FLYBOY

My Feedback: (11)

Lou, your confusing P factor and gyroscopic precession.

LouW

I can only suggest that you get a little gyroscope and experiment with it. If the propeller wasn't turning, the force would be a yawing force, however since it is turning, the force is 90 degrees out of phase which makes it a pitch rather than yaw. It's simple physics.

JimTrainor

Lou, both forces are present. The yawing moment doesn't dissapear just because there is a gyroscopic force as well.

Did you hear the one about the pilot who experienced a yawning moment and lost control of his plane? haw, I'd never make it in stand up.

skubydobdo

Major Tomski and LouW, thanks for that insight, just goes to show how much we think we know can still be just theory. You guys abviously have much more knowledge than I, though I comprehend all the aspects of p-factor and gyroscopic procession that have been tossed about here. Information gets passed on through the people intexts who think they "know" it all, and it seems similar to how history books are written by the winners. Maybe in another 100 years we will know a little more, or won't need to one day when all prop driven aircraft give way to turbines!!! There are many speculations here at school about the eclipse-type trainers taking over, but that would be a long time from now, hopefully when they do I will be right seat in a widebody...ahhh the dream!
Friends of mine are just now graduating from GA Tech, came very close to going for aerospace engineering degree from there, but decided that since the school was only 5 minutes from my house, and I didn't want to be that close to my parents I came to Daytona to fly instead.

pgitta

My Feedback: (3)

God. Now I'm afraid to fly. Thanks a lot.

Let me see if I have this straight:

The pro-pel-ler bone's connected to da...tail bone and the tail bone's connec-ted to da rudder bone and da...

Grampaw

P-Factor or Gyrpscoptic Procession? What ever it was or is, I saw the effects on a Cessna Skylane one morning. 3 of my flying buddies and an FBO were about to take off for a trip over a wooded area to find a downed R/C plane. The lost plane's owner was in the right seat, and the other two in back, one with his video camera running. The next evening as I watched the video I saw the Skylane's nose turn out onto the active runway and pause, then heard the pilot ask the guy in the right seat if he'd like to make the take off. Guess he thought he knew how to fly. He said quickly, "Me! Sure! Here we go! I saw his hand grab the throttle, saw the nose come up and swing quickly to the left and heard those in the back seat moaning "Oh NO!" The pilot yelled "Get your damned hand off the power stupid" just as the prop begin to shred corn! This all happened in a blink of an eye! After the Skylane was put back on the runway and was up and heading out, the pilot asked the redfaced guy, "Is that the way you take off with your R/C planes, slam full power to it?" The poor guy had started the take off the only way he knew, just like with his models, full power and go, never worrying about right rudder! His models were always off the ground too quick to get in trouble. He learned a good lesson that morning and afterwards always used a tad of right rudder while taking off an R/C model. Could have been an expensive lesson though. Oh yes, they found the lost plane, which we recovered that afternoon, with not a scratch on it. Lucky fellow.

Semi Retired Aviator

I always thought it was 'pucker factor'. You know, when your ass starts cutting washers out of the seat of your pants!!

And regarding which bones are connected, I can say with authority that the wallet is connected to the ass bone, because when the ass bone starts to pucker, you know it may be about to cost you money..

MajorTomski

Boy this one is FUN!

It'g GREAT to see this flow of educated opinions.

Lou, in answer to your kind answers let me offer the following.

[quote]ORIGINAL: LouW

You are correct in that the slipstream doesn’t “swirl” around the fuselage as shown in the typical diagram. Instead, it is a gentle twist more like the grooves in a rifle barrel. Looking at a top view as you suggest, it is obvious that the slight change in angle of attack at the wing root, working at such a short moment arm, can’t result in a significant rolling moment. In contrast, the change in angle of attack of the fin/rudder working at an arm that is the distance from the cg to the tail can result in a noticeable yawing moment. It’s not that the effect of the slipstream on the rest of the airframe is not addressed, it is that it is not significant. [quote]

Good point on the moment arm! But if the low intensity force on the fin is strong enough to support a yaw then multiplied over the five surfaces of the wings, stab and fin there should be some disruptive force rolling to the right.

[quote] In like manner, the smoke behind an aerobatic airplane does twist gently in the direction of prop rotation, is just isn’t real obvious.[quote]
Sorry but I've looked at a lot of photo's and videos and I haven't seen it

[quote] I just looked up some of the twins you mentioned, and notice that even though there is a net area above the propwash, they all have considerable sub rudder below the attachment. [quote]

I specifically mention these three aircraft because just the opposite of what you state is true. Lets look again:
For the B-25
http://www.fortunecity.com/marina/ma.../Mitchell.html
The Bf-110
http://www.fortunecity.com/marina/ma...72/me-110.html
and the BE-18
http://www.rcgroups.com/forums/attac...&postid=741909

Please not that on all three of these aircraft the fin and rudder lie completely above the thrustline of the engines, there may be sub rudder below the point of attachment but all of it is in the upper half of the supposed slipstream swirl.

[quote] Take another look at the propeller as a rotating wing. The downwash from the blade is not only rearward, but in the direction of rotation. [quote]

A propeller is no different than a wing. Downwash from a wing is down and aft. It does not wrap around the wing and go forward with the direction of the aircraft, why should it behave differently for a propeller?

[quote] The spiral patterns in the picture have nothing to do with the question under discussion. What is depicted is simply the vortex shed from the prop tips made visible by condensation and only shows the path of the tips. It shows nothing at all of the airflow to the rear of the propellers. I’m guessing that you included the picture as a joke.[quote]

Actually no it wasn't a joke. In one of those 57 text books I mentioned above it mathematically y proves that the vortex ring you're looking at defines the limits of the downwash from the propeller. Just as the vortex rings and disturbed cloud layer in this photo:
http://www.grc.nasa.gov/WWW/K-12/airplane/downwash.html

define the down wash from the wing. I include the C-130 photo to show that if you follow the math of the airfoil then the downwash from the prop turns the wrong way to support slipstream spiral.

[quote] The fact is that flow behind a rotating propeller is not straight to the rear but has a rotation in the direction of rotation. This has the effect, more pronounced at slower speeds, of causing a tendency to yaw in the direction opposite rotation.[quote]

Show me the math. !QUOT!In the direction of the rotation!QUOT! is the false assumption here, and it comes straight out of Stick and Rudder.

I agree with an earlier answer we're mixing P factor with gyroscopic forces and mixing them up. The applied force IS the increase in thrust from the propeller that occurs on the right side of the fuselage. And as you point out the REACTION on the aircraft is due to gyroscopic precession on the propeller which forces a nose up reaction. But the original force, the increased airflow is still there!

NOW THE GOOD NEWS I THINK WE!QUOT!VE ANALYZED OUT A BELIEVABLE ANSWER!

It is this localized increased airflow down the right side of the fuselage that is P factor. AND that same local higher velocity acting on the fin MAY just be the source of the lift to the right that causes our left yaw. and NOT the yet to be mathematically proven swirl!

Wacha think?

[X(]

MajorTomski

OOPS sorry for the funny boxes I didn't know the quotes would do that

Tom

FLYBOY

My Feedback: (11)

Gyroscopic precession is a force that makes the plane yaw left or right when you pull the nose up or down. If you pitch up or down, it puts a force on the plane that causes it to yaw. That is the only gyroscopic force they talk about on a plane. The pitch force you are putting on the prop acts in the same direction, but 90 degrees to where the force was applied. That is why it is a yaw. You don't slip turn airplanes, so the force is not acting on the side of a propellor making it a pitch problem. When you turn, you turn coordinated and it doesn't affect the pitch with gyroscopic problems. The 4 forces that are common on an aircraft that make it have left turning tendancies in a climb are just that, in a climb, not affected much in straight and level flight or turns.

Your theorie is correct if you walk the nose left and right with the rudder, but in coordinated flight, it doesn't work that way.

Flypaper 2

A ggod example is, when you were a kid and rolling a tire down the street with a stick, you steered it by pushing on the top. not the front, 90 degrees out of phase. Nowdays it would be done when someone was stealing your tires

LouW

I will make one final attempt to explain why the air downstream of a propeller must twist in the direction of rotation. It is first necessary to look at a wing. Convention always looks at the relative motion between the wing and the air from the viewpoint of an observer moving with the wing. The air is considered to “flow” past the wing. Testing is done in a wind tunnel where air is made to flow past the wing. This is quite valid when studying the effect of the air on the wing. However, in fact the air doesn’t flow at all. It is quite stationary while the airplane moves through it. This is acknowledged by the term used to define angle of attack, “relative" wind.

To see the effect of the wing on the air, it is necessary to view it as a stationary observer while the wing passes by. After the wing has passed, it will be seen that a mass of air has been put in motion (accelerated). The sum of all pressures generated by the wing’s passage has accelerated the air downward, and forward. From Newton’s law (f=ma) the reaction to the downward acceleration is the lift, and the reaction to the forward acceleration is the drag. For a low angle of attack, the drag (and the resulting forward motion) is small, whilst for a high angle, the drag (and the resulting forward motion) is larger. It is obvious that if the air were accelerated rearward, the wing would be experiencing not drag, but thrust. Viola! Perpetual motion, obviously not possible.

With that established, attach one end of the wing to a hub and rotate it. When viewed again as a stationary observer while the resulting propeller (or rotor) passes. It is now seen that the air has been accelerated perpendicular to the plane of rotation, producing thrust, and tangential to the plane of rotation in the direction of rotation, producing resistance to rotation (which is being overcome by the torque output of the engine). If the air had been accelerated opposite the direction of rotation, the reaction would have produced torque to drive the engine, again perpetual motion.

To our stationary observer as the propeller passes, the air is seen to have been accelerated a little to the rear with a slight twist in the direction of rotation. The smoke behind the air show airplane is not blown violently rearward, but seems to hang in the air almost motionless as the airplane moves by. It also slowly twists in the direction of rotation.

It is simply fact that a rotating propeller must generate twist in the propwash. To do otherwise would defy conservation of energy that is the cornerstone of physics. The attached picture illustrates the straightening vanes necessary in a wind tunnel to remove the twist so that accurate flow testing is possible.

Airplanes are in fact trimmed to eliminate the effect of this twist, which results in a tendency to yaw left. Aircraft such as the small Cessnas, have a ground adjustable tab on the rudder which is adjusted during acceptance flights at the factory to eliminate this effect at a normal cruise setting. Others, like my Piper, have a rudder trim in the cockpit that can be adjusted in flight. However when the airplane is slowed down at higher power settings (such as a climb) the twist increases and rudder must be added against the direction of rotation to prevent the yaw. The opposite is true when the airplane is dived at low power settings. Opposite rudder must be applied to prevent the original trim offset for cruising flight from yawing the aircraft in the direction of rotation.

Then there is torque. The torque driving the propeller produces a rolling force in the opposite direction. Granted for small low powered aircraft it isn’t much of a factor, but for high-powered aircraft with relatively short spans (such as WW2 fighters) it can loom pretty large. The pilot resists this rolling force with the aileron control which produces a yawing moment (due to adverse yaw) requiring rudder input to compensate.

As to P-factor, a force applied to the rim of a gyroscope and parallel to the axis, will only produce motion 90 degrees out of phase with the applied force. There is no exception nor additional force. The asymmetric thrust can only produce a pitching motion, not yaw.

The tendency of a conventional aircraft to turn left when climbing at high power below trim speed is due to torque, and increase twist of the propwash, period. Any other explanation violates the most basic laws of physics