A story about the MU-2

by Steve Smith

June 8, 2000

I and a few others were approached to make an assessment of how suitable the Mitsubishi MU-2 would be as a platform to mount a certain imaging device for some sort of surveillance activity. The device was a hemispherical "ball-turret" arrangement with an optical aperture, with a fairing on the downstream quadrant. It was to be mounted on the belly, on centerline, just aft of the landing gear pods. I do not know if it was a camera, a laser, a IR camera, or what....

The characteristics and history of the airplane were represented to me by the project folks as follows:

1) The airplane had a history of nose-over divergence. Several crashes were cited. One, in particular was written up in Aviation Week in the format of an NTSB report. I read that report. During early development, the airplane was designed to have a higher Vne (300, I believe, but don't know this for sure). At some point during development/certification, the Vne was reduced to something like 275 and then to 250, each time in response to some flight situation where a nose-over divergence occurred.

2) Oil-flow flow visualization images were presented to me from flight tests, showing the formation of a strong vortex attachment line/stagnation line along the centerline (keel) of the aft fuselage. It was clear from the oil flow visualization that the landing gear pods were promoting/controlling the formation of a pair of vortices shed from the aft fuselage at about the 5-o-clock and 7-o-clock positions of the round cross section, and reattaching along the centerline (keel). It is likely that some vortex shedding would occur here even without the landing gear pods, but they certainly seemed to intensify them. This reattachment zone normally is a region of high pressure, often almost stagnation (pitot) pressure.

3) Our hypothesis was that the vortex formation was strengthened by reducing fuselage angle of attack, and strengthened by increasing dynamic pressure (speed). At some speed, the high pressure region along the aft centerline produced enough nose-down pitching moment that the trim change could not be balanced by elevator control (either because of full elevator travel, or excessive elevator force, I don't know which). At that point, the nose would drop, increasing speed, further aggravating the problem until an in-flight failure occurred.

Both of these conditions can be aided by reducing the Vne. So the whole story seemed to hang together. The MU-2 has enjoyed a reasonable service life, so it is apparently a fine airplane provided you stay slow enough to avoid this pitch divergence. I would not consider its nickname "rice rocket" to be unique, nor negative. I have heard many people call it that, and it stems partly from its rather impressive climb performance.

The recommendation of our group was that there was a moderate chance that the installation of the optical turret would have some interaction with the vortex formation and pitch-over behavior. We could not say if it might increase or decrease the problem, without some wind tunnel test research to determine how the trim characteristics would change. We therefore recommended that if a wind tunnel test program could not be done, that they choose a different airplane for their platform.

I do not know what their actual decision was, if they followed our recommendations or not.

I do not know if the airplane was a long or short version. It may well be that an extended fuselage would weaken the formation of the vortex pair, and certainly would provide greater tail effectiveness to trim, so it may be able to dive to a higher speed before a pitch-over occurs.

So that is the basis of the statements I made several years ago. You should also realize that most all "conventional" low speed airplanes will experience some kind of pitch-over at high speed, usually from a shift in aerodynamic center as transonic flow forms on the airplane. Fast transports all have a "Mach-trim" mechanism of one type or another to help the pilot trim the airplane as this happens. Usually, the stabilizer angle will be changed.