Do I need a licence to fly an airplane?

I have a 20 pound model airplane how much thrust dose it need to fly..?

• i have a 20 pound model airplane, it has good dinamics its mad frome aluminum, i whant to know how much thrust dose it need and i am thinking of putting a pulse jet in it , is there a better engine...? please help...!! thanks...

• Answer:

  You would be better off knowing a few things first, as everyone else is saying. For starters, it's good to have an idea of the A/C drag polar. Also, for a given altitude & Mach number (flight speed) what thrust is required. Altitude & flight Mach determine the inlet conditions for an air-breathing engine, hence the proverbial boundary conditions. They also determine the nozzle exhaust ambient pressure, thus affecting nozzle expansion ratio & flow, hence engine thrust. A given wing design, for example it's thickness and lift over drag, determine what speeds (Mach number) are optimum. There needs to be sufficient speed for lift. But beyond a certain Mach no. the situation is one of diminishing returns; hence there is an optimum flight speed. Twenty pounds is a lot of metal for a model airplane, and it'll either take a lot of airspeed or a lot of wing to supply the required lift to keep it in the air. If you know the wing profile at say 50% span, you might look into a table of wing profiles to get the lift & drag coefficients for various angles of incidence. Then use Q = 1/2 rho V^2 = dynamic pressure and these coefficients for lift and drag to calculate actual lift and drag forces, and see if the A/C will stay in the air. This assumes the engine thrust matches drag. But you must also include total drag of the A/C, hence the importance of the drag polar. This is an exercise in matching engine to aircraft. You must also include the weight of the engine and fuel in calculating weight, else one could end up with an expensive taxi that needs a lot of runway, and never gets completely airborn.