Flutter testing

[RoarkS]

https://www.youtube.com/watch?v=ZymXOrPL1CQ

I've spent a fair amount of time shaking stuff for shock and vibe testing... But it's usually just a fail check running an ATP on the hardware after testing.

How does this work for flutter?

So he's got a stinger exciting the wing tip (I'm guessing on a spar/compression rib??)
Then a few sensors to "map" the response. Its sitting on it's tires/shock absorbers...

He didn't have a lot of resolution with just 8 accelerometers... but was able to see how each one was phased to the other. They were feeling by hand to find nodal lines.

He determined that a node line intersected an aileron (honestly before doing anything) and suspected, the aileron was being balanced on one side of the line was insufficient.

He also commented that there was slop/play in the aileron that... what if it was excited by airflow the slop **could** allow it to set up at a resonance.

What kind of analysis did he actually do with that equipment? Looked like a bunch of practical intuition and a show with vintage equipment.

 

[SWComposites]

Huh? Don’t think that is a flutter test. Flutter is a coupling of aerodynamic forces with structural resonance. I only recall flutter testing in a wind tunnel or actual flight test.

 

[rb1957]

it could be flutter, dependent on the mass properties of the wing (structural axis) and the mass and CG of the "stringer". "Flutter" (or aeroelastic instability) is the interaction between structural properties (the elastic axis of the wing), the inertial properties of the wing (weights and CGs) and aerodynamic loads. A classic instability is divergence, where the aerodynamic forces act on the wing structure to increase the angle of attack, which increases the aero forces, which increase the angle of attack ...

You're describing a GVT (Ground Vibration Test) which is usually done to valid the aero-elastic analysis for wing (and or fuselage flutter (or aerodynamic (or aeroelastic) instability).

read up at MIT's "opencourseware" site.



another day in paradise, or is paradise one day closer ?

 

[RoarkS]

I guess my point is... after watching that... I'm pretty sure the guy I first worked for would call this test a charlatan and throw him out of the hanger.

What did he actually do?

 

[BrianE22]

The adding of the artificial constraint (the stinger attachment) would make it an invalid test unless that type of actuator has zero stiffness. It would screw up natural frequency data and mode shapes so I don't see that you get anything out of this test. But I used to work with hydraulics so maybe stingers do have a near zero impedance.

 

[rb1957]

yes, when we did a GVT, the excitor was a hydraulic motor mounted in the fuselage.

another day in paradise, or is paradise one day closer ?

 

[Sparweb]

I have to agree that it looks exactly like a GVT to me, too. This video is at least 20 years old so you're not going to see LabView on a computer screen. Just get over it. The guy running the equipment is even older and if you wait for the end, you will see that he died in 2004. So I think you should calibrate your expectations; the test is not junk.

The GeeBee is over-powered, and likely to fly its wings off at full power, so you need to have some way to figure out what speed that is before you fly that speed. You don't fly the plane until the wings fall off and then decide to fly slower. You need to determine the stiffness, natural frequencies, and damping ratio under a range of conditions to see how the wings behave. The GVT is the tried-and-true method and this test isn't bad.

The frequency generator he uses to tune his measured excitation frequency is all I need to believe he knows what he's talking about. The guy running the test describes his system well. The equipment is a bit limited, and if he collected more than 2 channels simultaneously, he'd be able to give results much faster. But come-on, this guy's stuff is cool because it's so old and still works. Don't you miss your old Heathkit radio?

Did anybody notice Curtis Pitts in the audience?

Typical youtube: this comes from a TV show but the show credits were cut off.

 

[Sparweb]

I don't think they bothered to explain this, but at 13:40 when Kermit runs his fingers along the wing and finds a "node line" in 3 separate places, that means he is feeling a lack of vibration at those locations. That means the wing was oscillating in a higher mode. Maybe the 5th mode IIRC. And Leon says that the shaker is running at 24 cycles per second so that tracks for a stubby wooden wing. The first mode of vibration may be about 2 or 3 cycles per second. They'd want to find all of the modes while they equipment is set up.

Each of these frequencies will correspond to a certain airspeed, but that's hard to predict without a very elaborately worked-up aeroelasticity code. With computers it's now possible to leave the accelerometers on the plane when you fly it and measure the same stuff in flight. That will tell you what speed hits those natural frequency modes which can do the most damage.

 

[BrianE22]

The GVT is the tried-and-true method and this test isn't bad

SparWeb -

I have no experience with aircraft testing, just ground vehicles. Doesn't the added constraint of the actuator near the end of the wing change the wing's natural frequencies and mode shapes

 

[Sparweb]

It can, especially if the actuator is rigidly fixed to the wing, or is rather massive relative to the wing.

I didn't think of that while watching, so if you noticed simethung like that, it could undermine the results.

 

[BrianE22]

If this method has been used for quite a while then I'd guess some one did look at the effects and concluded they were negligible. But there's probably a bunch of caveats associated with that conclusion. Mass, springiness and damping of the actuator would all need to be looked at.

 

[WKTaylor]

The GVT is useful for understanding fundamental frequencies of the 'guts of the aircraft'... systems, mechanisms, installed equipment, engine/power components, fuel/oil slosh, etc... etc... as well as some vibration elements/harmonics of the primary and secondary structure. This knowledge during flight tests will help determine when flutter or resonance is emerging/increasing... and what to do to back-away from destruction.

Anything obvious that goes into harmonics/resonance within range of 'forcing functions'... such as engines/props/fans/etc … is a condition to be worried about.


Regards, Wil Taylor
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