The Turbine Duke : how to descend

[dominique]

I was quite interested by John Dow's lovely video  showing how he manages the descent and land 

 

John does what the RealAir manual says to do, get the power lever to idle (and he stays at 2000 rpm it seems). The Duke then descends at a relatively low  150 knots which is a shame for a plane "born to run"   ! 

 

What I do is to try to keep my speed up to 180/190 knots by getting the rpm back to 2200, reducing the power to only about 450 (torque) and monitoring with an hawkeye the engine parameters not to overspeed as the Duke is indeed a slippery son of a bird.... I've to reduce the power several times down to something like 300 (torque) at  the end. When I level off, the speed quickly bleeds to approach level, 140/120 knots.

 

I don't say at all that I'm right, mind you, I wonder how it is done in the real world !  The problem with the  Turbine Duke is that it is a conversion variant sold in small numbers and the internet resources are very few.

 

Maybe the pros here (and John !) may comment.

[Rob Newman]

One thing that always helps a descent in a twin with constant speed props (doesn't matter if it's piston or turbine) is the RPM setting.

 

Specifically, for a descent, I was always taught to set the RPM at it's maximum allowed for constant operation, just as you would have it set for takeoff.

 

Every twin I've flown since has followed that same premise, the only slight exception being the Q400 where the props remain at 850rpm for descent and landing whenever possible, but this is due to noise reduction... descent performance would certainly be better if they were higher and there are indeed certain situations (high winds, tailwinds, windshear etc) that it is required to have high RPM.

 

As for the reasoning, the higher the RPM is set, the more "face on" to the air flow those propeller blades are... Which increases drag, allowing you to descend faster (vertical speed) and manage airpseed much more easily.

 

Indeed flying the Q200 we used to call setting high RPM for landing "deploying the chutes"... the difference is quite noticeable (Of course the Q200/300 have huge props so at high RPM, it's like having two barn doors on the wings)

[John Dow]

ha ha  there was a mistake I made in that video that meant a slow descent from 10,000'.  I had restarted the flight to re-do the approach and the aircraft still had flaps down!  I discovered that close to the island.  If you look at the airspeed close to the island you'll see that it has gone back up to 200 knots (even got to overspeed briefly but we won't mention that)... so yes on a normal approach without flap the speed would have been around the 200 knots from the beginning

[dominique]

Rob, thanks for barging in. Now the issue of the rpm is settled. About the power lever/speed.  Would your target speed be the cruise speed with the power lever set at whatever torque is needed or lower  (to allow a smoothier cabin pressure change ?) ?

 

Another question  I've is prop sync. When would you put in on and off ?

 

John, it happens to the best of us  ! You see your video was closely watched !

[Patrick Cox]

One must remember with turbines. They take their time to respond to throttle inputs.

[dominique]

One must remember with turbines. They take their time to respond to throttle inputs.

On that chapter, a real issue when setting up a stabilized approach for the Duke specially with hard turns,  I'm afraid that in the absence of the performance tables in the RA manual, only trials and errors will work... Yet to find the magic number(s).