Re: 40% Stab size or 20%?

From: Geoff <gbower_at_gmail.com>
Date: Sat, 23 Oct 2010 05:55:54 -0000

It's the difference between static and dynamic stability. The positive static margin in the spreadsheet indicates that the model will tend to return to it's initial conditions if disturbed, this is static stability. What it doesn't tell you is if it will overshoot that initial condition and diverge, or if it will be well damped and return to the trim condition.

With small tail volumes (small tail and/or short boom length) the tail doesn't provide enough pitch damping. The damping is provided by the change in tail lift due to pitch rate. For a longer boom the tail will be moving faster in the vertical direction as the model pitches, this will lead to a bigger change in the angle of attack on the tail and a stronger force (well, moment) that is resisting the pitching motion. Similarly, a larger tail area will increase this force. This is where the tail volume coefficient comes from: Tail Area*Boom Length/(Wing Area*Wing Chord). If the volume coefficient is too small chances are the longitudinal dynamic modes go unstable -> airplane diverges. When stable these modes are known as the short period and phugoid modes.

Still lurking...
Geoff



--- In Indoor_Construction_at_yahoogroups.com, Don Slusarczyk <don@...> wrote:
>
> The models had slow/poor recovery. Poor recover and "unstable in pitch"
> are two separate things (at least to me). If someone says to me a model
> is "unstable in pitch" that means to me that the model is completely
> untrimmable and will not fly at all it, i.e. it can not be trimmed to
> fly at any speed or any angle of attack. That is not what happened, the
> models flew but when disturbed had poor recovery. So to me they were
> pitch stable (they flew) in calm air but had poor recovery when
> disturbed. For a small tail to work well in a duration model you would
> need a long tailboom, but 36" long EZBs with 15% tails are simply not
> practical. You can however do that on an outdoor FF model much easier,
> and long tail arms with "small" stabs seem to be popular.
>
> Don
>
> >
> >
> > The model is pitch stable it just had poor recovery.
> >
> > You lost me here. Please elaborate. Do you mean slow/marginal
> > recovery? Is this not the same as low SSM/pitch instability?
> >
> >
> >
> >
> >
> >
> >
>
Received on Fri Oct 22 2010 - 23:16:09 CEST