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[split thread] increasing Vne on the CH 640

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wsimpso1

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Revisiting this thread, I spotted something that I see repeated. Clearing an airplane to a Vd is a multi-part deal;
  1. Vd min is established in Part 23, and most decent homebuilts use that as a minimum even through they do not have to. Some will go much higher based on a time in a dive at a certain angle. And then some it is the full power nose straight down top speed. You want a speed? Sounds like that is your number;
  2. The aerodynamic design is analyzed using some sort of criteria and forcing function frequencies for your dive speed are estimated;
  3. The structural design is analyzed to check that fundamental frequencies are safely above the forcing function frequencies;
  4. Then the airplane is run through a vibe survey where a bunch of parts are run with vibe motors to see what the actual natural frequencies are - this is important - you need to have a pretty good idea what the forcing functions will be for this test to matter. You can always find natural frequencies, but they have to be safely above the forcing function frequencies (vortex shedding rates, prop blade wake frequencies, firing pulse frequencies, etc)
  5. Then they flight test the thing to demonstrate it will fly to Vdive.
Some folks may shortcut the process. Using a very similar airplane design with either known forcing function frequencies or or known very high natural frequencies may tempt some to skip steps. The vibe survey only has value if you know your targets for min natural frequencies because EVERYTHING has natural frequencies - you have to know the natural frequencies are safely above the forcing function frequencies. One way is for an engineer practiced in the art to know about what the forcing frequencies are for certain class airplanes, then they can check via a vibe survey or FEA for natural frequencies that are safely above.

Just going to flight test and gently exploring the envelope is the risky way.

One other point. Just because some particular part set the current Vdive does not mean that you can stiffen it and go. Even if that part or system has been driven out of range high on frequency, does not mean that there are not more waiting right behind it. Fixing the vertical fin still leaves modes on the horizontal stabilizer and the wings. Then there is the guy who substitutes a Continental six for a Lycoming four or a three blade prop for a two blade or mods the canopy and air outlets that change how air is flowing and your forcing function frequencies go up a bunch...

Billski
 

Mad MAC

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Dec 9, 2004
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Hamilton New Zealand
Hmm 2 pages and no one has pointed out that structurally GA aircraft are Gust load critical (except for some Aerobatic aircraft) and increasing the speed will increase the gust loading. Before even considering flutter one should make sure that the gust loads with in the limits of the airframe. Yes one can increase the empty weight to ensure the gust loads are maintained, but one will still need to check the wing bending load cases.
 
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