Braking techniques

Operating an F-16 on the ground or in the air - from the engine start sequence, over replacing a wing, to aerial refueling procedures
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Gums

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Unread post12 Apr 2008, 18:36

Salute!

On the other thread concerning the belly landing of a YF-16 we got into brake design and procedures.

For all who haven't worked on or flown the Viper, brakes are a big deal. The newer models are heavier, so they may not have the same problems we had in the early days........

RIT? What's that? Reduced Idle Thrust. That light little jet would get up to 60-70 knots if you didn't use lottsa brakes when taxiing. We once calculated our range if we could cruise on I-80 in idle at the legal speed limit. Turned out we could go lots further than any car any of us owned. 'course, that was over a thousand gallons of JP-4. Heh heh, when we were hit with bad fog and no legal alternates, we even considered taxiing to another base on the interstate (just kidding, but some of us DID talk about it).

Anyway, when you looked at the Dash-1, there was this note about taxi distance and brakes. Seemed that the further you taxied, the higher temperature the brakes and wheels got. That was 'cause we were keeping the thing under 25 knots or so. So we had a few incidents where the wheel rim plugs melted and tires deflated before they exploded. I don't recall any incident where the tire actually blew, so guess the design was good.

Being an instructor and all that, I had to look at the brakes, pass on clues to the yutes, and ......

I found from basic physics and from the tech data that the biggest thing contributing to brake energy being absorbed by the multiple disks and rotors was SPEED. Unless you landed 5 minutes from takeoff, with all your ord, then weight wasn't a big deal. How come?

Well, all the energy that the brakes absorb turns into heat. That energy is related to speed and mass ( mass*vee^2). The driver is the speed, which is squared. So if I am going twice the speed, I have to absorb 4 times the energy.

If you start "squaring" numbers, you find that reducing your speed by one-third cuts the energy number by half. So at 6 furlongs per fortnight, the number is 36. At 4 furlongs per fortnight, the number is 16!!!! That's way less than half, but good enuf for the discussion.

We looked at the brake data for the Viper, and sure enough, if you didn't put the brakes on until 80 knots, you used less than half than you did if you put the brakes on lightly at 120 knots. So preferred technique was/is wait as long as you can, then cram on the brakes!

Last point: So after this, I looked at some "delta" brake energy numbers. like "put brakes on from 120 to 80 knots", then coast to end of runway. GASP! Worse thing you could do. You were still doing 60 knots or so on a long runway. When your wheel plugs melted and tires went flat on the taxiway, it wasn't fun. I then decided to see how my truck would do.

So I used 70 mph and tapped the brakes to slow to 65 mph. Turned out that it was equivalent to slowing from 30 mph to a full stop using max braking. Heh heh. So my last two trucks have gone over 140,000 miles and still had/have a third of the brake linings left. Takes guts, but let the thing slow down until you can't stand it, then stand on the brakes. It ain't the pressure, it's the speed.

later,

Gums sends ....

P.S. Same technique works on your clutch. Have never replaced a clutch....
Gums
Viper pilot '79
"God in your guts, good men at your back, wings that stay on - and Tally Ho!"
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SnakeHandler

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Unread post13 Apr 2008, 04:52

I always aerobrake till the nose starts to fall and then I set it down as gently as I can and hold full aft stick and override the boards. I wait until I get to the 3 board which results in about 80ish knots and then I honk on the brakes. Being at the three board gives me time to lower the hook if the brakes don't work. I flew a jet everyday for a week and the brake indicators didn't move a milimeter in that time. I also do the rudder kick method just before touchdown and never get so much as a mark on the tires. The jets will last a lot longer and have less problems with them if we treat them better.
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MechFromHell

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Unread post13 Apr 2008, 05:00

Just curious how you measured brake wear from 70 to 65 mph versus 30 to 0 mph??
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Unread post13 Apr 2008, 17:55

Yo Ho!!! HellMech!!!

Great question, and I have been trying to get the Car Talk guys to get on this for years.

The energy our brakes have to absorb is related to the energy when we cram them down compared to the final energy we choose - like a full stop, where energy is zero, to a few mph slower, whatever that energy is.

What few of us had thot about was the math.

The energy is related to the square of our velocity. So e=v^2 on your excel sheet. Actual equation has mass and such, but I ain't counting the mass of the fibers on the brake rotors and pads we lose during braking, heh heh. So actual equation is e=(m*v^2)/2 or very close if we ignore Einstein's crapola.

The thing most of us forget is it ain't the square of the difference in velocity, it's initial velocity (v1) energy minus final velocity energy (v2). So five knots change at 5 knots is 25 units of "e". 26 knots to a full stop is 26^2 minus zero, or 676 units of "e". I use 26 knots as it is the actual energy the brakes absorb from 70 to 65.

My rule of thumb was 70 to 65 is about like 30 to zero, so eeeexxxxxcuuuuuse me!

The actual equivalent of 30 mph to zero is 71.6 mph to 65 mph.

So set up your Excel sheet and run the numbers.

From 120 knots to 85 knots is the same as from 85 knots to zero!!! So we were screwing our brakes if we used light pressure too soon. Don't make no difference how hard you press until you reach teh "brake energy" limit your disks and rotors can take without coming apart.

The Viper brakes were cosmic, so unless we were above 140 knots or so and heavy, that wasn't the problem. The problem was they would absorb all that "e" and the wheels and tires would heat up. After burning my crewchief's hands and my own, I became "kinder and gentler" on the brakes.

If you question I can get immense brake wear, try computing some small brake taps on the freeway with coming to a full stop from 20 mph. I ain't making this up! From 70 mph to 67 mph is the same as from 20 to zero!

So these tailgaiting folks who think they are in the Daytona 500 driving the 88 car are peeing away brakes like crazy. Ask the NASCAR drivers about using brakes at 180 mph. Those guys use brakes only as a last resort if they want any at the end of the race. Not only can they run outta brakes, but they can have serious pit stop problems like we did.

Think about hot pit refueling and having your brakes glowing red and the tires starting to burn.....

Gums sends......
Gums
Viper pilot '79
"God in your guts, good men at your back, wings that stay on - and Tally Ho!"
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asiatrails

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Unread post14 Apr 2008, 01:46

Gums, that’s exactly why the FAA Aborted Take Off Test is always done with brakes and pads which have been artificially worn so that only 10% of their useful life remains.

Here's a dyno test video: http://www.youtube.com/watch?v=m1dv_y_3EK0
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Unread post14 Apr 2008, 02:06

Gums,

Good theoretical analysis for your car or truck, but you're forgetting the combined effects of engine compression drag, rolling resistance, and aero drag ("other drag"). Take your truck up to 70 mph and just take your foot off the gas pedal. What happens? You slow down due to the above factors, with no braking at all. Let's say it takes 5 seconds to slow to 60 mph. Then get back to 70 and apply your brakes at whatever moderate pressure you want, and it takes 2 seconds to slow to 60. The deceleration rate from the first case is 2 mph per second. So in the second case, in the 2 seconds you were braking, of the 10 mph you slowed, 4 mph was due to "other drag" or 40%. So 60% of your energy loss goes into heating the brakes.

Then go back to 70 again, apply max braking, and it takes 1 second to slow to 60 (that's about .5g, you won't get much more than that from a truck). In that 1 second, the "other drag" causes you to lose 2 mph, or 20%, leaving your brakes to absorb 80%.

Point is, how hard you apply the brakes makes a difference in how much energy (heat) your brakes absorb. Lightly applying the brakes results in less brake heat than heavy application (for the same speed loss) because the "other drag" has a longer time to help you slow down.

Stupid automatic slushbox reduces engine compression drag. In my stick shift, I usually gear down to help unload the brakes. The Car Talk brothers say that increases clutch wear, but I've never had to replace a clutch, so what do they know?

Those numbers are realistic for cars and trucks, but maybe not for airplanes. Your F-16 at 120 kt has (relatively) less rolling resistance, more aero drag (especially if aero braking) and the engine thrust is trying to increase your speed, not slow you down. I'll go with whatever you say on that case.

Your recommendation is right, delay braking as long as possible to let "other drag" reduce the amount of energy the brakes have to absord.
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Unread post14 Apr 2008, 03:33

I think aerobraking would not work in this situation

http://www.youtube.com/watch?v=vZQ9pS1b ... re=related

The call "ACLS lock on, say needles" that you hear is a controller to pilot cross check to confirm that the glideslope information being presented by the automatic carrier landing system (ACLS) is operating correctly.

The call "You're a little low" is from the LSO. At 3/4 mile the image the pilot sees as a centered ball is just over 25 feet high, as you cross the ramp the aperture size is about 2.5 feet high.

Normally transition to the carrier scan of "meatball, line up, angle of attack" occurs at about the 3/4 of a mile point.
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Unread post18 Apr 2008, 19:32

Salute!

So what's the point, Asia?

The best thing you have for those landings is a rapid spool-up on your motor in case you miss all the wires.

later,

Gums sends ....
Gums
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"God in your guts, good men at your back, wings that stay on - and Tally Ho!"
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Unread post18 Apr 2008, 23:57

Same technique on blk52+, aerobraking(13 AOA) until 100, nosegear down-full aft -override and hit the brakes.
Dangerous speed is >120k for hot brakes(-1).
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Unread post19 Apr 2008, 02:51

Two of you have mentioned overriding the boards. I'm probably wrong, but I was thinking if you have full boards selected with gear down, they will extend only to 45 degrees, but when the NLG WOW switch closes, they will extend fully (60 deg). Do you still have to manually override to get full extension?
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Unread post19 Apr 2008, 09:47

johnwill wrote:Two of you have mentioned overriding the boards. I'm probably wrong, but I was thinking if you have full boards selected with gear down, they will extend only to 45 degrees, but when the NLG WOW switch closes, they will extend fully (60 deg). Do you still have to manually override to get full extension?


Hey John, the brakes extend to 43 degrees with gear down; to keep them at 60 degrees you have to hold the switch (as you know). When the NLG WOW switch closes you can actually open the brakes at 60 deg and it'll remain at 60. So you have to open them when the nose is on the runway (they don't auto-open), but we keep holding the switch just in case.
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Unread post19 Apr 2008, 16:48

Salute!

Great to have you here JAY-pol.

If you're a driver, we really need to get more of your type in here. John-boy and I are from the pre-cambrian epoch, and we don't fight those dinosaurs much anymore.

*************

The board limits were good, and sounds like they are still "good". The Sluf also had limits on the boards when gear was down. If you ever saw the full extension of the Sluf boards, you would go, "wow!". That sucker went down to reasonable deflection with a click, but to get full extension you had to hold the switch just like the Viper. Fully extended, they not only went down more, but then opened horizontally. Honest to God, you could go straight down in idle with full boards and stay at about 400 knots. Kinda felt like a Stuka, heh heh.

The "big tail" came about in our efforts to minimize chances of getting into a deep stall, as it gave us more "nose down" pitch authority. A side benefit was better turning in a slow knife fight, so we gained a little when dueling with a Hornet or Eagle, which didn't have the AoA limiter and had better nose-pointing ability than us.

The "notch" at the edges of the big tail was there just like the speed brakes limits. The thing could scrape if you pulled too hard with WOW. Original issue tails were not only smaller, but were "square" at the trailing edge. So we had a more "relaxed" stability than the Bk 15's and later that had the big tail. Bk 10's were retrofitted with the big tail, but I think a few Bk 10's and all of the Bk1 and Bk 5's kept the small tail.

We pilots always thot the small tail Bk 10's were the best A2A versions of the Viper, but ain't many of us here to support my view on that.

out,

Gums sends ....
Gums
Viper pilot '79
"God in your guts, good men at your back, wings that stay on - and Tally Ho!"
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Unread post20 Apr 2008, 00:37

Gums wrote:If you're a driver

At your service!
Gums wrote: John-boy and I are from the pre-cambrian epoch, and we don't fight those dinosaurs much anymore.

:lol: :lol: :lol:

Gums wrote:The Sluf also had limits on the boards when gear was down. If you ever saw the full extension of the Sluf boards, you would go, "wow!". That sucker went down to reasonable deflection with a click, but to get full extension you had to hold the switch just like the Viper. Fully extended, they not only went down more, but then opened horizontally. Honest to God, you could go straight down in idle with full boards and stay at about 400 knots. Kinda felt like a Stuka, heh heh.


My previous jet was the A-7H/E. The monster-brake of the sluf retracted automatically when you lowered the gear, cause if it didn't, you would split the runway in two!! Btw, really loved that jet.

Thanks for the warm welcome Gums!
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Unread post20 Apr 2008, 01:01

Gums wrote:The "notch" at the edges of the big tail was there just like the speed brakes limits. The thing could scrape if you pulled too hard with WOW.


Still happens … here is a recent souvenir. The nozzle flap is DIFM and boards were blended or I would have kept them as well.
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Unread post22 Apr 2008, 03:32

Hey Gums,

Remember those days of high closure rates in the T-Bird and thumbing those big "alpha" boards out and adding a fist full of forward stick to stay on level!!!
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