Showing my ignorance...

[goatherder]

I have a question and I was hoping that someone - maybe some of you aeronautical engineering types - might be able to answer it in a way that I can understand.

 

I think I understand the concept of "flutter" and why it occurs - basically the same principle as a reed on a clarinet. It's an oscillation induced by air pressure against the leading edge of the wing, right? And the pressure becomes so great that it overcomes the stiffness of the wing and makes it get all wobbly.

 

SO - assuming a constant amount of engine power (thrust), the higher you fly the faster you go. This due to the air being thinner and there isn't as much air pressure acting on the plane for the engine to overcome.

 

Iv'e read on this forum that flutter is said to occur at a certain speed for a given airframe. If flutter is a pressure-induced oscillation, and if the thinner air at higher altitudes is allowing flaster flight due to lower air pressure on the plane, then why would'nt flutter occur at a higher speed at higher altitudes??

 

Seems like it would take more speed to get the same amount of pressure on that leading edge when the air is thinner...

 

What am I not getting here?

[Drew Swenson]

Does this help any?

 

http://www.vansaircraft.com/pdf/hp_limts.pdf

[Vacabrava]

Hi,

For our Cozy Mark IV, Nat stabilished Vne of 220mph, it is at 20000' density altitude?

Our should we fly near 165mph IAS when in the 20kft?

[TMann]

Hi,

For our Cozy Mark IV, Nat stabilished Vne of 220mph..........

For HIS plane.

Your Vne may be different.

Are you using the exact same bulding materials and techniques?

[Marc Zeitlin]

For our Cozy Mark IV, Nat stabilished Vne of 220mph, it is at 20000' density altitude?

The DA for the MKIV Vne is not specified.

 

We do not know the limiting factor in the MKIV Vne - whether it's aerodynamic, structural, flutter, merely the limits of what it was tested to, or what. The most likely story is that it's what the Long-EZ's Vne was, and since the COZY MKIV is about the same as the LE, Nat used that number. Of course, there's no indication of what the limiting factor in the LE's Vne was, either.

 

Given that there are LE derivatives (Berkut, some E-Racers) that, using the same basic aerodynamics and structure have gone to speeds far above 220 mph IAS, and given that Richard Riley (RIP) has indicated that the flutter analysis on the Berkut showed a limit well above 300 MPH IAS, it is probably safe to say that flutter isn't the limiting factor in Vne, HOWEVER, this can only be said for low altitude flight - flutter (as Drew's VERY good link points out) is a function of TAS as well as IAS, so at 20k ft., things may be very different.

 

Our should we fly near 165mph IAS when in the 20kft?

If you have a normally aspirated COZY MKIV aircraft with a 180 - 200 HP engine, you won't be able to get NEAR 165 mph IAS. You'll be lucky to get near 100 - 120 mph IAS. And what are you doing at 20k ft, anyway?

 

In the low altitude, low Mach # world, Vne's are defined in IAS. So in theory, unless you're willing to point the nose almost straight down, you're not going to be able to get near Vne in a normally aspirated COZY MKIV at 20k ft. In fact, the only place where you CAN get near Vne in level flight is on the deck.

[Vacabrava]

Ok Marc,

Good point on the flutter.

I will not fly with oxigen. I hate cold temperatures, I live in one tropical country. My question was based on the Nat statment of 20k.

And is well aswered.

My concern is that mine CZ will be one turbo autoengine.

So I don`t want to hit the Vne, let`s say at 12k ft.