Where does the fuel go?

[Eamon]

I am flying behind a PT-6A 600 hp turbine in a Caravan.

It has an adjustable pitch prop that is at 1900 rpm at full forward & 1600 full aft.
I was told in training that normal is to leave it full forward all the time & that bringing it back doesn't do much.

My "throttle gauge" is read in Torque & you reach limits on the torque gauge before you run out of throttle.

The Tq gauge has the limit of 1658 at 1900 rpm & 1970 at 1600 rpm.

I have been experimenting.


So normally I am flying around at 1900rpm & 1658 tq and lets say I am truing 170 kts with a fuel burn of 330 pph

If I pull the prop back to 1600 the torque moves up to about 1850 but itself & the plane slows down a few kts and the fuel burn stays the same.

If I then bring the torque to the limit of 1970 (Still at 1600 rpm) , the plane goes back to 170 kts & the fuel burn goes to 360 pph.

It the prop eating up the fuel with drag?

What would Cessna put an adjustable prop on the Caravan instead of a fixed?

Why/when would you need to pull the prop back ever?

Eamon :dance:

 

[wsuffa]

Does altitude make any difference, Eamon?

 

[poadeleted20]

It's all about prop and engine efficiency. If you pull the prop back, you're increasing blade pitch, which means increasing AOA, which means changing the L/D ratio of the prop blades. The best place to be is the blade pitch that gives max L/D, as that will give the most lift (thrust) with the least drag (torque).

Based on your "experiment" (does your employer know you're playing with his power settings?), it would appear that pulling the prop back to 1600 RPM causes a reduction in "lift" (thrust) as evidenced by the reduced speed due to the slower prop rotational speed, while it increases drag as evidenced by the increased torque. However, since you didn't touch the power lever ("throttle"), you haven't changed fuel flow. The reduction in speed, then, is due to the reduced prop efficiency at the same engine power output.

Now, when you push the power lever up, you increase engine power, and the prop pitches even more to hold the RPM, which is why torque and fuel flow go up. Since the prop pitches more, blade AOA goes up and with it so does "lift" (thrust) and airspeed. However, the higher drag at that AOA is giving you sub-optimal L/D, so you don't go any faster with the higher engine power than you did at the lower power with better prop efficiency.

The reason for the adjustable prop is so you can maximize power when you need it to take off and climb without the tips going too fast, and then reduce power for cruise without having the prop turning too slowly to be efficient.

 

[Greebo]

Sorry - L/D ? Define please?

 

[Skip Miller]

Sorry - L/D ? Define please?

Lift to drag ratio.

-Skip

 

[Greebo]

Donkey shins!

 

[Eamon]

Based on your "experiment" (does your employer know you're playing with his power settings?),



The reason for the adjustable prop is so you can maximize power when you need it to take off and climb without the tips going too fast, and then reduce power for cruise without having the prop turning too slowly to be efficient.

Thank You


Actually it was put to me as... "There is no reason to pull the prop back. It doesn't do much. Some people do & think that is makes it quieter, but really all it does is lower the tone, not the noise level" I am allowed to except for the 5 min each day when I do "engine trend"

Ok I got it now.... I guess having "Reverse" is more the reason then prop eficiancy on the Caravan.

That I use all the time.


Eamon

 

[Eamon]

Does altitude make any difference, Eamon?

No. It changes the true speed a little, but everything else is the same.

I fly the first leg west (1/2 hr) @ 4K 2nd leg (1 hr) @ 8K

First leg east (1 hr) @ 7K 2nd leg (1/2 hr) @ 5K

When you fly higher, you reach the ITT/5 temp max at 700 C (Cessna has a 740 C limit, my company has a 700 C limit) before you reach max torque.

But if you compare max at 4K (1658 Tq) or max at 8K (700 ITT/5 which is about 1550 TQ) you still true about 170 kts.

You burn a little less fuel up higher but the speed is about the same at definitive max.

ITT/5 is read on a gauge just like EGT.

 

[Dave Siciliano]

Eamon:

Where I see slowing the prop on my Bo as beneficial is when changing from go fast mode to far mode. Reducing rpm can be accompanied by reducing power. High speed cruise for me is wide open throttle, 16.5 gph mixture and 2500 rpm. When I want to go far instead of fast, I reduce rpm to 2300 or even 2100. Throttle can stay open if lean of peak, with mixture being reduced automatically as you turn back the prop, or MP can come back (which reduces fuel flow) if ROP to get more range. Your prop may be set up much differently and, as Ron said, may not be efficient at lower rpm.

Have you ever tuned in a distant airport on the GPS and fiddled with MP, mixture and prop to see how eta and fuel remaining at the distant airport is affected? Very interesting experiment. In my plane, the biggest difference is derived by lowering rpm!!

Best,

Dave

 

[Let'sgoflying!]

"Fuel Rem @ KXYZ 0000.0 lbs" Don't you hate the look of that! But it often does say that as you climb out, at least until you get up to cruise altitudes. "OK its up to 200lbs now... 250....yay, 300lbs"

 

[Earthroamer]

"Fuel Rem @ KXYZ 0000.0 lbs" Don't you hate the look of that! But it often does say that as you climb out, at least until you get up to cruise altitudes. "OK its up to 200lbs now... 250....yay, 300lbs"

Dave, you should take off in the baby lear's like the LR24 or a LR31. It will actually say, -1100lbs and flash red at you on takeoff. Makes you feel warm and fuzzy.

Brent

 

[gismo]

I am flying behind a PT-6A 600 hp turbine in a Caravan.

It has an adjustable pitch prop that is at 1900 rpm at full forward & 1600 full aft.
I was told in training that normal is to leave it full forward all the time & that bringing it back doesn't do much.

My "throttle gauge" is read in Torque & you reach limits on the torque gauge before you run out of throttle.

The Tq gauge has the limit of 1658 at 1900 rpm & 1970 at 1600 rpm.

I have been experimenting.


So normally I am flying around at 1900rpm & 1658 tq and lets say I am truing 170 kts with a fuel burn of 330 pph

If I pull the prop back to 1600 the torque moves up to about 1850 but itself & the plane slows down a few kts and the fuel burn stays the same.

If I then bring the torque to the limit of 1970 (Still at 1600 rpm) , the plane goes back to 170 kts & the fuel burn goes to 360 pph.

It the prop eating up the fuel with drag?

What would Cessna put an adjustable prop on the Caravan instead of a fixed?

Why/when would you need to pull the prop back ever?

Eamon :dance:

First of all, your prop is changing pitch as you change power or airspeed even if you leave the prop control in the same position. A fixed pitch prop wouldn't give 1900 RPM except under certain conditions, and would likely be far less efficient in cruise and/or offer poorer climb rates. Overspeed in a descent would be an issue as well. Given that very little is needed to add a pilot control to the prop governor once you have a constant speed prop, I can imagine the control was included even if the benefit was small.

Second, if you have a "free turbine" engine (AFaIK the Caravan has a PT-6 which is a free turbine design), I think your engine's fuel consumption is pretty much unaffected by the prop RPM. With a "fixed shaft" turboprop all the turbine speeds are proportional to the prop RPM and when you slow the prop you decrease the airflow through the engine and the fuel control unit should compensate by reducing the fuel delivery.

Now the power going into the prop is exactly proportional to the prop RPM times the torque, so if the prop efficiency is exactly the same at 1900 as it is at 1600 (not likely) you would need to boost the torque at 1600 until it was equal to the torque at 1900 multiplied by 19/16. And IIRC the torque output of a free turbine turboshaft engine is pretty much proportional to the fuel flow so you are likely to see the best efficiency above 1600 RPM. This is very different from a piston engine where the fuel flow is roughly proportional to the power not torque output. It does seem likely that the prop's efficiency itself is better at 1600 than at 1900, but the difference is probably quite a bit less than the change in the engine's efficiency over the same range. There must be an RPM for a given condition (cruise speed or climb rate) that produces the maximum total efficiency and it would occur where the product of the engine and prop efficiency peaks. Chances are it's near 1900 RPM though for most conditions.