I am running a 20x8 propeller on my engine and find that I have to add right rudder on up lines. I will be going with a 21x8 or 22x8 propeller and I want to know if these new propellers will make the left thrust of the engine more pronounced or less?

It is my understanding that longer slower turning props have a more pronounced P-factor effect and will increase the pull to the left...

It's acting like full scale. Cool

Galen is correct, the larger the diameter the more leverage the prop has against the longitudinal axis, and hence more p-factor is experienced.

I was afraid of that, I guess I will try to double the existing angle and see how it behaves. I didn't want to have to fool with the engine.

I would just fly it with rudder. It will make you a better pilot, don't you think?

Put a couple of washers under the left side engine mounting bolts. Most people trim their airplane after selecting the prop.

Hey OscarPilot:
Move your aileron trim all the way to the left, move the rudder trim all the way to the right, move the elevator trim all the way down.......now just fly it. Does that make you a better pilot? Geez, flying an out of trim plane is like playing an out of tune instrument.

NO DOUBT, IT WOULD!

Diablo...So are you saying that the fact the place demostrates p-factor means it's out of trim??????

JEEZ! I guess we better put spacers behind the engine mounts of full scale aircraft so they won't experince left yaw due to p-factor. While we are at it, we can do way with counter rotating engines on twins just by installing spacers on the non-critical engine. I wonder why none of the engineers ever thought of that?

Huge difference between out of trim and normal aerodynamic characteristics.
What's he design call for and where is the thrust angle now?
Does it exibit left yaw when slowing down on a vertical climb or during the whole up-line?
Were you flying it directly into the wind when you noticed you had to add rudder?

I will just say this, when doing a humpty bump, I always throttle back slowly to make sure that I go over the hump slowly. I don't like the idea of having to add rudder as the plane drops below flying speed (where the rudder has effect due to airspeed). I don't mind it on take off and landings, just don't like it while flying maneuvers.

A bit of a note here as well, the Spiralling slipstream has a lot to do with that "left Yaw", possibly moreso than "P" factor or gyroscopic forces from the prop. It's my understanding that "P" factor comes into play during pull-ups, as the engines torque and airflow off the prop make the aircraft tend to yaw left during Pitch changes. During normal straight-ahead flight, spiralling slipstream washes against the vertical stabilizer, and since the slipstream is rotating L-R as it hits the stabilizer, it tends to push the tail to the right, making the aircraft yaw to the left. The more slipstream or thrust off the prop, and the higher the rpm, the more pronounced this is, thus an engine with right, and down, thrust (depending on what is required to "balance" things under different power settings) will exhibit less pitch and yaw changes. Why "pitch"? The stabilizer/elevator on the right side of the plane is "shadowed" from the slipstream, so the air spiralling into the fin on the left side puts a positive pressure on the top of the stabilizer as well, causing the plane to pitch upwards in some cases.

In Full Scale planes, the pilot automatically trims and compensates with the controls, and "Cruise Trim" of the controls is set up for hands-off straight and level at normal cruise power. Change power settings, though, and the plane will change it's flight characteristics, with more power initiationg a left yaw and possible pitch-up depending on the design of the plane, and less power yawing right as the props effect ont he airframe is reduced. On a Twin, however, there is no slipstream off the props hitting against the Vertical stabilizer and rudder, so any torque reaction or slipstream affects that would make the plane yaw either left or right woudl have no control surface to directly counteract them, so the yaw tendency would be extremely high if the engiens did not rotate in opposite directions. On that note, hoever, please note that not all Twins have engines rotting in opposite directions. The P-38 and Mosquito had it, but most twin-engine bombers or heavier aircraft did not.

As for why full-scale aircraft do not have "thrust" built into the airframe? Some do!! All one has to do is examine a T-28 Trojan to see the pronounced Right and Down thrust cranked into the engine so that at full power settings the plane tracks as straight as possible. This was done because the T-28 was meant to simulate Jet aircraft as a primary trainer role. I'm sure if you look at ANY high-performance single-engine planes out there you will see some thrust dialed into the engine.

Best bet is to dial in the engines thrust with shims until it flies straight, with no yaw or pitch, no matter what the power setting is. As to how it behaves on pull-ups or on lines other than straight and level. . .that's what those 2 little sticks on the transmitter are for.

Here ya go. I really like these trimming instructions from Piedmont Models.

http://www.piedmontmodels.com/images...structions.pdf

I would like to point out the Top Flite Monokote add on the inside front cover of Feb 2004 M.A.N. It shows Chip Hyde's Ultimate. Notice the pronounced right thrust in the engine. Realizing that Chip is one of the best and knowing that he is extremely proficient with the rudder, he still puts in right thrust to make the plane fly right.

Geistware, I realize that you don't like to fool with the engine, I don't either, but the place to start is with about 2 degrees of right thrust. Of course the rear of the engine will need to be moved left to make the prop come out in the right place. Yes, it's a lot of trouble but it has to be done if you want your plane to fly right.

I've done a lot of giant scale scratch building (from plans) in the past 20 years and I am still amazed that most of the major designers never mention right or down thrust. I would sure hate to fly my Z Corsair without right thrust. Trust me, I would have a bear by the tail.

Ken

Ken, following today's flight, I know I will need to add more but not sure of how much. I will measure the thrust angle and double it. From there, I want to see what happens. I now have the engine underproped so if it pulls to the right slightly, it may be ok until I get a new propeller. I am using a 22x6-10 and will be going to a 24x8

Geistware, I think when you get the correct amount of right thrust in you will know it right away. More than likely when you are at that point you should be able to change props around without noticable difference in yaw. Of course, some rudder corrections will still be necessary.

I'm not sure I could learn to live with a plane that went a little right on takeoff. That would be like having to push the stick forward a little to land. Some things just go against the grain.

Keep us posted.

Ken