Why alpha limit? Enchancing Viper's natural maneuverability

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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Patriot

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Unread post28 Jun 2017, 17:23

I have a few questions that kinda boggles my mind for quite a time, they are:


1. Was it neccessery from a concept (an ultimate dogfighter) standpoint to incorporate AOA limiter into the FLCS?
If so, what would have happen if that limit would be higher? 28, 30 or 35 degrees?
How other non-AOA-limited fighters are inferior in (sustained) turning areana to an F-16 with its build in alpha limiter?

2. What would it take to increase a roll-rate of the Viper? (It's not bad I know... but it's not as good as in M2000, Rafale or Hornet)
Enlarging Flaperons? Tail stabs? Adding an ailerons at the trailing edge would surely help to increase rolling rate I guess..

3. What would it take to improve slow speed handling in the Viper? My guess is... reduce mass - because it's a lot cheaper from redesigning the wings to have a lower wingloading - which itself is a key factor to success in this field, isnt it?

Hmmm... I think it would be great to have a Viper Block (70 ? ;) ) that would retain the weight of early production A/B models and have a thrust of Blk 50/52 or even above 30K lbs (Blk 60) .... then it would go to low orbit at MIL and reach an escape velocity on AB :applause: :notworthy: :crazypilot:
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saberrider

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Unread post28 Jun 2017, 19:13

My personal opinion is that if you want to be able to sustain higher G's longer period of time must have constant speed, so limited AOA is necessary for this task , pilot free handling is a must at the higher speed to not over stress airframe's ,then range of the speed for dogfights tell you what max. AoA gains is permitted ( high subsonic ).Higher AoA( 28 , 30 ,35 ) reduces time under max. sustainable G's .The shape of the wings plan form and thickness ( Delta , Tapered ,Swept Back- (Front)supercritical profiles is what dictate stall point .Mirage's 2000 from Greece is inferior to F16 .Drag is higher on delta wings. Roll rate increase is not practical ,because bombs are limited to 5 to 6Gs.Twist of the wings increases if you add aileron's and the end result is reduced effectiveness of them .For # 3 you are right but you looses an track for bombs. If you add.a second vertical tail and make LEF's dropping independent in roll (expensive)I believe you're going to enhance the low speed maneuvers more than now .This is my solution .
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basher54321

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Unread post28 Jun 2017, 21:46

1. Helped the pilot manage energy - sometimes less is more.

If you went to 35 degrees then you'll likley be facing directional stability issues - This is partly due to use of a single tail (note Eurocanards seem to have taken a similar approach to the F-16 with sub 30 degree AoA Limits AFAIK).

The only F-16 that had high Alpha capability (to 120+ degrees) by todays standards was the F-16 MATV that used thrust vectoring to deal with the stability issues.

If other fighters are inferior in a sustained turn (taking the pilot out of it) then that is down to their design not a limiter.


2. The F-16XL had a higher roll rate it seems so one answer is Cranked Delta wings.

3. Already has good slow speed handling, do you mean lower stall speed or post stall??
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botsing

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Unread post28 Jun 2017, 22:48

basher54321 wrote:This is partly due to use of a single tail (note Eurocanards seem to have taken a similar approach to the F-16 with sub 30 degree AoA Limits AFAIK).

IIRC this is indeed due to the lack of airflow over a single tailed aircraft at higher AoA.

̶I̶t̶ ̶i̶s̶ ̶m̶y̶ ̶g̶u̶e̶s̶s̶ ̶t̶h̶a̶t̶ ̶o̶n̶e̶ ̶o̶f̶ ̶t̶h̶e̶ ̶f̶u̶n̶c̶t̶i̶o̶n̶s̶ ̶o̶f̶ ̶t̶h̶e̶ ̶F̶-̶1̶6̶'̶s̶ ̶v̶e̶n̶t̶r̶a̶l̶ ̶f̶i̶n̶s̶ ̶i̶s̶ ̶t̶o̶ ̶a̶d̶d̶ ̶s̶t̶a̶b̶i̶l̶i̶t̶y̶ ̶a̶t̶ ̶h̶i̶g̶h̶e̶r̶ ̶A̶o̶A̶,̶ ̶a̶l̶t̶h̶o̶u̶g̶h̶ ̶d̶u̶e̶ ̶t̶o̶ ̶s̶i̶z̶e̶ ̶t̶h̶e̶y̶ ̶c̶a̶n̶ ̶n̶e̶v̶e̶r̶ ̶f̶u̶l̶l̶y̶ ̶r̶e̶p̶l̶a̶c̶e̶ ̶t̶h̶e̶ ̶s̶i̶n̶g̶l̶e̶ ̶t̶a̶i̶l̶s̶ ̶e̶f̶f̶e̶c̶t̶i̶v̶e̶n̶e̶s̶s̶.̶
(not correct, see post from johnwill)
Last edited by botsing on 29 Jun 2017, 17:47, edited 1 time in total.
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f-16adf

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Unread post28 Jun 2017, 23:08

I guess a Viper with 2 vertical tails would kinda look like this:

https://ibb.co/jJGX2k
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Unread post28 Jun 2017, 23:09

Possibly - I thought the Ventrals were for spin resistance and directional stability at high speed AoA - although probably not the whole story.

Strangely the single tail provided better directional stability than the twin tails at sub 30 degrees AoA especially at M0.8 according to C1.
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Unread post28 Jun 2017, 23:13

f-16adf wrote:I guess a Viper with 2 vertical tails would kinda look like this:




Or this

YF-16Evo51.png
YF-16-Twin
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Unread post29 Jun 2017, 01:58

Patriot wrote:I have a few questions that kinda boggles my mind for quite a time, they are:


1. Was it neccessery from a concept (an ultimate dogfighter) standpoint to incorporate AOA limiter into the FLCS?
If so, what would have happen if that limit would be higher? 28, 30 or 35 degrees?
How other non-AOA-limited fighters are inferior in (sustained) turning areana to an F-16 with its build in alpha limiter?

2. What would it take to increase a roll-rate of the Viper? (It's not bad I know... but it's not as good as in M2000, Rafale or Hornet)
Enlarging Flaperons? Tail stabs? Adding an ailerons at the trailing edge would surely help to increase rolling rate I guess..

3. What would it take to improve slow speed handling in the Viper? My guess is... reduce mass - because it's a lot cheaper from redesigning the wings to have a lower wingloading - which itself is a key factor to success in this field, isnt it?

Hmmm... I think it would be great to have a Viper Block (70 ? ;) ) that would retain the weight of early production A/B models and have a thrust of Blk 50/52 or even above 30K lbs (Blk 60) .... then it would go to low orbit at MIL and reach an escape velocity on AB :applause: :notworthy: :crazypilot:


1. It wasn't necessary, but it was desirable. I think I remember Gums saying he did not like it at first, but when he realized the AoA limiter helped him maintain energy in a turn by also limiting induced drag, he grew to like it. Above the limiter, directional stability is reduced, and so the limiter also helps to avoid spins, as basher mentioned.

2. F-16 has no roll rate spec. Instead it has "time to bank" spec, which is mostly controlled by roll acceleration, not maximum rate. So if you want increased roll performance, increased roll acceleration is needed, which can easily be obtained at higher airspeeds by moving the roll control surfaces faster at roll initiation, not more max deflection. But that runs in "twitchy roll command" at high speeds and roll/pitch coupling at low speeds. Rolling at low airspeeds means at a significant AoA, and since the airplane rolls around the flight path, not airplane axis, it wants to pitch up. With higher roll rates, it wants to pitch up more. Imagine a dumbbell (exercise weight, not human) rolling around an axis offset 15 degrees. The dumbbell wants to pitch up to a 90 degree angle from the rotation axis. As usual, everything is a compromise.

3.Depends on what you mean by low speed handling. If you put Block 70 avionics into a Block 15 airframe, it will weigh almost as much as a Block 70. And if you don't strengthen the airframe (more weight) you will have to reduce maneuver limits significantly. and if you do strengthen the airframe, you will have a Block 70 airplane.

Ventral fins don't have much effect at low airspeed, high AoA conditions - small area, and short yaw moment arm to CG. They are mostly needed to help with high speed directional stability, where vertical tail flexibility reduces its effectiveness, while the short and stiff ventrals still work.
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rheonomic

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Unread post29 Jun 2017, 02:42

johnwill wrote:2. F-16 has no roll rate spec. Instead it has "time to bank" spec, which is mostly controlled by roll acceleration, not maximum rate.


The original F-16 flying qualities would have been based off of 8785C, wouldn't they? The flying qualities specs for highly augmented aircraft weren't until 1797 which would have been about 10 years after the first Vipers if memory serves. Do you happen to know if 8785C was used directly or if the S&C team used different/additional standards?

All the people I know that worked on F-16 were Blk 60 and later; while I think I have the block diagrams for the really early CLAWs somewhere I don't they have anything for the desired flying qualities.
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Unread post29 Jun 2017, 03:08

I would posit that the AoA limit is not completely natural for the airframe if flown well with coordinated stick and rudder (we could open an entire rabbit hole about whether you actually need to coordinate rudder but that is neither here nor there). The risk with 30+ AoA is the deep stall regime the Viper is particularly susceptible to if you ham fist things and depart. The hornet, with a basically unlimited AoA (no limiter), is a more honest fighter real slow, at least from my experience flying both. In the Hornet, it is damn near impossible to lock into a true post departure gyration/spin unless you actively hold prospin control inputs and pro-spin split throttles and are already in a zero airspeed/negative airspeed tail slide. In the Viper, once you get into the deep stall regime, traditional FLCS won't get you out, only MPO engaged pitch buck and some patience. That's just my guess
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Unread post29 Jun 2017, 04:01

It wasn't necessary, but it was desirable.


An opening for another Cat 1/Cat 3 discussion? :shock:

:D
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Unread post29 Jun 2017, 05:00

rheonomic wrote:
johnwill wrote:2. F-16 has no roll rate spec. Instead it has "time to bank" spec, which is mostly controlled by roll acceleration, not maximum rate.


The original F-16 flying qualities would have been based off of 8785C, wouldn't they? The flying qualities specs for highly augmented aircraft weren't until 1797 which would have been about 10 years after the first Vipers if memory serves. Do you happen to know if 8785C was used directly or if the S&C team used different/additional standards?

All the people I know that worked on F-16 were Blk 60 and later; while I think I have the block diagrams for the really early CLAWs somewhere I don't they have anything for the desired flying qualities.


The F-16 has been around for a long time, but not that long. :D F-16 specs were written around 1975. I don't know the answer to your question. I come from the structures area, but had to be familiar with S&C requirements so structural design conditions would match S&C design, I borrowed a copy of their specs to study, but don't remember the doc number. The time to bank conditions were gear down 90 degrees, up and away 180 and 360 degrees, at 1g and elevated g, subsonic and supersonic. Structural requirements are from the 8860 series.
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Unread post29 Jun 2017, 05:27

johnwill wrote:The F-16 has been around for a long time, but not that long. :D F-16 specs were written around 1975. I don't know the answer to your question. I come from the structures area, but had to be familiar with S&C requirements so structural design conditions would match S&C design, I borrowed a copy of their specs to study, but don't remember the doc number. The time to bank conditions were gear down 90 degrees, up and away 180 and 360 degrees, at 1g and elevated g, subsonic and supersonic. Structural requirements are from the 8860 series.


Thanks.
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Unread post29 Jun 2017, 13:11

So, John Will, since we're dealing with a digital FLCS - to improve a roll acceleration a software upgrade is enough?

Im thinking... isnt it also a matter of roll authority - for example, if Hornet would have only the flaperons same as Viper it would have less surface to roll that body arround (?)
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Unread post29 Jun 2017, 13:48

If replace the ailerons with flaperons on F18 , engineers must
reshape the airfoil , platform ,wingspan , position of attachment (high, low,middle) , angle of incidence and twisting at the wingtip so the flaperons at max. angles of deployment not to be stalled .
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