Two conveyor belt scenarios

Actually, on a moving river, water speed matters and frequently speed
(well, distance really) relative to the ground matters. The ideal setup
is to have a headwind while pointing downstream. That way you have
slowest waterspeed and shortest run. BUT....local obstructions dictate
you concern yourself with distance of run relative to the land....

Like I said, it can get complicated.

Gary Drescher wrote:
"Doug" wrote in message
oups.com...
There are three speeds here. Speed relative to the air. Speed relative
to the belt. Speed relative to ground.

The only velocities that matter are that of the plane relative to the air,
and that of the air relative to the landing surface (in this case, the
constant-speed conveyor belt). The landing surface's velocity relative to
the surrounding ground has no physical effect on the landing (except perhaps
with respect to turbulence, but that's not part of the hypothetical
scenario).

And actually, this does have a real world analogy. Taking off in a
seaplane on a river that is moving. Now add in wind blowing upstream or
wind blowing downstream and the takeoff and landing upstream vs
downstream comparison gets quite complicated.

If the river is arbitrarily long (as we're assuming the conveyor belt to be)
and you don't care where on it you end up, then the landing isn't
complicated at all: you just ignore the land completely, and pay attention
to the wind speed relative to the water. (It helps to have a wind sock
that's riding on the river or on the conveyor belt.)

--Gary

"Doug" wrote in message
ps.com...
Actually, on a moving river, water speed matters and frequently speed
(well, distance really) relative to the ground matters.
....local obstructions dictate
you concern yourself with distance of run relative to the land....

Sure. That's why I said that *if* the river is arbitrarily long, and if you
don't care where you land, *then* you just ignore the land and care only
about the speed of the air relative to the water. (Those stipulations make
the situation analogous to the hypothetical conveyor belt scenario.)

The ideal setup
is to have a headwind while pointing downstream. That way you have
slowest waterspeed and shortest run.

The plane's speed relative to the water (the plane's waterspeed) depends
only on the plane's airspeed and the speed of the air relative to the water.
It doesn't depend in any way on the speed of the water relative to the land;
hence, it doesn't depend on whether you're going upstream or downstream.
Rather, it just depends on whether you're going upwind (relative to the
water) or downwind.

As for making the shortest run (relative to the land), wouldn't you want to
be going upwind (relative to the water) and upstream, rather than upwind and
downstream?

--Gary

I honestly don't know. I know that this is argued in the Seaplane's
pilot association newsletter ad nauseum. One thing to further
complicate things is with a Seaplane, water speed DOES matter. Faster
water speed gets you on the step quicker and that means shorter takeoff
runs. Also, TOO fast a water speed (on landing) can cause a bad
accident (this would be landing upstream with a tailwind). People who
takeoff on fast moving rivers do report shorter runs (relative to the
ground) going upstream. What is debated is whether they are getting
shorter runs relative to the water, than a downstream run with the same
headwind. But like I say, I dunno. The real world scenario is NOT
trivial. My bottom line is I need to have plenty of room to take off,
so I never really pushed the issue. I always wanted 2 or 3 times as
much distance as I needed. If I didn't have it, I found a longer lake
or river. Rivers tend to be plenty long, at least the ones I've dealt
with. My experience with rivers is limited to one or two and they were
fairly easy, plenty long. I always just took off into the wind.

Gary Drescher wrote:
"Doug" wrote in message
ps.com...
Actually, on a moving river, water speed matters and frequently speed
(well, distance really) relative to the ground matters.
....local obstructions dictate
you concern yourself with distance of run relative to the land....

Sure. That's why I said that *if* the river is arbitrarily long, and if you
don't care where you land, *then* you just ignore the land and care only
about the speed of the air relative to the water. (Those stipulations make
the situation analogous to the hypothetical conveyor belt scenario.)

The ideal setup
is to have a headwind while pointing downstream. That way you have
slowest waterspeed and shortest run.

The plane's speed relative to the water (the plane's waterspeed) depends
only on the plane's airspeed and the speed of the air relative to the water.
It doesn't depend in any way on the speed of the water relative to the land;
hence, it doesn't depend on whether you're going upstream or downstream.
Rather, it just depends on whether you're going upwind (relative to the
water) or downwind.

As for making the shortest run (relative to the land), wouldn't you want to
be going upwind (relative to the water) and upstream, rather than upwind and
downstream?

--Gary

"Gary Drescher" wrote in message
. ..
[...]
As for making the shortest run (relative to the land), wouldn't you want
to be going upwind (relative to the water) and upstream, rather than
upwind and downstream?

He didn't say anything about "relative to land". If you want the shortest
run relative to land, the only thing that really matters is the wind
direction and speed. But I think that's not what he was talking about.

Of course, since you're on the water, if you do it this way you ignore some
important factors. In reality, it's all a trade-off. There is no perfect
scenario the way there might be on a paved runway. For one, with the wind
against the water, waves can be an issue. So even though downstream with a
headwind minimizes water speed and maximizes the headwind, it means you're
operating in rougher conditions. A headwind going downstream may be better,
since it still reduces your water speed (helping compensate somewhat for
landing against the current), while not stirring up the water surface so
much.

And, of course, all of that ignores any local features of the landing area.
Terrain, rocks, course of the river, etc. all come into play as well.
There's almost never one absolutely best way to land a seaplane, though
there are usually more bad ways than good ways.

Pete