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Old Jun 16, 2012, 02:06 PM
Balsa&Tissue
payne9999's Avatar
United States, OR, Beaverton
Joined Jan 2011
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Question
P Factor & small planes

I have a 30", 9 ounce Howard DGA-6 Mister Mulligan. I am using a motor that is the equivalent of a "Blue Wonder". with a 10 amp ESC and a 3s 450 Lipo.

The problem is that it turns poorly to the left and requires very large rudder correction (like 60% rudder). Aileron turns to the right require a small rudder correction (more normal). The plane flies trimmed to the right slightly as if it needs more right thrust than it already has. Currently I am using a slow flyer 8X4 prop.

The question is: Is the 8 inch prop creating a lot of "P" factor making it turn poorly to the left?

Should I use a smaller prop such as a 7X3.5?

Thanks,

Dave
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Old Jun 16, 2012, 03:31 PM
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Moab, Utah, USA
Joined Apr 2003
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Generally things like propeller P factor and gyroscopic precession are relatively insignificant and transitory in nature. Also the direction of their effects, be it left or right, is dependent upon the aircraft attitude and pitch movement. The primary effect the propeller has is the spiral slipstream hitting the left side of the vertical stabilizer causing the airplane to yaw to the left. But that actually aids a left turn and makes turning right more difficult. So don't worry about the propeller and start looking at your flying and control surfaces for warps and misalignments.

Well, that is unless you are using a pusher prop and the prop is turning counter clockwise when viewed from the cockpit.

Larry
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Old Jun 16, 2012, 03:46 PM
Balsa&Tissue
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United States, OR, Beaverton
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Quote:
Originally Posted by Lnagel View Post
Generally things like propeller P factor and gyroscopic precession are relatively insignificant and transitory in nature. Also the direction of their effects, be it left or right, is dependent upon the aircraft attitude and pitch movement. The primary effect the propeller has is the spiral slipstream hitting the left side of the vertical stabilizer causing the airplane to yaw to the left. But that actually aids a left turn and makes turning right more difficult. So don't worry about the propeller and start looking at your flying and control surfaces for warps and misalignments.

Well, that is unless you are using a pusher prop and the prop is turning counter clockwise when viewed from the cockpit.

Larry
Larry,

I built two of the planes, one from the Dumas kit and one entirely from scratch. The two planes as near as I can tell were about the same. The surfaces do not appear to be warped or misaligned and I have built hundreds of small free flight stick and tissue models in the last 30 years. So, the interesting thing is both planes exhibited the same problem with left turns needing huge amounts of rudder correction and right turns needing almost no correction.

Prototype 1:


Prototype 2:


Thanks,

Dave
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Old Jun 16, 2012, 04:12 PM
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Moab, Utah, USA
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Dave,

Very nice looking planes. I'm really not sure exactly what you mean by rudder correction in a turn. Rudder use in a turn is usually required only to counter adverse yaw caused by unequal drag on the deflected ailerons. So the only other thing I can think of is that the right aileron has more down deflection when banking left than the left aileron has when banking right. Again, with a standard rotation direction, spiral slipstream and engine torque would only aid a left turn.

Larry
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Old Jun 16, 2012, 04:21 PM
Balsa&Tissue
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Originally Posted by Lnagel View Post
Dave,

Very nice looking planes. I'm really not sure exactly what you mean by rudder correction in a turn. Rudder use in a turn is usually required only to counter adverse yaw caused by unequal drag on the deflected ailerons. So the only other thing I can think of is that the right aileron has more down deflection when banking left than the left aileron has when banking right. Again, with a standard rotation direction, spiral slipstream and engine torque would only aid a left turn.

Larry
Larry,

When making a turn some planes require a little rudder to bring the tail around into the turn. Mostly it is just needed at the beginning of the turn. This plane will turn right with ailerons only quite easily. However, when turning left with ailerons the plane just banks a little and continues in basically the same direction. Adding lots of left rudder with left ailerons will make a smooth turn. So, why the difference?

Dave
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Old Jun 16, 2012, 04:45 PM
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Originally Posted by payne9999 View Post
However, when turning left with ailerons the plane just banks a little and continues in basically the same direction. Adding lots of left rudder with left ailerons will make a smooth turn. So, why the difference?

Dave
Well, adding rudder causes the plane to yaw. If the airplane doesn't need any yaw assist when in a right bank but does when in a left bank it seems that somethng is already yawing the plane to the right. The only thing I can think of is too much right thrust on the engine. Decreasing the right thrust should even out the amount of rudder required in either direction by requiring more rudder in a right turn and less rudder in a left turn. That may require more right trim for straight and level flight, but that trim sets the neutral point around which the rudder inputs operate in turns.

Larry
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Old Jun 17, 2012, 12:35 PM
Balsa&Tissue
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Originally Posted by Lnagel View Post
Well, adding rudder causes the plane to yaw. If the airplane doesn't need any yaw assist when in a right bank but does when in a left bank it seems that somethng is already yawing the plane to the right. The only thing I can think of is too much right thrust on the engine. Decreasing the right thrust should even out the amount of rudder required in either direction by requiring more rudder in a right turn and less rudder in a left turn. That may require more right trim for straight and level flight, but that trim sets the neutral point around which the rudder inputs operate in turns.

Larry
Ok, you were correct!

For whatever reason the plane wants no right thrust and 5 degrees of downthrust. It feels like even 3 down would be better. Such a light plane with almost a 300 size motor and it needs no thrust offset! well OK now it flies perfect.

Dave
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Old Jun 17, 2012, 03:32 PM
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Good to hear you got it sorted out. I think that removing as much down thrust as possible is also a good idea. Personally, I don't believe in down thrust. I much prefer adjusting wing vs horizontal stab incidence angles (decalage?) and using elevator trim. But, that's just me.

Anyway, great looking airplanes and I hope you get a lot of pleasure from flying them.

Larry
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Old Jun 18, 2012, 11:35 PM
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Gary, IN, USA
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Unless things have changed since I started building model airplanes 65+ years ago, and the aerodynamics taught at Embry Riddle Aero. are out-dated, The answers are a combination of "yes, no, & maybe". There is no One Rule fits All for airplanes. Are you aware that the Wright brothers crashed trying to learn to turn so often in the time following the 1903 straight flight that they were on the verge of giving up before they killed themselves? They admit that it was only with the 1905 model that both pilot and machine were flight worthy. Their interest in gliders also continued and in 1911 set a record of almost 10 minutes slope gliding with one of their biplane gliders.
Your model has a relatively small zone of balance, decalage and motor thrust alignment. A nice looking airplane but one that flys nice only after careful tweaking to find the right combination.
Electric power makes this much easier because there is no fuel weight burn-off balance change. A propellor is the same affector whether turned by a fuel or electric motor. There is a substantial difference between gyroscopic precession produced by fuel and electric motors. Model airplane fuel engines have much less because the rotating mass is basically the crankshaft. The comparable electric outrunner motor has the shaft plus a much larger cylinder ringed with magnets mass. Try this experiment; remove the prop and run the motor at full power; Hold the plane at the wing tips or near the balance point, apply yaw and pitch directions to the plane and watch how the nose reacts differently than the direction you apply. That is gyroscopic precession.
Humans have that wonderous motor skills ability to learn to stand upright...ride a bicycle upright.......and fly RC with hand-eye coordination.
It's nice to understand aeronautics, but even more fun to work on your flying skills and enjoy!
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Old Jun 19, 2012, 06:31 AM
Canadian Bacon
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Kingston, Canada
Joined Jun 2004
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You've probably already checked it, but make sure the right wing is not heavier than the left wing, causing it to want to turn to the right.

gord.
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Old Jun 19, 2012, 10:28 AM
Balsa&Tissue
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Originally Posted by flypaper 2 View Post
You've probably already checked it, but make sure the right wing is not heavier than the left wing, causing it to want to turn to the right.

gord.
It is flying just about as perfect as I could expect now. It just doesn't require any right thrust. The lateral balance is correct as well. The only change I want to try is slightly less down-thrust.

Dave
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Old Jun 19, 2012, 11:45 AM
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If it helps. The amount of downthrust is determined by how steep you want it to climb going from cruise speed to full throttle. Not enough and it can go around into a loop with enough power. Or you can have it not climb at all, just accelerate without climbing. Most high wing with a cambered airfoil climb at about a 10 or 15 degree angle. Hope this helps.

Gord.
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Old Jun 19, 2012, 02:35 PM
Balsa&Tissue
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United States, OR, Beaverton
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Originally Posted by flypaper 2 View Post
If it helps. The amount of downthrust is determined by how steep you want it to climb going from cruise speed to full throttle. Not enough and it can go around into a loop with enough power. Or you can have it not climb at all, just accelerate without climbing. Most high wing with a cambered airfoil climb at about a 10 or 15 degree angle. Hope this helps.

Gord.
I generally don't use a lot of down-thrust. I like it to cruise at sort of a neutral trim. On this model I over-estimated the amount of down thrust needed. Descending at less than cruise and ascending at more than cruise is what I prefer. I am not of the school that level flight is required at any speed above cruise (which is pretty much not achievable). When I was in full scale flight school we were taught that Throttle was for controlling ascent and descent and not so much for speed control.

Dave
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