Re: Popular rowing fallacies

Carl Douglas wrote:
Zbigniew A. wrote:
Carl Douglas wrote:
<snip>
If you achieve your required thrust by a 10% increase in velocity of a large mass flow of air you increase that air mass's KE by a factor of 1.21, so energy absorbed = mass flow. Now suppose for the identical thrust you double the velocity of a smaller mass flow of air? For the same thrust you will need only 1.1/2, = 55% as much air flow, but you will give that reduced flow four times as much total KE.

Whoops!

Something got edited out in error, & this should have read:

If you achieve your required thrust by a 10% increase in velocity of a large mass flow of air you increase that air mass's KE by a factor of > 1.21, so energy absorbed = mass flow x (Vout^2 - Vin^2)/2g where Vout & Vin are outgoing & incoming velocities & g is the acceleration due to gravity. Now suppose for the identical thrust you double the velocity of a smaller mass flow of air? For the same thrust you will need only 1.1/2, = 55% as much air flow, but you will give that reduced flow four times as much total KE.

Cheers -
Carl

--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Email: carl@xxxxxxxxxxxxxxxxx Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
.

Relevant Pages

  • Re: Rocket efficiency
    ... > thrust almost indefinitely, so the total delta-v can be quite high. ... > more things than rocket engines. ... > accelerating air downwards to produce a force to compensate gravity. ... rocket final velocity. ...
    (sci.space.tech)
  • Re: Rocket engineering: thrust calculation.
    ... Mookie relates engine thrust to mass flow, ... I turned the FIXED mass into a mass flow RATE, ... exhaust speeds by not wasting thermal and kinetic energy - with the ...
    (sci.space.policy)
  • Re: Rocket engineering: thrust calculation.
    ... Mookie relates engine thrust to mass flow, ... The first the the mass flow thru the nozzle of the engine. ... and evidently the bell had got a lot of design ...
    (sci.space.policy)
  • Re: Mobile S-Rotor!
    ... > If the intake is 8" diameter, at 88 fps flow, you have an air mass flow ... Atmos density = 1.22 kg/cubic meter ...
    (sci.energy)
  • Re: OT: Can anyone see any of this ash?
    ... a modern engine. ... The noise reduction is a handy byproduct of the thrust ... creation, in this case the thrust is related to mass flow times ...
    (uk.radio.amateur)