# How to compute thrust to weight ratio of a Cessna 172?

Cessna 172 has weight about 1 ton and it's engine produces about 213 Kg of thrust, and it flies great.

So if I make a plane with a weight of 100kg including pilot and fuel so will 15 kg thrust good for the plane, and if not please recommend me engine which is perfect and please also recommend me the best prop size.

What is thrust to weight ratio of Airplanes. Sorry for bad English. thanks

• The kilogram is not a unit of force, but of mass... May 10, 2020 at 20:57
• @xxavier you know units in aviation are confusing. Forces are often expresseed in Newton, often in lbs. length are in foot for altitude, meters for visibility, Nm for range. pressure in hPA or in inHg depending on location,... I think aviation people like to confuse others (I see no other reasonable explanation) May 11, 2020 at 8:26
• @ Manu H You're right... Even in 'metricated' countries, there are still people that use the kilogram (strictly speaking, kilogram-force or kilopond) as a unit of force. No problem there, provided that when a mass is involved, it should be reckoned in the obscure 'mass technical unit', sometimes termed 'hyl', TME, UTM... If someone insists in also using kg as a unit of mass within that context, the constant 9,8 must be present... All that is a mess, a source of errors, and fortunately, the 'technical units system' is today almost dead... May 11, 2020 at 9:01

You can see, from this picture, that in straight and level flight, the drag D is equal to the thrust T. You can see, also, that the lift L is the same as the weight W.

So you can easily calculate the value of thrust T, provided that you know the L/D of the whole aircraft for a given airspeed.

If, for example, that L/D is 9 for an airspeed of 35 m/s (as in many ultralights), and the mass of the airplane es 350 kg, then the thrust will be:

350 x 9.8/9 = 381 newton, where that 9.8 is the acceleration of gravity in m/s/s

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• So as not to confuse, the "weight" of the plane must be in Newtons as well, to generate a "ratio". But yes, expressing thrust in "lbs" is "old school". For your plane, the ratio is about .11 for level flight. May 10, 2020 at 22:02
• In SI units, weight is always expressed in newton, because it's a force... Drag and lift are obviously forces too, as is thrust, and they should also be reckoned in newton, within the SI. May 10, 2020 at 22:36

A good estimate for a personal aircraft thrust requirement would be more like 25% of aircraft weight, and remember, the "ton" of Cessna includes fuel and pilot too!

So: 1 ton x 2000lb/ton x 1kg/2.2 lb = 909 kg Cessna

213 kg thrust/909 kg weight = 23.3 % thrust to weight ratio.

But a lot depends what your plane design is. You can test thrust requirement for level flight by gliding, then multiplying weight x altitude lost/distance flown = thrust.

So it helps a lot to have a good, strong wing too, and more thrust will be needed to climb. 15 kg may not be enough for your plane, but as long as you are on internet, you may wish to study "ultralight aircraft" to get some better information.