F-16 stability at high AoA

Always wondered why the F-16 has a tailhook, or how big a bigmouth F-16's mouth really is ? Find it out here !
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ralfvandebergh

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Unread post04 Nov 2016, 12:49

Hi all,

In simulations I made at high AoA for stabilized approaches the F-16 stability changes toward the positive regions as angle of attack increases. The nose-dive usually happening after a pitch down disturbance due to the negative stability now is replaced by a series of oscillations towards the approximate original trimming; starting in level flight at pitch 10 degrees (high AoA) it returnes to around 7 degrees which is sufficient in terms of positive stability. (it is sufficiënt for a static stable aircraft to return to the approximate pitch angle and airspeed). You can even feel the aircraft flying more stable at high AoA as there is more pressure needed to push it up or down.

Now my actual question:

This should indicate that if AoA increases, the CL must move backwards as the CG must have a forward position to create static stability. What I generally learned is that normally at a higher AoA the CL moves in the other direction (forward). Is the CL position change different in the F-16 design?

The only clue of confirmation I could find on the web so far was in this old post from Gums (underlined) THANKS


PostThu Apr 29, 2004 2:24 am
Super post.

PLZ refer to the referenced posts above for more.

The relaxed static stability was the result of having the mean aerodynamic center of pressure very close to the center of gravity. If not pulling with some increased angle of attack, it was clearly 'unstable'. Only the FLCS control laws and resulting control surface movements kept the pointy end forward.
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ralfvandebergh

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Unread post03 Dec 2016, 13:15

I found myself the anwser after some experimenting:) Seems I was just confused with the positive tail lift.


Unstable: (positive tail lift)

>Distance between CL (positive force) and tail (positive force) is relevant.


Stable: (negative tail lift)

>Distance between CG (negative force) and tail (negative force) is relevant.


Thus as the AoA increases in an aircraft with negative longitudinal stability, the CL moves forward and distance to the positive tail lift becomes longer thus providing more stability.
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ralfvandebergh

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Unread post13 Dec 2016, 21:18

I think I was wrong in my first conclusion. I think its stabilator deflection what counts here: This is my idea now:

Principle of Negative Pitch Stability

In the case of an unstable aircraft with a forward CL and positive lift on the horizontal stabilizer the response on a pitch disturbance is totally different. A pitch down disturbance will result in more speed thus more airflow over the horizontal stabilizer. When flying in level cruise at zero angle of attack, the horizontal stabilizer of the F-16 is in neutral position thus providing positive lift. As a result, the tail moves upward bringing the nose even further down so the aircraft attitude moves further away from the original trimmed attitude. When flying at a negative angle of attack, a condition that is encountered for instance when flying at high speeds with flaps deployed, the horizontal stabilizer has a leading edge up attitude (comparable to descending attitude), thus a higher angle of attack. This higher AoA will cause the tail to raise even more (due to more lift), bringing the nose down even more so there is more pitch instability. In the case of deployed flaps however, there is some increased stability due to the flaps.

Increased Pitch Stability at High AoA

Several attempts to explain this condition aerodynamically as a result of CL position changes due to high AoA failed until I took a close look at the actual positions of the horizontal stabilizer during different phases in flight. This lead to my conclusion that the reversed situation occurs as explained in the previous section in zero or negative AoA conditions: In high AoA attitude, the horizontal stabilizer has its leading edge down thus a negative angle of attack. If there is a pitch down disturbance in high AoA configuration, the speed increases causing more airflow over the horizontal stabilizer with negative angle of attack with the result that the tail moves down more. This causes the nose to raise again thus the aircraft is able to return to its original trim attitude.
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johnwill

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Unread post14 Dec 2016, 04:03

I believe you will find the center of lift moving aft as AoA increases is due to LEF
deflection, which is driven by AoA and mach. As LEF deflects, it's local AoA is reduced, so lift on the LEF itself is reduced. But the LEF deflection causes more air to flow over the top of the wing, so total wing lift is maintained. Less lift at the front of the wing, total lift essentially the same, thus center of lift moves aft - stability increases.
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saberrider

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Unread post21 Dec 2016, 11:48

johnwill wrote:I believe you will find the center of lift moving aft as AoA increases is due to LEF
deflection, which is driven by AoA and mach. As LEF deflects, it's local AoA is reduced, so lift on the LEF itself is reduced. But the LEF deflection causes more air to flow over the top of the wing, so total wing lift is maintained. Less lift at the front of the wing, total lift essentially the same, thus center of lift moves aft - stability increases.

So more wetted area of wings due to deployment of LEF's,but horizontal tail leading edges must momentarely raise to counter added lift.Now you are in that region of flight envelope with a/ c want to hang there.It's this true?
Last edited by saberrider on 22 Dec 2016, 09:39, edited 3 times in total.
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johnwill

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Unread post21 Dec 2016, 16:00

Please become familiar with leading edge flap design. It does not deploy, it only rotates, thus does not increase area. Then read my previous post more carefully. Wing lift does not increase, it moves aft due loss of lift on LEF and increase of lift on aft part of wing.
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saberrider

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Unread post22 Dec 2016, 07:54

johnwill wrote:Please become familiar with leading edge flap design. It does not deploy, it only rotates, thus does not increase area. Then read my previous post more carefully. Wing lift does not increase, it moves aft due loss of lift on LEF and increase of lift on aft part of wing.

Thanks for clarification .

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