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Re: (ET) Mower blade speed
Maybe I am missing something here. Are we designing a lawn mower or an
airplane?
I don't see were the speed of a propeller for a winged air craft has any
relevance to cutting grass. For one, the environment is totally different.
If one was wanting to know what was the speed needed to chop up seagulls
in the air then there might be some correlation to cutting grass.
I think the original question of what is the proper blade speed for cutting
grass was a good one. I have been also searching for this with out any
luck so far. But a lot has to do with cutting edge of the blade, the lift
on the grass provided by the pitch and the density of the grass you are
cutting. The optimum is to find the ideal speed that is most efficient for
the cutting edge of the blade. I would think from the calculator that was
found that we would want to get the mach number as low as possible just for
noise sake. The higher the number the louder the blade noise will be.
For example if you used a 20' blade at about 1000 RPM you have a mach
number of 0.91 and a tip speed of 714 mph. The program says "Oh that is
Good!" problem is that is the specs of a helicopter. So much for silent
mowing. Personally I don't want my neighbors looking up into the air every
time I start mowing the grass. Sorry I am being sarcastic here.
The web program is doing nothing more than looking for a tip velocity of
721 mph which is the speed of sound at sea level and taking into account
the density of air via the temperature.
Keep a good sharp edge on your blade and you can cut at lower speeds or
lower power. If your blade is to the point of having a rounded edge then
your ripping your grass and use more power in the process. If your grass
is at all weak then you risk ripping your grass out. If your wondering if
your blade is sharp enough you can tell by looking at the lawn. If the top
of the grass blades are cut cleanly then the blades are in good shape. If
the top of the grass is ripped, torn, or raggy and white then your blade is
dull and need sharping. Mulchers are going to need sharping more than
discharge types just because they are cutting more. Also your blade can
appear sharp if you look at it but if the very tip of the cutting edge at
the outer most point is rounded or dull then you need to sharpen the blade.
This tip does most of the cutting and one stone or a plow through the sand
will dull this point.
From what I have heard in the pass is a reel type mower is still the most
efficient cutting device for grass. But it too has its own problems which
is why the deck mowers came about. For one it does not do well with tall
growth like weeds and long grass.
I am by far not an expert on this. These are just my findings and thoughts.
As a matter of fact a few weeks ago I sharpen the blades as I usually do
and notice when I got home from work my wife had mowed the grass for me.
The yard looked like heck, thatch was laying everywhere and it looked like
someone power racked the lawn. I figured she had cut it to low since she
hardly ever mows the lawn. Then I noticed that it looked like the deck was
cutting uneven. One blade looked about 1/2" lower than the other. I checked
the deck out and found I had one blade upside down and it was cutting with
the back side of the pitch. I flipped the blade over and the grass is back
to normal again. But I did figure this might be a good way next spring to
power rake the yard.
Dave Reuter
----- Forwarded by Dave Reuter/IAVINC/IAV on 08/28/2003 08:38 AM -----
"Christopher Meier"
<mr23 mn rr com> To:
<elec-trak cosmos phy tufts edu>
Sent by: cc:
elec-trak-bounces cosmos ph Subject: Re:
(ET) Mower blade speed
y.tufts.edu
08/27/2003 10:08 PM
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
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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