This is probably a good time to bring up flutter testing. I have attached some excerpts from emails with Bud Davisson about flutter testing on the Midget Mustang. Most of you will recognize who Bud is. Aside from being probably one of the best know aviation authors, quasi test pilot, owner of the Bearhawk company, and one of the leading experts on the Pitts, he is also an aeronautical engineer. He knows what he is talking about.
Most people flying experimentals don't know a lot about flutter or how to do flutter testing. Most people think you just get up fast and if flutter does not start that anything up to that speed is O.K. That is not the case. You can fly right past your flutter speed and not even know it. Then someday you can be flying at your critical flutter speed and you can get flutter. The Midget Mustang has a well doccumented precursor to flutter that is referred to as "shudder". Many people including me have had shudder in the stang. It is very scary, but it does not cause your airframe to get ripped apart like full fleged flutter. Most planes do not have this early warning. On most planes it just starts full fleged flutter and the next thing you know is that your wings have ripped off and you are kissing your ass goodbye. I have seen myself how improper flutter testing can fool you into thinking you are safe all the way up to red line. When I did my initial testing I did Vne testing going up a little at a time on a calm day and just inching my speed up and making sure I didn't feel anything unusual. I got up to 230 and figured that was all I wanted and as long as I kept it below that speed that I would be fine. A few weeks later I did a high speed low pass at 190 on a day with just a little turbulance. The shudder started and scared me near to death. I didn't know about shudder at the time and I fully expected the plane to disintigrate. That one ranks on the very top of my pucker list. I had the same thing happen on another occasion about the same speed in light turbulance and it was just as scary. If the Stang did not have this early warning I wold have probably been dead the first time from improper flutter testing. The correct way as Bud explains is to do the stick tap test. It was also explained by the RV test pilot at the first Mt Vernon gathering. You need to increase your speed 2 MPH at a time and give a quick slap to the stick to excite the ailerons. The stick should snap back then dampen out very quickly. You go up in speed slowly just until you see a change in the dampening response then stop. I have verified this on the Stang to be just a little slower than the speed where I saw the shudder. I have since made some mods to move the ballance weights a little further forward and decrease the aileron weight. I have also done some mods to take some play out of the system. I know have done the tap testing to 215 with no problems. I won't fly another plane again at a speed that I have not done proper flutter testing up to. Oh, and I am certainly not saying that anyone should use this technique to try flying at a higher than recommended Vne. I am just saying that you should properly test up to Vne before you just blindly go flying at that speed. Brian Kraut Engineering Alternatives, Inc. www.engalt.com -----Original Message----- From: m1-must...@yahoogroups.com [mailto:m1-must...@yahoogroups.com]On Behalf Of Budd Davisson Sent: Tuesday, August 30, 2005 1:35 AM To: m1-must...@yahoogroups.com Subject: Re: Shudder, was [M1-Mustang] engines/ No. What you're looking for is a change in damping. You'll be tapping it at the different speeds and every time the stick should return to neutral, overshoot, return and overshoot again but a little less each time. It'll probably take two or three cycles to damp out. What you're looking for is a change from the last increment. You tap it as before, but instead of damping out in two cycles it takes three or four. When that happens, don't go to the next increment. The damping is falling off and the next increment could be the one where it lets go. bd Brian Kraut wrote: I have heard of the tap test before. Will you generally always get a slight flutter that you will recognize before you get to the critical speed or can you creep up 2 knots, tap the stick, and wind up with the bad scary flutter that I had no intentions of seeing again? Brian Kraut Engineering Alternatives, Inc. www.engalt.com -----Original Message----- From: m1-must...@yahoogroups.com [mailto:m1-must...@yahoogroups.com]On Behalf Of Budd Davisson Sent: Monday, August 29, 2005 9:12 PM To: m1-must...@yahoogroups.com Subject: Re: Shudder, was [M1-Mustang] engines/ Flutter isn't caused purely by speed. It needs to be "excited" to kick off then something--generally flexing or sympathetic vibrations elsewhere in the airframe--feed it. That's why, when doing flutter testing, you creep up in two knot increments and try to artificially excite the flutter by bumping the stick one direction, leaving it hands free and watch it damp out. When the rate of damping changes significantly, is when you call it quits. It's entirely possible to fly right through a flutter range many, many times, but someday be passing through it when just a little turbulence or a twitch of your hand sets it off. You should never fly to a speed you haven't done a two-knot, incremental flutter test. Some will say five knot increments are plenty, but I'd rather take the time and make sure I haven't missed something. If a system has the flexing you're describing, there's all sorts of places for things to add up and feed the excited movement, making it divergent. Also, remember that aluminum isn't like steel. Steel you can flex back and forth, and as long as the loads are beneath a certain level (45,000 psi), it'll fly forever Aluminum will ALWAYS crack eventually if it is continually flexed. It is cycle limited, not just amplitude limited. So, if your fuselage is continually flexing where the tubes go through, not only is the flexing helping feed the flutter, it's a possible failure waiting to happen, even though that's not likely to be for a long time. 'My two cents. bd
