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streakeagle

Beating a dead horse-US fighter turn performance

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Anyone who hung out at SimHQ when SFP1 was originally released will know who Andy Bush is: F-104/F-4 pilot that helped develop the game.

Anyone who follows F-105s will know who Ed Rasimus is: F-105 pilot and author.

Check out this discussion: http://www.aviationbanter.com/archive/index.php/t-9918.html

 

My favorite quote is the perfect end to an ages old argument about the F-8 Crusader versus the F-4 Phantom by John Carrier, a pilot who transitioned from flying F-8s to flying F-4s:

 

As to the F-8 versus F-4, you presented the prevailing conventional wisdom

of the time. When I was an F-8 guy, I felt I pretty much could have the

Phantom for lunch. But there was a time or two when the individual I

opposed transformed the jet into a serious adversary, "Who IS that guy?"

 

The F-8 had superior PsubS under G than the Phantom at altitudes above

15,000 feet, so any kind of classic turning fight (oblique loop, etc was the

thing in the tacmans at the time) played to its advantage. The Phantom was

more controllable very slow and enjoyed superior unloaded acceleration.

That points to a VERY vertical fight.

 

When I finally transitioned to the F-4, I thought, "No wonder it was so easy

to beat up on this jet." But, by the 500 hour mark I had changed to, "How'd

we EVER beat up on this jet?" The F-4 was the antithesis of the

point-and-pull fighter and required a great deal of finesse to fight well

(skills that many never achieved IMO). Once mastered, you could

successfully engage just about any aircraft of its generation ... albeit a

roller with a Mig-17 was ill-advised (hear that, Duke?). Of course, once

the next generation appeared (F-14 and subsequent), there really wasn't

anyplace to take the fight they couldn't go.

 

But a more important part of the discussion is what the leading edge slats did for F-4s. Slats were added to solve the safety issues with pulling high-AoA. The side benefit was better instantaneous and sustained turn performance (which happens at high AoA). However, they also came with a penalty. At lower AoA, slats cause extra drag. This drag effects specific excess power, which in turn affects top speed, level acceleration, and sustained climb performance. In other words, hard winged F-4s could go faster, accelerate quicker, and climb better unloaded. But if you were pulling hard g, the slatted F-4s induced much less drag and therefore performed significantly better in high pitch/turn rate situations. The accident rate of hard wing F-4s ensured that slats were preferable, but a highly skilled energy fighter might actually do better if he could keep the fight away from extended hard turns and not depart controlled flight.

 

As modeled in the SF series, the slats make dogfighting much easier. I kill MiGs much quicker in the slatted F-4E/F-4F Phantoms much more quickly than any other variant even using just missiles. But I find it much more fun/challenging to take on agile MiGs in hard wing gunless F-4s, F-104s, and F-105s :)

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One item that seldom gets mentioned when looking at slats versus hard wing performance: the F-4E gained some serious weight with the addition of the gun and an extra fuel tank to the rear to counterbalance the gun weight in the nose. So, when you look at the performance charts of the slatted F-4E versus the hard wing F-4D, it doesn't look like the slats helped that much and it looks like the F-4E suddenly had a lower max g load compared to earlier F-4s. But if you rectify the comparison to equal weights, not only does the load limit become equal, but you see the true gain of the slats. Since the F-4S only suffers the weight/drag penalty of the slats and not the guns installation, It would be nice if the F-4J and F-4S manuals had the same sustained performance charts as the USAF F-4 manuals to get a true apples-to-apples comparison. The best comparison would be an early unslatted F-4E manual so that the only variable would be the slats with no other weight/drag changes. My manual is a late 70s manual after all F-4Es were slatted.

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Here is a nifty chart I made by overlaying the USAF F-4 turn rate charts for the F-4D, F-4E, and the F-4E scaled to match the weight of the F-4D. The black lines are the F-4D, the dark orange lines are the heavy F-4E, and the blue lines are the lightweight F-4E (same weight as F-4D). Up to low transonic mach numbers and up to medium altitudes, the F-4E is about 7% better than the F-4D (15% better with the same weight). At higher mach numbers, the F-4 doesn't have to pull as much AoA to get the same lift, so the slats actually cause a drag penalty that allows the F-4D to perform better. For reference, the F-14 is known to turn about 20% better than the unslatted F-4J. So, if the slats made the F-4S turn about 15% better, sustained turn rates would almost be pretty close between the F-14 and F-4S. The F-4E, being heavier, would still be significantly under the F-14. However, with numbers this close, pilot quality is everything rather than precise performance figures.

 

Note: After countless reviews/edits, the turn rate varies with (n^2 - 1)^0.5, which is not quite linear. So my linear shift of the turn rate is off by a bit at lower loads and almost dead on at higher loads: i.e. if the load was 2g, and the shift is weight decreases by 10%, the turn rate would increase by 13% rather than my shift of 10%. At 8g, the actual shift would be very close to 10%. So my original graph underestimates the gain at lower loads and is almost dead on at higher loads, hence I am reposting my original graph and my original text.

 

f4turnrate.jpg

Edited by streakeagle

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Here is a similar chart that compares the F-4C/D, slatted F-4E, and F-15A/C. I had to switch from KCAS to Mach to match the F-15 charts. One nice thing about using Mach is that you can see where the F-4E's slats become dead weight -- near the sound barrier. The weights are all similar, but the F-4s only have AIM-7s while the F-15 adds AIM-9s and a centerline pylon (useful data considering the F-15 almost always flies with its centerline tank). My only other choice was a clean F-15, and that would not have been anywhere near fair to compare. The F-15 uses two main changes to beat the F-4: thrust:weight and wing area. The F-15's wing is optimized to permit Mach 2+ speeds and does not have slats, so its lift curve is only marginally better than the F-4, but its wing area means a lower wing loading and less drag for the same amount of lift. With AIM-9s and the centerline pylon, the F-15 is a bit slower than the F-4D. If the F-4s were tagged with the same ordnance (a typical air-to-air loadout), their top speeds would be lower, too.

 

I don't have equivalent charts for the F-14 or F-18. The Navy didn't have sustained turn charts in their flight manuals, just V-n diagrams that allowed you to determine the max instantaneous load. In the case of the F-18, you actually have to cross reference the weight with the degrees of bank (g load) to get the stall speed, and there are only a few banks to pick from up to 60 degrees. I may have the F-16A data laying around somewhere. If I find it, I will make another overlay to show how it runs rings around them all.

 

f4turnrate2.jpg

Edited by streakeagle

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