Duo Discus XT

HI

Recently flew a 2008 Duo with the 5 bladed sustainer.

Started very fast and beautifully with very little height loss.

When it came to putting it away, it was, however, difficult to get it to stop rotating.

Yes, put it half way retracted and yes, used the decompression to also slow it down as advised. Only by reducing airspeed to close to stall and holding it there, did it finally stop.

The days was hot. OAT at 3,000' was 32C.

Of interest (as it seems counterintuitive), why does the decompression work to slow it down?

Regards

Richard

On Friday, December 21, 2018 at 4:02:35 PM UTC-8, Richard Frawley wrote:
HI

Recently flew a 2008 Duo with the 5 bladed sustainer.

Started very fast and beautifully with very little height loss.

When it came to putting it away, it was, however, difficult to get it to stop rotating.

Yes, put it half way retracted and yes, used the decompression to also slow it down as advised. Only by reducing airspeed to close to stall and holding it there, did it finally stop.

The days was hot. OAT at 3,000' was 32C.

Of interest (as it seems counterintuitive), why does the decompression work to slow it down?

Regards

Richard

Richard:
Unaware of that technique.
From the Duo XT manual I have, 4.5.3.

STOPPING AND RETRACTING THE POWER PLANT
O Reduce speed to about 90 km/h (49 kt, 56 mph)
O Ignition OFF
O CLOSE fuel shut-off valve
O RETRACT power plant for 5 seconds
O When prop has stopped, RETRACT power plant
at 90-100 km/h (49-54 kt, 56-62 mph) fully
O Switch ASI to pitot head in fin

Jim

A few points:
1. The reason it starts so easily is because the prop is larger than the single seat gliders and therefore spins up more quickly. This also means it spins down more slowly.

2. On the ground, extend the engine and then retract it and you will see that it can actually retract a very long way with no risk of the spinning blades hitting the doors. Perhaps you did not retract it far enough?

3. As I understand it, the use of decompress to stop the rotation is not going to work while the decompress is active (decompress handle pulled), but with the ignition off and the engine partially retracted a slow removal of the pulled decompress handle will act as a prop brake. A bit like using engine braking in a standard transmission car or popping the clutch too soon while trying to bump start a car and having it come to stop instead of firing.

On Saturday, 22 December 2018 14:57:32 UTC+11, Dave Springford wrote:
A few points:
1. The reason it starts so easily is because the prop is larger than the single seat gliders and therefore spins up more quickly. This also means it spins down more slowly.

2. On the ground, extend the engine and then retract it and you will see that it can actually retract a very long way with no risk of the spinning blades hitting the doors. Perhaps you did not retract it far enough?


yes, thats possible, being conservative and only did so for 5 secs.

will do as you suggest.

perhaps the state of the battery might affect the retract speed.


3. As I understand it, the use of decompress to stop the rotation is not going to work while the decompress is active (decompress handle pulled), but with the ignition off and the engine partially retracted a slow removal of the pulled decompress handle will act as a prop brake.

ahhh....that makes sense, will investigate that approach as well, thanks

A bit like using engine braking in a standard transmission car or popping the clutch too soon while trying to bump start a car and having it come to stop instead of firing.

On Fri, 21 Dec 2018 16:02:33 -0800 (PST), Richard Frawley
wrote:

HI

Recently flew a 2008 Duo with the 5 bladed sustainer.

Started very fast and beautifully with very little height loss.

When it came to putting it away, it was, however, difficult to get it to stop rotating.

Yes, put it half way retracted and yes, used the decompression to also slow it down as advised. Only by reducing airspeed to close to stall and holding it there, did it finally stop.

The days was hot. OAT at 3,000' was 32C.

Of interest (as it seems counterintuitive), why does the decompression work to slow it down?

Regards

Richard

Richard, perhaps you skipped a few critical steps, or did them but
neglected to mention this.

1. Either turn off the ignition, or shut off the fuel, or both.
2. Partially retract the engine, perhaps 30 degress tilt.
3. Open the decompression valve for a few seconds and then CLOSE the
decompression valve. If you continue to hold the decompression open
the prop will NOT stop.
4. When the decompression valve is then closed the prop should stop
pretty quickly.
5. Complete retraction.

Hope this helps

Bob (19 years flying a Ventus C turbo)

At 05:43 22 December 2018, Bob Gibbons wrote:
On Fri, 21 Dec 2018 16:02:33 -0800 (PST), Richard Frawley
wrote:

HI

Recently flew a 2008 Duo with the 5 bladed sustainer.

Started very fast and beautifully with very little height loss.

When it came to putting it away, it was, however, difficult to get it to
stop rotating.

Yes, put it half way retracted and yes, used the decompression to also
slow it down as advised. Only by reducing airspeed to close to stall and
holding it there, did it finally stop.

The days was hot. OAT at 3,000' was 32C.

Of interest (as it seems counterintuitive), why does the decompression
work to slow it down?

Regards

Richard

Richard, perhaps you skipped a few critical steps, or did them but
neglected to mention this.

1. Either turn off the ignition, or shut off the fuel, or both.
2. Partially retract the engine, perhaps 30 degress tilt.
3. Open the decompression valve for a few seconds and then CLOSE the
decompression valve. If you continue to hold the decompression open
the prop will NOT stop.
4. When the decompression valve is then closed the prop should stop
pretty quickly.
5. Complete retraction.

Hope this helps

Bob (19 years flying a Ventus C turbo)


I always find that pumping the decompression valve works best.

When the valve is closed the piston work like a bouncing spring, as it
moves towards TDC it uses the energy of the prop to compress the air in the
cylinder so slowing the props speed, as it goes over TDC the compressed air
is converted back into additional rotational movement of the prop. When you
pump the valve, depending on your timing, you keep removing the compressed
air from the system. This means the system has to use more energy to
re-compress the cylinder therefore slowing the prop faster than just
continually holding the valve open.

Works for me, Discus Bt and Ventus 2ct

At 16:42 22 December 2018, Jonathan Walker wrote:
At 05:43 22 December 2018, Bob Gibbons wrote:
On Fri, 21 Dec 2018 16:02:33 -0800 (PST), Richard Frawley
wrote:

HI

Recently flew a 2008 Duo with the 5 bladed sustainer.

Started very fast and beautifully with very little height loss.

When it came to putting it away, it was, however, difficult to
get it to
stop rotating.

Yes, put it half way retracted and yes, used the
decompression to also
slow it down as advised. Only by reducing airspeed to close to
stall and
holding it there, did it finally stop.

The days was hot. OAT at 3,000' was 32C.

Of interest (as it seems counterintuitive), why does the
decompression
work to slow it down?

Regards

Richard

Richard, perhaps you skipped a few critical steps, or did them
but
neglected to mention this.

1. Either turn off the ignition, or shut off the fuel, or both.
2. Partially retract the engine, perhaps 30 degress tilt.
3. Open the decompression valve for a few seconds and then
CLOSE the
decompression valve. If you continue to hold the
decompression open
the prop will NOT stop.
4. When the decompression valve is then closed the prop
should stop
pretty quickly.
5. Complete retraction.

Hope this helps

Bob (19 years flying a Ventus C turbo)


I always find that pumping the decompression valve works best.

When the valve is closed the piston work like a bouncing spring,
as i
moves towards TDC it uses the energy of the prop to compress
the air in th
cylinder so slowing the props speed, as it goes over TDC the
compressed ai
is converted back into additional rotational movement of the
prop. When yo
pump the valve, depending on your timing, you keep removing
the compresse
air from the system. This means the system has to use more
energy t
re-compress the cylinder therefore slowing the prop faster than
jus
continually holding the valve open.

Works for me, Discus Bt and Ventus 2ct

I had the later Duo XLT with the later ilec control.It was fully
automatic so little to do.When you switch off it cuts the ignition
and about 1 second after the engine starts to retract, it stops just
about 1/2 way down NO USE OF DECOMPRESSOR and when
rotation stops ,about 3seconds it goes down the rest of the way.
Thats it nothing else just shut the fuel cock when you are done.
The turbo prop will fit into the engine box at any point in its
rotation so there is no need to stop at any particular spot unlike
the 2 bladed props.

I now have a ventus 2ct with the older ilec and the same system
works.
You don't want to turn the fuel off because you loose cylinder
lubrication.
Ignition off
When the engine stops firing retract until its as low as is safe(you
need to know that position by memorising it on the ground)
When rotation stops retract the rest of the way and shut the fuel.

The only problem we ever had with the duo was catching a door
on the final few inches of the retraction and this was because the
door spring had lost tension and was allowing the engine bay
doors to blow partially shut in the slipstream.A new spring fixed
it.

On Saturday, 22 December 2018 02:02:35 UTC+2, Richard Frawley wrote:
Of interest (as it seems counterintuitive), why does the decompression work to slow it down?

Regards

Richard

No idea, but it works in cold temps also. Keeping deco pulled slows down prop considerably faster. It has nothing to do with opening and closing it all the time.

I used to ride a heavy dirt bike in the early 70s.Â* The compression
release was a Godsend when going slowly down a very steep hill in the
dirt.Â* It really kept the bike slow.Â* I sometimes wondered if turning
the ignition off would have been better or not, but, in the 70s we
didn't have electric starters on dirt bikes.

On 12/23/2018 1:28 AM, krasw wrote:
On Saturday, 22 December 2018 02:02:35 UTC+2, Richard Frawley wrote:
Of interest (as it seems counterintuitive), why does the decompression work to slow it down?

Regards

Richard
No idea, but it works in cold temps also. Keeping deco pulled slows down prop considerably faster. It has nothing to do with opening and closing it all the time.


--
Dan, 5J

A little later than you, but usually 125-250CC dirtbikes, thus, not as much engine braking (2-strokes), yes, technique on starting was a biggie......usually 4-stroke and Bing carbs........LOL......

Nope, never flew any type of powered sailplane outside of a Grob 109? May get to fly an electric ASW-24 if the conversion gets sorted.

Yes, fun to read what others deal with. Thanks for the info......