MFS said:
Not so, at least on our types.
a. Reverse thrust on our aft-mounted types causes a strong tendency for the nose to rise; for that very reason our advice to pilots is moderate forward stick force to prevent the nose from rising.
Can't dispute that authoritatively except to say that there are quite a few limitations on high aft-mounted engines as far as reverse goes (i.e. the restriction to 80% available reverse due to rudder blanking on MD80/717 types would also help limit any pitch-up due to reverse). So McDD put a forward stick recommendation in the Pilots' Handling Notes eh? I wasn't aware of that. Only goes to show that it's always horses for courses and no
one handling technique can be seen to be a universal panacea. I'd have thought that spoiler and braking would've easily overcome any such pitch-up tendency - with there being not much of a lever arm between those rear engines and the main-gear. The demise of the 717 may help write finis to this consideration in the fullness of time. But many newer design rear-engined smaller jets such as the CRJ seem to have similarly mounted engines....so perhaps someone can comment on whether the CRJ has a similar caution written into the Pilots' Notes.
link
Then what would happen next is that we (the OEMs) would take credit for the technique in our published distances which would result in every landing becoming potentially more marginal. The technique wouldn't be something in your back pocket for a bad day when you need it; it'd be something you'd have to use every day, and would be assumed to use.
At present the FAA is very very circumspect in defining techniques and configurations for establishing "book" distances - so I'd see that view as being unnecessarily alarmist. e.g. <<The standard curves (i.e., equations) of braking coefficient versus speed prescribed in §25.109(c)(1) are based on a tire tread depth of 2 mm. Since the tread depth of new tires is usually 10-12 mm, 2 mm represents no more than 20 percent of the original tread depth. FAA Advisory Circular 121.195(d)-1A, which provides guidance for determining operational landing distances on wet runways, specifies that the tires used in flight tests to determine wet runway landing distances should be worn to a point where no more than 20 percent of the original tread depth remains...etc>>. So the backstick braking technique would be no different to (say) not instinctively trying to pick up a dropped wing with aileron near the stall (another non-intuitive pilot-developed skill). I'm not sure that ALPA or IFALPA would agree with your continued portrayal of professional pilots as having to conform to your depiction of their abilities, skills and challenges necessarily being limited (and conforming) to a lowest common denominator.
any unloading of the nose gear will allow some extension of the gear, raising the nose and increasing AoA with the noted unloading of the mains. Unless your nose is basically burying itself in the ground, with oleo fully compressed, before you apply the back stick then the nose MUST lift. The question is, how much.
Not sure that this is a productive piece of imagery. It's one reason why my first inclination was to first use the analogy of the second-class lever (with the nose as fulcrum). Using that logic the backstick just lowers the tail and loads up the main-gear, with the nose-gear acting as a pivot-point. John Farley has endorsed the ability of backstick to "load up" the main-gear for more effective braking on mush - so perhaps we can leave this nicety to be thrashed out by the aerodynamicists (who will see it as the resultant of couples (nosedown pitching moment and taildown pitching moment). I've personally
never been able to raise the nose whilst under backstick braking. As a pilot it's obviously something that would be quite attention-getting. Because the ergonomics are optimal, you find that the more you haul back, the greater the braking that you can apply without wheel-skid. The dynamism of backstick braking is such that any minor degree of "nose-rise" induced mainplane AoA would be more than offset in the lift equation by the rapidly diminishing IAS.....However if anyone is into cheap thrills, try removing the backstick whilst under concerted backstick braking. I guarantee that you'll only try that once, if at speed on a wet or slushy runway.
Actually the contact area itself is of negligible impact on braking effectiveness; it might be counter-intuitive, but all that matters is contact download, not the area of tyre/ground contact.
The NASA study on a tyre tread's effect on hydroplaning and braking friction coefficients places great emphasis upon the grooved circumferential treads being in contact with the runway, and that being a function of weight-on-wheels. For more groovy contact I'd imagine that you'd need a larger "footprint". However we are all agreed (I think) that weight-on-braked-mainwheels is the name of this game.
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HYDROPLANING is a condition that can develop whenever a tire is moving on a wet surface. The tire squeezes water from under the tread generating water pressures which can lift portions of the tire off the runway and reduce the amount of friction the tire can develop. On a runway contaminated by rain or wet snow, it can be impossible for an airplane to accelerate to take-off speed and then to stop on the remaining runway in an aborted take-off. During landing, deceleration and stopping an airplane can be similarly compromised.
Just to be clear (due to some PM's Rx'd). There are three types of hydroplaning. We're interested in #2 below and R-R hydroplaning to a minor extent (because of heavy rubber deposits).
Viscous hydroplaning occurs when there is a thin film of water and relatively low tire speeds. The water lubricates the surface and decreases traction. A water film of only a tiny fraction of a centimeter will drastically reduce the friction between the tire arid the pavement and double the stopping distance.
Dynamic hydroplaning requires deeper water and results in complete loss of tire contact with the pavement. The tire lifts off the runway and rides on a wedge of water.
Reverted-rubber hydroplaning can occur when a locked tire skids on a wet or icy runway. Frictional heating raises the tire temperature causing rubber particles to shred off the tread. These particles accumulate behind the tire forming a dam that blocks the escape of water. The trapped water heats and turns to steam. The steam pressure lifts the tire from the surface.
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Your counter-analogy to the "ruler and pea" is totally and transparently specious. One nose-heavy moment versus a tail-heavy moment and the main-gear as pivot-point..... that is adequately illustrative of the forces involved.
worth reading
also worth a look