Re: Riding after heavy manual labor

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Sujet : Re: Riding after heavy manual labor
De : jeffl (at) *nospam* cruzio.com (Jeff Liebermann)
Groupes : rec.bicycles.tech
Date : 28. May 2024, 18:15:06
Autres entêtes
Message-ID : <hktb5jtc5gcloq387a5i8imvqv3c0c2j5f@4ax.com>
References : 1 2 3 4 5 6 7 8 9 10
User-Agent : ForteAgent/8.00.32.1272
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

Date Sujet#  Auteur
15 May 24 * Re: Riding after heavy manual labor204AMuzi
16 May 24 `* Re: Riding after heavy manual labor203Frank Krygowski
16 May 24  +- Re: Riding after heavy manual labor1AMuzi
16 May 24  +- Re: Riding after heavy manual labor1AMuzi
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16 May 24  +* Re: Riding after heavy manual labor76Jeff Liebermann
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16 May 24  ii`- Re: Riding after heavy manual labor1Jeff Liebermann
22 May 24  i`* Re: Riding after heavy manual labor73Zen Cycle
22 May 24  i +* Re: Riding after heavy manual labor6AMuzi
22 May 24  i i+* Re: Riding after heavy manual labor4Frank Krygowski
26 May 24  i ii`* Re: Riding after heavy manual labor3Jeff Liebermann
27 May 24  i ii `* Re: Riding after heavy manual labor2AMuzi
27 May 24  i ii  `- Re: Riding after heavy manual labor1Jeff Liebermann
22 May 24  i i`- Re: Riding after heavy manual labor1Radey Shouman
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28 May 24  i i+* Re: Riding after heavy manual labor9Frank Krygowski
29 May 24  i ii`* Re: Riding after heavy manual labor8AMuzi
29 May 24  i ii +* Re: Riding after heavy manual labor3Frank Krygowski
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29 May 24  i ii i `- Re: Riding after heavy manual labor1Zen Cycle
29 May 24  i ii `* Re: Riding after heavy manual labor4Radey Shouman
30 May 24  i ii  `* Re: Riding after heavy manual labor3Frank Krygowski
30 May 24  i ii   +- Re: Riding after heavy manual labor1AMuzi
30 May 24  i ii   `- Re: Riding after heavy manual labor1Radey Shouman
28 May 24  i i`* Re: Riding after heavy manual labor40Jeff Liebermann
28 May 24  i i +* Re: Riding after heavy manual labor37Zen Cycle
28 May 24  i i i`* Re: Riding after heavy manual labor36Jeff Liebermann
28 May 24  i i i +* Re: Riding after heavy manual labor9AMuzi
28 May 24  i i i i+* Re: Riding after heavy manual labor3Jeff Liebermann
29 May 24  i i i ii`* Re: Riding after heavy manual labor2Frank Krygowski
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31 May 24  i i i +* Re: Riding after heavy manual labor20AMuzi
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1 Jun 24  i i i ii  i  i+- Re: Riding after heavy manual labor1AMuzi
1 Jun 24  i i i ii  i  i`* Re: Riding after heavy manual labor3AMuzi
2 Jun 24  i i i ii  i  i `* Re: Riding after heavy manual labor2Radey Shouman
2 Jun 24  i i i ii  i  i  `- Re: Riding after heavy manual labor1Catrike Ryder
1 Jun 24  i i i ii  i  `* Re: Riding after heavy manual labor6Frank Krygowski
1 Jun 24  i i i ii  i   +- Re: Riding after heavy manual labor1AMuzi
1 Jun 24  i i i ii  i   `* Re: Riding after heavy manual labor4Catrike Ryder
1 Jun 24  i i i ii  i    `* Re: Riding after heavy manual labor3AMuzi
2 Jun 24  i i i ii  i     `* Re: Riding after heavy manual labor2Frank Krygowski
2 Jun 24  i i i ii  i      `- Re: Riding after heavy manual labor1Catrike Ryder
31 May 24  i i i ii  `- Re: Riding after heavy manual labor1Catrike Ryder
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28 May 24  i  +* Re: Riding after heavy manual labor9Zen Cycle
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31 May 24  i  i +- Re: Riding after heavy manual labor1Zen Cycle
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