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Old 10-31-2006, 09:23 PM
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DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 8,814

Ahhh guys, you don't flutter test by simply diving the aircraft to some speed in excess of Vne. You might find flutter this way, in which case you'll probably trash the aircraft and yourself. Even worse, you may well miss a flutter mode and think you have a safe aircraft.

The traditional method for light aircraft with direct control linkages is the control pulse method, aka the stick bump. The procedure goes like this; trim to a set speed in level flight, push the nose down to gain 5 mph, pull the nose up 10 degrees or so, then immediately release the stick and give it a sharp slap in either the pitch or roll axis to excite an oscillation. You then observe to see if the oscillation damps to zero quickly, dribbles away over a period of time, or grows in amplitude.

The beginning level flight speed was one previously checked.

The accelerate-by-diving part was to get you into a new, unknown range. Five mph was picked because we want to sneak up on any unstable speed range; you can use 2.5 mph bites or 10 mph bites if you wish.

The pitchup was to start the airplane on a decelerating speed trend, because if the oscillation doesn't damp, we want the airplane to return to the slower stable speed as soon as possible, all by itself. Most likely the oscillation will stop. If it doesn't you can add a throttle chop; unlike the stick, the throttle won't be flailing back and forth. Since you're already pointing up, the aircraft slows even more quickly.

You release the stick because your hands and arms serve as additional control system mass under normal conditions. We want to remove that mass, as well as the natural tendency to damp any oscillation with muscle.

The slap should be sharp and fairly hard. The literature says the upper limit of flutter frequency for this method is about 10 hertz; you need a nice clean whack to excite that.

And the observation? You're looking for a change in damping rate as compared to the previous speed check. Let's say you check speeds starting at 60, then do 65, 70, 75, and 80. Each time the stick does 3 or 4 oscillations and returns to rest. Then you do 85, and the stick oscillates 6 or 8 times. Hmmmm, decision time; do you move on to a higher speed? First you might want to repeat 85 and confirm the observation. If accurate, it is an indication that you are approaching a speed range with less flutter stability. The next 5 mph speed step may result in 10 or 15 oscillations, but still damp to zero. Or, it may push through the zero damping line on the graph and diverge into instability; the oscillation amplitude gets worse instead of returning to zero. This is where pros earn their pay. The pro will probably proceed in smaller steps, looking for the dragon. It may not ever become divergent within the practical speed range of the aircraft plus 10%, and the engineers may choose to do nothing about it. After all, it does always damp, and the pilot usually has his hand on the stick anyway.

Now consider the power dive method. Our fearless pilot tightens his jaw, his sphincter, and his meaty grip on the stick, pushes over, and does his best John Wayne imitation. His physical damping of the controls means that he doesn't find any warnings, not even a wiggle. If unlucky, he only finds the point where the system goes violently divergent despite his damping.

Now for the really sad part. Suppose Capt. Deathgrip finds nothing within the desired Vne plus 10% speed range. Later some poor sap flying the production version removes his hands from the controls to unwrap his sandwich, rides through a sharp bump in the air, and gets an unpleasant surprise.

You don't need a fast airplane to get flutter. Early Kolbs would flutter. The designer's successful cure was to add mass balance to the ailerons, generally thought unnecessary on ultralights. My friend had a Firestar II without the mass balance weights. You could dive it to 100 with no sign of a problem. However, if you started at 50 in level flight and pushed up the throttle slowly without a hand on the stick, it would burst into an amazing display of wing twist / aileron couple at exactly 53 mph (Kolb wings don't have much torsional stiffness). Grabbing the stick (or chopping the throttle, lots of drag available) would damp it. Very interesting.

Dan Horton
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