Another Stupid Newbie Question

Top_prop · #1 06-25-2008, 07:50 AM

Situation: Lots of time on Turbines (~3000hrs). Only 25hrs in censna's 172/152 from a few years ago sum up all my light civil experience.... So 0 time in a recip with an adjustable pitch prop.

From reading some stuff here its seems the inflight adjustable props on recips use engine oil to set the pitch, and it seems that their is some debate on how much authority one has over that pitch with the engine inoperative...

So I'm asking for an education. Please post replys or links to places I can get smart on this stuf... (I'm not in a position where building makes sense right now, so I want to start by getting smart).

Thanks,

Tom

Rick S. · #2 06-25-2008, 11:09 AM

It's my understanding that in order to generate enough oil pressure to move the prop, the engine has to be turning over at least 1700 rpm. So I translate that to no control over the prop unless you manage to get it to windmill past 1700 RPM if the fires out.

4kilo · #3 06-25-2008, 01:10 PM

The typical RV installation has a constant speed prop controlled by both hydraulics (oil pressure) and springs. The springs push the prop to low pitch, while oil pressure forces the blades towards high pitch. With this set-up, with no oil pressure the propeller will be at low pitch (high RPM).

In my installation, an IO-360 with a Hartzell constant speed, the engine will produce enough oil pressure to reduce the RPM to 800 with the throttle in idle. The oil pressure is down to about 45 PSI in this condition, while it runs about 65 to 70 in normal operations (2100 to 2700 RPM).

Some other propellers are configured differently. Aerobatic propellers and props for multi-engine airplanes typically use springs to drive the blades towards feather (low RPM), while oil pressure decreases pitch (towards high RPM).

Pat

Top_prop · #4 06-25-2008, 05:46 PM

Quote:

Originally Posted by 4kilo

<snip> Some other propellers are configured differently. Aerobatic propellers and props for multi-engine airplanes typically use springs to drive the blades towards feather (low RPM), while oil pressure decreases pitch (towards high RPM).

Pat

Thanks for the reply... I'm very familiar with two Turbo Prop engines, PT-6A and the T-56... The PT-6 has springs that drive it all the way to feather, while the T-56 has mechanisms that will lock the pitch where it is if the hydraulic oil pressure driving the prop from a low pitch towards a high pitch is lost (you may not feather but you will probably get some thrust out of it if the turbine is still turning)....

I want to get smart on what you'd strap to a Lycomming and how it works. So please continue to school me

I've got to admit having springs drive the prop to lower pitch is alarming... the rotation of the prop tends to do that itself...

JHines · #5 06-25-2008, 05:55 PM

The spring is necessary in the system to have a reliable "low pitch" force so the governing control loop will work properly.

And yes, it is scary when a C/S prop fails so it won't govern. I've had it happen so that I was probably limited to 50% power or so without exceeding redline.

gvgoff99 · #6 06-25-2008, 06:48 PM

Tom, There are a number of good reference books on this and a bunch of other stuff to do with small planes. William Kershner's book "The Advanced Pilot's Flight Manual" is one. There are any number of good references and I hope that after this post others will mention their own personal favorites. My son-in-law, Darryl Insley, instructed in the T-6A in the event you know him.

osxuser · #7 06-25-2008, 07:11 PM

Whats scary is an underpowered piston trying to push a near feathered prop at all. Trust me low pitch is a good thing.... Should be able to push about 18" of MP at full RPM at flat pitch, depending on what the low-pitch stop is.

Top_prop · #8 06-26-2008, 05:04 AM

Quote:

Originally Posted by gvgoff99

Tom, There are a number of good reference books on this and a bunch of other stuff to do with small planes. William Kershner's book "The Advanced Pilot's Flight Manual" is one. There are any number of good references and I hope that after this post others will mention their own personal favorites. My son-in-law, Darryl Insley, instructed in the T-6A in the event you know him.

I'll check this book out... what I'm looking for is probably more of a discussion on the various manufacturers and their methods, and the benefits and detractors of those methods.

As for T-6A's I've got zero experience with them other than seeing them on the flight line. I know a few fellas that fly them but not Darryl... We won't get getting our first T-6's until 2009. (when I mentioned PT-6's, it the engine in the BE-200 and the T-34, though the T-6 also has a newer ~1100HP PT-6 as well)

thanks for the help please keep it comming!

SabreKGB · #9 02-20-2009, 09:37 PM

Are there Constant Speed mechanisms out there for the engines RVs mount that act to feather the prop in cutoff/engine failure? I'm coming from a background with most of my time in T-6s (which can feather in the event of engine failure, and the prop reacts like a big speed brake in idle when it's not feathered) and wondering about the effects of a failed engine with a fine pitch on the prop. I know glide distance in the T-6 is severly reduced without feathering...same for a C/S Van's? Is failure of the pitch control mechanism a common emergency? Would you be able to maintain level flight in such a case?

Of course, all my time in pistion powered aircraft was in fixed pitch prop planes, so i'm relatively unfamiliar with C/S reciprocating ones. Apologies for any ignorant questions.

osxuser · #10 02-20-2009, 09:59 PM

Yes, you can get a propellor and governor setup for Multiengine operations, as most of those use pressure to DECREASE pitch. No pressure, full pitch/feather. These props have interlocks to prevent them from going into feather if they loose oil pressure at idle, since starting a piston engine with the prop in feather is nigh to impossible.