Downwind Faster than the Wind - Successful Run by manned cart
- Thread starter Pierrome
- Start date
nimbusgb
Well-Known Member
Quite easily once you get your mind around it actually!
nimbusgb
Well-Known Member
oops. Silly me. of course that would work much better but the switch from 'forward' to 'reverse' pitch ( windmill to drive ) will have to be acheived quickly methinks. Turning those plades in 'feather' pitch will try to brake the machine although the energy stored in the propellor 'flywheel' might be enough.. The other option would be to disengage the drive as the pitch is transitioned.
Good luck to the guys! If they get 3* windspeed it could get hairy in an f6
RAI
Well-Known Member
![[32511]](https://ybw-data.community.forum/avatars/m/32/32511.jpg?1576058889){kind=avatar}
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Well, with all the controversy, I decided to pee on the electric fence, or rather knock up a test rig myself, and see what happens. The results were not altogether as I expected.
The little model has three wheels, the rears being 100mm dia. The prop is 400 mm dia, driven at the same rotational speed as the back axle by a belt and pulleys. Construction is mostly balsa, with the odd bit of carbon fibre, the weight being just over 30 grammes. As a builder of indoor duration model aircraft, I can build light.The prop pitch can be adjusted.
The test ground was the local car park this morning, the concrete block paviors not being the smoothest of surfaces. Wind speed was about 6-8 mph, but a bit variable and gusty. Lack of R/C steering limited the length of many of the runs. A small tell-tale was fixed on a mast at the front of the rig, to show the relative wind direction.
FIRST RUNS
The first runs were carried out with a coarse pitch to the prop, i.e. greater than the ground distance travelled by the wheels. Every time, the rig accelerated smoothly and positively backwards, i.e. directly UPWIND.
SECOND RUNS
The second runs were with the prop in fine pitch, i.e. less than the wheel travel. Starting from rest was marginal, a nudge or a gust helping. Once moving, acceleration was slow initially, improving as the speed built up. the rig eventially ran with the tell-tale limp, and, on two occasions, definitely streaming backwards, proving travel at FASTER than the wind.
THIRD RUNS
As the belt drive had come off, I tried running with the prop freewheeling as a windmill. The results were surprising- acceleration from rest was much improved, and continued until the rig was running faster than the wind speed!
CONCLUSIONS
If you think about it, a freewheeling prop will always be travelling slower than the apparent wind speed, slowing the wind and absorbing energy. This is true, whatever the relative speeds are; you will always get an energy input. The gearing of the prop to the wheels will work very well at certain speeds, but will not self adjust without a complicated variable pich mechanism, and is un-necessary.
Gentlemen and theoreticians, I await being shot down!
The little model has three wheels, the rears being 100mm dia. The prop is 400 mm dia, driven at the same rotational speed as the back axle by a belt and pulleys. Construction is mostly balsa, with the odd bit of carbon fibre, the weight being just over 30 grammes. As a builder of indoor duration model aircraft, I can build light.The prop pitch can be adjusted.
The test ground was the local car park this morning, the concrete block paviors not being the smoothest of surfaces. Wind speed was about 6-8 mph, but a bit variable and gusty. Lack of R/C steering limited the length of many of the runs. A small tell-tale was fixed on a mast at the front of the rig, to show the relative wind direction.
FIRST RUNS
The first runs were carried out with a coarse pitch to the prop, i.e. greater than the ground distance travelled by the wheels. Every time, the rig accelerated smoothly and positively backwards, i.e. directly UPWIND.
SECOND RUNS
The second runs were with the prop in fine pitch, i.e. less than the wheel travel. Starting from rest was marginal, a nudge or a gust helping. Once moving, acceleration was slow initially, improving as the speed built up. the rig eventially ran with the tell-tale limp, and, on two occasions, definitely streaming backwards, proving travel at FASTER than the wind.
THIRD RUNS
As the belt drive had come off, I tried running with the prop freewheeling as a windmill. The results were surprising- acceleration from rest was much improved, and continued until the rig was running faster than the wind speed!
CONCLUSIONS
If you think about it, a freewheeling prop will always be travelling slower than the apparent wind speed, slowing the wind and absorbing energy. This is true, whatever the relative speeds are; you will always get an energy input. The gearing of the prop to the wheels will work very well at certain speeds, but will not self adjust without a complicated variable pich mechanism, and is un-necessary.
Gentlemen and theoreticians, I await being shot down!
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snowleopard
Well-Known Member
One question - what was the direction of rotation of the prop in each of the 3 experiments?
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The same, and the same as the one in the manned cart video. The freewheeling runs were in fine pitch.
snowleopard
Well-Known Member
That's odd. In the DDWFTTW experiments the prop turns in the opposite direction to the way it would if allowed to freewheel and when going upwind.
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Pic of model- prop is 400mm dia. Sorry about rather poor photo- It could do with a nice dark background!
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Keen_Ed
Well-Known Member
RAI
Well-Known Member
Congratulations
On one of the youtube spork videos there are some comments, one asked about the gear ratio on the spork tricyle. He answered saying the gear ratio has to be about 0.7:1 IIRC.
I don't know whether that is the total ratio or just the gearbox ratio, but I can see the wheels must have an initial mechanical advantage over the propeller to ensure that the cart does not go into up wind mode.
Good to see someone with initiative.
On one of the youtube spork videos there are some comments, one asked about the gear ratio on the spork tricyle. He answered saying the gear ratio has to be about 0.7:1 IIRC.
I don't know whether that is the total ratio or just the gearbox ratio, but I can see the wheels must have an initial mechanical advantage over the propeller to ensure that the cart does not go into up wind mode.
Ubergeekian
Well-Known Member
That would also depend on the windage of the rest of the cart, although I presume they minimise that for speed runs.
Pierrome
Well-Known Member
That's well done! Nice to see someone who has got the sense to actually find out for themselves.
I notice the sceptics are keeping a low profile nowadays!
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I've had further trials this afternoon, and I have to admit that the free-wheeling prop was a red herring- I can't get it to go faster than the wind. The first time, there was probably a gust, followed by a lull, and the momentum kept it going. When I think about it again, once you get past wind speed with a freewheeler the drag will be from the front.
BUT- I have replicated the results again a couple of times with the wheel-driven prop, and I am convinced that the cart has been travelling faster than wind speed, downwind.
With the coarser pitch, it heads directly upwind every time. So it is effectively going faster than the wind speed in this case too, although not in terms of the over the ground speed.
BUT- I have replicated the results again a couple of times with the wheel-driven prop, and I am convinced that the cart has been travelling faster than wind speed, downwind.
With the coarser pitch, it heads directly upwind every time. So it is effectively going faster than the wind speed in this case too, although not in terms of the over the ground speed.
Kelpie
Well-Known Member
That's fantastic. I hope those who were so rude and called Pieromme, Ubergeekian, and SL 'trolls' are ashamed of themselves.
Now, of course, can we build a functional and practical boat using this technology?
Now, of course, can we build a functional and practical boat using this technology?
Ubergeekian
Well-Known Member
With one very honourable exception, they seem to have gone rather quiet. As in "totally silent". Odd, that.
Tommyrot
Well-Known Member
colingr
Well-Known Member
As a sceptic, then a waverer, now a 99.725% convert I can see why the die hards have gone quiet. It's never easy to be proved wrong.
I'm still struggling to see why it works but it clearly does. I was intending to build one over Easter and try it for my self but I don't think it is necessary now that tobermoryphil has done the deed.
Nice job there by the way.
The 0.275% is just there because I've not seen it live for myself by the way.
Just one observation. This is great if you want to go downwind. I presume to get home you have to wait until the wind changes? (only joking, really).
I'm still struggling to see why it works but it clearly does. I was intending to build one over Easter and try it for my self but I don't think it is necessary now that tobermoryphil has done the deed.
Nice job there by the way.
The 0.275% is just there because I've not seen it live for myself by the way.
Just one observation. This is great if you want to go downwind. I presume to get home you have to wait until the wind changes? (only joking, really).
RAI
Well-Known Member
It is one thing to have expressed scepticism and another to insult the believers. The first only involves admitting a change of mind, the second deserves an apology.
mikemonty
Well-Known Member
The "sceptics" just can't be arsed and have moved on.
I liked the part where the machine worked better with a freewheeling prop!
I'll be delighted to hear your theories...