[rv969wf]

What are other RV's seeing there IAS or GPS cruising speeds on there RV's at say 23" MAP x 2,450 rpm at 3,000 ft MSL compared to running at 28.7" MAP x 2,680 rpm at the same altitude of 3,000ft MSL???? What I'm seeing with my -6 is either engine/prop or reduced drag, or all the above. My feelings are mixed and I'd like to see what others are seeing as I've been testing a lot of things and NO I'm not an engineer, but would like to know if anyone else is coming up with the same speed increases from cruise to Wide Open Throttle and Max RPM with a constant speed prop at the 3,000ft MSL range.

I was indicating today 185mph at 23" x 2,450rpm x 3,000ft MSL burning 10gph in Very smooth nice 65F air.

Cranked the -6 up to full throttle and was seeing 28.7" MAP x 2,680 rpm x 3,000 MSL and burning 18 gph. Same smooth air and indicating 214mph. All testing done with alltitude and heading autopilot on for 5 miles each.

29 mph speed gain between Cruise and Wide open throttle. No I don't fly my -6 wide open and max rpm all the time, only on takeoff a few times if needed, just curious what others are seeing with the same apples and apples test.

Yep I'm aware IAS is not always correct but the GPS ground speeds verified the 29 mph average gain.

Do most RV's increase 29 mph with these conditions?????? Or is this normal or behind the curve?????? Nope I'm not a racer!! But I'm sure the racers out there might give me a clue if this is norm, good or great, engine whatever. I'm just asking as I don't know. Any response would be great. AJ

 

[Bob Axsom]

I don't think that is typical

Let's face it your plane is not "typical". Flying at 6,000 ft density altitude on November 9, 2006 at 07:02, was an actual altitude of 4,200 ft MSL. The temperature at 6,000 MSL was 18C. Surface winds that morning were light and variable. I flew heading of 360, 120 and 240 until I got GPS speeds within 1 kt of each other at 5 consecutive 20 sec. intervals. I flew the 2450 RPM run first and the manifold pressure was 26 the average speed was 194.98 MPH. Then I increased the RPM to 2700 re-leaned for max speed and repeated the run. The MP was 25.5 and the average speed was 197.47 MPH. roughly 2.5 MPH speed gain. The engine is a stock Lycoming O-360-A1A and the prop is a the standard (not blended airfoil) Hartzell 72" diameter prop.

My theory on your performance gain focuses the test method and your engine. From your previous posts I know you have many special features in your engine, special coatings, very high compression cylinders, etc. The write-up here indicates that you increased the throttle opening for the high speed run and the fuel burn increased from 10 GPH to 18 GPH and the MP increased by 5.7 inches. This shows that the two runs you made were at different POWER levels and different RPMs. In the first test you were limiting the power and the prop speed so relatively speaking the engine was putting out less power and you were applying an RPM level that called for a greater load (pitch). In the second run you pulled out all the stops on power and prop loading. With the apparent power available from your engine the prop pitch is higher than on stock engines to load the engine to 2680 RPM. The higher the pitch the farther the prop travels forward in the fluid air with each revolution. Basically your plane is exceedingly fast but the two speeds are not a valid comparison of performance at the two prop speeds because the power available was not the same in each run.

Bob Axsom

 

[Kevin Horton]

Alan - your numbers make sense to me.

I know you don't have a stock engine, but the closest I could come was to use a standard IO-360-A power chart. If I assume a stock engine, 23" MAP x 2,450 rpm at 3,000 ft MSL at 65 deg F would have the engine producing 139 hp. 28.7" MAP x 2,680 rpm would be 212 hp, with both values assuming you were leaned for best power. 212 / 139 = 1.53, and the cube root of 1.53 = 1.15. The speed for the first power was 185 mph IAS. 1.15 x 185 = 213 mph IAS.

So, given the various errors associated with all the above (modified engine, tach errors, MAP gauge errors, ASI errors, etc), the speed you observed (214 mph IAS) was within the expected range.

 

[rv969wf]

Thankyou.

[

QUOTE=Kevin Horton]Alan - your numbers make sense to me.

I know you don't have a stock engine, but the closest I could come was to use a standard IO-360-A power chart. If I assume a stock engine, 23" MAP x 2,450 rpm at 3,000 ft MSL at 65 deg F would have the engine producing 139 hp. 28.7" MAP x 2,680 rpm would be 212 hp, with both values assuming you were leaned for best power. 212 / 139 = 1.53, and the cube root of 1.53 = 1.15. The speed for the first power was 185 mph IAS. 1.15 x 185 = 213 mph IAS.

So, given the various errors associated with all the above (modified engine, tach errors, MAP gauge errors, ASI errors, etc), the speed you observed (214 mph IAS) was within the expected range.

[/QUOTE]

I agree! The next test is to run at max RPM ,,, 2,680 at say 23" versus 28-29" and then do another run. I will do this and see what she does. I'm still learning OK. I'm a rookie and only trying to see what other are seeing. Anyone with feedback or ideas are appreciated. Alan