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Author Topic: Effect of wind on maneuvers  (Read 3706 times)

Offline Matt Piatkowski

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Effect of wind on maneuvers
« on: November 01, 2019, 07:28:07 AM »
Hello,
A single propeller stunt plane has a tendency to speed up in maneuvers when it is windy. 

The plane with the counter-rotating propellers does not show this behavior or shows very little of it.

Why?

Thank you,
M



 








Online Dave_Trible

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Re: Effect of wind on maneuvers
« Reply #1 on: November 02, 2019, 12:24:58 PM »
I doubt it really makes any difference .  The speed build up comes from the wind pushing the airplane from behind and added side forces from wind on the fuselage sort of like wind in the sails of a boat.  Most of any help comes from the prop if the power is suppressed somehow like a piped IC engine set up where the pipe acts like a governor to prevent the engine gaining rpm from being forced forward and unloading with the tailwind.  The direction of rotation can have other affects but not much in regards to winding up in the wind.  If you fly a twin that seems to act that way I'd suggest perhaps the second prop disc is offering a little more forward air braking affect when pushed,  like pushing a ping pong paddle into the wind.  If the engines aren't winding up very much they can seem to have some braking ability in strong winds once the prop disc isn't showing it's broadside into the wind. 

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Online Brett Buck

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Re: Effect of wind on maneuvers
« Reply #2 on: November 02, 2019, 12:48:44 PM »

Hello,
A single propeller stunt plane has a tendency to speed up in maneuvers when it is windy. 

The plane with the counter-rotating propellers does not show this behavior or shows very little of it.

Why?

   Probably because of something else being different, unrelated to counter-rotating props. The same effect can almost certainly be replicated or exceeded with a single prop. There's some other factor or factors (or actually, a lot of them) that are undoubtedly different, too, causing the difference you see. Because you think you have found a new "killer feature", you are attributing any differences you find to that, and that alone. Basically, it's not a controlled experiment.

    Brett

Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #3 on: November 02, 2019, 06:23:46 PM »
Hi Brett,
In 2019 season, I have seen at least fifty F2B practice and competition flights in the wind with contras and the speed of these planes in maneuvers was almost constant.

Hi Dave,
The wind indeed adds the side force on the fuselage - we all can feel it as increased lines tension. The effect I have written about is undoubtedly caused by the wind velocity (Vw) added as vector to the plane velocity (Vp) but I cannot understand looking at the vectors diagram what part of Vw causes speeding. Lets say you fly a single prop model and perform the inside loop. The plane of this loop is tilted 22.5 deg. w/r to the wind direction. Vw = 5 m/sec (16.5 ft./sec). Should this value be multiplied by sin 22.5 deg. and arithmetically added to the model's speed of 25 m./sec? This will give the plane's speed of 26.9 m.sec. Of course the velocity of the model is not constant while it flies the loop but the example, if correctly described, shows the increase of Vp.

I am, however, completely at loss trying to quantify the behavior of the models with contra in the wind. What causes the contra powered F2B planes to fly the maneuvers with almost constant speed in the wind?
The answer for this question will shed a new light on the flight dynamics of the C/L models.
Best Regards,
M

Online Brett Buck

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Re: Effect of wind on maneuvers
« Reply #4 on: November 02, 2019, 07:03:44 PM »
Hi Brett,
In 2019 season, I have seen at least fifty F2B practice and competition flights in the wind with contras and the speed of these planes in maneuvers was almost constant.

Hi Dave,
The wind indeed adds the side force on the fuselage - we all can feel it as increased lines tension. The effect I have written about is undoubtedly caused by the wind velocity (Vw) added as vector to the plane velocity (Vp) but I cannot understand looking at the vectors diagram what part of Vw causes speeding. Lets say you fly a single prop model and perform the inside loop. The plane of this loop is tilted 22.5 deg. w/r to the wind direction. Vw = 5 m/sec (16.5 ft./sec). Should this value be multiplied by sin 22.5 deg. and arithmetically added to the model's speed of 25 m./sec? This will give the plane's speed of 26.9 m.sec. Of course the velocity of the model is not constant while it flies the loop but the example, if correctly described, shows the increase of Vp.

I am, however, completely at loss trying to quantify the behavior of the models with contra in the wind. What causes the contra powered F2B planes to fly the maneuvers with almost constant speed in the wind?
The answer for this question will shed a new light on the flight dynamics of the C/L models.
Best Regards,
M

   Again, you are jumping to conclusions and the experiment is uncontrolled. The obvious answer is that the contra-rotating systems have a different response to load and velocity than single-prop systems you have seen/used. But it's very easy to come up with single-prop systems, even with IC, that range from virtually no speed stability, to far too much speed stability. I guess that for reasons unrelated to the fact that the props contra-rotate, you have seen systems with better speed stability.

     I don't know how the various systems you are comparing respond; your IC systems never worked even to first approximation, not even close, so I am not surprised that if you got or saw better systems that were working correctly, and they happened to have contra-rotating propellors, that they worked better.

    BTW, the dynamics of the "whip-up" are a lot more complex than you describe. For example, the induced drag is much lower at the top of the loop than it is at the bottom, because it requires far less deflection at the top of the loop than the bottom, since at the top, the wind want to decrease the radius, whereas at the bottom it has no effect.  Another effect is that if you had perfect *airspeed* stability, it would go much faster "down" than "up" relative to the ground, because of  the vector sum of the airspeed and wind speed along the X axis - not Y or Z. The side force you describe is nearly trivial with regard to speed stability. That's also why you don't actually want perfect airspeed stability in many cases of wind.

   The value of contra-rotating propellors is undeniable in the area of reducing axial coupling; that *is* a good reason to do it, if there was no other factor.  I expect you are attributing a better system to the fact that it is contra-rotating, rather than the many other aspects that are infinitely more likely to cause the effect you notice. It's hardly a new phenomenon - most new tuned pipe users attribute every effect they see to the "pipe length", and Rabe rudder users tend to attribute every trim effect to "rudder adjustments". In any of these cases, there are a myriad of other effects that may also be relevant, but the new enthusiast to a particular gadget focuses in on that to the exclusion of everything else.

     Brett

Offline Air Ministry .

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Re: Effect of wind on maneuvers
« Reply #5 on: November 02, 2019, 09:19:14 PM »
Quote
Practical Airscrew Performance Calculations
F. M. Thomas, F. W. Caldwell and T. B. Rhines
DOI: https://doi.org/10.1017/S0368393100100951Published online by Cambridge University Press: 28 July 2016
Summary
To the practical designer the cumulative knowledge on airscrews seems at present to be in an unsatisfactory state of disorder. He wants to get reasonably accurate estimates reasonably quickly, without taking the time to become a specialist.

Several airscrew theories exist, but they either give wrong answers or take too long to work out, or both.

The great mass of test data that have been accumulated is so confused in a tangle of variables that the practical designer is often forced to select a test report almost at random and use it as if it applied to his particular problem; usually it does not and the calculation is seriously in error.

The present study attempts to clarify airscrew practical knowledge by coordinating data in a form suitable for use in performance calculations. It embraces the whole body of test data, but places particular reliance upon large scale wind tunnel results.

The arrangement of the analysis is such as to permit quick estimates to be made, the degree of accuracy obtained being a function of the amount of time available to the calculator. It is also designed to be as foolproof as possible so that it may be readily applied by inexperienced calculators.

It is especially adapted to calculations of the performance of aluminium alloy variable pitch airscrews.

One of the points brought out in the paper is the need for more data on certain subjects. They are specifically designated, and suggestions are made for carrying out research.


https://www.cambridge.org/core/journals/aeronautical-journal/article/practical-airscrew-performance-calculations/667FBFD377C3356DBF699C2A7390BAC9

An Oxymoron . = "  where ultimate performance is more important than efficiency. " they would say .

Quote
A contra-rotating propeller or contra-prop places two counter-rotating propellers on concentric drive shafts so that one sits immediately 'downstream' of the other propeller. This provides the benefits of counter-rotating propellers for a single powerplant. The forward propeller provides the majority of the thrust, while the rear propeller also recovers energy lost in the swirling motion of the air in the propeller slipstream. Contra-rotation also increases the ability of a propeller to absorb power from a given engine, without increasing propeller diameter. However the added cost, complexity, weight and noise of the system rarely make it worthwhile and it is only used on high-performance types where ultimate performance is more important than efficiency.

Contra Props work somewhat like a dual stage supercharger , in some respects . Rather than relying on atmospheric ( 16 psi ish. )input alone .

As we want consistancy rather than output , with variable inputs , the dynamics may be outside convention .
« Last Edit: November 02, 2019, 09:39:00 PM by Air Ministry . »

Offline Air Ministry .

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Re: Effect of wind on maneuvers
« Reply #6 on: November 02, 2019, 09:24:44 PM »
Theres some fancy algebra here :
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19820006176.pdf

that may be remotely related ( to a sore head ) . sounds like its for a prop that rotates both ways tho not simultaeneous .

ROTOL was De Man wot come up wiff fancy props for Spitfires & Seafires particularly , Circa 1943/45 . So might know a few things .
https://www.emerald.com/insight/content/doi/10.1108/eb030872/full/html?skipTracking=true
https://www.emerald.com/insight/content/doi/10.1108/eb030872/full/html

Stratospheric codswallop .
https://pdfs.semanticscholar.org/4fd6/01fb895f9da0a75b247f89e21d1416a3709e.pdf

Or CONSIDER theres a INTERFACE . The Air In , the between prop disc , & the air out .Sortof sheared in there .

Did a prop to bludgeon the air , in preferance to cleaveing it . With rather usefull results .

Also , you shouldnt be get much spiral airstream corkscrewing the fuselage .

A bright thead of say 20 inches , on the end of say a 20 inch dowl/rod/wire ,
held aft of the prop , should be indicative of airflow , no matter what the prop .

Held at various positions aft of / adjacent to etc , the running prop , to find variables .
Dont get your arm sucked into the airscrew .



http://www.aerodynamics4students.com/propulsion/blade-element-propeller-theory.php

That should keep him tied up for a while .  VD~
------------------------------------------------------

Seems there may be a usefull link , here . too . in there .
 https://forum.kerbalspaceprogram.com/index.php?/topic/186322-propellers-changing-vectors-at-different-speeds/



« Last Edit: November 02, 2019, 09:41:48 PM by Air Ministry . »

Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #7 on: November 03, 2019, 05:23:03 AM »
Air Ministry,
Thanks for the links.

Brett,
Perhaps you would be so kind and describe the "controlled experiment" of your choice to gather the test data?

My idea of "controlled experiment" is as follows: https://www.eagletreesystems.com/index.php?route=product/category&path=64 shows the airspeed sensor that can be used to measure the model's speed in maneuvers and eLogger can record it w/r to the time of flight. Then, a very good stunt pilot will fly the plane powered by a single prop and then by contra, the flights will be recorded and timed by a stopwatch and the speed in, say, inside loops will be compared in no wind and in windy conditions.

Best Regards,
M

Offline Howard Rush

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Re: Effect of wind on maneuvers
« Reply #8 on: November 03, 2019, 02:25:35 PM »
My guess is that the braking you’re seeing is because you have more prop than usual, not because of the directions it’s turning.
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Offline Lauri Malila

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Re: Effect of wind on maneuvers
« Reply #9 on: November 04, 2019, 10:16:13 AM »
Sorry if I say something silly but does a contra prop have twice the the normal prop disc area?
I have found, that when my engine is set correctly to reach the top of power curve at top of a loop (or just a bit before), the model does not really want to accelerate more. More prop dia, more it reduces acceleration.
Or maybe it is a question of prop efficiency, I guess contra prop is more efficient.
I don't use pipe and I use more pitch that is generally accepted here, and I have found that pitch is actually quite irrelevant to speed stability.
It all boils down to engine running stability which I have improved with other stuff than pipe or running at high rpm.
What I try to say, is that according to my observations I tend to agree with Matt. I have not seen very much of flying with contra props but from that little that I have seen, I have been quite impressed with the constant speed they fly at. Both in calm and wind.
But as Brett said, you'll need a more controlled test conditions. L

Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #10 on: November 04, 2019, 11:19:09 AM »
Hi Lauri,
Yes...the contra indeed has twice the prop. disk area but these areas are in tandem. I have no idea how much the second (aft) prop. disk area contributes to the overall air resistance as there is so much turbulence between the contra props. that "normal" air drag formulas most likely do not apply.

I am waiting for Brett to describe the "controlled experiment" he was writing about.

Are you coming to the WC 2020 in Wloclawek, Poland?
If yes, see you there.
Happy flying,
M


Offline Lauri Malila

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Re: Effect of wind on maneuvers
« Reply #11 on: November 04, 2019, 01:56:32 PM »
I guess you'll have to fly the same model with both setups and see how it goes. maybe the one-prop in both running directions.
I haven't decided about coming to the Wch yet, hopefully it does not collide with some more important contest. L

Online Brett Buck

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Re: Effect of wind on maneuvers
« Reply #12 on: November 04, 2019, 03:19:00 PM »
Hi Lauri,
Yes...the contra indeed has twice the prop. disk area but these areas are in tandem. I have no idea how much the second (aft) prop. disk area contributes to the overall air resistance as there is so much turbulence between the contra props. that "normal" air drag formulas most likely do not apply.

I am waiting for Brett to describe the "controlled experiment" he was writing about.

    I am not going to devise your experiment for you. You are the one making claims for otherwise unknown or unique physical phenomenon. Use the scientific method: hypothesize a mechanism, make predictions based on that mechanism, test your predictions against your theory, evaluate the results and update or verify your hypothesis

   Right now, you are comparing apples and oranges, some airplanes flew better than others, and they happened to have contra-rotating props. I can probably find airplanes that fly better still, that have single props. It could be literally anything.

 Howard's observation is about right, it's almost certainly because you have 1.5x to 2x the blade area. The rear prop is doing something, otherwise all the other, acknowledged, value of the contra-rotating system wouldn't exist, either. But it could be the gain on their accelerometer is different, their lower threshold is different, or a million other things unrelated to contra-rotating. A potential experiment is to get a prop with the same blade area and set it up optimally, get a contra-rotating prop and set it up optimally, and compare which is better. Of course you are going to need to define what "optimal" means in engineering terms.

    Brett

     

Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #13 on: November 04, 2019, 03:31:16 PM »
Brett.
I am tired of your comments, your tactless style and I do not like to be patronized.
Have a nice day,
M

Offline Howard Rush

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Re: Effect of wind on maneuvers
« Reply #14 on: November 04, 2019, 07:44:02 PM »
I haven't decided about coming to the Wch yet, hopefully it does not collide with some more important contest. L

If you go, I'll go.
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Online Brett Buck

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Re: Effect of wind on maneuvers
« Reply #15 on: November 04, 2019, 08:59:03 PM »
I am tired of your comments, your tactless style and I do not like to be patronized.

   My original (several years ago) advice stands - establish a baseline working system, experiment carefully around that baseline, and take advantage of the vast experience that Stunthanger has to offer. I am sorry if that inflames you, but that's the best advice I have for you, or anyone else.

    Brett

Offline Air Ministry .

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Re: Effect of wind on maneuvers
« Reply #16 on: November 05, 2019, 10:09:14 PM »
Or put in a FP 25 .


" Today I made the first flights on a double motors. Motors work absolutely synchronously. The operation and regulation of the electronics is the same as with one engine. I liked the very uniform speed of the figures and the excellent braking in a strong wind. The balance of the TP 2800 6s 25c battery after the whole complex of figures and with a cable length of 21.3 m was 25..30%"

This is antique of Rand Smiths Dual Prop  ( Rear Freewheels at warp factor six . scotty ) recorded for posterity by one W Urtnowski Esq.





« Last Edit: November 10, 2019, 08:53:01 PM by Air Ministry . »

Offline PJ Rowland

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Re: Effect of wind on maneuvers
« Reply #17 on: November 06, 2019, 05:48:24 PM »
There is one issue with contra rotation for CL stunt thats not really spoken about.
The GP effect of the prop rotation. How many guys actually run electric with the prop spinning the opposite direction?

Not many...

There is a reason for this.. the GP in normal operation aids in line tension by torque rolling the wings.
Contras have almost neutral gp effect.. the line tension becomes a problem..
Sure you will get slightly more brake with more disc area, the compression effect between the 2 discs forces air to slow.

I dont see contra being useful.. but thats just my opinion based on talking to the guys in F3A who use it..
If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them.” - Bruce Lee.

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Offline Howard Rush

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Re: Effect of wind on maneuvers
« Reply #18 on: November 06, 2019, 11:43:23 PM »
There is a reason for this.. the GP in normal operation aids in line tension by torque rolling the wings.

Show your work.

Sure you will get slightly more brake with more disc area, the compression effect between the 2 discs forces air to slow.

If it did, it would be really loud, and the battery would have to be really big.
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Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #19 on: November 11, 2019, 11:18:56 AM »
Hello,
Please see the attached.
The sketches and formulas shown explain why the C/L model powered by the counter-rotating propellers does not speed up or, more likely, speeds up much less in maneuvers than the model with a single propeller.
Both C/L models are affected by wind but the model with contra has more air resistance (Rc), therefore its flight velocity (vel.of model) remains more stable. I am working right now on 3D vectors analysis that will show the change in the C/L model flight velocity as a function of wind speed and air resistance.

Please remember that the explanation I am including is simplified to show only the basic mechanism of what, in my opinion, is going on.
The air in the narrow gap between contra propellers has the averaged pressure much higher than the surrounding air. Consequently, the gap air pressure forces the gap air outside, creating the "cushion of air", having the diameter (Dc) larger than the contra propellers. Dc converts to "Effective (drag) Area" and this area has the air resistance (drag) defined by Rc.

Lauri wrote: "...but does a contra prop have twice the normal prop disc area?...". After I read this, I started thinking about air resistance as the main cause of the unusually stable velocity of the contra powered models.
Thanks Lauri!
M

Offline Lauri Malila

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Re: Effect of wind on maneuvers
« Reply #20 on: November 11, 2019, 01:17:39 PM »
Please don't get me wrong Matt; I know nothing about this subject :)
But this reminds me of my experiment with prop tip shapes, I have tried many shapes in order to reduce noise. But the fancy looking swept tips allways caused more speed-up so I went back to rather square tips, or elliptic blades.
Perhas some induced drag is necessary? L

Offline Howard Rush

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Re: Effect of wind on maneuvers
« Reply #21 on: November 11, 2019, 04:02:59 PM »
Matt, I think you are confusing drag with rate of change of drag (or thrust) with airspeed.  The net thrust in the direction the airplane is going relative to the air (thrust component - drag) is the same for a given airplane and flight condition regardless of how many propellers are providing it, how big they are, and how they are arranged.  It's the thrust it takes to make the airplane go that speed. 

As the airspeed increases, the angle of attack of a propeller blade element decreases, thus decreasing thrust. The flow through the front prop would thus slow down relative to the free stream, which would if anything, I think, give the opposite effect of what you show above.  Nevertheless, I shall go to Wloclawek to see for myself. 

Lots of prop can help speed stability.  On the US West coast, we are using multiple propellers, but not stacking them in tandem.  Below is David Shorts's wonderful trimotor.  Under construction are stunters with four and even six propellers. 
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Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #22 on: November 12, 2019, 06:16:59 AM »
Hi Lauri,
There indeed exists induced drag generated on the propellers tips. One prop., at certain RPM and flight speed, will have, how shall I put it, one induced drag "value" and two props will have two induced drag "values".
Let me think about it.

Hi Howard,
I can assure you that I am not confusing drag with the rate of change of drag. You wrote also: "The net thrust in the direction the airplane is going relative to the air (thrust component - drag) is the same for a given airplane and flight condition regardless of how many propellers are providing it, how big they are, and how they are arranged.  It's the thrust it takes to make the airplane go that speed.
. I agree.
You wrote also: As the airspeed increases, the angle of attack of a propeller blade element decreases, thus decreasing thrust.. I agree. Regarding this, contra is no different than a single prop or any number of single props. working independently. Comment: independently means that props. do not affect each other like the props. in tandem (contra). The trimotor you are showing has three props. working independently.
You wrote also: The flow through the front prop would thus slow down relative to the free stream, which would if anything, I think, give the opposite effect of what you show above.. Let me think about it. There is such an "air mess" in the gap between the contra props....
See you in Wloclawek. We will see at least two contra powered planes (Krystian Borzecki's and Sylwester Kubik's). Regards, M

« Last Edit: November 12, 2019, 07:45:18 AM by Matt Piatkowski »

Offline Air Ministry .

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Re: Effect of wind on maneuvers
« Reply #23 on: November 12, 2019, 08:25:19 PM »
Excuse me if youve already coverd it , but ;

What is the ' battery drain ' / current - compared to similar 2 blade tractor propellor .

( Will drain / draw vary - Tractor to pusher ( R H to L H ) conventional set up ? )

Do yo run the SAME PITCH on the front & the rear propellor ?

Was halloween here - cans of gloop , on special . last day . Flouresent . Would have done for a spray & film test - to evaluate flow .  S?P

It would be intresting to observe the airsteam at the tips of the propellors and its interaction . Do you have a wind tunnel .  %^@

A few guys have done ' on plane ' cameras . This would give ' in service ' evaluation .
Youd just need a can of smoke & a tube on a stick to forward of the props !


Offline PJ Rowland

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Re: Effect of wind on maneuvers
« Reply #24 on: November 12, 2019, 08:27:39 PM »
Howard.

Well.. I did a whole series of prop tests when I was testing trying to get a stalker 76 to run in my nobler series. The torque role of the engine causes a very pronounced wingroll outboard wingtip rolls up and reduces your effective line te sion.
There was only a few ways to solve this effect and certainly props and flap area were the keys.


As for the second statement of its volume being loud have you HEARD these contras ?? Lol that would answer it clearly for you.

As for battery drain they need to run bigger battery anyway because its 2 motors !!


Fun topic.. dead end tho..

I first put a camera on a plane in 1994 nothing much to find out.. trim is still best done at the handle.
If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them.” - Bruce Lee.

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Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #25 on: November 13, 2019, 04:14:55 AM »
Hi PJ Rowland and Air Ministry,
Please see https://stunthanger.com/smf/open-forum/counter-rotating-propellers-for-stunt for the information about the issues you mentioned here yesterday.
I would suggest to stay on topic of this thread and the topic is " Effect of wind on maneuvers".
Thank you,
M

Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #26 on: November 14, 2019, 06:42:51 AM »
Hello,
Please see the attached vectors analysis. The inside loop was selected for this analysis. 5 m/sec. wind will cause the model velocity increase of 1.91 m/sec. while the model is flying down at 9 o'clock.
Please note that the air resistance (drag) of any flying object grows with the square of velocity. This is why the models with the counter-rotating propellers speed-up less in maneuvers. The effective drag area for contras is larger and, because the air resistance (R) is proportional to the velocity squared, the effect of air resistance for contras grows faster than the increase in the contras velocity caused by the wind.

Happy (indoor) flying,
M




Online Brett Buck

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Re: Effect of wind on maneuvers
« Reply #27 on: November 14, 2019, 08:31:51 AM »
Please note that the air resistance (drag) of any flying object grows with the square of velocity. This is why the models with the counter-rotating propellers speed-up less in maneuvers. The effective drag area for contras is larger and, because the air resistance (R) is proportional to the velocity squared, the effect of air resistance for contras grows faster than the increase in the contras velocity caused by the wind.

     This has nothing to do with contra-rotation - the  rate of change of "drag" forward velocity could be the same with two props running the same direction, or with one larger prop. The rate of change of "drag"  with the same size prop would be higher if it was lower pitch and spinning faster, or if the engine fed back more with load, or if the Igor gadget had the gain turned up higher. It's not "drag", anyway, it's reduced thrust, not "negative thrust" - it's not really slowing you down, it's speeding you up less than it would have.

   The "area" difference you postulate doesn't necessarily exist, it might in some cases, and not in others, depending on the characteristics of the forward prop and the rear prop. You could easily choose a rear prop that squeezes your "free stream" diameter smaller as much as it would be larger. Setting it up as you describe suggests that you are blowing a lot of power having the rear prop impede the flow from the front prop - which is inconsistent with your previous observation that it was more efficient and used less battery. You are computing this mythical "area" and treating it like a pure drag source, which it certainly is not.

 What you are searching for, as previously noted, is the partial derivative of the propulsion thrust with respect to forward velocity. The higher that is, the more it controls the airspeed. There's a lot more going into it than just the "area" of the prop.  It may well be higher with two props having twice the area of the same single prop, which is likely the cause of your observation, but could easily be replicated with a single prop that is larger, has more blade area, or with two of them turning the same way. You are treating it like it was just a drag source, which it is definitely not, it actually changes *much more* than with the square of the velocity on properly-operating IC engines, and if you have an accelerometer feedback system, it can be function of more than the current apparent airspeed -since the accelerometer acts with respect to the change in inertial velocity, not airspeed.

     Brett

p.s. as an aside, adding induced drag, as suggested earlier, would make the problem much worse, because a very large part of the whip-up issue is a function of the variation of the induced drag from the top to the bottom of a loop - lots of lift/induced drag at the bottom, and a whole lot less lift/induced drag at the top. With constant thrust, you would get tremendous velocity increase, just "dynamic soaring". You would be better off in this regard by adding *parasitic* drag, and adding enough extra power to overcome it - that's what everyone did with ST60 airplanes, the most famous example being the Patternmaster, and Ted and I probably overdid it on the Trivial Pursuit and Infinity. It's overdone because it, combined with the engine response partials noted above, *works too well* to maintain the airspeed in some situations.
« Last Edit: November 14, 2019, 09:00:03 AM by Brett Buck »

Offline Massimo Rimoldi

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Re: Effect of wind on maneuvers
« Reply #28 on: November 14, 2019, 08:56:12 AM »
I apologize but I am a little confused.
If I'm not mistaken you have shown that two propulsion systems (which have two different "drag areas") give different accelerations to the stunters on which they are installed.
However, this does not establish in any way that the two systems are suitable for equipping the same model.
I believe that (if all else is true) it is necessary to show that (always) the XXXXX model needs a contras system that has (always) a greater "drag area" than the normal prop system.
I'm sure I'm wrong but given the large amount of setups available and conceivable I don't think this is possible and this brings us back to the first answers of Dave and Brett.

Massimo

Typed later: My answer does not consider what Brett says above and that I share 100%


Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #29 on: November 14, 2019, 10:11:32 AM »
Guys,
I have provided my explanation, formulas and graphs.
If you have another one, please publish it but be specific.
Best Regards,
M

Offline Chuck_Smith

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Re: Effect of wind on maneuvers
« Reply #30 on: November 14, 2019, 10:56:17 AM »
Propulsion is propulsion. Newton applies to props. At the end of the day the thrust is the time rate change of momentum of the air affected by the prop.

Contra rotating props allow smaller disks to get the same power absorption with smaller prop, exactly the same as switching from a 2 blade to 4 blade prop. A contra rotating prop is usually chosen for efficiency - provided the design trade-off of the extra weight and thus loss in payload supports the exercise. By using smaller contra rotating props the tip speed which is the vector sum of the R*omegadot (rotational speed in radians/t times blade length) PLUS the forward velocity of the airframe is reduced and compressibility affects are similarly negated as the tip speed moves lower than sonic.

How does this apply to CLPA ship? Doubtful it means much EXCEPT that the rotation in the slipstream *may* be less and thus the rolling moment due to the slipstream's impingement on the tail surfaces may be less.

It should also be fairly torque neutral regarding the moment that gets passed from the motor to the mount. Thinking about that in basic Newtonian physics - that would have to mean the net change in angular momentum of the slipstream should be new zero as well. I have held an electric Contra rotating system in my hand there was no noticeable torque I had to overcome. That makes me think the slipstream will be pretty straight IF the rear prop pitch is correct.  Choosing the correct pitch for the rear prop is a real science in itself and I'm certainly not the guy to speak to that.

Chuck
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Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #31 on: November 14, 2019, 12:12:54 PM »
Hello Chuck,
There is no doubts that the Newtonian Mechanics applies to the C/L flying. You are also right about the straightening of the air flow behind the counter-rotating propellers. This is the main reason that people using this propulsion system can use the LiPo batteries of lower capacity and still fly the entire pattern. They are simply getting about 15% more thrust for the same battery use. I fly my single prop. Big Red using the 6S 2800 mAh. ThunderPower battery and, after the pattern, there is about 800 mAh. left. The best Polish F2B pilot, Krystian Borzecki, uses 6S 2300 mAh. battery for his contra plane and there is about 600 mAh. left after his pattern. You are also right regarding the torques - they indeed cancel each other. Consequently, the gyroscopic yawing moment in corners is zero. Six pilots flying the contra system in Poland, use either two 12x6 two blade carbon composite propellers or one 12x6 and one (aft) 11.5 x 6. I believe the aft prop. should have a slightly smaller diameter but, at this moment, nobody really knows for sure what is better for the pattern. New propellers, having sometimes "weird" shapes are being constantly tested by the top F2B contra fliers in my old country and, in general, there is a lot of development work before the WCh in C/L in Wloclawek 2020.
Best Regards,
M



Offline Air Ministry .

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Re: Effect of wind on maneuvers
« Reply #32 on: November 14, 2019, 07:11:49 PM »
NOW ! .

Everyone knows if you hold a spinning bicycle wheel by the axle , and lift it it goes sideways , & V c V .

So we need to hold TWO bicycle wheels and see if the do the same thing spinning opposite directions .  VD~

Or we could get two bicycle wheels on ONE AXLE  , get Brett to hold it . And spin them  in opposite rotations .
To see what happens . When he lifts or moves it L & R or drops it .  ;D

Gyro & P would by canceled or substantially negated .

I think the only way to observe the airflow is with visable air , & whilst airborne by camera on airframe. Aimed at props .
Id thats whats being investigated .
Clunk tanks , and flap movement through manouvres would be of intest also .

==========================================================================

" How does this apply to CLPA ship? Doubtful it means much  "

On nasty little British Aircraft Carriers , it meant a great deal . Like with a Jet .

When the more powerfull Griffon Spitfire was introduced , The throttle had to be progressively opened . . .
as the swing W F O on the ground was uncontrolable .
However . You dont get ' swing ' with the Contra Props .

NOW , WIND .

Is comparable to Acceleration & deceleration .
Where the Torque Reaction is prominant .

WIND - Varying Airspeed . Even if its just steady wind . It'd be Headwind & tailwind .
Thus the BALANCE of the forces on the airframe at the start & exit of manouvres ,
As well as those through them ,
Would be significant .

Most will have noticed a reluctance for the nose to raise from level , down wind . If it blowing past 18 Kts .
Also the entry to Clover & the turn into the top leg of the hourglass can be problematic . In severe wind .

Be intresred in Matt P's observations there .

Been looking for a ' everything down ' Picture of a Seafire 47 on approach . Contra Props .
A Go Around with a conventional prop , from ' on the verg ' is problematic in the extremme .
Whereas with the contra props it is almost routine .

IN WIND , particularly choppy air , for AEROBATICS , the contra props likely are a significant advantage .

================================================================

A aside , was that the forward area increase ( from dual propellors ) needed compensateing tail area for directional stabilty ,
the rear area being a bit marginal on the Supermarines , bar the very last series .
« Last Edit: November 14, 2019, 07:51:06 PM by Air Ministry . »

Offline Howard Rush

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Re: Effect of wind on maneuvers
« Reply #33 on: November 15, 2019, 12:48:02 PM »
I think Sina Goudarzi should have one of these systems.  He'll be his country's stunt team at the WC. 
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Offline Chuck_Smith

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Re: Effect of wind on maneuvers
« Reply #34 on: November 16, 2019, 06:58:46 AM »
Matt,

Cool data there. What we need is test on an airframe with and without.

Me, put in a big block with a pipe. If it speeds up in the air lengthen the pipe. If it slows down, shorten it.  Duh.

From that point on, kick tire and light the fire. Needle will never move.

Much simpler.

I have built electric and what I've found is that the batteries are so inconsistent it's a crapshoot. "C" ratings aren't worth the ink they used to print them. On an R/C ship it gets hidden by the left stick. On a Stunt ship it becomes glaringly obvious.

Chuck
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Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #35 on: November 16, 2019, 09:00:05 AM »
Hi Howard,
What country Sina Goudarzi represents?

Hi Chuck,
I hope to test the contra powered model next season. Then we will know a little more.

Regarding "C" rating: please read https://www.rcgroups.com/forums/showthread.php?1608752-A-simple-way-to-estimate-the-true-C-rating-of-a-LiPo-battery.
Thanks, M

Offline Howard Rush

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Re: Effect of wind on maneuvers
« Reply #36 on: November 17, 2019, 08:44:16 PM »
What country Sina Goudarzi represents?

I regret that I have made a joke that you would not be expected to understand: it's an American history joke.  I am somewhat disappointed that no Americans responded, but it's also not very funny.  Other jokes in this thread relate to aerodynamics or dynamics. 

On the subject of American history, do you recognize the insignia on the side of my posts (not the rotating gear)?

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Offline Howard Rush

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Re: Effect of wind on maneuvers
« Reply #37 on: November 17, 2019, 08:47:17 PM »
On the US West coast, we are using multiple propellers, but not stacking them in tandem.  Below is David Shorts's wonderful trimotor.  Under construction are stunters with four and even six propellers. 

No joke here:
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Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #38 on: November 18, 2019, 06:06:41 AM »
Hi Howard,
Many history jokes are "country specific" and indeed hard to understand for the "outsiders".
I visited https://www.warhistoryonline.com/history/rule-one-book-war-not-march-moscow-unless-polish.html and educated myself about Merian C. Cooper and Polish 7th Air Escadrille. Extraordinary piece of history!

Does this six motors Score fly the entire pattern? What is it's RTF weight?

Best Regards,
M

 

Offline Howard Rush

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Re: Effect of wind on maneuvers
« Reply #39 on: November 18, 2019, 06:11:46 PM »
No pattern yet, 2438 grams.
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Offline Matt Piatkowski

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Re: Effect of wind on maneuvers
« Reply #40 on: November 19, 2019, 09:41:23 AM »
Hi Howard,
Score's wings area with flaps is: 677sq in. ==> 677/144 = 4.70 sq. ft.
2438 grams = 86 oz.

Wing's loading is: 86/4.7 = 18.3 oz. / sq. ft.

Most currently flying competition C/L stunt planes, fully fueled or with batteries, has the wings loading in 13.2-14.2 oz. / sq. ft. range. Their level velocity is around 25 m/sec. and the lap times are within 5.0-5.3 sec.

The electric, six motor Score will have to fly really fast to maintain the lines tension in the pattern and certainly it is not a competition plane.

You probably know that Paul Walker built B-17 with four glow engines in 2000. Please see . It is a unique example of the very good pattern flown by the almost scale control line, multi-engine model.

Best Regards,
M


Offline Howard Rush

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Re: Effect of wind on maneuvers
« Reply #41 on: November 19, 2019, 03:20:12 PM »
You probably know that Paul Walker built B-17 with four glow engines in 2000.

I think I heard about it from the fat guy flipping switches on the battery box in that video.
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Online Brett Buck

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Re: Effect of wind on maneuvers
« Reply #42 on: November 20, 2019, 11:11:08 PM »
This is the (greatly simplified) basis for speed stability based on the prop and (for sake of simplicity) a constant RPM. It shows the drag as a function of velocity, which goes up as the airspeed increases, and the thrust from this simplified situation for two example cases, one with identical props, but one with 6" of pitch and the necessary RPM to get the required airspeed, and 4" of pitch and the much higher RPM required to get the same inflight speed.



     I will give you the basis of this (and a way to model it {using numerical integration of the lift at each point along the radius} that seems to have worked well over the years), but it takes a fair bit of background work to fully explain. It's not that complicated, but it takes a lot of preamble. It's also largely as previously discussed at extraordinary length on SSW.

  What you see is the expected static thrust difference (that anyone who has launched a ST46 and 40VF airplane back-to-back), much higher for the 4" pitch prop. A fair bit of the 6" pitch prop is stalled on the ground near the root, so as the velocity increases, it progressively unstalls and the thrust rises for a while, then starts falling as the angle of attack of the prop blades goes down does as the airspeed goes up. Less of the 4" pitch prop is stalled on the ground, so it both has much more thrust (due to the much higher dynamic pressure, ~67% more) and less "knee" in the curve at low velocities.

   The result is that the thrust falls much faster with respect to the airspeed for a 4" pitch prop than it does for a 6" pitch prop, for the same inflight thrust/speed. Since the acceleration at any particular is the difference between the thrust and the drag (divided by the mass), a steeper slope (partial derivative of the thrust with respect to forward velocity) means more acceleration for "perturbed" velocity , or a smaller change in velocity for a given perturbation. That, ST60 fans, is why it doesn't work as well as a 40VF, and why a Fox doesn't work nearly as well as a 20FP.

    Note that this is for identical props, the only difference being the pitch. To first approximation, doubling the blade area doubles the static thrust, meaning when set to get the same thrust at the same airspeed, the slope is much steeper. This is like using a 4-blade prop instead of a 2-blade - or having to nominally identical props spinning in opposite directions.

   There are of course, significant simplification to make this amenable to analysis, for example, it assumes a constant RPM, which essentially never happens, even with an RPM governor electric, although it's closer than any other system. It considers nothing at all about the engine torque response, and there is *much* more happening with either a IC engine or  accelerometer feedback system. But it does illustrate the basic and strongest effect in principle.


    Systems with better airspeed control have a larger slope of this curve at the operating point.

    Brett
     
« Last Edit: November 20, 2019, 11:32:49 PM by Brett Buck »

Online Brett Buck

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Re: Effect of wind on maneuvers
« Reply #43 on: November 21, 2019, 01:37:07 AM »
The reason the curves shown above exist is because the angle of attack of the prop changes with forward speed. To understand why, and to begin to be able to approximate the effect analytically, we need to understand the angle of attack at various points along the prop radius. Of course, it is different depending on the pitch of the prop.

   The pitch is defined as how far the prop would travel forward in one revolution, if it didn't "slip". Of course it always does, but the angle of "something", usually the back of the blade, has to be something, and it's not the same angle from root to tip - its much steeper at the root than the tip. The due to the fact that in one revolution, different parts of the blade travel different total distances.



   The drawing shows the points at a radius (distance from the prop shaft) of 2", 4", or 6") with a hypothetical 6" of pitch. In one revolution, they all go the same distance up and down (direction of flight) , but closer to the shaft, it moves less around the circumference of circle. Imagine "unwinding" each spiral and flattening it out to get wedges:



The angle of the wedge gets lower as you go out the prop. The angle is the blade angle associated with the back of the blade. That's why it gets flatter near the tips.

 At a given radius r, the "length" of the wedge is 2pi*r, and the height is the pitch. So, in this example:

r=2"        length = ~12.6"
r=4"        length = ~25.2"
r=6"        length = ~37.7"

The tangent of the blade angle (alpha)  = rise/run = 6"/length, so, alpha = arctan(pitch/length)

r=2"   alpha = 24.5 degrees
r=4"  alpha = 13.4 degrees
r=6"  alpha = 9.04 degrees

    This is the angle measured by a prop pitch gauge - it's really just a protractor calibrated in inches. Of course, a 4" prop pitch has lower angles at the same point:

r=2" alpha = 17.6 degrees
r=4" alpha = 9.02 degrees
r=6" alpha = 6.05 degrees

Of course, it works at any radius, and can be computed at as many points as you want (which will come in handy later):



    Brett





« Last Edit: November 21, 2019, 10:11:38 AM by Brett Buck »

Offline Air Ministry .

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Re: Effect of wind on maneuvers
« Reply #44 on: November 21, 2019, 07:10:05 PM »


You forgot to put some on the tailplane ! .  ;D

Offline Howard Rush

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Re: Effect of wind on maneuvers
« Reply #45 on: November 22, 2019, 01:23:17 PM »
I just received a report that the above six-motor airplane has done all the maneuvers. I also learned that the motor rotation direction differs between wings.
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Online Tim Wescott

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Re: Effect of wind on maneuvers
« Reply #46 on: November 22, 2019, 02:06:34 PM »
The result is that the thrust falls much faster with respect to the airspeed for a 4" pitch prop than it does for a 6" pitch prop, for the same inflight thrust/speed.

OOH!  So we should use zero-pitch props.  Because they go so fast, we can drive them by zero-displacement engines!
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Online Brett Buck

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Re: Effect of wind on maneuvers
« Reply #47 on: November 22, 2019, 03:15:43 PM »
OOH!  So we should use zero-pitch props.  Because they go so fast, we can drive them by zero-displacement engines!

  Of infinite RPM and HP.

   In point of fact, we started out with 46VFs running 3.25" of pitch and 12,000 rpm, they would seem as if they were barely moving, and were almost immune to high steady winds - if you were willing to wait for it . That's the basis of my first post above  - you can build systems of such speed stability that it is excessive, counter-rotating or otherwise.

   Brett

Offline Chuck_Smith

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Re: Effect of wind on maneuvers
« Reply #48 on: November 22, 2019, 04:59:30 PM »
Good stuff Brett.  Yep that's true of all air breathing propulsion systems, even turbojets. The faster you go, the more the thrust drops off. Even if you can control the pitch you end up at some point with the flow through the combustion chamber thermally choked and you hit the momentum exchange limit.  That's when you light the rockets, since their thrust isn't velocity dependent.

The engineering paper is a nice touch too!

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Online Brett Buck

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Re: Effect of wind on maneuvers
« Reply #49 on: November 22, 2019, 09:06:06 PM »
The engineering paper is a nice touch too!

   It's the paper I have...

   If I really wanted to show off, I would dig out a "<<large aerospace contractor>> Stress Sheet", which was something similar to the generic engineering pad that we had custom-printed for us, on yellow paper, and was probably the place almost every new idea in the space age started (long before I was here - for instance, someone has the yellow stress sheet where they scribbled down the first block diagram for orbital gyrocompassing in 1956, framed, up on their wall).

   I actually prefer the green generic version, but I save the yellow ones (which haven't been produced for about 15 years now) to lend credence to whatever stupid idea I come up with.

    Brett


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