Difference between landing and takeoff?

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saberrider

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Unread post11 Dec 2016, 10:38

In landing configuration flaperons are dropped at 20degrees angle in F 16 .At takeoff the same but speed is higher than landing also is more heavy.What is for an a/c to act so different in this two instances? Speed,LEF position,AoA,air density of initial set up configuration?I presume that is glide slope .
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Gums

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Unread post11 Dec 2016, 19:16

Salute!

Firstly, most of the increased speed for takeoff is due to weight. An empty Viper in A2A configuration might only weigh 18,000 pounds or so versus 28,000 + at takeoff. That is one helluva lot of weight fraction, requiring a lot more "lifies".

There are also changes to some FLCS functions once WoW ( weight on wheels).

If you look carefully at pictures of a Viper when taxiing, the LEF are not down 15 degrees. In fact, they are up about 2 degrees. AoA/airspeed data appears to become reliable around 60 knots if you look at my LEF HUD failure vodeo. I never paid much attention, but the LEF's are definitely down once WoW on takeoff. I need a current pilot/crewchief to verify. My feeling is that there is less drag and a shorter roll.

Gums sends....
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saberrider

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Unread post11 Dec 2016, 21:35

First batch of Vipers(YF16) have LEFs in down position in taxi -takeoff roll 25 down not 15.Guess FLCS logic's changed in time.Don't now where to look for video can you provide me a link.By the way thanks for your help with my aqward questions https://www.youtube.com/watch?v=cPYjmJynE_0
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vilters

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Unread post11 Dec 2016, 22:59

A/C configuration for T/O and Landing are basically the same.
Gear and Flaps down, and the transfer between ground => air or between air => ground is done by the WOW switches.

Landing config is +/- 125 kts (+ 5kts for each 1.000 lbs) and between 11-13 AOA.

Tale off speeds are computed for weight but the A/C configuration is exactly the same as for landing.
Roll starts with LEF 2° UP and they come down when Weight goes OFF wheels.

Gear and Flaps come up together by pilot gear handle action, and LEF controlled by FLCC as from WOFFW.
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Unread post11 Dec 2016, 23:12

Salute!

TNX, Vilters.

The LEF angle changed with the FSD version after the YF-16 flyoff and initial testing. So it went to 15 degrees. There were other changes to the FLCS computer functions such as max roll rate you could command and so forth.

Once the golden arms discovered that we did not have enuf pitch authority to recover from a deep stall, they added the MPO for direct control of the stabs. Then the Bk 5 had the slight movement in the stick, and we got the video recorder versus old 16mm film. Honest!! My first A2A introduction flight had a day delay for complete debrief so we could develop the film. Heh heh.

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johnwill

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Unread post12 Dec 2016, 00:41

Like Gums says, the control functions change with time. The last time I worked F-16 (1986) the LEF position was determined by mach number and AoA for up and away flying. Taxi was with LEF at 2 degrees up. For takeoff roll the LEF stayed at 2 degrees up until Nose Gear WOW was off, then LEF moved to Mach/AoA schedule. Less drag during roll, more lift and stability from rotation onward.

For landing, LEF controlled by mach/AoA until Main Gear WOW, then driven back to 2 degrees up. That provides more drag and keeps the nose up longer during aero braking. There was a plan to try bringing the TEF to 20 degrees up after Nose Gear WOW to provide more down force (better wheel braking) and drag. Don't know if that ever happened.
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Gums

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Unread post12 Dec 2016, 00:54

Salute!

TNX, John-boy.

I also seem to recall that the LEF went up a bit and/or the trailing edge flaperons went up a bit once supersonic. My reference booklets from GD were written during FSD, and some things were obviously modified even for the Bk 5.

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johnwill

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Unread post12 Dec 2016, 01:36

You are correct about the supersonic. From 0.95 to 1.05, LEF and TEF moved to 2 degrees up. The purpose was to flatten the wing and counteract the aft shift of the center of lift. That shift created a nose down pitch moment and required the horizontal tail to push down to balance it. That down load meant more lift (and drag) from the wing and fuselage was needed.

LEF up resulted in increased lift on it. TEF up resulted in a down load on lt. That pitch moment was enough to balance most of the moment from the supersonic shift. Lower tail loads, lower wing loads, less drag, everybody wins.
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saberrider

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Unread post09 Jan 2017, 20:47

Look like I'm not putting right questions. So if an F16 which is at AGL3000feets and about to land ,pilot drop the the landing gear at speed of + 200knots then TO/Landing gain configuration setup is in place the speed is enough to climb ,AoA 11 degrees ,why at this speed a/c drop and not climb ,leave aside glide path of -2degrees?
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Unread post10 Jan 2017, 00:30

Salute!

Well, Saber, you need to write to LM and ask/pay for a tech order or three.

We do not understand your questions. And we sure as hell don't know where you are going with all this.

The gear and flaperon and LEF doofers work according to all the FLCS laws and the position of the gear handle or the ALT flaps switch ( don't ask about it here!!!)

Look like I'm not putting right questions. So if an F16 which is at AGL3000feets and about to land ,pilot drop the the landing gear at speed of + 200knots then TO/Landing gain configuration setup is in place the speed is enough to climb ,AoA 11 degrees ,why at this speed a/c drop and not climb ,leave aside glide path of -2degrees?


Good friggin' grief!!! You can lower the gear handle or use ALT flaps thru a large range of speeds and altitudes. Where in the hell is the 2 deg descent angle coming from? What does 3,000 ft have to do with anything?? At 200 kts IAS and most any altitude up to maybe 25,000 feet you can climb or descend or stay level depending on the position of that big handle in your left hand!! Sheeesh. You can lower just the flaps ( flaperons) with the ALT flaps switch and may or may not switch to landing gains, depending on block and other mods. I forget and am tired of using the FSD tech stuff I have and do not trust my fading memory 90% of the time anymore. You can climb or descend at many various AoA degrees depending on the position of that big lever in your left hand, so 11 deg AOoA is irrelevant. I flew the original 13 deg AoA for approach as I did not like a long, drawn out flare and was slower to reduce rollout so was not an 11 deg AoA guy.

Gums sends....
Gums
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saberrider

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Unread post10 Jan 2017, 04:53

Numbers I have put on display is informative to show a general picture .A plane coming to land it was a glide slope .So FPM is under horizon about 2-,2.5degree.To be clear if the flaperon are lower at a value 20 degree s and landing gear is dropped the same settings is at takeoff not another set for flaps,what make the plane to climb if the camber of the wings is the SAME.Another types of the plane put flaps at landing setting not at take off setting, but F16 have only one settting 20 degree.
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saberrider

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Unread post10 Jan 2017, 09:09

Gums wrote:


We do not understand your questions.

Good friggin' grief!!! You can lower the gear handle or use ALT flaps thru a large range of speeds and altitudes. Where in the hell is the 2 deg descent angle coming from? What does 3,000 ft have to do with anything?? At 200 kts IAS and most any altitude up to maybe 25,000 feet you can climb or descend or stay level depending on the position of that big handle in your left hand!! Sheeesh. You can lower just the flaps ( flaperons) with the ALT flaps switch and may or may not switch to landing gains, depending on block and other mods. I forget and am tired of using the FSD tech stuff I have and do not trust my fading memory 90% of the time anymore. You can climb or descend at many various AoA degrees depending on the position of that big lever in your left hand, so 11 deg AOoA is irrelevant. I flew the original 13 deg AoA for approach as I did not like a long, drawn out flare and was slower to reduce rollout so was not an 11 deg AoA guy.

Gums sends....

Thanks, so throttle is the key to mentaine glideslope not stick, when you are about to land.It is hard to capture 11*-13*AoA ?Do you have to adjust throtle AND the Stick to mentaine it? You mentioned there are more gains you can cycle while flaperon are down,aside To/landing.Can you tell me more about ?If the plane is heavy at landing it is not indicated to fly at 11*AoA and higher speeds?
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The-2_-2,5 degrees FPM under the horizon line in HuD of F16 on landing
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saberrider

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Unread post10 Jan 2017, 11:26

vilters wrote:A/C configuration for T/O and Landing are basically the same.

This is why I'm not sure how the plane is change (gain alt. at take off and drop at landing ).
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Unread post12 Feb 2017, 00:02

Let's go back to basics. During takeoff, the objective is to get to flying speed as quickly as possible. Flaps lower the flying speed, but they also increase drag and therefore reduce your acceleration. So it's all a tradeoff; typical compromise is to use partial flaps.

During landing, the lower the airspeed the better, so this is where high-lift devices pay off. Since your speed is stable (not accelerating) the added drag of these devices is of little importance because one usually has excess power in the landing config. Until you wave off...then it's critical to clean the bird up as quickly as possible.

Enough stating the obvious...
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