Pending new revised Sky Arrow POH, revised Operating Limitations and SB

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MrMorden
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Re: Pending new revised Sky Arrow POH, revised Operating Limitations and SB

Postby MrMorden » Thu Dec 28, 2017 8:56 am

FastEddieB wrote:Andy,

If there is a flaw in your reasoning, I think it’s covered in that first link to that Australian site.

In part...

“4. Air density
Thick, dense air damps oscillations and delays the onset of flutter. It’s like oil in the shock-absorbers of your car. Obviously you have direct control over the density of the air in which you fly. High altitudes and hot temps mean less damping and a greater chance of flutter.

Since you used the reducto ad absurdum of outer space, try the other direction - if you submerged your plane and tried to push it through water, I think you’d see the damping effect of the thicker medium, which would make flutter very unlikely.

Right?


That goes back to what was in the Pipestrel manual; that IAS is a good indicator below 13,100ft (the number they used) and above that one should use TAS. I can certainly buy the idea that there is some threshold air density below which oscillations are inadequately damped to use IAS as a reliable measure. If that is the case, and the aircraft is capable of flight above that critical altitude, then I think the manufacturers whould be making that crystal clear in their POH. My CTSW has a Vne by the POH of 145KIAS, and there is no mention of of an altitude limit on using that number. The POH lists the service ceiling for the CTSW as 14,000ft and change.

So I'm perfectly willing to concede that at higher altitudes the Vne listed in IAS might not work very well. I don't fly at those altitudes, so at least for me I think using the listed Vne in IAS is fine...but it's certainly something I will keep in the back of my mind when flying at 9500ft or higher to clear terrain.
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

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CharlieTango
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Re: Pending new revised Sky Arrow POH, revised Operating Limitations and SB

Postby CharlieTango » Thu Dec 28, 2017 10:10 am

MrMorden wrote: My CTSW has a Vne by the POH of 145KIAS, and there is no mention of of an altitude limit on using that number. The POH lists the service ceiling for the CTSW as 14,000ft and change.



The flutter criteria in 23.629 are quite complex. They attempt to take into account externalities like wear and tear, hidden damage and even in-flight failure of various components. But the kind of testing and validation that the FAR envisions is outside the scope of amateur builders and, I suspect, of many manufacturers of kits, ultralights and LSAs. Flight flutter testing is expensive and dangerous, and ground vibration testing, which goes a long way toward ensuring freedom from flutter, eliminates only the danger, not the expense. I haven't conducted a poll, but I suspect that the flutter testing of most homebuilts and LSAs has been limited to "pulsing" the controls in flight — that is, hitting the stick or kicking the rudder — at gradually increasing speeds while looking for any softness in the damping of the resulting control movements, and while wearing a recently packed parachute. This method is fine as far as it goes, but it's far from perfect.

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FastEddieB
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Re: Pending new revised Sky Arrow POH, revised Operating Limitations and SB

Postby FastEddieB » Thu Dec 28, 2017 10:22 am

Anecdote #1: As delivered, my Sky Arrow had a definite oscillatory feel to the ailerons - bump them or hit a gust and they would take several cycles to settle down. My dealer adjusted them so they’re ever-so-slightly reflexed down. That did the trick, I suppose by lightly “loading” the ailerons.

Anecdote #2: I had the ailerons on my Tiger begin a subtle but definite “wag” at least once when flying in a light rain. This was caused by the trailing edge of the ailerons being hollow. Just a bit of water in them could apparently move their cg enough to cause that. One fix was to remove the channel, but that adversely affected the sporty feel of the Grumman line. I think an alternative fix was to drill drain holes in the trailing edge, but it’s been a long time.

Anyway, control surface balance and rigging is critically important, and not to be taken for granted*.


*A Cirrus owner actually mounted a GoPro to his rudder. Lived to tell about it, but ignorance of this sort of thing can be and has been fatal.
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
CFI, CFII, CFIME
FastEddieB@mac.com

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CharlieTango
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Re: Pending new revised Sky Arrow POH, revised Operating Limitations and SB

Postby CharlieTango » Thu Dec 28, 2017 10:50 am

Here's where Vne comes into play, for me, in the real world.

Sierra mountain waves can sometimes provide huge unwanted lift that will suck you into cloud base or the flight levels. What do you do with your pitch attitude?

Its better to explain to ATC why you are in the flight levels without a clearance or better to be IMC during your time when the wave is in control than to break up in flight. Be clear on this going in.

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MrMorden
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Re: Pending new revised Sky Arrow POH, revised Operating Limitations and SB

Postby MrMorden » Thu Dec 28, 2017 10:51 am

For those who have not seen it, here is my flutter event. This is actually the second time it happened -- the first time, I said "surely that isn't flutter" since it was far below Vne. So I strapped a GoPro to the top of the airplane pointed at the tail to make sure, and repeated the conditions. Sure enough, it was fluttering at ~125KIAS. It's fairly mild as flutter goes, but carbon fiber doesn't flex so it tends to fail spectacularly when it does so.



I worked with the factory to resolve this. Apparently there were two different stab designs; the earlier one had less stiffening in the aft area just ahead of the trim tab hinge. This caused flutter events on a very few (two or three that I know of) CTSWs in the 125kt speed range. The full-span anti-servo tab would get differentially loaded (I personally think due to rotating propwash pushing down one side and up on the other) and flex the stab and cause the flutter as seen in the video.

Flight Design was very good in working with me to resolve this. The stab had to come off for a trip back to Connecticut, where they added the full span composite reinforcement that is on the newer stab design. That solved it, and I have since flown to 140KIAS (Vne is 145, but I don't see the need to push it) with no problems at all.
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

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Jim Hardin
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Re: Pending new revised Sky Arrow POH, revised Operating Limitations and SB

Postby Jim Hardin » Thu Dec 28, 2017 11:02 am

SCARY!

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Re: Pending new revised Sky Arrow POH, revised Operating Limitations and SB

Postby Cub flyer » Wed Feb 14, 2018 5:12 pm

I’m curious did Flight Design send you a written letter that your serial number range airplane
has this problem and it’s a safety of flight issue or did they issue a Service Bulletin about the issue?

If it’s a known flutter problem that was resolved by stiffening the stabilator after they went to full span servo tabs and
the problem has caused some accidents I think the owners should be notified. Quickly

Did they charge for the “upgrade” modification?

The self notification by the manufacturers is how it’s supposed to work with known problems since there are no Airworthiness Directives
on SLSA.
"Perfection is finally attained not when there is no longer anything to add but when there is no longer anything to take away." Antoine de Saint Exupery

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FastEddieB
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Re: Pending new revised Sky Arrow POH, revised Operating Limitations and SB

Postby FastEddieB » Fri Feb 23, 2018 8:25 am

I received the decal with the new Operating Limitations yesterday:

Image

Kinda hard to read, but in the plane it’s white letters on a red background.

As a surprise, what I got in the mail from Italy was a full set of cabin placards:

Image

Nice. Most of mine are in pretty good shape, but a couple are showing their age. Nice having spares regardless.

Now I just need to placard or mark the airspeed indicator with the new redline and make the appropriate logbook entry per the SB.
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
CFI, CFII, CFIME
FastEddieB@mac.com


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