
MFS said:
"assuming that there is NO aerodynamic download, which is an optimistic assumption for this method...."
MFS then goes on to draw invalid comparisons of parked a/c and the effect of significant winds, disregarding the nose-down pitching moments that result from reverse and spoiler-assisted early braking. His conclusions are invalid because we are talking about 2 completely different scenarios:
a. static parked weight distribution with nose-up stab trim combatting local wind effects and
b. a 1.1.Vs or greater handling technique with engines running and reversing, spoilers up and a pilot making appropriate control inputs).
.
It is difficult to clearly discuss the nuances of handling techniques with engineers (even flight engineers) - or so I've found. Handling is not like placing a switch at A, B or centrally OFF. Handling is about moderated input tempered by observed reaction. An OFF/ON selection of instant full backstick is not what's being advocated at all. However MFS does introduce (and remind us of) that all-important "square of the speed" factor - which is also a player when considering the effect of early backstick after touchdown. It is certainly what empowers that backstick-induced early breakthrough (to the bitumen) for the main-gear wheels. "EARLY" is also the point at which the effect of reverse thrust (and its pitchdown moment) is maximal (then gradually tapering off).
I'm as certain as I can be that you WILL lift the nose under those conditions. Well if it wasn't for the four additive factors producing the strong (and opposing) nose-down pitch during deceleration, I might concede that point. But MFS is single-mindedly stuck in his engineer's view of the airplane as a static entity. After all, that's how you achieve rotation, no? Well no, not really. You have to consider that there's a Thrust/Weight-Lift/Drag couple at work there during lift-off, ably assisted by leading- and trailing-edge devices (and minus spoilers, braking and reverse). If you fuel-chopped all four at the point of pre-rotate (V1) would it still rotate??
"We are scared of someone trying this out with pax in the back and either departing the side of the runway due to loss of NWS/directional control, or raising the nose and actually losing braking effectiveness."
A mite harum scarum -but that has been your [MFS] underlying theme. I'd suggest that it's only MFS with the trepidations. All the Xperimental TP's that I've known in the RAF, BAe and elsewhere would say:
"well let's do some sums and then go find out. Anything that can stop this wasteful "off the end" bizzo is worthwhile exploring and exploiting. It may even be a great marketing tool if it was automated. Let's go talk to some aerodynamics people and perhaps get them to write a program for the iron bird. The simulator should tell us what's what."
RBF said:
"Static loads while parked and dynamic loads while braking are different cases with greater nose wheel loadings under braking."
Bravo for keeping an open mind and identifying the unsubtle difference.
MFS said:
1. "The problem is that the case where the nosewheel is most heavily loaded by the braking forces is the case where the brakes are ALREADY WORKING WELL!" MFS -Please see a+b+c+d below for your illogical sequitur
2. "Not enough to overcome full back stick; not even close for some types."
Five bucks for whoever can get MFS off his FULL backstick bandwagon.... and start entertaining the full vista of forces at play during the dynamics of landing deceleration. i.e.
,
a. REVERSE - greatest effect at the outset => strong nose-down pitch (
NDP)
.
b. Braking - It is agreed that despite spoilers assisting, that braking is minimal at the outset on a slippery surface (but then again,
that is the actual problem) but still =>(
NDP) (
initially only equivalent to only another +5% of a/c mass - according to MFS)
.
c. Center of Gravity - I won't venture whether this is MUCH more pro or con at landing (versus take-off) but I suspect that in the landing configuration it's in most cases helping our quest (for a counterbalance to introduced backstick). But no matter how far it's moved as fuel burns off, it will still be telling the nose firmly
"to stay down" => (
NDP)
.
d. Spoilers will be up and, per the T-W/L-D couple, should also be helping kill lift and admonishing the nose to stay down. i.e. =>(
NDP)
.
a+b+c+d => a powerful counterbalancing (but varying) total nose-down force against which we can introduce our proposed progressive backstick
.
So throughout the early landing evolution, speed dissipates at "some rate" and braking will improve "somewhat". Once in reverse, early pilot introduction of progressive backstick will have the desired effect and
will improve braking effectiveness (which will in turn enable
more backstick, etc etc). Certainly "forward" stick has more than its fair share of perils
(link) ("..it appeared that the captain’s technique for landing at Gibraltar differed from that at other destinations, with an evident, if unconscious, propensity to apply full nose-down elevator right after main landing gear touchdown. Immediately after the incident, Monarch modified its FDM software to include nose-down elevator as an "event."). G-MONC was out of service for over 2 months and required >$6M in repairs.
.
The "effective" weight-shift toward the nose will be that imparted by aircraft momentum and retardation forces acting around various axes (and thus summed in ft/lbs). Overall, the
early "stickiness" of the nose very much depends upon the total magnitude of the nose-down couples mentioned above. MFS chooses to ignore the cumulative effect.... and why that should be is a mystery - but perhaps it just helps his argument. It is conceded that the diminishing effect of reverse as the a/c slows will be countered by the increased effectiveness of braking.... probably making it a zero-sum game. However that's of no real consequence because the pitch-up authority of the (by that time) full backstick will have faded to insignificant.
.
The theory disregards all the other factors acting for and against a successful stop within runway available as being irrelevant - however it should be noted that it will be particularly effective at combating high groundspeeds caused by landing with the wind (rather than against it). Sometimes it is expedient to accept a tailwind component, but always it is less safe to do so. At such times, the value of backstick braking will be to restore the balance in favour of the pilot not running out of career or runway. Not to be forgotten (also) is the stabilizing effect of getting the weight shift redistributed to the tricycle geometry that gives best directional control. Stick forward can be destructive (G-MONC) but the wheel-barrowing effect of any unopposed weight-shift forward can also cause great directional instability, particularly on a wet runway in a crosswind.
.
Ultimately the aim is to have the theory revalidated and assessed for effectiveness. It is nothing more than a handling technique that seeks to minimize the intervention of anti-skid on inhospitable surfaces by increasing the weight-on-wheels as soon as possible in the landing roll. If it is assessed as worthy, then everybody is better off....and at nil cost. It's a principle called
regrets management....and it contrasts well with risk management (which lets us down more often than not).
Overtalk
Pilot Retard Advocate