Airliners.net

This is probably a dumb, dumb question, but I've always been a turbofan man not really a prop dude.

When an propeller aircraft cruises, the engine is probably running at 2000-3000 RPM or something. At that speed, the propeller wouldn't be very efficent right? Is there some sort of reduction gear that works to keep the propeller spinning at a reasonable speed? Is this anything like a car transmission?

Someone mentioned a "PSRU" to me, but at the time it went right over my head.

Thanks,
DFW

When an propeller aircraft cruises, the engine is probably running at 2000-3000 RPM or something. At that speed, the propeller wouldn't be very efficent right?

Not necessarily; power is proportional to engine speed (RPM), but efficiency isn't always. The efficiency can be plotted against a dimensionless value 'J' (which incorporates engine-speed/RPM), but the graph isn't linear and it slopes both up and down at various points along the x axis - therefore, efficiency can decrease with an increase in engine speed, but not for all flight regimes. High RPM values also bring with themselves the problem of the propellor tips reaching sonic/supersonic speeds, another consideration. In short, it all depends...

Is there some sort of reduction gear that works to keep the propeller spinning at a reasonable speed? Is this anything like a car transmission?

It varies from engine to engine. Sometimes the engine is "direct drive", but there are certainly engines that have reduction gears which, as you said, keep the propellor rotating at a reasonable speed.

Cheers,
QantasA332

Edit: Sorry, I forgot to address "PSRU"... PSRU stands for "propeller speed reduction unit," and it's basically just a name for a gear reduction device. This should help a bit. I'm in a rush, so anyone feel free to expand on what I've said already...

Edit #2: Sorry, the link didn't work...it should be okay now...

[Edited 2004-06-21 04:29:37]

There are a few engines/aircraft out there which use a reduction gear, which simply allows the engine to operate at a higher RPM than the prop. The 'G' prefix on piston engines (eg: GTSIO-520 on Cessna 404) designates that a gearbox is fitted.

Propeller efficiency, however, is more a function of forward airspeed, RPM and the correct blade angle which ensures the maximum amount of engine power can be absorbed and imparted to the air. This is the advantage of a constant speed prop compared to a fixed pitch.

Then consider turboprops... PGB (Propeller Gear Box) very much a required item! 

Cheers,
Fred

On the Saab 340, the prop speed is maxed at 1384 rpm, I think the Saab 2000 is much lower...some where in the 700-800 rpm range...

Anyone remember the Continental Tiara engine with the prop driven off the nose of the camshaft? Turns half the speed of the crankshaft. No further gearing down required.

The prop blades will also have variable pitch. As velocity increases, it changes the angle of attack of each blade, which is a small airfoil. The blades will vary their pitch for different flight regimes to acheive better performance.

Another fun note, efficiency really starts going down by Mach 0.6 - 0.7 when shockwaves begin forming on the blades. This is one of the limits on the speed of prop planes.

Efficiency and power production are also two different things. All propeller aircraft with variable pitch props t/o at lowest pitch-highest rpm. However at high speeds, higher altitudes etc. its more efficient to operate the prop and engine at a larger pitch lower speed. IE a Q400 has three prop speed choices and 4 rating torques which tend to correspond to prop speeds. For normal t/o you would operate the engines at NTOP (normal take-off power) 90% tq and props at 1020rpm, then for climb moving the condition levers to the 900 rpm detent would both slow the props to 900 rpm and increase tq at the power lever detent to I believe its 91%. And finally for cruise you would move the c/l's one more detent slowing the props to 850 and increasing tq to 93%. Besides feather and beta range these are the only available prop speeds, 1020, 900 and 850. However complicating things worse is the ability to change rated tq. at any prop speeds, by pressing a engine control switch. MTOP, NTOP, MCL, and MCR (Max T/O, Normal T/O, Max Climb, and Max Cruise.) And of course adjusting your power levers between flight idle and the rating detent would also adjust tq. Anyhow hope that's somewhat helpful I've always found these computerized aircraft amazing such a different way to control the engines and props then the old school cables. Have to admit even a monkey could run these engines.

Late
PC

EMBQA,
950/1100 low/high RPM on the 2000. The 340 has variable prop RPM from 1180 to 1384.


The 340 PGB gear ratio is 15.9:1 while the 2000 uses 13.98:1.

Cheers,
Fred

I'm not a prop, turboprop fan myself, but actually you "change" the propeller speed with PROP RPM handle that is usually between throttle and mixture. Though DC-3 has it in the left :P. Sure it doesnt change gears or something like that, but it changes the prop blades angle and basically you set prop rpm with it.

EMBQA:
"On the Saab 340, the prop speed is maxed at 1384 rpm, I think the Saab 2000 is much lower...some where in the 700-800 rpm range"

The Saab 2000 uses Rolls Royce AE2100 turboprop engine which is also fitted to Lockheed C130J.

The prop spins around 1024rpm, with core engine spinning around the 14700rpm mark. the prob gearbox has a ratio of 13.8:1.

the prop RPM remains between 99-101% unless in LSGI - low spd gnd idle.

The new J Hercs does not have prop "condition" levers as such, each eng having a single pwr lever with LSGI button behind it. I'd imagine the Saab 2000 being similar


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Undies- Please explain further, the new hercs do not have adjustable prop speed? Only normal setting and this low speed ground idle, you speak of? Or does prop speed simply adjust with power setting. I'm very confused.

Late
PC

As I stated above, the Saab 2000 only has high/low prop RPM, 950/1100 RPM, depending on mode of operation as selected on the power management panel.

I dug the books out for more useless info: The power turbine at 100% is at 15.375 (as was indirectly specified above) while the gg reaches 100% at 15.265 RPM.

Undies,
as you can see, the fact that it is the same basic engine doesn't mean that the RPMs etc are the same. Unless I'm mistaken the C130J uses AE2100D3 engines while the 2000 has 2100As.

Regards,
Fred

Fred.
Correct, we use the AE2100 - D3 variant. I'd be interested in learning more about the Saab2000's eng management if you have any info.
The "low prop RPM" selection would be for BETA (ground) range? & "high RPM" could be used in both; BETA & ALPHA (flight) range?

PC. This may take a while, I'll do my best using words.
Like Fred mentioned about the Saab2000s, we also have a similar system - normal ground idle & low speed ground idle (NGI/LSGI). We start them in LSGI which give us a prop RPM (Np) around 72%Np in BETA range.

IN A NUTSHELL:
- in BETA; pwr lever request blade angle.
- in ALPHA; pwr lever request fuel flow (horsepower).

Np - prop RPM
Ng - gas generator (compressor) RPM

BETA range (excluding reverse):
The pitch control unit - PCU, is commanded by the FADEC.
Thottle position inputs (elec signals) to the FADEC will directly control/determine the blade pitch via FADEC output signals to the PCU.
The fuel pump metering unit - FPMU, regulates fuel supply to maintain 99%Np.
There is a high pres pump/overspeed governer - HPP/OSG which has been isolated from the prop control (oil) circuit by the BETA enable valve - GBEV, within the PCU (it's switch is in the throttle quadrant).

ALPHA range:
Throttle position inputs to the FADEC now directly control/determine FPMU's fuel delivery. The prop HPP/OSG now maintains 100%Np by; regulating the air load (maintaining optimum AOA) on the prop blades.
drive coarse = more load, decrease Np.
drive fine = less load, increase Np.
Try this in a manual car to demonstrate the ALPHA range;
cruise @ a set speed, as you start going up a hill maintain speed with throttle but don't shift down gears = your fuel flow will increase (more fuel req'd to compensate extra load) with speed & eng RPM unchanged. Inverse effect for a decsend.

BETA reverse range:
same as ALPHA range with blades driving fine to increase air load.

hope this clears it up.

At least 99% of turboprop fixed-wing aircraft engines have always been geared; is it 100%? Has there ever been an ungeared turboprop?

Like he said, American-made light-aircraft piston engines have a "G" in their designation if they're geared. But bigger engines-- radials for airliners-- were always? geared. The crankshaft turned 2 to 3 (?) times the speed of the prop.