The F-35: What Will Happen While Exploring ‘High Angle-of-Attack’, Part 4

‘Old School’ F-15: Entry to the modern world of ‘High-Performance’

Part 4 (and last) in a series of posts where we document The Profound Truth of High Angle-of-Attack (AoA) flight testing of high performance aircraft. What is “The Profound Truth”?

Discovery and rectification of undesirable aircraft behaviors during High Angle-of-Attack testing of High Performance Aircraft is not only the ‘Norm’, but those behaviors needing rectification/mitigation are usually complex, sometimes bizarre, and often ‘spectacular’.

The F-15 and AoA: A Surprisingly Short Story

My whole family had just moved to Antelope Valley to be near my Dad while he worked another flight test program at Edwards AFB-- only the month before the F-15’s first flight at the same location. I joined the Air Force before the whole family moved back to Texas six months later (Dad would often refer to himself as a “migrant aerospace worker” or “Aero Bracero”).



F-15 First Flight, July 1972 With Square Wing Tips Shown to Good Effect (US AF Photo)

I joined the Air Force before Dad’s assignment ended, but still got to see the first F-15s in the skies over Eddy and the AV quite a bit before I left. In a way, the F-15’s ‘career’ began the same time mine (paying anyway) did, and I got to watch most of the F-15’s maturation, successes, and evolution in ‘real time’ before and after I retired in 1993.

From knowledge gained going way back to the early 70’s and the heady days of the F-15’s flight test and OT&E, I thought I’d just be able to search up some early scholarly papers using a select few keywords like ‘F-15 flutter’ and ‘F-15 buffet’ and then just quote the source data as to what actually ‘happened’. But my research surprised me. There was almost NOTHING written in peer-reviewed or official literature on F-15 High AoA exploration and behaviors while they were being discovered that I could find in the public domain.

Some of this void could be due to the time frame: we were going toe to toe with Russian fighter designs in Vietnam until at least six months into the F-15’s flight test program, and the Cold War was still ‘freezing’. Absence of hard data could also be due to the priorities given to the challenges, controversies, and manufactured scandal surrounding the F-15’s engine development history. That was always in the news at the time, and the news was (usually) wrong about what was really happening with the F100 engine and why.

From forensic examination of the larger body of available ‘retrospective’ literature, it seems for the most part the Air Force was just extremely happy to be able to do what the F-15 design was intended to do: fly much deeper into the High AoA regions before onset of buffeting and stall at higher speeds than its predecessors.

In the past 10 years, U.S. military aviation has progressed from the generation of F-4/F-8 air superiority fighter to that of the F-15/16/18 aircraft, which are demonstrating significant improvements in maneuver performance. These improvements result from more sophisticated aerodynamic design, lower wing loading, and higher thrust-to-weight ratio, and they permit the newer fighters to maneuver as well at 7 to 8 g’s as the earlier aircraft did at 4 to 5 g’s. The limited assessment to date of the newer fighters indicates that they also track as well at 7 to 8 g’s as their predecessors did at 4 to 5 g’s. This is attributed largely to their improved aerodynamics and more sophisticated control systems, which permit them to operate at higher load factors with lower levels of buffet intensity and wing rock than their predecessors. – Precision Controllability of the F-15 Airplane, T.R. Sisk and N.W. Matheny, NASA Technical Memorandum 72861, 1979.

Sisk and Methany’s 'Precision Controllability of the F-15 Airplane' is particularly valuable to our examination because it recounts NASA’s experience exploring F-15 handling qualities using a pre-production F-15 (Airframe number 8). Sisk and Methany offers some insight into what needed to be ‘fixed’ on the early F-15s and why, when after 6 of 10 Gunsight Tracking tests, key systems had to be upgraded to ‘production standard’. The ‘Gunsight Tracking’ test involved making a windup turn from 1 g (trimmed) to the “maximum allowable load factor or angle of attack”.
That “adverse pilot comments concerning the airplane’s handling qualities” in the early tests drove the testers to upgrade the control system to “meet production standards with regards to friction, hysteresis, and breakout forces”, ‘replace roll and trim actuators”, and replace the ARI (Aileron-to-Rudder Interconnect) with a production unit, among other changes is a pretty good indication that the first F-15s were hairy (or at least hairier) beasts at higher AoAs.

Findings of particular note are 1) that the F-15’s buffet as experienced in the cockpit is considerably higher than that experienced in the YF-16 and YF-17 prototypes and 2) the F-15 wing buffet is “severe” at higher AOAs, with mild to moderate wing-rock at airspeeds of interest when the AoA is above “Approximately” 10 degrees.[This raises an interesting question: If the comparison holds for F-16s and F-18s, and the F-35 exhibits buffeting somewhere in between the spread, may we expect F-15 pilots transitioning to the F-35 to think of the F-35’s buffeting as “meh” and pilots transitioning from the F-16 or F-18 react to the same as “OMGWTFO!”?].

A key point to remember here is that the F-15 has no lift augmentation devices, either trailing or leading edge, so there is no way to alter wing high AoA performance without redesigning the wing itself or tweaking your primary flight controls.

Fighter Weapons Center F-15C with Conformal Fuel Tanks and Speed Brake Deployed. When I was at Nellis in the late 70s, F-15 Crew Chiefs were kept 'moist' with cases of beer that pilots would have to buy them after pushing their aerodynamic braking 'show' too far and dragging their tail feathers.

Obvious Changes as a Result of Initial Flight Test

The two most obvious changes made to the F-15 as a result of flight testing were the wingtip design and the speed brake design and operation. I can find no detailed scholarly or otherwise authoritative references to the hows and whys behind the changes were made, but the reasons seem to just be presented now in the ‘everybody knows’ matter-of-fact manner. It is common to find references to the raking of the wingtips due to transonic flutter after the third F-15 was built, but only some sources mention the flutter was occurring at higher g’s and AoAs.

The speed brake design changed at some time during the F-15’s fielding, but I haven’t found an authoritative source I can corroborate as to exactly ‘when’. It is now apparently common knowledge that the dual changes of increasing the size of the speed brake and greatly reducing the angle it can be raised on a different schedule allows the same effectiveness at higher AoAs without adding to the F-15s total buffeting.

Other Issues for Perhaps Another Time

There’s a lot more ‘back story’ on the F-15 development and later discovery of peculiar flight characteristics that have nothing directly to do with ‘just’ high angle of attack. There is the twin vertical stabilizer buffeting issue [1], that seems to affect ALL twin-tail fighter designs to one degree or another, but caused severe cracking and redesign of the F-15 vertical surfaces. There is the “Bitburg Roll” [2] phenomenon, whereby the effects of aerodynamic asymmetry caused by having the 20mm Gatling gun only on one wing root “first surfaced in 1990 as an uncommanded yawing and rolling motion on a F-15C at Bitburg” Air Base in Germany. The Bitburg Roll manifests itself as either an uncommanded roll “up to 60 degrees per second” to the right or yawing motions at higher altitudes between 250 and 350 KIAS. There is also the horizontal stabilator flutter problem [3] that was ‘solved’ by putting a ‘snag’ in the leading edge of the stabilators. The solution was found by trial and error in a wind tunnel, and only recently has the art of computational fluid dynamics reached a point where, 40 years later, it is believed the ‘why’ of the solution can be understood.

There's more back story, but the point is made that:

Operation and test has shown that aerodynamic performance can and will remain unpredictable to any exactness when aircraft are operated in regions where non-linear (the definition of ‘turbulent’) airflow occurs.

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Note: Apologies for the posting drought. I've been way 'out of pocket' part of the time on business travel and been working killer hours at work and home catching up on priorities that could wait no longer.

[1] F-15 Tail Buffet Alleviation: A Smart Structure Approach; Georgia Institute of Technology; WPAFB Contract Number F33615-96-C-3204; 1998.

[2] An Investigation Into The Effects Of Lateral Aerodynamic Asymmetries, Lateral Weight Asymmetries, And Differential Stabilator Bias On The F-15 Directional Flight Characteristics At High Angles Of Attack; D.R. Evans; AFIT/GAE/ENY/96M- 1; 1996.

[3] Flutter Mechanisms of a Stabilator with a Snag Leading Edge; R. Yurkovich; 19th Structures, Structural Dynamics and Materials Conference, 4 - 7 April 2011, Denver, Colorado; AIAA 2011-1847