Problem with Grob-103

Interesting video with the testing of plane
https://www.youtube.com/watch?v=vel-qoc0UY4

On 2/7/2019 4:47 PM, Elena Malanushenko wrote:
Interesting video with the testing of plane
https://www.youtube.com/watch?v=vel-qoc0UY4

Hmmm...it's been a while since I've looked at a spring-trimmed G-103
specifically, but - assuming no obvious W&B issues muddying the picture -
things I'd look hard at upon noting (in the same flight?!?): a) insufficient
forward trim effect at tow speeds, and b) repeatable step-function-uncommanded
forward trim/stick-inputs at some higher/arbitrary speed would be:

1) "something 'fixed-in-place' not right" in the trim system;

2) "something capable of slipping under tension" in the trim system;

3) the entire primary pitch control system.

Of course, my version of Occam's razor supposes these possibilities have
"long-ago been considered/inspected" (based upon calendar-info in the film
snippet), but inferring as to what - if any - actions have occurred, based
upon the commentary, raises more questions in my mind than it answers.

Me not "doing" YouTube Q&A, my curiosity cat be definitely interested in what
ultimately proves to be the culprit...

Bob W.

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On Thursday, February 7, 2019 at 8:35:08 PM UTC-6, Bob Whelan wrote:
On 2/7/2019 4:47 PM, Elena Malanushenko wrote:
Interesting video with the testing of plane
https://www.youtube.com/watch?v=vel-qoc0UY4

Hmmm...it's been a while since I've looked at a spring-trimmed G-103
specifically, but - assuming no obvious W&B issues muddying the picture -
things I'd look hard at upon noting (in the same flight?!?): a) insufficient
forward trim effect at tow speeds, and b) repeatable step-function-uncommanded
forward trim/stick-inputs at some higher/arbitrary speed would be:

1) "something 'fixed-in-place' not right" in the trim system;

2) "something capable of slipping under tension" in the trim system;

3) the entire primary pitch control system.

Of course, my version of Occam's razor supposes these possibilities have
"long-ago been considered/inspected" (based upon calendar-info in the film
snippet), but inferring as to what - if any - actions have occurred, based
upon the commentary, raises more questions in my mind than it answers.

Me not "doing" YouTube Q&A, my curiosity cat be definitely interested in what
ultimately proves to be the culprit...

Bob W.

---
This email has been checked for viruses by AVG.
https://www.avg.com

Spring trimmer? our Club grob G 103 twin II has an aerodynamic elevator trim tab.
Scott

Thank you, Bob Whelan !

At 23:47 07 February 2019, Elena Malanushenko wrote:
Interesting video with the testing of plane
https://www.youtube.com/watch?v=vel-qoc0UY4

I would check for loose seals.

At 00:57 11 February 2019, Scott Calvert wrote:
At 23:47 07 February 2019, Elena Malanushenko wrote:
Interesting video with the testing of plane
https://www.youtube.com/watch?v=vel-qoc0UY4

I would check for loose seals.


I have replaced loose seals after experiencing similar in flying one.
Problem never returned.

Between the speedup and pitchdown, there's a growling noise.
Don't think it was the pilot.

I've heard vibrating mylars, but this pitch is too low.
Maybe a bi-stable one?

I wonder if you could get enough 75Knot air with a garden blower in the shop.

On Sunday, February 10, 2019 at 8:00:05 PM UTC-5, Scott Calvert wrote:
At 23:47 07 February 2019, Elena Malanushenko wrote:
Interesting video with the testing of plane
https://www.youtube.com/watch?v=vel-qoc0UY4

I would check for loose seals.

Scott has a good point.
I would suggest a few things.
1) Determine if weight and balance is as it is expected to be. A quick check is to level the glider and get the tail weight. Compare this to weight shown on the wt & balance being used. If different, investigate. Sensitive pitch on tow, especially nose up tendency, and pitch sensitivity in glide is consistent with an aft CG condition. Mid range weight in seats should not require full down trim.
2) Carefully look at all seals on the elevator, external and internal. A shifting seal at a particular air pressure, could easily cause pitch change.
3)If the elevator is trimmed with a tab verify smooth progressive function with no slop or binding.
Good luck
UH

Early Twin-2’s had aerodynamic elevator trim (trim tab), but later models went with spring trim. The jumping back reminds me of a slipping trim bar where the desired trim position doesn’t hold its setting. Recommend you pull the front seat and see if there is any slipping of the desired trim position. Can’t remember exactly what the trim setup is on the Twin Two, but a look under the seat will reveal any malfunction.
Hope this helps,
JJ

Normal bias configuration. Springs attached pushrod.
Front spring fully fwd on pushrod, rear spring 110 mm
from the rear end of pushrod


t 18:02 11 February 2019,
wrote:
Early Twin-2=E2=80=99s had aerodynamic elevator
trim (trim tab), but later
=
models went with spring trim. The jumping back
reminds me of a slipping
tri=
m bar where the desired trim position
doesn=E2=80=99t hold its setting.
Rec=
ommend you pull the front seat and see if there is any
slipping of the
desi=
red trim position. Can=E2=80=99t remember exactly
what the trim setup is
on=
the Twin Two, but a look under the seat will reveal any
malfunction.
Hope this helps,
JJ