Mini Challenge: Sailing with a blower

The intuitive answer is that the thrust goes up since the air is blowing against something solid.

But, I'll guess that the thrust went down due to blocking the flow of air - it's the same as if you put the board on the suck side (I believe that is the correct technical term).

And I don't believe for a second that you actually did this.

it's the same as if you put the board on the suck side (I believe that is the correct technical term).

Actually when I said "intake" I meant the suck side.

Yeah, I really did this. And worse than that, I actually did a whole string of experiments (described around posts 340 and after in the original "blowing across the lake" challenge question). Clearly, I've gotta get a life. Then today, I wandered around with dynamic earphones on, with my cell phone in my pocket, in hopes of hearing those weird audio burst described elsewhere. (I told my family I was wearing the earphones to keep aliens out of my ears, so they wouldn't worry.)

WHo will be the first to propose a water driven generator type of free energy device?...

We're almost there. There are several electric auxiliary sailing catamarans, that use the spinning props dragging through the water when sailing to generate power to use when the wind dies. There are those who think of this as free energy. In fact it slows the boat while it sails, so that by the time you get near port the wind has died -- and now you have to motor in. Without the drag of the props, you would have been there earlier, and could have sailed in.

(OK -- that's a pretty jaundiced view. It's really not such a bad idea.)

The intuitive answer is still mostly right.

As you move the plate gradually closer to the intake, the force
should rise untill the fan 'stalls` at which point the force would
suddenly show a marked decrease.

Not enough info to figure where your particular fan would stall
and that's too much work anyway.

I'd guess substantially more: you've now got a partial vacuum between the board and the fan.

I would guess lower. The fan generates thrust by imparting momentum to the surrounding air and you're blocking its supply. It's the same effect as stopping up a vacuum cleaner hose. The motor speeds up because it has less air to push.

The apparent thrust would be higher. There is less pressure in front of the fan when the board is in place. If you also measured the tension on the board you're holding, there'd be a difference from a static moment. No?

400 g, due to lateral lift by interferance [slight pressure drop] jbo

The answer:

Thrust went up substantially, to about 600 grams. Prior to doing this, I was thinking in terms of throttling the mass flow, so that thrust would go down, because the whole system of accelerating air rearward would be made less efficient. But in the process of doing it I could feel that the fan was "pulling" on the board, which the scale confirmed. Per Bernoulli, the higher speed flow should create lower pressure between board and fan.

I would bet a pint of stout that if you fixed your plywood screen to the table so as to capture the total combined thrust, you would find that the suck of the plywood was matched by the equal and opposite suck of the fan. Physics sucks, no?

RichH

I'd happily buy you the pint of stout.

I've been planning to do that setup, next time I play with this thing. I'll tape the board to the fan with some small blocks of wood in between. (The fan's weight keeps it on the table quite well -- although if the resulting net thrust is 600g, then the fan will probably topple.)

When the board is held (in the same non-contact way) on the other side of the fan (outlet) side, the thrust also increases, but not by as much. Seems, at first glance a little counter-intuitive, but on reflection, it makes sense.

Also counter-intuitive is the fact that on the low pressure side, where velocity without the board is very low, the effect is relatively more dramatic than on the output side, where the velocity is much higher. Given that most aerodynamic effects vary with the square of speed, you'd expect the opposite, at least at first glance.

In all of this, it is clear that getting the fan and any sail to cancel perfectly (as in the original question's official answer) is very difficult. The boat will certainly go forward or back, even if one were actively trying to make it stand still (ignoring the outlandish "dynamic friction").

Physics doesn't suck. It blows.

Ken

I would guess that the negative pressure on the face of the plywood, on the supply end (throttling) produces a distributed and cumulative to the extent that the peak negative pressure would be at or near the center of the plywood. Somewhat similar to the lift generated by the 4 6 in by 20 foot helicopter rotor blades. A 2 ft by 20 foot wing would not generate sufficient lift for flight, but as a rotor, they do. Howe come?

On the discharge side, the same accumulation to a peak at the middle of the plywood probably wouldn't be as great. The blade has a maximum pressure differential, the would prevent the movement of additional air at the center, whereas on the supply side, the exit velocity reduces the equivilant pressure on the discharge side, so that the increase in negative pressure can aggregate more. Plus, the discharge side will suffer from a rebound effect of the pressure wave from each blade. Probably causing a "thump thump" (in American English) audible as the distance to the plywood varies.

Enjoy your toys while you can. Someday you won't, and your children probably won't have any clue what you were up to.

RichH

Gadzooks. You can see how this goes. Your assumption re pressure distribution is different than mine, (I'm guessing there is a venturi effect near the edges on the inlet side) but your assumption seems pretty reasonable. So now, I'm thinking "I could set up a simple water tube manometer..."

I'm working on developing that wild look of the guy in "Back to the Future." So far I haven't managed to blow up anything in my shop.

Ken