Pulling a Stop to Runway Overruns
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Absolutely right about the racing car.
However, an airplane differs from the racing car by having a huge inverted spoiler forward or the aft spoilers, which is creating lift!
If a wing is capable of creating a 50 tons lift, after spoilers are extended and pitch is zero when on the runway, it will still generate, for example, 10 tons of lift.
Now, the tailplane is designed to create a constant negative lift.
By pushing the yoke fully forward, I think you can reduce this negative lift, or even create a positive one - it doesn't matter - by, say, 3 tons (imaginary figures).
Doing so your pitch will decrease by a couple of degrees.
This may reduce the residual positive 10 tons on the wing to 5 tons.
You see from all this simple math that the net result is in favor of pushing, because the apparent weight of the airplane has increased by 2 tons (-3 on the tail, +5 on the wings).
Thus more weight on the wheels.
Of course all this is type and conditions dependant.
If conditions permit a further compression of the nose oleo, for example, it's better to push, otherwise it's better to pull.
If the imaginary math above is in favor of pulling on a certain type, so be it, pull.
And the list is long.
That's why it's almost impossible to establish an official procedure.
Ps: an instructor once criticised me for pushing too much after touchdown.
He said Boeing's official technique was to neither push or pull.
Just to leave the yoke neutral.
I can't recall such a statement in any book. Anybody has got the official reference?
LEM
However, an airplane differs from the racing car by having a huge inverted spoiler forward or the aft spoilers, which is creating lift!
If a wing is capable of creating a 50 tons lift, after spoilers are extended and pitch is zero when on the runway, it will still generate, for example, 10 tons of lift.
Now, the tailplane is designed to create a constant negative lift.
By pushing the yoke fully forward, I think you can reduce this negative lift, or even create a positive one - it doesn't matter - by, say, 3 tons (imaginary figures).
Doing so your pitch will decrease by a couple of degrees.
This may reduce the residual positive 10 tons on the wing to 5 tons.
You see from all this simple math that the net result is in favor of pushing, because the apparent weight of the airplane has increased by 2 tons (-3 on the tail, +5 on the wings).
Thus more weight on the wheels.
Of course all this is type and conditions dependant.
If conditions permit a further compression of the nose oleo, for example, it's better to push, otherwise it's better to pull.
If the imaginary math above is in favor of pulling on a certain type, so be it, pull.
And the list is long.
That's why it's almost impossible to establish an official procedure.
Ps: an instructor once criticised me for pushing too much after touchdown.
He said Boeing's official technique was to neither push or pull.
Just to leave the yoke neutral.
I can't recall such a statement in any book. Anybody has got the official reference?
LEM
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Originally Posted by UNCTUOUS
MFS - I'll Try and Simplify it for you
I hope that you will be able to see that to be a fact......
But I have absolutely nil faith that you will......
I hope that you will be able to see that to be a fact......
But I have absolutely nil faith that you will......
Please actually READ what I wrote, and work out the moment balance about main, nose and tail with and without download. It's not rocket science, it's not even 'flight' science.
You will find that ANY download at the tail MUST produce a LARGER downforce at the maingear and a corresponding, proportionate, DECREASE at the nose. That is weight transference.
to address the racing car analogy
You may have noticed that F1 and similar cars, which do use aerodynamic downforce, have both FORWARD and REAR spoilers, all of which are intended to produce downforce. The ideal is to achieve an aerodynamic downforce distribution, courtesy of this feature AND of the car body design, which is proportionate to the existing weight distribution, as that means that the handling is more consistent. Very few aircraft have the luxury of a 'front tail' analogous to the F1 front spoilers.
Also, those vehicles with large rear spoilers alone are often using them to counteract the large aerodynamic lift being exerted on the back of the vehicle (in a typical 'saloon car' configuration) rather than to generate 'extra' downforce; they're just trying to keep the back wheels on the ground.
It is highly amusing that the 'backstick lobby' bandies around accusations of closed-mindedness, yet refuses to even do any even mildly dissenting view the courtesy of considering its points.
Last edited by Mad (Flt) Scientist; 23rd Mar 2006 at 11:53.
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Yes LEM, you are quite correct in believing that reverse thrust does not give a pitch-down moment. Neither do spoilers for that matter. But I suspect we have moved onto the point of religion. And you know what happens then.
Anyhoo, some airlines might have this 'technique' in their books, some might not. I suspect those who do, have so because those wielding the pen are ex-warriors. Is the military the sole proprietor of the gospel? I could have sworn I've seen the camouflaged ones off the runway too.
No matter, I think the 'pro-lobby' here is not a large as appearences might suggest.
Anyhoo, some airlines might have this 'technique' in their books, some might not. I suspect those who do, have so because those wielding the pen are ex-warriors. Is the military the sole proprietor of the gospel? I could have sworn I've seen the camouflaged ones off the runway too.
No matter, I think the 'pro-lobby' here is not a large as appearences might suggest.
The front and rear spoilers of F1 cars are mounted very close to the two sets of gear. I suspect that the downforce vectors are located between the axles, in which case there is no unloading of an axle. In the case that spoiler(s) are located beyond an axle:
- the moment arm is enormously less that that from a tailplane so that the result is very little unloading of the opposite axle -- or the car goes off the road
- Any unloading from one spoiler is more than counterbalanced by the action of its opposite number.
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The most important things in my humble is opinion are:
1. Is the runway safe for landing LAW, X-wind Tailwind etc etc? I do not care what the last landing pilot said. If the information about runway status is not clear then why would one want to try if not sure. This is not a game where you try but must make solid judgements and decisions.
2. Make a firm landing at the correct touch down point and speed. Do not try to impress the passengers your copilot or new flight attendant you are daydreaming about, by hunting for a smooth landing and floating.
3. Apply recommended stopping technique; auto brakes, spoilers reverse as recommended.
Lastly I have flown with pilots who apply so much brute force on the reverse levers so hard, that once the conditions are met for reverse they cannot get reverse because they are applying such a high force that the solenoid or whatever prevents reverse on (your aeroplane) they cannot get anything beyong reverse idle. Only after this white knuckle stage are they able to get reverse which is not as effective at low speed. (this may not apply to all aeroplane types but applies to all I have flown.)
1. Is the runway safe for landing LAW, X-wind Tailwind etc etc? I do not care what the last landing pilot said. If the information about runway status is not clear then why would one want to try if not sure. This is not a game where you try but must make solid judgements and decisions.
2. Make a firm landing at the correct touch down point and speed. Do not try to impress the passengers your copilot or new flight attendant you are daydreaming about, by hunting for a smooth landing and floating.
3. Apply recommended stopping technique; auto brakes, spoilers reverse as recommended.
Lastly I have flown with pilots who apply so much brute force on the reverse levers so hard, that once the conditions are met for reverse they cannot get reverse because they are applying such a high force that the solenoid or whatever prevents reverse on (your aeroplane) they cannot get anything beyong reverse idle. Only after this white knuckle stage are they able to get reverse which is not as effective at low speed. (this may not apply to all aeroplane types but applies to all I have flown.)
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Specious and Spurious Argument
MFS
In your 23 March 03:51 post you completely and purposely neglected to factor in (to the couples at work or "in play") the nose-down cumulative effects of spoilers, reverse and braking (as well as the c of g position that's tending to keep the nose on the ground).
.
If you go back to the beginning of this thread, the original (and very reasonable) postulate was that
sufficient backstick could be progressively fed in so that the nose would stay down and the main-gear benefit
from the aerodynamic downforce of the horizontal stabilizer under backstick - thus gaining considerable
braking traction upon contaminated runways at higher speeds (i.e. not long after nosewheel on).
.
Using convoluted math that excludes the most significant factors or (like LEM) hypothesizing with completely
imaginary figures and drawing bogus conclusions from them, is to cobble together an "argument" without any useful
substance.
.
There may well be weight transference that's cumulatively in favor of the technique, but there is also (quite unquestionably) an aerodynamic downforce (on the maingear) courtesy of the backstick.
.
I suggest that you should now reflect upon the fact that backstick braking is a proven methodology - and then
drive yourself even maddder trying to figure out WHY.
In your 23 March 03:51 post you completely and purposely neglected to factor in (to the couples at work or "in play") the nose-down cumulative effects of spoilers, reverse and braking (as well as the c of g position that's tending to keep the nose on the ground).
.
If you go back to the beginning of this thread, the original (and very reasonable) postulate was that
sufficient backstick could be progressively fed in so that the nose would stay down and the main-gear benefit
from the aerodynamic downforce of the horizontal stabilizer under backstick - thus gaining considerable
braking traction upon contaminated runways at higher speeds (i.e. not long after nosewheel on).
.
Using convoluted math that excludes the most significant factors or (like LEM) hypothesizing with completely
imaginary figures and drawing bogus conclusions from them, is to cobble together an "argument" without any useful
substance.
.
There may well be weight transference that's cumulatively in favor of the technique, but there is also (quite unquestionably) an aerodynamic downforce (on the maingear) courtesy of the backstick.
.
I suggest that you should now reflect upon the fact that backstick braking is a proven methodology - and then
drive yourself even maddder trying to figure out WHY.
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Originally Posted by TheShadow
MFS
In your 23 March 03:51 post you completely and purposely neglected to factor in (to the couples at work or "in play") the nose-down cumulative effects of spoilers, reverse and braking (as well as the c of g position that's tending to keep the nose on the ground).
In your 23 March 03:51 post you completely and purposely neglected to factor in (to the couples at work or "in play") the nose-down cumulative effects of spoilers, reverse and braking (as well as the c of g position that's tending to keep the nose on the ground).
Whether the effect of reverse thrust is nose-up or nose down is irrelevant. The question is, what is the incremental effect of the technique: does it cause more load on the main gear or not? Whether there is an offset in overall pitching moment or not is utterly irrelevant, provided we do not have to consider the catastrophic case of causing the nose to rise such that we actually lift off again.
As long as the characteristics remain linear - which is why I keep pointing out that oleos are NEVER fully compressed, please note - the consideration must be between the DIRECT effect of increased download on the main gear, arising from the pure lever action of the tail download, and the INDIRECT effect of increased pitch attitude, increased AoA and hence increased overall aerodynamic lift. If the former is greater, there's a net benefit; if the latter, an net degradation is decel capability.
As with many things in life, there is no single right solution; on some types, depening on the characteristics, it will work. On others, it won't. On certain types it may be dependent on other factors - like weight or cg - as to what is the better option. To pretend that there is some undiscovered holy grail of truth, if only the stupid OEMs would recognise it, is to be entirely misguided. As I mentioend a long time ago, if it was that simple it'd be in the AFM data AND we'd be taking credit for it already.
