I have seen a lot written lately on why our planes pull left. It has been called torque, and then another writer says, "No, no, it's P-factor." Still another claims prop blast. Well, it really doesn't make much difference what causes it, my plane is still going to pull left. No matter, let's look at what may cause it, call it whatever.
First, there's torque. Now with the little glow model engines and their short, high rpm props, there isn't much torque. Oh, let me interject that torque is a measure of the twisting force of the engine and is usually measured in units like foot-pounds (ounce-inches for servos). Horsepower is calculated by multiplying torque times rpm and dividing by a constant, depending on the units. It doesn't take much torque for some impressive horsepower when you are turning 14,000 rpm. On the other hand, if you are turning 7,000 rpm, you need twice as much torque to get the same horsepower.
All the big recip pilots know about torque. You have to use right rudder because the prop turns to the right, which means that the reaction of the plane is the opposite, left. The question here is whether the actual torque if the engine causes the pull to the left, or is another factor involved. The other factor I am referring to is the prop blast or wash which flows, not straight, but in a spiral.
Those of you who have a background in free flight or control line may remember the term, "Spiral Prop Blast." Others of you may have heard of propeller vortex, or some other bit of technical jargon like that (I heard on author call it "prop and wash tornado." That's a mouth full!). Years ago, when I flew control line, we all called it a spiral prop blast. Since the prop rotates, it gives the air a spin rather than pushing it straight back. This curved airflow hits the left side of the vertical fin and rudder more than it does the right side, causing a yaw to the left. This caused both control line and also, free flight, planes a turn to the left. We would normally just offset the rudder in CL, although a few actually flew clockwise, so the left pull would help them.
The free flight guys also compensated. A few people noticed that if the fin were on the bottom, lo and behold, the plane went right, instead of left. I seem to recall a Goldberg kit like this.
Please note that the spiral prop blast is dependent on rpm, you know, high rpm, more prop blast, less rpm, less prop blast. If you are coming in at idle, get messed up and hit the power, bang, the old prop blast hits the tail and swings the plane to the left. If your plane is heavy and slow to accelerate, you'd better get on the rudder.
Most of the time in the air, we don't notice the spiral prop blast. When we trim out a plane, we add in whatever rudder trim we need and forget about it. Maybe the plane has a little right thrust, which most planes need anyway, so we don't need any trim.
OK, the next thing I mentioned in the opening was something called P factor or P effect. Actually the "P" stands for "prop." (This was an early try at jargon to keep the uninformed in the dark. We're much better at this now, we even have government departments to make up acronyms.) What, then, is P-factor? There are lots of technical explanations with terms like 'asymmetrical angle of attack," but if you will grab a prop, I'll explain it to you. Hold the prop like it was on an engine with the blades horizontal. Now pretend it's on a plane. Bend your elbow up and down a few times and watch this happen. The right side blade with the higher angle gets a better bite of air than the left side which is at a lower angle. Better bite, more pull, so the plane pulls to the left. This is where you get the pull to the left when you pull up steeply, like in a loop.
This will always happen, but it only happens while the nose is moving in the pull up. The quicker the pull up, the more bite the right side gets and the more left pull there will be. Doing those squares makes the plane pull left. And, the longer the prop, the more pull you'll get. This is why we notice it a lot more on giants than we did on little planes.
Let's look at a stall turn as an example. As you are pulling up, you need right rudder to compensate for P-factor. I always notice this on my giants, and I use right rudder sometimes. (A lot of the time I am lazy and don't do it.) After you establish the vertical line in the stall turn, the plane begins slowing down. As it slows, the rudder loses effectiveness and the spiral prop blast overcomes our trim, causing a pull to the left. This makes most planes like to stall turn to the left, unless they have right thrust or a crooked fuselage/tail. For you guys who have flown pattern, your 60 sized plane had enough power se that it didn't slow down enough before you put in the rudder, and you were using a pretty short prop relative to your planes size.
There we have it, torque, spiral prop blast and the prop factor. The bottom line to all of this is twofold. First, it doesn't matter what causes it, you have to trim and compensate for these aerodynamic forces which pull the plane to the left. Second, use some right rudder when you pull up and when you pop the power to it on a go-around.
Oh, boy. I could go on and on. One of the guys in the
club once said, "Ed, don't you ever run out of BS?. I guess not. You guys
have a good time and keep it safe.