T/W Ratios vs Takeoff Distance and Takeoff Speed

[CaseyErickson]

I got a bunch of questions here mostly pertaining to thrust/weight ratios vs takeoff distance, and to an extent takeoff speed.

Okay, for example the SE-210 Caravelle I've been told has a pretty good handling and takeoff performance. However I've looked at the T:W figures and if I recall correctly (at least an early model) had a T:W of 0.2 : 1.0 when fully loaded which is actually quite poor. My only guess is that it's payload was only 3,000 pounds and it probably took a lot more fuel to propel those 3,000 pounds of payload, so even small reductions in pax weight would dramatically reduce fuel weight and thus lighten the plane up a bit, but I'm still not entirely sure there. How does it still manage a good takeoff performance? Was it's takeoff speed considered high, average, or low for a given takeoff weight? (say fully loaded, medium loaded, take your pick)


The CV-880 (Non M) is another plane that really baffles me. It had no LED's, had a very high takeoff speeds, and it allegedly can takeoff in 5,000 feet and land in 5,350 feet at certain weights, and it's T/W ratio was 0.244 : 1.000 fully loaded. While i'm pretty sure its T:W ratio was better at light to moderate loads, to my knowledge there were some planes with far better T:W ratios, and still had the same takeoff distance but also had far lower takeoff and landing speeds. What accounts for these?


Casey Erickson

[Mad (Flt) Scientist]

One possibility: required takeoff distance has to account for several factors - both AEO and OEI, and both "stop" and "go" cases. An aircraft with apparently poor T/W may have very good brakes/spoilers, and so very good "stop" performance. Therefore it might have a good takeoff distance, even compared another aircraft with better "go" characteristics.

[CaseyErickson]

What does AEO and OEI mean?

The part I don't seem to grasp is that the airplane would still have to accelerate to a given speed to takeoff in a given distance right? The CV-880 had takeoff speeds on the order of 170 kts if I recall on an average load and 180 - 190 kts on a full-load, speeds higher than most modern airliners.

Casey Erickson

[Mad (Flt) Scientist]

Sorry, AEO= All Engines Operating and OEI = One Engine Inoperative.

The key to what I was getting at is what conditions you need to accelerate under.

Let's assume two aircraft, one with a VR of 130 knots, a good T/R but shoddy brakes, the other with a VR of 150 knots, a poor T/R and good brakes.

Takeoff distance has to account for both the "go" case, where an engine fails at V1 and the balance of the takeoff is conducted with one engine inoperative, and also the "stop" case, where the aircraft has to reject at V1 and halt on the runway. The higher I set V1, the easier I make the "go" case (because I'm closer to takeoff speed) and the harder the "stop" case (because I'm stopping from a high speed). Conversely, a low V1 makes the "stop" easy but the "go" hard, because I have to continue with one engine inop.

So, to be somewhat extreme about the example, I can conceive of a low T/W, good brakes a/c setting V1=Vr, while a good T/W, bad brakes a/c will set V1 as low as it can (basically, close to Vmcg). In such circumstances its possible for the bad T/W aircraft to still have good performance in terms of certified data, even though day-in/day-out the good T/W a/c will of course be more sprightly. Because it's the failure case where the two aircraft are balanced, and you (hopefully) don't see that too often.