F-16 Deep-Stall

KJ_Lesnick

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How did the F-16 manage to win the LWF program with such a serious problem? I thought high-alpha capability was important?
 
Of course not. Blame a British sense of "humor".

I think it goes like this. In the LWF competition, Northrop offered an aircraft with a conventional mechanical flight-control system that therefore had to be designed to be aerodynamically departure-resistant. General Dynamics offered an aircraft with a full-time fly-by-wire control system that could be programmed to prevent departure. I think it is impossible to design an high-performance aircraft that doesn't have any vices. The question is can you design around them so that you lose as few aircraft and pilots as possible? To that end a lot of aerodynamic refinements were made to the F-18 and a lot of software refinements to the F-16.
 
This article has a good description of the F-16 deep stall problem. It only occurs in very rare specific circumstances, and has specific remedies to get out of. Its hardly a showstopper of a problem. Would buying the YF-17, a fighter that was inferior in many performance areas, on this one criterion, have been a sensible decision?

http://www.codeonemagazine.com/archives/1986/articles/july_86/deep_stalls/index.html
 
wasn't F-16's sparkling performance due to its flight control systems which maximased the potential yet in time principally due to increased payload requirements the tail size was increased . Won't claim anything but isn't the F-16 on par with with the '17 now ? F-18 is hardly defensible ı know but the claimed advantages of the Falcon are nowhere in sight now .
 
The Viper of today is really not what was intended (at least by Boyd, his followers and I suppose you can also say by General Dynamics) when the LWF program was run. The original horizontal stabilizer was fine if the design had remained as a true lightweight fighter (with secondary air to ground) but once the mission changed to that of a bomb truck with a secondary A2A mission, the limitations became readily apparent.

When you look at a photo of the YF-16 and look at an F-16D or the later iterations available today, There's obviously a familial resemblance but the differences are pronounced.

Enjoy the Day! Mark
 
Overscan,

This article has a good description of the F-16 deep stall problem. It only occurs in very rare specific circumstances, and has specific remedies to get out of. Its hardly a showstopper of a problem. Would buying the YF-17, a fighter that was inferior in many performance areas, on this one criterion, have been a sensible decision?

http://www.codeonemagazine.com/archives/1986/articles/july_86/deep_stalls/index.html

Very good explanation. I actually understand exactly why you would rock the plane out of the problem (pushing back and forth on the stick matching the oscillations)

As an aside, I always thought the requirements for supermaneuverability necessitated alphas in excess of 60-degrees.


R16,

wasn't F-16's sparkling performance due to its flight control systems which maximased the potential yet in time principally due to increased payload requirements the tail size was increased . Won't claim anything but isn't the F-16 on par with with the '17 now ? F-18 is hardly defensible ı know but the claimed advantages of the Falcon are nowhere in sight now .

I'm confused...

I thought the YF-16 had the YF-17 outperformed in most areas? Was the F-16 less capable a performer than the YF-16? Was the F/A-18 was better than the F-17?


Mark Nankivil,

The Viper of today is really not what was intended (at least by Boyd, his followers and I suppose you can also say by General Dynamics) when the LWF program was run. The original horizontal stabilizer was fine if the design had remained as a true lightweight fighter (with secondary air to ground) but once the mission changed to that of a bomb truck with a secondary A2A mission, the limitations became readily apparent.

Why did they switch from lightweight fighter with secondary air-to-ground capability to a bomb-truck?[/quote]


KJ Lesnick
 
without trying to give too much of a r16 speech , the LWF programme was indeed different in aims to what was really done . The original idea was to have technology demonstrators which will explain the twin tailed , Viggen like thrust reversing GD projects . It was by the edicts of US lawmakers that it became a production programme which will explain the J-79 engined Mirage F-1 by Boeing ; there was a war going on in Vietnam and the French import might well have been chosen , by how long a shot ı wouldn't really know . It was the judious mix of newness ( say the highly angled seat ) and the ability for "operational translation" ( the high canopy which would have allowed a more conventional angle for the seat if it the original didn't work out ) that won the competition for General Dynamics .

in a similar vein the YF-16 was simply meant to beat the '17 to win the flyoff and while it did so , ı say it on Mike Spick's authority ( or more understandably ı read it in one of his books ) that the gap between the Viper and the Cobra was not that big . Gaining weight doesn't do much for flight performance and Greeks definitely prefer Block 30s to 50s when messing with our flights in the Aegean. The test articles of both camps in the LWF competition will outperform any of the operational aircraft that descended from them .
 
One has to wonder about the tactical validity of a day fighter based around the sustained turn, rear aspect missile and guns fight when that type of engagement already had one foot already in the grave. The YF-17 would have been the superior solution if the US had moved toward a VTAS / AIM-95 WVR missile baseline, but interservice squabbling took care of that.
 
The actual reason the YF-16 won was due to it's Ps (Specific Excess Energy) capability, which was based on Boyd's work, which the YF-16 exemplified. Whereas, the YF-17 was originally designed as an F-104 replacement and was running behind the YF-16, in terms of being ready for the fly-off. As such, there was a problem with the YF-17s horizontal tails that didn't allow it to take complete advantage of it's maneuvering capability.

Having said that, in the close in dogfight there wasn't anything that could touch the YF-17. By the same token, the YF-16 was definitely superior in the energy maneuvering regime.

In fact, both prototypes were so maneuverable that the Air Force wouldn't allow them to dogfight the F-15.
 
Sundog,

The actual reason the YF-16 won was due to it's Ps (Specific Excess Energy) capability, which was based on Boyd's work, which the YF-16 exemplified.

What exactly do you mean specific excess energy? Like more maneuverability than it would ever practically need?

Whereas, the YF-17 was originally designed as an F-104 replacement and was running behind the YF-16, in terms of being ready for the fly-off.

How far behind was it?

As such, there was a problem with the YF-17s horizontal tails that didn't allow it to take complete advantage of it's maneuvering capability.

What problems did the stabilators have? Also, how good would it's maneuverability have been if it could take full advantage of it's maneuvering capabilities? (How would it compare to the YF-16)

Having said that, in the close in dogfight there wasn't anything that could touch the YF-17.

Wait... even the YF-16 couldn't beat it in a close in fight?

By the same token, the YF-16 was definitely superior in the energy maneuvering regime.

I'm confused. I thought a fighter that was best as an energy maneuverability fighter was automatically the best fighter in a close in fight? Yet, from what you said the YF-17 couldn't be touched pretty much in a close in fight.

In fact, both prototypes were so maneuverable that the Air Force wouldn't allow them to dogfight the F-15.

Because it would endanger the F-15 program I would guess?
 
Specific Excess Energy is given by ((thrust - drag)/weight) * airspeed. It is a measure of your ability to accelerate and/or climb.
As you know, any sort of maneuvering will bleed off energy (unless you are diving). If you write that equation in level flight, chances are you have a positive number, but consider that in a hard turn drag (in particular induced drag) and weight (as mass times however many g's you are pulling) can quickly decrease that number.

You can probably find a better explanation in any number of aircraft performance books, like Robert Shaw's Fighter Combat, for example. The relevant pages are available on Google books.
 
The essence of Boyd's energy maeuverability concept is that the airplane that can transition form one maneuver the and dump / recover energy the quickest - doing it faster than the bad guy - will win. That takes light weight (low wing loading) and lots of thrust.

The LWF / F-16 was optimized from the outset with that in mind. A well-aft CG with Fly-Through-Computer (aka Fly-by-Wire) stabilty augmentation was essential. There were, however some regimes where a deep stall could be encountered.

As an airplane gains strucural weight in order to carry more "stuff", wing loading goes up and maneuverabilty suffers. Each square foot must produce more life and induced drag increases as well. An engine can be upgraded only so much. There was an idea in the 80's to refit the F-16 with a "big wing", as part of a mid-life update,to restore some of the lost agilty but it did not get too far.

The CL-1200 (?) Lancer project had the wing the F-104 should have had all along. The -104 gould go like stink in a straight line (low form or parasite drag), but try a tight turn and that little wing generated lots of energy-eating lift induced drag.

An airplane of the F-104 / F-4 generation took a LOT of altitude to recover after departing controlled flight!

In 1977 at the Point Mugu air show, I recall seeng a prototype F-17 execute a pull up to the vertical, slow to zero knots, do tail slide and recover to level flight - all within about 5,000 ft. Back in "the day" - THAT was spectacular!
 
Okay...

So Specific Excess Energy is:

-High Sustained-G capability
-In the event airspeed is lost it is very little
-Even if the loss of airspeed is major it can get back up to speed very very fast

Sound right?
 

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