Custom props and they know
elippse said:
All of these theoretical formulae of Ct, Cp, J, etc. do not address the really important aspects of propeller design and efficiency, namely: does the prop have a klunky shape where it enters the spinner or is it a correct airfoil at the correct helix angle. Is it sealed to the spinner or does it form a separate prop tip with loss of thrust. What is the tip Mach. Is the airfoil optimized to produce the best L/D. Is the blade Cl optimized to give the best blade loading. What is the thrust/torque ratio of each radius along the blade; is it constant or is most of the lift concentrated in the outer 20% of the blade so that the inner portion contributes drag only. One European outfit actually uses that as something to brag about, rather than hiding their heads in shame! Those are the formulae that are missing in the texts on propeller design!
I agree prop design verges on almost ART and a little black magic. That is why, "the proof is in the eating of the pudding". You don't know until you fly it. Airfoil area, diameter, twist distribution, chord, tip shape, plan form shape and so on are all interrelated variables that defy quantitative absolutes. It gets you 90% of the way, but 10% is that magic of designer. Like the RV has wings and all just like any plane, but the combo is just right and hits some magic balance of compromises.
Believe me Hartzell and Sensenich know as much or more about props than anyone. What every the european Prop company is doing is not new or unknown. Hartzell/Sensenich understand lift distribution varies along the prop span due to rotation. The fact is at the blade ROOT is un-aerodynamic in shape for one reason, strength and attachment to the HUB. Its just practical considerations.
Look at
any GA prop near the hub, they have the aerodynamics of a 2x4 or baseball bat. It's just the way it is for any small plane props. Now look at a Hamilton Standard prop on later P-51's, which had an airfoil right up to to the spinner. The spinner was a huge diameter deal they could hide the blade attachment inside. Our spinner is not large enough in diameter and is just enough to hide the hub and blade shanks. You could make a bigger spinner, but it would change the whole cowl and plane.
Look at the evolution of props on the C130 military transport. First three blades, than 4 fat square paddle blades. Now I think its 5 or 6 scimitar blades (scimitar - curved like a turkish sword). The new C130 curved blades are no doubt an improvement in some areas of operation and not so much in others. In part they have them because manufacturing process allows them to do it. However I am sure they cost is huge. With fast turboprops they run into tip speed issues (which is build into analysis theories). In fact tip speed is one of those limiting factors that brackets the design.
When you get into Mach 0.90 prop tips speed, you start to take a big efficiency hit. The scimitar blades have more sweep at the tip which improves high speed drag. Big turboprop wind makers are turning something like 1900 rpm, not 2,700 rpm, but their TAS is higher, HP is higher and they have larger diameters, which all increase tip speed. Therefore they have more blades because they need them, not because it is cool.
RV's are in the Mach 0.82-0.83 range at high speed full RPM. RVs don't need swept blades and if you could get them, they would cost more than the plane. At some point going flying is the best prop. For the money the Sensenich fixed prop is one of the best values. However I do admire the handmade wood or wood/fiberglass props, but they cost as much or more than a Sensenich. The advantage of a wood prop use to be they cost $600-$800. Those days are over. Whats a new Catto cost? $3,000?
There are several "theoretical" prop analysis approaches and design methods. That is why companies like Hartzell and Sensenich have an advantage, 50-80 years of experience each. They have developed their ability to correlate the theoretical analysis and design to actual performance of light plane props. The little guy whittling some wood in his shop can make a nice prop, but it is that last few percent, 2%, 3% or 4% that is key. They are not going to make 10 props and test all of them just for the RV.
Both Sensenich and Hartzell make special RV PROPS. These props are tailored to the airframes aerodynamics, a custom fit if you will. They did not get there just by analysis. Much of it was flight tested, so analytical and empirical data was correlated. Once that's done, the analysis and design are fine tuned. This is typical of engineering and aerospace. My criticism of other props was the blades sold to RV'ers where what they had in production for other (slower) planes.
That is the KEY people don't understand. Most other props are just approximate generic props. The old Hartzell HC2YK/F7666 was good for the Mooney and Piper Comanche, and it worked pretty good for the RV, which has similar performance and HP. However the blended airfoil is 3 mph faster. That does not sound like much, but that 1% or 1.5%, represents a breakthrough. Prop efficiency gains are measured in fractions of a percent. To get that you need to hone every prop aspect for that specific airframe. No other prop company makes a real RV prop in my opinion. Are the other one good or close enough? yea sure I guess, but I want that last 1.5%.