Exhaust fan on velomobile doubling as propulsion

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SaladFish   1 W

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Exhaust fan on velomobile doubling as propulsion

Post by SaladFish » Sep 14 2020 6:40pm

If you have a 100w electric exhaust fan at the rear of a velomobile will you get 100w of forward propulsion?

I remember exhaust fans were used in pedal car races before they were banned because the racers were getting an extra 5-6km top speed from their use. They were using rc airplane motors.

I want to know if an exhaust fan could be used as forward propulsion and how efficient it is.

I'm asking in this forum because people here may be familiar with the workings of propellers.

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Re: Exhaust fan on velomobile doubling as propulsion

Post by Hillhater » Sep 14 2020 7:37pm

I suspect an “exhaust fan” is design optimised for moving air volume quietly and efficiently, and the generation of maximum thrust is not a priority.
If its thrust you want primarily, you would be better off configuring the same power motor with a rpm and propellor purpose designed for maximum thrust at the speeds you will use.
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SaladFish   1 W

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Re: Exhaust fan on velomobile doubling as propulsion

Post by SaladFish » Sep 15 2020 7:27am

Hillhater wrote:
Sep 14 2020 7:37pm
I suspect an “exhaust fan” is design optimised for moving air volume quietly and efficiently, and the generation of maximum thrust is not a priority.
If its thrust you want primarily, you would be better off configuring the same power motor with a rpm and propellor purpose designed for maximum thrust at the speeds you will use.
Yes thrust.
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Re: Exhaust fan on velomobile doubling as propulsion

Post by fechter » Sep 15 2020 8:19am

The exhaust velocity has to be much higher than the vehicle speed in order to get any thrust. I don't think the efficiency will be very good compared to driving the wheels. You could use something that looks like an electric ducted fan they use on RC model aircraft and probably get as much as you can get.
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Re: Exhaust fan on velomobile doubling as propulsion

Post by topspeed » Sep 15 2020 10:06am

There may be more to it that meets the eye !

I mean the tail vortices may be reduced by this thrust propellor as well.
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Re: Exhaust fan on velomobile doubling as propulsion

Post by e-beach » Sep 15 2020 10:25am

Do a wind tunnel test. Or model it if you can.

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Re: Exhaust fan on velomobile doubling as propulsion

Post by Balmorhea » Sep 15 2020 12:52pm

I doubt you'll get any identifiable amount of thrust from something like that, but you could possibly do some boundary later control, or reduce the low pressure area behind a Kamm tail.
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Stealth_Chopper   100 mW

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Re: Exhaust fan on velomobile doubling as propulsion

Post by Stealth_Chopper » Sep 15 2020 6:37pm

You seem to equate 'fan' and 'propeller', fans are far less efficient than propellers under most conditions.

At equal power: A large diameter propeller will produce more thrust, a fan, less because of the protective duct that surrounds the fan.

Propellers are ancient, like Archimedes Screw, yet efficient.
SaladFish wrote:
Sep 14 2020 6:40pm
If you have a 100w electric exhaust fan at the rear of a velomobile will you get 100w of forward propulsion?

I remember exhaust fans were used in pedal car races before they were banned because the racers were getting an extra 5-6km top speed from their use. They were using rc airplane motors.

I want to know if an exhaust fan could be used as forward propulsion and how efficient it is.

I'm asking in this forum because people here may be familiar with the workings of propellers.

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Re: Exhaust fan on velomobile doubling as propulsion

Post by Balmorhea » Sep 15 2020 11:18pm

Ducted props avoid vortex losses at the tips.
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Re: Exhaust fan on velomobile doubling as propulsion

Post by Stealth_Chopper » Sep 16 2020 10:16pm

So?
& You seem refer to a ducted FAN
Balmorhea wrote:
Sep 15 2020 11:18pm
Ducted props avoid vortex losses at the tips.
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The propulsive efficiency of a propfan engine is increased by the high bypass ratio achieved using external fan blades. The efficiency boost is maintained even at high speeds because the fan blades are curved like scimitars to prevent shockwave formation on the outer tips of the blades. The increased propulsive efficiencies translate into overall propulsive efficiency gains of as much as 20-25% over turbofans.

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Re: Exhaust fan on velomobile doubling as propulsion

Post by Balmorhea » Sep 16 2020 11:23pm

Stealth_Chopper wrote:
Sep 16 2020 10:16pm
So?
& You seem refer to a ducted FAN
Call it what you will. The terminology is more a judgement call about blade aspect ratio than anything. But just like wings, prop blades have tip vortex losses, and just like winglets, propeller ducts can minimize or even eliminate these.
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The Toecutter   10 kW

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Re: Exhaust fan on velomobile doubling as propulsion

Post by The Toecutter » Sep 17 2020 7:57pm

SaladFish wrote:
Sep 14 2020 6:40pm
If you have a 100w electric exhaust fan at the rear of a velomobile will you get 100w of forward propulsion?
The only way that happens is if the blades of the fan are 100% efficient and the motor driving it is 100% efficient. You might get the motor to be around 95% efficient if you are careful in your selection thereof, but the blades are going to run up against the Betz coefficient, which in theory is about 59% efficiency, but in practice, you will be lucky to get 30-40% efficiency.

Then you have to account for any drag of the air being exhausted out and what it runs up against.

You'll be lucky to get 25W of forward thrust from a 100W exhaust fan.

However, the cooling of the rider from said fan may more than make up for it by allowing the rider to sustainably generate more power without overheating, but that isn't something that could be readily modeled because rider physiology and ergonomic needs are so widely variable.

If the goal is to use the fan for rider cooling, a hole and the fan could be placed at the front of the velomobile where the high pressure stagnation point of the airflow will be found, and with proper consideration of airflow in the velomobile, the air could be vented out at the rear of the velomobile, providing effective cooling, while very slightly reducing drag. The Milan SL has a vent in the rear that is functional, and the DF velomobiles have a front air intake at the stagnation point.

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