Rob2160

LOL Coyote finally did eat the road runner. https://www.youtube.com/watch?v=EuE-GpNV0sY

Watch the movie "Gravity".

Frame of reference is relevant when the bolt drifts off harmlessly, and Bullock reaches out to grab it.

Though it is actually moving at thousands of miles per hour in orbit and could punch a hole through another spacecraft travelling in the opposite direction.

https://www.youtube.com/watch?v=d6sk3HudM4k

JPerrone

- I've never been to space and I don't expect to, so I don't think I'll ever be able to actually experience what they portrayed in that movie
- Sometimes movies are accurate depictions of reality, and a lot of times they aren't
- But it looks like a lot of the behavior in the movie corresponds to the physics

I think people can get some interesting insights into mass vs weight; momentum; acceleration; and some of the repercussions of that, from the movie. Maybe those insights are useful in other areas as well. A long time ago, I came across some images of what a face looks like in space when there's no gravity working on it for awhile. Incredible!

Regards

Kwigen

This does cause problems if you are flying downwind, then turning final and suddenly losing 15 kts of airspeed in the last part of your approach (due to the wind changing significantly in a few hundred feet)

However most of these forum discussions on the down wind turn involve a perfect steady state wind where turning through 360 degrees will not change your airspeed at all.[/QUOTE]

I know all that. The theory, math, and scenario all make sense to me. I'm just saying that the real world experience of a guy who made down wind turns every couple of minutes for 10 to 12 hrs per day took the resolve out of my argument.

dirtybird

My Feedback: (5)

I know all that. The theory, math, and scenario all make sense to me. I'm just saying that the real world experience of a guy who made down wind turns every couple of minutes for 10 to 12 hrs per day took the resolve out of my argument.[/QUOTE]

Wouldn't wind be a better word than resolve?

radfordc

My Feedback: (14)

Isn't that what you're doing, too? If I say that 2+2=4 and you say 5 what's the point of continuing the discussion?

W7APD Alan

Hear. Hear.

The airplane's momentum (see post #1) is not relevant until said airplane comes into contact with something on the ground. The frame of reference, when calculating airspeed, is the air.

Our problems as RC pilots arise because we are fixed to the ground and using visual cues, meaning we can ONLY "see" the model's groundspeed. Trouble follows our attempts to maintain a constant speed both upwind and downwind.

sensei

We call that sudden stoppage around these parts and the only thing I think about when turning downwind is very soon I will need to turn upwind again, beyond that I don't much think about it.

Bob

Top_Gunn

My Feedback: (6)

OK, one more post.

The fact that someone on a train moving at constant speed cannot detect the difference between being on that train and being on one that is standing still without referring to the world outside is not something that is so because I said so. It is something that Galileo pointed out in the 17th century, though he used ships rather than trains as his example, because we didn't have trains yet. You cannot "feel" the motion of the train just by walking in it and changing direction. This principle, now called "Galilean relativity," is basic to all Newtonian physics (and Einstein, who did use trains in his examples, ran with the notion and extended it to develop special relativity). To deny this plain fact, which everyone who has studied even basic physics knows, makes as much sense as claiming that the world is flat and has sharp edges, off of which people will fall, in a discussion of geography. Some facts are so basic to any discussion of physics that it is absurd to ask for reasons why they are true. This, just like things like f=ma and "don't push a rope," is one of them. The OP started this discussion with a real physical formula, but after several people pointed out his mistake he has simply turned to assertion. If we are going to talk about physics, we have to use physics, and denying that elementary physics is so because your father was a railroad engineer does not amount to using physics.

JPerrone

I think the concept that the airspeed of the aircraft doesn't change has been stated, proven, established beyond any doubt. That means that the OP's original equation proves that momentum is conserved, from the standpoint of the aircraft's momentum relative to the wind.

I think most have agreed that ground speed is different on upwind vs downwind runs, for the same airspeed.
That means that the airplanes momentum relative to the reference plane of the ground has changed.

I think it has been established that ground reference has no influence over the flight dynamics; therefore, while there is an observed (not yet explained) difference in momentum relative to ground, it should have no effect on how the aircraft behaves.

I have no explanation for where the momentum comes from or goes to in this ground reference frame. The rules about conservation of momentum must apply, and most of the explanations that come to mind are contrary to what makes sense in the moving wind reference plane, so these can't be correct. Some will continue to say "groundspeed has nothing to do with airplane dynamics." I'll concede that even agree with it!!!

- But the aircraft still does have a groundspeed;
- It does change;
- That change implies a change in momentum relative to the ground!
Theory: If I were flying just above the ground at 1 mph and hit the ground, there would be less pieces to pick up than if I hit the ground at 10 mph. I think this is because of momentum but maybe it isn't....

I would say until the explanation for this apparent discrepancy is found, there will remain lingering questions about "why is that?"

Regards

Bozarth

My Feedback: (15)

No discrepancy. Review the principle of conservation of momentum. Momentum is conserved WITHIN an isolated system. In your case, your isolated system is the air mass and does not include the ground...enter the "conservation of energy." It is all relative. Einstein took your question a few steps deeper. You are in good company.

Kurt

Rob2160

You beat me to it. (I was out flying RC all morning)

Totally correct, calculation of momentum also depends on frame of reference. The Ground has zero interaction with the aircraft when it is flying in the air.

The only time the aircraft momentum becomes relevant to the ground is when landing (or crashing) . At which point a downwind landing will be much faster than an upwind landing.

The difference is due to the acceleration imparted to the aircraft by the wind shortly after take off.

Plenty more reading here about momentum- skip to the part on "Dependence on reference frame"

And this video covers it pretty well.

https://www.youtube.com/watch?v=iHfCwJwdIv8

JPerrone

My premise is that
1) the moment the aircraft crashes into the ground, it is now part of the frame of reference that includes the ground
2) There is a difference in the result on the airplane if that groundspeed is 1 mph vs 1000 mph

Regards

JPerrone

I like the moving walkway!! I use those a lot in airports. Here's some observations about speed and momentum on a moving walkway:

Let's say you are walking at a brisk pace on a walkway, 5 mph. You have a certain momentum relative to the walkway.
Now, compare that situation with walking on a different walkway, moving at a different speed. As long as you are walking 5 mph, your momentum relative to that walkway is the same as the momentum relative to the other walkway.
To prove that: Imagine the result of you not noticing that someone is standing still in front of you; you walk into them on walkway A vs B. You and they won't detect any difference in the impact. Regardless of the speed of the walkway, both of you feel as though you've just had a 5 mph collision

All good things come to an end, and when you step off of the walkway, your body is moving at the speed that you are walking plus the speed of the walkway.
When your foot touches the ground, it is going to feel different to you than if you were walking on the stationary ground.

And that feeling will be very different if the walkway speed was 30 mph as opposed to 3 mph.

Rob2160

Was there a walkway in that video? I kept replaying 0:28 to 0:31 (There were dozens of suitable videos but I liked Sarah)

JPerrone

LOL! There's this great photo of a young lady in a near invisible bathing suit, posed on a boat. The caption is "and if you stare at this long enough, a boat will mysteriously appear!!!"

.

eddieC

My Feedback: (2)

If you're 'deferring' to someone flying overgross with a sick engine, that says more about your (poor) judgment than it does about the science and physics involved.

Why he probably dreaded the downwind turn was the fact that he would hit with more momentum flying downwind than he would upwind.

cfircav8r

My Feedback: (1)

Just because someone has been at it a long time doesn't mean they are competent. The only pilots I have ever met that believe the down wind turn myth are the ones that rely on the ground for their sole reference. The sad part is that if they truly understood what is happening they would be a safer pilot.

Rob2160

This is very true.. I had just started flying a new corporate job, I had about 7000 hours total time and about 1500 on jet.

I was flying with another pilot who had 15,000 hours and 5000 on jets. (We operated as Co-captains, flying command on alternate legs)

One leg was mine, Perth to Sydney, at FL390, M.75 with a 100kt tailwind and a plane full of passengers.

Two days later, it was his leg, flying Sydney back to Perth, FL360, Mach 0.75, no passengers into a 120 kt Headwind.

The indicated airspeed was higher due to the lower altitude. but the other guy said. "We have a higher airspeed now, it must be the headwind"

I thought he was joking, but he wasn't.. I thought about it for 5 seconds and just said, "Yeah must be"

If he didn't get it after 15,000 hours he never would.

cfircav8r

My Feedback: (1)

You won't ever win with some. You can trot out all the scientific proof in the world and the retort will always be the same "I, have x # of hours, fly ag, bush fly, seen it every day for x # of years in my models, etc..." All we can hope for is the new ones see the light and ignore the amateur "experts."

radfordc

My Feedback: (14)

Ag pilots fly in the wind gradient near the surface. This what makes turning downwind a problem.

Bozarth

My Feedback: (15)

So you mean in the same area 99% of RC planes fly...?

Kurt

tailskid

My Feedback: (34)

Could you explain the 'wind gradient' as I have not heard of that term before. Thanks,

cymaz

Wind gradient , the lower you get to the ground the less the wind strength. I think it is something to do with the friction of the ground slowing the air current down.

cfircav8r

My Feedback: (1)

So flying up into the gradient you get a slight boost in airspeed then turn down wind and dive into a lessor tail wind, win-win. Doesn't explain the "dreaded" part that ag pilots fear. It is a lack of understanding combined with flying by reference to the ground. There are times when you need to be aware of the wind speed, direction, and the possibility of shear but unless you understand the actual reasons why you need that information you will eventually get caught off guard.

Bozarth

My Feedback: (15)

cfircav8r is spot on - especially regarding referencing the ground.

Kurt