Warbird Information Exchange

I did a short local flight today from Denver Jeffco to Boulder. The weather was about as fine as it could be. It was perhaps 75*, calm wind, blue skies and some of those precious Fall days that we call Indian summer. If the Indians are responsible then they are indeed the noble savage! I just did some rolls and some simulated engine out glides. I didn't shut the engine off, just reduced power to idle and 1800 rpm so the prop has less drag. One thing that might surprise a pilot is how flat the nose attitude is, the nose is barely below level flight, just a few degrees down. The best glide speed is 130 knots and the descent rate is almost 2000 feet per minute. The book does not specify a glide ratio, I assume it is around 12 or 14 to 1 about like a Mustang. If one lost oil pressure and the prop was in flat pitch it would be a much steeper descent. Where the skill is needed is to transiton from 130k down to the 85k approach speed, at the runway if one is lucky enough to have one handy. There were other people out flying and lot's of gliders up also. Too fine.

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Bill Greenwood
Spitfire N308WK

Bill Greenwood wrote:

If one lost oil pressure and the prop was in flat pitch it would be a much steeper descent.

Bill, can you tell me more about how that whole setup works? If you lose oil pressure will your prop automatically go into flat pitch or are you saying that if you happen to be in flat pitch and then you lose oil pressure, you can't change the pitch?

Thanks.

From what I'm learning in the commercial license process, most adjustable props are kept in pitch with oil pressure, or some other mechanism that forces them to stay at the right angle of attack - but with a loss of oil pressure, depending on the design, the props will automatically move toe the pitch at which they receive the least resistance from the system.
On twins, most often the propellers are designed such that in the event of an engine out situation or oil pressure loss to the system, they automatically adjust to the lowest possible angle of attack relative to the aircraft's normal flight path - and tend to go the the "feathered" or full pitch position in order to reduce drag on the aircraft so that the remaining engine has less "work" to do. In single-engine aircraft this is less desirable because it would make the engine harder to restart - because you'd have more resistance. Think of being in a canoe, and trying to paddle - your paddle will do your canoe the most good speed wise if you use the flat of the paddle to push the canoe along, but if you just wanted to get the paddle moving, you'd be better off going skinny. So, they design single engine aircraft with adjustable pitch props to go to a "flat" pitch that's easier to restart.
I hope that makes sense.

Ryan

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Aerial Photographer with Red Wing Aerial Photography currently based at KRBD and tailwheel CFI.
Websites: Texas Tailwheel Flight Training, DoolittleRaid.com and Lbirds.com.

The horse is prepared against the day of battle: but safety is of the LORD. - Prov. 21:31 - Train, Practice, Trust.

WarbirdI, I believe that if you lose oil pressure in most singles the blades will go to flat pitch, that is less bite, most rpm, and most drag. I think it takes oil pressure to move the blades to coarse pitch against the wind resistance. Your question is a good one, and frankly I am not sure, I intend to find out. Loss of oil pressure could come from an large engine leak or seizure so the pump didn't rotate. Not a good, situation! You'd be going down faster than a bucket of fried chicken at Oprah's house! This happened to Dan Martin racing his P-51 at Reno. He said it took a dramticaly low nose angle to hold the best glide speed, which from memory is 150 mph in a 51, and the descent rate is so much that it was hard to catch it in the flare.

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Bill Greenwood
Spitfire N308WK

I was told during instruction that if you couldn't get a restart and you have some altitude that it would be helpful to carefully raise the nose until the prop stopped, you would cut the drag dramaticly. You would only have the small drag of the frontal area of the individual blades. This would extend your glide considerably because you wouldn't be fighting the flat plate area of the rotating prop. Of course it would only be applicable in the right scenario.

Everything follows the path of least resistance, especially me!

There is a balance between the centrifugal twisting moment of the prop blades and oil pressure. Depending on the specific prop design a loss of oil pressure can cause the blades to stabilize at the mechanical stop in one direction or another.

Multi-engine piston planes generally have a feather pump so it doesn't matter how much engine oil pressure you have. I think that many turboprops end up feathered with a loss in oil pressure.

Generally speaking...

Centrifugal twisting moment (CTM) and aerodynamic twisting moment (ATM) both always want to move a prop towards fine pitch. CTM is a function of RPM (higher RPM = more CTM) while ATM is a function of airspeed (higher airspeed = greater ATM) The oil pressure acts against these two forces. This is why a runaway prop tends to head towards a finer pitch/high drag angle and an overspeeding condition instead of heading towards the higher blade angle and lower drag position with it's more favourable underspeeding and easier feathering. If you've ever heard a prop really overspeed uncontrollably it'll make the hair on the back of your neck stand right up (but then, I scare easily!)

But that's only the physics of it, and a only a very general description. Smarter guys than me can explain it much more thoroughly.

Dan and Ryan.
Both true.
In most piston powered single engine aircraft with a loss of oil pressure the blades will go to low pitch high rpm.
In most piston powered multi engine aircraft the blades will go to high pitch low rpm with the loss of oil pressure. The prop will also have a lock out so the blades don't feather overnight in the hangar. It is normally set so if the blades don't feather by X rpm they won't feather. An unfeathering pump or accumulator on small twins is not unheard of but not a regular item. On large aircraft it becomes much more standard.

I had a Two position Hamilton Standard 2D30 that had counter weights that took it to high-pitch-low RPM for cruse and oil pressure took it to low-pitch-High RRM for take off or climb. All props are not created equal and you need to know your prop, some use oil or air pressure some are electric !!

Steve

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"Any excuse is good enough if you're willing to use it!"

Some aerobatic CSU's work in the reverse sense with a loss of oil pressure (some have a pressure accumulator so that it doesnt happen inadvertently in a low pressure state such as prolonged knife edge or inverted) which tends to bring the prop to low RPM to avoid overspeed.

Generally speaking oil pressure would normally drive a piston within the prop hub that moves the prop through to coarse pitch (low RPM) and the removal of that pressure will allow centrifugal twisting moment and the assistance of the counterweights if fitted through to fine pitch (high RPM).

I think Ive got that the right way around but its late and maybe my brains asleep.

Guys thanks for the info. It seems my guess is correct for Merlins and probably most singles. The prop seeks flat or fine pitch (high rpm) and needs oil pressure to drive it to coarse pitch (more bite, low rpm). There are numerous exceptions like the Ham Stand. I am flying the Spitfire up to Ft. Collins Loveland this morning for its annual. Right on schedule a cold front is moving in for Haloween. A little freezing rain or snow will toughen up all the kids out. AND ONE FINAL THING ABOUT WIX; THERE IS ALWAYS PLENTY OF RPM ON THE SITE, SOMETIMES A LACK OF MANIFOLD PRESSURE

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Bill Greenwood
Spitfire N308WK

Yeah, but we don't fault you for that, Bill.

Gary

I am glad I got the plane in for annual yesterday while the weather was good. Today, Halloween started off cold and wet, even some snow and icy roads up in the mountains. It cleared up this afternoon, but is still about 45* F. I've been without a plane one day and am already having withdrawal symptoms. Flying has been so nice lately with Indian Summer, and it looks like we have a string of more good days coming through the weekend. I did not do any actual engine landings, as the airport was swarming. With their 3 mile finals it is sometimes enough just to find a spot to get in the pattern.

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Bill Greenwood
Spitfire N308WK

I can sympathise easily with you Bill as my 140 is also at annual. It was really nice and warm last Friday when I took it in. I flew a good three hours before landing at my AMO.

Haven't flown since (commercially doesn't really count) and I am beginning to feel the withdrawal symtoms too ! Hmmm, winter will be looooong !

The only difference I guess will be the thickness of our respective wallets at the end of the day ! I get away with about $1200 for a complete annual without snags on the trusty 140 which is nothing, really !

I can't imagine how much it is going to be for the Spit so I guess it is a clear sign that I could not afford it ! hehehehe...

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Michel C
Thousands of a/c pics at Passion-Aviation

Michel, good luck with your Cessna. That $1200 will about take care of the oil change on the Merlin. After you remove the four cowlings and the air filter assembly, there are 3 cleanable oil filters and 2 large disposable cartridge ones. One oil analysis kit, and 12 gallons of Phillips 25w-60. Mix with an expert A&P for a day or two. While they are at it, they remove the valve covers and check the torque on the heads and banks. We rarely find this loose, but I understand the Packard in the 51 does. I don't know why it differs on similar planes. Also important is a visual check of all the valve gear and interface between the the cam lobes and the followers. If all is ok, make new valve cover gaskets and reassemble all, with an engine wash down. Say a prayer of thanks, then on to the gear swing, and all the other inspection areas and checks.

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Bill Greenwood
Spitfire N308WK