Power to weight ratio??

[Robini]

Hello everyone!
I was looking for engine thrust on the net and then thought about
power to weight ratio on som aircraft...Anyway i been pretty surpriced
when i saw some aircrafts power to weight ratio.
PS. took the strongest engines and HGW version of the planes :E

B752: 116t/87=1.333...

A330: 233/142,2=1.638...

B744: 420/248=1.693...

And now to see that the B752 has a very good power to weight ratio,
while the A330 and B744 is pretty close to each other...
I mean can this be true??
For example take the B757 and the A330.
1,333...*142,2=189,9
i mean if the B752 are at its MTOM and the A330 are at 189,9 t
would they use the same amount of runway??
And the same with the A330 it's pretty close to the B744..
Or has it something more to do with construction on the wings etc...??
Thanks for all help! :ok:

[Mad (Flt) Scientist]

That's certainly not the way you usually express thrust to weight ratio - usually it's T/W, which routinely gives a number below 1.0 unless you're a fightery type thing.

The next thing to watch for is that the thrust used is the "static thrust" - the force that the engine provides at zero airspeed. That force drops off quite quickly with airspeed, so the "real" T/W at the end of a takeoff run may be noticeably less. There's also increased drag on the aircraft, further reducing the acceleration.

Finally, will two aircraft at the same T/W have the same performance? No. Because they may have very different takeoff speeds, so despite having the same acceleration, if one takes off (to be extreme) at 100 knots and the other at 150knots, the second one will need 2.25 times the runway of the first one (roughly).

(Furthermore, much takeoff performance is actually based on the "what if" case of an engine failure. Losing one out of four is different to one out of two, obviously. Looking at the OEI (one engine inoperative) T?W values may in some cases be as meaningful...)

[chornedsnorkack]

So, comparing the different airplanes, the relevant factors are:

Wing loading/Stall speed (The second is not completely given by the first, because there are some differences in dirty Coefficient of lift). This is the only part relevant to landing performance in first approach (obviously, because the engines are not producing much thrust on landing whether they are functional, installed but failed or never installed on the airframe).

And thrust-to-weight ratio (which changes with airspeed somewhat). This is what allows aircraft to take off (obviously a glider lands just like any powered plane but cannot take off, and a plane with engines failed or underpowered by design will need a long runway to gather speed, but lands just as easily as an overpowered plane or a glider).

Some numbers for thrust/weight can be seen at

http://www.airliners.net/discussions/tech_ops/read.main/212727

See Post 9 for twins, including 757-200 (high at 0,341, but less than what 717-200 has at 0,347, also less than ERJ-170, ERJ-135 and CRJ-200) and A330-300 (low at 0,280, only MD-80 has as low as 0,263).

Post 21 has trijets and quads, including B744 (0,286 is high for a quad).

Post 48 has small jets: note that Citation Sovereign has as much as 0,380.

Why higher thrust for twins?

Because of the "what if" cases.

Engine performance does have some relevance to landing, because pilots who are not flying gliders like the ability to go around in case they miss the approach. And the number of engines is relevant on takeoff, because pilots like to be able to climb out of a takeoff if one engine fails late during takeoff run (when it is too late to abort and stop before end of runway).

So, twins have more thrust installed, to keep some spare for one engine out. Which means that with all engines functional, they climb faster than trijets and quads.