A520 Commander Mixture and Prop question...

[Guest abent]

I love Milton Schupe's Commander - my new favorite plane to fly.I usually fly jets so I need some help.When cruising at 10000ft, I cannot seem to get more than 140mph IAS out of the plane. I don't know, maybe this is normal for the plane. It says that the high speed cruise is around 197 mph. I have played with different mixture settings and prop feathering with limited success in terms of increasing speed.So my questions...In cruise at say 10000ft what percent mixture and prop feathering should I have?Also, I'm not clear on this manifold pressure stuff. At 10000ft the manifold pressure doesn't seem to go over 20psi with the throttles pinned. Is this normal?Anyway, can someone give me a little guidance on how to properly use the prop and mixture settings?Thanks, Adam

[LAdamson 240]

Check your ground speed with the GPS & no winds. You will find that it's much higher than indicated airspeed. Air molecules which control the airspeed indicator are less dense at higher altitudes/and or higher temps., and will show lower readings than actual speed. But it's always the indicated speed that you'll use for manuvering the aircraft, as this relates to the amount of air molecules that flow around the wing.I don't have specs on this plane, so I'll leave comments on other settings to others.L.Adamson

[Bearracing 3]

Adam,Operating a recip engine is a great deal different than a jet engine, so bear with me (pun intended)! First of all, gasoline must be mixed at a very specific ratio with air to burn properly and efficiently. The mix ratio is stated as, for every 1 lb of fuel you need to mix it with 14.7 lbs of air, no matter what your altitude is. As you gain altitude, you must reduce the volume of fuel entering your engine because the air is becoming less dense (weights less) and that mix ratio of 14.7 to 1 must be maintained or a loss of power will occur. The general rule-of-thumb for adjusting your mixture is by watching your engine rpm or by listening to your engine. You should bring back your mixture setting (pull toward yourself or toward a leaning condition) until you note a loss of engine rpm or misfire, then gently move the mixture setting back in a bit or until the engine runs smoothly. As you increase prop bite (noted by the reduction of the prop speed), you may have to slightly increase your mixture setting toward a rich setting, otherwise your EGT (Exhaust Gas Temperature) may increase to too high a reading and engine damage can occur. Reducing your prop speed is kind of like driving your car up a hill, your engine has to work harder, so the burn temperature will increase, but you can check this temperature increase by increasing your fuel input.On your situation about not being able to acheive a cruise speed greater than 140 knots, when the stated cruise speed of the Commander 520 is 197 knots is probably the difference between IAS (Indicated Air Speed) and TAS (True Air Speed). Because the outside air pressure is reduced by an average of 10 inches of mercury at 10,000 foot elevation, your "indicated" air speed will also be reduced, but I bet if you checked your ground speed (with the default GPS) you would note that you're clicking across the terra-firma at the advertised speed. About your manifold pressure gauge not indicating anything higher than "20 inches" (you incorrectly stated it as 20psi, but no worry, that's a common mistake), but 20 inches of manifold pressure is normal at 10,000 feet. If you had a barometer in your hand, while standing at sea level, and the day was a normal day (59

[Guest abent]

Thanks Larry. I'm aware of the lower density of airat higher altitudes and resultant decrease in displayed airspeed. I did have a approximate 28Kt headwind at the time so I expected a lower ground speed. I just can't get the IAS over 140 even with a headwind.Adam

[Guest abent]

Thank you Bear for an excellent explanation. That certainly demistifies the manifold pressure thing for me.TAS was something I didn't consider. I just assumed they meant IAS. Also, I guess I just assumed as well that the airplane would be able to reach, or at least come near, its yellow or caution zone on the airspeed indicator in level cruise.I'll see if I can hunt down an operations manual or at least some data on this.Thanks again, Adam

[Guest abent]

Larry and Bear...in case you are interested...I did some digging and found - actually in the docs I overlooked that came with the Commander - and the speeds are in TAS. I guess that would account for the lower percieved speed. I got some stuff on rpm. 2600 for economy cruise and 3000 for high speed.I've gotta be more observant. I do really appreciate your explanations though. Still gonna look for an operations manual and checklists etc.I like your reviews Bear. I think I'm going to seek out some nice bush flying airstrips and take a brief break from the jets. The Commander looks like a good rugged plane for just such a purpose. I hear Alaska offers a lot of nice scenery.Thanks, Adam