Friday, August 18, 2017

Constant Speed conversion - 30 day review

As you both (my 2 readers) know, last month I converted my airplane from a fixed pitch Catto 3 blade prop to a Whirlwind 2 blade constant speed prop.  I have been flying the new combination for a little more than a month now and am getting comfortable with the new configuration so I thought I would write up a 30 day review.

My original review posted several Pro's and Con's to the conversion.  I have started to develop/evolve a few opinions about the conversion and that will be what I focus on for this discussion.  Keep in mind that this is only after about 30 hours of flight time behind the Whirlwind so I'm sure there are aspects that I am neglecting or that will change as I get more experience.

Pro's:  These have not changed much.  I am still impressed with the climb rate on takeoff.  The thrill of climbing out at Vx is almost scary!

I did have one fact reinforced on my recent trip to Oshkosh.  RPM has a huge impact on my oil temperature (go figure huh?).  I was climbing out in 90 plus degree days and I found that if I climbed at anything above 2400 RPM my oil temperature would climb up quite rapidly.  On one takeoff I pulled the prop back to about 2500 RPM for the first 5 minutes and my oil temperature hit 230 degrees.  At that point I remembered that I had a constant speed prop and could decrease the engine RPM and still have outstanding climb capacity.  I count this as a plus since I can now manage my oil temperature more precisely with the ability to control engine RPM and not impact climb performance significantly.  Its ironic that I never have issues with CHT's but oil temperature will bite me if I'm not careful.  Rarely do I see CHT's greater than 370F.

Another pro that I encountered was a significant decrease in cruise noise level.  I found that if I pulled the prop back to about 2100 rpm in cruise I could maintain pretty close to the same speed but the noise dropped considerably.  On the other end of that spectrum the noise on takeoff is louder simply because the engine is developing full RPM.

Con's:  Again these have not changed much but I have noticed a new con that I didn't mention before.  When I am taxiing with my canopy pulled back for cooling air I noticed that the canopy jumps around a LOT more than it used to.  This has to be because the two blades are pulsing the air evenly on both sides of the fuselage (something my 3 bladed prop never did).  This concerns me a little due to the flexing that occurs.

Previously I mentioned having a harder time with speed control because I have been using my engine noise level as a good indicator.  I will say that I am getting used to this change and its not as drastic of a change as originally thought.  I am getting used to hearing air noises on the airplane and using those to manage speed but I still find its darn hard to hold a constant airspeed when approaching an airport as I slow down for pattern entry.  Once the governor gets down to the low rpm stop things are fine but I see my airspeed varying by as much as 20 knots if I don't pay attention to the ASI.

Glide ratio.  Yesterday I got a chance to do a little glide ratio testing with the engine at idle.  Its not as bad as I originally expected once my speed has bled off to about 85 knots.  Pulling the throttle really slows the airplane down but at best glide speed the glide ratio is slightly greater than 8:1.  Doesn't sound too bad until you compare it to what I got with the fixed pitch prop at closer to 12:1.  8:1 is roughly equivalent to the RV-7 and RV-8's I have seen.

Conclusion:  Ok, so I have been asking myself lately if I thought the conversion was worth the cost and honestly I don't think so.  My 3 bladed Catto performed well in every aspect of flight (including formation flight) with the sole exception of takeoff static RPM.  The extra $6000 I spent to increase my takeoff climb from 1000+ fpm to 1500+ fpm was probably not money well spent.  Density altitude was never really an issue before so the improvement is negligible.  The fixed pitch takeoff roll was longer but never as long as 90% of the GA fleet so its not really an issue either.

No I won't be pulling my Whirlwind off and putting the Catto back because I have already spent the money but if I were going to build another RV (except the 10) I don't think I would do it again.


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