And/or why do we add down thrust? What happens if a particular design calls for right and down thrust and I get one or the other off a bit? I guess said another way, how do you know when you've got it right, or wrong. I think it has something to do with the advancing propeller, but beyond that I'm clueless. Then, does a 3 blade prop affect the equation somehow?

A lot of our models for whatever reason tend to climb & turn left under power, also more power = more up & left. This could be trimmed out at any one speed ---- but then a dead stick will leave U trimmed to go down & right. Thrust line ofset use to
correct this up & left tendency varys the correction with the amount of thrust & leaves a straight ahead trim if U dead stick. A lot easier to deal with when U are already busy.

Unfortunately the thrust "correction" is power sensitive -- more power = more effect. If you adjust thrust angles for cruise, or full throttle, there will be trim changes at low power settings. There are no free rides.

This link should help http://www.rcuniverse.com/forum/m_3683941/tm.htm

Thanks, I should almost qualify for a PhD if I can assimilate all that! Should have known that had been discussed in depth, but maybe some others will also benefit.

What is the size of your plane ?

Do you have a C O G, or Balance Point for it ?

Mainly downthrust is to compensate for the fact that most of us are too lazy to use the elevator trim control as a primary control much like they do on full sized stuff. Add enough downthrust and the thrust vector makes up for the extra speed and the model only wants to climb a bit at full throttle and go into a powered glide at low throttle rather than try to loop or to dive like it would if the downthrust was not there and you didn't constantly play with the elevator trim.

Right thrust is just a way to make up for the torque reaction.

If you don't quite get the angles right don't sweat it. But I would not suggest taking it all out or you'll find that you NEED to learn to use the rudder and elevator trims as a basic and constantly used flight control.

Full sized stuff sets the throttle and basically trims the plane and flies for hours at a time in one direction and one trim. However our models are all over the sky and the throttle is on and off like a light switch at a rock concert. So we try to compensate by using thrust angles.

The reason is P-factor, the torque produces by the prop blades and the slipstream from the prop hitting the left side of the vertical fin/rudder. Twist a door knob to turn it, and the door knob will want to twist your hand the other way. For every action, there is a reaction. Twin engine planes with counter rotating props do not tend to turn left. Also jets do not tend to turn left. Do a google search for "P-factor" to get more detailed explanations.

Cyclops: it's a Ziroli Stuka, building with a 'short' kit. The plans give me a CG, and calls for right thrust. The way the firewall was precut, it appears to have a bit of downthrust-though not specifically mentioned. Bill

B Matthews told it straight -
to add-- all of these "side thrust /down thrust angles are simply efforts to compensate for a model which is likely going to fly a rather high angles -getting going -or slowing down
Compensation is the key word
IF the model was extremely light (extremely low wing loading) non of this would make any difference.
When the prop (a spinning thing) is forced to slide as it spins-- slide up or down or sideways , it resists this motion --you can test this for yourself with any small electric model held in your hand - or using a toy gyroscope top you will see one of the forces involved
so --IF the plane could always fly at a very low angle of attack - these forces would never amount to a hill of beans
Scale warbirds typically end up with high wing loadings - -and so the plane is often operating on the ragged edge of flying well - -it is at these angles -the prop skidding forces are at maximum.
If flying speed is kept up and th plane is gently maneuvered -th forces are at minimum.

Down and right thrust is mostly a carryover from free flight where there is no control available to correct for changes in flight condition. A free flight model had to climb under full power as long as the engine was running, then transition to a slow glide with minimum sink rate after the powered phase of flight. It further had to glide in a circle to keep it in the vicinity as it came down. The down thrust prevented too steep a climb (or even looping) when under power. Since the airplane was trimmed to circle in the glide, the side thrust made the airplane circle the opposite direction under power. Using power to circle under power in the opposite direction as the glide helped in avoiding a spiral dive onder power since there was no elevator control. A skillful freeflight modeler, by adjusting thrust and some other tweaks could reproduce a consistant flight pattern flight after flight.

With radio control, where all axis control and trim is available, side and down thrust is not actually required. BMatthews answer is right on the mark. It can be helpful in flying a model that wants to climb or turn excessively but the angles are not critical.

You must have not followed up on http://www.rcuniverse.com/forum/m_3683941/tm.htm

I'm going to put my two cents in by saying that a prop creates three things known generically as p factor. They are differential blade thrust, slip stream over the tail and gyroscopic precession. These three forces act on the plane depending on your flight configuration Without going into a long winded discussion, the right thrust helps compensate for these factors and generally you will have an easier time flying your plane. Its like putting washout in your wings; Don't have to have it, but it makes the plane easier to fly. If you want to know how a plane fly's, read "Stick and Rudder", an old book but probably one of the best on the subject.

P Factor is another name for Asymmetric Propeller Loading.

There are four turning forces: Torque, P Factor, Spiral Slipstream and Gyroscopic Precession, all of which have different causes, and can act in different directions under different conditions.

In some texts, they are all referred to generally as torque, which is technically incorrect, but it is sometimes done in the interest of simplicity. I have never seen all four forces referred to as P Factor.

If P stands for -- phorse- then it all works ---
I think that is a phine way to look at it
(you may say --phooey)

Then the fact that a WW II fighter had a huge fan slamming air on 1 side of the fuselage as the prop approached the upper vertical part of the fusulage ( the pilots left hand side ) and creating a vacumm on the opposite side after it passed the vertical, has nothing to do with pulling the nose ? That fusalage areas may not be equal above and below the prop centerline.
The larger area could be above or below.


Ok, Cyclops. Then why do radial nosed planes suffer the same fate?

Why is it a 10 : 1 or greater geared glider can be almost impossible to fly if torque is not the major factor.

Yet a ducted fan model that is equally over powered has no noticeable torque effet or " P " factor ?

Does the above mean to imply that the spiral component of the slipstream is very compact or dense compared to its aftward component?

If you are referring to torque as the force causing the plane to rotate counterclockwise if the prop is rotating clockwise, it is a force although I really don't know if right thrust would do anything to help compensate for it although it is so minimal, it may not be an issue. I realize my jargan is not completely right but I do understand how it works. I have also heard the forces related as torque but by my above statement you could see how that would be very confusing due to most peoples understanding of what torque is. Thats why I refer to it as P-factor although not technically correct. Right thrust help make plane go down straight down runway and track good through air to put it in an easy to understand way.

Mesae. I do not know.
But I do know all motion pictures of WW II fighters flying away at full power and pulling up steeply show the nose way off center to the right constantly.

I've done a lot of pull-ups, pull-downs, push-ups and push-downs in Decathlons and I don't understand what you are referring to.

As you know, by the rigorous definition, torque is a rolling reaction, so right-thrust doesn't help with that. I was trying to be general, yet somewhat complete IAW aeronautical texts.

In the 3 blade prop area, you have broken up the air with the previous prop a bit more so, the thicker the air, the more effiecient the prop, it's kind of like putting 2 motors and props operating independantly just behind each other, the rear prop only has the left overs from the front one, this is why you don't see them too pushed with RC plane designs. The same principal is demonstrated with RC combat where the front plane prop washes the other, causing the opponents plane to fall out of the sky since it doesn't have dense enough air to pull in enough air to transfer to it's prop, that and of course the instability caused by the wash.

That does not actually happen in flight & only occurs under particular conditions on a test bench.

Mesae. The planes are pulling a vertical climb and the nose is constantly off center to the right.
I have seen it on a couple of clips. The plane is crabbing or cat walking to the right. The engines were at or close to full RPM's.

If a plane has enough power to maintain speed the need for any offsets is lost.
Not to poo poo the old fighters -they were the king of their day - current unlimited aerobatic stuff tho is far more powerful -power to weight
Example -- a Pitts 12 will pull away almost vertically from a vertical stopped position.
On models of bipes we designed for TOC- one problem was thrust setup - some needed up thrust - why?
simple - not enough power -- they were always slowing on verticals and requireg a bit of upTRIM -even tho it was not readily apparant.
add more power - the need goes away . We did this year later using more powerful engines - not available in 1990