DDWFTTW - Directly Downwind Faster Than The Wind

That's very interesting! But it's not clear to me that the gearing is going to decide the outcome at the static start position. It seems that this will be a straight tug of war between the drag which will tend to push you downwind and the lift which will tend to turn the prop (the wrong way) and pull you upwind.

By the way, I read your blog, and I couldn't find a quick link to any summary or design or explanation of what you are doing. Did I just miss it?

 

Thanks Spork

I sail high performance beach cats so I can understand 'dragging the apparent wind forward' as we call it and obviously an ice boat has less drag than a cat so do it can do it better.
Can you make that diagram with the prop blade inserted or is its camber as is the sail so its identical, but it needs to show DDW TWA
Now I'll try to read and re read the above to get it to sink in.

Powerabout

 

Maybe its just like a ram jet and needs to be doing a couple of thousand MPH before it works...only joking

but I also asked that question.

Its seems like a delicate balance of wind drag of whole unit versus power developed by the prop thru the gearing to the wheels that might want to wind it up the wind?
So if you start on the flat it might go upwind or if you start on a down wind slop it will happily go downwind and continue on the flat FTDDW?

Maybe with a prop connected to a gen that didnt care which way it rotated but would only drive an electric motor one direction you might get a different result?
Its a bit like trying to understand the first chapter in your gyro compass manual and then being asked what it will read when you are on one of the poles.

 

Forgot I had taken these a year or so ago at the club in Lake Garda
It did rotate quite fast but I never saw it moving

 

I have attached a table that gives conditions for a prop with a pitch of 1m geared to advance through the air 0.8m for every meter the cart travels.

One way to think about this is to forget about the propeller and how it works.

Think of the cart being towed by huge kite. It is easy to envisage a cart getting close to windspeed in this condition. The cart would be doing almost 5m/s, the same as the windspeed.

Now mount a winch on the cart with a very long line attached to the kite. Connect the winch to the wheels so that the gearing gives mechanical advantage to the wheels. This means that the wheels can reel in the kite.

In one second the cart travels 5m without the winch but the winch has reeled in 4m so the air behind the kite has only travelled 1m rather than the 5m it would have done without the winch. So this system has the ability to generate a huge excess force to keep the cart accelerating.

The advantage of the propeller is that it is like an infinitely long winch line that can pull as long as the cart is moving. Its weakness compared with the kite is that it is not as well connected to the air and suffers greater slip than a huge kite.

The mechanical advantage of the wheels over the prop is a function of the propeller efficiency. The higher the efficiency the higher the end speed relative to the windspeed. With an effective ratio of 0.8 the cart could get to 5x windspeed. In practice it is unlikely that the gearing to get it to work will be higher than 0.5 and this limits speed to twice windspeed.

Rick W

 

Many thanks Rick

almost got it,
How do those figures look from cart speed of 1 down to zero?
Could you do a plan view showing the top of one blade, angled blade, rotation direction and all wind directions on a cart?

 

The initial thrust is created by the drag on the cart and blade. This gets the cart moving. Once the cart is moving the prop is turning and it establishes a near field flow backwards through the disc swept by the prop.

With any propeller or turbine you have to distinguish between the near field air flow and far field airflow. The prop is accelerating air through it.

I have attached the flow field image for a prop similar size to what the guys are building and the conditions I have shown in the cart drawing.

The speed of airflow in front of the cart is 1m/s toward the cart. The prop accelerates the airflow rearward. I have shown it as 4.8m/s relative to the cart but for a real prop the near field will be around 4.4m/s through the prop. So rather than the wake moving at 1.2m/s it will be more like 1.6m/s with an imperfect prop.

The wind has given up energy because it is moving at 5m/s over the ground in the far field but in the wake of the cart it has dropped to 1.6m/s. So the prop has slowed it down by 3.4m/s. The force generated by a 4m diameter prop to do this is about 140N (say 30lbf).

JavaProp shows the power absorbed by the prop is 650W. The power produced by the cart wheels is 140N x 6m/s = 840W. So there is some 190W excess power to overcome rolling friction, air drag on the cart, mechanical losses in the transmission and a bit left to keep the cart accelerating.

As the cart speeds up the propeller efficiency improves.

Rick W

 

Hey guys, very sorry I missed these posts. I'll take a look at them now and see if I can answer anything I missed.

 

Alrighty - first of all thanks Rick for covering those questions.

Now, as far as describing the starting condition, that's kind of messy. It's fairly easy to get reasonable prop data from static thrust on up to the design advance ratio. So that starts us out at wind speed. Below wind speed, the prop is working in tailwind. You could certainly do a CFD run to compute the behavior in this situation, or you could take measurements (which we intend to do with our prop), or you could make some reasonable guesses based on the airfoil of the prop and what percentage of it will be stalled vs. lift producing at each cart velocity from 0 up to wind speed.

In any event, we don't specifically make any claim about the cart being self-starting, but it's worth noting that all the carts we've built to date (small scale) have no trouble at all self-starting. Because the behavior of a prop is pretty non-linear (particularly when a large portion is stalled), it's a bit hard to say exactly what gear ratio would be required to insure self-start. But clearly a prop with 0 pitch would self-start in any case as there would be no tendency to produce a torque in the opposite direction. So we know it's just a matter of gearing, but we don't know the exact value that would make it start downwind. If it were a kinematic constraint rather than an airfoil, we'd know the dividing point would be at a speed ratio (prop/wheels) of 1.0.

As far as picturing the prop in the above diagram, just imagine the tips of the prop as the sails on two boats on opposite sides of the globe - each on a downwind tack. Does this make sense?

Rick - my first guess would be the same as yours regarding the speed ratio needing to be 0.5 or less, but when I simulated the whole thing I was surprised to find the best performance and acceleration with very high speed ratios (close to 1.0). I sure hope I didn't do something horribly wrong.

 

spork
The Javaprop data above is for a ratio of 0.8 and it still has excess power so you should be able to get better than 0.5 - that is very conservative. If you squeeze up too close to 1 it will just sit there. The wheels will not have enough leverage over the prop.

As a matter of interest my windward going boat was very reliable with the air turbine pitch at 1m pitch and the prop at 0.3m. I tried a prop of 0.625m pitch and could get it moving if I spun the turbine to get it moving and the wind was strong enough but the blades on the prop were a bit small to be efficient. A boat is a much more difficult proposition than a cart. I determine with the windward boat that a ratio of 0.6 prop to turbine pitch was close to the limit. The advantage with the windward condition is that the turbine is in a good working regime at reasonable Re# even though the blades are quite small.

Rick

 

Understood Rick. Using the prop performance data from wind speed to 2X wind speed we found the best acceleration to actually occur at a speed ratio slightly greater than 1.0. This had me plenty perplexed, but I checked and double-checked the computations. I think it may simply be the difference between a prop and a kinematic constraint. The prop slips, doesn't truly have constant pitch, and I assumed you measure it's beta angle relative to the 0 lift line. I don't know if that's truly the best assumption.

My fallback plan is to simply change the gearing (and perhaps prop pitch) if I bungled this deal. We have so many hours into this ridiculous prop now that I really don't want to scrap it and start over. But certainly our second effort would be better than our first.

ETA: We don't have prop data for below wind speed. I agree that self-starting with such a high speed ratio becomes dicey. We're currently thinking in terms of big deployable drag devices until we approach wind speed.

 

I just happened upon your post a couple pages back where you suggested getting a sponsor. As you probably know, Joby Energy was kind enough to sponsor us. We also got a call recently from a second sponsor that would like to be a part of this silliness. I hope we can announce it soon.

 

The gearing is the thing to tweak. It would be ideal to have variable speed.

Once the vehicle is moving the prop is spinning and creating its own near field flow.

If a boat is going backward and you kick forward the flow reverses through the prop before the boat goes forward. Same thing with an aeroplane taxiing in a tailwind. The apparent wind on the fuselage may be from behind but the flow through the prop is from ahead. The prop establishes its own flow field as soon as it turns. The only problem with be the initial kick of the wheels. They might skid if there is not enough weight on them.

Rick W

 

Additionally to the vector drawings the wedge analogy might be helpful. Here it is for a tacking sail-vechicle:

http://www.youtube.com/watch?v=H_OKNr120t4

And here for the propeller cart:

http://www.youtube.com/watch?v=zPFzHoubQzg

As you see, it is pretty much the same: The sail/blade is mechanically constrained to move along a fixed path relative to the ground, which is not directly downwind. It is squeezed out from between the air and that constraint, like a wedge.

 

So a vehicle is in the works? What size? Any idea when it might be ready?