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Author Topic: Spiral flow, truth or myth?  (Read 211771 times)

Offline Wolfgang Nieuwkamp

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Spiral flow, truth or myth?
« on: November 12, 2014, 03:08:28 PM »
For my new plane I was interested in the torque effect of spiral flow (assuming it exists) on the airplane.
I suspended one wing on a fixed balsa edge, the other wing on a balsa edge on a scale. Wings horizontal, fastened at the rear exactly in line with the motor shaft.
Then started the motor, and wrote down the reading of the scale. The reading, multiplied by the distance between the edges, gives the torque on the model.
Assuming a motor efficiency of 87 % (Plettenberg data sheet), the torque measured was within a few percents of the calculated motor torque!

From this I assume that there is no noticeable spiral flow effect.
A picture of a smoke flow experiment with two counter-rotating props, found on the internet, seems to support my assumption. No traces of tangential flow...
Comments welcome,

Wolfgang

Offline Tim Wescott

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Re: Spiral flow, truth or myth?
« Reply #1 on: November 12, 2014, 03:12:57 PM »
I sincerely hope that you mean the difference in reading between motor off and motor on, multiplied by the distance between edges, gives the computed torque of the motor.

(Or that you zeroed out the scale -- which does the same thing, only with the scale doing the math for you).
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Offline Steve Helmick

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Re: Spiral flow, truth or myth?
« Reply #2 on: November 12, 2014, 07:19:39 PM »
I would suggest a completely different experiment. Take a stick of balsa or CF arrow shaft and pivot it with a fixed vertical axle, as free a bearing as you can come up with...maybe magnetic, like a prop balancer? Make it so you can turn it upside-down. Put a rudder on the top of the stick, about 1/2 to 1 meter aft of the pivot. Set it up with a fan blowing straight at the pivot axle. Table fan, box fan, or Plettenburg fan, no matter. Does the rudder on top make the stick shift one direction...or the other? Turn the stick over, putting the rudder on the bottom, without moving the fan. Is it now different direction? I'd bet that it is. I think the mass of your model is causing some problems with your setup, along with motor torque reaction.  H^^ Steve 
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Offline Chris Wilson

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Re: Spiral flow, truth or myth?
« Reply #3 on: November 13, 2014, 01:43:29 AM »
I assume that prop driven wind tunnels use straightener vanes to eliminate swirl and cross flow for a reason.
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Offline Wolfgang Nieuwkamp

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Re: Spiral flow, truth or myth?
« Reply #4 on: November 13, 2014, 02:00:34 AM »
Tim,

of course I set the scale to zero before starting the motor :)

Steve,

If there was spiral flow, I should have measured less torque.

Chris,

I think straighteners in wind tunnels are more to dampen turbulence, and to get equal velocity for the complete cross-section.

Regards,

Wolfgang

Offline Igor Burger

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Re: Spiral flow, truth or myth?
« Reply #5 on: November 13, 2014, 07:44:17 AM »
Assuming a motor efficiency of 87 % (Plettenberg data sheet), the torque measured was within a few percents of the calculated motor torque!

From this I assume that there is no noticeable spiral flow effect.

may be the first thing could be how you got torque from motor max. efficiency (that is that number 87% - highest POSSIBLE efficiency) ... I cannot very well imagine how to figure torque on shaft up from that number with precision necessary for such experiment

and then ... depends what is noticable and what is spiral flow effect ... it will need to define some vector and some units, you are measuring motor reaction moment minus some aerodynamic forces which depends on many deatails and resulting net effect could have even negative values :- ))) ... did you ever see r/c plane hinging on prop and rotating right or left, depending what pilor does?

and for the question, if we are speaking about side force comming from asymetric fuselage in swirl, it is well known effect with well known and used solution so it will probably exist if we can sucessfully compensate it, so we not only see it, we can also trim it out :- ))))

Offline Wolfgang Nieuwkamp

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Re: Spiral flow, truth or myth?
« Reply #6 on: November 13, 2014, 10:58:10 AM »
Igor,

regarding data: The incidences of Motor, Wing, Flaps, Stabilizer, Elevator, Fin and Rudder all are 0°.
Power to the motor 431 W. At 9234 rev/min the theoretical torque to the shaft would be 0,446 Nm. Plettenberg eta at this operating point 87 %, so the torque should be 0,388 Nm or 39,55 g.m.

The distance between the suspension points was 65 cm, the reading at the scale was 63 grams. Torque on the model: 41 g.m.

I admit that it would be more accurate to have torque sensor between fuselage and motor, but I do not have access to such equipment… But if there were any spiral flow effect, I would have expected a lower reading.

If we define hinging as a sudden simultaneous rotation about all three axes, then it would be nice if we could sort out the cause regarding each of the three axes. Well, with my measurement I tried to find out if there is any unexplainable torque about the longitudinal axis.

Maybe somebody should start to make a mathematical model of F2B flight??



Offline Ted Fancher

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Re: Spiral flow, truth or myth?
« Reply #7 on: November 29, 2014, 10:46:16 AM »
I've often wondered how the proponents of spiral airflow effect on roll due to the vertical stabilizer being (generally) above the axis of rotation of the prop manage to ignore the existence of much larger surfaces (the wing) which would, if spiral airflow is actually an issue, produce a much larger roll component than the puny (in comparison) vertical fin????

In addition, it seems logical that the wings themselves would act as almost perfect "stators" or, as some have called them, flow "straighteners" well in advance of the vertical surface and, at normal RPMs would almost certainly impede spiral flow for one or more revolutions prior to the "spiral effect" reaching the trailing edge of the wing.  Full disclosure, I haven't done the math of RPM versus forward motion at 55 or so vehicle MPH but I do know that "narrower in relative chord" stators in the compressor section of jet engines seem to do the job.  Of course, there are, I believe, multiple rows of the stators/diffusers aft of each row of rotating compressors sections, each angled to an optimum for the design RPM of the  engine based on its mission...probably balls to the wall for high performance fighter types and at cruise levels for greater efficiency in commercial applications, although that is a total guess.

Ted

p.s.  Kudos to Wolfgang for a fascinating experiment.

Offline Will Hinton

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Re: Spiral flow, truth or myth?
« Reply #8 on: November 30, 2014, 07:45:09 PM »
After a total winter of research with Purdue University in Florida, a local ag pilot and the University arrived at a sure fire solution to uneven spray patterns as tested on paper stretched across the runway and spraying colored water.  I then made the same spray tests myself and found them to be accurate.  As a result, a typical ag spray setup will have the nozzles closest to the fuse on one wing spaced quite far a part and the root of the other had extra nozzles spaced quite close together. (I seem to remember the right wing had the fewer nozzles, but don't quote me - that was very long time ago.) All this was done to compensate for spiral effect.  Test spray patterns proved the concept.
I am quite sure new systems have taken the place of those, since this was in the late 70's, and so much has transpired since then in the ag field.  However, I feel those tests and my success as an aerial applicator were conclusive proof of spiral flow.
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Offline Wade Bognuda

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Re: Spiral flow, truth or myth?
« Reply #9 on: November 30, 2014, 08:40:25 PM »
Agreed. As an ag pilot also. I think what is being called spiral flow here is what is also known as spiraling slipstream. And it is as real as it gets. They still pattern test the same way. Because it works. They also spray a string (positioned just prior to or after the cards) with the same colored water and feed it into a computer which generates a graph to aid in nozzle placement to optimize spray deposition. As I am sure you know, torque, spiraling slipstream and P-factor are all at work in all piston powered fixed wind aircraft. Especially at higher angles of attack and lower airspeeds.

Offline Wolfgang Nieuwkamp

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Re: Spiral flow, truth or myth?
« Reply #10 on: December 01, 2014, 09:36:10 AM »
Regarding torque and P-factor, I fully agree. I have flown single engine aircraft for ten years, and sure I had to give right rudder during take off. But still I think that the reason for that is the P-factor, and the port U/C wheel being pressed on the runway.

Offline Ted Fancher

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Re: Spiral flow, truth or myth?
« Reply #11 on: December 01, 2014, 01:22:45 PM »
Agreed. As an ag pilot also. I think what is being called spiral flow here is what is also known as spiraling slipstream. And it is as real as it gets. They still pattern test the same way. Because it works. They also spray a string (positioned just prior to or after the cards) with the same colored water and feed it into a computer which generates a graph to aid in nozzle placement to optimize spray deposition. As I am sure you know, torque, spiraling slipstream and P-factor are all at work in all piston powered fixed wind aircraft. Especially at higher angles of attack and lower airspeeds.

Just to support my comments about spiral slipstream/ wings as stators, etc. in the earlier post vis a vis the ag comments...

The RPM typical of modern stunt ships--i.e. 7 to 11K depending on pitch--is significantly greater than I suspect is typical of most Ag ships, especially those powered by the big, low revving radial engines.  Thus the spacing between the "spirals" would be, at least at first blush, significantly greater.  At the same time the airspeed of the Ag ships is greater than stunt ships by a factor of probably 1.5 to 2.0, say 75 to 110MPH or so versus 50-55 for our stunters (semi-educated guess based on lots of flight time but zero of which included spraying anything but jet fumes).  Thus the ratio of spiral spacing to the forward movement of intervening stator surface forces prior to reaching the tail becomes of greater interest.

Got to admit, however, that the Ag example and the apparent reality of spray pattern adjustments designed to adapt to it make a good argument in favor of the "spiral is viral crowd".  I would still, however, like to see some discussion of the seemingly obvious "stator effect" of large surfaces between the prop and the supposed  source of the yaw/roll components which result from the spiral effect...the vertical fin.

Ted fancher

Offline Will Hinton

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Re: Spiral flow, truth or myth?
« Reply #12 on: December 01, 2014, 04:04:27 PM »
I think it would be so neat to get to fly a stunter in a heavy fog and film the ship from different angles so we could watch the different effects, if any, for not only the possible spiral flow, but the effect of flap deflection, etc..
I know wind tunnels can do a lot, but I've always been a fan of the real invironment experiments.
Any takers?
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Offline Howard Rush

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Re: Spiral flow, truth or myth?
« Reply #13 on: December 01, 2014, 04:45:54 PM »
I think it would be so neat to get to fly a stunter in a heavy fog and film the ship from different angles so we could watch the different effects, if any, for not only the possible spiral flow, but the effect of flap deflection, etc..
I know wind tunnels can do a lot, but I've always been a fan of the real invironment experiments.
Any takers?

Google "flow visualization" for ideas.
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Offline Howard Rush

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Re: Spiral flow, truth or myth?
« Reply #14 on: December 02, 2014, 01:28:14 AM »
I switched from props that turn clockwise looking from the front to (traditional) props that turn counterclockwise looking from the front.  The airplane then banked to the right: opposite of what torque would do.
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Offline Chris Wilson

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Re: Spiral flow, truth or myth?
« Reply #15 on: December 02, 2014, 03:38:48 AM »
Surely during a manevour the model will decelerate and acelerate thus rolling both directions and the severity dependant on the rate of change?
And isn't that more critical than level flight 'crop spray' patterns?

And with the original smoke test, presenting a solid column of smoke the same diameter as the props is probably not going to show any internal spiralling - try placing a single narrow smoke stream to one side and repeat.
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Offline Igor Burger

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Re: Spiral flow, truth or myth?
« Reply #16 on: December 02, 2014, 03:52:42 AM »
I switched from props that turn clockwise looking from the front to (traditional) props that turn counterclockwise looking from the front.  The airplane then banked to the right: opposite of what torque would do.

.. :- ))) ... didn't I write that we not only see it we also trim it out? :-P

Offline Howard Rush

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Re: Spiral flow, truth or myth?
« Reply #17 on: December 02, 2014, 12:48:25 PM »
.. :- ))) ... didn't I write that we not only see it we also trim it out? :-P

And that the net effect could have either sign. 
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Offline Air Ministry .

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Re: Spiral flow, truth or myth?
« Reply #18 on: December 02, 2014, 04:22:11 PM »
If you get a 10 inch piece of wool on a stick , and hold it behind the prop ( with the engine running )
it will stand straight up and go and fetch you a cup of tea , %^@ S?P er disregard this bit . ;D

We eagerly await your observations . H^^

Offline Chris Wilson

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Re: Spiral flow, truth or myth?
« Reply #19 on: December 02, 2014, 06:51:39 PM »
If you get a 10 inch piece of wool on a stick , and hold it behind the prop ( with the engine running )
it will stand straight up and go and fetch you a cup of tea , %^@ S?P er disregard this bit . ;D

We eagerly await your observations . H^^
Finding it difficult to run around at 60mph in a 70' circle whilst holding the stick at a constant distance Matt!
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 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Igor Burger

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Re: Spiral flow, truth or myth?
« Reply #20 on: December 03, 2014, 12:20:24 AM »
And that the net effect could have either sign. 

yes, means classic CW prop will need motor offset out, while CCW pusher will need motor in = lowering line tension, another reason why I do not like pushers :- )))

Offline Air Ministry .

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Re: Spiral flow, truth or myth?
« Reply #21 on: December 06, 2014, 09:40:49 PM »
Finding it difficult to run around at 60mph in a 70' circle whilst holding the stick at a constant distance Matt!

Buy a bicycical , and use a down hill circle !  ;D n1

ACTUALLY you could stick a stick on and fly it ,
with a three foot wool strand .

If you video it we can see the crash when it gets caught in the controls .

WOOL TUFTS were around when NOAH was testing the ARK . So are proven tecnology .




Offline Chris Wilson

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Re: Spiral flow, truth or myth?
« Reply #22 on: December 07, 2014, 02:48:46 AM »
I don't know about the tuft test being accurate Matt, if you had your piece of wool trailing back from a wing tip would it show the well proven spiral vortex found there?  I think not as the surface drag would tend to pull it in an averaged straight line.

The best visual example that I have seen showing a spiral flow around a fuselage involves the oil trail left from an exhaust that coalesces on any surface.
« Last Edit: December 07, 2014, 11:29:16 AM by Chris Wilson »
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
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Online Dave_Trible

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Re: Spiral flow, truth or myth?
« Reply #23 on: December 07, 2014, 06:39:43 AM »
It's been a few years since I flew much free flight but this is a pretty well established science with them.  Discussions if not near- wars over this covered a series of magazine articles between Carl Goldberg and Bill Winter in the early days.  Cabin vs. pylon,  rudder and sub rudder vs. under slung rudder,  high thrust line vs. low thrust line.  A good study if you care to go find it.  I've not specifically considered this in our application but like Ted, I'd have to think our huge wing right behind the prop would have to straighten most of the spiral other than a trail around the top and bottom.  I would think that with our normal counter clockwise flow the larger net effect would be a right-rolling force contacting the wings that counteract engine torque to some degree.  The quantity of that force would be a function of RPM,  prop size,  shaft torque at various output power and airspeed. Maybe we'll see a few sub-rudder experiments.

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Offline Howard Rush

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Re: Spiral flow, truth or myth?
« Reply #24 on: December 08, 2014, 12:20:46 AM »
I would think that with our normal counter clockwise flow the larger net effect would be a right-rolling force...

Never a dull moment.
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Offline Igor Burger

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Re: Spiral flow, truth or myth?
« Reply #25 on: December 08, 2014, 12:45:44 AM »

 Maybe we'll see a few sub-rudder experiments.


Where I only saw it ? ? ?  ;D


Offline Howard Rush

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Re: Spiral flow, truth or myth?
« Reply #26 on: December 08, 2014, 03:53:06 AM »
For my new plane I was interested in the torque effect of spiral flow (assuming it exists) on the airplane.
I suspended one wing on a fixed balsa edge, the other wing on a balsa edge on a scale. Wings horizontal, fastened at the rear exactly in line with the motor shaft.
Then started the motor, and wrote down the reading of the scale. The reading, multiplied by the distance between the edges, gives the torque on the model.
Assuming a motor efficiency of 87 % (Plettenberg data sheet), the torque measured was within a few percents of the calculated motor torque!

From this I assume that there is no noticeable spiral flow effect.
A picture of a smoke flow experiment with two counter-rotating props, found on the internet, seems to support my assumption. No traces of tangential flow...
Comments welcome,

Wolfgang


Back to the original experiment, one thing I didn't notice, nor did I notice any comment on, was the airspeed of the airplane.  You should do the experiment in a wind tunnel or on an automobile at F2B speeds.  Superposition does not apply to airflow over stunt planes.  My own experience of taking off with an electric motor running at full speed in either direction on my stunt plane is that motor torque predominates at low speed.

I made some rigs for measuring reaction torque of engines using ball bearing mandrels such as http://www.advancedbearings.com/category/Ball_Bearing_Mandrels.html .  To eliminate swirly-air effects, I put a vane on the rotating part and blew air on the test rig with a propeller mounted on a router rewired to run backwards.  I adjusted the vane until there was no net moment on the rotating part.  Torque on the test rig using a similar prop at similar RPM should therefore have been only from the engine.   
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Offline Howard Rush

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Re: Spiral flow, truth or myth?
« Reply #27 on: December 08, 2014, 03:54:39 AM »
Where I only saw it ? ? ?  ;D

I saw one like that somewhere in Poland, I think.
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Offline Wolfgang Nieuwkamp

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Re: Spiral flow, truth or myth?
« Reply #28 on: December 08, 2014, 08:20:14 AM »
Howard,
it will not be easy to find a wind tunnel which has a circular airflow of radius 20 m…..

Offline phil c

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Re: Spiral flow, truth or myth?
« Reply #29 on: December 10, 2014, 04:50:43 PM »
I did a little research and found this picture        https://www.google.com/search?q=spiral+prop+airflow+picture&rlz=1C1TSND_enUS575US575&es_sm=122&biw=1093&bih=534&source=lnms&tbm=isch&sa=X&ei=T9SIVP_MEpHjsAT8tYL4BA&ved=0CAcQ_AUoAg&dpr=1.25#tbm=isch&q=fuselage+airflow+tests&facrc=_&imgdii=_&imgrc=ogHnepIb9BwifM%253A%3BtHuEzsptKJTTTM%3Bhttp%253A%252F%252Fcontrails.free.fr%252Fimages%252Fsnapshot20051219035220.jpg%3Bhttp%253A%252F%252Fcontrails.free.fr%252Ftest_aero_huile_en.php%3B720%3B576

It's  an if flight picture from just below and behind of an sport plane.  It shows no airflow spiraling around the fuselage.  Lots of other drawings showing air spiraling around the fuse, and many photos of tip vortex spiraling behind the prop.

I think the so-called spiral airflow effects are due to the direction of the air as it leaves the prop and interacts with the fuse, wing, and tail.  When on the ground the air comes off the prop at a small angle since the prop blade is twisted.  Once it leaves the prop it heads straight away, at an angle to the prop axis.  When the plane picks up speed the flow still is at some small angle to the fuse.  This angled flow would tend to hit the bottom of the left wing and stab and the top of the right wing and stab the left side of the rudder.  It probably is a pretty small force and ony really affects the plane when it is going slow.

phil Cartier

Offline Air Ministry .

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Re: Spiral flow, truth or myth?
« Reply #30 on: December 15, 2014, 06:59:13 PM »
As all us hillbillys know , a dusty car shows the windflow , if its bolted together right so it moves .
You tend to get rock dings in the front of the rear footwell past 80 mph , so dont fly in hail . ???

RIGHt , if you had a dusty aeroplane , or she was a bit tackey and you spinkled frech chalk on it.

after say youd lightly sprayed it with fuel ( castor ) and wiped it off ( not CLEANED it off ).

the chalkey dust should be aligned in flow patterns with the airflow . ( Patented - all payments to . . .  ;D )

Might pay to put one of those exhaust extender things on if the muffler dumps on the surfaces , or it could be a frightfull sticky mess .
Actually , people in dusty areas who never wash there planes down should already have the evidance .  :o S?P

 H^^


Offline EddyR

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Re: Spiral flow, truth or myth?
« Reply #31 on: January 18, 2015, 07:51:10 AM »
I know nothing about spiral flow but what I have read. But here is something I do know about high performance Free Flight models and the direction they fly when launched.Up to about 1957 most Contest FF models had thrust lines at the center of the body,the wing was on a pylon. Along came the Tbird and then the Stardustar. These model had the thrust line at the wing level 5" above the body on a 1/2A class model. Models had been trimmed to power to the right and glide left. The high thrust models flew left under power. Excepted rule of the day low thrust models goes right,high thrust model goes left. A very smart guy named Woody Blanchard put his thrust line at the center of the pylon and they fly straight. WoW Amazing but there was a problem. Models flown straight do not have a good transition to gliding. I flew against Woody a few times and he won every time but he had a lot of gadgets to make his model work. Technically I do not know why these forces worked as they did but I did know how to use them. Maybe it has all been explained since I left powered FF in the 1970's. I continued to fly Tow Line FF with the work champ Matt Quin in central  florida for a few years.

As a side note On FF models a small tab 1/4 x1" on the rudder has total control over the  powered part of the flight but no effect on the glide. Here is what I am saying about the rudder. Under power is is super critical but under glide it has almost no effect until very large amounts of movement are used. The rudder is not used for gliding turn,wing warp is. 

EddyR  AKA Eddie Haskell >:D
 
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Offline Chris McMillin

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Re: Spiral flow, truth or myth?
« Reply #32 on: February 21, 2015, 12:24:17 AM »
Having flown a couple of different real single engines airplanes with very big props and short, tall fuselages, the yaw from slipstream effect can almost be felt. It's very speed and attitude sensitive, to me. Also, when standing near or on said large propeller airplanes when run static at high power the air goes straight up one side and straight down the other. A long skinny stunt model with a short prop would minimize the effect.
The B Proto guys went to sub rudders and forward canopies to help with yaw on takeoff. Since they sit pretty level (little P-Factor with low AOA) and don't roll much (wings were level), I always went with the slipstream effect making them head in at the pilot since the sub ruder and forward area made them pretty much easy to take off without dramatic pulls on the lines necessary with a topside rudder and rearward canopy.
Chris...
P.S. Another practical slipstream effect evidential item I thought of is that my dad would have a fit if I left the pilots's (left) window down on start, run-up or take off on his Staggerwing Beech because the oil would come in and streak from left to right across his mohair headliner.   

Offline phil c

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Re: Spiral flow, truth or myth?
« Reply #33 on: February 24, 2015, 03:07:03 PM »
Looks like it is a myth, even I was just guessing.  Nothing is better than a guess than evidence.  From Alex Voicu on rcuniverse.

go look for yourself:

http://www.rcuniverse.com/forum/rc-pattern-flying-101/11585841-airflow-visualisation.html
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Offline Chris Wilson

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Re: Spiral flow, truth or myth?
« Reply #34 on: February 25, 2015, 07:00:30 PM »
I don't think that closely tied tufts are going to demonstrate anything other than boundary layer derived laminar vs turbular  (sp?) flow since the prop wash is an expanding cone that radiates outwards - plus it has a slight effect compared to forward thrust.

Look for the spiral much further outwards away from the fuselage and away from lifting surfaces.


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Re: Spiral flow, truth or myth?
« Reply #35 on: March 12, 2015, 02:46:13 AM »
There is an article here about spiral flow in engine tests -

http://adriansmodelaeroengines.com/catalog/main.php?cat_id=57
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Offline Howard Rush

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Re: Spiral flow, truth or myth?
« Reply #36 on: March 12, 2015, 05:30:29 AM »
There is an article here about spiral flow in engine tests -

http://adriansmodelaeroengines.com/catalog/main.php?cat_id=57

Yes, it mentions it, but doesn't tell you what to do about it, which you have previously been told above.  So what does this add?
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Offline Howard Rush

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Re: Spiral flow, truth or myth?
« Reply #37 on: March 12, 2015, 05:33:58 AM »
... boundary layer derived laminar vs turbular  (sp?) flow ...

So tell us what that means, Chris.
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Offline Chris Wilson

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Re: Spiral flow, truth or myth?
« Reply #38 on: March 12, 2015, 07:47:29 PM »
So tell us what that means, Chris.

Trying (probably ineptly) to say that tufts affixed to the cruciform surface of an aerodynamic body are not the best testing medium to show a rapidly expanding cone from a propeller because they only exist on that bounded surface.
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Re: Spiral flow, truth or myth?
« Reply #39 on: March 12, 2015, 07:48:47 PM »
Yes, it mentions it, but doesn't tell you what to do about it, which you have previously been told above.  So what does this add?
Whatever you accept.
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Offline Howard Rush

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Re: Spiral flow, truth or myth?
« Reply #40 on: March 12, 2015, 08:53:20 PM »
Trying (probably ineptly) to say that tufts affixed to the cruciform surface of an aerodynamic body are not the best testing medium to show a rapidly expanding cone from a propeller because they only exist on that bounded surface.

Rapidly expanding?  It contracts: http://www.mh-aerotools.de/airfoils/propuls4.htm .  Indeed, tufts may not tell you much about prop wake on cruciform fuselages, such as Igor's foam board indoor stunters, but we're mostly interested in-- and the pictures Phil referenced are for-- conventional, convex-cross-section fuselages.  You may be saying that swirly flow on the fuselage surface doesn't matter, because it doesn't impart much force to the airplane: what matters is the angle it causes the air to hit the wings and empennage.  That's what Ted was saying, too, I think, and what I think I observed with the not-obvious trim change I had to make when going from a negative- to positive- rotation prop.   
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Re: Spiral flow, truth or myth?
« Reply #41 on: March 12, 2015, 11:39:31 PM »
Hi Howard,
thanks for that web site link, very informative but that page really concentrates on the contracting air around the prop disk and not the airframe.

Once contracted the air must expand again, as evidenced by the original post's smoke trails and the swirl pattern that effects the airframe expanding with it forming a cone.

Thanks again.
« Last Edit: March 29, 2015, 08:10:07 PM by Chris Wilson »
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Re: Spiral flow, truth or myth?
« Reply #42 on: April 17, 2015, 01:21:22 PM »
I switched from props that turn clockwise looking from the front to (traditional) props that turn counterclockwise looking from the front.  The airplane then banked to the right: opposite of what torque would do.

I did this again yesterday on a different airplane (another Impact) with the same results.  Takeoff became much sportier because torque is dominant at low speed  Tim has a fix for the takeoff rolling moment, which I shall install shortly. 
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Re: Spiral flow, truth or myth?
« Reply #43 on: May 16, 2015, 05:23:03 AM »
As long as air has viscosity, the slipstream will spiral. If air ever loses it's viscosity, airplanes will cease to fly. Viscosity is what sets the snowball rolling. It's also why Bernoulli won't predict the lift of an airplane wing.
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