Mark,
When I did the analysis versus mass (and also versus
throw angle) I was aware that assuming a constant
throw speed wasn't realistic, however I didn't have
a better alternative. I like the approach you've
presented and have redone the analysis versus mass
and aspect for this throw model. The website has been
updated with the new results.
http://www.rcmodels.com/clubs/asfcnz/
You're right up to a point in saying that lighter models
will inherently launch higher using this model. The results
indicate that there is actually an optimal weight for a
given design, and if you go much lighter then launch height
suffers badly. However, with current construction techniques
I don't think you can build down to this weight and still
have a model strong enough to throw at full power (at least
for a 1.5m span RC HLG).
Also, the optimal weight for max launch height increases
with wind speed, so now we can justify ballast for HLG's
on the basis of launch as well as penetration when the
wind gets up.
Ciao - Chris
******
Chris Kaiser
Auckland, NEW ZEALAND
http://www.rcmodels.com/clubs/asfcnz/
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]
> Sent: Wednesday, 16 August 2000 7:53 p.m.
> To: [EMAIL PROTECTED]
> Cc: [EMAIL PROTECTED]
> Subject: RE: [RCSE] Results from a HLG launch analysis
>
>
> Chris,
>
> Nice presentation.
>
> The only beef I have is the assumption of a given
> velocity independent of the glider weight. This
> is clearly inappropriate. I think a better assumption
> is to assume a fixed muscular energy output, which
> would correspond to fixed (maximum) muscle forces
> acting over some fixed throwing motion. This energy
> shows up as kinetic energy of the glider and the arm
> at the moment of release.
>
> E = 0.5 (m + m_arm) V^2
>
> This gives the launch velocity in terms of glider mass m:
>
> V = sqrt( 2 E / (m + m_arm) )
>
> I estimate m_arm = 1 kg as the effective mass of the
> hand and forearm, and E = 300 J seems like a reasonable
> energy for a strong throw. You can get fancier
> and assume that there is also a running speed
> which adds to the throw independent of the glider weight.
> In this case you would have
>
> V = sqrt( 2 E / (m + m_arm) ) + V_run
>
> Whatever.
>
> In your parametric variation of the glider weight,
> I suggest fixing m_arm and E, and set V from one of
> the relations above rather than fixing it. You'll find
> that lighter gliders launch inherently higher now,
> which I think is what most people observe.
>
>
> --
> - Mark Drela
>
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