Thrust to weight ratio and lift

[ayoyoayoyo]

hello

a plane would need a thrust to weight ratio of 1 of fly straight up without losing speed
but how can a plane in level flight maintain its altitude with <1 TWR

since to maintain a level flight you need to have lift = weight
so your engine needs to supply enough power to generate enough lift, how can it do this with <1 TWR

thanks

 

[rcgldr]

It's because the lift to drag ratio of a wing is much less than 1. In the case of a high end glider, lift to drag ratio of the glider is 60:1. If glider and pilot weigh about 1500 lbs, then while in a steady decent with a forward speed of 60 mph the decent rate is 1 mph, which translates into the fact that it only takes 4 hp (the power from gravity on a 1500 lb object decending at 1 mph) to go forwards at 60 mph.

The idea of a wing is to minimize the amount of power it takes to generate lift force, with some compromises due to cost to build and size. The force is related to momentum change of the air, mass flow times velocity, while the power is related 1/2 mass flow times velocity². By increasing the amount of mass flow (moving more air, usually with a longer wing span), the required power is reduced.

 

[ayoyoayoyo]

:) thanks for your answer