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Hi All

 

As you know I replaced the prop on my Gazelle. The old one was very fine pitched and I was getting 5,800 rpm. The new prop is coarser and I am now only getting 5,000 rpm.

 

With the old prop I would be at 5,800 till climb was well established and about 800ft AGL I would pull the throttle back to 5,300 rpm due to the 5min max at full throttle limitation.

 

With the new prop only allowing 5,000 rpm does this mean that I can leave the throttle on full all the time as I am not even getting my old 5,300 rpm on cruise?

 

 

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Hi Ian,

 

I guess a 5000 rpm your Rotax is not producing peak power for the climbout. Although it may appear easier on the motor(and it may well be) your not getting the optimum climb performance which means if anything goes wrong you may not be high enough to prevent a heavy landing or worse.

 

My Jabiru J160c will reach 3300 rpm easily on the level, but climbs out at around 2900 rpm max.

 

At this speed the mixture is full rich and helps cool the motor. You may find the Rotax over heating a little? at less than full throttle in the climb.

 

regards

 

 

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This is where it gets a little tricky and a job for the engineers to explain.

 

You are talking FULL THROTTLE, versus FULL POWER here.

 

(@ 5,000rpm) At full throttle you will have Low Vacuum in the inlet manifold which means that the engine will be under load and only measurable via manifold pressure.

 

You will find that at FULL THROTTLE that the engine will use a lot more fuel, however it won't necessarily get hot as the mixture will be rich.

 

Perhaps one of the engine people can elaborate further.

 

 

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Guest micgrace

Hi all

 

 



 

 

 

I think you may have a problem if only obtaining 5000 rpm with wide open throttle. This, of course is not max power which is obtained at 5800 rpm 5,500 being max power that can be sustained. Best cruise at max torque, 4800 rpm.

 

 

 

 

 

 

 

Having put a few hours in a Gazelle (GA reg) these rpm ranges are what should be aimed at.

 

 

 

 

 

 

 

Micgrace

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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  • 14 years later...
Posted (edited)

Yes this is interesting. Lots of Gazelles have 3 blade carbon props fitted. With a VP prop, you set the RPM with the prop pitch, the boost of manifold pressure, with throttle opening. With the fixed pitch carbon prop an the Gazelle: If you are only getting 5000 RPM, but have the throttle fully open....could this be slightly over boosting the engine, particularly at sea level, (manifold pressure will reduce with height)? Yes you will not be getting peak power for the climb, but should be getting good prop thrust (coarser pitch) in the cruise? Would Bolly props have some answers?

Edited by F10
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F10, this thread is 14 years old! I think it's perhaps better to start a new thread, I don't know how many of the posters above are still around (apart from Ian).

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Yeah I know...but still a valid topic? Gazelles been around for a while now....

playing old man GIF

 

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12 hours ago, F10 said:

Yeah I know...but still a valid topic? Gazelles been around for a while now....

playing old man GIF

 

I'd suggest talking to the Rotax dealer; Gazelles as you say have been around for a while, and I'd be surprised if the optimum hadn't been found a few years ago, so no need to go through expensive prop changes.

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On 24/05/2021 at 10:37 PM, F10 said:

Yes this is interesting. Lots of Gazelles have 3 blade carbon props fitted. With a VP prop, you set the RPM with the prop pitch, the boost of manifold pressure, with throttle opening. With the fixed pitch carbon prop an the Gazelle: If you are only getting 5000 RPM, but have the throttle fully open....could this be slightly over boosting the engine, particularly at sea level, (manifold pressure will reduce with height)? Yes you will not be getting peak power for the climb, but should be getting good prop thrust (coarser pitch) in the cruise? Would Bolly props have some answers?

Rotax say any pitch setting that gives 5,200 rpm or less WOT you need to reduce by 100 rpm after climb or max 5 mins at 5,200 then drop back to 5,100. For Rotax info good information is sourced from Rotax owner.com as a guest or member.  I set my pitch to achieve 5,500 rpm WOT S&L.

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When you are reading the Rotax manual, be aware there are separate sections for FIXED and VARIABLE pitch RPM and power recommendations and limits. Don't get them confused- easy to.

 

 

 

 

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9 hours ago, RFguy said:

When you are reading the Rotax manual, be aware there are separate sections for FIXED and VARIABLE pitch RPM and power recommendations and limits. Don't get them confused- easy to.

 

 

 

 

Ok, have the Rotax manual, that came with the aircraft. Will see what it says about props!

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33 minutes ago, RFguy said:

section 5 of OM_912 Series_ED4_R0.pdf
....there is quite a bit in there,

and also :

 

this is the good stuff  in this service letter :

 

https://legacy.rotaxowner.com/si_tb_info/serviceletter/sl-912-016.pdf

 

fair bit on probs and engine loading, in a nutshell : section 3.1.2  is useful data also

 

-glen

Also bear in mind that this is one of the more draggy aircraft that the Rotax goes into, so you have to add that factor into the equation, which is why I made my earlier recommendation to learn from all the failures.

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Posted (edited)

If it was a school aircraft certainly at one time, if not still, only the original make and type of prop was permitted..   On a warm day with 2 up often climbing above 3,000 ft required patience. in a Gazelle .  Nev

Edited by facthunter
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  • 1 month later...

My Gazelle has a 3 blade Bolly on it, maybe this drops the max RPM? Performance seems good, acceleration on take off pretty brisk and solo, climbs out at around 600-700ft/min. Cruise at 4800 RPM is 70KIAS. Also pretty ballpark. Anyone else have a 3 blade Bolly? Flew to Tyabb from Yarram recently with a mate on board, retired Esso A139 driver. He really enjoyed it. I got normal fuel burn figures as per the flight manual. Book says 12.7 l/hr at 4800 RPM. I flight planned at 13 and it seemed accurate, but that’s trying to see fuel levels on the sides of those tanks….they really needed to Put in a small clear tube level on the side of the tanks! 

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What static RPM do you have  at start of TO roll , nil wind ?

 

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Just on 5000. So probably pitch set a bit coarse, but then thats good for cruise. As I said, take off performance is as expected, so no problems with this. The only concern is I guess, going fulk throttke on take off, could be a overboost, if RPM is low? 

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yeah certainly too heavy. as previously discussed in rotax factory letters, 5200 min static. engine has excessive load and labours.

you'll get improved everything with the higher static rpm 

 

Our club Brumby was propped for 5200 and fell to 4850 gradually as the propellor aged and became pitted (1800 hours I think now) . We recently re pitched it half a degree to restore the static RPM of 5200 . Despite the prop damage, repitching for 5200 again was like getting a extra 10 HP in flight and TO

 

https://legacy.rotaxowner.com/si_tb_info/serviceletter/sl-912-016.pdf

 

too fine a pitch and you will overspeed a little easily on WOT at fast flight. or descent. might not happen with the gazelle. 

 

  • Informative 1
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3 hours ago, F10 said:

Just on 5000. So probably pitch set a bit coarse, but then thats good for cruise. As I said, take off performance is as expected, so no problems with this. The only concern is I guess, going fulk throttke on take off, could be a overboost, if RPM is low? 

My post above (May 27) is what you need to consider doing.

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18 hours ago, F10 said:

My Gazelle has a 3 blade Bolly on it, maybe this drops the max RPM? Performance seems good, acceleration on take off pretty brisk and solo, climbs out at around 600-700ft/min. Cruise at 4800 RPM is 70KIAS. Also pretty ballpark. Anyone else have a 3 blade Bolly? Flew to Tyabb from Yarram recently with a mate on board, retired Esso A139 driver. He really enjoyed it. I got normal fuel burn figures as per the flight manual. Book says 12.7 l/hr at 4800 RPM. I flight planned at 13 and it seemed accurate, but that’s trying to see fuel levels on the sides of those tanks….they really needed to Put in a small clear tube level on the side of the tanks! 

I ran a 3 blade bolly on an 80hp Rotax in a CA21  (25-0606); it was good.

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