Can a single seater airplane fly with 50 lbs of thrust
A key thing you need to provide is the ratio of L/D, which is also equivalent to the ratio of Cl/Cd, where Cl and Cd are the lift and drag coefficients.
Thrust required for level flight = D/L ratio * Weight.
Without knowing more information, including L/D ratio and Weight, the answer to your question can only be &quot;maybe&quot;.
If the L/D ratio is 10:1 or better and the total weight is 500 pounds, then the answer is &quot;yes&quot;.  Increase the weight or decrease the L/D ratio, without changing the other variables to compensate, and the answer becomes &quot;no&quot;.
Also, naturally, we have to know the thrust available at the airspeed that we are actually going to fly at, which may, or may not, have been what you were referencing-- (I assumed it was, in the preceding paragraph.)
Edit: since you've now provided the total max weight at 400 pounds, the answer is &quot;yes, as long as the L/D ratio is at least 8:1, and the full 50 pounds of thrust is actually available at the airspeed at which you are flying.&quot;  An 8:1 L/D ratio is certainly doable.  Of course there would be no excess margin to allow for climbing, if the thrust were exactly equal to drag (50 pounds in the case of an 8:1 L/D ratio) and no more.  If you can get the L/D up to, say, 15:1, the performance will be a lot better.  (I'll leave the actual calculation for achievable climb angle in that case up to you, but all the required information is present here.)