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912ULS Propellor Pitch


bobcharl
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I have set the prop pitch on my Sav XL VG to achieve 5200 RPM at full power, stationary, in still air.

 

However, at cruise at 5000 RPM I am only utilising manifold pressure of 23 in. I am tempted to increase the pitch to get nearer the 26in./5000RPM stated in the Rotax manual, but am concerned at reducing the 5200 figure and possibly creating an overload situation on take-off and climb. It presently climbs like a rat up a rope so I am prepared to lose a bit of climb performance in order to improve cruise performance.

 

What sort of manifold pressure/RPM are you guys out there running, and what cruise speed is being achieved?

 

I have no intention of turning my Sav into a pylon racer, but if I have 3in. of manifold pressure to play with, an extra few knots would be nice. What think you all?

 

Regards,

 

Bob

 

 

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For what it is worth, I've done a few tests at various altitudes & power settings in my Eurofox 3k with a 912 ULS. I don't have any stationary info, but here's one data point: Altitude 5500 Feet, RPM 4990, Manifold Pressure 22.5, Fuel flow 19 l/hr, True airspeed 103 Kt.

 

For the record, I'm happy with this as a good compromise pitch setting for my 3 blade ground adjustable FITI. If I want to push it a bit harder, 110 Kt TAS is easily achieved, and climb performance at MTOW is still a tad short of 950 fpm at full throttle from YLIL.

 

 

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Hi Bob

 

Its a suck it and see thing depending on the size of the prop and the brand. I am using a Bolly Optima 72 inch and I have set mine for a full power climb at 60 knots for just under 5300 but thats how it worked out so that flat out full power straight and level in super calm conditions the engine eventually gets to 5600 rpm. If the throttle spring breaks the engine goes to full power and the manual says 5800 for only 5 mins but 5600 all day if you need it. This is how I set mine up and seems to give a good compromise between takeoff performance and cruise. At 5000 I cruise at 85 knots flat out at 5600 I can get to 97 knots . A lot of guys cruise at 5200 but I don't I prefer to stick to 5000 . The savannah will just be wasting fuel for that little bit extra performance

 

 

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There's just been some discussion on this topic in the VAF forum for the RV-12, which uses a 912 ULS and a composite ground-adjustable Sensenich (70" from memory). Van's recommend (remembering that they are aircraft manufacturers, not engine builders) setting the pitch to get 4950 on the ground in still air, giving a cruise climb (75 kts) at 5100RPM. However, there is also a Rotax service letter (recommendation only, not mandatory) that says the pitch should be set so that take-off RPM is above 5200 WOT (presumably all phases of takeoff, including stationary at the start of roll). Van's are aware of the Rotax recommendation. The RV-12 does about 120kts WOT at sea level with this setup. I'm not saying Vans is either right or wrong, but I'd be interested to hear how others have set up their 912S/ULS props.

 

rgmwa

 

Edit: probably answered my own question. Found this useful piece on Rotax Owner: http://www.rotax-owner.com/rotax-blog/item/9-understanding-the-ground-adjustable-prop

 

 

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I set mine to give 5600rpm straight and level at 5000 feet. seems to be a good compromise between climb and cruise and then there is that broken throttle spring possibility, so it should not be more. Not a Sav, but I think the principle is the same.

 

 

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Guest Maj Millard

image.jpg.3c06715e085f64db377473b58ef08e0b.jpg For the past coupke of years I've had my 3 blade Powerfin pitched fairly course for best cruise, as that is what I was doing most of. Was only getting about 5100 wot at takeoff heavily loaded, with 700 fpm climb, ( flaps) and cruising level at 5000 RPMs straight up.

 

Just de-pitched the prop a couple of degrees and now getting 5300-5400 at take off with good climb, 5000-5100 cruise RPM, slightly better fuel economy and same or better airspeed.

 

Prop now seems much more lively and happier allround. Haven't lost anything, but gained more safety RPMs at takeoff.

 

The 100 HP Rotax likes to be loaded in my opinion and you know your using all the HP available. My engine has just turned 900 hours and always run on the cooler end of the scale with both radiator and oil cooker half blanked off. Sometimes I'll remove the oil cooler one here in the northern summer but leave the coolant one on year round. They seem to be a little happier if running warm.

 

 

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Thanks for all the info everyone. I am convinced I need some fine tuning to achieve a better balance between climb and cruise. Will make some alterations over the next few weeks and post the results.

 

Re the throttle spring, breakage of the spring would put that carb at the mercy of whatever linkage is used. It is only if the linkage breaks/disconnects the spring the gives full throttle.

 

Cheers,

 

Bob

 

 

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5800 flat and level. 5500 clmb (60kts). Cruise 5250 give or take at 90kts.

 

Lots of people say the 912' s love to rev and I totaly agree. I have read somewhere about the cam being setup/tuned for 5000 plus.

 

Spring-on carb redundancy alows me to climb and then kill mags to glide and repeat if required.

 

Forgot to add that my fuel economy averages about 16lph.

 

 

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  • 3 months later...

I have older VG model and usually fly at 4400-4800 RPM , 65-75 kts

 

My green arc is 74 kts and do not push it to yellow very often because of the rough weather in Tassie.

 

Occasionally achieved 85 kts just above 5000 RPM.

 

Climbs pretty well at 4800 RPM about 700-800 ft/min.

 

Any comments on flying in rough weather speed wise??

 

George

 

 

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Guest Maj Millard
I have older VG model and usually fly at 4400-4800 RPM , 65-75 ktsMy green arc is 74 kts and do not push it to yellow very often because of the rough weather in Tassie.

Occasionally achieved 85 kts just above 5000 RPM.

 

Climbs pretty well at 4800 RPM about 700-800 ft/min.

 

Any comments on flying in rough weather speed wise??

 

George

I ferried a VG classic Casino to Townsville during hot summer WX .......found during middle of the day turbulance that it was more comfortable to just back off, reduce the cruise speed a bit and enjoy a more comfortable ride. Still did the trip in 9 hrs flying time via Chinchilla and Dingo. ( long- range tanks ) It's a STOL wing after all and you can't have everything.

 

 

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  • 1 month later...

Just an update on where I'm at on this one. I have tried various prop settings and have settled for the moment on one which is 2 deg coarser than where I started. Performance is as follows:

 

Commencement of T/O run 5100 rpm'

 

Climb -- 50 Kts IAS

 

5200 rpm

 

1200 fpm

 

26.5" mp

 

Cruise---- 84 Kts IAS

 

5100 rpm

 

23"mp

 

WOT Straight and level

 

5800 rpm

 

94 Kts IAS

 

26.5"mp

 

I have no definitive measurement but I feel I have extended my takeoff run a few metres, I have certainly lost 200 fpm in the climb, but have gained 4 kts in cruise. WOT is higher than I expected. All considered its a compromise that I am satisfied with. A few more revs ( hence power) on takeoff and climb would be nice, but that would be at the expense of cruise speed. An in-flight adjustable prop would be handy, but for a small benefit the price is way too high for me. And as Maj so correctly said, "its a STOL wing after all".

 

Thanks to all for your comments.

 

Bob

 

 

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In my previous post I said Vans recommended 4950 static rpm for the RV-12. However, I've just been through the prop adjustment process and their notes actually say 4960-5020 static rpm as a starting point to get good all-round performance. Sounds like your numbers are in the same ballpark, even though it's a different aircraft.

 

rgmwa

 

 

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Just an update on where I'm at on this one. I have tried various prop settings and have settled for the moment on one which is 2 deg coarser than where I started. Performance is as follows:Commencement of T/O run 5100 rpm'

Climb -- 50 Kts IAS

 

5200 rpm

 

1200 fpm

 

26.5" mp

 

Cruise---- 84 Kts IAS

 

5100 rpm

 

23"mp

 

WOT Straight and level

 

5800 rpm

 

94 Kts IAS

 

26.5"mp

 

I have no definitive measurement but I feel I have extended my takeoff run a few metres, I have certainly lost 200 fpm in the climb, but have gained 4 kts in cruise. WOT is higher than I expected. All considered its a compromise that I am satisfied with. A few more revs ( hence power) on takeoff and climb would be nice, but that would be at the expense of cruise speed. An in-flight adjustable prop would be handy, but for a small benefit the price is way too high for me. And as Maj so correctly said, "its a STOL wing after all".

 

Thanks to all for your comments.

 

Bob

Those figures look pretty good I recon. It might actually be more efficient climbing at 60kts though......just a thought.

 

If those are "summer flying" figures then you'll probably see better performance in winter.

 

 

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