Forum: F-16 Procedures

Enthusiast Joined: Oct 01, 2007 - 08:23 PM Posts: 29

Some questions about ground handling that I hope you guys can set me straight on.

When NWS steering is engaged, the orientation of the strut is controlled by the pilot using the rudder pedals. Application of one toe brake more than the other (to get a differential brake force pressure between the two mains) can add to the turning force and decrease turn radius if combined with rudder bar input. I hope I'm on solid ground so far in my understanding Smile

My next question is what happens when the NWS is turned off. Hypothetical: Say you are at normal taxi speeds with NWS off and you press on one toe brake only keeping the rudder centered. In this case what would you expect the jet to do?? How free is the NLG strut to rotate in this case?? Is is like the tail wheel on a GA tail-dragger aircraft just in that it will turn freely in this case?? Or is it in some sense damped or held centered??

For bonus points part of the reason that I'm wondering about the above questions is curiosity about landings. When you are firmly on the ground but aerobraking, I note that in the Semper Viper articles (for example) there's a suggestion to apply main gear braking in combination with the aerorake, holding up the nose with increasing back pressure on the stick. The nose wheel is still in the air in this case but again I wonder about how sensitive the jet is in this attitude to unequal pressure on the toe brakes and/or how easy it is to apply pressure equally enough to get effective wheel braking in this case without snaking all over the runway! Smile

I'd really appreciate any insights you guys might be able to offer.

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Quote:

My next question is what happens when the NWS is turned off. Hypothetical: Say you are at normal taxi speeds with NWS off and you press on one toe brake only keeping the rudder centered. In this case what would you expect the jet to do?? How free is the NLG strut to rotate in this case?? Is is like the tail wheel on a GA tail-dragger aircraft just in that it will turn freely in this case?? Or is it in some sense damped or held centered??

From a technical standpoint the NWS actuator will remain in its current position whenever NWS is disengaged, it is not free to rotate the aircraft will track on the NWS actuators last position while in NWS, although im sure differential braking could influence this somewhat but not very gracefully. When NWS is reengaged the NWS actuator will center without any pilot input. From an operator standpoint someone who drives the thing will have to answer that part.

Enthusiast Joined: Oct 01, 2007 - 08:23 PM Posts: 29

Thanks for the information, sweetpete! Hopefully an "operator" will drop by and fill in the rest of the picture.

Forum Veteran Joined: Jul 01, 2007 - 07:22 PM Posts: 620

The approved landing solution is to aerobrake to 100 knots and then smoothly fly the nose to the runway. Then, mash down on the brakes as firmly as possible while maintaining control with differential braking till the jet is at a normal taxi speed. Maintain the center of the runway until you engage the NWS. Then engage NWS and drive it like normal. If you have not reached the turnoff or the end of the runway, move the jet till the wingtip is over the edge of the side of the runway you intend to turn off toward (cold side). We don't brake in the aerobrake because if the anti-skid feature isn't working, you'll probably not know it until you hear two loud thumps and some wicked vibration as your main tires explode. Also, braking above 120 knots could lead to hot brakes. The whiz kids determined that the most efficient braking technique with the least amount of wear is to get it all done at once as quickly as possible, then taxi at a normal speed.

As for turning the jet with just braking, the best turn radius comes from NWS off, about 10-15 knots of speed and full brake pedal deflection. The nose wheel will caster and the jet will almost pivot around the main gear to which the brake is applied. It won't pivot around one of the tires because if it did, then you would create a flat spot where the tire was grinding against the ground.

I like bonus points. The jet is very sensitive to rudder and brake inputs while landing and taking off. As such, since the rudder is still very effective above 100 knots, then use the rudder for lateral control on the runway. There have been many landings in which I've been cruising down the runway at as low as 80 knots in ground effect (block 25), waiting for the jet to finally run out of airspeed and sit down. In cases like this, the brakes aren't effective and if you apply them as you land, you could skid the tires and end up in front of an accident board at best. The Viper is the easiest jet to land but the most difficult to land well. Cool

Enthusiast Joined: Oct 01, 2007 - 08:23 PM Posts: 29

SnakeHandler -- thanks very much: that is very useful.

Can I get greedy and ask a clarifier or two, please??

For aeroraking, it seems holding 13 AoA is the most effective attitude. As you slow from touchdown speed to 100 knots and prepare to fly the nose to the runway, does the amount of back pressure required on the stick to hold 13 AoA increase or stay fairly constant?? I can see that once you get well below 100 and the nose wants to fall on its own for lack of airspeed that trying to hold the nose up would require more pull as the critical airspeed threshold approaches (and of course in the limit you can't hold it forever), but I guess behavior above 100 knots isn't quite so obvious to me.

Secondly, given what you say about the nose wheel castering with NWS off, does that mean the normal braking procedure is to fly the nose wheel to the runway (presumably nose wheel touching at something less than 100) and then engage NWS and only then apply toe brakes as required to get down to taxi speed?? I'm guessing applying the brakes *before* engaging NWS in this case might be a it more excitement than one wants on a normal day?? Wink

To steer the jet on landing you have to use the steering wheel, keep your hands at 10 and 2 and PLEASE for god's sake, use your blinker when exiting the runway (failure to do so may result in serious traffic jams and fist fights. trying to get to the horn is also a bitch on this jet). Please refrain from sticking your head out the window and yelling "GO AROUND IDIOT!!!!" (he can't hear you anyway) EOR will then check your brake lights/head lights for servicability and clear you to taxi back to your spot. Smile and wave at the losers on the ground and don't forget to honk (if you can find the horn) at any good looking chicks (give a quick arm flex for added effect on your way past). When you get to your spot detach the steering wheel (NASCAR style) and egress the jet in a manner befitting the Dukes of Hazzard (bonus points if flight lead sees you do this). Lastly hand the keys to the jet to the crew chief for safe keeping until the next sortie. Tell the CC "Ran like a champ" shake a specialists hand and go to the bar.

Something else fun to do is when you're in a knife fight at 30K feet with your flight lead, get him on a head on pass and then......TURN ON YOUR BRITES AND FLASH THEM A FEW TIMES!!! haha, scares the hell out of him and he will be blinded for a few seconds allowing you to reverse and blast him out of the sky. Just makes sure to turn off your headlights before you send the 'winder.

BTW Snake, NWS is automatically disengaged on takeoff when the wheel speed sensors read 60 knots and will not let you engage on landing until speed drops below 60 knots. This is part of the take-off/landing config.

Enthusiast Joined: Oct 01, 2007 - 08:23 PM Posts: 29

Raptor_DCTR wrote:

To steer the jet on landing you have to use the steering wheel...

ROFL -- nice one Raptor_DCTR -- should be cake to model all that Wink

Quote:

BTW Snake, NWS is automatically disengaged on takeoff when the wheel speed sensors read 60 knots and will not let you engage on landing until speed drops below 60 knots. This is part of the take-off/landing config.

Really?? That's a surprise. The T.O. I've seen for blk5X jets says that NWS cancels when the NLG strut decompresses and the only requirement to re-engage it again is to get the compression back on the strut.

Could it be that NWS control is different on different age jets or is the T.O. text just "full of it" in this case??

it's prolly the different blocks...I'm not E+E so I could possibly be completely full of sh*t which is entirely possible if you read my above post.....I could swear though that on blk 25s it's the way I explained it.

Enthusiast Joined: Oct 01, 2007 - 08:23 PM Posts: 29

I think I read in a different thread here when I was first searching for this type of info that the Blk4X and Blk5X jets have WoW switches on the NLG and the earlier jets do not. Could be that the control of NWS migrated to the NLG when that WoW switch was fitted -- that scenario might make your memory and the T.O. text that I read consistent Smile

blk 25s have NLG WoW switches. They are a roller type instead of the plunger style of the 4X or 5X but the switch is still there on the blk 25/30/32. I think you're right though about the strut compression, i seem to remember helping out on a NWS job once that they could not complete because there was not enough weight on the strut but the wheel speed sensors have something to do with it too.....I don't know, I'm going to work the F-22 in a couple months so who knows.

The wheel speed sensors have nothing to do with NWS. The pilot has NWS up untill the nose gear is off the ground and that is due to the NLG WOW switch.

roger, thanks for the clarification...I'm not sure where I got the wheel speed sensor thing from. Confusing it with something else.

Enthusiast Joined: Oct 01, 2007 - 08:23 PM Posts: 29

OK well good to get that cleared up Smile

I'd still like to hear what the story is for the questions on back pressure for aerobraking and when NWS is generally re-engaged during rollout...if we can tempt an "operator" out to play again here Wink

Forum Veteran Joined: Jul 01, 2007 - 07:22 PM Posts: 620

Stick pressures are always pretty light in the Viper. 25 pounds of aft stick pressure, 16 pounds forward and 17 pounds lateral (12 pounds in Takeoff and landing gains) is all the sensors will recognize anyway. I got those straight out of the -1. I honestly don't feel the aft stick input because I'm focused on keeping the gun cross at 12 to 13 AOA. She's actually very responsive down to 80 knots or so. I spend most of my time on the rollout using both fore and aft stick to keep 12-13 AOA and the rest of my brain bytes fighting the wind to keep the wings level.

The FLCS in this jet commands flight control movements as required to maintain performance levels based off of takeoff and landing gains, cruise gains or standby gains. Thus, ten pounds of aft stick pressure results in the same nose up authority at 350 knots as it does at 500 knots in cruise gains. The slower the jet flies, the bigger the control surface deflection. The theory is essentially the same for the other gains as well (without going into great detail).

To your second question, leaving the NWS off till taxi speed is the preferred method (not to mention that max NWS speed is 70 knots). Once the nose wheel touches the ground, we overrride the boards and simultaneously stand on the brakes. When I'm done flying I want that jet at a safe taxi speed (sub 25 knots) yesterday. I routinely touch down between the 1000 and 1500 foot markers on the runway and I am ordinarily ready to taxi off a few thousand feet before the early turnoff here at Luke. Applying the NWS at high speeds can lead to a flipped jet due to the sensitivity at high speeds. With the NWS off the nose wheel will just caster like a shopping cart's front wheels and in the jet this usually means perfectly straight.

To Fixin': I looked it up, rub1con is right. But these jets are gettin' old. I've had some interesting gear malfs lately. The only other thing I have for you is in regards to your post about after landing checks. Anytime we are taxiing and pass either Eagles or Raptors we have to remember to put the engine to SEC to close the nozzle. You never know what those boys are likely to try to stick in there. Good luck on the Dark Side and don't drop the soap! LMAO

SnakeHandler, if you left the stick alone on approach, would you have to put back pressure on the stick for the jet to maintain 12-13 degrees AoA or would the jet maintain that AoA without any inputs from the pilot, assuming no winds affecting it? Also, how much does wind affect the jet during landing?

Forum: F-16 Procedures

Ground handling: Braking, NWS and nose gear strut behavior