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The following came from Ted Terner " )


Don discusses propeller effects in detail...

    He flew the A-36, (Dive bomber version of the Allison engine, early P-51A).

From : Don Stackhouse

Yes, I'm familiar with it, there's one on display at the Air Force Museum in Dayton, Ohio, not too far from here. Its Allison engine performed very well at low altitudes, and was excellent for a ground attack aircraft. It's primary shortcoming was its supercharger, which couldn't quite get the job done at higher altitudes. The RR Merlin engine used in the fighter versions of the Mustang had a supercharger designed to provide extra boost that kicked in at about 16,000 feet, so power could be maintained at the higher altitudes where fighters did most of their work.

    He took exception to your mention of ailerons and torque. When they handed in their P-40's in Italy for the A-36, they were taught to control the stronger take off torque with the rudder and not the aileron. In fact they often cheated and used a little brake as well.
Yes, rudder is typically required on takeoff and climb in most airplanes. The need for this is often attributed to "torque" by pilots, including instructors. However, this use of the term "torque" is incorrect. The tendency to yaw on takeoff and climb is due almost entirely to P-factor and slipstream effects. The only direct contribution that torque makes to yawing in the early part of takeoff roll is perhaps a slight increase in the rolling friction of one main wheel because the torque is causing it to carry more weight than the other wheel. Strictly speaking, torque applies a rolling moment to the plane, NOT a yawing moment.
    Anyway--I pointed out to him that you were only making an illustration and I didn't think you were suggesting that pilots should correct low speed yaw with ailerons. People do tend to take things out of context and old WW-II pilots do get persnickety. In his defense, I guess he has seen too many R/C pilots try to horse a tricycle ship out of tall grass with insufficient flying speed and try to correct the inevitable "torque roll" with only ailerons.
Of course I'm not suggesting that yaw should be corrected with ailerons! Ailerons control roll, not yaw. I do maintain however, that torque is not normally the direct cause of yawing effects. The slipstream effects and P-factor are the culprits there, even though torque is often unfairly blamed for something it was not directly responsible for.

However, to be completely fair, the torque of the motor, applied to the air by the propeller, is what makes the air in the slipstream swirl, and that swirl is ultimately responsible for many of the slipstream effects. Without torque, there would be no swirl, but the torque itself is not directly responsible for the yawing.

Also, P-factor, one of the major contributors to yawing, is related to the angular difference between the inflow direction of the air and the axis of the prop shaft; torque is not related to P-factor.

v Just an aside, there are often some P-factor and slipstream effects in cruising flight, and it's not unusual on airplanes that don't have in-flight adjustable rudder trim, for the fin to be offset a little to counteract this. For example, the fin on the Aeronca 7AC "Champion" I used to fly was welded to the fuselage with the leading edge offset to the left (i.e.: so it acted like right rudder), about 1/2" if I remember correctly. The P-factor and slipstream effects were greater than this offset could counteract when the airplane was taking off and climbing, so some right rudder was necessary in that flight mode. The offset was just about right in cruise (the mode where the plane supposedly would spend most of its time), and the airplane would fly coordinated with essentially zero rudder. In a power-off glide, the P-factor and slipstream effects were much less, but the fin was still offset, so LEFT rudder was required to keep the airplane coordinated in a glide.

    Would you be interested in doing a re-write to make it into a short article?? I would love to include it on our club web site-- http://www.andersonparkflyers.org Or better still, perhaps a link to a page on your web site.
Just so long as you don't insist that I say the yawing is the result of torque! ;-)

Seriously though, we've always extended a standing invitation to magazine columnists, club newsletter editors. club webmasters, etc., to use anything on our website, especially in "Ask Joe and Don", that they might find useful. All we ask is that: 1. You indicate where you found it. 2. You don't take anything so out of context that it changes the meaning, or "puts words in our mouths". If you need something re-worded to fit the context of where you plan to use it, let us know and we'll work with you.

Also, it's nice to send us a note if you decide to borrow something from there. Besides the pleasure of knowing that somebody is actually reading these dissertations, it also helps us get a feel for what sorts of information seem to be the most useful for all the readers out there.

There seems to be some confusion about prop effects floating around here again. I think it might be helpful to review some definitions and clarifications.

Torque is the twisting effect coming from the motor that makes the prop spin around. In accordance with Newton's third law (the one about action and reaction), when the motor (which is mounted to the airplane) applies a torque to the prop to make it spin, the prop reacts by applying an equal and opposite torque back onto the motor and airframe. A right-handed prop (i.e.: rotates clockwise when viewed from behind) will try to roll the airplane to the left.

Right-handed props follow the "right handed rule", just like right-handed screw threads. Make a fist with your right hand, then stick out your thumb like you're about to give a "thumbs up" sign. A right-handed prop, when rotated in the direction that your fingers are curled, will make thrust in the direction your thumb is pointing. Left-handed props follow a similar rule except you use your left hand. Most American engines (and therefore their props) tend to turn in a right-handed direction when mounted in a tractor (i.e.: prop on the front of the engine) installation. If you mount it as a pusher, you need to use a left-handed prop to make the propwash blow aft and the airplane fly forwards. Installing a prop backwards does not turn it into a pusher, it just makes it less efficient because the airfoils are now backwards and upside-down.

Props on tractor-mounted older British engines tend to be left-handed. The prop on the DeHavilland Chipmunk I took my aerobatics training in had a left-handed prop. On takeoff and climb it needed left rudder to fly straight (because of slipstream effects and P-factor, more on that in a minute), whereas the J-3 Cub that I'd flown for my primary training required right rudder on climbout, like most American designs. The Chipmunk felt a little strange at first, but I got used to it surprisingly quickly. BTW, the Chipmunk is a truly delightful airplane to fly, despite the prop's direction of rotation.

Screws follow similar rules, and are normally right-handed. This can be handy to know when you're working on a stubborn rusty bolt in an awkward orientation under your car and you want to be sure you're trying to turn it in the right direction.

Torque tries to roll the airplane. Theoretically you counteract it with some aileron, although for most airplanes the amount of aileron required to do this is almost too small to notice. In some cases the airplane may be rigged with a little more incidence on one wing relative to the other to help counteract torque, although this is rare (it tends to create some funny stall characteristics). Airplanes with way too much power and too little airplane, such as WW II fighters and some aerobatic airplanes are some exceptions.

One of the advanced training exercises in a P-51 was to take it up to a safe altitude (maybe about 20,000 feet!), extend the gear and flaps, slow the airplane down to final approach speed, then quickly apply full takeoff power. Even with the stick against the stops on the right side of the cockpit, the massive amount of torque would inexorably roll the airplane over to the left. Novice Mustang pilots quickly learned to respect all those ponies that resided inside that throttle, and to be very careful about waking up too many of them at once at the wrong time and place.

There are slipstream effects that may tend to roll the airplane as well as yaw the airplane. Some folks call this P-factor, although as I was taught, P-factor is something else (be patient, I'll get to P-factor in a moment). Rolling and yawing slipstream effects are due to the helical swirl that the prop imparts to the slipstream interacting with the various parts of the airplane behind it. The classic example is the slipstream of a right-handed prop swirling around the fuselage and then striking the left side of the fin and rudder. This tends to shove the tail to the right, which therefore yaws the airplane to the left. Slipstream effects are influenced by power and airspeed (these influence how much swirl the prop imparts to the airflow), but not very much by angle of attack.

P-factor is something else. Both it and slipstream effects tend to be constant, continuous forces at any given airspeed and power setting, but P-factor forces are generated directly within the prop disk by the interaction between the blades and the airflow. P-factor occurs when the prop disk is not exactly perpendicular to the incoming airflow. Power and airspeed are important, but (unlike slipstream effects) the airplane's attitude is a major determining factor.

For example, imagine a Piper J-3 Cub at full power and in a max-performance climb. The nose is high and the prop disk is therefore tilted up quite a bit. It's a right-handed prop, so the blade on the right side of the airplane is descending. The angle of attack of that descending blade on the right side is a function of the prop's pitch PLUS the angle of attack of the airplane, and the local airspeed that each location along the blade sees is a function of the rotational speed at that radius PLUS the component of the airplane's airspeed that acts in the plane of the prop disk.

Meanwhile the blade on the left side is rising. Its angle of attack is a function of the pitch angle MINUS the angle between the inflow and the propshaft. Its local airspeeds along the blade are a function of the rotational speed at each location MINUS the component of the inflow airspeed that acts in the plane of the disk.

If the airplane were flying with the propshaft parallel to the plane's flight path, there would be no differences in the blade angles of attack and the blade local airspeeds. There would still be swirl, so there would still be slipstream effects, but there would be no P-factor.

However, since the Cub is climbing with its nose high relative to the airflow, the descending blade on the right sees a bigger angle of attack AND a slightly higher airspeed than the rising blade on the left, and so the blade on the right makes more thrust than the blade on the left. This tends to yaw the airplane to the left.

There's another factor that arises from this same effect. Since the blade on the right is seeing both a higher airspeed and a higher angle of attack, it also makes more drag than the blade on the left. This results in a net upward force acting in the plane of the disk. In this case it's trying to pull the nose up. For a plane with the prop ahead of the C/G (such as a typical nose-mounted tractor), this is destabilizing in pitch. Likewise, if the plane yaws, you get a sideways force from the prop that tries to make the yaw worse.

On an aft-mounted prop (such as most pusher installations), these forces tend to fight a yaw or a pitch excursion, so a pusher prop tends to increase pitch and yaw stability (one of the very few good things about pusher props!). For example, when Northrop converted the propeller-driven XB-35 flying wing into the jet-powered YB-49, they had to add four little fins to replace the yaw-stabilizing effects of the props.

This effect is especially important on the V-22 Osprey. When the rotors are tilted down for cruise, the lift to support the airplane is made by the wings. When the rotors are tilted up for "helicopter mode" flight, the rotors provide the necessary lift. However, there is a regime of flight about halfway between those two modes where the combined lift of the half-tilted rotors plus the low-speed lift of the wing is still not enough to support the entire weight of the aircraft. The additional required lift comes from the lateral force in the plane of the rotor disks caused by the difference in drag between the rising and descending blades.

Note, P-factor and lateral forces are continuous. There's another force, gyroscopic precession, which folks sometimes get confused with P-factor. Gyroscopic precession occurs when the propeller disk is being yawed or pitched to a different position, and ONLY exists while the disk's position is changing. It's related to the spinning mass of the blades, and has nothing to do with aerodynamics. A propeller spinning in the near-vacuum of the moon (now there's a useless exercise in futility of ever there was one!) would have gyroscopic precession forces, but no P-factor or slipstream effects.

Precession forces happen whenever you try to change the tilt of a spinning mass. You've probably observed them if you've ever played with a gyroscope. When you try to tilt a gyroscope one way, it reacts by trying to tilt in a direction 90 degrees from the direction that you tried to tilt it. A spinning propeller works the same way.

v Imagine a right-handed prop on a tricycle-geared airplane on takeoff run. The airplane reaches rotation speed, and the pilot pulls back on the controls to raise the nose for liftoff. At that particular instant, lets assume that the plane's right handed 2-bladed (or in propeller industry lingo a "2-way") propeller is vertical. The blade at the top is headed toward the right, and the blade at the bottom is headed toward the left. When the airplane starts to rotate nose-up, the top blade has to accelerate aft, and the lower blade has to accelerate forwards. This means that by the time the blades are horizontal, the formerly top (now right) blade wants to be a little behind the original prop disk, and the formerly lower and now left blade wants to be a little further ahead. The net result is that the prop disk wants to momentarily yaw to the right, and take the plane with it.

Note, this is only happening while the plane is changing its pitch attitude, the effect stops as soon as the plane reaches the new pitch attitude and stops pitching up.

If we yaw the airplane, we get a pitch-up or pitch-down precession from the prop, depending on the direction of the yaw and the direction that the prop is spinning. This is probably one of the biggest culprits behind the somewhat checkered safety record of the Sopwith Camel.

The WW I Sopwith Camel, like many airplanes of that period, used a rotary engine. This rather bizarre variation of the radial engine (i.e.: the cylinders are arranged in a circle like the spokes on a bicycle wheel) had the prop bolted to the crankcase, and the crankshaft bolted to the firewall. The whole engine spun around with the prop! One of the biggest problems of engine design in those days was cooling, especially on the ground, and spinning the cylinders was a very effective way to deal with this problem. The power-to-weight ratios of the WW I vintage rotary engines would not be bettered by conventional non-spinning engines until many years after the war. However, this meant that those tiny and extremely lightweight airplanes had a spinning gyroscope of an engine in their noses that might weigh several hundred pounds. More importantly, the enormous mass of that spinning engine could create some extremely powerful gyroscopic precession effects. Which is part of the explanation as to why there were far more Camel pilots killed in training accidents than were lost due to combat.

The Camel had a relatively large and heavy Clerget rotary in the nose. In addition to its being a rotary, with all the quirks that go with that, it also had a little problem with its carburetor. About 200 feet of altitude after takeoff (just about the time the plane would be making its first turn after takeoff), it needed to have its fuel mixture adjusted a little, or else it would start to sputter and misfire.

Now imagine that you're a new Camel pilot, taking off for your first time. You're climbing out, at minimum airspeed and holding a whole bunch of rudder to counteract the P-factor. You reach the altitude for your first turn (about 200 feet above ground), the engine starts to sputter. Your attention is immediately drawn to the sparse instrument panel and the motor controls, and the sudden mental workload causes your leg muscles to relax on the rudder pedals (studies done for human-powered aircraft demonstrated that the work a human can do drops quite dramatically if they also have to think about something at the same time). The rudder deflection decreases a little and the airplane creeps into a slight yaw.

Meanwhile the plane is still turning, changing heading, which means there are gyroscopic precession moments being generated. It just so happens that you're turning in the direction that creates a nose-up precession effect, and you're already nose-high and at low airspeed due to the climb. The still-sputtering engine is losing power, and it plus the nose-up effects of the precession cause airspeed to decay, until the plane stalls. The nose drops suddenly, and the combination of precession, torque, P-factor, etc. causes the plane to stall one wing first, and the plane goes into a spin. However, you're so low that when you suddenly look up from the engine controls to see the ground coming up VERY fast, you don't see the spin's rotation. In a final moment of panic you instinctively yank back on the stick, sealing your fate (although from 200 feet you probably don't have room to recover anyway, even if you did everything correctly). The plane has time to do about a quarter turn before impacting the turf and turning you into another sad statistic. You probably don't even realize that it was a spin that ended your career, as well as everything else for you.

However, years later an astute reader looking at an old photo of your Camel's wreckage in a history book will see that it was a spin that resulted in your untimely demise. Clearly visible in the picture, one set of wings is wrapped slightly around the top of the fuselage and the other wrapped around the bottom, indicating that the whole airplane was rotating when it hit.

There are a number of other prop effects, but we'll leave those for another time.

In general, prop effects are not likely to be all that huge for our models, at least not in comparison to what's seen in full-scale aircraft. The can influence the plane's flight trim, but (as Ted alluded to) for low powered models such as the Pico Cub, other things such as warps in the wings and problems with the alignment and the other flying surfaces are likely to be just as important.

Don Stackhouse
DJ Aerotech



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