Otter prop management

[Wing Nut 0]

I'd like to know more about prop management. How various settings effect speed, fuel comsumption, vertical speed, etc. Where should the prop be set for landing?

These issues dont seem to be covered in the manual. There are just settings for takeoff, climb, and cruise, but now how performance is effected.

[pbearsailor 9]

Full forward on takeoff.

96% for climb.

76% for cruise.

Mostly what you're doing is lowering the noise level in cruise.

Full forward for landing, usually up when you add your final flap setting.

Real one will not go into reverse unless the props are fully forward--note the "Reset Props" light on the annunciator panel if you don't.

I tended to fly cruise on fuel flow with the power levers--300 pph each side. That's roughly 90 US gallons an hour. You can push it up more, but you don't go much faster and you're not going fast in a Twin Otter anyway.

Brakes you'd tend to be very careful with as you've still got lift in the landing roll and can slide the tires and blow them out easily. Don't need them usually anyway. If you have to use max reverse and max braking, you'll likely not have room to takeoff anyway.

cheers,

steve

[gmark 0]

pbear

If I haven't messed up the picture, are these gauge values roughly right?

Gmark

[Wing Nut 0]

gmark,

According to the manual your prop should be 75% as you appear to be cruising flat and level.

Listen to me ... I'm asking questions and giving answers

[Ken Stallings 1]

In a real aircraft, you have a pilot's manual which among many other items includes a section on power settings. Normally it includes either a charted graph or tables with various suggested combinations of power (manifold pressure in a piston and torque in a turbine) and RPM.

The short answer is that you get better fuel economy the more your lower the RPM setting at a given power setting (manifold pressure or torque).

The charts and tables show various settings that can provide best cruise speeds, best cruise fuel economy, or best combination of the two.

Sometimes, pilot preference based upon actual experience results in slight variations of the manual's suggestions.

However, there's a limit to everything. If you run with a high power setting and very low RPM setting then you are putting enormous stress on the engine. This can lead to blown cylinders, camshafts, or other failures.

It is also possible to overspeed the prop, exceeding the max rated RPM value that the gears in the prop assembly can endure for long periods of time.

To achieve max power at stand still on the ground, you want full RPM set. As the speed of the plane increases you have to progressively flatten the blades to maintain the same efficiency. This is because as the flow of air increases due to airspeed, the prop at a given angle of attack loses thrust. So, to keep the thrust constant, the blade must flatten out the angle of attack. A constant speed prop has a governor that automatically rotates the blade angle to compensate for increases and decreases in airspeed. It does this by detecting the RPM value the pilot manually set with the prop lever, and then maintaining that RPM for chances in airspeed and air conditions.

Ken

[gmark 0]

Aye, Wing Nut. She came around to 76% and straight and level heading toward Portugal. I've issued two life jackets and called for volunteers to lighten the load - with the promise of a two bottles of Rothchild '98 should they reach the shore... evil laugh.

Thanks!

GMark

[Ken Stallings 1]

One final point.

You can set max RPM with high power even at red line for short durations of time, such as takeoff. But once airborne, you should throttle back and reduce RPM's both to lower noise and also reduce stresses on the engine and propeller gearing.

You set max RPM's prior to landing for two reasons. First, if you have to go around, at the low speeds you initiate go around a max RPM setting will give you the best possible thrust. Second, while on final approach, a max RPM setting with low power will actually act like a mini speed brake, helping you to steepen the approach some without gaining unwanted airspeed.

It's all about the relationship between power and RPM settings. In a complex aircraft they are used together to achieve various desired outcomes and various phases of flight.

Cheers,

Ken

[pbearsailor 9]

Yes, gmark, that's right about where I run it.

Most of what is in this thread is right, but I just want to add a couple more thoughts. Really want to separate pistons from turboprops in the thinking. The PT-6 on the Twin Otter really doesn't care if you push it hard as far as torque and RPM go. What wears out turbine engines is heat. If your EGT or ITT or T5 or however you measure it is cool and not up by the redline, it simply doesn't care what you do. Likely in most cases, the engine maker would rather you run RPM up pretty high, but the airframe maker is thinking passenger comfort and wants to reduce RPM in cruise due to cabin noise (and the Twin Otter is very noisy).

Like I said, my power levers are likely set on fuel flow as more fuel burn just won't make the Twin Otter quick. You can't do much else as it's unpressurized and pretty well limited to flying under 10,000. The Cheyenne we've been talking about in the other thread has a huge advantage in fuel burn even though the engine is virtually identical. That's because it can climb and here a turboprop is like a normally aspirated piston--the higher you go the lower the fuel burn. The turboprop like the Cheyenne can get pretty high, so it's normal to be up in the flight levels, up to FL250 or so. There, you'll have a total burn of about 60 gallons an hour at 250 knots or so, while the same engine in the Twin Otter is flying on fuel burn to limit it to 90 gallons an hour at 140 knots or so. Big big difference.

cheers,

steve

[Wing Nut 0]

Thanks Ken and steve. I knew I could count on both of you for definative answers.

[Ken Stallings 1]

Thanks, Steve! I wasn't really sure what was the critical power indicator for the PT-6 -- torque or ITT.

I knew in the C-130's that over torquing the engines was a huge no-no. We monitored the ITT (that's what we called it) but the torque was how we rated power output for the Allison engines on the Herc.

As always I add your information to my growing list of stuff about operation of P&W PT-6 engines.

Cheers,

Ken

[Hiropemonkey 0]

I know the otter has the prop pitch change. Does the visual model change the pitch also. I've seen in some screen shots that on the visual model the props are feathered with the engines off. So far, I've not been able to recreate this. Any one have any ideas? I'm running 1.11 with FSX. Thank you in advance.

[Finn 873]

Nope

The propeller blades are not animated in order to save FPS.

But it seems that it is hard for You to tell anyway, so I hope it isn´t a big issue ;-)

But there is a cheat:

When rotating they are shown flat, and when stopped they ar shown feathered (apart from the float version, that doesn´t feather whilst stopped in real life)

I hope that I didn´t spoil Your imagination :-D

Finn

[Hiropemonkey 0]

no, it doesn't spoil it at all. I just noticed that some of them do it, and others don't. As far as being flat when spinning and feathered when stopped. I don't really mind it not transitioning in between, as you don't really see it any way.