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Question on propellers?

 
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jbalonso777
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PostPosted: Sat Jan 04, 2014 1:33 pm    Post subject: Question on propellers? Reply with quote

Hello everyone!
I just flew on a JetKonnect ATR 72.
A few months ago, I flew on a Helitours Xian MA60.
Both completely different aircraft types, but with similar propellers.

However, I found quite a significant difference in the start up procedures of the two.

Onboard the Xian, the props start spinning slowly, before finally gaining enough speed and then the whole 'engine' comes on.
http://youtu.be/ZdsSCkDlzCI

However, on the more common ATR, the sound of the engine suggests that they are being switched on, while the propellers start spinning much later, and seems to have gained full speed to start spinning.
http://youtu.be/dVe_56LHcLk

Are there any significant reasons for this?

Thanks in advance!
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The_Goat
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Posts: 2871
Location: South of France

PostPosted: Mon Jan 06, 2014 1:52 am    Post subject: Reply with quote

There are two things, the propeller itself and the turbine engine that drives it.

In the ATR, I believe that the two can be decoupled. So the turbine engine can be kept running while keeping the propeller stationary. This is useful when there is no ground power available or if the APU of the airplane is not functional. The running turbine can provide electrical power to the aircraft and the non-spinning propeller won't cause any inconvenience to ground ops.

What you heard in the ATR was the turbine engine revving up first, followed by the propeller getting coupled to it later. So there is a lack of sync between the engine revving up and the prop spinning.

I don't know if this feature is universal. Maybe it doesn't exist in the Xian, so the prop speed and the engine speed would be perfectly in sync.

Maybe some of the resident techsperts can shed more light?
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ALT STAR
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Joined: 24 Dec 2009
Posts: 70

PostPosted: Wed Jan 15, 2014 12:26 pm    Post subject: Reply with quote

Nice trip report. I thoroughly enjoyed it,especially the ATR flight.Takes me back to the good times flying that airplane.

Regarding the prop not spinning while the gas generator is running is a feature on the ATR ,available only on the no 2 engine.
I believe the previous reply has pretty much summed it up but i will add a few more details to it.
They call this feature "Hotel Mode".
Instead of having an additional APU installed, the no 2 engine prop can be disconnected from the gas generator via hydraulic pressure and supply electrical power and bleed air on ground when the aircraft is parked.
There is guarded switch on the overhead panel which can be used to disconnect/reconnect the prop to the shaft as required, provided you have blue hydraulic pressure and prop feathered(iirc..long time since i flew the airplane).
So what you saw was the no 2 engine running with prop brake "on" and later with the prop brake "off" the prop is reconnected to the gas generator unit.

Quote:
Also, there is a part of the flap which comes down...I hope you notice that in the video!
HD: JetKonnect ATR 72-500 take off from Bengaluru


What you see is not part of the flap but part of the aileron.It is called a balance horn or tab.On the ATR it is on the elevators and rudder as well.Since the ATR has no hydraulically assisted control surfaces(only one hydraulic roll spoiler on each wing i think) these tabs reduce to the amount of force required to move the control surfaces in flight(although on some occasions during gusty winds and engine failures it does require quite a bit of force ).Forces acting on the control surface and horn are opposing so it cancels out some of the effort required to move the main control surface.

Hope that answers your questions.
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Optimus.Prime
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Joined: 21 Dec 2009
Posts: 302
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PostPosted: Sat Jan 25, 2014 4:38 am    Post subject: Re: Question on propellers? Reply with quote

jbalonso777 wrote:
Hello everyone!
I just flew on a JetKonnect ATR 72.
A few months ago, I flew on a Helitours Xian MA60.
Both completely different aircraft types, but with similar propellers.

However, I found quite a significant difference in the start up procedures of the two.

Onboard the Xian, the props start spinning slowly, before finally gaining enough speed and then the whole 'engine' comes on.
http://youtu.be/ZdsSCkDlzCI

However, on the more common ATR, the sound of the engine suggests that they are being switched on, while the propellers start spinning much later, and seems to have gained full speed to start spinning.
http://youtu.be/dVe_56LHcLk

Are there any significant reasons for this?

Thanks in advance!

The Infamous "Hotel" mode rears it's head again!
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jbalonso777
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PostPosted: Mon Mar 10, 2014 7:06 pm    Post subject: Reply with quote

Thank you all for your inputs!
All of this makes a lot of sense now Smile
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