Operational Performance Comparison: Viper, Beagle and Stubby

mixelflick · by **mixelflick** » 22 Apr 2019, 14:02

Everything I've ever read says the Eagle is NOT an unstable airframe.

That may change with the F-15EX, as FBW was incorporated into the latest birds for Saudi Arabia, Quatar etc.. It really is a new bird under the skin...

sprstdlyscottsmn · by **sprstdlyscottsmn** » 22 Apr 2019, 15:22

So first you have to understand that stability is not a binary thing. Traditionally "STABLE" aircraft have a static margin (location of the CG in relation to the center of lift as a percentage of the Mean Aerodynamic Chord) of 25-30% Even since the P-51, and possibly sooner, fighters have been built with this being closer to 0% to improve pitch response.

The static margin changes with fuel, munitions, Mach, angle of attack, and pitch control movement. In general, the F-15 is on the positive side of Neutral stability. Think a static margin of ~5% or less. The Hornet is on both sides of Neutral, say, +3% to -2%. The SHornet is less stable than the Hornet, say, +2% to -3%. The F-16 is fully unstable, say -10%. There are angles of attack where the F-16 becomes stable and there are angles of attack where the F-15 becomes unstable.

viper12 · by **viper12** » 22 Apr 2019, 22:50

If my memory serves, in a documentary series from the late '90s about the Air Force's history, at least from WW2 (if not earlier) till the Gulf War, which mainly (exclusively ?) interviewed the top brass, one of the generals recalled that when flying in the F-15, he could see the flaps move on their own multiple times a second, or rather ordered by the flight control system, which goes to show that stability isn't a clear-cut definition.

Now if someone knows which documentary I'm talking about, I'd be happy to know the name, as my Googlefu couldn't retrieve it. In particular, one of the generals said something like "a treasure of experience" which was learned during the Vietnam War and applied to the Gulf War.

sprstdlyscottsmn · by **sprstdlyscottsmn** » 22 Apr 2019, 23:03

Which brings up an excellent point. Lowing trailing edge flaps makes more lift at the rear of the wing and shifts the center of lift back, increasing nose down moment.

johnwill · by **johnwill** » 23 Apr 2019, 02:36

But stability is not defined by the relation of center of lift and CG. It is defined by the aerodynamic center in relation to CG. Aerodynamic center is the location of delta lift with a delta change in AoA. So if you go from 5 deg AoA to 6 deg, the delta lift may not be the same place as center of lift at 5 deg. If you hit an up gust that causes a delta AoA, the nose goes up if the delta lift is forward of the CG (unstable) and it goes down if the delta lift is aft of thee CG (stable).

zero-one · by **zero-one** » 23 Apr 2019, 08:20

sprstdlyscottsmn wrote: There are angles of attack where the F-16 becomes stable and there are angles of attack where the F-15 becomes unstable.

Curious, how does an F-15 remain under control when it becomes unstable. isn't the FCS of the Eagle a manual hydraulic control system, no FCC involved

sprstdlyscottsmn · by **sprstdlyscottsmn** » 23 Apr 2019, 14:07

johnwill wrote:But stability is not defined by the relation of center of lift and CG. It is defined by the aerodynamic center in relation to CG. Aerodynamic center is the location of delta lift with a delta change in AoA. So if you go from 5 deg AoA to 6 deg, the delta lift may not be the same place as center of lift at 5 deg. If you hit an up gust that causes a delta AoA, the nose goes up if the delta lift is forward of the CG (unstable) and it goes down if the delta lift is aft of thee CG (stable).

I know that, but I often get burned by using accurate descriptions that other people don't understand. As such I'm thinking about the AC but using the term CoL. You are also touching on dynamic stability where most people are only thinking about static stability. Hence why I said stability is not binary. The AC changes when anything changes. Any change in dynamic pressure, airflow angle, or surface movement will impact the AC in some way. Thanks for adding another level of detail to the discussion johnwill, I think it was well timed.

johnwill · by **johnwill** » 23 Apr 2019, 18:29

Fully agree with your approach to these discussions. I hope it was obvious my comment was not directed at you. :salute:

sprstdlyscottsmn · by **sprstdlyscottsmn** » 23 Apr 2019, 18:52

All good johnwill! :salute:

zero-one wrote:Curious, how does an F-15 remain under control when it becomes unstable. isn't the FCS of the Eagle a manual hydraulic control system, no FCC involved

I cannot answer definitively on the presence or absence of an FCC, but I suspect there is one. Gums mentions having both an AoA and G-Limiter all the way back to the F-101. Having an FCC is very different from having FBW. At high AoA an Eagle pilot has to push the stick forward and hold it there to get the nose down. One of the big advantages the Super Hornet brought was the ability to RAPIDLY get the nose back down from high AoA.

h-bomb · by **h-bomb** » 24 Apr 2019, 00:16

sprstdlyscottsmn wrote:All good johnwill!

zero-one wrote:Curious, how does an F-15 remain under control when it becomes unstable. isn't the FCS of the Eagle a manual hydraulic control system, no FCC involved

I cannot answer definitively on the presence or absence of an FCC, but I suspect there is one. Gums mentions having both an AoA and G-Limiter all the way back to the F-101. Having an FCC is very different from having FBW. At high AoA an Eagle pilot has to push the stick forward and hold it there to get the nose down. One of the big advantages the Super Hornet brought was the ability to RAPIDLY get the nose back down from high AoA.

The F-15 always had a FCC but it was parallel to the pilot and mechanical controls. It was more of an augmentation of the mechanical system. Similar to what is proposed for the CH-47F Block 2 aircraft.

See the last paragraph: http://www.joebaugher.com/usaf_fighters/f15_2.html

marsavian · by **marsavian** » 27 Apr 2019, 19:15

Does anyone know how many USAF F-15 and in what variants have 29,000 lb engines if at all ? Tried looking for the information but unlike F-16 upgraded engine model information it's hard to find. TIA.

sprstdlyscottsmn · by **sprstdlyscottsmn** » 27 Apr 2019, 20:24

F-15E only. I don't know how many have -220s and how many have -229s.

marsavian · by **marsavian** » 27 Apr 2019, 21:11

PW or GE ?

sprstdlyscottsmn · by **sprstdlyscottsmn** » 28 Apr 2019, 01:01

All USAF F-15 models are PWs. F100-PW-22(0/9).

marsavian · by **marsavian** » 28 Apr 2019, 09:50

Thanks and with your cue have found out that 3 squadrons out of 8 have the -229.

https://web.archive.org/web/20180612205 ... 01-engines

The PW-229 variant was introduced in 1992, the first jet to be equipped with it was 90-0233. The most visible difference between PW-220 and PW-229 is the color of the flame coming out from them in afterburner mode. PW-220 produces a yellowish flame, while PW-229's afterburner flame is blueish.

333rd FS "Lancers" - F100-PW-220
334th FS "Fighting Eagles" - F100-PW-220
335th FS "Chiefs" - F100-PW-220
336th FS "Rocketeers" - F100-PW-220
389th FS "Thunderbolts" - F100-PW-220

391st FS "Bold Tigers" - F100-PW-229
492nd FS "Madhatters" - F100-PW-229
494th FS "Panthers" - F100-PW-229