What minumum thrust is needed ?

[dw1122]

Hi all - I searched this forum for "thrust weight" but found no answer. What is the minimum static thrust to ROG and cruise a 50 oz twin motor scale plane ? Or is this a stupid question that has no answer?

I've been told the watts/lb rule but I found from Pcalc that very little watts can generate half the AUW worth of thrust - eg. 2 x Mega 16/15/6 on 6 cells drawing 108W, spinning two 7x5 APC props at 9300 rpm generating a total of 29.2 oz thrust. Wouldn't that be enough to get the 50 oz plane off the ground and cruise comfortably? By going to 7-cell it goes to 158W total, 10600 rpm generating 37.8oz. static thrust. Both cases at about 80% efficiency according to Pcalc.

Thanks in advance.

[Ollie]

For level flight, thrust equals drag. The drag force increases with the square of the air speed. So, a lot depends on the minimum airspeed with a little stall margin. Doubling the airspeed increases the thrust requirement by a factor of four. Because of the drag's dependency on airspeed, the thrust requirement depends on the coefficient of lift of the wing and its airfoil. High lift cambered airfoils fly slowerand therefore require much less power. Also, airspeed depends on the square root of the wing loading. The lower the wing loading the slower the plane will be able to fly and the less power to keep it airborne.

In addition to the thrust required for level flight, even more thrust is required to climb. The greater the surplus thrust, the greater the rate of climb. The surplus thrust required to climb is inversely proportional to the weight. The lower the airspeed, without stalling, the greater the surplus thrust, if any, then the greater the rate of climb.

These are the relationships (with assumptions about airfoils, motor efficiency and prop efficiency) that support the power and weight recommendations that you mention. If you can reduce weight, improve prop thrust and reduce drag, then climb performance will improve.

[jmbig]

Take a look at a trial version of www.motocalc.com
It may answer a lot of question's. CU