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rob
To give you an idea of what the challenge is I looked at a NACA0010 profile on a bicycle type vehicle weighing 30kg plus 75kg rider. I assumed drag equivalent to an upright bike. This is probably optimistic given the size of the prop.
This vehicle is intended to really perform. The design objective was to do 10m/s (say 22mph) in 5m/s wind (say 10kts). This wind should be reasonably easy to come across. The vehicle drag at this speed is 17N when apparent wind is 5m/s.
The check condition is zero apparent wind and vehicle moving at 5m/s. The vehicle drag is 6N.
The prop needs to 5m (16ft) in diameter and 3-bladed with 190mm chord (almost 8 inches) operating at 100rpm. (Will need to be made from CF to keep the weight down of course). Efficiency at design condition is 76%.
The design point requires a prop input power of 390W. This will be a bit over 400W required at the wheels allowing for transmission losses. So extra drag with a bit of wheel slip is say 42W to drive the prop when vehicle is at 10m/s. Total drag is now 59W. The prop will generate thrust of 60N in 5m/s wind with 400W. So it does the job.
When the vehicle is doing 5m/s the apparent wind is zero and prop is doing 50rpm if the gearing is fixed. The prop power absorbed is 96W - say 100 at the wheels. This will add about 21N extra drag at the wheels so total drag is 27N. The prop will be producing 37N so the vehicle will be accelerating - no problem here.
If you were to use an asymmetrical blade - eg NACA16-610. The diameter can be reduced to 3.5m. Prop power reduces to 310W running at 110rpm. Still 3-bladed but with slightly larger chord. It also gets through the zero apparent wind condition. This prop would probably be OK for going upwind as well and would automatically depower because it would stall early if it was loaded up when working as a turbine.
So for a duel function prop/turbine I think something like a 3.5m asymmetric prop/turbine would work OK.
To give you an idea of what the challenge is I looked at a NACA0010 profile on a bicycle type vehicle weighing 30kg plus 75kg rider. I assumed drag equivalent to an upright bike. This is probably optimistic given the size of the prop.
This vehicle is intended to really perform. The design objective was to do 10m/s (say 22mph) in 5m/s wind (say 10kts). This wind should be reasonably easy to come across. The vehicle drag at this speed is 17N when apparent wind is 5m/s.
The check condition is zero apparent wind and vehicle moving at 5m/s. The vehicle drag is 6N.
The prop needs to 5m (16ft) in diameter and 3-bladed with 190mm chord (almost 8 inches) operating at 100rpm. (Will need to be made from CF to keep the weight down of course). Efficiency at design condition is 76%.
The design point requires a prop input power of 390W. This will be a bit over 400W required at the wheels allowing for transmission losses. So extra drag with a bit of wheel slip is say 42W to drive the prop when vehicle is at 10m/s. Total drag is now 59W. The prop will generate thrust of 60N in 5m/s wind with 400W. So it does the job.
When the vehicle is doing 5m/s the apparent wind is zero and prop is doing 50rpm if the gearing is fixed. The prop power absorbed is 96W - say 100 at the wheels. This will add about 21N extra drag at the wheels so total drag is 27N. The prop will be producing 37N so the vehicle will be accelerating - no problem here.
If you were to use an asymmetrical blade - eg NACA16-610. The diameter can be reduced to 3.5m. Prop power reduces to 310W running at 110rpm. Still 3-bladed but with slightly larger chord. It also gets through the zero apparent wind condition. This prop would probably be OK for going upwind as well and would automatically depower because it would stall early if it was loaded up when working as a turbine.
So for a duel function prop/turbine I think something like a 3.5m asymmetric prop/turbine would work OK.
