Maybe a silly question: side thrust - why we need it ?
 

It may be a silly question but I want to now your opinions about side thrust and/or down thrust: why we need it, is it 100% necessary, why some planes need it and others don't ?

Thank you !

RE: Maybe a silly question: side thrust - why we need it ?
 

Engines are usually mounted with a few degrees of right and down thrust. The right thrust aids in directional stability. For example, insufficient right thrust will manifest itself when pulling vertical as the model will want to yaw left. This is due to a number of phenomenon including asymmetric propeller loading.

RE: Maybe a silly question: side thrust - why we need it ?
 

Yup,
The downthrust is because the usual engine thrust line is below the centre of gravity of the aircraft (for high wingers) and the plane will want to pich up as soon as you open the throttle.
Even if it's through the centre line, the secondary effect of throttle is to raise the nose by way of longitudinal stability.
The side thrust is because the propellor loading will want to turn the aircraft. Even in straight and level flight, the propellor causes a twisting of the air that acts on the tail of the aircraft that makes it want to yaw left.
Full-size pilots have to correct for this with a bootful of rudder during take-off along with an application of rudder trim. We can't usually apply rudder trim in flight or sense the yaw during pitching manoeuvres, so the designer makes it easier for us by applying a little side thrust (usually about 3 degrees).

RE: Maybe a silly question: side thrust - why we need it ?
 

All of the above is correct. But to answer you question about why some planes don't have the offset, if you fly the plane inverted, the offsets will work in the wrong direction. That's why most aerobatic planes have no thrust offsets. The pilot compensates for the directional issues with the radio inputs. Takes a lot of subtle stick work from a proficient acro pilot to keep the maneuver smooth and straight.

RE: Maybe a silly question: side thrust - why we need it ?
 

Pull up and the airplane turns left, push down and the aircraft turns right. Its called gyroscopic precession. You probably studied it in 10 grade physics.

Another force known to turn the aircraft is called P-factor. If the aircraft is travelling through the air mass at a positive angle of attack, then the prop is also. In this case the decending blade's pitch is greater than the accending blades pitch. The decending blade is on the right side of the engine and thus turns the plane left.

There is also a third force working here. Torgue. The engine is spinning one way. The airplane wants to spin the other way, left.

Fly any real airplane and you will learn that you must retrim the airplane when you change speeds. If the airplane has a very large speed envelope, then there might be a good deal of trim changes as you speed up or slow down.
The airfloil also plays a role. Flat bottom airfoils require much more tirm changes as speed is changed. Symmetrical foils require less trim changes throughout their envelope. High wing aircraft are much, much more susceptible to large trim changes with throttle changes than are low wing aircraft.

Down thrust helps reduce the trim changes throught the speed envelope . Thats why we use it.

RE: Maybe a silly question: side thrust - why we need it ?
 

Right thrust is used to attempt to minimise the effects of 'P' Factor and it occurs most pronounced at high deck angles at slow speeds and high power settings such as in a sustained climb or shortly after takeoff.

P factor is thrust disymetry caused by the desending propellor blade producing more thrust than the asecnding blade when at a high deck angle (relative wind to the propellor disc angle) causing a a yaw to the left with normal rotation engines.

This also affect full scale aircraft and many general aviation aircraft use right thrust. Its also the reason all full scale aircraft that do not have rudder trim must sustain right rudder imput during a climb (high deck angle, high power and slow speed).

John

RE: Maybe a silly question: side thrust - why we need it ?
 

Thank's for your answers !

RE: Maybe a silly question: side thrust - why we need it ?
 

This weekend I found how much influence the side thrust actually has on a plane. After the rebuild on my firewall I took the plane out - pulled vertical and the plane yawed to the left and in flight it did the same if you looked carefully. I went home, added a washer at the back to give it more right thrust and WHALLA...you pull vertical and it goes and goes and goes straight up. I do however see my plane has a tendency to pull to the right on take off roll - but then it tracks straight as soon as it gains speed - this happened after the thrust mod. Oh well - it flies :D

RE: Maybe a silly question: side thrust - why we need it ?
 

All these answers are correct. However the most significant factor in the left-turning tendancy is the spiral propellor wash. It swirls over the fuselage and impacts the left side of the vertical stab. The causes a left yaw, and the right thrust offsets this. The other above mentioned effects of precession, p-factor, and torque also contribute, although to a lesser degree.

Regards,
George