nigelspeedy
Well Known Member
So after spending the last 50 hours breaking in the engine and fixing all the little things that turned out not to be as perfect as I thought they were during construction I have finally set my mind to experimenting with the performance of the aircraft.
My first thought was to try and reduce some of the variables involved in cruise performance. So what is the best RPM to use in level flight with a constant speed prop, in my case a Whirlwind 74RV. How does one define best? My priorities were speed, efficiency, vibration & noise, roughly in that order.
So the aircraft is an RV-8, the exterior is pretty much built to plans with the exception of the Sam James Cowl. The engine is a 380 CID stroker parallel valve engine with 10:1 and dual P-Mags and fuel injection.
I did my prop RPM experiment by flying straight and level at 8,000' pressure altitude, on my test day it was ISA +10C. I had the throttle wide open for each point and did a mixture sweep from full rich (approx 300F ROP) to as lean as it would run without shaking apart (approx 80F LOP). After each mixture adjustment I let the airspeed stabilize. I corrected all the recorded IAS values to TAS by adding the instrument error and position error (I assumed compressibility error was zero). After each mixture sweep at a constant RPM I changed the RPM and repeated the exercise. I flew 2600, 2500, 2400, 2300, 2000 RPM. For this prop 2700 RPM is the max for takeoff and can't be used continuously so I did not test this as it can't be used for cruise. The prop also has an avoid range from 2050 to 2300 RPM, which is the reason I did not test 2200 or 2100 RPM.
What I was expecting was that as prop RPM was decreased the efficiency would increase by virtue of less friction in the engine. Turns out this was not the case.
As shown in the graph below for any given fuel flow between best economy and best power mixture setting the highest airspeed is with the highest prop RPM. So what I suspect is happening is that the change in ignition timing with RPM has a bigger effect on the engine efficiency/power than the change in RPM has on the efficiency of the prop.
So what RPM to choose then? For max speed choose 2600 RPM, but this is a bit noisy. At 2000 and 2300 RPM there is some perceptible roughness, not bad just not as smooth as 2500. The engine/prop were dynamically balanced at 2500 and it just a little smoother here than at 2400 RPM.
So based on efficiency, speed, vibration and noise 2500 RPM is my cruise prop setting.
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My first thought was to try and reduce some of the variables involved in cruise performance. So what is the best RPM to use in level flight with a constant speed prop, in my case a Whirlwind 74RV. How does one define best? My priorities were speed, efficiency, vibration & noise, roughly in that order.
So the aircraft is an RV-8, the exterior is pretty much built to plans with the exception of the Sam James Cowl. The engine is a 380 CID stroker parallel valve engine with 10:1 and dual P-Mags and fuel injection.
I did my prop RPM experiment by flying straight and level at 8,000' pressure altitude, on my test day it was ISA +10C. I had the throttle wide open for each point and did a mixture sweep from full rich (approx 300F ROP) to as lean as it would run without shaking apart (approx 80F LOP). After each mixture adjustment I let the airspeed stabilize. I corrected all the recorded IAS values to TAS by adding the instrument error and position error (I assumed compressibility error was zero). After each mixture sweep at a constant RPM I changed the RPM and repeated the exercise. I flew 2600, 2500, 2400, 2300, 2000 RPM. For this prop 2700 RPM is the max for takeoff and can't be used continuously so I did not test this as it can't be used for cruise. The prop also has an avoid range from 2050 to 2300 RPM, which is the reason I did not test 2200 or 2100 RPM.
What I was expecting was that as prop RPM was decreased the efficiency would increase by virtue of less friction in the engine. Turns out this was not the case.
As shown in the graph below for any given fuel flow between best economy and best power mixture setting the highest airspeed is with the highest prop RPM. So what I suspect is happening is that the change in ignition timing with RPM has a bigger effect on the engine efficiency/power than the change in RPM has on the efficiency of the prop.
So what RPM to choose then? For max speed choose 2600 RPM, but this is a bit noisy. At 2000 and 2300 RPM there is some perceptible roughness, not bad just not as smooth as 2500. The engine/prop were dynamically balanced at 2500 and it just a little smoother here than at 2400 RPM.
So based on efficiency, speed, vibration and noise 2500 RPM is my cruise prop setting.
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