Rick Cavallaro wrote:

On Oct 5, 9:04 am, Ronald Raygun
wrote:

That's a pity, isn't it, because the object of the analysis was to
explore the feasibilty of building a *real* (and therefore finite size)
vehicle that can outrun the wind.

I don't see that as a pity at all. I think his analysis does that
quite nicely.

I'm sure that's what it was meant to do.

But at first it doesn't, because his analysis made the explicit assumption
of a loosless propeller, and hence the implicit assumption of an infinite
propeller. Yes, he slipped in a rider at the end allowing for 85%
efficiency, but in a subsequent exchange of views he was adamant
that, in the initial analysis, he was not simply ignoring losses in the
calculations despite acknowledging that they would of course exist, but
instead he was assuming there were none, that all components were 100%
efficient. This philosophical distinction seemed important to him, even
though (it seems to me) the implication of this gets in the way of
promoting understanding, given that most readers (here) would be unaware
that 100% efficiency implies infinite size.

When we got to discussing the static case and when I said:

## Suppose car speed and wind speed are *both* 55ft/s. The
## wheels are still delivering 1hp. The headwind is zero, so you
## would calculate the power needed by the prop to produce 10lb
## of thrust as 0hp. Can't be right.

His reply was:

# But it is right, because at that point in the calculations we are
# still describing the scenario using theoretical, lossless components.
# We've defined the transmission method as lossless and the propeller
# also as 100% efficient.

And it breaks it into two very convenient parts. The
first part proves that it's only an engineering problem (no laws of
physics need changing), and the second part shows that it can be done
with readily available gear.

That's a good approach, but the prop efficiency stuff ought to have gone
into the engineering part, whereas he seems to have it initially in the
physics part. It seems to me that the physics analysis would be more
illuminating if it allowed us to work with finite values of air throughput.

It seems to me that therefore it would have been
far better to say "let's ignore the losses and see how much spare power
there's going to be, and then think about whether it's enough" than to
say, as he did, "let's assume there will be no losses, that all the
components are 100% efficient, ..."

Go back and read his analysis again. He did exactly as you suggest.

I have done, and he didn't. See above.

especially if that was said with full knowledge
that this would require an infinite propeller. Moreover if it was said
in a forum where most readers would not have that specialist knowledge,
it's not a very effective way to try to explain things to non-experts.

There's no need to imagine or be concerned with infinite propellers.
If the first part of the analysis showed that you needed a 100%
efficient propeller you'd probably just dismiss the whole idea right
there as uninteresting at best. If you wanted to consider what a 100%
efficient propeller was - you'd come to find that it had to be
infinitely long. But an 85% propeller, which does just fine, is quite
realistic.

OK

Of course choosing different definitions for efficiency cannot change
the outcome of an experiment, but it can lead you to an incorrect
result if you use a different definition than what is assumed by the
person that did the analysis.

Does this make sense?

Sort of, er, um, to be honest, no, not really, by which I mean it doesn't
make sense to me because I don't understand propellers well enough to be
comfortable with the implications associated with various definitions of
their efficiency.

Sorry, the definition used relates to "actuator disk theory" - which
is a method of analyzing propellers. Basically it's kind of like
saying what if we could push air with some specified efficiency
without regard to how we do it specifically (i.e. kind of ignore the
propeller). So this would seem to go in the direction you're looking
for - which is to try and understand the thing on first principles -
rather than having to understand propeller design. In any event, it's
unfortunate if the prop terms and efficiency terms were confusing. I
assure you that's the opposite of our intention.

What actually got me going was that he stated the prop's power output
would equal thrust times the incoming airspeed. I thought, by considering
the work it had to do on the air, that it should be more than that.
something like thrust times a speed value somewhere (probably halfway)
between the incoming and outgoing air speeds.

If that is wrong, I'd like to know why it's wrong. I'm sure you could
just tell me, but please don't just yet, as it'll be better for my
soul if I can work it out myself.

Frankly, I don't think efficiency is really the main problem giving
rise to my misunderstanding. I think it's more of a relativity problem,
i.e. looking at things in different inertial frames and then trying to
tie them together.

Energy is a particularly nasty little item. Very few people seem to
stop and realize that it's not an intrinsic property that something
has. The amount of kinetic energy something has depends entirely on
the frame of reference from which you measure it. Any frame will do
to give you the same results, but you sort of have to pick a frame and
stick with it.

Indeed.

The other day you had a quibble with something TNP said about static
propellers doing no work, and you rightly pointed out that while a
hovering helicopter's rotor thrust is doing no work on the helicopter
itself, it is nevertheless doing work on the air.

I don't think there was a quibble. I was merely clarifying a point
that he also readily agreed with.

It was a minor correction; I call that a quibble. If you understand a
quibble to be more of a dispute, that's not what I meant.

I'm hopeful that with some more concentrated thought I can sort out
the misunderstanding, and I've a feeling the prop efficiency issue
is a red herring and that its complications can be side-stepped by
going back to the (very useful) skateboard analogy.

I assure you the prop efficiency thing was only intended as a way to
do the analysis. But if you think the skateboard analogy will be more
intuitive let's have at it. I'll ask you to lead.

OK. Watch this space.