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blended airfoil propeller versus variable pitch propeller aircraft

The older Hartzell prop blades that are not blended airfoil is an older design and slightly slower top speed. The latest Hartzell blended airfoil has less yellow caution rpm speed range that it can operate in. There are at least two different Hartzell blended airfoil blades used on the 180 - 215 HP engines that I know of. I forget the complete blade number ???6 and ???7. The ???7 is the newer of the two and has the prefered smaller list of restrictions of any that are used on the 180 - 215 HP engines. The oldest blade used on the 180-215 engines that I know of was the 7666 blade with square tips.

If you are talking fixed pitch vs constant speed prop, then that is a endless debate.

The constant speed compared to the fixed pitch, will provide the most performance.
 
One point not obvious is that the Sensenich fixed pitch prop has a 2600 RPM limit but the Hartzell constant speeds are at 2700. At altitude, where you may only be able to get 21" of manifold pressure, the constant speeds lets you get more RPM, hence more HP, hence more speed, but of course, power costs gas.
 
Info

As I recall, there was an excellent article in a recent edition of Kitplanes magazine, by Paul Dye. I believe it will shed much light on your question.

It may be available at Kitplanes online?

Good luck, Jim
 
Before the comments and answers get too far out of hand, perhaps we should settle on terminology, to make sure we answer the correct question. As stated, the question is a bit like “do you like apples or screwdrivers?” On the surface, the comparison doesn’t make sense.

A “Blended Airfoil” prop is really an aerodynamic blade design. It could be used on a constant speed, fixed pitch, or ground adjustable prop. A “Variable Pitch” prop is a bit ambiguous - it could be a constant speed (which varies its pitot maintain an RPM set by the pilot), it could be a ground adjustable (in which you pick a pitch on the ground and set the prop for that flight), or it could be one of those flexible props which actually changes pitch due to load.

In order to answer the question properly, we need to understand the question - so perhaps the OP can expand on what they are asking?

Paul
 
The intent of the question I posed regarding blended airfoil props was to help determine the best choice of propellers to provide optimum performance for both climb and cruise speed. Most of my flying is in the mountainous NW where many airstrips are at high altitude with high air temp DA and are sometimes short. Hence the absolute need for best climb performance. I also fly relatively long distances where high end cruise speed is a given. So it is essential to have both features. In reference to a constant speed prop I mean in flight adjustable pitch to maximize both climb and cruise performance. As such a fixed pitch prop is not a viable alternative. Not wanting things to get out of hand in responses I am looking at buying an RV6 or RV7 aircraft that may have installed other prop blade designs. So simply put does the blended airfoil profile on a fixed pitch prop perform better than a CS in flight adjustable variable pitch prop. The aircraft I fly now is a Cessna U206G so I am familiar with the operating parameters of the engine/propeller configuration and in flight management.
 
I think the original post is causing a bit of confusion due to terminology.

You can in fact have a constant speed prop with a blended airfoil. As is the current crop of Hartzel constant-speed versions found on many RVs. The blended airfoil prop blade is not limited to fixed-pitch propellers.

So propellers. We have fixed pitch. Ground adjustable. Variable Pitch (no governor, the pilot is the governor). And Constant Speed (the modern de facto changeable prop blade angle design). We even have what has been coined "almost constant speed" where a flexible fixed pitch prop changes shape due flexing due to loading. Bernie Warnke kind of pioneered this type of prop in the 70's and 80's and they are still around today. As Paul Dye pointed out these modes of operation have little to do with the propeller geometric planform, like "blended airfoil", "scimitar" or legacy straight and square. They are two different things. The former being mode of operation and the latter being a description of geometry based on aerodynamic objectives and aerodynamic design improvements.

There isn't any way a fixed pitch prop, of any planform profile, could provide the climb and cruise mission profile as you have specified in your requirements. There isn't anything wrong with the simplicity and light weight of well-designed fixed pitch propellers but it sounds like you are aiming for all the performance possible from an RV. One of the most endearing qualities of Van's aircraft is the spread between low-speed performance and top end cruise performance so it makes sense for you to strive for both. RVs are docile and safe with good pattern behaviour without surprises. So taking that operational range spread into account, in order to meet your specified mission profile you will definitely want a constant speed propeller.

It is rare to run into a "variable pitch" propeller as they were replaced by "constant speed" props. The variable pitch propeller depended on the pilot to constantly move the propeller control to keep the pitch desired without over speeding the engine. I flew a Stinson 108 with an O-435 with constant speed a few times and the prop control was almost a primary flight control. So a variable pitch prop is analogous to driving a manual transmission automobile where the driver shifts gears based on speed and power requirement. A constant-speed prop is a step up and is more analogous to an automatic transmission vehicle where the pilot sets whatever RPM he wants and the governor takes over the tedious task of automatically adjusting the propeller blades to maintain that RPM. The governor vastly reduces the pilot workload and can prevent engine damage due to overspeed in a dive. At the same time pushing the constant speed prop control can help to add drag in the pattern while configuring the aircraft for a proper go around. There is a reason why most high performance piston aircraft go constant-speed.

So that's the answer if you are asking about constant speed versus fixed pitch. If you want to find out about the geometrical attributes, blade loading and geometric design of a propeller blade irrespective of operating configuration that is a wholly different discussion. There is plenty dynamic physics, aeroelastics and aerodynamics involved with about as much math as empirical data obtained through testing. The most successful propeller designers can get the reputation of practicing a black art as it can appear to the casual observer to be about as much art as science. Suffice it to say they are very passionate, capable, smart and driven to get the most performance possible through lessons learned.
 
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The intent of the question I posed regarding blended airfoil props was to help determine the best choice of propellers to provide optimum performance for both climb and cruise speed. Most of my flying is in the mountainous NW where many airstrips are at high altitude with high air temp DA and are sometimes short. Hence the absolute need for best climb performance. I also fly relatively long distances where high end cruise speed is a given. So it is essential to have both features. In reference to a constant speed prop I mean in flight adjustable pitch to maximize both climb and cruise performance. As such a fixed pitch prop is not a viable alternative. Not wanting things to get out of hand in responses I am looking at buying an RV6 or RV7 aircraft that may have installed other prop blade designs. So simply put does the blended airfoil profile on a fixed pitch prop perform better than a CS in flight adjustable variable pitch prop. The aircraft I fly now is a Cessna U206G so I am familiar with the operating parameters of the engine/propeller configuration and in flight management.

OK, so I THINK that what you have said is that you want a constant speed prop, and the question is, would you want the Blended Airfoil design or the older “paddle blade” design - and the answer I can give you is that the Blended Airfoils were sort of designed with the RV speed range in mind, so they are optimized for planes like the -6 or -7. Testing by a few folks years ago showed about a 4-5 knot speed increase with the Blended Airfoil constant speeds over the older design, with some increase in climb performance as well.

Paul
 
Thank you Paul for your polite response to my question which is in essence quite clear and understandable. Simply put will a fixed pitch prop give better performance than a variable pitch prop. The blended foil design if incorporated in a variable pitch prop may have even better performance but if on a fixed pitch prop it will not perform as well over the full range from climb to high end cruise. So I would not want to choose a blended pitch prop.
 
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