MajorTomski

For a spiraling slipstream to exist, you are saying that the airfoil of the propeller is accelerating the airflow behind it in the same direction as the blade is moving.

So does the air moving over the wing at some point below and aft of the wing now have forward motion in the same direction as the aircraft?


If the air flow does not spiral, but because of P factor is simply moving faster down one side of the fuselage than it is the other would you not see the same effect on the fin? i.e. the fin will yaw towards the higher airspeed side?

Darrel on your free flight comment. I still have to look into that one. Most free flight designs with the pylon do not have a 1 to 1 counterpart.

I do know that many supporters of the spiral theory will point out that several multi-engine aircraft have rudders burried in the slipstream to elimitat the yaw. Examples the Beech 18 or Me/Bf-110. but if you look at the side profile of either of these aircraft then by their own argument the slipstream effect on these two espcially should be TWICE as bad becaue the fins are at the worst location relative to the slipstream.

da Rock

For me? Why thanks, but I know whether it exists or not. And have been on the ramp with lots of planes, even DC3s. (Which of course has 2 engines out on the wings.) BTW, back in the day, starting those gasoline engines provided some smoke that was ejected right into the prop blast. Well, often there was flame too, but that event was over before the prop blast could be called 'blast'.

If it was for me, it'd be a PICTURE of the phenomenon and could have strings or not.

Rotaryphile

Torque, of course, is inevitably produced by the engine in converting fuel chemical energy to mechanical energy. Any engine will exert a torque on its engine mount, and thus, try to roll the airframe in the opposite direction to prop rotation.

The spiral slipstream can nearly cancel the twisting effect produce by the engine, by interacting with the wings and tail. A good example is a 3-D model hanging on its prop. Its engine is producing plenty of torque, which tends to roll the airplane in the opposite direction of prop rotation, while the spiral slipstream tries to roll the airplane in the same direction as prop rotation. If the ailerons are deflected a bit to the right, the airplane can hover motionless, with engine torque completely counteracted by the interaction of the propwash with the wings and tail feathers.

eddieC

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Otter Guy

You are correct with the example of the gyro. The same thing happens with floats as you come on step as well. Ive flown a certain 172 that really needed a ventral fin, you needed full rudder just to hold the P-factor (high AOA on the decending propellor blade) then when you drop the nose over onto step you have nothing to stop it carving left, bad enough to turn about 10 degrees and depending on the wind you needed to plan for it to put you on your takeoff heading. 185's seem to have enough rudder without extra fins, but the 206 has a ventral usually. seems to be enough for the propellor effects. turning right during a takeoff doesn't seem to be a problem.

the most significant phenomenon on floats relating to this topic is another taxi problem. on a very windy day aircraft are very tough or sometimes completely impossible to turn out of wind and turn around. you can get nearly 90 degrees and full rudder, proper aileron, water rudders, and power setting are just unable to counter the force of the wind on the tail weathercocking you. the otter is the absolute worst, the beech 18 is one of the best. the beech has a few cheats though, assymetric thrust being one, but having 2 small tails means only 1 at a time is really getting the full force of a cross wind. some airplanes like cubs, only have 1 water rudder to limit your ability to turn out of the wind so you dont flip over from a gust under the tail. buuuuuut. I have noticed this the most in the 206, and some in the 185, assuming the water rudder rigging isn't all screwed up, turning left is faster, and more likely to be succesfull turning around in wild winds. this is with a power setting barely above idle so P-factor is negligable, the only thing that is helping is the swirl of the slipstream keeping the airflow over the left side of the vertical and rudder alive on the downwind side.

I tried to look up some videos of those prop trails, but motion photography just doesn't seem to capture it very well. when you get to see them up close you can see them generate and make maybe a quarter turn around the aircraft before they disipate. also had a quick look for some grungy pc 12's. seems sales websites only feature polished examples lol. if you watch some videos of one running you can see the exhaust heat from the left side come out and up, and the right goes out and down, tough to get good angles and backgrounds that show it well. the soot on the sides after 20 hours is unrefutable.

I've lost track of who mentioned reving an engine to illustrate torque. that shows a different sort of torque. you're accelerating a bunch of heavy parts relative to the chassis its bolted to. if you hold a car at 4000 rpm in neutral, your not producing very much torque, just enough to overcome the drag of oil pumps and accesory drives, if the engine was sitting on the ground relying on you to hold it upright, sitting there at 4000 rpm, you will just be leaning on it keeping warm. now bolt it to a load, like wheels resting on the ground, or a propellor, goooood luck! like the example of the top fuel dragster, with a load on that shaft whatever the cylinders are bolted to is going to be stressed. Turbo prop engines have a gauge in the cockpit literally reading out the torque pressure being put out. its usually an arbitrary unit, like %, or the oil pressure exerted by the measuring device, but the really big radials often had a BMEP guage that was calibrated in such a way as to tell you your actual power output. kind of a wierd idea really. mesures the strain on one of the ring gears in the reduction box and reads in brake mean effective pressure of the cylinders. I hear it was a great tool for knowing when you were about to blow a cylinder.

the stators in turbofans and turbo jets are much more complex than just straightening out swirling airflow, it does result in a net torque of zero though. a jet stator is a divergent duct ment to reduce the velocity, increase the density, and smooth the airflow before the next rotor has to bite it thus making the next rotor more efficient and also less likely to be shaken apart by vibrations made by turbulent airflow. if you put a stator behind a propellor you may reduce the torque effect on the airframe to near zero, but your just moving the part that gets stressed over it.

I would like to clarify, are we debating the existance of a spiraled slip stream? or the resulting forces from an alleged spiraled slip stream? or just the relative contribution of a spiraled slip stream in the bucket of complex turn tendencies inharent to a propellor?

eddieC

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OG, great post.

MTK

Windtunnels also have flow straighters in front of the sucking fan. Remember that a rotating wing will turn the air immediately in front of the blades as the air is pulled in and thrown back.

In models precession and torque are minor. Just not enough mass being turned. Spiral air streamplays a greaterrolefor abouttwo wing chord's (MAC) lengths, damping beyond that to the point of nonexistence in the free stream.

If a model had lets say, 2 degrees of right thrust and one were to add small cannards (just a few square inches (10) are enough area)just behind the prop blast, the plane would turn right following it's thrust vector. The cannards are flow straighteners. Take the cannards away and the model flies alongthe more expected straight path.I've actually done this experiment. Very enlightening

eddieC

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MajorTomski

As you say, respecfully disagree.

The cub is an example of P factor and P factor only. Torque by definition cannot cause left yaw. The precession P factor, by definition, results in nose up pitch.

eddieC

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charlie111

Just wanted to add some input.Any Tubulane can affect control surfaces.Hard right rudder is explained with (P) Factor.The Guy with the Pusher still has His Tail Behind the Prop. So it would have an affect on the Vertical Stabilizer.I use Prop Wash to Steer My Electric Rotowing! Check it out.Next is a Version that is powered By an Electric Car Driven into and Pluged in!! A Bird Cage (Platform).Roll Over Sigorski Todays Heli's are Dislecsic!

eddieC

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Another good explanation:

Quikturn

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+1 from a former flight instructor!

da Rock

You guys know there is gyroscopic precession and it's not p-factor.

Also, both gyroscopic precession and p-factor can yaw both left and right.

Gyroscopic precession happens when you attempt to pitch up or pitch down. P-factor happens when the prop is pitched up or down. The Cub can be used to illustrate the difference. When it's taxiing there is no force couple acting on the prop so there isn't any gyroscopic precession going on. However, there is definitely a significance in AOA difference between the left side of the prop fan, and the right side. So there is p-effect working at it's best/worst. As the tail comes up, that's a force into the prop that brings the gyroscopic precession to life. It also removes the different AOAs, reducing the p-factor / p-effect to zero. As the Cub rolls along, the gyroscopic precession dies away since there isn't any pitch change going on. Both forces wander off and look for other pilots to bedevil.

Then when the pilot of the Cub pulls the stick back, he introduces a force couple into the prop and wakes the gyroscopic precession up. Whatever pitch up exists also wakes up the p-effect and give it something to do.

Both forces are somewhat more complex than many people give them credit for.

da Rock

The Cub takeoff starts with an acceleration with the tail down and the pitch stabilized. There will be p-effect. It will be greatest while the tail wheel is down and will go away as the tail rises.

As the tail rises, the gyroscopic precession strength tracks the rotation.

While the acceleration continues with the Cub level and on it's mains, ain't nothing happening with our two devils.

At lift off, both devils come alive.

Keep in mind that gyroscopic precession will try to yaw the Cub in one direction while the tail is coming up and will want to yaw the nose the other way when the pilot pulls the stick.

da Rock

BTW, p-factor will try to pitch the nose up or down when the rudder is trying to yaw the plane.

And of course, gyroscopic precession will work it's magic whenever anything tries to point the nose anywhere other than straight ahead. 360 degrees......

How anyone can come up with sensible force estimates for the effect the spiral slip stream exerts, and separate the amount of force for it from all the other forces is beyond me. Maybe that's why they invented wind tunnels.

Quikturn

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This has been discussed in another thread so I thought I would copy and paste it here.



RE: Is left yaw induced by fore or aft force? - 10/16/2012

This is an interesting thread and I'd like to add to it by listing and talking about the 5 left turning tendencies in single engine airplanes.

1. P-Factor - is the asymmetrical thrust produced by the propeller. Every airplane needs some positive angle of attack (AOA) in order to fly and this AOA (angle of the wing vs relative wind that the airplane flies in) gives the propeller blade on the downstroke a higher AOA that the propeller blade on the upstroke. Since the propeller turns clockwise as seen from behind more thrust is produced on the right side causing the airplane to turn left. P-factor is pronounced at high AOA, low speed and high power settings. I doubt there is much p-factor during a perfectly vertical climb because AOA is minimal and gravity is at the tail. However, if you're at full power and pulling vertical you'll likely have p-factor.

2. Spiraling Slipstream - is the twisted airflow that strikes the left side of the tail as it flows back causing the airplane to turn (yaw) left. Think of it as a slinky around the airplane. Spiraling slipstream is more pronounced when airspeed is slow and power is high because more power pulses from the prop hit the tail. As speed increases (the slinky is stretched) fewer power pulses from the prop strike the tail reducing the effect. Also, flying surfaces become more authoritative with speed. I believe some of the newer pattern ships experimented with a horizontal airfoil above the canopy in order to minimize the effect of the spiraling slipstream. A lot of kits will have about 3 degrees of RH thrust angle built into the firewall to minimize the effects of this as well.

3. Gyroscopic effect - as stated earlier a gyro will react to a force 90 degrees ahead of the plane of rotation. That is, if you push on a gyro at the 12 o'clock position it will also react at the 3 o'clock position. So as a tail dragger rotates off its tail wheel (12 o'clock) on takeoff it will have a tendency to rotate left (3 o'clock) as well. This effect is pronounced with a bigger and/or heavier propeller. Think warbirds with 4-strokes and big props.

4. Torque - Since the propeller turns clockwise as seen from behind the counter effect is to turn the airplane counterclockwise. This is really a roll to the left. As with other forces, torque effect is more pronounced at low speeds and high power settings. A large and/or higher pitch propeller will a increase torque effect. You can see this in action when someone is in a 3D style hover a little right aileron is needed to counteract torque to keep the airplane from rolling left.

5. Drag - this is actually a side effect of torque. Torque on a takeoff run will put more pressure on the left main tire causing more drag and a turn to the left. I'm not sure how much this would affect us rc modelers but if you fly a small single engine airplane you may want to check tire pressures before flying next time.

As you can see, there is a lot of stuff going on here. It's all very dynamic and none of it can really be isolated. The first 3 turning tendancies can be compensated for by using the rudder. If you want to eliminate the left turning tendencies the most practical way is to go with jets.





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AA5BY

And... I've argued that there could be possible pressure differences on sides of fore sections inducing a yaw force. This based on observation that planes that have round symmetrical cowls or fore sections with center thrust line seem to exhibit less left yaw tendencies than planes having a high engine thrust line on asymmetrical cowls or fore sections.

Think on it. Think about your planes that show little left yaw and if they have a center thrust line on a vertically symmetrical fore section and compare to those that suffer significant left yaw, do they have a high thrust line on a vertically asymmetrical fore section?

MajorTomski

In a high AOA high power low airspeed situation where there is maximum P-factor thrust running down the right side of the fuselage, the load from that thrust, acting through the propeller due to gyroscopic precession will result in a pitch up moment on the airplane.

Think about it. The wind is blowing faster down the right side. The resultant force pulls the nose up.

Oh, Aero Engineer since 1977, priviate pilot and A&P since 1976.

Quikturn

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But maximum P-factor is during high AOA and HIGH power.

MajorTomski

Sorry typo, fixed

drac1

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That's exactly what i was going to say Matt. You are spot on.

While there would be some spiraling of airflow as it comes directly off the prop, by the time it travells down the nose and over the wing (which would act like vanes and straighten the airflow), by the time it reaches the tail i can't see how the airflow would be still spiralling. Just as with contra rotating, how can the airflow be straight? The rear prop would still spiral the airflow albeit in the opposite direction. I think that torque is the biggest cause which is being confused with spiralling airflow. That's why contras don't need right thrust, props turning in the opposite direction cancel out the torque effect not spiralling airflow.
Of course this is just my opinion as i see it.

dbsonic

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I was going to mention that, like wind tunnels, ducted fans have stator vanes in the shroud. In particular EDF guys are tending to add more shroud stators to increase performance. You can get away with 4 stators(synergistic with mounting the center body), but the newest shrouds have as many as 12, presumably to straighten the flow for greater efficiency? The fan blades tend to have very significant pitch btw(at least 14 degrees).

Otter Guy

on thing a stator does is trade air velocity for pressure using divergent ducts. a side effect of slowing the flow of a fluid in a smooth section is a drastic reduction in turbulence, which would help the efficiency of the stators behind them. i would imagine that a EDF thats only going to top out at 100 and somthing mph, maybe, isn't going to be all that efficient about putting its power to the atmousphere and stators slowing and condensing its gas stream ads a little more weight to its EPR, maybe increasing its speed through the tip of the tailpipe.

the force reacting to the thrust on the downgoing blade, would act on the bottom of the proppelor arc, adding a pitch input, conteracted by trimming the tail, adding an opposite pitch input, according to that logic, all the off angle, non 12 3 6 9 oclock positions cancel each other out every 180'. so no we are left with no yaw input.

if you look at a heli rotor and the obstacles it tackles automatically ever 360 degrees it can show you something. to adjust the rotor disk to move forward you push forward, this sends an input to add angle of attack to the rearward going blade, cornholious, raises the rotor disk at the rear, 90' to the original input, but the heli slides forward. as it picks up speed, the rearward sweeping blades lose airspeed as they are trying to go through air fast enough to make lift, while at a deficit of 100 plus knots. your going to need to add a roll input, add AOA on the rearward blades, and reduce it on the forward going blades. luckily this is usually automatic, as the forward blades raise the disk on that side, it causes the blades to receive an adjustment from the swashplate or similar mechanism, to cause the appropriate action to stop rolling the helicopter. this is all great up to a certain speed, usually not overly fast, big limitation of the heli. you hit a speed where the rearward blade stalls. but when the blade stalls on the side, that should cause a pitch input 90' off. curiously, it responds like a wing drop.

but your already going really fast, so its seems like a crazy place to be if you ask me, im guessing you lower the collective and pitch up to recover, but im not a heli guy.

playing with a contra would be really fun. that airflow is all kinds of ****ed up. but the torques and P factors would be very close to equal, and i sense some agreement among the yay sayers on this existance that whatever force the spiral really has, is small, and less active with forward speed. i think its greatest power is in making a mess no matter where you put your drains and exhaust pipes

rmh

anybody want to trade a spiral prop flow for a pair of downwind turns?
I'll throw in a high lift , no drag wing -