P-Effect
 

I've been flying Contra equipped planes from late 2012 until the end of last year. This year I've been trimming and trimming a Nuance with a single 21x14 Falcon prop.

One thing that is really bugging me is the amount of rudder required during pulls and pushes. Once the plane is vertical it stays put very well but it really veers to the left during a pull up. I'm attributing most of this to P-Effect. I've noticed a lot of guys have gone to 3 bladed props ( I'm not sure if it's for this reason) but I would think this might reduce the P-Effect. So my question is, can anyone tell me if they have determined that 3 bladed props are really better, for curing this or other problems?

Jim O

Hi Jim,

I mix power with right rudder until goes straight up. Mix start from around 1/2 power up. I know that many don't like this. I don't do it anymore because I have been using the contra since 4 years ago. Don't ask me why but it worked for me for many years when I flew 2 and 3 blade props. With 3 blade prop the amount of mix is a little more. 3 blade are good to reduce the down-line speed. Well this is one method. I am sure that there are other options. Let's see what other pilots will suggest.

P? As in your eyes turning yellow?

Have you tried a bigger fin/ rudder, or (similar in effect) 2,or 4, tail fences?
A further-forward c.of g. might also help---but of course this can squeeze the balloon somewhere else.

I run a 2 blade prop, have no throttle to rudder mix and the plane goes up straight and doesn't veer off during pulls or pushes.

As I understand it, (correct me if I have it wrong), P factor is caused by a high angle of attack attitude, so maybe try a more gentle pull or push?

P factor is when a plane is in a high pitch angle the descending blade has a higher angle of attack than the ascending blade.
This will make the p[lane turn to the direction of the ascending blade, usually left.

What I think you are experiencing is gyroscopic precession.
The more rotating mass you have the stronger the effect.
Pushes downward or upward will result in an attempted turn.

The guys in the aerodynamics section could possibly solve your problem.

An APC 21 X 10 prop is 165 grams and I don't have that happen. Electric props are a great deal lighter, so I doubt that's the problem.

Sounds like it could be the stabs aren't aligned.

Precession acts 90 degrees aft of input in the rotation. It's how you control a helicopter.

I've got 2 fences on the stab, perhaps 4 would be better.

drac 1:

Yes lower angle of attack would help. Higher velocity and a larger radius.

pd 1:

Yes in a pull the descending blade which is on the right (looking forward) has a higher angle of attack than the one on the left so that causes a yaw to the left and that is what I am experiencing.

drac 1:

My Jeti transmitter provides a 13 point servo balancer on all servos, so my elevators are matched perfectly. The plane had a bowed aileron that I fixed as well as I could but I still carry a little right aileron trim. That could contribute.

Then again it could be the pilot. I think I'll take a Contra plane out and see what it does.

Jim

But what about the stabs themselves? Even though the elevators are matched perfectly, the stabs could be a bit out still. Just a thought.

My 2 cents worth.
Try increasing your expo on ailerons.
Increase the stiffness of aileron on your Tx stick or replace springs.
I have my aileron spring much harder than elevator Mode 2

hope it works for you

Some years ago, I had a similar frustration. Twiddled with several things until I was encouraged to add to wing tip weight. Something I was dead against as the aircraft was meticulously balanced. Needless to say, it cured the problem and had no noticeable negative effect on everything else.... which in it's self presented another can of worms; Why. It was one I never bothered opening.

As I understand it, 3 bladed props are good for braking, nothing else.

P-factor is probably the biggest factor making your plane pull to the left when pulling up.

Gyroscopic effect (probably extremely negligible with our light props) during a pull up acts with a yaw to the right, so is not involved.

http://www.rapp.org/archives/2008/09...ic-precession/

If you can envision a two bladed prop when it is horizontal and the aircraft is at a positive angle of attack you can see the descending blade on the right will exert a greater forward force and the blade on the left will exert a lower force than they do when the plane is at zero angle of attack, and that is why the plane yaws left. With a three blade prop the blades are never horizontal at the same time so I would expect the force couple would be lower. However, from the comments in this thread it doesn't sound as if anyone experienced this or even expected it.

Jim O

When considering the gyroscopic effect shouldn't the spinning motor be taken into account?

Esp if its an outrunner.

I'd like to hear Brenner's thoughts on this entire thread.


J e r

It doesn't matter how many blades you have, 2, 3, 4, 20 makes no difference. At some time the blade will be on the right side descending it will have an increased angle of attack and more pull.
The blades on the left side ascending will have a reduced angle of attack and will pull less. The blades don't have to be positioned opposite each other statically, they just have to enter the increase or decrease zone.

I mis-understood the question when I first read it, somehow I thought you were flying electric .
If you are, Gyroscopic precession may or may not have a noticeable effect, but it will have an effect.
A push down will cause the plane to turn left. A pull up will cause the plane to pull to the right.
Since this is not what is happening I think it might be P-factor not precession.

Paul

Guys it isn't P factor or Torque. Torque is a twisting motion which is what causes an airplane to try to ROLL during a torque roll. The problem you are experiencing is from the Spiral Slipstream caused by your spinning propeller. Your propeller is sending a spiraling blast of air across your rudder. The spiraling air is hitting more on the left side of the rudder and less on the right. This is what makes the model turn left on take off roll. The most common correction for this is to add right thrust to the engine.
JERRY

The old "Spiral Slipstream" myth.

LOL

It's Coriolis force. If you're flying South of the equator, the pull will be to the right.

Geeze, do I have to tell you guys everything?

I'm not with he P & Gyro factor. I mentioned tip weight some posts ago and I'm still going for that. Here's the deal based on what I've experience with a meticulously laterally balanced aircraft; Perfect hand of vertical verifying the right thrust but a peculiar slight yaw (or what appeared to be yaw) to the left then pulling or pushing. As mentioned, this was cured by the addition of 15g of right wing tip weight. Expecting some noticeable trim changes on the first flight out, I experienced non, nothing, nada... except the yaw problem had vanished. Now I know the typical problems you get with a heavy wing and the tests, and cures to fix it, but as mentioned, this aircraft was perfectly balanced previously and here I am now flying and aircraft that is distinctly unbalanced... yet now flew superbly! Why?

Here's what I think: The natural effect of the engine torque is to rotate the airframe in the opposite direction, to the left when viewed from the back. 99,99 % of the time this is unnoticeable, the wings/physics countering this and holding the aircraft in equilibrium. Now for some reason, could be wing shape, prop choice, cg or what ever, that equilibrium I can be very slightly disturbed during a pull/push causing the airframe to slightly roll left. We're talking minuscule amounts... but it's enough to appear that the aircraft is yawing left. Why this happens, how 15g of wing tip weight can result in no noticeable trim changes is as mentioned, a can of worms I never bothered opening.

Jim,

Your problem is nothing to do with the contra or three bladed/ two bladed prop.

Your problem is most likely related to misalignment or twists in the surfaces. I would get yourself some accurate and repeatable measuring tools and check the wing/stab incidence at the root, mid span and tip.

Straighten your model and the rudder corrections will mysteriously vanish.

Cheers,
Jason.

Make sure both elevators are going up equal distance on both sides.. An easy way is to tape a carbon rod to the top side of each elevator angled towards the rudder.
pull up elevator and make sure they track each other and adjust.. Then do the same to the down elevator..

Check all incidences first and get left and right wings at the same incidence first then the stab halves, then check elevator throws..

Well I believe I've received the answer to my question because apparently no one has experienced a 3 blade prop reducing P-effect. At least no one on this forum. In fact no one suggested any benefits of a three blade. Guess I'll save $200.

Jim O

:)

Brian

As P factor is most prevalent in high angle of attack situations and pattern flying is about smooth/gentle flying, then I doubt P factor is the cause of your problem.

I had the same issue where I was applying rudder during pull ups. I found the stabs were not exactly aligned. I reset the stabs to perfect alignment and the problem disappeared.