What is Engine Down Thrust?
Remote controlled airplanes need the so called technique “Engine down thrust” to their engine installation in compensation to the force that generated from the wing. High-wing and Shoulder-wing type of plane are common for this reason.
Engine down thrust angle is related to the Pitch Moment that effect the airplane. Pitch moment results from the drag on the wing, which active along the wing and thrust line.
Firstly the wing with high lift airfoil generally yields higher drag which therefore giving high Pitch moment.
Secondly the distance between the wing and thrust line is also adding to higher Pitch moment, the High-wing type of plane has more distance than other type of wing and, therefore, results in higher Pitch moment.
Thirdly the Dihedral – wing angle, at its wing tip having higher value than the wing root, is adding to the distance from the Thrust line and, in return, making more Pitch moment.
These three significant force are highly engaged in the Pitch moment on the remote controlled airplanes. An excessive lift generated by the Pitch moment makes it hard to level the flight – with the higher revolution added to the engine the plane tendency to climb up is higher along side. So the Down thrust is slightly applied to the engine so that it will compensate the unnecessary lift caused by Pitch moment.
A down thrust angle is always made to the remote controlled airplanes before first flight. A 3 -5 degree of engine down thrust is justified for the High wing Trainer plane with Aileron.
A 4 – 5 degree of engine down thrust is good for the High wing Trainer plane with more dihedral without Aileron.
Shoulder wing plane can apply 0 – 3 degree engine down thrust depends on its dihedral and airfoil. High dihedral and high lift airfoil should be compensated with 3 degree down thrust.
A Shoulder sport wing with Symmetrical airfoil and with small dihedral needs no engine down thrust.
The Low wing plan can either applied small down thrust or without down thrust according to your justification on the amount of dihedral and type of airfoil used on your plane.
If you find that the information is not yet clear and more aspect should be added in please feel free to leave me a comment and I will be happy to get back to you.