Quite some time ago David wrote a flight training briefing for us on Spins. This is a followup to his earlier work.

You're at 30,000ft, on your first mission, escorting B-29s deep into North Korean airspace. Sunlight glints off the wings of MiG-15s rolling in from above. As they come screaming in, you wonder if you'll meet a Russian instructor from "Antung University" or one of the freshmen today.

Turning hard into the MiGs while staying with your lead, he calls, "I'm padlocked!" Checking around, you pick up four MiGs bearing down from the opposite direction to the first group. You call, "Viper lead, not clear!" just as the lead MiG fires. Too late, Viper Lead is hit and loses his tail feathers. He ejects and you make a "May Day" call for him. Time to get out. Single birds are dead meat in MiG Alley.

The victorious lead MiG begins reversing his pass as you roll into a steep right hand descending turn to head home. Anxiously checking your six, the lead MiG is now bearing down on you. Instinctively, you continue into a high-speed, high-G defensive spiral to shake him off. He stays with you. So now you know - you've met an instructor from Antung University!

25,000...21,000...17,000...the altimeter continues winding down merrily. Checking the G-meter, you see and feel all of the 8.5Gs. You know your bird is feeling the Gs as well - it's that funny groaning sound from the airframe!

Your breathing is difficult and heaving, your sight is greying and tunnelling...9,000ft and still falling - time to recover. You roll hard back to your left while continuing to pull hard on the stick. BANG! Happy lawn darting because you've just lost your right wing!

MiG Alley

Huh? Welcome to the new world of structural failure modelling in flight sims. PCs and sim codes have now become sufficiently fast and sophisticated to simulate such realities. Sims like MiG Alley certainly provide a rude shock to those PC fighter jocks who have a habit of heavy-handed stick hauling. No fancy fly-by-wire G-limiters to molly coddle you. Time to rediscover the light touch or life will get short for heavy-handed yank and bank jocks!

A Question of Factors

First, a little terminology to square away. Most accelerometers (G-meters) in non-zillion dollar aircraft, measure load factor (G) along the aircraft centreline. The maximum value shown on the meter is typically called the limit load factor or maximum G. So why don't you hear the dreaded explosive failure when you pull the maximum G?

Without going into a lengthy discussion about limit and ultimate/design load factors, the short answer is that the maximum G is not the calculated failure load factor of the airframe. The ultimate or design load factor is the point at which failure is expected. Its value is 1.5 times the limit load factor or maximum G. The 1.5 multiple is known as the factor of safety; and is a deliberate reserve buffer.

So for example, if the limit load factor is 6G, then the ultimate load factor is 9G (6G x 1.5). In MiG Alley, the F-86 accelerometer shows 9G as the maximum value (ie limit load factor) which means that the theoretical ultimate load factor is 13.5G (9G x 1.5). In practice, as an airframe ages, its ability to withstand load factors up to the original design specification becomes increasingly doubtful.

Failure Under Symmetrical Loads

Symmetrical loads occur when both wings produce equal lift - for example, straight and level flight as shown below.

What happens when one pulls up into a loop at say, 6Gs, in an aircraft which has a limit load factor of 6Gs? The load profile would perhaps look like:

If the pilot decided to pull to the ultimate load factor of 9Gs (6G x 1.5), it would theoretically result in both wings failing simultaneously. In practice, variations in construction and/or material properties would probably mean one wing failing sooner than the other.

So does it mean that as long as one pulls Gs up to but not beyond the limit load factor, everything will be okay? The answer is that it depends on the situation. In certain circumstances, pulling up to the limit load can still result in structural failure. The introductory anecdote alludes to this.

Failure Under Assymmetrical Loads

Suppose you're in a situation described at the start of this article - a high-G defensive spiral. Without roll inputs, the lift loading would be pretty much symmetrical as in the previous diagram except that the aircraft is banked and pitched nose down.

Now, suppose the pilot decides to aggressively roll out of the bank while still pulling at the limit load factor (measured along the centreline). What happens then? Lift on the down-going wing is reduced while lift on the up-coming wing is increased as shown.

As far as the pilot is concerned, the accelerometer still shows 6Gs. But the up-coming wing experiences not only the 6Gs measured along the centreline but also the additional load from the downward deflected aileron. If the combined load factor exceeds 9G (ultimate load factor in this example), then the up-coming wing will fail.

Rolling G Limit

Let's look at another situation. If an aircraft has a maximum limit of 6Gs, what is the G limit for a barrel roll? Is it still 6Gs or something different?

The first step is to recognise that barrel rolls involve simultaneous pitching and rolling of the aircraft - similar in concept to rolling out of a downward spiral. So this is a situation where an unwary pilot may overstress the aircraft, if he/she is unaware of the danger described in the preceding discussion.

Without the benefit of fancy G-limiters, a simple rule of thumb to avoid becoming a lawn dart is to take 66% of the maximum G limit as the rolling G limit. For example, if the limit load factor is 6G, then the rolling G limit is 4G (6G x 0.66). The 33% reduction gives sufficient buffer to avoid structural failures during aggressive and simultaneous pitch/roll manoeuvering; where load onset can be rapid and catch unwary pilots offguard.

What do you do if you want to roll aggressively but are already above the rolling G limit? Simply unload (move stick forward) the aircraft to the rolling G limit before whacking the stick to one side. Rest assured the aircraft flies much better with both wings attached!

Until the next time an Antung University instructor bounces you, keep your wings attached and trousers dry!

Background on writer

David holds a bachelor's degree in aeronautical engineering with first class honours, and double master's degrees in aeronautics & astronautics, and business administration. A non-practicing commercial pilot, he regularly flies aerobatics in a SIAI Marchetti SF260 to keep his sinuses clear.