"Dan Thomas" wrote in message
om...
"Gary Drescher" wrote in message
news:bnzLb.6520$8H.20195@attbi_s03...
"Dave S" wrote in message
. net...
Now... a question about realities.. The POH nazi's will say that the
Word as written is good, praise be to the POH... if I base flight
decisions and speeds on MY calculated numbers rather than the max
weight
sea level standard day numbers published in the almighty POH.. am I
going to be asking for trouble here?

It depends on what you mean by 'trouble'. The laws of physics prevail
over
the POH in determining whether your engine mount will break...

Why do folks worry about engine mounts breaking? They are far
stronger, in most cases, than the rest of the structure. For
production airplanes, the legal standards for certification include a
9G strength for fuselage/cabin structure for crashworthiness, and I
have seen other specs calling for the same 9Gs specifically on engine
mounts.

Are those regulatory specs?

In any case, it's just an example. The crucial point is that Va is a speed
that limits the _acceleration_ that the control surfaces can impose before
the plane stalls, whereas Vno is a speed that limits the _force_ that the
wings can develop before the plane stalls. Therefore, staying below Vno is
what keeps the wings attached and intact, whereas staying below Va is what
keeps _other_ parts of the plane attached and intact (because the plane's
acceleration determines the force exterted upon other structures). This
distinction is key to understanding why Va is proportionate to the square
root of weight, whereas Vno is independent of weight. (Whether or not the
engine mounts are the weak link in the rest of the plane presumably varies
from one aircraft to another.)

--Gary


Dan