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From: Jeff Liebermann <jeffl@cruzio.com>
Newsgroups: rec.bicycles.tech
Subject: Re: Riding after heavy manual labor
Date: Tue, 28 May 2024 09:15:06 -0700
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On Mon, 27 May 2024 22:07:53 GMT, Tom Kunich <cyclintom@yahoo.com>
wrote:

>A lawn mower is DESIGNED to lift the grass up so that the blades can trim them to a constant hieght. Do you have even the slightest notion what aerodynamic lift is? Good God, you are absolutely unbelievable. The trimings are blown out ABOVE the blades.

That's true.  However, the lifting power of the blade "fan" is very
limited.  The static thrust of the propeller section of the mower
blade (not the lift of an airfoil) has to lift the grass off the
ground as you describe, so the blade can cut as close to the ground as
possible.  The blade can do that because the grass is quite light,
even when wet.  The static thrust is quite small.  I don't have a
proper model for a mower blade, so I'll assume a standard propeller
which will be much larger than the static thrust of the mower blade.
See:

Static Thrust Calculator
<https://www.poweredparagliders.com.au/Calculators/Static_Thrust_Calculator.htm>
Propeller Dia:  19 in
Pitch: 7 in (shallow pitch and not full length)
Propeller Type:  Standard Prop.  
Effectiveness Coefficient(CF): 1  (very optimistic)
RPM: 3200  (my guess)
Air Temp and Density:  20C and 1.2kg/m^2

Result:
Static Thrust:  3.63 lbs (1.64 kg)
Required engine power:  0.203 HP

3.63 lbs of static thrust will lift 3.63 lbs off the ground.  That's
adequate for lifting a small amount of wet grass, but hardly enough to
lift a grass bag and "suck" it into the blades.
<https://www.mtdparts.com/en_US/riding-lawn-mower-attachments-and-accessories/grassbag-21inch/764-04082D.html?fitsOnModel=11A-A2MR766#start=5>
Without the steel framework, the cloth mower bag weighs about 1.5 lbs.
However, with the steel framework add about 5 lbs.  3.63 lbs of static
thrust is not going to lift about 6.5 lbs.  (Note: I had to guess at
these weights using weights of similar grass bags.  When your new bag
arrives, I would appreciate it if you post your bag weight).  

There's also some question how a rotary blade can cut a "1 ft square"
hole in the bag while being "sucked" into the mower.  My guess is that
the blade will grab the cloth bag, become tangled in the drive shaft,
and stall the engine.

Looking down at the mower, if the deck exit port to the bag is on the
right side, the blade rotates clockwise.  If on the left, it rotates
counterclockwise.  This is because the blades only partly "blowing"
the grass into the bag.  With only 3.64 lbs of static thrust, there
isn't much force available to shove a mass of wet grass though the
relatively small exit port.  What really happens is the blade cuts the
grass, which sticks momentarily to the blade.  The blade then pushes
the mass of wet grass into the exit port using centrifugal force,
which is larger than the static thrust.  Something like this:

"Rocks vs. Bullets: The Surprising Power of Lawnmowers | Mythbusters"
<https://www.youtube.com/watch?v=i0O17079BeY>  (8:11)

If the blade can launch rocks, it can certainly launch sticky grass.


-- 
Jeff Liebermann                 jeffl@cruzio.com
PO Box 272      http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann      AE6KS    831-336-2558