Turboprop Torque and Drag

[Pugachev Cobra]

Hello all, this question goes to the fellow turboproppers out there!

The ATR 72-200 has 4 blades, while the -600 has 6 of them.

Based on a colleague description, could it be that in the -600 with the added pair, you would need less torque than in the -200, say, on a final approach? If the torque you needed was around a number like 22, is it correct to assume that in the -600 you would need just 20?

Also, when you reduce power, is it safe to assume that the increased drag from the extra blade pair translates to reducing less the power lever than you would on the -200?

In other words, smaller thrust corrections in the 6-blade than in the 4-blade engine, since it generates more thrust if you add more power, and more drag if you reduce power, than you would do in the 4-blader?

I hope that wasn´t too confusing!

Thanks in advance for your valuable inputs!

[VinRouge]

If you were to add 2 more props, you MAY get more power for the same torque; it all depends upon the design. The benefit of higher solidity ratios are higher maximum total power with a corresponding increase in torque. The only way you could otherwise increase power would be an increase in prop speed (or prop area but then you get a lot of wetted drag, requiring more torque!), but then transonic and compressibility effects will then limit your np (prop efficiency) as you start to lose a tonne of energy in weak shocks (unless you have a very advanced and difficult to manufacture prop design).

As to your second question, its a bit difficult to say as most modern turboprops are FADEC controlled, my type (J hercules) wouldnt let you go into negative torque (min in flight is around 100 torque per engine) which is a nightmare in an aircraft that has no spoilers, its really difficult to get draggy if you are high on approach. Hercules does NOT like sideslip due to nasty and possibly fatal rudder overbalance at slower speeds.

The FADEC takes control of lag and responsiveness, the ideal being care free and also similar handling between marks of a similar type. You wouldnt want changes in thrust output for the same throttle motion imho, as it may introduce dynamic feedback (PIO of throttles or FADEC saturation where FADEC can lag input power requirements).

As for traditional FCU controlled turboprops, again, the designer will have set criteria for throttle displacement/increase in torque. I may be wrong on this, dont take my word for it as the only FCU controlled aircraft I have flown is B200 and Jetstream!
I have never flown in the types you mention, I suspect the reduced torque on finals has more to do with a more efficient prop design or more aerodynamic airframe.

Edit:

having thought about it, I suppose you COULD get a drop in torque as the tip speed for 6 props will be lower than for 4 and thus you will have less parasitic drag/compressibility effects and as a result, and thus require less torque. You have to weigh this against 6X sources of drag as opposed to 4X, as with all design, there will be a compromise for your design spec as defined. A modern composite design will crap all over your old school canoe paddle like on the H model herc; the majority of the extra power on the J comes from the prop design and its increased propulsive efficiency, not the bigger donks (the core/LP are derated by nearly 30% from their original max design value for longetivity).

[Shawn Coyle]

Power + torque x RPM X some constant. Need to know that you're measuring the same things.
Engine power is the power required to turn the props - thrust is a useful byproduct - you can't determine the thrust from the torque being used.

[aerobat77]

thats correct shawn coyle !

torque is just the turning force on the proshaft . a given force with a given rotational speed is the power given to the prop.

how much thrust the prop generates from this power is a question of its efficiency. in general speaking props with a lower amount of blades are more efficient due to less interferences from the blades.

props with more blades tend to be quieter in cruise.

[Pugachev Cobra]

I knew eventually someone would get to me about messing with different terms.

I'm no turboprop pilot, and just relaying a colleague's doubts.

In essence, what I want to know if in a pilot's perspective, the "power lever" corrections especially in a final approach scenario, would need to be smaller and within a smaller range, for a 6 blader than that required for a 4 blader.

Would 2 more blades increase both the produced thrust in a add power situation, and also increase the drag in a decrease power situation?

[VinRouge]

You can't determine from the information provided.

Is it fadec or not? If not, what design guidelines did the manufacturer use to determine throttle displacement/power increase?

[Roger Greendeck]

The ATR uses and 'Engine Electronic Control' EEC to regulate power (think of it as a poor man's FADEC) so you are not directly controlling one engine perameter with the power lever. Like most turbo-props power is set by using a torque figure but as mentioned above this a measure of turning force on the shaft and for it to be a meaningful indication of thrust output other perameters must be in the intended range. For example if the prop is feathered your can get lots of torque and no meaningful thrust.

From a pilot perspective I doubt there is a significant difference in power lever feel when flying.

In regard to the added drag of the extra bladed, there is a significant difference to the shape of the blades in the six blade version. This has a significant effect on the drag. I highly doubt that the manufacturers selecected a new prop that has more drag for the same thrust output.

[no-hoper]

The power lever is mechanically connected to the hmu and regulating the power.
There is no fadec installed at any ATR.
On the -500 and -600 a feathered prop shows zero torque.

[Tu.114]

The formula for the power delivered by the engine P=T*rpm*c has been mentioned.

Consider two airframes equal except for a 4 vs 6-bladed propeller slapped to the same basic engines. Put them in the exactly same, unaccelerated flight situation, let´s say approach at one defined IAS, configuration, mass and weather. The thrust requirement is a function of drag and will therefore be equal on both of the aircraft.

Now seeing that the thrust both aircraft require is the same, the torque required to achieve this thrust will be based on the individual airframes propellers efficiency. The more efficient propeller system will require less torque, as less energy will go to waste in the system.

It may therefore well be that the 6-bladed prop installation shows less torque required for an approach in otherwise equal conditions. But this is more of an indication of better efficiency in that flight phase than anything else.