The Wind Powered Sail-less Boat

Thanks, ThinAirDesigns, I think you have put me right. The propellers are indeed acting as airscrews rather than turbines. Goodman's statement that the car was self-starting downwind (not 4mph of wind, you are right again-it was 4mph on the treadmill) that threw me off. In his video he states that the demo is taking place in 5-6mph wind, yet the car had to be pushed to get it going, even after the brake was released. So the purpose of the wind is mostly to overcome the stiction and mechanical losses? I suspect the car (and boat) would benefit from a variable pitch prop. Perhaps actuated by a flyball rig...

 

Actually, I think the cart was beginning to self-start when he released the brake. Having spoken to Jack I know that he never anticipated that video being posted on the internet, and certainly never causing such holy wars. As such he said "give it a push" just because it seemed like an obvious thing to do. I've wished 1000 times that he'd let it start on its own.

There are a million things people complain about regarding his video. He could have very easily addressed most of them. Had he known that such criticisms would be raised he might well have done so.

What's more surprising is how naive JB and I were. We set out to make our videos to be above reproach. By putting the cart on a treadmill we eliminated the questions of gusts, directly downwind, downhill, etc. We never imagined that people would argue for hundreds of pages that a treadmill is somehow different than the road (yes, we were that dumb).

We made several videos in response to skeptics concerns. We still believed we could adequately address these concerns in a video-taped demo. I think it's fair to say that we've learned there are a certain percentage of sceptics that could be put in the seat of the cart with GPS, wind instruments, telemetry, and high speed cameras from every angle. They could bring their mother, a cop, and a nun along as eye-witnesses, and their intuition would still trump the evidence. If you know it's impossible, evidence isn't going to sway you.

But I'm well aware that this is a small percentage of the sceptics, and that I've gone off on a bizarre tangent. The bottom line is that I think Jack's cart was actually self-starting when his wife gave it a little push.

EDIT: I just went back to review it. Jack had the brakes on. The cart (and prop) came to a full stop. He removed the brake (via remote) and at 18 seconds it had started moving downwind - before his wife pushed it. In any event, we know that such a cart will definitely self-start as ours has demonstrated, even if light wind.

 

A variable pitch prop could be very beneficial for things like braking and self starting, but for maximum performance every position other than its design pitch gives compromised performance since you can only twist each blade as a unit rather than change the twist of the blade as you change pitch.

 

I assume you're addressing me.

Certainly helicopters get significant dynamic stiffening from the head RPM. Whether that would be a significant factor here I couldn't say without trying to run some numbers - and even then I'm certain Mark's top of the head guess would be far better than my analysis.

 

I'm betting he was going for Rick Willoughby.

JB

 

A second group do not comply with the completeness principle, which means that there are no true sentences in the system that cannot, at least in principle, be proved in the system.

Some others departure from the going upwind situation to then proof the DDFTTW.

All this is confounding me.

I would like to focus in the cart over the road pushed by the wind experiment, not the treadmill one, if possible. Thanks.

Cheers.

 

If you know it's impossible, evidence isn't going to sway you.



So true. I once mentioned on the Mythbusters forum that a septic tank contractor in Toronto routinely used a dowser to help locate buried pipes in old drain fields. Lit the place up for three pages, a lot of it in all caps. I am no more naive than I should be, but I never suspected that DDFTTW was not true. I myself have built things that I did not have the training to explain, so my inability to puzzle it out right away was no barrier to believing what I could see.

 

D'OH! I'm sure you're right.

Still no one has told me which proofs fail in any of these regards. I'm happy to provide not only analytical proofs (as others have done), but analogies to bring the concept across, and plain old video taped evidence if our current videos somehow come up short. On top of that I built a pile of these carts and sent them to people that requested them for their own testing (for the cost of parts). I've also posted detailed build videos. The last thing we're tying to do here is keep any secrets.

That's a sticking point we have to figure out how to address. The two are one and the same - as such the analysis is identical in either case. As Galileo, Newton and Einstein tell us, there is no distinction between a moving road under still air vs. a stationary road beneath moving air (even this sentence uses terms that have no true absolute meaning since "moving" and "still" are strictly relative).

I think the best way to proceed is for you to tell us which proofs fall short for you, and how. I think that will help us understand how you look at it and what evidence you would consider satisfactory.

 

Lets start with the sailboat propeller. It has been measured and calculated (e.g. CFD) many times, that a typical sailboat propeller has a much (e.g. 3x) higher drag when locked compared to free spinning. This is due to low pitch/diameter and wide baldes. With extremely narrow blades of heli and airplanes the story is different.

Then to autogyros and heli emergency landing. I don't know how a heli actually does the emergency landing, hut autogyros need the horizontal velocity given by the motor to be able to fly. The autogyro is actually very close to a wing providing lift from the horizontal velocity AND the horizontal velocity is providing the moment to turn the rotor. This moment comes from the very different drag of the blade when going upwind (profile in correct direction) compared to when going downwind (profile reversed) NOT from negative pitch.

Is a helicopter emergency landing based on the same principle as autogyro? If yes, then it must have a horizontal velocity AND the pitch is not reversed. If not, it falls faster than the same heli with a parachute equal in size to the rotor. If the flow past the rotor is purely vertical, the drag can not be even equal to 1/2*rho*Adisc*V^2.

Joakim

EDIT: Here is a link to a paper about sailboat propeller drag: http://strathprints.strath.ac.uk/5670/1/strathprints005670.pdf

 

They need that horizontal thrust to gain and maintain altitude, but not to glide. They could also soar in ridge lift (I've flown R/C auto gyros and seen them ridge soar).

Things get a bit complicated when you look at the lift profile on the disk in translational flight, but suffice it to say, you quite definitely don't have to have any horizontal translation of the aircraft or air to autorotate. I've done hundreds of them in R/C helis, and have been along for the ride when my instructor did a few full scale. Autorotation is more efficient with horizontal velocity for reasons vaguely similar to the reason you can remain aloft in forward flight with less energy than is required to hover. I think it's fair to say that in autorotation the horizontal translation actually helps you sort of "side-step" the Betz efficiency limit.

Again, the story gets more complicated with a horizontal velocity component, but a low or negative pitch is quite sufficient to perform a vertical auto with. In my experience you need a minimum of about -3 degrees pitch for model helis, while full-scale helis can auto with slightly positive pitch (1/2 to 1 degree if I recall correctly) this is due to the airfoil shape of the blade (rounded L.E. in particular).

Yes. And in both cases we can complicate things with flapping blades, lead/lag, etc. But we can also consider vertical unpowered flight and leave all that out.

 

Certainly autogyros can glide, but if there is no horizontal flow, they are not gliding, they are just falling. When falling there has to be enogh vertical velocity through the rotor disc to keep it rotating and the rotor must push down the air, thus the falling velocity must be rather high. I would think they would fall too fast for safe landing. That depends on the weight vs. disc area, which probably is very diffent in RC-scale.

At low negative angle of attact the cambered profile still has a positve lift and it is trying to rotate the rotor in the wrong direction, which must be comensated with horizontal flow in steady state.

Joakim

 

I could not open your link Joakim.

It is interesting that you say a typical aux. prop should have less axial thrust when autorotating than when locked. I anticipated this effect but did not expect it to be so great as to actually reverse the situation. I need to see at what pitch the situation reverses (is critical)

<<<< the drag can not be even equal to 1/2*rho*Adisc*V^2.
>>>>

Here i disagree. The blade of the heli prop is gliding down with its negative collective just like a glider is gliding down and lift balances weight


The critical number is S = (D/L)^2 where S is solidity. As long as S is greater than (D/L)^2 the autorotating prop will be better than a parachute of the same size.

However , heli pilots get instructed to avoid descending straight down (power on or off) as they can get caught in their own wake (kind of a torus with the center rolling downwards) and start a vicious cycle of greater and greater sink speed.

Also descending diagonally improves efficiency.

And yes i meant Rick W, long time proponenet of the strutless prop. My question is would a water turbine exhibit the same kind of self aligning attributes?

 

Tcubed, right click and save file as or save link as.... it is more than 1mb I think, downloaded OK for me...

 

As long as their blades are spinning and there is laminar flow over them, they're gliding.

I'm actually not following this completely. I'm thinking of blades that are either symmetrical or flat bottomed. In this case even a slightly positive AOA will generate torque in the proper direction.

 

I just tried the link and it opened the pdf just fine. You can find it also by the name:
Sailboat propeller drag
P.M. MacKenzie, M.A. Forrester

The drag of a locked propeller can be calculated from the total blade area using Cd around 1 (depends on pitch and aspect ratio). In marine propellers the area ratio is typically 30-100%, thus the drag will be rather high. A heli prop probably has the area ratio well under 10%, which makes the locked drag very low.

For free rotating marine propeller Larsson and Eliasson have used Cd of 0.3, but in the paper linked they measured clearly higher values, but still much less than the locked. I would think here Cd depends very much on the propeller geometry and also the friction of the shaft.

I didn't quite understand where do you actually disagree. I think the situation is quite different, if there is no horizontal air speed. In that situation the rotor must let some airflow through the rotor disk and thus can not have drag equal to a parachute. A parachute stops all the flow and would have a Cd about 2, thus drag about rho*A*V^2.

If there is horizontal air speed, the situation is totally different and the rotor can achieve a good glide ratio and much higher lift than the drag purely vertical situation.

An analogy would be a frisbee dropped (spinning or not) vs. thrown. The last one will stay much longer in the air.

Joakim