chornedsnorkack:
Quote:
Most planes are unable to hover and fly and take off by lift.
There's a comma missing:
Most planes are unable to hover [on thrust alone], and fly and take off by lift.
In other words, the vast majority of the force generated to oppose the weight of an aircraft is aerodynamic lift; thrust generally provides only a small amount of the "anti-gravity".
If you took a typical airliner thrust-weight ratio of 25-35% at max power, and considered a relatively nose-high attitude of 15 degrees, the component of force in the vertical direction due to thrust is sin(15 deg)*(25-35% of tyhe weight) - i.e. about 6-8% of the aircraft weight. In cruise, where the pitch attitude is close to zero, virtually ALL the weight is carried by lift, and all the engines do is overcome drag.
If you assumed for a general case that about 95% of the weight was lift, you'd be fairly close, especially given other factors (like uncertainty of the weight, and trim lift)
Am i right to assume that the structural limits of an aircraft far exceed those that the aircraft is capable of producing on takeoff? i.e. you said transport aircraft are certified to +2g, but on takeoff the g's on the wing will be nowhere near 2g right?
So with regard to structural mtow, is this a useful concept for pilots day to day, it would seem not? Since although the wings could take 2g's, it may be not be possible to load them with that much anyway, and hence impossible to generate that much lift, so a plane loaded to structural mtow, may not even be able to take off right?
It would seem like a more useful one would be performance limited mtow (is there a term for this) since the pilot can then deduce from the weight of the aircraft whether it can actually take off, is this correct?
At a normal takeoff speed (once the plane is safely airborne) it will be flying about 25% faster than stall speed, and will be capable of generating about 1.5'g', so it's true to say that you can't structurally break a plane on takeoff with lift alone. However, the maximum permitted speed in a takeoff configuration (Vfe) will be about 75-100% higer than takeoff speed - say twice the stall speed. Which means that you could in theory generate 4'g' if you went as fast as permitted then pulled as hard as you could. That WILL break something, I'd expect.
MTOW for structural purposes has to cover ALL design manouvres - not just during takeoff, cruise and landing normal operations, but various extreme cases. The extra margin you get in normal operations provides additional safety margin and allows for a decent fatigue life. And, yes, a plane at its structurally certified MTOW can takeoff, certainly on a 'normal' day - otherwise there'd be no point in designing the structure to take the relevant loads!
For simplicity, MTOW (= 'structural MTOW') is the usually quoted value when doing simple comparisons because it's (generally) a single value and is indicative of aircraft performance limits (since most designs will balance out strure vs performance limits to optimise the design). But "MTOW as limited by [brake energy/climb/runway length/whatever]" - there are various performance limits on takeoff weight - is necessarily a complex beast, usually presented on charts in the flight manual as a function of things like temperature, altitude, runway slope, configuration of the aircraft, etc., so it's VERY difficult to choose a single case to take as a datum.
These charts are provided in order that the pilot doesn't have to deduce whether he can takeoff safely - he looks up the conditions of the day, and it'll tell him whether his current weight is ok or not.