Dear Bob
Bob Axsom said:
The pilot has no way to directly change the pitch which was my original concern. However, if you reduce the diameter the load on the engine is reduced and the pitch must be increased to maintain the same RPM. Up to a point, the aircraft speed will increase with each reduction in diameter. For a given prop/engine/airplane combination there is one diameter that will give maximum aircraft speed. This can be found experimentally as done by Sulivan Propeller but the cost is to find the optimum you have to go beyond the peak and that means to get the peak aircraft speed you have to buy replacement blades and cut them down to the diameter that produced the maximum aircraft speed. Bob Axsom
Yes that is true, dia is critical. In general for RV's with Hartzell's 72" are more efficient (faster) than a 74" for high speed cruise. Going less than 72" is academic because most Hartzells have a min 72" dia limitation. Hartzell does make some 68" dia props for the Lancair's. So Hartzell is very aware of twist and dia in optimizing for the airframe. However when we talk about optimization we are talking about fractions of a percent.
Look at the old standard, the C2YK/F7666-4 used for RV's for years. This was a common prop hub/blade used on early Mooneys and Piper Arrows, which have similar cruise speeds as a RV, so it was a good match. Now enter the Blended airfoil PROP, C2YR-1BF/F7496-2 or -4. This is a gain of about 3.5 mph over the older prop. That is fantastic. Here is Vans test data (click to enlarge):
So from 205.4 to 208.9 mph, about 1.8%, is phenomenal. The BA prop should be called the RV-prop, because that is exactly what it was optimized for, RV's with nominal 180 hp engines and cruise speeds in the low 200's mph at typical cross country altitudes (8,500 feet).
The point is anyone claiming 10 mph increase for their prop are either comparing it to a prop that's a poor match for the airframe, either because it is just a bad match or the airframe/engine is highly modified or is exaggerating.
Hartzells are about as good as it gets in efficiency. You have to be very radical in HP or drag reduction (clipped wings, chopped canopy and so on).
When you have a company like Hartzell, with their background, set out to make a prop spacifically for a particular airframe its going to be a good match. Now Hartzell no doubt made some compromises to give good overall performance, with emphasis on normal cruise at normal altitudes with a normal engine. Altitude BTW makes a difference in optimal prop design. So even with a given airframe there are many factors to consider. No prop is optimal at all conditions, however C/S props are have a way broader sweet spot than a FP which is truly a one trick pony, all other operations are significant compromise. Still a C/s prop can't change its twist, airfoil shape or diameter in flight. So if you choose to make some radical change in your C/S prop for say WOT, max speed at sea level, you will take a hit in all other flight regimes. Its all about trade-offs.
Bob unless you are going to Reno and had a radical plane, prop changes will not make much difference. Now as I said, keep the prop leading edge and back smooth as glass and waxed. This will gain a MPH or two.
Bob you plane is fairly stock, but if you start to significantly reduce airframe drag, radically decrease drag or increase HP, than the prop you have would start to became even less ideal. Obviously the new BA prop is a good start. I am still running the old C2YK/F7666, but the BA prop is what I'd like to upgrade to.
I am not sure why you don't make your cowl into a Holy cowl, or a cowl with small, wide spaced, round inlets, with a smooth sealed connection to a plenum, aka Sam James cowl? That is worth 8 mph, plus/minus a few. The cowl change would give you most bang for your buck. You can MAKE your own cowl from your existing one and your own plenum with your existing baffles. Cost will be small, time and effort who knows. A 100 hours? Of course fiberglass is involved to glass in new round inlets. (I can help size them for you.) The plenum can be made with sheet metal top and some fiberglass transitions (diffusers) to lead into the plenum box.
Just suggesting the cowl as the best mod you could make, and I would not fret on your prop too much, it will only gain you so much speed. A BA prop will be good for 3.5 mph over what you have; it cost less than $6,000 minus what you can get for your own prop. The cowl may be a few $100.
As far as Randy's comments, he did great work. Van's tables uses some of Randy's data, but as you can see the 200RV prop is slower than the BA prop and not much better than the good old HC-C2YK/F7666. However again to make my main point, difference in prop performance is very small between the top contenders. The MT's of course are slower simply because their wood core material requires a thicker airfoil. Many MT props marketed for RV's are sold very slow planes. One size does not fit all. A prop must be optimize for a spacific airframe and engine to get the most out of it. The 200RV and Hartzell BA are about as good as it gets, with the old HC-C2YK/F7666, like you and I have, is still pretty good after a few decades.
Just keep your prop clean, smooth, waxed and make sure the leading edge profile is perfect. What perfect means is a bit of a secret, however as the leading edges wear as they get dressed out from nicks, it looses its airfoil shape. When I say secret, some have different profiles they file into the prop. Warning! If you don't know what you are doing I'd not whittle on your prop, get someone to do it. You only have so much meat to remove, take great care to remove minimal material, no sharp edges and blend ratios of 10 to 1. Prop design is ART and Science.
but for you buddy. Don't tell anyone. 
