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
----- Original Message -----
From: Paul McAllister<mailto:paul.the.aviator@gmail.com>
To: europa-list@matronics.com<mailto:europa-list@matronics.com>
Sent: Friday, March 20, 2009 5:00 PM
Subject: Re: Europa-List: Constant Speed Props, etc.
Hi Bud,
I am not an aerodynamicst, not even an amature one but I was wondering
why you didn't think the Kiev blades will be much of an improvement
over the Warp Drive blades. The reason I ask is that I do know that I
get an increase in TAS up to about 10,000' with my AP332 Warpdrive /
914T combination, but after the speed doesnt increase much.
I hear from the Whirlwind folks that this is not the case for them,
higher = faster.
Given that something isn't happening as expected I am concluding that
there is some design parameter(s) that are not appropriate for a an
aircraft that flies at Europa speeds / altitudes.
Any insight that you could share with me would be appreciated.
Thanks, Paul
On Fri, Mar 20, 2009 at 11:32 AM, ALAN YERLY
<budyerly@msn.com<mailto:budyerly@msn.com>> wrote:
Frankly I doubt there will be a significant change over the WD
blades, but that is what testing is for. As you are aware, wider chord
blades require a somewhat larger base .
As you well know I am not a politician.
Bud
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