CNN's Grumman X-29: The impossible fighter jet with inverted wings contains several interesting photos, and says:

It was unflyable -- literally -- without a digital flight computer on board, which made corrections to the flight path 40 times a second," said Christian Gelzer, chief historian at the NASA Armstrong Flight Research Center in southern California (where the plane was tested) in a phone interview.

"An F-18 fighter jet has an instability factor of only 5%. The X-29, on the other hand, was 35% unstable," said Gelzer.

Question: What is the "aircraft stability factor" and what does it mean exactly when an aircraft is "35% unstable"?

The Grumman X-29 had a wingspan of 27 feet and was 48 feet long. It could reach Mach 1.8 (1,100 mph).

The Grumman X-29 had a wingspan of 27 feet and was 48 feet long. It could reach Mach 1.8 (1,100 mph). (NASA)

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    $\begingroup$ It's just a pessimistic way of saying it's 65% stable. ;) $\endgroup$ Aug 23 at 14:52

At the most fundamental level, aircraft stability is governed by the relative positions of the center of gravity and the neutral point. The neutral point being the point about which the pitching moment experienced by the aircraft is independent of the aircraft's orientation.

A useful number that quantifies the relative position of the CG and NP and allows comparisons to be made between aircraft of different size and shape is the Static Margin.

It is defined by the distance from the neutral point to the center of gravity normalized by the mean aerodynamic chord.

$$ \text{Static Margin} = \frac{x_{np} - x_{cg}}{\bar{c}} $$

In the case of the X-29, it being 35% unstable means the static margin is -35%, or the neutral point is ahead of the center of gravity by 0.35 of the mean aerodynamic chord.

What this practically means is that in a similar manner to how an inverted pendulum will fall over with the slightest perturbation, the X-29 will almost immediately tumble out of the sky without constant corrections by the flight computers.


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