Prop Synch...when? Mixture? RPM?

[jonss]

After a couple of years pressing buttons to fly an aircraft, I decided to get a fastish twin engine B60 Duke.

My questions are, and I want to speak to 'real' pilots and 'Simpilots': Balancing the aircraft, at cruise:What am I searching for here. It's a 'sweetspot', right? Is there a sequence for achieving the best aircraft performance, rpm v prop speed v mixture?

Jon

[Guest J van E]

Usually you set rpm and then prop to a specified number (when you reached cruise level, that is), adjust mixture if necessary (above a certain height and usually a bit rich of peak) and then you simply adjust trim to 'balance' the aircraft. You don't use rpm of prop or mixture to do that: just trim. So when you can't stay level, don't change rpm or prop or mixture: keep them at the right numbers (check the charts for those: quite often the numbers are 'squared' so rpm and prop at 24/24 or 23/23) and adjust trim to stay level. AFAIK.

And btw you always lower rpm first and then prop (the 'prop-number' always has to be higher than the 'rpm-number', so to speak). So when you need more power. first change the prop to a higher number and then rpm.

If you are only talking about 'best performance' (and not balancing as in flying level): as I said, check the charts/manuals for specific cruise settings and stick to those. You don't have to look for a sweet spot somewhere.

But hey, I'm only a simpilot, so...

[level 7]

i was looking around a lot about this lately ...

for the best settings, you need to look into POH charts included with the duke for the optimal settings

in an airplane the power controls are in order from left to right MP (manifold pressure), RPM (revolutions per minute), and mixture (mix).

as to the order, to increase power, you go right to left (mix, rpm, mp).

to decrease power you go from left to right (mp, rpm, mix). this is because with constant speed propellers, the plane will automatically adjust itself to the correct rpm after the sequence.

you can read more about it here, feel free to correct me if i'm wrong

http://www.stoenworks.com/Tutorials/Throttles,%20mixtures%20and.html

[jonss]

Thanks for your explanations and the link. It's just that I'm an economy tragic. 

Jon

[macca22au]

J van E:  I know it was an accidental slip, but prop and rpm are the same thing in an aircraft with constant speed propellers.

Throttle is Manifold pressure, the Prop control gives rpm, and the Mixture adjusts the fuel/air mixture which naturally alters with altitude.

Yes read the POH carefully to get the recommended settings for altitude and temperature, and set your Manifold Pressure first, then the Prop RPM.  The mixture can be set roughly, by pulling the lever back until the engine suddenly begins to lose power then move it forward a little to regain the full engine noise.  A more exact way is to pull the lever back while watching the cylinder head temperature.  When it peaks push the lever forward a small amount to bring CHT off the top.

Manifold pressure for normally aspirated engines reduces with altitude so you will cruise the Duke with full throttle but way below the red line.  Turbo-charged engines of course perform better to higher altitudes, but unlike jet turbines they still decrease with altitude until you reach a height where it maxes out.  On long flights most pilots challenge themselves to get the best fuel economy from their enginges.  (Good engines are always needed on takeoff!)

[Guest J van E]

J van E:  I know it was an accidental slip, but prop and rpm are the same thing in an aircraft with constant speed propellers.

Throttle is Manifold pressure, the Prop control gives rpm, and the Mixture adjusts the fuel/air mixture which naturally alters with altitude.

Very true, thanks for the correction. I didn't think deep enough (or not at all ) when I read 'rpm v prop speed v mixture'.

[Maurice_King]

There is a myth that you should NOT operate over square but this is not the case.

Many engine manufacturer's actually allow over square operation at cruise.

For matching rpm NEVER rely on the Tacho's, at least for syncing that's what your ears are for.

by matching the harmonic sounds just like tuning a guitar. then you can pull the throttles back to your calculated cruise power and let things settle.

Now you can trim back your mixture because you would be cooling currently with fuel and that is costly and inefficient, depending on the engine manufacturer you can safely trim back to 50-100 degrees lean side of peak (Air is cheaper than fuel)

So you can see at cruise altitude, it's a balancing act to get economy and efficiency.

Running a twin is no different than a single engine constant speed apart from the asymmetric thrust for operating unbalanced should you have an engine failure.

Managing the engines in no different though you just need to do it twice.

Hope this helps

[DavidP]

Manifold pressure for normally aspirated engines reduces with altitude so you will cruise the Duke with full throttle but way below the red line.  Turbo-charged engines of course perform better to higher altitudes, but unlike jet turbines they still decrease with altitude until you reach a height where it maxes out.  On long flights most pilots challenge themselves to get the best fuel economy from their enginges.  (Good engines are always needed on takeoff!)

I think you'll find the Duke has turbo charged engines.

David

[tim_A]

Over-square was only ever a problem for old radial engines, where it could wear out the bearings very rapidly (over square is where the MP number is higher than the RPM number - eg 28"/2500RPM aka 28/25). Modern horizontally opposed flat-fours and flat-sixes ddon't have any problems with it. Normally aspirated engines don't have the MP grunt to develop more than an inch or two of over-square, but turbos like the Duke can easily get 9.. 10.. 12" over.

Here'a a couple of good articles on engine management. They explain amongst other things why, when given a choice of settings in the POH for (say) 65% power, you should always go for the one with the lowest RPM...

http://www.avweb.com/news/airman/184483-1.html

http://flighttraining.aopa.org/students/solo/skills/engine.html

[jonss]

Over-square was only ever a problem for old radial engines, where it could wear out the bearings very rapidly (over square is where the MP number is higher than the RPM number - eg 28"/2500RPM aka 28/25). Modern horizontally opposed flat-fours and flat-sixes ddon't have any problems with it. Normally aspirated engines don't have the MP grunt to develop more than an inch or two of over-square, but turbos like the Duke can easily get 9.. 10.. 12" over.

Here'a a couple of good articles on engine management. They explain amongst other things why, when given a choice of settings in the POH for (say) 65% power, you should always go for the one with the lowest RPM...

http://www.avweb.com/news/airman/184483-1.html

http://flighttraining.aopa.org/students/solo/skills/engine.html

So, after reading that, I'd like comments on these 'cruise numbers'. Just to ascertain if I'm in the ballpark. http://picasaweb.google.com/lh/photo/8jOfpll6hkA-ihnoRkQE7S0z-nW0hc_DWPwevjDR1y8?feat=directlink. After reading the article, I'm not too sure whether I've got it but I was happy. The only thing i was a little uncomfortable about was the temps, even with the cowl flaps fully open. They'd rise higher but come back after a while.

Jon

PS: Please :0...and it's a Beechcraft B60 Duke.