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Possibly needing prop adjustment


Rich

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Good afternoon,

 

Rotax ULS 912 100HP (2012 engine)

DUC three blade Helices, Swirl, Pitched at 23.5 degrees

AC is a Sting S-4

 

I'm getting 4950 RPM max, WOT on take-off and climb. This is at altitudes from 400-2000 MSL

 

I'm thinking that I may be putting to much strain on the engine/gearbox. Also planning an X/C from FL to San Diego in mid to late January and want to be sure I'm getting the propper RPM for the higher density altitudes.

 

My cruise is very good, averaging 115Kts from PA>GA and return at 5250 RPM. Approx 75% power setting.

 

Thoughts please.

 

 

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...  My cruise is very good, averaging 115Kts from PA>GA and return at 5250 RPM. Approx 75% power setting.

 

Thoughts please.

 

With a ground adjustable prop you can only optimize for one altitude and one RPM limitation otherwise there is performance that is not available at that prop pitch.  I generally want to see 5,500 RPM at 75% settings i.e. 7,500' DA and WOT.  This way I figure I have more performance available to me.

 

I can generally cruise at 123-127kt TAS at less than 75% setting and it costs me at least 5gph.  I can still throttle back and save gas if I want.

 

Of course if the Duc works as advertised it unloads and coarsens the pitch for cruise and become a whole different animal.  Bottom line is if you can firewall your throttle and are not getting 5,500 than there is performance not available till you flatten the pitch.

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Do they give you a certain station at which to measure? Or is it marked at the hub?

 

Just curious, because my Sky Arrow manual just calls for 13.5° for my Warp Drive prop mounted on a 912ULS2, but that's at a certain distance from the hub - though I forget how far.

 

I took 1° out when I became Experimental. Before that felt locked into what the manual called for. Used to get just under 5,000 rpm on takeoff and climb, now get 5,200 rpm, for much better performance there.

 

I do, however, think my fuel efficiency, and hence my range, at cruise has suffered a bit. I can go about the same speed, but it takes more rpm, and fuel, to do so. Or so it seems.

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Do they give you a certain station at which to measure? Or is it marked at the hub?

 

Just curious, because my Sky Arrow manual just calls for 13.5° for my Warp Drive prop mounted on a 912ULS2, but that's at a certain distance from the hub - though I forget how far.

 

I took 1° out when I became Experimental. Before that felt locked into what the manual called for. Used to get just under 5,000 rpm on takeoff and climb, now get 5,200 rpm, for much better performance there.

 

I do, however, think my fuel efficiency, and hence my range, at cruise has suffered a bit. I can go about the same speed, but it takes more rpm, and fuel, to do so. Or so it seems.

 

Interesting.  When I first got my airplane it would only reach about 4700-4800 on takeoff roll and 5200rpm WOT.  The airplane would not go faster than 108kt indicated, and seemed doggy in climb compared to the CTSW I trained in.

 

I had the prop flattened by about 2° at Roger Lee's suggestion, and it was a night and day difference.  I now get 5100-5200rpm on takeoff, and WOT at 3000ft is about 5700rpm.  The airplane will now cruise at 105kt @ 5000rpm, 112kt @ 5200rpm,  or 120kt @ 5400rpm...and I gained about 300fpm in climb.  I don't think that my economy suffered, but I don't have any detailed numbers to back that up.

 

I flew 500nm to Sebring FL at 5400rpm the entire flight and burned 5.6-5.8gph at 5500ft.

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If using the DUC pitch adjusting tool, it's 20CM from the blade tip to the front edge of the tool.

I have a different pitch finder and imagine if it's set at the same spacing on the blade it would work just as well.

 

DUC says 1* of pitch change = 200 RPM. I think I'll try removing 1* to start and see what that does.

 

Thanks Dan, FE and MM for you replies.

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What the actual pitch doesn't really matter and it differs from plane to plane and prop to prop. Set the pitch (what ever that number may be) to get around 5600-5650 RPM WOT in flat and level flight at your AVERAGE altitude. If you live at seal level and never get over 2K then set it up for that. If you always fly at 9K set the pitch to get the 5600 rpm at that altitude.

We are just trying to find the best balance point for our pitch to give good climb, good cruise, good engine temps and good fuel economy. Unless you have special circumstances the 5600-5650 is a pretty good spot to be. If you like a tad better climb use 5700 rpm WOT. Don't worry about what the take off rpm is. That will follow with the setting of the WOT at altitude. Most people that have around 5600 rpm WOT are around 4950-5000 on take off. There are other factors that may come into play, but for the average pilot these numbers work pretty well across the board.

 

If you tell me what the WOT rpm is now at your average altitude I can give you a pretty good guess at how much to add or subtract from your pitch now to make it in the 5600 area ballpark.

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Rich,

The DUC manual says 24 degrees (using their gauge) (pg 8/26) for the 912 S. I believe this is the pitch that mine is at. I recall the 2 coinciding.

 

When I took delivery of the plane WOT was ~ 5200 (my memory may not be correct). Anyway I asked some pitch to be taken out according to what Roger recommended, not specifically for me but just in general and it only took one adjustment to hit the 5500-5550.

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Hey Rich,

I was finally able to get out this morning. At WOT on takeoff rollout, I hit 5330 rpm (GS below 25 kts). At DA 3000' at WOT, I hit 5560 rpm. It took a minute or 2 to get there, but 5500 was hit right as the throttle was pushed forward.

 

Hope this helps.

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Hello Rich,

Funny enough, I was in Harrisburg/Pennsylvania last week (terrible weather in I78!). Now back in Burgundy/France...

My CT was set with 22,5° and got 4900 WOT at take off, and 5400 at DA 3000'. Following suggestion from Roger, I have removed 2,5 ° using my Iphone 5 with Ihandy level apps: this worked fine on my Neuform propeller and allow a 0.1° accuracy.

Now I'm getting 5100 WOT at take off, and 5650 at DA 3000'. To get the same speed (210 km/h TAS) I need to add 200 RPM to my cruise setting. Take off distance seems now improved, but didn't really measure it. One thing I didn't fix yet is that above 5300 RPM , engine got significant vibration: I though that changing the prop pitch would help, but no improvement noticed. i have no idea where this could come from...

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I'm very surprised that the guys getting 5100+ on take off are not up at 5700+ in flat and level WOT.

 

Vibration can because by many factors. prop blades out of pitch or track. prop out of balance, carb sync, carb floats starting to sink, carb vent tubes not getting equal pressure, air leaks, worn and old carb parts,

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DUC claims there ground adjustable props display "constant speed effect". Maybe there is something to this claim. I spoke with someone that switched from a Woodcomp to a DUC and he claimed significant performance improvement.

 

My plane came with the DUC.

 

http://www.duc-helices.com/contenu.php?id_contenu=22&id_dossier=10〈=2#_Toc377570073

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S3,

 

Thanks for the info. Thats a good KTAS.

I haven't been up and am awaiting a head cold to finally get out of my system.

 

Your 220 RPM,s better than I on take-off.

I need to get up to 5500 MSL and see what RPM's I'm getting at WOT.  According to Roger, that should be the tell all as to what I may have to take out of the prop.

 

Thanks again

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I'm very surprised that the guys getting 5100+ on take off are not up at 5700+ in flat and level WOT.

 

Vibration can because by many factors. prop blades out of pitch or track. prop out of balance, carb sync, carb floats starting to sink, carb vent tubes not getting equal pressure, air leaks, worn and old carb parts,

Hi Roger, Based on your comment, I have done again some test today:

600" OAT 7°C : got 5100 RPM at take off

then at 5000" and OAT 5°C:

  • 4800 RPM - 103 KIAS - 108 KTAS
  • 5000 RPM - 106 KIAS - 112 KTAS
  • 5200 RPM - 112 KIAS - 119 KTAS
  • 5700 RPM (WOT) - 122 KIAS - 130 KTAS

So, you were right, I have underestimated the RPM at WOT.

Regarding vibration, I'm considering prop out of dynamic balance, since I got this after more than 500 hours, (bought second hand at 600 hours) and went through several carb  balancing, Carb full kits changed twice, with all parts replaced by brand new one... Will keep you posted when done... Thanks anyway for suggestion.

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Hi Roger, Based on your comment, I have done again some test today:

600" OAT 7°C : got 5100 RPM at take off

then at 5000" and OAT 5°C:

  • 4800 RPM - 103 KIAS - 108 KTAS
  • 5000 RPM - 106 KIAS - 112 KTAS
  • 5200 RPM - 112 KIAS - 119 KTAS
  • 5700 RPM (WOT) - 122 KIAS - 130 KTAS

So, you were right, I have underestimated the RPM at WOT.

Regarding vibration, I'm considering prop out of dynamic balance, since I got this after more than 500 hours, (bought second hand at 600 hours) and went through several carb  balancing, Carb full kits changed twice, with all parts replaced by brand new one... Will keep you posted when done... Thanks anyway for suggestion.

 

Marco, those speeds look correct, assuming you have flaps at -12° and a ground-adjustable prop.  My CTSW gets speeds just a few knots less than those number with the -6° US flaps.  At 5700rpm I am usually about 125 KIAS, and 112-115 KIAS at 5200rpm.

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