What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Engine out best glide CS prop

Some interesting testing has been done to answer this question.

The summary statement would be pull prop to full COARSE pitch for maximum reduction in prop-induced drag and engine pumping losses.

For most hydraulically-controlled constant speed props, loss of oil pressure results in the props being spring-loaded to the fine position. Should the engine failure be accompanied by a loss of oil pressure most props will default to fine pitch, becoming very nice aerodynamic brakes.

Aerobatic props generally are the opposite, being spring-loaded to full coarse pitch in the event of oil pressure loss. This is done to prevent a major overspeed condition which could result in severe mechanical damage (think prop or engine coming off the airplane - the engine is already toast so we're now only concerned about saving the souls on board). Keep in mind that a loss of oil pressure could occur when the aircraft is on a down-line, rapidly converting altitude into speed, speed which would cause a full-fine pitch prop to turn at crazy-high RPM.
 
Well, if you have no oil pressure you won't be able to change a CS prop. If the engine is free wheeling in the wind you should have some oil pressure, pull the blue knob for course pitch. If your engine is seized don't bother pulling the blue knob. Fine pitch had more drag.
 
Educate me. I know my CS will go to fine pitch with loss of oil pressure. I'm not sure about the drag penalty of a fine pitch prop being worse IF the choices are:

a windmilling prop in coarse pitch or a stopped prop in fine pitch.

So my side-question is will the prop stop if the engine has lost oil pressure and the prop goes as flat as it can, or will it keep turning - because if so the coarse pitch is clearly the better option, but if not the fine pitch may well be less drag.

I know a seized engine won't windmill but I am speculating about situations where pressure is very low but not zero. Are there any such, that could last more than a few seconds?
 
Did some testing this morning in the RV-4, IO360, Hartzell CS. Downwind at 100 mph, about a wingtip (1 /1/4 mi) from runway. Pulled power, prop high rpm/fine abreast of numbers and immediately turned base, trimmed for 90 mph/best glide, no flaps. Didn't make the runway by quite a bit. Same scenario but full low rpm/coarse, was about 100 feet over the numbers, dumped full flaps and made a nice 3 pt.

Prop setting makes a big difference!

I'm thinking the default with no oil pressure is fine/high rpm, not sure. I'm pretty sure the prop will windmill unless efforts are made to stop it like nosing up to stall. Not sure if a windmilling prop will produce enough oil pressure to operate prop.
 
Last edited:
Well, if you have no oil pressure you won't be able to change a CS prop. If the engine is free wheeling in the wind you should have some oil pressure, pull the blue knob for course pitch. If your engine is seized don't bother pulling the blue knob. Fine pitch had more drag.

Plus one, in case of an engine related emergency one of my first task is to pull the blue knob all the way and drag reduction, if wind milling is very noticeable


Educate me. I know my CS will go to fine pitch with loss of oil pressure. I'm not sure about the drag penalty of a fine pitch prop being worse IF the choices are:

a windmilling prop in coarse pitch or a stopped prop in fine pitch.

So my side-question is will the prop stop if the engine has lost oil pressure and the prop goes as flat as it can, or will it keep turning - because if so the coarse pitch is clearly the better option, but if not the fine pitch may well be less drag.

I know a seized engine won't windmill but I am speculating about situations where pressure is very low but not zero. Are there any such, that could last more than a few seconds?

Good point and my test has shown the wind milling a prop, even at full coarse, is more drag than a not turning prop. However, it does take a bit to get the prop to stop wind milling and that is the time that is spent during an emergency. It does take some practice, at least for me, to do that with the calm head. I have shut down my engine, even though high and above an airport, the rush and nervousness is still there.
 
However, it does take a bit to get the prop to stop wind milling and that is the time that is spent during an emergency. It does take some practice, at least for me, to do that with the calm head. I have shut down my engine, even though high and above an airport, the rush and nervousness is still there.

In my airplane, I cannot stop the engine rotation even at stall speed with the prop in full coarse unless I close the throttle also. The difference in pumping losses across the throttle plate is that significant. With the throttle open, you are simply not going to stop it, not enough drag.

IO360 with Whirlwind CS.

If the engine fails down low and theres no time to jack with it, then yes just pull the blue knob back and fly the airplane into the crash. If you're up higher at a decent cruise altitude, then you have time to pull up and stop rotation, to improve sink rate and pick a landing site.
 
If the engine fails down low and theres no time to jack with it, then yes just pull the blue knob back and fly the airplane into the crash. If you're up higher at a decent cruise altitude, then you have time to pull up and stop rotation, to improve sink rate and pick a landing site.

The most important message here is that your individual aircraft may vary. I know, for instance, that in my aircraft the altitude loss associated with trying to stop the prop is greater than the altitude loss associated with letting it windmill.

Know your airplane and fly it for what it is, not for what applies to somebody else's airplane.
 
The FAA handbook says, commercial single engine PTS now ACS (Airman Certification Standards) for power failure at altitude and associated emergency approach and landing procedures to "Configure the airplane in accordance with the POH/AFM and existing conditions." If the POH/AFM says to go to Low RPM/Coarse (Blue knob full aft/back) you have to do it on a check ride.

Some factory POH/AFM for complex planes with controllable pitch props do not mention this and therefore in a check ride you don't need to do this. However some examiners might ding you for that. Some POH/AFM do mention going to coarse/low RPM.

It has been awhile since I played with this. Coming into the pattern slow, power back, going to high RPM/fine pitch nothing happens, RPM stays the same, because Prop is already at fine pitch. I recall it would make a difference with engine at idle on a Piper Arrow. I am not sure you have enough oil pressure to make the prop move?
 
Last edited:
I agree with Canadian Joy.
In testing I stopped the prop, took slowing down to just over stall, than speeded up to a best glide speed-- the prop started windmilling again...
Best to just fly the plane to best glide speed and look for the best place to set it down (usually a spot right under you!). In that situation you will probably be so rattled, even the basics will be overload.
 
Last edited:
had an engine out experience a few years ago on a Falco equipped with a Lyco O-320 coupled to a Hartzell prop, pics here: http://www.aerofun.ch/falco-1.html
Conrod failure during climb out with associated oil loss. Prop kept windmilling and the resulting braking effect had us rejoin Terra Firma in a short time. The ROD was way higher than anticipated = rapidly diminishing options...

Still thanking my guardian angels :cool:
 
Zürich W, Switzerland, Europe, Earth, Milky Way, known Universe...

well said. it is estimated that the known universe is only 1%, the rest is unknown. some hypothesize the the big bang is a recurring cycle and we could be on the billionth big bang cycle. knowing this, we should all be flying more.

should I prime or not prime, that is the question. :)
 
Last edited:
Test your plane

I did glide testing both ways, fine and coarse in my 9A with Hartselle CS with engine at idle. Coarse was actually a little worse, assumed not enough oil pressure to hold and it kept cycling from fine to coarse causing more drag. On my 7 I just did worse case glide tests at fine. Stopped a fixed pitch on my 12 and did not notice a significant increase in glide distance. As someone mentioned, probably lost any gains by slowing to near stall to stop the prop.
 
had an engine out experience a few years ago on a Falco equipped with a Lyco O-320 coupled to a Hartzell prop, Still thanking my guardian angels :cool:
WOW... Great job flying the plane to the ground. Beautiful Falco. Beautiful country to fly over. Glad you are OK.

Zürich W, Switzerland, Europe, Earth, Milky Way, known Universe... well said. it is estimated that the known universe is only 1%, the rest is unknown. some hypothesize the the big bang is a recurring cycle and we could be on the billionth big bang cycle. knowing this, we should all be flying more. should I prime or not prime, that is the question. :)
Pretty deep for RV forum and a slight thread drift, but 1%, my estimate is that is correct divide that by infinity (indeterminate). The theoretical physicist say a lot of things, because it is "SCIENCE!" Scientist are humans and have ego at play. I'm of the mind that what we do know about creation of earth, life on it and all that surrounds from solid scientific standpoint (Observable Phenomenon, Mathematics, Data supported by repeatable experiments) is infinitesimal. We may never know, except for theories, which is fine as long as those theories are not "believed" on faith only. Big fan of general and specific theories, but to your point we know little about the universe.
 
Last edited:
Pretty deep for RV forum and a slight thread drift, but 1%, my estimate is that is correct divide that by infinity (indeterminate). The theoretical physicist say a lot of things, because it is "SCIENCE!" Scientist are humans and have ego at play. I'm of the mind that what we do know about creation of earth, life on it and all that surrounds from solid scientific standpoint (Observable Phenomenon, Mathematics, Data supported by repeatable experiments) is infinitesimal. We may never know, except for theories, which is fine as long as those theories are not "believed" on faith only. Big fan of general and specific theories, but to your point we know little about the universe.

matter and anti-matter. particles and anti-particles. per the laws of physics: if you prime there will be an equal and opposite aircraft that exists somewhere that is not primed.
 
My feeling is that it doesn't matter whether your aircraft glides 1.3nm/1000 or 1.5 - provided you know and train that way and know the correct techniques for judging and adjusting the glide profile. I can't imagine training with my prop way coarse because if you forget to put it to fine before a GA it is going to ruin your whole day.

There are, of course, certain situations where you require a better glide - a few miles off-shore or over inhospitable terrain for instance. In that case, I see nothing wrong with trying the coarse setting if there is some oil pressure to move the prop. But you are never going to make much difference. Maybe a couple of miles from 10,000'. Otherwise, I think it is preferable to concentrate on the landing rather that messing around with controls to achieve a questionable advantage.
 
Back
Top