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Author Topic: Weathercocking into wind  (Read 666 times)
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billdennis747
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« on: June 03, 2019, 05:02:21 AM »

Can anyone explain why a scale model - the Bucker Jungmann - will almost invariably come in to land heading into strong wind, occasionally going backwards? The only other time I have seen this behaviour is the Douglas 0-36. I always thought that, once a model is flying, it doesn't care about wind strength or direction.
If I can find out why, I might be able to find a similar subject - it certainly helps!
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Russ Lister
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« Reply #1 on: June 03, 2019, 05:32:49 AM »

I've had this discussion with Steve Bage over a decade ago on SFA ... me being a "weathercocking believer"! Steve was not! I almost dare not comment! (Steve was more conversant with aerodynamic theory than me)
I think that the thing that most will agree on is that gusting will cause a weathercocking effect. Acceleration of the air mass will cause the model to align with the direction of the airflow.
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Pete Fardell
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« Reply #2 on: June 03, 2019, 05:48:25 AM »

I don't know the answer, but my big Fairchild does this sometimes. At North Luffenham a couple of years ago it hung in the air for ages in something almost akin to a howling gale. It's not consistant, but seems much more likely to initially weathercock and eventually land back into wind after a circuit if it can be persuaded to go left rather than right. When it goes right it still flies well but just disappears downwind into the distance. Which way does the Jungmann go, I forget?

As you say, such behaviour is a huge boon on a windy day. Not only less walking for retrieval, but much less liklihood of being dashed into the ground. (Also, lots of your competitors wimp out entirely, thereby instantly moving you up the order!)
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cvasecuk
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« Reply #3 on: June 03, 2019, 05:50:07 AM »

I suspect it is a very complicated combination of model layout, wind speed, gusts and wind shear close to the ground. I doubt if anyone could even start analysing the mathemathatics!!!
Ron
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Starduster
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« Reply #4 on: June 03, 2019, 06:19:34 AM »

I've always wondered about axes (or knives) being thrown at a target. the axe is flying through the air in a circular manner, and it does not "know" how far away the target is, so, one would logically think that the axe would hit the target at least 50% of the time handle first, but this is not the case....
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RalphS
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« Reply #5 on: June 03, 2019, 07:03:01 AM »

Flying big r/c gliders some time ago we (the club members) would often say that a particular model would weathercock.
Later I said that one of my new coupes weather cocked and only started to behave when I cut the fin height down.
This was taken up by Hepcat (John Barker) who said that can't happen as there is nothing for the model to react
against - or something like that.  He quoted the full size glider competitor in the 1920's Lympne (I think) glider
trials who had fitted a sail to his glider and claimed that it enabled him to fly great distances, but it didn't.
Sadly we can't ask John to explain things for us any more and most of us wouldn't understand the explanation anyway.
I think that I will continue to say "it is not turning - no idea why not".
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lincoln
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« Reply #6 on: June 03, 2019, 07:48:55 AM »

I'd like to see an experiment where the direction of at least 100 landinhs were recorded in some objective way.
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billdennis747
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« Reply #7 on: June 03, 2019, 07:57:28 AM »

Thanks all. Yes it doesn't make much sense but my rubber Jungmann always does it (given sufficient height) once the prop starts to freewheel. My diesel version did it when the engine stopped and finally the original designer, Eric Coates, referenced this behaviour in his blurb for his plan in Aeromodeller, so it's definitely a 'thing'.
And yes, I have long wondered about the knives thing. In cowboy films, the baddie would hurl a knife spinning end over end to land in the victim's back. If I tried it, the victim would be hit by the handle and he would then come back to me and remonstrate. I don't even think it would strike 50% of the time - more like 10%. I'd like to see an experiment where 100 throws are recorded in an objective way, such as the number of baddies lying on the floor.
« Last Edit: June 03, 2019, 08:07:31 AM by billdennis747 » Logged
Pete Fardell
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« Reply #8 on: June 03, 2019, 08:43:05 AM »

I supect that real baddies and assassins etc. don't actually spin the knife. A quick search of youtube videos using the words "knife throw slo mo" (thereby probably putting me on a terrorist watch list somewhere) suggests that although the knife might turn 180 degrees initially, after that it just keeps going pointy end first. Eg. https://www.youtube.com/watch?v=7I1O58ojoMg

(Not sure how that applies to Bucker Jungmen though!)
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billdennis747
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« Reply #9 on: June 03, 2019, 08:51:16 AM »

Hmmm...not sure about that. What stops it rotating after 180 degrees? Who was the knife-thrower in Magnificent 7? There must have been a knife thrower. I'm going a-googling; anything to make this gardening stop.
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billdennis747
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« Reply #10 on: June 03, 2019, 09:04:22 AM »

James Coburn, and he throws it straight, underhand. before his adversary has even 'cleared leather'. Oh well, back to the garden
https://www.youtube.com/watch?v=DbP5iWKYH3A
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Pete Fardell
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« Reply #11 on: June 03, 2019, 09:18:35 AM »

I found that video too. He says that you always throw it by the lighter end, be that blade or handle. But how on earth does the knife know to turn itself around only if its lighter end is the sharp bit?

(I’m also busy gardening by the way, carefully checking that the flowers are growing properly with the aid of a cup of tea and an aeroplane magazine.)
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tross
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« Reply #12 on: June 03, 2019, 09:38:31 AM »

Not sure about knife throwing.
And I'm not saying you should goggle directional stability instead of knife throwing of course.
Or swept wing directional stability for sure don't do it. Roll Eyes
It's a sure way to make sure the gardening makes the top of the priority list. Grin Smiley Cheesy Grin

I've heard you have good weather this year Bill. Smiley
We have wind and rain. Tongue

Tony

Trying not to be a kill joy.
Please continue with the knife throwing it's fascinating.
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dputt7
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« Reply #13 on: June 03, 2019, 09:46:47 AM »

When I saw Tony had replied I though he was going to tell us how his fantastic CatJets do it.
 But while I'm here, You fellas are too easily distracted  Grin There's folk out here that want to know about weathercocking so concentrate please  Grin
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billdennis747
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« Reply #14 on: June 03, 2019, 10:17:12 AM »

Tony, it could be worse - we could be discussing penetration into wind.
Not too much rain but plenty of wind. Not like the 1950s
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tross
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« Reply #15 on: June 03, 2019, 11:43:40 AM »

Touché. Cheesy
A little wind is ok but I could use something under 20 (mph)
Maybe a little basket weaving while I wait. Or trajectory study. Grin

Tony
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WarhawkP40E
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« Reply #16 on: June 03, 2019, 01:04:16 PM »

Consider that we trim our models to circle.  This means that the model does NOT want to fly straight.  This works very well for maintaining that rotating torus of rising air we call a thermal, but what happens when the model exits?  It continues to circle.  Now enter the breeze which is straight-line.  The model still wants to circle, and there is a possibility that the combination of a long fuselage and large enough vertical stabilizer will submit to the wind and remain aligned with the wind direction.  It takes a bit longer flying into the wind than with it, as it decelerates ground speed, and accelerates downwind (making the turn a bit more effective), so the likely-hood is that the model will land into the wind if it weather-vanes.

Justin
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TimWescott
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« Reply #17 on: June 03, 2019, 03:35:13 PM »

The only way that the thing could stick in a direction with respect to the ground is if it's somehow reacting to the ground-level wind shear.  Which may well be possible, although I'd have a bigger tendency to believe it if you could show that it is more pronounced the closer the plane gets to the ground (and hence the more wind shear there is).  I can't imagine why this may be, but if it does happen I'd bet that there'd be markedly different reactions between a low-wing high-rudder plane and a high-wing low-rudder one.
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tross
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« Reply #18 on: June 03, 2019, 04:22:37 PM »

It's hard to imagine that one would believe an increase in directional stability would result in a greater distance.
Directional stability is mostly influenced by the vertical tail and fuselage. A coupe wouldn't have much of either.
The cat jet has an enormous amount of both, and it comes with a huge drag penalty in a turn.
One might imagine the vertical fin as a wing, and in a side slip (yaw) the wing is at a higher incidence.
Drag is therefore increased and for a glider this is not the desired effect.
I'm still not sure I fully understand the original question, but only to say I'm not a big fan of too much of anything.

Tony
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Yak 52
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« Reply #19 on: June 04, 2019, 07:51:18 AM »

If such an effect were to exist a model would have to turn into the wind from a launch on both sides of the wind - ie from a launch to the left AND a launch to the right of the wind direction.

This is one of the big controversies and a complicated one to get you head around, but a few established facts help:

1. In a stable 'parcel' of moving air, an aircraft will not feel any influence of the wind whatsoever. It will appear to move differently from a ground based perspective but in reality it is flying exactly the same as it would in dead still air. Thought experiment: indoor flying in the enclosed hangar of an aircraft carrier travelling at 20 knots. Or flying a model from a hot air balloon at altitude - you don't feel any wind because you are moving with it. (A model in a steady turn does not spend any more time going up wind, it's just circling with no knowledge that the ground is moving beneath it.)

2. Weathercocking, in terms of the models stability characteristics, refers to the way it responds to a change in side slip angle of attack. This combined with dihedral and yaw damping is what provides 'spiral stability'. A model flying straight and level across a steady wind will feel the wind as straight on the nose but in comparison to the ground will be crabbing somewhat. A gust or lull will change the side slip angle and so a stable model may well change course visually from a ground perspective as it responds.

3. Wind gradient and shear. The wind speed and direction varies with height so if the model is ascending or descending through shear layers it may well experience changes in side slip angle as a result and change course. A straight on the nose, up wind descent through a wind gradient will look like a loss of airspeed. I think this may also apply in some way to side slip angle because when descending across the wind, the sideways component will be changing with wind gradient - so may be this is responsible for the phenomenon described to some degree?


Jon
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tross
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« Reply #20 on: June 04, 2019, 10:12:52 AM »

So true Jon and well said. Smiley
I don't know about any such controversy.
I'm just a dumb farm boy so pardon me as I must inquire.
Is it the fact that it only rotates about one axis the issue.
I wonder if the weathercock realizes how much we admire his simplicity. Grin

Tony
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Yak 52
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« Reply #21 on: June 04, 2019, 11:22:53 AM »

Is it the fact that it only rotates about one axis the issue.

It's a frame of reference thing. The weathercock is attached to the ground so the ground is the frame of reference.

After launch/takeoff the ground doesn't exist as far as the model is concerned. It can only react to relative changes in local air velocity or direction such as thermals or wind sheer and gradient. Just like a jet flying in turbulence, they have an effect and cause accelerations or rotations of the model. The axis for these forces or moments is the aircraft's centre of gravity (in 3D).

But when you are on the ground watching it's hard not to make the ground your frame of reference mentally.

Looking at it another way - a model circling in a steady wind (with no sheer) would look exactly the same as  a model circling in dead still air while you moved past it on a travelator Smiley
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tross
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« Reply #22 on: June 04, 2019, 01:34:08 PM »

Hi Jon Thanks. Smiley
You aero modeler's have had controversy over the silliest things.
IMO. Grin

Tony
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TimWescott
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« Reply #23 on: June 04, 2019, 02:26:01 PM »

Hi Jon Thanks. Smiley
You aero modeler's have had controversy over the silliest things.
IMO. Grin

Tony

Totally pointless and silly until you start crashing planes or losing contests because you misinterpret reality -- then it's important.
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« Reply #24 on: June 04, 2019, 06:12:40 PM »

After launch/takeoff the ground doesn't exist as far as the model is concerned.

That might be true in the textbook world.

We often forget that free flight (duration) models fly near the stall, near the critical RE number and do have a aerodynamic behaviour which is all but linear, but in most cases dictated by hysteresis behaving.

Think also about gravity and connected to it inertia which will have influence on how the models turns in relation to the ground. All the time the model is prone to wind shear, wing gusts in both planes (horizontal and vertical) and as said, also inertia induced higher or lower airspeed than optimal. In these cases the behaviour of the airfoil/model is hardly predictable and is certainly not like a model flying in a "box" of still air.

Urs
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