Hi guys, I attended the EAA Flyin at Oshkosh, WI the last week. A new electric scooter trike mobillity assist chair thingy (Pride Revo manufactured) rescued my heart and knees and made prowling the miles of airplanes a realistic thing this year. I shot a lot of photos and have put up some of the more interesting (to me at least) on my Fotki site at

http://public.fotki.com/benlanterman/2004_eaa_flyin/

One of the interesting things I saw was a result of a formation of T-28 trainers flying over with smoke systems on. The result was that if you consider the formation as being an equivalent big airplane that at every 3 or 4 airplane lengths behind the formation the smoke formed into long rectangles like links of a chain.

The ends of the chain away from the front of the formation formed the very tight vorticity action shown in the photo (I hope it will). The loose end is upwind toward the nose position. It would seem that the concept of bound vortex than extends behind the wing is real. The interference between several of the natural frequencies involved seems to have resulted in the smoke filling the center of the vortex located at every 3 or 4 equivalent fuselage lengths.

Things like this make aerodynamics interesting.

I'll also attach a photo of a warbird pulling high g's on a moist day (it rained for part of 2 to 3 days). The tightness of the wing tip vortes is a joy to see.

Also is a condition where only one tip of a four blade propellor is leaving a vortex trail. Very interesting.

Nice pictures Ben!

The pic of the Super Corsair should make anyone a believer in the spiral slipstream effect

This was only the second time I've missed Oshkosh in my life.

Ben, thanks for sharing the pictures with us. I envy your opportunity to attend Oskosh.

Baron, the spiral around the Corsair doesn’t show spiral slipstream effect. It only shows the path of the propeller tip. It is the vortex being shed from the tip of the propeller like the vortex shed from the tip of the wings in the other picture, and it traces the path of the tip. The twist of the slipstream is more like the grooves in a rifle barrel than threads of a screw. It is only a slight twist and is result of downwash from the propeller like downwash from a wing.

LouW - You're probably right on this pic since it's moving, but I've seen a picture that looked very similar to that, although less dramatic, when the plane was running up at a stand-still... I know that wasn't the path of the prop. I suppose it was the path of the prop 'through the air' just not 'across the ground'. Makes for a cool picture either way!

Don't forget, when the airplane is stationary the air is still moving past the propeller disk with good velocity. The vortices from the prop tip also move rearward with it making the spiral pattern, just as the wingtip vortices move downward after the wing has past. If the twist of the slipstream was actually as tight a spiral as the pictures suggest, control, and indeed level flight would likely be impossible. The actual twist of the slipstream is quite gentle by comparison.

You have to consider 4 of those vortices for each prop revolution...

Watching vapor trails from high altitude jets is an interesting way to laze away an afternoon, especially when the type of jet leaving the trail can be identified.
I've seen the "donuts on a rope" said to be characteristic of the mythical "Aurora" quite frequently behind Lear Jets and T-38s.

LouW, we may have crossed paths before, if not let me ask your input. Have you ever seen a mathematical proof or even controlled photo graphic evidence that that he spiral slipstream, even the gentle one you allude to exists?

I ask because I have found text books that state that the down wash off the prop is defined by the spiral off the tip as a sheet of backward flowing are between that spiral and the hub.

And the !QUOT!the airplane yaws left on takeoff!QUOT! theory due to slipstream doesn't logically hold up if you look at the top view of an aircraft under the same conditions. If the slipstream spiral is strong enough to cause enough left yaw so that the pilot has to hold as much rudder as shown in the above picture then the same amount of forces would also be acting the wing and horizontal stab causing a roll to the RIGHT. Something that doesn't happen in the climb out.

Just curious
Tom

very nice pictures; thanks for sharing. the condensation off the ends of the prop only highlights the core of the vortex. The actual vortex is larger in radius. Only portions of the vortex where the pressure has dropped sufficiently, will you see it visibly through the condensation effect.

A propeller is merely a rotating wing. What is seen in Ben's picture is the core of the vortex shed off the tip like teryn1-RCU said, not the path of the swirling slipstream. Just as a finite wing has downwash, the finite propeller also must have downwash except instead of moving down, it is moving tangential to the radius of rotation. This tangential velocity is added to the rearward velocity of the prop wash resulting in a slight swirl in the direction of propeller rotation as shown in the sketch.

Notice that the slipstream past the rudder in fact results in an angle of attack at the rudder causing a force tending to yaw the aircraft to the left. You are correct that the same swirl increases the AOA of the crosshatched portion of the wing root on the left and decreases it on the right side. However the moment arm (and the resulting rolling moment) is quite small. The torque from the engine driving the propeller produces a rolling moment in the opposite direction and in most cases this is large enough not only to cancel out the rolling moment due to the different AOA’s but tends to roll the airplane in the opposite direction.

In the late 1930’s free flight rules were changed to limit engine runs making the contest flight profile a high powered climb followed by gliding flight. The free flight model airplane in essence became an engine-launched sailplane. Designs evolved quickly to control the suddenly significant effects of high power slow speed climb. Two features quickly appeared, a pylon mounted wing, and a large sub-rudder. From the sketch of a typical example, with just a little downthrust, the area above and below the thrust line become approximately equal. This eliminated the left turn tendency due to slipstream swirl. The swirl passing the pylon causes a rolling moment to the right to cancel out the remaining torque effects. With this setup the model could be trimmed to climb to the right and glide to the left with great efficiency. Such features do not carry over to man carrying machines (or R/C models) since the forces are not great and are easily controlled.

Any idea why only one visible vortex trail off the four-bladed prop?

Jim

I am guessing that there is some small bit of dirt or roughness that made the difference. Just enough to set it off.

It's strange how we think. At least the way I think (according to my wife). As I stand behind of a prop blast I imagine a completely turbulent mess of air mass moving aft. The vapor trails would seem to indicate that the propwash is a very coherent set of vortex actions and interactions that have strong boundaries many feet aft of the prop.

Speaking of vortex actions, we had an electric fly for fun at the Boeing field here in St. Charles, MO. The corn at the edge of the runway runs east and west and is 7 feet high and thick. The wind was blowing from the north fairly strongly. I took off and turned to fly about 3 or 4 feet high parallel to the corn and about 10 feet from the corn but drifting toward the corn. Suddenly the airplane did a perfect axial roll (hard to do with a GWS Tiger Moth) and then some less than perfect rolls impacting the ground. I figure I flew right into the vortex. Neat to see at only the cost of a new plastic cowl.

Looking closely at the corsair pic i can make out 3 of the 4 vortex trails coming off 3 of the prop blades. Very cool none the less. As an side note, while plane watching at YXU London, Ont. at the end of the runway. When an lear jet crosses the threshold He is at about 100' or less. and once the plane passes above your head, at about 5-6 plane lengths later you can hear the vortices cross over your head and feel them. Very cool especially at night.

Grinder.

The -downwash- from a 747 or Tristar after it's passed overhead is quite impressive, if you can get to about where the middle marker is.. the planes are about 250' up then, and the vortices really stir the ground up...

That's a fact!

My girlfriend and I used to have a strange hobby, back in about 1978. We were in Minneapolis, and would go out to Minneapolis/St. Paul airport, where you could stand with your back up against the blast fence, as big jets passed overhead on final approach.

When they first passed over, you could hear (off to the sides, below the wingtips) the vortices "snapping" like whips. Then, a couple of seconds later, we'd be sucked up against the blast fence. (not hard enough to hurt, or be dangerous, but an interesting effect, nontheless) Th effect was most pronounced with 747s.

At an airshow I attended last year the humidity was pretty high. A friend of mine came and we saw the same vortex trails from an AT-6 prop. He also mistook them for the spiral airflow caused by the prop. A debate ensued. It was pretty easy to disprove because the vorticies spirall in the opposite direction of rotation if the prop. It is not too hard to visualize the reason why they spiral in the opposite direction. Just imagine the position of the moving prop with respect to the rearward airflow. For any disbelievers take another good look at that photo of the Corsair.

Those were the days. I used to spend a lot of time at the observation area at the airport before they closed it. It was right at the end of the runway off to the side and there was a trail that took you to the very end of the runway. It was fun to stand there as a plane landed. It is amazing how powerful the wake of the plane seems when standing in it.

I was watching the documentary the other day on the Wright flyer that was recreated for the anniversary of flight. The image that stands out is the propeller being tested in the wind tunnel. They had a smoke trail running through the prop. I know that the prop was not turning all that fast, but there was no noticeable change in the flow after the prop. The twist in the airflow was not visible and it didn't look like the flow behind the prop was all that turbulent. Just an observation.

Say a prop airplane is accelerating down the runway. Does the mass of the airplane x the acceleration of the airplane = the mass of the air x the acceleration of the air? Neglecting drag, of course. If so. what do you think the numbers would be for the Corsair in question?

Jim