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Guest Ron Freimuth

Engine Revs/HP Output

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I've been trying to better model the Rotax 912 for my Allegro project I am working on with Don Halcom.I have a question regarding how output is measured in an engine. On the ground, at full throttle, I can get output to hit 100HP at the max rpm of 5800 for the Rotax. However, as soon as I go airborne, the output continues to climb, topping out in the 120-130HP range as the engine revs higher than the 5800rpm redline. The solution is to scale power output back so 5800rpm isn't reached until in the air, trimmed for level flight.My question is, how should I calibrate the engine? Should I calibrate it for 5800rpm/100HP once trimmed in flight, or are HP readings meant to be measured on the ground at full throttle? If the latter is true, would any aircraft engine overrev in level flight beyond rpm redline if the throttle's going full bore? I am using an outside air temp of 59 degrees and pressure of 29.92 in my testing.Thanks in advance for your help-John

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>I've been trying to better model the Rotax 912 for my Allegro>project I am working on with Don Halcom. I modeled a Rotax 914 powerplant from the Rotax curves posted at their site. Which give torque vs throttle and RPM. A bit over 114 HP at 5800 RPM, MP ~ 39.5".>I have a question regarding how output is measured in an>engine. On the ground, at full throttle, I can get output to>hit 100HP at the max rpm of 5800 for the Rotax. However, as>soon as I go airborne, the output continues to climb, topping>out in the 120-130HP range as the engine revs higher than the>5800rpm redline. The solution is to scale power output back>so 5800rpm isn't reached until in the air, trimmed for level>flight. My Rotax has a CS prop attached. So, it can't exceed 5800 RPM at the crankshaft. The prop shaft is geared down by 2.43, so the prop is limited to about 2400 RPM. Powerplants with fixed pitch props won't run up to rated RPM on the ground. RPM increases significantly at max TAS. One only gets rated power at red line when MP is at rated maximum. Which would be at SL for non-turbocharged engines. The FAA TCDS generally gives the RPM one must be able to get when run up on the ground. This is a strong function of prop diameter and pitch. 2350 RPM is about right for a powerplant that develops full power at 2700 RPM. While the Tach should only hit red line at full throttle once the AC accelerates to maximum TAS. That means, for a fixed pitch prop, that the Coefficient of Power table (512) in the AIR file needs to be appropriate over at least that range. Increasing Cp loads the engine more, so RPM drops and so does HP. Cp depends on J and Blade Angle (fixed in this case). J = V/N*D [(ft/sec)/(rev/sec)*ft]. The table Cp value at J = 0 is applicable to 'run up'. While a lower Cp at maximum V (TAS) and RPM/60 defines the powerplant loading at maximum TAS. I see Cp ~ 0.065 for J=0 and 0.037 at 20 deg for a 5.7 ft prop on my Rotax powered AC. However, it is CS, so the blade angle changes as required to set the appropriate Cp.>My question is, how should I calibrate the engine? Should I>calibrate it for 5800rpm/100HP once trimmed in flight, or are>HP readings meant to be measured on the ground at full>throttle? You should only get rated RPM and HP with a FP prop at SL after the AC accelerates to maximum TAS. On runup you might see 80% of rated HP. If the latter is true, would any aircraft engine>overrev in level flight beyond rpm redline if the throttle's>going full bore? I am using an outside air temp of 59 degrees ..>-John Unless a lower pitch prop than normal is used, I'd expect to see the Tach get to red-line or just slightly higher in level flight with full throttle. However, in a shallow descent it would go higher. It's up to the pilot to pull back the throttle if RPM gets too high. Ron

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Thanks Ron--it sounds like I got it right then--I'm getting roughly 80 pct. of the rated HP before takeoff roll. It's interesting though that using the same .air file and aircraft.cfg, I got higher performance in FS2002 vs. FS2004 and had to scale down the power a bit for FS2002. I'd like to be able to share the same FDE between both sims, so I wonder if FS2002 is ignoring a parameter that's avail in FS2004, or the other way around. -John

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>Thanks Ron--it sounds like I got it right then-->interesting though that using the same .air file and>aircraft.cfg, I got higher performance in FS2002 vs. FS2004>and had to scale down the power a bit for FS2002. I'd like to>be able to share the same FDE between both sims, so I wonder>if FS2002 is ignoring a parameter that's avail in FS2004, or>the other way around. >-John I get identical performance in FS9, FS8, etc. if the parameters in the AIR file are consistent with those later set in aircraft.cfg. Note the two IAS calibration factors near the top of REC 1101 are not automatically exported into the FS9 aircraft.cfg. I add something near this for lower speed prop AC:[airspeed_indicators]airspeed_indicator.0 = 1.111, -10.0 //Reads 100 kts at CAS = 100 kts Also, FS9 hard coded a few tables, such as that 'throttle table' and 'mixture table' so I see the same profile of SFC in FS9 as I could get in FS8 and earlier. The default mixture table in earlier versions resulted in too much of a drop in HP at full rich at low altitudes. Regardless, leaning for maximum HP gives virtually the same max value. Since the 'throttle table' no longer has an effect I may end up adjusting the 'torque vs RPM' table at low RPM to get an appropriate idle. Note, in aircraft.cfg: max_rpm_mechanical_efficiency_scalar=1.00idle_rpm_mechanical_efficiency_scalar=1.00 //Higher inc. idle RPMmax_rpm_friction_scalar=1.00 idle_rpm_friction_scalar=1.00 //Higher drops idle RPM Ron

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