# What should be the thrust weight ratio for takeoff? [closed]

I am doing some calculations for preliminary design of an airplane and I was wondering how I would go about determining the minimum thrust to weight ratio needed for the aircraft to takeoff. Which factors play a role and how are they limiting the thrust to weight ratio?

I admit the question is vague but it is intended to be that way. My team and I are tasked with building a Distributed Electric Propulsion based aircraft and the only other requisite apart from it being fully electric is that it must have a wingspan of at least 3 ft. Everything else is completely up to us to estimate and design.

I am looking for guidance on what factors I have to consider or what mathematical methods I would have to employ in order to estimate a necessary Thrust/Weight ratio

• It depends how long your runway is... – Michael Hall Mar 3 at 4:26
• @Michael Hall: And the airfoil used for your wings, and how much drag your fuselage &c has. – jamesqf Mar 3 at 5:02
• Start with 0.3 and check against your take off run, top speed and climb requirements. Clearly, a glider-like airplane can fly with less thrust/weight than a supersonic fighter. – Peter Kämpf Mar 3 at 6:30
• Honestly, it looks like you haven't done any. What do you expect us to do? Safe rate of climb is a good place to start, lest you spend lots of time following early Wright Brothers research. Plenty of power is available these days, particularly with electric motors. – Robert DiGiovanni Mar 3 at 12:11
• A little information on what you are designing would help. – Robert DiGiovanni Mar 3 at 12:18

• after take-off, you typically need to achieve a certain rate of climb and a certain climb angle to clear obstacles. This is affected by the thrust to weight ratio. If you require a climb angle of 10°, you need a thrust to weight ratio of $$\sin(10°) \approx 0.17$$ just for the climb, even if you have no drag. With a lift to drag ratio of 8 at take-off conditions, you would end up at a thrust to drag ratio of at least. $$\frac{D}{L}+sin(\gamma) = \frac{1}{8}+sin(10° )\approx 0.3$$