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found this interesting discussion on Spiral Slipstream

Old 02-28-2012, 09:47 AM
  #1  
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Default found this interesting discussion on Spiral Slipstream

Take from it what you will! I really enjoyed the information the discussion offered up. There have been years of posts concerning Spiral Airflow, P Factor, Torque Effects etc... This is a neat discussion that touches on all.
http://www.eng-tips.com/viewthread.cfm?qid=224619
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Old 02-28-2012, 09:59 AM
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Default RE: found this interesting discussion on Spiral Slipstream

Yep, I started that over there to see the issue through another set of eyes and experiance
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Old 02-28-2012, 11:02 AM
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Default RE: found this interesting discussion on Spiral Slipstream

Well,, I'll be damned!!
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Old 02-28-2012, 04:00 PM
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Default RE: found this interesting discussion on Spiral Slipstream

Couple of quick comments.

First, MajorTom, I read through your previous thread on propeller myths when it was active, as well as the thread linked by gdaddy. I notice one of your biggest arguments is that there should be a roll in the opposite direction (to the right). Look at a top view of an aircraft, and see how much wing area is in the prop wash, percentage wise......not very much right?? Notice also that the small percentage of wing that is in the prop wash is right at the root, therefore not much leverage. Also not that on most low or high wing aircraft, most of the propwash passes above or below the wing, therefore not a whole lot of thepropwash actually affects the wing. Now look at the percentage of the tail that is in the propwash, quite a bit more. Also note that the tail is generally fairly centered in the propwash. And finally, look at the leverage that the tail has to push the nose from side to side. As for why the spiral on the elevator doesn't make it roll, well, remember, it is fighting against the whole wing again, same as the slipstream that does affect the wing.

Second, Iwork in the full size helicopter business. When our pilots are slinging a load on a long line, they quite often need to put either a swivel in the line so that it does not get all twisted up, or use some sort of "stabilizer" to prevent it from spinning in the first place. Sometimes, it is as simple as a longer line so the load is not affected as much.

Now, Ifound a cool video you might like to see. Probably will erase, or at least drastically reduce any doubts, check it out:

http://www.pond5.com/stock-footage/1...-water-un.html
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Old 02-28-2012, 05:07 PM
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Default RE: found this interesting discussion on Spiral Slipstream


.... biggest arguments is that there should be a roll in the opposite direction (to the right). ...
Actually the prop-stream has commonly been used to help generate a roll-force to the right. Free flight modelers have been doing it for decades. The tall pylon used to mount the wing was incorporated to position forward, side-area in the prop-stream to help counteract the engine torque and to moderate the severe (often fatal) left turn (spiral dive) that had previously been encountered during the power-on phase. An additional feature for reducing the power-on left turn was the use of a sub-fin to position more of the vertical fin area below the engine thrust-line. They also found that moving the entire fin to the bottom of the fuselage would generate a power-on right turn. The bottom fin however would reduce the flight score because it touched the ground sooner. Then evolved the very-high-thrust-line configuration with the engine and wing mounted on top of the pylon, and the fin mounted on top of the fuselage; to generate the power-on right turn.
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Old 02-28-2012, 05:37 PM
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Default RE: found this interesting discussion on Spiral Slipstream

Actually the prop-stream has commonly been used to help generate a roll-force to the right. Free flight modelers have been doing it for decades. The tall pylon used to mount the wing was incorporated to position forward, side-area in the prop-stream to help counteract the engine torque and to moderate the severe (often fatal) left turn (spiral dive) that had previously been encountered during the power-on phase. An additional feature for reducing the power-on left turn was the use of a sub-fin to position more of the vertical fin area below the engine thrust-line. They also found that moving the entire fin to the bottom of the fuselage would generate a power-on right turn. The bottom fin however would reduce the flight score because it touched the ground sooner. Then evolved the very-high-thrust-line configuration with the engine and wing mounted on top of the pylon, and the fin mounted on top of the fuselage; to generate the power-on right turn.
Huh, wellisn't thatcool, thank you gyrocptr!!. I did not know that. But then, all of my free flight planes back inthe day were just sport rubber power or slingshot launched gliders. Never got into any competition stuff beyond me and dad seeing who's plane flew better.

But, it still goes as more evidencein favor of thethe spiral slipstreamtheory
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Old 02-28-2012, 06:51 PM
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Default RE: found this interesting discussion on Spiral Slipstream

Guys for the spiral to be true the propeller has to induce airflow in the same direction as the rotation of the propeller.

This is the same as saying, if we place a probe somewhere below and aft of the trailing edge of a wing, we will find air moving in the same direction as the airplane. Never heard that one taught either.

As to the exhaust bubles from the prop. still not proof the bubbles are acually spiralling in the prop's wake.

As to the free flight stuff, I'll have to do a little more research.
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Old 02-28-2012, 07:51 PM
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Default RE: found this interesting discussion on Spiral Slipstream

Guys for the spiral to be true the propeller has to induce airflow in the same direction as the rotation of the propeller.

If you watch the video again, you can see the direction of rotation right at the start of the video. I admit, it is hard to tell with the bubbles, but sure looks to be the same direction to me.

This is the same as saying, if we place a probe somewhere below and aft of the trailing edge of a wing, we will find air moving in the same direction as the airplane. Never heard that one taught either.

Can't say as though Ihave ever seen evidence of that in any wind tunnel tests either....but Icould be wrong. There definately is some "forward" moving air below and aft of the LEADINGedge.

As to the exhaust bubles from the prop. still not proof the bubbles are acually spiralling in the prop's wake.

Really??? Do you honestly think that there is something else there that is causing the bubble to spiral, and what would such a thing be?? The boat is barely moving, and you can sort of see the flow pattern off the back of the boat hull by watching the seaweed stuck to the bottom, and other floating particles in th water.
See my comments above in red. I hate to be rude Major, but it seems to me as though you are grasping at straws now, trying desperately to cling to your theory Just kidding, I know how hard it can be to accept things sometimes when your brain is certain of something else, I can be pretty stubborn myself sometimes!!!!!!
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Old 02-29-2012, 11:06 AM
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Default RE: found this interesting discussion on Spiral Slipstream

Wind tunnel tests for this are worthless - -in a freeair environment - the non spiraling surrounding air quickly changes the "spiral " to just a high vs low pressurecolumn- which fades fast -
wing tip vortices are another thing .
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Old 02-29-2012, 11:43 AM
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Default RE: found this interesting discussion on Spiral Slipstream

Well, the simple fact is that since propellers blades pass through the same air column so much the drag from the blades does tend to produce a spiralling stream which is rotating in the same direction as the prop. The skin friction drag of a wing likely also tends to pull some of the air forward with the wing in much the same manner. Just not as noticably since the wing isn't passing through the air volume as often as the blades of a propeller.

Free flight power models use this by allowing the slipstream to work with the typical wing pylon to aid in producing the right hand spiral climb.
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Old 02-29-2012, 01:12 PM
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Default RE: found this interesting discussion on Spiral Slipstream


ORIGINAL: BMatthews

Well, the simple fact is that since propellers blades pass through the same air column so much the drag from the blades does tend to produce a spiralling stream which is rotating in the same direction as the prop. The skin friction drag of a wing likely also tends to pull some of the air forward with the wing in much the same manner. Just not as noticably since the wing isn't passing through the air volume as often as the blades of a propeller.

Free flight power models use this by allowing the slipstream to work with the typical wing pylon to aid in producing the right hand spiral climb.
May I not agree ?
most Free Flight Unlimited types are NEVER climbing in the same direction that the engine is pointed and thrust lines are often far below wing position
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Old 03-01-2012, 03:37 PM
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Default RE: found this interesting discussion on Spiral Slipstream

I too am a bit of a cynic regarding 'spiral slipstream'. My reasons are simple, and backed by empirical experiment. Observe the smoke injected into the prop slipstream on any IMAC/fullsize aerobatic aircraft and see if you can observe any 'spiral flow'. Stick a pin and a long bit of cotton just behind your own models engine, and see if you can detect any spiral flow. Finally, the theory does not explain how the same yaw/roll reaction occurs on pusher 'wings', another experiment you can do for yourself with your electric Zagi type. And last, what continues the 'spiral flow' given that, aft of the prop, no further energy is supplied to the air? And that's notwithstanding that most model have a great big airflow straightener (wing) right behind the prop. And as the scale guys will tell you, the exhaust stains each side of those WW2 fighters tend to be the same, one side to 'tother. OK, it's only empirical, visual evidence, but it would take a bit of explaining away if there was a spiral flow...
Evan, WB #12.
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Old 03-07-2012, 12:55 PM
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Default RE: found this interesting discussion on Spiral Slipstream


ORIGINAL: rhall999

See my comments above in red. I hate to be rude Major, but it seems to me as though you are grasping at straws now, trying desperately to cling to your theory[img][/img] Just kidding, I know how hard it can be to accept things sometimes when your brain is certain of something else, I can be pretty stubborn myself sometimes!!!!!!
Well, you see, I've been an aero engineer for 35 years now, and I've been looking for an answer to this particular question for 25 of those. I'm not grasping at any straws. Simple facts

1. slipstream spiral was not mentioned in any of the aerodynamics texts in the FAA library in OKC (about 30 aerodyanamics books) until after the publishing of "Stick and Rudder"

2. It is the ONE and ONLY one aerodynamic property that is not quantified ANYWHERE! The ONLY mention of an acutal measurment of the supposed effect is mentioned in ONE NACA wind tunnel report. AND THAT report is an imperical statment that the fin needed 3 degrees of offset. Again no math to support it, just what it took to make the model have zero yaw. Which they arbitrarily attributed to slipstream spiral.

3. as posted by pimmnz above, there just isn't any photo graphic proof of it either.

Everyone who believes in it states it because that was what was told to them. The ultimate irony of this belief was found in the intro to one of the aerodynamics books menttioned above says to the effect; Question everything; until YOU are satisfied with the answer! Then he goes on, in a VERY deep mathematical analysis book to just state the spiral slipstream exists.

As for the boat prop video, sorry but due to the quality and brevity of the video I just don't see any spiral
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Old 03-07-2012, 01:01 PM
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Default RE: found this interesting discussion on Spiral Slipstream

I have no dog in this fight. But I can't offhand think of any physical mechanism that could create a spiral slipstream. The propeller sends particles of air off in various directions. That's fine. But what force could then cause them to change direction to make a spiral? Not saying it can't happen, but if it can, what would this force be?
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Old 03-07-2012, 03:20 PM
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Default RE: found this interesting discussion on Spiral Slipstream

Well fellas I'm no expert, I will admit that. But if you can not see the bubbles swirl right aft of that boat prop, maybe you need your eyes checked. Granted, the prop rpm is pretty quick, and the bubbles swirl pretty quick, but the presence of theswirl is obvious. I found an interesting report by Dr.Michael Selig on R/C airplane props, and it makes mention a few times of the "swirl" in the slipstream. Here is the report. (Incidentally, I found a few reports exploring the slipstream effects on V/STOL aircraft, and how the slipstream affects the wing, interesting but extrememly technical reading....made my head hurt!!!)
http://www.ae.illinois.edu/m-selig/p...ropAeroSim.pdf

My reasons are simple, and backed by empirical experiment. Observe the smoke injected into the prop slipstream on any IMAC/fullsize aerobatic aircraft and see if you can observe any 'spiral flow'. Stick a pin and a long bit of cotton just behind your own models engine, and see if you can detect any spiral flow.
When you are looking at the smoke, keep in mind the volume of air blowing the smoke straight back due to the speed of the aircraft even on the takeoff roll. To really be able to see the smoke follow any sort of spiral from the prop, it would need to be under a very high power setting, at a very slow airspeed or basically stopped.........NOW, on that note.........

I also found an interesting video of Sean D Tucker flying his Challenger biplane. Most of this video was taken from another aircraft. Now, there is a chunk of this video that is of particular interest, and that is from about the 00:30 to 00:45 second mark. In that section Sean has the aircraft in his hover, and the camera aircraft passes around behind. You can see quite nicely the flow pattern of the smoke. Take note which side of the horizontal stab is obscured by smoke, and which side is not. Near the end of that segment, observe the smoke pattern, as the smoke on the left side of the aircraft rises considerably higher than the the tail. Meanwhile, the smoke on the other side stays almost "flat", almost in line with the trailing edge of the wing. For those who don't know, the prop on this airplane will rotate the same as most model airplane props, in other words, clockwise when viewed from the pilots seat.

http://www.youtube.com/watch?v=H8nKz...eature=related

Another interesting video I found is someone testing their R/C Yak 54 smoke system. Watch the smoke pattern here as well, and you will see that the left wheel is obscured by smoke, and the right is not, indicating the smoke is being sent to the left a bit......which is consistent with the "spiral slipstream theory". If you watch closely,youcan also see the smoke behind the left wing begin to curve upwards an da little bit back towardsand over the tail. I thought at first that perhaps there was a wind blowing when this guy was doing his test, but watching the smoke farther back behind the tree, that does not appear to be the case.

http://www.youtube.com/watch?v=lDNreZke7iM

Anyway, like I said, I am not an expert, but there is enough evidence in favour of the spiral slipstream for me
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Old 03-08-2012, 09:14 AM
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Default RE: found this interesting discussion on Spiral Slipstream

Stumbled upon another neat one. This is a video of airshow pilot (and R/C pilot too!!) Skip Stewart flying his highly modified Pitts. In this video there is a very short...only about 2 seconds....example of the swirling slipstream. As it is pretty brief, you need to watch closely, in fact I had to watch it 2 or 3 times to be sure. If you skip to the 6:00 mark in the video, Skip is performing a torque roll. Just as the airplane stops climbing, he switches of fthe smoke, so it forms a nice cloud behind him. Then, as the airplane start to back up into the cloud, right around the 6:12-6:15 mark, you can see the top of the smoke cloud all of a sudden start to fairly rapidly rotate, in the direction of propeller rotation.Once the airplane gets into the smoke and all kinds of other strange air movement from a sliding backwards airplane takes over. you can not see it anymore, which is to be expected considering what is going on.

Check it out, the important part starts around 6:00

http://www.youtube.com/watch?v=ST0rhX0BKiE
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Old 03-08-2012, 10:46 AM
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Default RE: found this interesting discussion on Spiral Slipstream


ORIGINAL: rhall999

Well fellas I'm no expert, I will admit that. But if you can not see the bubbles swirl right aft of that boat prop, maybe you need your eyes checked. Granted, the prop rpm is pretty quick, and the bubbles swirl pretty quick, but the presence of the swirl is obvious.
There are also plenty of photographs of props creating vapor trails as they taxi.

Same phenomenon. The vapor trail and the bubbles are right where the prop tip created them. They are particles/objects left behind, right where they were created. They show you where the prop tip was as it passed through.

The swirl and the vapor trails are obvious. But really don't prove anything more than the tip was there.
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Old 03-08-2012, 12:23 PM
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Default RE: found this interesting discussion on Spiral Slipstream

Same phenomenon. The vapor trail and the bubbles are right where the prop tip created them. They are particles/objects left behind, right where they were created. They show you where the prop tip was as it passed through.
Well, without wanting to sound rude....did you even watch the first video Iposted of the boat prop?? I only ask because in that video, the bubbles are nowhere near the tips of the prop, they originate out of the hub, and start to swirl well aft and inboard of where the prop tips are.In fact, right at the "start and end" of the bubbles, you can see thatrightnear the center of the prop is where they start their swirl.

In this case, the "vortex" at the tips in which you are referring to is definitely not what we are looking at. I have been fortunate enough to have seen that on full size Dash 8 and Dehavilland Twin Otters at the airportwhere I work
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Old 03-08-2012, 01:12 PM
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Default RE: found this interesting discussion on Spiral Slipstream

Spiral Slipstream calculations found!!!

Yes, I seem to have too much time on my hands Anyhow, I have found not one, but 2 papers in which there is a mathematical formula to calculate the angular momentum....or rotation....of the propeller slipstream.

http://www-mdp.eng.cam.ac.uk/web/lib...ler/prop1.html
http://www.mh-aerotools.de/company/p...erhepperle.htm

Enjoy!!
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Old 03-08-2012, 04:54 PM
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Default RE: found this interesting discussion on Spiral Slipstream

Nope - the only spiraling air I know of is generated from wing tips
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Old 03-08-2012, 08:20 PM
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Default RE: found this interesting discussion on Spiral Slipstream


ORIGINAL: rhall999

Spiral Slipstream calculations found!!!

Yes, I seem to have too much time on my hands[img][/img] Anyhow, I have found not one, but 2 papers in which there is a mathematical formula to calculate the angular momentum....or rotation....of the propeller slipstream.

http://www-mdp.eng.cam.ac.uk/web/lib...ler/prop1.html
http://www.mh-aerotools.de/company/p...erhepperle.htm

Enjoy!!
Good find, I'll read them when I'm awake!

T
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Old 03-09-2012, 04:46 AM
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Default RE: found this interesting discussion on Spiral Slipstream

start with a pusher prop setup- there is NO fuselage interferrence -now try and find the spiral flow -
you will find an interesting flow pattern.
As for calculated spiral - gimme a break -
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Old 03-16-2012, 05:06 AM
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Default RE: found this interesting discussion on Spiral Slipstream

You can find lots of information in Hoerner: Fluid Dynamic Lift!

Enclosed some pictures from one of my presentations I made for the F3A community in Germany ...
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Old 03-16-2012, 08:40 AM
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Default RE: found this interesting discussion on Spiral Slipstream

ORIGINAL: RainerSeubert

You can find lots of information in Hoerner: Fluid Dynamic Lift!

Enclosed some pictures from one of my presentations I made for the F3A community in Germany ...

I hope you didn't use the pictures as proof of spiral slipstreams.

The vapor trails in the pictures show where the prop tips that are producing those stationary trails have been as the airplanes taxi. If they were actually slipstreams, they would have displaced aft. The planes displaced forward and those clouds remained in place. They prove there isn't really any moving air where they are.

They don't prove that spiral slipstream doesn't exist, btw.
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Old 03-16-2012, 09:39 AM
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Default RE: found this interesting discussion on Spiral Slipstream

I hope you didn't use the pictures as proof of spiral slipstreams.

The vapor trails in the pictures show where the prop tips that are producing those stationary trails have been as the airplanes taxi.
Da Rock is correct Ranier. What you are seeing in the Hellcat and Transall photos is essentially the "wingtip vortices". If you have ever gone to an airshow and watched a modern fighter jet, you can see white vapour trails coming off the wingtipsin high G manuevers. Basically all that is (from my understanding), is the low pressure in the center of the wingtip vortex that is cooling and condensing the moisture out of the air into a visible vapour trail. The pattern you see coming off those props is exactly the same thing, the center of the "wingtip vortex".

Last week I did a bunch of research on this spiral slipstream for this thread, to try and find as much info as I could for those that do not believe it exists. In that time I found a few different research papers that indicated that the spiral flow originates further inboard on the blade, where the blades are wider.

As for calculated spiral - gimme a break -
I guess you never actually looked at those links I posted. Seems to me that it is pretty clear. Complicated bit of math, but nonetheless it can be calculated. The one paper was written by a Dr. Eppler and Dr. Hepperle, two pretty well known and respected aerodynamicists.
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