PVC blades - lift style or drag

[Perry1]

noob question but are PVC and curved aluminum blades considered lift or drag style?

Thanks,

Perry

[Flux]

Depends on how you make them.

For HAWTs the blades are normally built to work on lift. The things may have a tsr of somewhere between 3 and 5 if properly built and will work on lift. Probably when you get down to the solid wheel type wind pump you are in the grey area between lift and drag.

Even a plank set at an angle to the wind will exhibit lift and the curved single surface things such as some of the pvc and aluminium blades will have quite a high lift coefficient. The thing that normally determines success or failure is the lift to drag ratio.This is where the plank doesn't come off too well. decent curved blades correctly set and intended for low tsr don't do so badly. Even the Bergey pultruded blades are of this single surface form ans thet work well at something like tsr6.

True drag machines are mainly Savonious and a few other vertical axex machines with a tsr less than 1. Measuring devices such as anemometers also fall into this category.

Many vertical axis things such as Darrius are lift devices.

Flux

[Perry1]

Thanks for taking the time to explain it Flux.

Perry

[gizmo]

I've seen this mentioned before, HAWTs using drag. I wonder it thats really the case, are not all HAWT's lift based. The turbine is at right angles to the wind, it can not be dragged by the wind. The blades are changing the angle of the wind, which is providing a force against the blade at an angle other than downwind, driving the blade around the turbines axis. If it were a drag machine, the turbine would remain stationary.

I dont think TSR is a indication of lift or drag. Just thinking out loud, but I wonder if there is such thing as a drag HAWT. Is not a plank of wood at the correct angle of attack the same as a nice airfoil at its corect angle of attach, even though one is much more efficient than the other.

Glenn

[Flux]

I wonder about this as well, it's probably a case of using terminology in different ways.

No normal HAWT uses drag.  I have seen it claimed that the vane anemometers with flat vanes at 45 deg or more are drag devices. Certainly when TSR falls below 1 then there is not a lot of reason to look at it from the lift point of view.

I am sure a true drag machine has to be something like a Savonious or a cup anemometer where the blade is pressure driven but with the vane anemometer it is probably more the vector component of thrust on the vane that is turning it than lift in the normal sense. It will still have a lift component and a very high drag one.

I am not sure where the wind pump fits into this , with its curved vanes it is most likely more driven by lift, but the early wooden slatted ones were much more like the vane anemometer.

Anyway for all normal HAWTs whether with curved sheet vanes or aerofoil sections they are lift machines. Yes I do agree that normally the plank is a very poor lift machine if set at low angles. If you set the angle of attack high enough to stall then it may be different. In fact this may be the point where these vane anemometers are considered drag , it may be more related to vane angle than anything else.

Flux