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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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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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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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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Re: Exhaust fan on velomobile doubling as propulsion

Post by SaladFish » Sep 19 2020 5:14pm

The Toecutter wrote:
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.
This is exactly the sort of information I am looking for thank you.

A tightly enclosed velo (or car) can get hot so I'm looking at solving that with ventilation while benefiting performance of the vehicle. Also I am thinking the thrust produced by a ventilation fan/propeller would be excluded from power limitation legislation which is a bonus.

Just found this concept car (Ford Probe 4) has side mounted vents and a blown wake. Very interesting.

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Re: Exhaust fan on velomobile doubling as propulsion

Post by Solarsail » Sep 24 2020 6:44pm

Hi -- off topic

This subforum on electric aircraft seems to be dormant. I wonder if participants have moved to another forum, and what that may be?

Or is it that all interest in electric aircraft has ceased everywhere? :o

Where can I find electric aircraft projects being discussed?

My apologies for interrupting this thread -- just didn't want to start a new thread.

SolarSail

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Re: Exhaust fan on velomobile doubling as propulsion

Post by Balmorhea » Sep 24 2020 10:29pm

Solarsail wrote:
Sep 24 2020 6:44pm
Hi -- off topic

This subforum on electric aircraft seems to be dormant. I wonder if participants have moved to another forum, and what that may be?

Or is it that all interest in electric aircraft has ceased everywhere? :o

Where can I find electric aircraft projects being discussed?

My apologies for interrupting this thread -- just didn't want to start a new thread.

SolarSail
Well, there was burst of activity recently when longtime contributor Farfle was killed in his formerly electric ultralight.
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Re: Exhaust fan on velomobile doubling as propulsion

Post by Solarsail » Sep 26 2020 9:39pm

e-beach wrote:
Sep 15 2020 10:25am
Do a wind tunnel test. Or model it if you can.

. :D :bolt:
How do you wind tunnel test? At your local car wash center? :D

Is there a modeling package for props that is easy to use?

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Re: Exhaust fan on velomobile doubling as propulsion

Post by Solarsail » Sep 26 2020 9:40pm

Balmorhea wrote:
Sep 24 2020 10:29pm
Well, there was burst of activity recently when longtime contributor Farfle was killed in his formerly electric ultralight.
Sorry to hear that. R.I.P.

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Re: Exhaust fan on velomobile doubling as propulsion

Post by Solarsail » Sep 26 2020 9:47pm

Stealth_Chopper wrote:
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.
Wait a minute. I have been told that propellers with a duct (is that a fan?) are more efficient because the inflow and outflow is axial, and thus all the momentum add up. Without a duct, there is some radial momentum both in and out that do not contribute to thrust.

Why are turbofan engines ducted and not ductless like a turboprop? Aren't turbofans more efficient? Why is turbofan preferred to turboprop?

Note: I am speaking of fans and props of the same diameter.

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Re: Exhaust fan on velomobile doubling as propulsion

Post by Solarsail » Sep 26 2020 10:01pm

Stealth_Chopper wrote:
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.
Can you tell me the difference between a ductless fan and a prop?

And the difference between a prop and a propfan? Thanks.

You are also confusing the question. We are all assuming the same diameter for the fan and the prop. What we are saying is does a prop generate more thrust with a duct or without a duct (prop diameter does not change).

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