Counterweighted, inverted oil system or both?

Andy_RR · #1 10-30-2007, 09:58 PM

When I was doing my skills test for my Australian PPL conversion, my examiner did his best to distract me by talking about RV's once he discovered I was planning to build one. One of his comments was that if I was planning on doing any aerobatics with a CS prop, make sure I fitted a counterweighted version to prevent revving the **** off the engine when inverted.

Now, it seems that this is due to oil pressure loss due to starvation, so does an inverted oil system (Christen, Raven etc) solve this problem or is counterweighting still required.

My aim is for moderate aerobatic capability for sunday afternoons, but not hard core competition, for which the RV probably isn't the best choice.

Any thoughts from the VAF collective?

Cheers,

Andrew

Kahuna · #2 10-31-2007, 04:38 AM

Quote:

Originally Posted by Andy_RR

My aim is for moderate aerobatic capability for sunday afternoons, but not hard core competition, for which the RV probably isn't the best choice.

Any thoughts from the VAF collective?

Cheers,

Andrew

With this goal state then you need neither. Generally the first thing to go is the fuel. Meaning if your carb'd, then you will loose fuel (engine quits) before oil and before prop. Either way, none of this is required for Sunday joy ride acro.

Norman CYYJ · #3 10-31-2007, 08:58 AM

Stay with positive 'G' manouvers and you won't need anything fancy. No oil loss and no fuel starvation. Go minus 'G' engine quits and when you land you will have a major belly washing to do.

asav8tor · #4 10-31-2007, 09:23 AM

Are you sure he didn't say (or meant to say) "do not get a counter weighted" engine?

Counter weighted crank can be detuned with rapid throttle movements (not the kind of acro an RV should be doing anyway). This can cause crankshaft cracking and other problems.

Russ McCutcheon · #5 10-31-2007, 06:16 PM

No they did mean a counter weighted CS prop, with this type of prop it goes to course pitch when you lose oil pressure rather then going to fine pitch with a non weighted prop. With that said I have a standard MTV-15-B CS prop and a Christen oil system, if and when I do on occasion lose oil psi it never goes very high, my tack records the highest RPM during the flight and the take off rpm is always the highest rpm. I just do acro at cruse power and 2400 rpm and only lose psi if the balls in the valve have not been worked for a wile and I don?t give it a quick bump inverted the first time, once there free than you never lose psi it does fluctuate but not enough to affect the prop. I?m only speaking for the MT prop and the christen system, also a Woodward governor (YMMV).

Russ

Andy_RR · #6 10-31-2007, 09:14 PM

Russ, that's the answer to my question! Thanks.

I did mean counterweighted prop. If you look on the Whirlwind website, you will see that their aerobatic 200C and 400C props are counterweighted, but they are also between 9lb and 37lb heavier than their non-counterweighted bretheren - not that all of this weight would be the counterweighting.

I know you don't 'need' any of this for simple aerobatics, but you could live with a carb-fed O-320 and a FP prop too... (if you know what I mean)

A

RVjim · #7 11-01-2007, 08:31 AM

From what I have heard, loss of oil pressure most commonly occurs in zero G maneuvers. And for the inverted oil valve mounted flat on the firewall, a vertical climb would create the same condition. (The inverted oil valve ball doesn?t know where to go for the oil pickup.)

The standard blade CS propeller will go to low pitch (high RPM) with the loss of oil pressure. This can easily over speed the engine and propeller.
The counterweighted blade CS propeller will go to high pitch (low RPM) with the loss of oil pressure. The will cause the engine RPM to decrease.

Also, in the event of an engine failure, the counterweighted blade CS propeller has 2/3 of the drag of a CS propeller in flat pitch. This isn?t too far off from feathered blades that have 7/8 of the drag of a flat pitch CS propeller.

Regards,
Jim Ayers

frankh · #8 11-01-2007, 10:50 AM

with an IO360 parrallel valve and a Raven invert system I have never lost oil pressure to the prop and have pointed thing straight up several times and of course fly inverted.

I do have a friend who did starve the prop in competion aerobatics doing knife edge passes (not in an RV)

I can see how this might happen but the knife edge's are boring anyway so I don't bother with 'em.

Oh remember when doing a hammerhead to leave the power ON until AFTER you have rotated...A few weeks back I decided to see just how slow I could let the speed drop before rotating.

Unfortunatly I also pulled the power while pointing upwards.

Can we say "suspended animation"..Hold the stick firm and small deflections for the subsequent tail slide...

Not a big deal (as long as you don't let go) but certainly an odd feeling!

Frank