# Where, not how, is VNE calculated. Does VNE go up with altitude?

Discussion in 'General Experimental Aviation Questions' started by Faralon, Jan 26, 2017.

1. Mar 7, 2017

### Marc Zeitlin

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No sweat - hard to tell emotional content if you don't know someone. I've got thick skin.

Which I now apparently need, as I get needled for not knowing more about flutter, even though I worked for Scaled, although not as an aerodynamicist .

And a bunch of searching that I did previously was reasonably opaque to me, hence my question.

Yeah, and since the last time I futzed with ODE's was... 37 years ago, I might as well be staring at martian.

So I appreciate the pointers - although I'm not totally clear on the deeper math, I do understand more than previously. Let me try to summarize what you've said and what I gleaned from the docs you pointed to so we can see if I actually understand it.

Flutter is a function of many parameters - air density, TAS, TAS^2, and structural stiffness in multiple modes, and more. The interactions of these parameters are exceedingly complex and non-linear. GENERALLY, the effect of altitude (density) on flutter speeds can be approximated by using the average of the EAS (IAS, at low Mach #'s) and the TAS as the flutter speed limit, but this is only a rule of thumb - it is NOT an analytical limit. While that will USUALLY be a reasonable limit to use for approximating how fast one can go before things flutter, given a known flutter limit at a known altitude (density), it is by no means a guarantee, and even using a straight TAS line as the flutter limit, no matter what the density, is not necessarily a conservative position.

Due to the fact that flutter is an interaction of multiple modes of vibration, even GVT and analytical calculations are not always accurate in predicting flutter speeds at all altitudes, and only real flutter testing can determine whether or not the analysis is accurate.

So, in conclusion, using the EAS+TAS/2 limit, due to the density dependence of the driving frequencies, will usually give a reasonable limit for flutter speeds. Using TAS only will give a more conservative limit (but NEITHER are guarantees of flutter resistance).

My error, then (and the error of the Van's article to which I and others have pointed) was in claiming that flutter was a function of TAS, without acknowledging that it's more complicated and not truly dependent on either EAS or TAS, but on density and the other factors mentioned above.

Close?

Last edited: Mar 7, 2017
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2. Mar 8, 2017

#### Moderator

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Yes.

I'd put it down differently though.

This is only applicable to conventional airframes at speeds were trans sonic effects don't play a role, using conventional structures and practices, be it wood, cloth-covered tube, composite or aluminium sheet construction:

All classical flutter modes will vary att altitude with Vflutter ∝ (EAS+TAS/2) with the exception of unbalanced or under-balanced control surfaces, notably if their Re number if small, so very low flutter speeds, or very narrow chords.
The latter is notably problematic at high (20K+ ft) altitudes where flutter can occur well below sea-level flutter speed. It's a biggie in wave-soaring sailplanes. Without exception the fix is to balance the ailerons (and flaps).

That's where the advise to balance all controls at higher (say 200+ kts VNE) comes from. The late Orion and Raymer for example stipulate this.

I've discussed the above with several designers of light airframes (mostly certified, sailplanes and conventional piston airframes) and all seem to use that. Not aware of any light (non-turbine) airframes where they did the full aerodynamic flutter analysis in the design phase. At most companies they just use conventional practices, check the modes with GVT and then validate with dive testing.

Fortunately, structural modes are pretty much constant, so unless you have really complex variable mass in the wings (water ballast, fuel), flutter analysis is surprisingly straightforward weren't it for those nasty PDE's

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3. Mar 8, 2017

### Himat

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Should it be noted that this for airplanes with ”free” control surfaces?

From pictures and reports, the fastest radio control sailplane that surpassed 500 mph speed did not have balanced control surfaces. The control servos and linkage on the other side was stout. Same if memory serve me, ailerons on the Northrop F-5 where not balanced, but operated by hydraulic jacks.

4. Aug 21, 2019

### BJC

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I posted this response here rather than in the thread about turbo-normalizing.

See discussion above for greater detail, especially wrt unbalanced control surfaces. This is intended to be a simple, somewhat intuitive, response to Steve’s question.
Super simple (not rigorous) way to think of it:

The excitation of the structure has to be at a frequency close to the resonance frequency of the structure.

The excitation frequency depends on the speed of the air molecules passing over the structure. That speed, by definition, is the true air speed.

The fluid surrounding the structure provides viscous damping to the movement of the structure. As altitude increases, the viscosity of the air decreases, and the damping decreases.

Decreased damping (increased altitude) lowers the frequency (speed of the passing air / TAS) at which the structure resonates.

Summary: The driving force is a function of TAS, but the damping that opposes resonance decreases with decreasing air density / altitude. Therefore, the speed at which the structure will flutter reduces as altitude increases.

Now, just for fun, go and calculate the EAS of the STS that broke up on re-entry.

BJC

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5. Aug 22, 2019

### TXFlyGuy

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What is the difference between vibration and flutter?

A reed in a woodwind instrument vibrates to produce a musical tone, but I don’t think it flutters.

Can an aircraft have vibration without flutter?

6. Aug 22, 2019

### BJC

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Aeroelastic flutter is vibration of a part of an airframe caused by aerodynamic forces.

Flutter is typically reserved for describing aeroelastic flutter, ie, the excitation energy is from aerodynamic forces. Wings flutter. Flags flutter. Audio speakers vibrate. Circuits and mechanical systems resonate.

A reed vibrates at a resonate frequency (musical tone or note) determined by the the length of the reed that is free to vibrate, assuming adequate airflow from the musician’s breath. One may say that the reed is resonating, vibrating or fluttering.
Every airplane that I have flown in has some degree of vibration, typically excited by the engine or propeller or turbofan vibration. Engine/propeller forces can initiate destructive resonance, such as that experienced early in the history of the Lockheed Electra. IIRC, the common description used there was “nacelle whirl flutter.”

BJC

Last edited: Aug 23, 2019