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arwel.pritchard(at)powerc
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 Posted: Sun Jan 22, 2023 2:49 am Post subject: Warp Drive Propeller Diameter |
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Good morning,
I have recently checked the pitch on my warp drive prop and as a result
found that one blade was significantly different from the other two.
I have re-pitched and it seems to run a lot smoother.
Looking in the manual, it suggests a 17 deg pitch for a 912ul (62" prop)
and a 19 deg pitch for a 912ULS (64") prop.
I have a 912 ULS but its fitted with a 62" prop. I have pitched it to 18
deg for now.
Question, is having a smaller prop on the ULS a disadvantage and why was
a 64" prop specified for the ULS over a 62" one?
Thanks
Arwel G-BYPM
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budyerly@msn.com
Joined: 05 Oct 2019
Posts: 282
Location: Florida USA
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| Posted: Sun Jan 22, 2023 6:20 am Post subject: Warp Drive Propeller Diameter |
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The prop discussion for the 80 horse is a good reminder of mistakes I've made.
The Rotax 912 is a great engine. Low on power and torque, but reliable and light and as Creighton would say "it'll run on cat pee" although I wouldn't recommend it as it is hard to collect! 87 RON Octane or regular gas is all it needs, but it does like higher octane premium gas. Even ethanol that is not older than say a month is OK provided you can get it started.
Matching a prop to it is not difficult but can be costly if one forgets that the engine and prop are the powerplant and the prop is a torque converter of sorts.
For a fixed pitch Warp Drive, the 80 HP can only easily drive a 62 inch taper 3 blade Warp.
At Custom Flight, we tried the wide chord Warp 3 blade at 64 inch and the engine could not pull the prop. (Same issue on the 80HP Jab and VWs. Longer is not better.)
The 62 inch range prop does not appreciably affect speed, in fact it is about a wash with most props. If you want to go faster, prop length is not terribly critical from 62-67 inches. If you want initial takeoff acceleration longer is better, but soon after liftoff, the prop unloads and frankly the plane is sluggish and cruise speed is lost.
Even by lowering the pitch for the 64 inch wide chord Warp, we found we could get 5000 RPM but the low pitch on the prop resulted in terrible acceleration and disappointing cruise.
So, the Warp taper blade set to a pitch that will yield 5000 RPM at WOT with premium gasoline was used for years as a good all around prop for the 80HP engine. That was about 17-18 degrees depending on cowl inlet MP, engine compression and fuel octane.
If you lower the octane of your fuel to 87 RON (regular grade) Rotax recommends setting the pitch to achieve 5200 static RPM at WOT to preclude detonation.
That pitch from experience is between 17 and 19 degrees at a manifold pressure of 26 to 28 inches for most prop brands. If I recall (remember I have CRS aka, can't remember s__t), on a cold air plenum XS style cowl I found a touch over 17.5 degrees is about all I could get for 5200 static WOT comfortably on regular gas. Most of my testing was done using 100LL so the engine octane was not an issue.
Performance with the Airmaster tapered Warp Drive AP332 required the tapered blade for the stock 912 operations. By setting the fine pitch adjustment to 5200 at WOT then locking it in manual mode was an acceptable takeoff and climb setting but speed at cruise was tolerable at about 125-127 KTAS at 5000-7500 feet and with the constant speed set to cruise setting of 5000, and near WOT we got about 134 KTAS. Takeoff using the 5700 WOT was very comfortable with the Airmaster and Warp Drive taper blade on the 80 HP Rotax 912.
Cold air intake, engine condition, and of course altitude and leaning are all factors that affect the fixed pitch prop performance. If you look into new propeller swept tip and narrow chord designs you can get a bit of a boost over the rather simple Warp, but not much. In testing at Airmaster we found the blade difference in speed of the various brands was about 3 knots on most light sport aircraft. The cleaner the airframe design of the aircraft, the more twist you need in a propeller. Sensenich has a great three blade ground adjustable for the 912S (100HP) and in testing was a game changer for that engine. The CT guys love it o the 912S. I never tested one on an 80HP Europa. What I found was the newer blades with modest twist like the Whirlwind and Sensenich blades have a lot less residual thrust in the flare than the Warp Drive or the Wide Chord Sensenich. For the fixed pitch Warp Drive, I'm afraid that the residual thrust in the mono or trigear is noticeable when landing. If setting a prop for cruise, I lower the idle RPM to 1600 to try to reduce the float but that affects idle during start and taxi to takeoff due to gearbox and of course carb balance roughness. Just put it to 2000-2500 for warm up and taxi at 1800 -2000 or throttle slightly cracked.
As for basic prop maintenance, please do a check at least annually (I do a 25 hour check) on your aircraft to not only service the engine, and check compressions and balance of the carbs, but also check the torque of your prop and do a thorough check of prop blade fixation torque on all ground adjustable and constant speed propellers.
Even just using your digital level (or cell phone level) with each blade at the same position ( I use horizontal and cut a piece of wood to set the prop tip at the same spot but a level will do also) to the horizon will tell you if you are having blade creep. These blades today are tough, but over time glass compresses and moves so the prudent owner operator/maintainer will do a proper check as part of an annual condition inspection IAW the prop manufacturers checklist. At a minimum, during the 5 year inspection, fix the prop dings, repaint if necessary (properly do your coats evenly), and balance the prop to assure the blade angle, all torques and balance are within tolerance. I like to pull the prop, hang it from a string (as described in my paper on prop balancing) and get a good static balance. You can’t dynamically balance a prop without it being very close statically balanced
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