Not long after I purchased my flying 7A in October of 2017, I took the plunge and ordered a Hartzell Composite propellor and aluminum spinner through Van’s. The decision to buy was downright scary, as several pleas for opinions and experiences here on VAF went unanswered, and searches of prior posts turned up very little info. Were pilots having bad experiences they were not revealing? Was I making an expensive mistake?
The blades have an incredibly large surface area and chord widths from 3.5” at the squared-off tip to over 8” at the widest point. Long after I’d paid, my old friend and propellor guru Jack Norris told me that classical propellor theory would suggest nasty tip vortices, and I wondered if my plain-vanilla 180HP, 8.5 compression Lycoming clone would be able to pull it, or if top speed would be severely limited. I’ll jump ahead and declare a resounding YES to the first and NO to the second. Evidently the lack of info on VAF was mainly due to very few of these being installed on RV’s because of the higher cost.
The only assurance I got was from potentially biased sources. Hartzell tech support told me it WAS a good match, that there were sound reasons for the 8”+ max chord, that I could expect fully 100 pounds more static thrust with my engine than the aluminum Hartzell BA could generate, and that there were absolutely no RPM restrictions or blade-life limitations. I also talked to Mike Stewart, lead pilot for “Team Aerodynamix”, which Hartzell had sponsored to install them on all of their RV’s. He said that they may have lost perhaps a knot of top speed compared with the aluminum Hartzell, but that the other advantages completely outweighed this. I was also encouraged to learn that this is the same blade(with 1” lopped off)offered as an optional threesome for the 300 HP Cirrus SR-22. That sled does not enjoy quite the same speeds as our RV’s, but it is in the same ballpark and has about the same power loading per blade area.
I promised myself I’d post a thread describing my own experiences with this beast for the benefit of others. I’ve not had a chance to do much in the way of controlled study of absolute performance, but I can relate my anecdotal experiences so far with about 50 hours including considerable aerobatics and cross country time. I’ll supplement this later as I acquire more real data.
Additional photos of my installation can be found here, please be sure it is in “roll view” and do read the captions appended to each photo:
The fixed pitch aluminum propellor I replaced on my bird was a Sensenich 72FM889-1-85, designed specifically for the 180 HP Lycoming variants. The last two digits refer to the pitch, 85” being the max available without re-pitching and the one Sensenich recommends for RV’s. It was actually a pretty good performer, producing brisk takeoff and climb and acceptably good cruise performance. I was actually tempted to have it re-pitched to 86” or so as I was unable to operate full throttle in cruise even at high altitude without over-reving the engine, and found myself cruising at an uncomfortable and noisy 2550 to 2650 RPM’s if I really wanted to cover some ground.
The entire Hartzell assembly including installed spinner weighs in at just under 44#, so I was “pleasantly shocked” to discover that the Sensenich assembly, including the 2” aluminum spacer, the stock Van’s spinner, and the long heavy bolts weighed in at fully 42.5 pounds! This was almost too good to be true, I’d be getting a huge performance boost with almost no weight penalty at the nose! My days flying numerous experimentals for the CAFE Foundation had taught me the deleterious importance of excess polar mass. In short, for any given airplane, polar mass(the “dumbell effect) is inversely proportional to maneuverability, ease of spin recovery, and fun. It is noteworthy that installing an aluminum Hartzell BA instead of the composite would have added 17# at this most forward station instead of what turned out to be a little over a pound!
Installation was pretty much a bolt-on affair, as Hartzell had generously created a hub just for RV’s that extended the blade-plane 1” forward, and an awesomely well built spinner with significant fore and aft positioning adjustability. Fortunately, my engine was set up for CS, so all I had to do is add a governor, an oil line, and a blue knob to make it work. I did have to remove a forward expansion plug from the crankshaft and install a smaller interference fit plug about 8” back to create the pressure chamber that drives prop pitch as described here in this Lycoming document:
I’ve never liked the propellor oil line installed on Lycomings, which contorts its way through hostile territory below the cylinders and is difficult to inspect. I was also disinclined to use a flexible oil line. I decided to fabricate and install my own heavy-wall stainless steel line routed much more favorably and fully visible during preflight. That story can be found in a somewhat contentious VAF thread that I posted. I respect the opinions expressed but absolutely stand behind my own design with great confidence. See:
Take-off and climb performance with the new prop installed is stunning, and I now understand why Sean Tucker calls the variants of it he has flown “the claw”. I set the RPM limit to 2720 RPM. About one second after reaching full throttle, the prop optimizes its “bite” and the acceleration is thrilling. My takeoff run is roughly half of what it was before- basically you apply power, pause briefly, and rotate. Sustained climbs solo with half-full tanks around 2,200 FPM are possible and I typically reach pattern altitude while still over a 3,000’ runway. I’m still exploring this and will report back later, but I seem to be getting max ROC at speeds well below Vx! I’ll leave it to the aerodynamicists to refute this possibility, but it feels like this thing has imparted a bit of helicopter mojo into my airplane! One advantage of the large, stiff, ultra-thin carbon fiber blades may by that they see far less flow separation along portions of their spans than do skinny and/or thick blades at some stations during high-power/low-airspeed operation. Also, cooling in climb is greatly improved. I think Hartzell has found a way to produce far more thrust with the inner third of the span than conventional propellors have ever managed, and some of that blasts right into the cooling inlets.
In cruise, and in concert with the new PCU5000 governor I installed, the prop is silky smooth, super stable, and very well behaved with no trace of hunting or oscillation. I generally feel that my engine and prop combo is happiest in the 2450-2500 range and well under square, but I tend to cruise at a bit more aggressive settings than many pilots seem to choose.
My very favorite thing about this prop, and I’m sure this is also true for the other composite CS props, is the impact of the very lightweight blades on nimbleness when doing aerobatics. Most of the weight is concentrated at the hub, so the reduction in gyroscopic inertial forces during abrupt maneuvers is profound. You don’t realize how much control force must be applied (at 90° to the desired change in direction!) to overcome the precession resulting from displacing a heavy aluminum propellor out of axis until you have flown lightweight blades. It really feels like a different airplane compared to the metal Sensenich.
One Big Giant Proviso! :
There is one thing that you must know and consider that is unique to this propellor: If you pull the throttle in flat pitch, it feels like a drogue chute has been deployed, and you are coming down fast with a best L/D of somewhere around 3:1! This is quite startling at first but I’ve come to regard it as an asset. On the flip side, with the blue knob pulled out, the thin-bladed prop goes to a very coarse pitch and, with the ultra-thin blades, the airplane feels more like a glider than most RV’s ever do. The big proviso is that it takes a LOT of oil pressure to hold the prop in coarse pitch, so to obtain it with a windmilling prop you MUST have a governor that can generate high pressure at reduced RPM, AND your installation must be relatively free of any oil leakage from the propellor pressure section into the crankcase. Mike Stewart admitted that his governor cannot do this, so if he has an engine failure with a windmilling prop, it goes to flat pitch and his landing site options will reside within a very small circle of real estate. If anyone can handle that it is probably Mike, but most of us would not be happy campers in that situation!
Thanks to the PCU5000 and an engine with a nice tight vascular system, mine can maintain coarse pitch when windmilling at approach speed just fine. I’m a glider pilot so I’ve come to enjoy controlling my approach angle by simply leaving power at a very low setting and trimming airspeed constant while using the blue knob like the spoiler control on a sailplane to dial in the glide-path. One plus of all the available drag is that you can easily put an approach entered at very high speed and altitude on the numbers at full stall with impunity. I’m not talking about a minor difference from other constant speed props here, this is profound.
To put it in a nutshell, if I opened my hangar one day to find that someone had put the FP Sensenich back on my airplane(and of course deposited about $15K back into my bank account!), I’d be ordering another Hartzell Composite without the slightest hesitation, but I appreciate that it is not the prop for everyone. I’d be happy to respond to any questions or comments.- Otis