Originally Posted by OVERTALK
I think that we have lost the physics bubble here. The four retardation scenario factors discussed (a-b-c-d above) are producing a strong nose-down pitching moment.
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.
b. Braking. As I noted above, the best you can hope for is 5% or so additional mass on the nose - under relatively good braking action. With poor braking action - say 0.10'g' or lower decel - you'll be lucky to see more than 1 or 2% mass 'transfer' occur.
c. c.g. - not sure exactly what you allude to here. Are you assuming that the landing cg is further forward than at takeoff? It hardly matters, for most practical aircraft configurations this is basically the static nosewheel load - usually, as discussed above, of the order of 5-10% of aircraft weight.
d. spoilers. Depends on their location on the wing, degree of sweep etc; entirely possible to have a nose UP moment due to spoilers, since they are AFT of the cg and above or aft of the main gear.
The best you could hope for is the 5-10% basic, static, nose loading, plus an additional 5% due to decel 'weight transfer' under good braking. There are other factors acting to raise the nose, on our types at least, so that represents an OPTIMISTIC assessment of the force on the nose gear.
If you multiply that tyre/bitumen area interface increase on one tyre by all the tyres on a plane's main-gear, you can appreciate that the overall effect will be worthwhile.
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.
This is a little confusing but I imagine that you meant to say that up elevator would cause the nose oleo to extend and consequently the wing's AoA to increase, thereby acting in an opposite sense to what is being claimed (i.e. reducing the weight upon the main-gear).
Absolutely; 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.
That that is a fallacy can be exposed by a simple illustration. Put a soft pea (or jelly-bean say) under the midpoint of a 12" rigid ruler and press down on both ends of the ruler. Do it with roughly equal force, simulating a pilot tempering his backstick input to just below what "could" (not saying "would") possibly cause the nose to rise. In other words, braking "for effect" by flying the attitude out the front window. The pea gets squashed of course. That simple experiment replicates the 4 retardation factors producing a nose-down moment at one end of the ruler and, at the other end, the opposing nose-up (i.e. tail-down) moment induced by the backstick. It adequately demonstrates that the resultant will be a down-force upon the maingear (the pea). Now do it with a new "soft pea" yet without the simulated pilot's up-elevator input. The nose drops, tail rises and the pea is unsquashed. That simulates the lack of anything other than the ruler's weight acting upon the "maingear" pea. It emphasizes the need for early introduced backstick braking when the chips are down (i.e. you suspect that you may have landed too far in on a slippery runway and urgently need max effective braking ASAP).
OK, let's stick with the 12" ruler, and make it more like the actual aircraft conditions.
Put the "mainwheel jelly baby" in the middle, and stick something fairly heavy on top of the ruler, pretty much right above it. Say a coffee mug or something. That squashed the baby a bit - representing the bulk of the aircraft weight on the mainwheels.
Now put a smaller jelly baby - just a head, perhaps? - near the "front" of the ruler, to represent the nose gear, and then slide the mug forward a little until there's a little bit of load on the "nosegear jelly baby" too. That represents the aircraft under normal static conditions.
Now take a small glass - say "shooter glass" sized and place it above the nose gear. That's our "weight transferece" due to various effects. Fill it with water (it might tip off, so don't risk alcohol, no sense in wastefulness) to simulate the effect of increased braking/increased nose weight transfer. What should happen is that the "nosegear jelly baby" gets more squished due to this increased load.
Now, apply back stick by pushing down at the back of the ruler with your finger. As you do so you will both squish the "mainwheel jelly baby" AND "unsquish" the "nosewheel jelly baby". The only circumstances under which you will NOT unsquish the "nosewheel jelly baby" is if the load was so high that it totally flattened it first. In which case you'd have some ability to apply tail download before affecting the angle of the ruler.
And THAT is why I mentioned complete nosegear oleo compression.
You seem to have a picture of pilots as motor morons incapable of coping with a simple manipulative task. Landing a heavy-weight asymmetric airplane on a wet runway with 50 kts across used to make me salivate with anticipation..... not dread.
That's all very well for a skilled pilot; procedures are written, have to be written, so that a pilot of minimal skill can apply them reliably. I'm afraid I've seen enough cases where everyone is sat round the table - our own training and test pilots included, I might add - looking at each other, wonder what the HELL was pilot X thinking when he did whatever the FDR is showing.
And, incidentally, let's assume that I'm wrong, and that a reliable and simple technique can be demonstrated to provide a, say, 10% reduction in landing distance. 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.