Lancair Legacy Design Flaw?

On Fri, 31 Oct 2008 12:09:27 -0700, Alan Baker
wrote:


The one thing not quite right is that there is no important difference
between tractor vs. pusher configurations with respect to directional
stability.

Like some of the early rocket designers (e.g. Goddard), you are falling
into the fallacy that somehow pulling is more stable than pushing. This
is not so.
Pusher or tractor makes virtually no difference, but the vast
majority of pilon mounted engines ARE pushers - and the pilon mounted
engine does have that nasty quirk.

In article ,
wrote:

On Fri, 31 Oct 2008 12:09:27 -0700, Alan Baker
wrote:


The one thing not quite right is that there is no important difference
between tractor vs. pusher configurations with respect to directional
stability.

Like some of the early rocket designers (e.g. Goddard), you are falling
into the fallacy that somehow pulling is more stable than pushing. This
is not so.
Pusher or tractor makes virtually no difference, but the vast
majority of pilon mounted engines ARE pushers - and the pilon mounted
engine does have that nasty quirk.

Yup, but that's because the pylon (note spelling) places the thrust line
far above the centre of mass, creating a long moment arm.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg

In article ,
"Gregory Hall" wrote:

France? I don't live in France. I built and used to fly a Rotec Rally 2B
many years ago. It was a tail dragger with a high wing and the motor was
mounted atop the win[g] with a pusher prop.

When I got it trimmed out correctly at cruise speeds I could lean forward in
the seat to nose it down and lean back in the seat to nose it up.

Sort of like a Taylorcraft or Cessna 140 or similar small aircraft.

Even as
well-balanced as it was at about half throttle, when the engine quit it
would pitch up immediately and drastically because the high engine placement

.... Placed the thrust line enough above the center of drag that adding
power caused a downward pitch moment, and reducing power resulted in a
upward pitch.

and pusher prop had enough leverage so that the proper trim at the tail
counteracted the nose down force of the engine and prop.

Which is what the horizontal stab/stabilator is for.

If you didn't
immediately push the stick way forward when the engine quit it was a matter
of seconds before it would nose up fast and stall and then you would have no
control at all from the stick until it fell for a while and the nose dropped
(thank god for that) so you could gain speed provided you had enough
altitude to get control of it again. But it didn't glide too well being a
single surface wing with wire bracing. Perhaps 2:1 glide ratio. But it was
easy to land with no power but you had to come in hot and steep and at the
last second pull back on the stick and flare it.

It looks to me like the Legacy would act pretty much the same if the engine
quit.

Except for the Legacy not incorporating those design elements that
result in the pitch/power response of the Rotec Rally. The Rally needs a
lot of upward pitch dialed in for level cruise (which ought to be
contributing a lot of drag as an added bonus), giving you some nasty
response to losing power.

In particular, both thrust and drag components in the Legacy are much
closer in alignment, resulting in much less pitch change when power
changes.

The two aircraft behave very differently in many aspects, and the Legacy
not much at all as you've asserted.

"Steve Hix" wrote in message
...
In article ,
"Gregory Hall" wrote:

France? I don't live in France. I built and used to fly a Rotec Rally 2B
many years ago. It was a tail dragger with a high wing and the motor was
mounted atop the win[g] with a pusher prop.

When I got it trimmed out correctly at cruise speeds I could lean forward
in
the seat to nose it down and lean back in the seat to nose it up.

Sort of like a Taylorcraft or Cessna 140 or similar small aircraft.

Even as
well-balanced as it was at about half throttle, when the engine quit it
would pitch up immediately and drastically because the high engine
placement

... Placed the thrust line enough above the center of drag that adding
power caused a downward pitch moment, and reducing power resulted in a
upward pitch.

and pusher prop had enough leverage so that the proper trim at the tail
counteracted the nose down force of the engine and prop.

Which is what the horizontal stab/stabilator is for.

If you didn't
immediately push the stick way forward when the engine quit it was a
matter
of seconds before it would nose up fast and stall and then you would have
no
control at all from the stick until it fell for a while and the nose
dropped
(thank god for that) so you could gain speed provided you had enough
altitude to get control of it again. But it didn't glide too well being a
single surface wing with wire bracing. Perhaps 2:1 glide ratio. But it
was
easy to land with no power but you had to come in hot and steep and at
the
last second pull back on the stick and flare it.

It looks to me like the Legacy would act pretty much the same if the
engine
quit.

Except for the Legacy not incorporating those design elements that
result in the pitch/power response of the Rotec Rally. The Rally needs a
lot of upward pitch dialed in for level cruise (which ought to be
contributing a lot of drag as an added bonus), giving you some nasty
response to losing power.

In particular, both thrust and drag components in the Legacy are much
closer in alignment, resulting in much less pitch change when power
changes.

The two aircraft behave very differently in many aspects, and the Legacy
not much at all as you've asserted.


Thanks guys. I think I understand the differences now. The part about the
forces being in better alignment makes sense to me and pusher vs. puller.
I'll have to retract my ill-conceived statements about the Legacy. By bad!

--
Gregory Hall

In article ,
"Gregory Hall" wrote:

"Vaughn Simon" wrote in message
...

"Gregory Hall" wrote in message
...


It looks too much like an irresponsible, hot rod, stunt plane to me.

Well, you sucked me in at first, so on a troll scale of zero-to-10 you
rate at least a five. How are things in France?

Vaughn


France? I don't live in France. I built and used to fly a Rotec Rally 2B
many years ago. It was a tail dragger with a high wing and the motor was
mounted atop the wind with a pusher prop.

When I got it trimmed out correctly at cruise speeds I could lean forward in
the seat to nose it down and lean back in the seat to nose it up. Even as
well-balanced as it was at about half throttle, when the engine quit it
would pitch up immediately and drastically because the high engine placement
and pusher prop had enough leverage so that the proper trim at the tail
counteracted the nose down force of the engine and prop. If you didn't
immediately push the stick way forward when the engine quit it was a matter
of seconds before it would nose up fast and stall and then you would have no
control at all from the stick until it fell for a while and the nose dropped
(thank god for that) so you could gain speed provided you had enough
altitude to get control of it again. But it didn't glide too well being a
single surface wing with wire bracing. Perhaps 2:1 glide ratio. But it was
easy to land with no power but you had to come in hot and steep and at the
last second pull back on the stick and flare it.

It looks to me like the Legacy would act pretty much the same if the engine
quit.

Greg,

What was happening to you was not caused by the *weight* of the engine,
but the change from higher than CoM thrust creating a pitch down torque,
to higher than CoM drag creating a pitch up torque.

Congratulations: you've just rediscovered one disadvantage of having a
thrust line that doesn't go through the centre of mass.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg