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budyerly(at)msn.com
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 Posted: Sun Aug 28, 2011 7:51 am Post subject: 914 Prop Question One more thought |
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<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]--> Tony,
One more thought on matching the 914 with a fixed propeller.
All advice given by our learned companions is true. I thought I'd throw a couple of numbers I have seen for your decision process.
In accomplishing the initial flight test of the LSA version of the Europa, I flew 12AY with two fixed pitch props attached to my 914 Trigear with Classic Wings and XS Fuselage mods...
Keep in mind propeller theory is a bit of a zen art as each airframe, engine, turbo system, fuel injected vs carburated, etc. greatly affect the performance as does the blade width, length, twist taper and flexibility. Many times you are comparing apples and oranges. This prop with this engine works great to one guy and is disgusting to another. The prop twist and taper of one custom fixed pitch prop on a trigear is no problem, and in the event of a ground loop in a mono, a very costly mishap. In selecting a prop, the strength of a prop should never be compromised and its ability to absorb abrasion from rain, grass and dirt must be considered in the choices as well as cost and insurance coverage for a prop strike. As all things in an airplane, compromises must be made..
I have tested 64 inch Sensenich ground adjustable two blade and the Warp Drive 64 inch three blade ground adjustable propellers.
Both were adjusted to work by minute pitch adjustments. I tweaked until the whole engine/prop combination could be repeatable, dependable, and acceptable to my standards. These standards are: it can't allow the engine to overheat (ground or air), or over speed, or over boost in any normal operation, give a minimum climb of 700 FPM fully loaded at 75 KIAS at full power, cruise at least 120 KIAS at sea level, and climb at max continuous to 10,000 feet (7500 minimum) in 20 minutes or about 500 FPM average.
For testing here at my shop it have found the following:
Set the pitch so as to get 5200 RPM static at 40 inches of Manifold Pressure (38 if you have the older 914). This drops your power available from 115 HP to about 100 HP. This is a recommendation by Rotax and prevents over boosting and eventual damage. It also is a good all round start for a pitch setting. For the newer 914 engine, that is about 21 degrees (20.5 for the old 914) prop pitch at the tip of the Warp Drive.
Takeoff allows the prop to unload while accelerating to about 5500 RPM while at 38 inches and gets you away from the ground quickly at 75 KIAS.
Once airborne and above 1000 MSL in a 75 KIAS Climb, begin pulling the throttle back to 100% (out of turbo), the RPM drops disgustingly to about 5000 at 34 inches MP. This greatly reduces the climb, read as dropping the nose a lot. However if the climb speed is increased to 90 KIAS the RPM comes up to about 5300, the MP pressure stays close to the mid 30s and the rate of climb is tolerable and the transition from the 75 to 90 with throttle retard seem natural.
Climb to 7500 MSL is reasonable at approximately 700 fpm with one on board and a bit labored at 500 average loaded.
Cruise at 5500 MSL tops at about 122 KTAS at 5000 RPM and 31 inches. Max at 5500 RPM and about 35 inches only goes up to about 127.
I am working from memory here on Sunday morning so the numbers are ish.
I have found with a basic Airmaster AP 332, climb after takeoff is about 1400 FPM, Climb at max continuous starts at 1000 FPM and drops to 4-500 FPM at 10000 MSL. Climbs above 10,000 with the Airmaster start to drop off using the narrow chord Warp Drive blades. Cruise is up from 120 ish to 135 ish at 7500 MSL. Airmaster has now completed and is selling a reverse or Beta with the AP332 for sea planes. Much was done to keep this mod safe and full proof. So for you guys that must have everything, (sea or ski operations), beta is now available. Use with much care.
My experience has shown me that the 914 needs a constant speed prop with automatic adjustment (rather than variable pitch) to be properly used and give the full potential of the engine. The constant speed prop eases cockpit workload in high cockpit task operations (takeoff and land), provide a safety margin to prevent over speed, or over boost possibilities in go around or spirited flight situations, improves cruise speed and fuel mileage, and reduces the thrust for short field operations when coming into a short field fully loaded.
That said, based on the repair rates of new low time mono pilots and propeller tip damage I have experienced as an Airmaster dealer, a fixed pitch prop used during the initial flight test and learning phase is not a bad decision. It saves initial cost to the mono guys until they are comfortable with the landing characteristics, techniques, etc. It also gives a spare prop for the eventual tip strike or other maintenance action after upgrading to a constant speed. Use care for the CG shift and engine/gearbox/oil system changes during a prop swap.
My further testing of the Airmaster two and three blade has been cut short by LSA testing and damage from Sun n Fun taking forever to fix with the backlog of customers in my shop and the painters (customers come first). My apologies to all, but circumstances have delayed any further fixed pitch testing.... Jabiru engine testing with various propellers has been curtailed also, until we can get the Jabiru 3300 in an airplane that will stay cool enough throughout the performance envelope to allow a full test profile.
Regards to All,
Bud Yerly
Europa Tech Support
Custom Flight Creations, Inc.
www.customflightcreations.com
(813) 653-4989
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davidjoyce(at)doctors.org
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| Posted: Sun Aug 28, 2011 9:59 am Post subject: 914 Prop Question One more thought |
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Tony, To compare with the figures Bud has produced so meticulously for fixed
pitch props, you may like to compare those I have found with my XS/mono/914
/Woodcomp CS SR 3000 high twist prop and the low twist SR2000 pro at 90 %
Max AUW loading:
Ground run - Not accurately measured but around 100 metres
Climb from1000 to 2000ft: SR3000 40 secs(= 1500fpm) SR2000 42 secs Top True
airspeed with100% power
at 2000ft SR3000 156 kts SR2000 152 kts
at 6500ft SR3000164kts SR2000 160kts
The warp drive blade is low twist and I would guess on a CS hub it would
give equivalent figures nearer the SR20000 than the SR3000, but even so you
will see that climb rate and top speed are dramatically better than with a
fixed pitch prop. One other point about the high twist prop is that it
produced dramatically better cooling than the SR2000, effectively curing
what had always been an issue with my plane ( this is what you would expect
theoretically as with low twist blades the inner 1/3 or so of the blade
effectively operates in Beta mode at cruise speeds, effectively diverting
air from the Europa air inlets!)
Regards, David Joyce, G-XSDJ
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