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Power settings for 75% power in cruise VP prop?

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JonathanMilbank

Joined: 14 Apr 2012
Posts: 395
Location: Aberdeen area

Posted: Mon Jul 04, 2016 10:19 am Post subject: Power settings for 75% power in cruise VP prop?

My question is addressed to those with a Rotax 912S and VP prop combination. I believe that 75% power equates to 18.5 litres / hour in the cruise. In my Classic Europa tri-gear, this gives 115 to 120 KIAS cruise and is generally achieved around 4700 rpm, which agrees very well with Kevin Dilks' suggested rpm "sweet spot". Certainly I don't enjoy the louder and less smooth cruise at 5000 rpm.



So my question "What cruise power settings do you use and how do you know when you're at 75% power with a VP prop?"

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dg.watts(at)talktalk.net
Guest

Posted: Mon Jul 04, 2016 10:41 am Post subject: Power settings for 75% power in cruise VP prop?

Jonathan,



This is all done using rpm and manifold pressure.



The Rotax Operators Manual is the place to go for this information in the Performance Data section.

There it says that 75% is 5000 rpm and 27.2 inches manifold pressure.

65% is 4800 rpm and 26.5 inches manifold pressure.



I tend to run 4900 rpm and 24 inches for general messing around or increase to 25 inches if I am going somewhere or if I'm in a hurry then 26 inches.



Dave Watts

G-BXDY Classic Monowheel 2230 hours



 	  Quote: 	
		   On 4 Jul 2016, at 19:19, jonathanmilbank <jdmilbank(at)yahoo.co.uk> wrote:

 

 

 

 My question is addressed to those with a Rotax 912S and VP prop combination. I believe that 75% power equates to 18.5 litres / hour in the cruise. In my Classic Europa tri-gear, this gives 115 to 120 KIAS cruise and is generally achieved around 4700 rpm, which agrees very well with Kevin Dilks' suggested rpm "sweet spot". Certainly I don't enjoy the louder and less smooth cruise at 5000 rpm.

 

 So my question "What cruise power settings do you use and how do you know when you're at 75% power with a VP prop?"

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=457716#457716

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Burrilla

Joined: 25 Apr 2015
Posts: 188

Posted: Mon Jul 04, 2016 10:57 am Post subject: Power settings for 75% power in cruise VP prop?

Jonathan.



The ROTaX manual provides a power setting tables hopefully attached. 



The 75% power equates to 5000rpm and 26 MP.



Alan

GOBJT



Sent from my iPad





 	  Quote: 	
		   On 4 Jul 2016, at 19:19, jonathanmilbank <jdmilbank(at)yahoo.co.uk> wrote:

 

 

 

 My question is addressed to those with a Rotax 912S and VP prop combination. I believe that 75% power equates to 18.5 litres / hour in the cruise. In my Classic Europa tri-gear, this gives 115 to 120 KIAS cruise and is generally achieved around 4700 rpm, which agrees very well with Kevin Dilks' suggested rpm "sweet spot". Certainly I don't enjoy the louder and less smooth cruise at 5000 rpm.

 

 So my question "What cruise power settings do you use and how do you know when you're at 75% power with a VP prop?"

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=457716#457716

 

 

 

 

 

 

 

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dg.watts(at)talktalk.net
Guest

Posted: Mon Jul 04, 2016 11:20 am Post subject: Power settings for 75% power in cruise VP prop?

Jonathan



Oops, sorry I quoted the 912 figures and not the 912S. I have got both operators manuals because my daughter has got the 80 hp in her Europa.



Dave Watts



 	  Quote: 	
		   On 4 Jul 2016, at 19:55, Alan Burrill <alanb(at)dpy01.co.uk> wrote:

 

 Jonathan.

 

 The ROTaX manual provides a power setting tables hopefully attached. 

 

 The 75% power equates to 5000rpm and 26 MP.

 

 Alan

 GOBJT

 

 Sent from my iPad

 

 <image1.PNG>

 

 

> On 4 Jul 2016, at 19:19, jonathanmilbank <jdmilbank(at)yahoo.co.uk> wrote:

> 

> 

> 

> My question is addressed to those with a Rotax 912S and VP prop combination. I believe that 75% power equates to 18.5 litres / hour in the cruise. In my Classic Europa tri-gear, this gives 115 to 120 KIAS cruise and is generally achieved around 4700 rpm, which agrees very well with Kevin Dilks' suggested rpm "sweet spot". Certainly I don't enjoy the louder and less smooth cruise at 5000 rpm.

> 

> So my question "What cruise power settings do you use and how do you know when you're at 75% power with a VP prop?"

> 

> 

> 

> 

> Read this topic online here:

> 

> http://forums.matronics.com/viewtopic.php?p=457716#457716

> 

> 

> 

> 

> 

> 

> 

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> 

>

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JonathanMilbank

Joined: 14 Apr 2012
Posts: 395
Location: Aberdeen area

Posted: Mon Jul 04, 2016 11:58 am Post subject: Re: Power settings for 75% power in cruise VP prop?

Guys, thanks for the replies. Firstly I don't have a manifold pressure guage, but only an old car economy guage to give warning of carburettor ice build-up. Permission was given for this and it works very well, although the needle moves in the opposite sense. Because my 912 ULS engine isn't turbo boosted, there's no chance of "overboosting".



Now to my puzzlement about the consequence of using a constant speed Airmaster propeller and changes in horsepower without changing rpm. In my simple brain, if I have 5000 rpm selected for cruise and move the throttle within the governed range, then the horsepower MUST increase as I move the throttle from lowest position at which 5000 RPM can be maintained to the fully forward position, still at 5000 RPM.



Therefore the tables provided in the engine manual become nonsensical regarding RPM vs horsepower. It's more likely to me that the only immediate indication of horsepower being produced must be manifold pressure.



So assuming I generally cruise at 4700 RPM for aural comfort, indicating 115 to 120 KIAS and at a fixed throttle position, what remains for me to have an idea of what % power I'm using is fuel consumption. Somehow I've acquired the idea that 18.5 litres/hour suggests about 75% power.

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Rick Moss

Joined: 21 Sep 2014
Posts: 96

Posted: Mon Jul 04, 2016 1:26 pm Post subject: Re: Power settings for 75% power in cruise VP prop?

Does anyone know if altitude (or more specifically barometric pressure) effects the fuel consumption at the specified 5000rpm/26" map? Or is it just that at higher altitudes require increased throttle settings to achieve 26" map?

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Burrilla

Joined: 25 Apr 2015
Posts: 188

Posted: Mon Jul 04, 2016 3:10 pm Post subject: Power settings for 75% power in cruise VP prop?

The VP prop will adjust itself within the limitations of the propellor pitch adjustment to maintain the rpm setting although if the throttle is not open sufficient there will be insufficient power to even maintain the set rpm on full fine and the rpm will drop at lower throttle settings. Therefore power is related to the throttle setting and this will adjust the MP until Wide Open Throttle is achieved. Full consumption will rise as you open the throttle and therefore produce more power.



Not sure where you have obtained the 18.7 number from but the Power/Fuel Consumption performance graphs in the ROTAX Operating manual should be your guide and more important I suggest a MP Guage to provide you with the performance settings you want to achieve and also measurement of the fuel consumption for your aircraft at chosen cruise altitudes so you best able to determine the performance and range of your aircraft. Relying on someone else's figures is never wise.



Alan

Sent from my iPad



 	  Quote: 	
		   On 4 Jul 2016, at 20:58, jonathanmilbank <jdmilbank(at)yahoo.co.uk> wrote:

 

 

 

 Guys, thanks for the replies. Firstly I don't have a manifold pressure guage, but only an old car economy guage to give warning of carburettor ice build-up. Permission was given for this and it works very well, although the needle moves in the opposite sense. Because my 912 ULS engine isn't turbo boosted, there's no chance of "overboosting".

 

 Now to my puzzlement about the consequence of using a constant speed Airmaster propeller and changes in horsepower without changing rpm. In my simple brain, if I have 5000 rpm selected for cruise and move the throttle within the governed range, then the horsepower MUST increase as I move the throttle from lowest position at which 5000 rpm can be maintained to the fully forward position.

 

 Therefore the tables provided in the engine manual become nonsensical regarding RPM vs horsepower. It's more likely to me that the only remaining approximation to horsepower being produced must be manifold pressure.

 

 So assuming I generally cruise at 4700 RPM for aural comfort, indicating 115 to 120 KIAS and at a fixed throttle position, what remains for me to have an idea of what % power I'm using is fuel consumption. Somehow I've acquired the idea that 18.5 litres/hour indicates about 75% power.

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=457721#457721

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Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

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