RCC Supreme Contributor
I am: Ronm
Join Date: Dec 2002
Location: Mission, B.C.
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# of RCs: 10
Total Props: 0
Here is an article from an expert.
His explanation matches my own beliefs, but there are other experts with different explanations. The most important thing to remember is that it is not "Engine Torque" that we are dealing with by changing engine thrust.
He explains that in this article.
Aerodynamics for Modellers by Don Stackhouse
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 propwashblow 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 andairspeed 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 wouldstill 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 wasone!) 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.
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 theprop 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 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 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
(former full-scale propeller engineer in a "previous lifetime")
Flying Tigers RC Club
"Flying an airplane is just like riding a bike...except it's harder to put cards in the spokes"