flyncajun

http://www.rcuniverse.com/forum/m_11...m.htm#11429493

Lnewqban

Yes, it is real.

speedracerntrixie

My Feedback: (29)

I'm going to say full scale yes, models no. I have seen hundreds of models with smoke systems, I have never seen the smoke try to spiral around the fuse.

MajorTomski

Ah another diciple drinks the coolaid.

Been arguing spiraling slipstream is a myth that was created when "Stick and Rudder" was published.

The true exsistance of a spiral has never been proven.

Folks love to explain P factor as the source of the yaw, but the same folks ignore that the physical manifestation of P factor is higher velocity air flowing down the right side of the fuselage, BUT the result of the force from that increased lift is a pitching UP moment on the prop shaft.

The high speed air from P factor will have EXACTLY the same effect as the suposed spiral.

But if the spiral truely existed the aircraft a low airspeed and high alpha would roll to the right almost uncontrollably, which it doesn't.

Rotaryphile

The spiral propwash definitely exists, but the helix angle is quite small at normal flying speed, just a couple of degrees, and much larger at standstill. For visual proof, try holding a piece of string about six inches long behind the prop with model standing still. I have noted a spiral angle of about 12 degrees with high pitched props (16x13), and about eight degrees with 12-8 prop, both during static testing. (Zero airspeed.)

da Rock

Sounds cool.

In this day of digital cameras it would be nice to see a picture. It would mean a lot more than the pictures of taxiing planes and their prop tip contrails that are hanging in the air.

BTW, the fact that those tip contrails are hanging there destroys their value as proof of any velocity or direction of airflow. The airplane is simply taxiing away from them and they aren't going anywhere. On the other hand, a picture of a string that showed direction.....

da Rock

There have been a number of prototypes all "tufted up" to show airflow. The famous "Fuzzy" F6F was one of them iirc. Wonder where the pictures are now, when they would at least show boundary layer flow.

AA5BY

An interesting observation is the exhaust droplet deposits from a gas engine. I've a mixture of pitts and port side down stacks and observe that the port side stacks deposit more droplets than the pitts.

The droplets collect on port side horizontal stab both under and over surfaces and slightly on the port side of vertical stab. Usually, the ratios will approximate 60% under, 35% over and 5% vertical.

Do these deposits tell any thing about whether there is spiral slip stream or to what degree?

da Rock

Those would tell something. However, my pitts exhaust deflects back into the underside while the plane is sitting and while it taxiis out and back in after landing. So the residue might not tell much.

Most of my planes show a bias just as yours, however.

MajorTomski

And I've got shots of the exhaust off a BE-300 that people will swear comes off in a spiral but my shots show it's symetrical. Someday I'll figgure outhow to get them posted here.

MTK

Another thing that I've found on aerobatic models is this:

The SINGLE prop turning CCW when viewed from the front (to the right when viewed from the back) would require a reactive force in the opposite direction (to the left when viewed from the back). Yet, on every aerobatic model I've flown over the past 35 years, rolling right is always faster than rolling left. Assume that I'm very fussy about setting the model up with identical throws, left and right, during the initial set-up on my bench

Considering that engine reaction force should enable left roll to happen faster, it doesn't stand to reason that right roll in facthappens atfaster speed for a common amount of throw left and right roll. In every case I adjusted throws to compensate.

This effect is hard to see on a trainer and some sport models since thrust to weight is usually at most 1. Aerobats on the other hand usually sport thrust to weight >1 and in most cases much much >1 (2-3). This doesn't prove the existence of the spiral air stream from the prop, but doesn't disprove it either.

But air is air and it tends to re-establish steady and smooth conditions rather quickly. As fluids go, it damps quickly. By the time air hits the tail, it's far smoother but not 100% I don't think. Hence the oil droplets differential stab right to stab left.

In a different thread on a similar subject I suggested to anyone that,has an E-aerobat to drive the motor in the opposite direction. It assumes that a proper propeller is available to allow this of course. I'b willing to bet dollars to donuts that rolling speed difference would be reversed .....

Darryl Usher

As a builder of free flight models before RC, the same model with the rudder on bottom will turn
one way and with rudder on top it turns the other way.
Darryl

Otter Guy

The spiral is in fact real. full scale, as well as scale aircraft use thrust angles as well as crooked verticle stabalizers to trim it's effects out. as was mentioned about residue on the aircrafts, so far all of my models have had an exhaust on the right side, nearly all the grease is on the bottom on a PT 40, an 80'' cub and a 72'' beaver. Im soom to maiden a 72'' otter that ive set up with a functional scale exhaust exiting the left side and im expecting a mess up the left side.

having spent years with the pilatus pc 12, i can tell you with semetrical exhaust stacks, the grime is very different on each side of the aircraft. the left side has a black stripe up the fuselage that follows the contour of the wing and disperses down the tail, and the right wing root and root fairing and landing gear bay get pretty gross, also following down the tail on the bottom side coating the right strake. also the gunge ends up inside the tail through a large vent in the door on the belly.

yes things like the wings and the tail surfaces have a tendency to straighted out this twisting flow, newtons third therefor means to straighten out the twist is to absorb its energy, resulting in a yaw and roll tendency.

the point about the prop trails on a moist day is also moot, they dont show up at a taxi power setting, only when you throw the coal to it, and half way to takeoff speed they fade away, you dont taxi through them, they are propelled very quickly, its pretty cool to see a loaded otter throw prop trails, or a big Herc. P factor is a much larger component in all the left turn tendencies and the slipstream is quite small, but its there, undeniably

da Rock

One way is to edit your post. Click the EDIT button. Then look below the editing window for:
Click here to upload images and files

Click on that and use the window that opens to browse into your pc and select one of the pictures you want included. etc

rednekk58

If your talking "wingtip vortex?" It sure is real on full scale planes.

MajorTomski

Well Otter I got first interested in this issue about 20 years ago. I am and aero engineer and for most of those 20 years I've been hunting for ONE textbook that mathematically defines the spiral so that the design of the crooked tail feathers can be established.

Yet every book I've ever looked at only re itterates Stick and Rudder.

BMatthews

In that picture of the C130 prop trails and in other similar pictures keep in mind that you're seeing a vapour trail off the tip of the rotating propeller. The air may well be moving straight back from the prop with NO spiral motion but the prop would still produce that corkscrew vapour trail because it's rotating within a moving airflow. It simply is NOT evidence of spiral airflow or not. It's simp;y a vapour trail that is being produced in a moving body of air which happens to look like a spiral because it's being generated by a rotating propellor blade within that airflow.

Having said this I do believe that there is SOME amount of spiral airflow from the prop. The prop blades have drag and that drag, if nothing else, is going to pull the air with the blade to some extent. Now how long it exists before being damped out by the rest of the airflow I'm not sure. But if there was no spiral component to the air right behind the prop disc then why are turbojet compressors equipped with airfoil shaped stator blades to catch and straighten the air off the back of the upstream blade in a shape which would only be done to counteract the spiral component?

Rotaryphile

If the propwash was not spiraled to some degree, the engine would exert no torque on the airframe. (Newton's Third Law says that for every action there is an equal and opposite reaction.) Torque is caused solely by the spiral propwash. Fixed vanes could be mounted behind the prop to straighten the flow, which would reduce the engine torque to zero, but they would cause more drag that could be justified. The wing and stab of a normal tractor configuration airplane tend to straighten out the flow to some extent, reducing the effect of engine torque. Deflecting full-span ailerons can totally remove the spiral of the propwash, and thus, reduce net engine torque on the airframe to zero, enabling an airplane to hover motionless, with its nose facing vertically upward.

The tendency that many aircraft have to swing to the left on takeoff is almost solely caused by the spiral propwash. I frequently use large sub-fins and sub-rudders on my own designs, which, being under the prop rotational axis, tend to oppose the force exerted on the upper portion of the fin and rudder. I have found that the sub-fins and sub-rudders cancel the side force on the vertical tail to the extent that no rudder deflection is needed on takeoff.

GREG DOE

Proof positive. Wind tunnels have vanes to straighten out the air flow. Some wind tunnels also force the air around a cornor for the same reason: to eliminate the spiral in the air flow. As already stated is small, but it's real.

oian

My Feedback: (2)

Spiral airflow (against the side of the pylon) is why pylon free flight models tend to fly to the right under power which gives a very stable climb. Trying to (or allowing one to) go left under power will usually end up in a crash.

John

wellss

It is physically impossible for it not to exist. That would imply that there is ZERO drag from a propeller blade or any other surface moving through the air.

http://www-mdp.eng.cam.ac.uk/web/lib...ler/prop1.html

AA5BY

I'm not aboard on that theory. While I'd never argue that it isn't a player, I don't think it is a major one. I've argued here before that I think the greater left yaw force is forward on the aircraft rather than aft.

Lnewqban

Excellent post and link, wellss !!!

Demonstrating its existence is as simple as placing a string behind a rotating propeller or observing the bubbles produced by a boat propeller under water.

MTK

"Torque" is caused by the engine's rotating mass (crankshaft). It is always in the direction opposite rotation. Torque and air flow caused by prop action are not the same thing. Straightening vanes immediately bihind the prop will straighten flow immediately behind the prop and the free airstream when the plane is flying will dampen out what remains. But torque will still be there. If you're not sure, just rev your car engine and observe torque in action. OR if your model engine is soft mounted, simply rev it and see where the cylinder goes. OR, simply install a flat pitched prop on the shaft, one that generates zero air flow. "Torque" is still there.

eddieC

My Feedback: (2)

MTK beat me to it. While the prop affects it, torque is produced from the rotating mass of the engine components. Stand behind a dragster as it blips the throttle, you'll see the whole car try to twist longitudinally. As it takes off, it unloads the right side momentarily.

P-factor isn't as big a factor on our models as it is full-scale, and hopefully Otter Guy can chime in here. In full-scale tailwheel aircraft, it's easy to raise the tail too early in the takeoff run and veer left. The more abrupt the pitch (down), the more abrupt the left turn.
I attribute this to the gyroscopic precession aspect in P-factor, wherein the force acting on the gyro (prop) is from the top of the prop and behind it. Precession reaction is 90 degrees in the direction of rotation, clockwise from the pilot's seat, so it's as if something pushes hardfrom behindthe right side of the prop disc. This is further exacerbated by the low airspeed, which means little rudder effectiveness to straightenthings out (right rudder, in this case) and also spiralslipstream, because the spiral is tighter at low speed and the left side of the vertical finfeels greater pressure from the prop blast.

(Rock, this is for you - no wimpy string needed. ) Spiral slipstream is easily felt behind any prop plane, from a 150 to a DC-3, by standaing behind the plane while the pilot revs the motor (of course, first discussing this with the pilot. I would suggest doing this facing rearward, or with goggles for eye protection), Standing behind the tail at idle, there's a pronounced blanking effect by the left side of the vertical fin, such that there's a calm area close to the right side. Move to the left side, and there's a gale of air assaulting you. As the aircraft moves forward and gains speed, the spiral is elongated and has proportionately less effect. As speed is gained. the rudderand vertical fin have much greater authority and spiral slipstream has almost no effect.

It does exist, especially at slower speeds.

Major Tomski, the 300 is one of my favorites. Just wished the control column came out of the floor, like the Merlin's.