Assymetrical Thrust proper tech in x-wind?
08-20-2009 | 10:28 PM
#111
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I suspect the discounting of reverse thrust is again for the KISS rule of thumb. It would be incredibly taxing on ones mind to work with reverse thrust and the rudder pedals with proper coordination while scared your going to slide off the runway.
08-20-2009 | 10:48 PM
#112
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I have a few issues with what you typed to my response. The things I listed weren't just my opinion, but also from a flight manual book from the FAA.
Drag isn't a big issue if you have autofeather (preferred) or the ability to feather/windmill an engine in case you lose one. Which leads me to discuss VMCA. The fear here is that a pilot uses this technique poorly, goes around, applies power asymmetrically to the engines (downwind engine spooling up to go-around thrust first) while you were banked slightly into the upwind engine to kill the drift from the crosswind. This is EXACTLY when VMCA is going to kill you. Read this article. AvioConsult - Accidents after engine failure
Drag isn't a big issue if you have autofeather (preferred) or the ability to feather/windmill an engine in case you lose one. Which leads me to discuss VMCA. The fear here is that a pilot uses this technique poorly, goes around, applies power asymmetrically to the engines (downwind engine spooling up to go-around thrust first) while you were banked slightly into the upwind engine to kill the drift from the crosswind. This is EXACTLY when VMCA is going to kill you. Read this article. AvioConsult - Accidents after engine failure
For my crosswind, I WANT the sideslip.
As I said before, drag isn't that big of deal. The stall issue applies here too. This manuever doesn't seem to be one that you're performing in the flare. It sounds to me that you have to set it up first for it to work correctly. Shdw admitted that he did it from 2 miles. Not faulting him. But I wouldn't be applying forward-slip controls that far out either. So, drag, not that big of deal. Stall, yes. If I crab to a forward-slip, I'm doing it at the runway. And all I have to do is let go of the controls and I neutralize everything instantly. In this scenario, I have the get the power symmetric or pull the downwind engine -- neither of which is going to be as quick as the flight controls.
Look, we aren't going to agree. And that is ok. It's what makes the world go around.
But you are correct. It is all about risk management. What is more dangerous? Using this technique to give you extra crosswind capability or crashing because you had your aircraft in an odd flight manner when an engine failed?
Both are stupid because you should have never tried to land on a runway without having the crosswind limits in the first place. Which was my original point about 4-5 pages ago ... and is what other's were saying you could use this technique for. If you can't land using a traditional crab to a foward-slip, then don't land. Otherwise, you are needlessly addiding risk.
But you are correct. It is all about risk management. What is more dangerous? Using this technique to give you extra crosswind capability or crashing because you had your aircraft in an odd flight manner when an engine failed?
Both are stupid because you should have never tried to land on a runway without having the crosswind limits in the first place. Which was my original point about 4-5 pages ago ... and is what other's were saying you could use this technique for. If you can't land using a traditional crab to a foward-slip, then don't land. Otherwise, you are needlessly addiding risk.
08-20-2009 | 10:53 PM
#113
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From: Burning the Agitprop of the Apparat
" blightingly " -flyingchicken
Ok, this explains a great deal that was heretofore unknown.
"Not something I think I'll do 2 miles out either... for me this is something I think I'll file away in the back of my mind, just in case one day I really really (really) need the airplane to scoot sideways another 50 feet but I'm out of bank and about to scrape a pod or wingtip." flyingchicken
There now, that's a sensible lad. Not that we have seen any real evidence that it will, but it's the thought that counts. Scarebus logic below 100 feet AGL may come into play here too.
Ok, this explains a great deal that was heretofore unknown.
"Not something I think I'll do 2 miles out either... for me this is something I think I'll file away in the back of my mind, just in case one day I really really (really) need the airplane to scoot sideways another 50 feet but I'm out of bank and about to scrape a pod or wingtip." flyingchicken
There now, that's a sensible lad. Not that we have seen any real evidence that it will, but it's the thought that counts. Scarebus logic below 100 feet AGL may come into play here too.
08-20-2009 | 10:59 PM
#114
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I am unsure about the phrasing, "pulled," but the net effect will be a turn to the left.
He is right and here is how you can see it. Imagine your in a boat rowing along, you drop the left ore into the water (your reverse thrust), the boat turns left. Similarly if you have a left to right crosswind, the upwind side being the left side, if you drop the reverse thrust on the left engine (putting the ore in the water) the aircraft will turn left, or upwind, assuming it has forward motion when this happens.
Edit: Reverse thrust is nothing more than massively increasing drag on whichever side it is applied, usually both equally, just like dropping an ore in the water massively increases drag on that side of the boat.
Make sense?
Sorry for the triple posts by the way, just saw this.
He is right and here is how you can see it. Imagine your in a boat rowing along, you drop the left ore into the water (your reverse thrust), the boat turns left. Similarly if you have a left to right crosswind, the upwind side being the left side, if you drop the reverse thrust on the left engine (putting the ore in the water) the aircraft will turn left, or upwind, assuming it has forward motion when this happens.
Edit: Reverse thrust is nothing more than massively increasing drag on whichever side it is applied, usually both equally, just like dropping an ore in the water massively increases drag on that side of the boat.
Make sense?
Sorry for the triple posts by the way, just saw this.
Yes, it makes 100% sense. Just because the aircraft is pointed into the wind, doesn't mean it is tracking upwind. Nose wheels have been known to slide. Wind body forces (pushing you downwind), the reverse thrust (pulling you downwind) ... and if you are pointed into the wind, reverse thrust is pulling you off the runway. But don't believe me, take it from McDonnel Douglas. Why would they put a warning into the flight manual then? This is especially dangerous on wet or icy runways. That is why Boeing, McD, and I would suspect, Scarebus, all say to discontinue revers thrust if directional control cannot be maintained.
It doesn't matter if you are single engine or which direction the wind is from. You can use reverse thrust or beta (not restricted in any aircraft I've flown). However, if you get yourself turned into the wind, which could have happened from asymmetrical reverse thrust, the opposite of what you expected (going off the runway on the downwind side while pointing into the wind) may very well happen.
I believe the yawing moment generated and rudder sideforce required to counter the resultant yaw in the asymmetrical case should be much greater than the downwind component of any thrust line offset from reasonable crab angles.
The end result is you end up with a boat going at a sideslip through the water.
08-20-2009 | 11:09 PM
#115
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But it flies on the same basic physics - its just another airplane.
Like I tell the girls in the back - "Just work the (side)stick, baby!"
Looking forward to see what happens when I get some time in the sim.
08-20-2009 | 11:24 PM
#116
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08-20-2009 | 11:34 PM
#117
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From: Burning the Agitprop of the Apparat
08-21-2009 | 01:00 AM
#118
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Like I said earlier, I applied it 2 miles out so I could stabilize the approach in a sideslip. As I, and my CFI at the time, had never done this, we both wanted to explore the possibilities further out instead of slamming it in 100 feet away with pure guess work. Just like you teach the first time student to slip it the entire way in for a first crosswind, then teach crab to slip transition, I think it is best to slip it in with this technique all the way in the first few times, then you will know what is necessary for transition.
As you will likely not use it for common procedures I think you would be out of your mind to use it for the first time only 100 feet off the ground. You have no clue what to expect from that aircraft, why not figure that out on the way in and deal with a little extra drag. Again remember this is only 5 - 10 percent of the drag that would be experienced in an OEI go around, it isn't like your dropping out parachutes.
HAHEYYYY!!!! Annddddddd thus allowing you to reduce your crab into the current (wind) slightly, just like asymmetrical thrust with an applied slip. You stumbled on the answer yourself that time buddy. 
I see what you mean about the rudder by the way, I can't believe I missed that. Nose goes right aircraft drifts left if rudder is used properly, makes perfect sense, thanks.
As you will likely not use it for common procedures I think you would be out of your mind to use it for the first time only 100 feet off the ground. You have no clue what to expect from that aircraft, why not figure that out on the way in and deal with a little extra drag. Again remember this is only 5 - 10 percent of the drag that would be experienced in an OEI go around, it isn't like your dropping out parachutes.
Get your boat and ores as before. Switch on motor and boat moves forward. Stick left ore into water - boat yaws left, and will continue to yaw left until..... you put in some right rudder.
The end result is you end up with a boat going at a sideslip through the water.
The end result is you end up with a boat going at a sideslip through the water.
I see what you mean about the rudder by the way, I can't believe I missed that. Nose goes right aircraft drifts left if rudder is used properly, makes perfect sense, thanks.
08-21-2009 | 08:16 AM
#119
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From: Burning the Agitprop of the Apparat
Just one minor point, since most jet manuals will direct you to close the throttles or "thrust levers" starting at 50-25 feet prior to touchdown and land ideally with idle thrust, how exactly is this magic going to assist in rollout control?
What happens at touchdown, or just prior, as one or two engines go to idle prior to the other(s)?
What happens at touchdown, or just prior, as one or two engines go to idle prior to the other(s)?
08-21-2009 | 10:03 AM
#120
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So just do your landing normally below 50 feet, the only difference is the rudder required to get the aircraft back to centerline will be less since your crab angle is less.
F=ma where a = dv/dt, or change in velocity over a given time, acceleration.
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