Flutter: how to react

[Crakob]

Hi everybody
this question goes out to the more experienced pilots than i am. So almost everybody

Since i fluttered and crashed yesterday right on the startline (at the eurobattle contest)

So, what ist the best reaction to counteract the flutter. (i know, shouldn't happen in the first place)

Thanks

Christian

[yautay]

Slow down (pull up) - quickly but gently.
Brakes also work at least in sim

[yautay]

and...
expect that at higher altitudes less IAS needed for flutter.
i.e. Nephi is quite elevated so "easier" for flutter.

Good luck.

[sadatoni]

and...
expect that at higher altitudes less IAS needed for flutter.
i.e. Nephi is quite elevated so "easier" for flutter.

Good luck.

Experiments seem to indicate it's based on actual ground speed.

[yautay]

Ground speed has nothing to do with flutter. Glider soar in air not ground...

TAS is a factor => True Air Speed.

but...

we have no indication of TAS here.

We have IAS. Indicated Air Speed.

IAS needle will report less speed as you are higher due to lowered pressure (generally speaking).

FYI we have:

IAS=>CAS(calibrated for instrument and NAVPOS error)=>EAS(Compressibility corrected)=>TAS(Density corrected)

Point to remember is that higher you are less speed (from cockpit gouge) needed to test flutter.

Cheers

[Lenticular]

I pull the brakes as by the time I have pitched up, I've ripped the wings off.

As for flutter - it's TAS related, not IAS.

In Nephi, flutter occurs around 135 kts indicated (around 5000 ft), rather than 150+ kts at sea level (Antares 18S).

I certainly don't want to ever test this for real!

[stevemyall]

Would anyone be kind enough to explain why Flutter is independent of indicated air speed as I'm confused?

On a powered aircraft, I understand why true airspeed is important for flight at high altitude due to compressibility but I didnt think this would be the cause of flutter in a glider travelling at relatively slow speeds?

My expectation is that flutter would occur at the same indicated airspeed regardless of altitude due to the instrument measuring an equivalent mass of air acting on the controls.

Hope someone can lend a explanation to reduce my confusion on this one.

Thanks

Steve (GR1)

[Rotareneg]

Both dynamic pressure and damping effect flutter. Assuming we're not getting anywhere near any transonic speeds, IAS and dynamic pressure should be equivalent. Damping however is more dependent upon density. Thus, as you climb your dynamic pressure has the same effect on flutter but there's less damping to control oscillations.

Apparently a good rule of thumb is to assume the Vne at altitude is half-way between the sea-level Vne and the same value converted to IAS at that particular altitude. So, assuming a 100 knot Vne at sea level, 100 knots TAS at 20,000 feet being 71.4 knots IAS, you take the average of 100 and 71.4 and get 85.7 knots IAS Vne at 20,000 feet.

[stevemyall]

Thanks for taking the time to reply rotareneg.
I'm still not quite there with static pressure/density. Would this not chnage in proportion to the dynamic pressure also? I had always assumed that the pressure curve over the wing surface would not change shape idependantly of air density if dynamic pressure was maintained (by maintaining a constant indicated air speed). Hope I don't come across as a flat earther. I genuinely just want to clear this up in my mind.

Steve

[Rotareneg]

Here's a blog post that has all sorts of info about the subject: http://flyingdonald.blogspot.com/2011/0 ... utter.html

And a quote from a forum post that's relevant:

The classical flutter equations contain forcing terms that are proportial to dynamic pressure ( q = 1/2 rho V^2) and damping terms that are proportional to just density and velocity ( rho V). This produces the altitude dependence, BUT it is not strictly related to TAS either. I have found, and others also have shown, that a good approximation to a realistic flutter boundary as a function of altitude is a line roughly half way between a constant EAS line and a constant TAS line. On a plot of speed vs altitude, you can lable the X axis as EAS or ( or IAS) and the y axis as altitude. So a constant EAS line is vertical. A constant TAS line curves over to the left, corresponding to lower EAS at higher altitude. Now, draw a second curve about half way between the two. This has been found to be a fairly good approximation to typical flutter boundaries vs altitude.

Some manufacturers go with a conservative method (the flight manual for the DG-808C does this, for example) and just give the IAS conversion at each listed altitude from the single TAS VNe value.

[Bre901]

I have made a graph of TAS as a function of IAS and altitude, assuming standard atmosphere.

The metric version is available there : https://drive.google.com/open?id=1lJMGd ... kLgkS4lVzW

Comments are welcome

If someone is interested by a version for the metrically challenged , please feel free to ask, it would be rather simple to make.

Edited to add that the graph can be read for whichever speed units you are using (m/s, mph, kts, fathoms/minute), which means you only have to convert metres to feet (1 m = 3.25 ft appx.)

[Bre901]

Following yesterday's thoughts, here is an "oecomenical" version.
I forgot to mention that I have neglected compressibility effects.

https://drive.google.com/open?id=17beGU ... jXw0g0Tipw

I have changed the plot orientation as to reflect what can be found on the web : some interesting plots, however there are intended for airliners and they usually start above the gliders' VNE

[jjmaloss]

You do not open airbrakes above Vne because they will increase the load on the wings that will separate (see Danse avec le vent JM Clément). If you are in a spiral dive, you correct it (ailerons outside) and pull gently gently until getting below manoeuvring speed. After this, the aircraft is no longer airworthy and you need a structural inspection.