CS Prop, Recip Enging, How do I recognize when RPM too low?

kontiki

kontiki

Cleared for Takeoff
Joined
May 30, 2011
Messages
1,124
Display Name
Display name:
Kontiki
Hi all,

Went for my first ride in a complex aircraft today, (PA28R-201) now working toward a commercial. My only other CS flying has has been getting to practice area and back in a Super Decathlon.

I understand that the CS prop is all about efficency.

So far, it's always been recipies, full RPM & PWR for takeoff, 25 squared for climb, 23 squared for cruise. From a pratical standpoint, if you are adjusting for best efficency is it all in the performance charts or can you tell from the cockpit? What's the warning sign if the RPM is too low?

Thanks,
 
kontiki said:
Hi all,

Went for my first ride in a complex aircraft today, (PA28R-201) now working toward a commercial. My only other CS flying has has been getting to practice area and back in a Super Decathlon.

I understand that the CS prop is all about efficency.

So far, it's always been recipies, full RPM & PWR for takeoff, 25 squared for climb, 23 squared for cruise. From a pratical standpoint, if you are adjusting for best efficency is it all in the performance charts or can you tell from the cockpit? What's the warning sign if the RPM is too low?

Thanks,
Click to expand...

The airplane's Pilot Operating Handbook gives a range of settings and associated airspeeds. The pilots stays within those if he wants to cruise efficiently and not damage the engine.

Like this:

cruise_power_setting.gif


Dan
 
Last edited:
Squaring everything up is an urban myth. However, it was derived from the fact that there are a number of manifold pressure and RPM combinations (typically manifold pressure limitations for given RPMs) that are restricted. These are for various reasons, but typically it came down to either detonation or crankshaft/propeller vibrations.

Check your POH. If it says it's good, it's good! What's good on one aircraft may be bad on another aircraft, so there's really not much in the way of "rules" that you can use.
 
kontiki said:
Hi all,

Went for my first ride in a complex aircraft today, (PA28R-201) now working toward a commercial. My only other CS flying has has been getting to practice area and back in a Super Decathlon.

I understand that the CS prop is all about efficency.

So far, it's always been recipies, full RPM & PWR for takeoff, 25 squared for climb, 23 squared for cruise. From a pratical standpoint, if you are adjusting for best efficency is it all in the performance charts or can you tell from the cockpit? What's the warning sign if the RPM is too low?

Thanks,
Click to expand...

The Arrow POH's all contain this snippet:

"When selecting cruising RPM below 2400, limiting manifold pressure for continuous operation, as specified by the appropriate "Avco-Lycoming Operator's Manual", should be observed."

For your question about efficiency, it is best to consult the performance charts.
 
Last edited:
The problem with waiting for symptoms of the RPM being too low for the amount of power being developed (i.e., the onset of detonation) is that by the time you see them, the damage may already be done. See http://www.sacskyranch.com/deton.htm for more on that.

The way to avoid that is, as those above have said, to know and use the power tables for your engine/prop combination in your AFM/POH/Owner's Manual (as applicable to the plane you're flying).

If you can't find that (say, because it's one of the really old owner's manuals from the 60's or before), get the Operator's Manual for your engine from the manufacturer. That will have power tables for your engine with minimum RPM's for each MP setting and/or max MP's for each RPM setting. If it's a Lycoming engine, see Lycoming Service Letter 114AT for ordering information.
 
Last edited:
Ron Levy said:
The problem with waiting for symptoms of the RPM being too low for the amount of power being developed (i.e., the onset of detonation) is that by the time you see them, the damage may already be done. See http://www.sacskyranch.com/deton.htm for more on that.
Click to expand...

That is correct. The other thing you might notice is one of your propeller blade falling off or crankshaft breaking in two. Those would be quite unpleasant consequences.
 
Ted DuPuis said:
That is correct. The other thing you might notice is one of your propeller blade falling off or crankshaft breaking in two. Those would be quite unpleasant consequences.
Click to expand...

Detonation damage is more typically scrubbed or holed piston crowns, broken piston rings, cylinder heads cracked or broken off the cylinder, or cylinder studs pulled out.

Dan
 
Dan Thomas said:
Detonation damage is more typically scrubbed or holed piston crowns, broken piston rings, cylinder heads cracked or broken off the cylinder, or cylinder studs pulled out.

Dan
Click to expand...

That is correct. I wasn't referring to detonation damage, I was referring to torsional vibration damage.
 
You must log in or register to reply here.
Back
Top