adjustable props

[mjoekingz28]

I was thinking back to either when I was in class studying different configurations or maybe it was when I took a ride in a Bonanza, but I have a question about adjustable props.

I think when your prop is adjustable and you need to climb or add power, you don't use the throttle......is that correct? I remember something about manifold pressure being what is adjusted.

This could be for turbochargers that I am thinking about, but it was on my mind and just thought I would ask and see if anyone could provide information or point me in the right (or left) direction. Depending on which way you are facing.....ah my sad attempt at humor.

 

[Cujet]

My Cessna has a constant speed prop. The prop will operate, within reason, at the selected RPM. (can't expect 2700RPM with an idle throttle setting, for example)

So, on takeoff, I push the throttle to max, and sadly, can't feel any acceleration. The prop setting is "max" and the engine gladly spins the prop to 2700.

As I move forward, the speed increases, and the prop automatically increases blade pitch to maintain 2700 RPM. Remember, the prop is "screwing" through the air. The faster you go, the more blade pitch required.

After 10 minutes, I've only made my way to 500 feet above the ground and I can safely reduce power to save fuel. I can pull the throttle back OR reduce RPM OR both! Any combo is acceptable in my airplane.

After another hour or so of turtle slow acceleration, I can reach 165 or more MPH in level flight. The prop blades have automatically increased dramatically in pitch via the prop governor pumping oil to the prop itself. I've configured the aircraft to run at 2500 RPM, and 25 inches of manifold pressure. It's a good combination that results in a nice smooth ride, and makes me feel good about saving fuel.

The reality is that my engine is rated for continuous full output of 200HP at 2700 RPM.

Not all engines are rated this way. Some have a "Max Continuous" rating. An example would be 2500 RPM and 25 inches manifold pressure. It's typical for engines to have a 5 minute take off rating, and then a max cont rating.

Some turbocharged engines have really interesting max cont ratings. An example might be 37 inches of manifold pressure and 2550 RPM.

In each case, the pilot selects the RPM via the prop control.

The pilot then selects manifold pressure via the throttle.

Yes, there are many aircraft that do this complicated task for the pilot and they have a single power lever.



EDIT: In climb, it's generally more effective to use maximum RPM. But some props and some aircraft do not show much benefit from max RPM in cruise. My Cessna will gain only 1Kt at high altitudes if I choose 2700 RPM vs 2550, full throttle.

 

[DeepFriar]

Originally Posted By: Cujet


Yes, there are many aircraft that do this complicated task for the pilot and they have a single power lever.




We are finally, at long last, seeing single lever break into somewhat widespread use. Cirrus, up to now, has used a simple mechanical system within the throttle quadrant to track along with throttle input. Diamond uses a very sophisticated FADEC on their system to control the Mercedes core engines they use which also yields single lever power/prop control. GE is preparing to make a strong run at the dominant P&WC PT-6 with a single lever throttle/prop turbine. Evolution, now that they're going to be a full Part 23 builder, is going to use Lycoming-s electronically controlled 540 iE2. I think this will contribute much to IFR safety most but in other uses as well. I could even hold out hope that we might someday see autothrottles based on evolution of these same technologies making IFR that much more safe. Here are some constant speed data points for the OP:

{url}https://m.youtube.com/watch?v=9nzwhg6obM8{/url} Cirrus system


I don't have the url for it but the FAA's Aviation Maintenance Technician Handbook, Powerplant, Vol. 2 has a good discussion of how they work. Those handbooks are excellent by the way and provide a good, if basic, technical overview. Worth looking at.

A search for "constant speed prop basics" turned up a practical guide/discussion in Flying Magazine If you would care to go looking.

 

[mjoekingz28]

Originally Posted By: Cujet
My Cessna has a constant speed prop.

In each case, the pilot selects the RPM via the prop control.

The pilot then selects manifold pressure via the throttle.




Exactly what I was wanting to hear! Thank you.

 

[tom slick]

On the C-130 with T-56 turboprop engines the engine runs at 100% constant speed (13,000 rpm) and the propeller adjusts. In fact the throttle give the propeller input and the propeller coordinates with the engine.

 

[Cujet]

Just a side note about the "single power lever" and constant speed prop. It seriously limits your options, as you cannot choose individual settings.

Take a look at this picture carefully. I have "chosen" to run 2000RPM, and about 17 inches manifold pressure. You can see my throttle and prop controls pulled way out. The mixture control is set to operate "lean of peak". You can see the EGT gauge on the left, and the bars are quite low (lean of peak EGT)

Look at the fuel flow gauge, 5 gal per hour.

Look at the speed, 124 MPH.

This is outside of typical operations for my airplane of 165-175MPH. However, it's a wonderful and quiet speed to operate if you want, say, 25 miles per gallon!

In addition, pulling the prop control all the way back (low RPM) allows me to nearly feather the prop during descent or engine failure, for significantly reduced drag, and/or increased glide. My airplane has a 9 to 1 published glide with the prop windmilling and prop control full forward. I don't know the "spec" with the prop control back, probably around 13 to 1. From 9500 feet, I can glide 20 miles with plenty of reserve altitude, with the prop control pulled back.

A single power lever will not allow any of the above. You cannot, for example, operate at 2000 RPM, partial throttle, lean of peak. (perfectly OK for the engine under such low loads) and unless an additional lever or system is added, you cannot feather the prop on a single engine aircraft to extend glide.

While the Cirrus is an amazing aircraft, it cannot be "forced into efficiency" by careful management. It won't matter if all the pilot wants to do is cruise at high speed. However, I find that 50 mile trips use far less fuel when flown as shown above. The 165MPH and 12GPH saves only 6 minutes but uses twice the fuel.

 

[mjoekingz28]

OT

What are the gauges on either side of the vertical speed dial?

One is heading 260 and the other 130.

 

[Cujet]

Originally Posted By: mjoekingz28
OT

What are the gauges on either side of the vertical speed dial?

One is heading 260 and the other 130.


To the left is the directional gyro (DG) . It's powered by vacuum, has an internal gyro and when I remember to set it to the correct heading, will display my heading at all times. Such as during a turn. So I know when to roll out of a turn. A magnetic compass behaves badly during acceleration of any sort, hence the "DG".

To the right is the VOR/ILS indicator. The vertical needle called a CDI (course deviation indicator) , when set up properly, shows whether the course is left or right of the aircraft. The horizontal needle is for "glide slope" for an approach to an airport. It indicated whether the glide path is above or below the aircraft.

So, if the glide slope needle is at the top of the gauge on an approach, the path is above you, and you are WAY TOO LOW. Good idea to fly back up to the path.

 

[DeepFriar]

Very good points Cujet. The loss of flexibility appears to be a real drawback. Makes me wonder if other methods have been engineered into the systems to address some of those concerns. For instance with FADEC electronic fuel injection as practiced by Diamond and Lycoming the system, just like the ones in our cars, is going to be working constantly to maintain the most efficient burn for a given power setting. Want it quieter/more fuel efficient? Dial it back. The engine is going to tune to the most efficient burn for that. I wonder if the pilot will be able to put in a bias, lean or rich, if he wants to depart from the stock mapping. The capability is there to do so. The prop control is where the flexibility loss, as you point out, seems most acute. Here I wonder if the system could go to alternate mapping when you're operating in the different regimes - climbing, descending, landing, etc. The ability to constantly monitor and adjust to temp, density altitude, power requested by the pilot, etc. Would seem to allow for pretty fine grained control.

 

[DeepFriar]

I followed my own advice (for once) and looked up that FAA technician manual mentioned earlier. Lo and behold, in the chapter just before the one on props is the one on lubrication and cooling systems. And its very good. Clear language, good illustrations (oooo, color) and discussion of both recip and turbine oiling/cooling. If you want to look the document is:


FAA-H-8083-32-AMT-POWERPLANT-VOL-2.PDF

 

[DeepFriar]

AvWeb reported today that IS&S has been granted an STC for autothrottles on the PC-12 which, of course, has the PT6. So even without FADEC, the PC6 is still hydromechanical,apparently IS&S has figured out how to control prop torque and temps along with the throttle. This is the first application I know of for autothrottles on a turboprop. I always thought it would happen on a single shaft engine first because of the better throttle response but kudos to IS&S and Pilatus for leading the way. First aircraft equipped will be the Prez of the PC12 Owners Association.

 

[tom slick]

The C-130J with AE2100 engines has just throttles, no condition levers.

 

[Cujet]

Originally Posted By: DeepFriar
Very good points Cujet. The loss of flexibility appears to be a real drawback. Makes me wonder if other methods have been engineered into the systems to address some of those concerns. For instance with FADEC electronic fuel injection as practiced by Diamond and Lycoming the system, just like the ones in our cars, is going to be working constantly to maintain the most efficient burn for a given power setting. Want it quieter/more fuel efficient? Dial it back. The engine is going to tune to the most efficient burn for that. I wonder if the pilot will be able to put in a bias, lean or rich, if he wants to depart from the stock mapping. The capability is there to do so. The prop control is where the flexibility loss, as you point out, seems most acute. Here I wonder if the system could go to alternate mapping when you're operating in the different regimes - climbing, descending, landing, etc. The ability to constantly monitor and adjust to temp, density altitude, power requested by the pilot, etc. Would seem to allow for pretty fine grained control.


At this point, FADEC does not seem to allow any of the operational modes I discussed. The manufacturers simply choose to operate rich of peak EGT and burn more fuel than a well tuned and well managed, fuel injected Lycoming operated carefully.

Rotax has newer series of engines with FADEC and conventional automotive style fuel injectors. Their carburetor engines have modest output, and therefore modest fuel consumption. However, their carb engines fall far short of injected Lycoming BSFC numbers, until now. The FADEC versions come very close, but that is largely because the ignition timing is allowed to advance, not because the engine runs lean of peak, in the "most efficient zone".

An well tuned, injected Lycoming with magnetos will regularly achieve a BSFC number of 0.38 pounds of fuel per HP hour in cruise flight and even better with electronic ignition! That is very difficult to beat, no matter what you do. Automotive engine conversions fall far short, as do the carb Rotax engines.

 

[DeepFriar]

Yes, I agree with you, it doesn't answer how to have more prop control. I keep thinking there must be a way to "command" the prop function in these schemes that we don't know about. I'll be very interested in seeing how Pilatus has mechanized the autothrottles on the PC12. Nextant has had a long ordeal with the FAA while certifying single lever on the twin GE engined (Beechcraft) G90xt. It'll also be interesting to see how they manage the props. The issue reminds me a little of the automatic versus manual transmission debate - more control and involvement with the manual versus ease of use with the automatic. I'm mostly agnostic about it except for one thing. On that proverbial ugly, cold dark night when your teeth are getting rattled and the pucker factor is maxed and even your curb feelers are out, I would take all the help I could get.

 

[Cujet]

The PC-12 has a conventional prop governor, that is simply "locked" in the full RPM position. Gas turbine engines do well operating at a single, effective RPM. No magic to the PC-12's single lever control setup.

The PC-12 does have a selection for "ground idle", to reduce noise.

I could, for example, operate my Cessna this way, by leaving the prop control full forward all the time. It's completely acceptable on my engine to run at 2700 RPM all the time.

 

[DeepFriar]

Originally Posted By: Cujet
The PC-12 has a conventional prop governor, that is simply "locked" in the full RPM position. Gas turbine engines do well operating at a single, effective RPM. No magic to the PC-12's single lever control setup.


You know more about the PC12 than I do so I defer to you. And yet...and yet...it has taken 50+ years to qualify a prop autothrottle? I'm guessing that there is more going on than firewalling the prop. A lot more. But it's just a guess from me at this point, I have no way of knowing. I'm sure we'll find out more as time goes on.

 

[Cujet]

Originally Posted By: DeepFriar

And yet...it has taken 50+ years to qualify a prop autothrottle? I'm guessing that there is more going on than firewalling the prop. A lot more. But it's just a guess from me at this point, I have no way of knowing. I'm sure we'll find out more as time goes on.


I'm sure the FAA and other authorities have some "difficult to achieve" criteria with regard to prop/turboprop autothrottles. I am unaware of the requirements. Certainly, once configured for takeoff, setting the throttle for takeoff in a PC-12 is just a matter of pushing it smoothly, all the way forward. Any servo motor could do this. And that same servo motor could reduce power lever position for the climb, cruise and descent settings, as necessary.

In cruise, we fly 720 degrees on the PC-12/45 and 780 degrees on the PC-12/47

 

[Yah-Tah-Hey]

In 1969, I went to take my P exam and the FAA guy took my out to a rather worn out looking Martin 404 with puddles of oil under each engine and I thought I was in for a real @** kicking. He
ask me a few basic questions about the Hamilton Standard Hydromatic propeller (I think that's what they were called) pitch and feathering mechanism and I got them all right. Paid the exam fee and that was that. Shortly after I finished my A&P I learned that the tired old 404 was the plane that crashed on Loveland Pass in Colorado killing the starting team, coaches and AD. Seems the crew decided to take the "scenic" route over the Front Range rather than fly up to Laramie,Wyoming and then proceed west to Logan,Utah. The accident report reads like a comedy of errors.A 404 coming up Clear Creek Valley must have been quite a sight for those on I-70. As a side note: The 707 had been in service since 1958 and the FAA exam was still on recips.

 

[DeepFriar]

Atlanta-Huntsville, Huntsville-Atlanta
Atlanta-Huntsville, Huntsville-Atlanta
Atlanta-Huntsville, Huntsville-Atlanta....

Southern Airways Martin 404

I recall one day looking at a pin or bolt or something vibrating and backing out of its hole in the wing root. It then departed the aircraft as they say. I called the Stewardess (yes snowflakes, Stewardess). She didn't even look out there and just said, Don't worry sugar, happens all the time.

 

[Yah-Tah-Hey]

Originally Posted By: DeepFriar
Atlanta-Huntsville, Huntsville-Atlanta
Atlanta-Huntsville, Huntsville-Atlanta
Atlanta-Huntsville, Huntsville-Atlanta....

Southern Airways Martin 404

I recall one day looking at a pin or bolt or something vibrating and backing out of its hole in the wing root. It then departed the aircraft as they say. I called the Stewardess (yes snowflakes, Stewardess). She didn't even look out there and just said, Don't worry sugar, happens all the time.
I grew up in Albuquerque, and in February of 1955, TWA Flight 260 404 struck the west side of Sandia Crest only minutes and thirteen miles from the Sunport. The Crest was in the clouds and faulty navigation instrument took the Martin into the Sandias rather than up the Rio Grande and a short flight to Santa Fe. Thirteen passengers and the three crew members were killed. Some of the wreckage is still visible from the Sandia Peak tramway. The site is now popular for hikers. Very steep and rocky climb.