atr 42 torque rating

[khurram butt]

I am confused with the atr42-500 torque available graph given in the FCOM. It shows that take off torque and RTO with 100%NP is less than climb and cruise torque with 82% NP. Why is that ?

[stubby1]

Power = torque * rpm.

do a simple exercise with the above formula and you will get rto power max, then t/o , then climb & last cruise.

So the torque does not give the full picture of the power developed.

hope it helps.

[khurram butt]

Thats fine but my question is that how is the torque at take off with 90% NP and RTO with 100% NP less than torque of Cruise and Climb with 82% NP. The atmospheric conditions being the same. (The graph given in the power plant chapter).

[Escape Path]

Doesn't it have to do with increasing density altitude, decreasing temperature and the effect of ram air at climb and cruise?

Similar effects that makes max EPR on a jet engine higher at cruise than at takeoff

[khurram butt]

I have been looking in different books but cant seem to figure out the answer. You have a point but I think it is more to do with the efficiency range of a prop. A prop being more efficient at a faster forward speed then at a slower forward speed, keeping a constant rotation. I dont know if that is the correct expanation !!

[Clandestino]

What stubby1 has tried to explain with his complicated mathematical formula is that torque is neither power nor thrust and it's entirely possible that lower torque at high RPM requires more power from engine and delivers more thrust than higher torque at low RPM.

[khurram butt]

Isnt torque and power output directly proportional, higher the torque higher is the power output ? or am i wrong !!

[no-hoper]

khurram butt,pls read and try to understand the answers given by stubby1 and Clandestino.
A friend of mine made this calculation 15 years ago for the 42-500:

SHP=NP*TQ/4,17

Back to your example:
SHP for TO: 100% NP* 90% TQ/4,17 gives 2158 SHP
SHP for Climb2%NP*110%TQ/4,17 gives 2163 SHP

The reduced NP for Climb and Cruise is a function of the Powemanagement selector switch helping to reduce the noise level.

[Clandestino]

Isnt torque and power output directly proportional, higher the torque higher is the power output ?

As power is proportional to both torque and RPM, this hold true only if RPM (Np for turboprops) remains constant. As takeoff and climb use different prop RPM setting, you can not simply compare power by comparing torque.

Khurram, are you rated on ATR, in ATR groundschool or just curious?

[khurram butt]

Under going transition on the atr. Was flying jets for the last 15years, they are so much simpler to fly and understand.