Bad landing

On Mar 6, 3:25*pm, Chris Reed wrote:
Frank Whiteley wrote:
This might make you feel better about a few of your landings.

http://www.youtube.com/watch?v=eamnTyfkUBY

Song is a bit off color.

Even those landings where the rudder was used didn't seem to have much
rudder *to* use, nor did the rudder seem to make a lot of difference to
where the aircraft was pointing.

That plus, apparently, no aileron control after touchdown, plus the fact
that full flap gives about the same float as a no airbrake landing in a
conventional glider, suggests this is rather a handful.

Would the aerodynamicists know whether these characteristics result from
optimisation for high altitude flight?

I think the rudder is adequate - people do fly it successfully.
Rather, what I saw was a pilot trying to 'steer' with aileron. If the
rudder was used, it was far too late to be effective.

Indeed, flap retraction once on the ground would have helped but how
slowly do they retract? If they are hydraulic or electric, it might
take more time than they have after touchdown.

There are two typical 'gotchas' for transitioning power pilots. They
tend to forget that there is only one wheel and not three so let a
glider tip over before responding and they are usually not too quick
with the rudder. I think I saw that in the video too.

Eric,

For the purpose of the discussion, let's separate stability from
maneuverability. The two are coupled in that a very stable design will
require large control surfaces to overcome stabilizing forces.

I would think for ease of landing you would want good stability. We're
all aware of how gusts and changing winds have an affect on us during
landing. Stability is essentially a measure of how quickly an an
aircraft will return to it's static state after an upset. If the
stability is high, it would return to this state quickly, and if it is
low, it won't. If it is unstable, it will diverge and get worse. I
can only imagine that would help during landing. Rather than having to
correct for each and every upset during approach, the pilot could
focus more of their tasks on execution rather than correction. Does
that seem a reasonable hypothesis?

However, too much stability would be bad as well. There would have to
be a balance. Perhaps because the controls are non-boosted, and it
seems that an effort was made to properly balance the airplane for
it's mission. At mach 0.71@ 70,000 feet or greater, pilots report that
it's a delight to fly and handle at these conditions. I would suspect,
that the vast difference in these two extremes, probably resulted in
some compromises to it's stability and maneuverability during landing.
Of course, this is all speculation on my part because I was not
involved with the design, but it seems reasonable and fits with my
understanding of stability and control.

-Kevin

PS We've had some soarable weather here on the West side of the
cascades, but I'm itching to start flying in Ephrata. The SGC is
hosting a 3 day racing encampment during Memorial day. Any plans to
attend? I've met you before on a couple of occasions, but haven't sat
down and talked at length. You're one of the local resources of
knowledge that I would enjoy learning from.

KevinFinke wrote:
Eric,

For the purpose of the discussion, let's separate stability from
maneuverability. The two are coupled in that a very stable design will
require large control surfaces to overcome stabilizing forces.

I would think for ease of landing you would want good stability. We're
all aware of how gusts and changing winds have an affect on us during
landing. Stability is essentially a measure of how quickly an an
aircraft will return to it's static state after an upset. If the
stability is high, it would return to this state quickly, and if it is
low, it won't.

Would you agree it's also a indication of much it will react to upsets?
Generally, I think, the more stable the aircraft, the more it will react
to gusts. So, an aircraft with mild/neutral stability won't react to the
gust, and there will be nothing to correct. I think that's better in
gusty conditions, because there is always another gust, so if aircraft
is "stable", you spend a lot of time correcting.

If it is unstable, it will diverge and get worse.

Well, maybe, except you have the pilot to damp the motions, easy to do
if the divergence is slow. And, at landing speeds, things are happening
slowly.

I
can only imagine that would help during landing. Rather than having to
correct for each and every upset during approach, the pilot could
focus more of their tasks on execution rather than correction. Does
that seem a reasonable hypothesis?

I'm still thinking a mildly/neutrally stable aircraft is going to be
easier to handle in gusts. I don't know how stable the U2 is, but I
don't see how we can tell, just by looking at the dihedral. There are
other factors, like the amount of sweep back and the effective rudder
size, so I fussing about picking just dihedral as an important factor.

However, too much stability would be bad as well. There would have to
be a balance. Perhaps because the controls are non-boosted, and it
seems that an effort was made to properly balance the airplane for
it's mission. At mach 0.71@ 70,000 feet or greater, pilots report that
it's a delight to fly and handle at these conditions. I would suspect,
that the vast difference in these two extremes, probably resulted in
some compromises to it's stability and maneuverability during landing.
Of course, this is all speculation on my part because I was not
involved with the design, but it seems reasonable and fits with my
understanding of stability and control.

My guess is it has so much yaw inertia (and maybe roll inertia) and slow
response (that's the maneuverability compromise, I'm sure) to both
rudder and aileron during landing, that the pilots were way behind the
aircraft. That's typical of power pilots used to flying short wing
aircraft with good response to control inputs at landing speeds. I
suspect that's a more likely explanation than the effect of dihedral.

-Kevin

PS We've had some soarable weather here on the West side of the
cascades, but I'm itching to start flying in Ephrata. The SGC is
hosting a 3 day racing encampment during Memorial day. Any plans to
attend? I've met you before on a couple of occasions, but haven't sat
down and talked at length. You're one of the local resources of
knowledge that I would enjoy learning from.

We're planning a safari to Golden, BC, the Memorial Day week and
weekend, but we plan to come up to fly during (not in) the contest for a
couple days. We'll see you then!

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* Sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

On Mar 7, 12:24*pm, KevinFinke wrote:
Paul, that's exactly what I was thinking. In watching the videos, it
looked like the aileron effectiveness was very poor, so if a wing
started rolling, it just kept going. That combined with the low tip
clearance meant that the airplane looked very prone to easy ground
looping. If the airplane had better roll stability, I think it would
be easier to land.

In the flying article, they comment that in order to save weight, the
airplane has un-boosted controls. The plane really only lightens up in
control forces at it's design mission. The rest of the time, it takes
a lot of strength and force to move them. Can't imagine that's very
easy after flying for a long mission, now I have to do a strength
workout just to land.

-Kevin

IIRC, the only 'boost' control was the vernier control on the engine
exhaust nozzle. Controllable to within 1.5deg F.

Frank