RE: (ET) Mower blade speed

You slipped a decimal somewhere.

The speed at the tip distance divided by time. At 3600 RPM and 24" dia (to make it even, 2' dia and 1' radius) you get 3.14159 * 1' or

3.14159' per revolution *2 for the circumference. 3600 revolutions per minute is 21,600 revolutions per hour * 2* 3.14199' per revolution is

1357166' per hour. There are 5,280' per mile, so 135,7166/5280= 257 miles per hour. If you take a different diameter or RPM you will get a proportionally different speed.

With 29" and 7000 RPM I get 29" / (12"/ft) * 3.14159 *7000 (RPM) *60 (Minutes/hour) /5280 (feet/mile)= 603 MPH .... REAL FAST, the speed

of sound is about 720 MPH at sea level and 70 deg. F!

BTW, as an ex-pilot, tip speeds on a 72-84" dia prop (small plane) start going supersonic at about 2800 RPM depending on altitude (the

speed of sound drops with altitude). Nobody wants a mower turning THAT fast!

Larry Elie

-----Original Message-----
From: Christopher Meier [mailto:mr23 mn rr com]
Sent: Wednesday, August 27, 2003 10:09 PM
To: elec-trak cosmos phy tufts edu
Subject: Re: (ET) Mower blade speed

I tossed in 29" diameter and found that it took over 7000 rpm to make

the recommended speed; what blades can be run at that speed and

last? Or won't they fatigue? I've seen car water pump fan blades

that fatigued (developed cracks, some came apart destructively to

their surroundings) when run in excess of 7000 (engine rpm, don't

know what the blade rpm was, anyone have a vehicle with the older

style water pump mounted fans that could measure the pulley ratio?).

----- Original Message -----

From: Elie, Larry (L.D.)

To: 'Tim Gulden' ; elec-trak cosmos phy tufts edu

Sent: Wednesday, August 27, 2003 9:30 AM

Subject: RE: (ET) Mower blade speed

The speed is right when the batteries are doing well and the voltage hasn't been drooped by excessive current.

Yes, re-winding will add speed, and take a ton more current to power. For electro-magnets, the governing factor is the

FIELD. The field is proportional to the current times the number of turns of winding. Reduce the number of turns and

the current has to go up by the same amount to preserve the field. The power is just the product of the current times

the voltage. Since would still be powering with the same voltage, and the current would be higher, the power would be

higher.

Double blades do up the current a bit. Whether it lets you cut faster is questionable; you are now cutting at two heights and

mulching as well. There isn't any free lunch.

Larry Elie

-----Original Message-----
From: Tim Gulden [mailto:rimmer59 hbci com]
Sent: Tuesday, August 26, 2003 8:44 PM
To: elec-trak cosmos phy tufts edu
Subject: (ET) Mower blade speed

Does anyone know what the minimum blade tip speed should be before the blade tears the grass versus cutting it clean?

I found the following links that calculate this speed. It looks like our Elec-Trak blades max out at 155 mph.

Should a person rewire the motor for 24 volts and run it at 36 volts to raise the rpm? Maybe run 2 blades per motor in a +?

Any thoughts?

http://www.hoverhawk.com/propspd.html

http://www.pponk.com/HTML%20PAGES/propcalc.html

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