vibration of duodiscus after winch launch

Hello everybody,
we in my Club(Pavullo in Italy) are suffering(2 times) a very strange vibration of the duodiscus for a minute or so after the glider is released from the rope,after one minute the vibration is finishing.
We are trying to investigate why it happened but we are not able to find out the reason.
Does someone already experienced the same problem?

Thanks a lot to everybody

Paolo Guardigli

On Wednesday, May 2, 2018 at 3:32:35 PM UTC-4, Paolo Guardigli wrote:
Hello everybody,
we in my Club(Pavullo in Italy) are suffering(2 times) a very strange vibration of the duodiscus for a minute or so after the glider is released from the rope,after one minute the vibration is finishing.
We are trying to investigate why it happened but we are not able to find out the reason.
Does someone already experienced the same problem?

Thanks a lot to everybody

Paolo Guardigli

Hello Paolo - can you be a bit more specific about that vibration? What is the estimated frequency of that vibration - low, high?
What is vibrating? The wings, the tail-boom or ??
Uli
'AS'

On Wed, 02 May 2018 13:58:42 -0700, AS wrote:

On Wednesday, May 2, 2018 at 3:32:35 PM UTC-4, Paolo Guardigli wrote:
Hello everybody,
we in my Club(Pavullo in Italy) are suffering(2 times) a very strange
vibration of the duodiscus for a minute or so after the glider is
released from the rope,after one minute the vibration is finishing.
We are trying to investigate why it happened but we are not able to
find out the reason.
Does someone already experienced the same problem?

Thanks a lot to everybody

Paolo Guardigli

Hello Paolo - can you be a bit more specific about that vibration? What
is the estimated frequency of that vibration - low, high?
What is vibrating? The wings, the tail-boom or ??

Could it be that the nosewheel got spun-up during the ground run and is
still spinning after release? I'm pretty certain I've noticed that once
or twice in an ASK-21 when the glider wasn't balanced on its main wheel
soon enough.


--
Martin | martin at
Gregorie | gregorie dot org

Il giorno mercoledì 2 maggio 2018 22:58:44 UTC+2, AS ha scritto:
On Wednesday, May 2, 2018 at 3:32:35 PM UTC-4, Paolo Guardigli wrote:
Hello everybody,
we in my Club(Pavullo in Italy) are suffering(2 times) a very strange vibration of the duodiscus for a minute or so after the glider is released from the rope,after one minute the vibration is finishing.
We are trying to investigate why it happened but we are not able to find out the reason.
Does someone already experienced the same problem?

Thanks a lot to everybody

Paolo Guardigli

Hello Paolo - can you be a bit more specific about that vibration? What is the estimated frequency of that vibration - low, high?
What is vibrating? The wings, the tail-boom or ??
Uli
'AS'

According the feeling(that is difficult to say precisely it seems the vibration comes from the tail (bottom)

I can't imagine a scenario where the nose wheel of a Duo Discus would touch the ground on a winch launch. The tail wheel, though -- you might be on to something.

I'd try to replicate the scenario on the ground. Have two [heavy!] people get into the cockpit, close the canopy, get the tail boom up off the ground and find a way to get the tail wheel to spin up to a good speed. See if the tail wheel is what's making the noise. If after the take-off the tail wheel is still spinning, and slowing down, making noise the whole time.

On Wednesday, May 2, 2018 at 5:45:19 PM UTC-4, Martin Gregorie wrote:
On Wed, 02 May 2018 13:58:42 -0700, AS wrote:

On Wednesday, May 2, 2018 at 3:32:35 PM UTC-4, Paolo Guardigli wrote:
Hello everybody,
we in my Club(Pavullo in Italy) are suffering(2 times) a very strange
vibration of the duodiscus for a minute or so after the glider is
released from the rope,after one minute the vibration is finishing.
We are trying to investigate why it happened but we are not able to
find out the reason.
Does someone already experienced the same problem?

Thanks a lot to everybody

Paolo Guardigli

Hello Paolo - can you be a bit more specific about that vibration? What
is the estimated frequency of that vibration - low, high?
What is vibrating? The wings, the tail-boom or ??

Could it be that the nosewheel got spun-up during the ground run and is
still spinning after release? I'm pretty certain I've noticed that once
or twice in an ASK-21 when the glider wasn't balanced on its main wheel
soon enough.


--
Martin | martin at
Gregorie | gregorie dot org

Il giorno giovedì 3 maggio 2018 19:51:17 UTC+2, Piet Barber ha scritto:
I can't imagine a scenario where the nose wheel of a Duo Discus would touch the ground on a winch launch. The tail wheel, though -- you might be on to something.

I'd try to replicate the scenario on the ground. Have two [heavy!] people get into the cockpit, close the canopy, get the tail boom up off the ground and find a way to get the tail wheel to spin up to a good speed. See if the tail wheel is what's making the noise. If after the take-off the tail wheel is still spinning, and slowing down, making noise the whole time.

On Wednesday, May 2, 2018 at 5:45:19 PM UTC-4, Martin Gregorie wrote:
On Wed, 02 May 2018 13:58:42 -0700, AS wrote:

On Wednesday, May 2, 2018 at 3:32:35 PM UTC-4, Paolo Guardigli wrote:
Hello everybody,
we in my Club(Pavullo in Italy) are suffering(2 times) a very strange
vibration of the duodiscus for a minute or so after the glider is
released from the rope,after one minute the vibration is finishing.
We are trying to investigate why it happened but we are not able to
find out the reason.
Does someone already experienced the same problem?

Thanks a lot to everybody

Paolo Guardigli

Hello Paolo - can you be a bit more specific about that vibration? What
is the estimated frequency of that vibration - low, high?
What is vibrating? The wings, the tail-boom or ??

Could it be that the nosewheel got spun-up during the ground run and is
still spinning after release? I'm pretty certain I've noticed that once
or twice in an ASK-21 when the glider wasn't balanced on its main wheel
soon enough.


--
Martin | martin at
Gregorie | gregorie dot org

thanks

I had a similar event after a auto launch last weekend. After release there was a weird vibration/buzz/whine/etc. and I figured it was the main wheel so squeezed the brake handle. No change. After landing just we rolled to a stop and the tail dropped back to earth I heard that sound again. Obviously it was the tail wheel spinning down, and in the case of the Duo XL that component is a heavy brass item (for cg) and takes a long time to unwind.

If your Duo has a brass tailwheel I'd bet that's what you heard.

KS

On Thursday, May 3, 2018 at 10:51:17 AM UTC-7, Piet Barber wrote:
I can't imagine a scenario where the nose wheel of a Duo Discus would touch the ground on a winch launch. The tail wheel, though -- you might be on to something.

On a winch launch you want to get to your climbing speed before laying back into the stick. In other words, no early back stick.

And even then, a winch launch lasts all of about 40 seconds. Assuming your were back on the stick too much too early and spun the tail wheel - it will stop being spun when you leave the ground (about 2 seconds into launch) and should be mostly spun down at time of release.

Regardless of technique, a balanced wheel will spin down smoother. Any way to check the wheel balance on the tail wheel?

On Saturday, May 5, 2018 at 9:43:15 PM UTC-7, Jeff Morgan wrote:
On Thursday, May 3, 2018 at 10:51:17 AM UTC-7, Piet Barber wrote:
I can't imagine a scenario where the nose wheel of a Duo Discus would touch the ground on a winch launch. The tail wheel, though -- you might be on to something.

On a winch launch you want to get to your climbing speed before laying back into the stick. In other words, no early back stick.

And even then, a winch launch lasts all of about 40 seconds. Assuming your were back on the stick too much too early and spun the tail wheel - it will stop being spun when you leave the ground (about 2 seconds into launch) and should be mostly spun down at time of release.

Regardless of technique, a balanced wheel will spin down smoother. Any way to check the wheel balance on the tail wheel?

It may have to do with bearings used than the balance.

Tom

On Saturday, May 5, 2018 at 9:24:20 PM UTC-4, Karl Striedieck wrote:
I had a similar event after a auto launch last weekend. After release there was a weird vibration/buzz/whine/etc. and I figured it was the main wheel so squeezed the brake handle. No change. After landing just we rolled to a stop and the tail dropped back to earth I heard that sound again. Obviously it was the tail wheel spinning down, and in the case of the Duo XL that component is a heavy brass item (for cg) and takes a long time to unwind.

If your Duo has a brass tailwheel I'd bet that's what you heard.

KS
We regularly hear a similar "spin down" noise from the tail wheel of our ASK-21 after takeoffs and landings.
UH