Bud,
Thanks very much for the excellent and concise description of the
problem.
Bob Borger
Europa Kit #A221 N914XL, XS Mono, Intercooled 914, Airmaster C/S
http://www.europaowners.org/N914XL
Aircraft Flying!
3705 Lynchburg Dr.
Corinth, TX 76208
Home: 940-497-2123
Cel: 817-992-1117
On Mar 21, 2009, at 0:22, ALAN YERLY wrote:
> Paul,
> I am only a junior aerodynamicist. Propeller blade design is still
> a zen art. Too many theta, beta sigma deltas to patiently wade
> through and design a prop. The math is not beyond me, ( I have a
> computer program for doing the gymnastics and iterations, (but my
> kid the aero engineer has to get me in to it...go figure). I can
> say that most designs take into account the assumptions of where the
> max efficiency of their design will be optimized. When designing a
> prop blade, it is hard to get a prop that does all things well. We
> buy a CS prop to accelerate fast, give max climb and optimum
> efficiency at the exact torque/power curve for the specified
> aircraft at a specified cruise speed and altitude, but propeller
> blade designers don't know your plane, engine or the altitudes you
> intend to use, so they assume for a specific engine and speed/
> altitude, and don't tell anyone. For you Paul, the next two
> paragraphs are a bit basic, but somebody less experienced may glean
> some knowledge or correct me.
>
> Example: For max static thrust and quick acceleration of say a
> float plane/airboat, you design the prop so the entire prop blade is
> pulling very close to the stall angle based on the rotational speed
> for takeoff power/torque and forward speed. This gets the plane on
> the step quickly and off the water. But, once you pass about 80 Kts
> the blade angle of attack falls off due to forward speed, cowl
> stagnation point / flow interruption and the reduction of power to
> max continuous for the climb. To design a perfectly optimized fixed
> pitch propeller, the following is optimized by trial and error: For
> takeoff, the blade bites as described above. During the transition
> to climb, the tip unloads slightly and the root takes up the
> difference. At cruise the tip unloads further leaving the root to
> take up the slack. Balancing this twist means the propeller
> designer and aircraft/power plant designer must cooperate to achieve
> this goal. So the fixed pitch designer at WD makes his blade so
> that there is extra pitch at the hub decreasing to the tip so as
> speed increases the root pulls the load the tip can't, but only to a
> point. Typically 7500-9500 feet and 120 Kts is about max you'll
> ever get unless you've got a clean airplane like a Europa and you
> get 130 Kts...
>
> Take the Airmaster with the Warp Drive (WD) blades. On a slow
> aircraft like a Rans S-7 or a Kitfox. Let's say their optimum
> cruise will be 110 Kts at 7500 feet for that plane. A fixed 68 inch
> WD prop can be twisted to give max static thrust, and high climb,
> but then the pitch is too low for optimum cruise, and the plane only
> makes 105 Kts because the tip is unloaded and prop efficiency
> drops. So we put the WD blades on a CS prop hub. Now the static
> thrust and high climb are there but once at cruise we adjust the
> pitch to coarse to allow the prop to bite more and be at the optimum
> pitch for 110-115 Kts or a little higher.
>
> Now let's go to our 914 and 15,000 foot cruise. The WD blade is a
> good strong blade, but the assumed efficient cruise altitude of the
> blade may not have been optimized for 15,000 feet and 175 KTAS
> pulled by an engine making 70 HP and 70 ft lbs of torque at
> altitude. As altitude goes up, we know the rules about how density
> affects the TAS. The blade (wing) of the prop is now at a higher
> Reynolds number which results in a lower lift curve slope. In the
> rarified air up there, the dynamic pressure is less so less lift (or
> pull) is provided, and the velocity (True) is higher, so the vector
> made by the rotational angle of the disk and the Velocity vector
> forces the blade to a higher angle of attack to bite (course prop
> pitch). The higher altitude decreases the thrust (or lift of the
> blade) and you find yourself with the blade generating max lift, and
> or in fact stalling at some point and not operating efficiently. I
> believe it is a high probability the torque of the 914 engine is
> higher than the prop requires for max efficiency at altitude. Hence
> the CS mechanism adjusts the prop to a more coarse setting to absorb
> the torque demanded by the throttle, but that puts the blade at an
> angle which lowers the overall efficiency of the blade. Therefore
> you experience the effect of no appreciable increase in speed the
> higher you go. The WD blade is most probably being over driven by
> the 914 at altitude. I presume you experimented with many manual
> pitch settings, manifold pressures and RPM settings looking for max
> performance. I am well behind you as my schedule does not allow a
> lot of time to climb to altitude and test, but I am hoping for a
> break this spring. My gut feeling will be that I will need to
> throttle back to gain efficiency and be satisfied with 40 mpg
> instead of 30 and get to my destination a little slower.
>
> From my Airplane Aerodynamics reference by Dommasch, Sherry and
> Connolly, "Because of the factors discussed above, we find that the
> over all shape of a propeller is determined by the maximum speed at
> which it must operate efficiently. A low-speed planform should be
> slender with well rounded tips, whereas a high speed planform should
> have a large chord, with the maximum amount of blade area
> concentrated in the minimum diameter. Because the major portion of
> the thrust is derived from the outer portion of the blades, a high
> speed propeller is generally characterized by paddle shaped tips
> that place the area where it can best be utilized.
>
> What's the fix? To translate the above academic explanation, the
> best example is to look at WWII prop designs of the VDM propeller
> used on the ME 109. Early versions had the thin profile blade we
> see on our WD narrow chord blades. But as engine performance and
> the demand for higher altitudes increased, the designers went to a
> wider prop of more surface area to absorb the torque, and to create
> more thrust out of the blade normally lost due to the effects of
> higher altitude. They were limited by the diameter of the prop due
> to ground clearance, and production forced them to stick to three
> blades so they fattened the blades, increased the pitch of the root
> and went to war (see pictures of the Ta 152 for an even higher
> altitude and speed prop). Takeoff performance wasn't that much
> better because the larger blade area demanded more torque/power than
> available and the pitch was reduced lowering efficiency at takeoff,
> but mid altitude climb and acceleration, as well as cruise was
> improved. By the way the VDM was a variable pitch prop, with a
> visual indicator of prop pitch in the cockpit, not a constant speed
> prop. Talk about pilot work load. In the US we did the same for
> the P-47 by changing the Hamilton Standard prop from a thin blade to
> a thicker blade as WD did and made the prop Constant Speed to reduce
> pilot work load. God I love history...
>
> Right now Airmaster is looking at a number of blades that fit our
> hub and future hubs. Problem is, the blade designs are many, and
> twist features are not that much different than the WD. My comment
> on the Kiev blade is that it doesn't appear to have much more twist,
> but it does seem to have more area. It is in use on lower altitude/
> low performance ultralight and trike aircraft. It is light, but, is
> its hollow design tough enough for the 914 at high Q, and for our
> Europa, all the Kiev prop blades are longer (67") than we can
> normally use. Sensenich (fixed) and Whirlwind (oil pump driven
> hydraulic) have wider chord blades than the WD and may hold an
> advantage in some areas, but give a thumping sound as the pressure
> wave hits the aircraft. Airmaster does use their own Europa to test
> prop blade performance. In my opinion, if some blade was
> significantly better for the three blade AP332, it would be on there
> already. Since we have new meat in the prop market, there are more
> choices in the blades, and the Rotax / WD is very popular so the new
> blades favor that LSA speed/altitude market now and use a similar
> hub attachment and blade design. I'm afraid there is not a lot of
> call for high altitude low power high speed props like we need for
> cruise at 15,000 feet. Also the market has to be satisfied, many
> lower performance aircraft enjoy the Airmaster and aren't in need of
> a major change, while other markets need a longer two blade prop for
> static thrust. Wider blades restrict feathering or cowl clearance
> in some aircraft. These are the things being evaluated at this
> time. It takes time, money and testing.
>
> Good Night,
>
> Bud
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