Well, I really like my rotary and I am happy that I chose that engine.  It is really smooth and I like the sound of it, but I have to admit that I was led down the primrose path a bit on what to expect in performance.  I also think a lot of others are as well.  We toss out HP numbers with abandon but they have little to do with what you really get with the engine.

 

The rotary is an RPM engine.  It makes very little torque and gets its HP from RPM.  These RPMs are in ranges that we just don’t achieve with the airplane.  Nobody cruises at 8500 rpm, or even hits that number on takeoff, yet that is the rpm that the HP numbers are reported.

 

When I first cranked my engine, I had a Performance Prop that was designed to give a 200 mph cruise at 6000 rpm.  I could only get a static rpm of about 4500 out of it.  I was concerned that I would not have enough power for a take off!  I lucked out and found an MT electric and I now start my takeoff run at 7100 rpm.  I think that with my homemade intake and exhaust, that I am somewhere between 180 and 200 HP at that rpm.  We are talking sea level here.  When I moved to Texas I noticed a definite loss of power taking off from a 1000 ft field vs a field elevation of 55 ft.  if you pull the rpms back to 5500-6000 for cruise and climb up to 8000 ft, you are talking about a serious drop in power.  I am probably in the 125 hp range up there.  I really would have to think about it before I would land at a field that was 6-7K feet of elevation.

 

I have attached a file of dynos of various engines.  The top graph is a 20B that is boosted to 45 inches.  I doubt any of us would boost above 38-40 inches, so all those numbers would be lower for us.  The middle is a 1987 NA 13B that is making about 165 HP at 6500 and the last is a Renesis that is using the factory bells and whistles on the intake that I had to remove and throw away.  It is also a 6 port which breathes better than the 4 port that I have.

 

So if you looked at one of those graphs and the power produced at your rpm and reduced it by the altitude you are flying at, you will see that you are not getting the power that you were probably thinking about when you made the decision to install the engine.  I am not saying anything negative about the engines or my decision to install it.  I am just saying we should be realistic and understand what we are doing.