Getting out of the clouds in a Ventus C

On Sunday, May 31, 2015 at 7:04:28 AM UTC-7, wrote:
I find this thread very interesting.

Matt--

So the yaw string was streaming to the right, and the ship was slowly turning right?

What would you estimate the bank angle to be, and in which direction?

I was banked left about 15-20 degrees (guessing from memory), nose was significantly down. I had to hold full right rudder with some effort to keep it in a slip. Yaw string was to the right. No spoilers or you loose pitch control due to turbulence over horiz. stab.

Matt

Update on test #2

Logger showed descent rate to be between 1500-2900 fpm. Turn rate was about 45 seconds per turn.

Matt

And if you try the more generally recommended approach of airbrakes out, hands and feet off? That seems to work in both the gliders I most often fly.

You should enter the recovery mode from an unusual attitude since you will probably be in an unusual attitude very soon after losing external visual reference.

Unless I have reason to think the cloud goes down to the ground, I would not consider bailing out unless/until the wings come off.

You shouldn't be in an unusual attitude soon after losing visual
reference unless you immediately begin wiggling the stick around. If the
controls stay pretty much where they were, the glider should maintain
the same attitude, more or less. But then I only have a paltry 42 years
experience with instrument flying...

On 5/10/2015 11:13 AM, waremark wrote:
And if you try the more generally recommended approach of airbrakes out, hands and feet off? That seems to work in both the gliders I most often fly.

You should enter the recovery mode from an unusual attitude since you will probably be in an unusual attitude very soon after losing external visual reference.

Unless I have reason to think the cloud goes down to the ground, I would not consider bailing out unless/until the wings come off.

--
Dan Marotta

On Sunday, May 10, 2015 at 9:26:20 AM UTC-7, Matt Herron Jr. wrote:
Update on test #2

Logger showed descent rate to be between 1500-2900 fpm. Turn rate was about 45 seconds per turn.

Matt

Matt,

To be realistic and get a true sense of what will happen please do this with you eyes closed.

Richard

You first, smart guy.

On Sunday, May 10, 2015 at 5:50:09 PM UTC-7, Matt Herron Jr. wrote:
You first, smart guy.

Matt,

Seriously once you go into a cloud you will have no visual references. After about 30 seconds especially with the maneuver described you will have vertigo. What happens when you get out of the cloud? To establish your visual reference again may take another 30 seconds depending how serious the vertigo and what you do when you experience this. Things may get much worse. It is not just the aircraft that is the issue.

Richard

And your direction of bank was to the left I presume...

I've experienced something very similar in Challenger ultralight, climbing under power.

In hindsight I wish I had experimented with seeing what would happen if the bank angle were perturbed significantly in one direction or the other. If find myself flying a sailplane that exhibits these characteristics, I'll be sure and check that out. Would be good to know, if one is contemplating using that technique for a blind descent in a situation with some turbulence. My concern is that if the bank angle is somehow perturbed past wings-level into a right bank, you might end up in a really tight spiral to the right. But I don't know for sure.

Btw this technique wouldn't work in a 2-22. I'm sure you can guess why. The ailerons are too powerful, in relation to the rudder.

It's interesting to think through the dynamics at play here. Why is this situation stable? What are the dynamics that act to restore the bank angle, after a slight perturbation? Are they significantly different than the dynamics of a benign spiral? I suppose the basic dynamics of a stable benign spiral are-- increased bank angle also increases sideslip which interacts with dihedral to create more rolling-out torque. Decreased bank angle also decreases sideslip, which allows other competing rolling-in torques to increase the bank angle.

In the cross-controlled spiral you'd have something kind of similar going on-- you have a strong sideslip (skid?) from the rudder (yaw string streams toward inside of turn, which is toward the high wingtip), creating a roll torque that tends to increase the bank angle. Any decrease in bank angle will increase the turn rate which will tend to increase the drag of the outboard (low) wingtip and decrease the slip (skid?) angle, decreasing the roll torque toward the high wingtip, and allowing other competing factors to drive an increase in bank angle. Any increase in bank angle will decrease the turn rate which will tend to decrease the drag of the outboard (low) wingtip and increase the slip (skid?) angle, increasing the roll torque toward the high wingtip, and driving a decrease in bank angle.

In a normal spiral, the high descent rate creates a requirement for roll toward the low wingtip just to hold the bank angle constant, so roll damping tends to drive a decrease in bank angle. That's why opening the spoilers helps keep the bank angle from increasing.

But in this cross-controlled spiral, the direction of turn is toward the HIGH wingtip, not the low wingtip. The high descent rate creates a requirement for roll toward the HIGH wingtip just to keep the bank angle constant, so roll damping tends to drive an INCREASE in bank angle. (The opposite would be true if the maneuver were performed while climbing under power.) So opening spoilers would NOT be expected to create a stabilizing effect in roll-- this helps to explain how a similar dynamic could exist even in powered flight. Likewise the drag from the sideslip, and the resulting high sink rate, would NOT be expected to contribute a stabilizing effect in roll.

In a normal spiral, the outside (high) wingtip is moving faster than the inside (low) wingtip, which tends to generate a rolling-in torque, increasing the bank angle. In the cross-controlled spiral, the high wingtip is also the inboard wingtip, and it is moving slower than the low wingtip which is also the outboard wingtip, creating a roll torque that tends to decrease the bank angle.

It's a curious animal. I wonder what other gliders show exhibit this stable cross-controlled spiral behavior-- and how stable it really is?

S

Now I see you did say left, thanks.

On Tuesday, June 2, 2015 at 6:10:53 AM UTC-7, wrote:
Now I see you did say left, thanks.

not sure about the full aerodynamic affect of roll stability, but I imagine if the wings started to level off the turn rate would increase. This would give more speed to the outboard wing which would tend to drop it back down due to full upward deflection of the aileron on that wing. I did observe pitch stability during this maneuver, with the nose coming up, aircraft slowing down, then nose dropping again, speeding up.

Worth noting that with a 45 second turn rate and an average decent rate of 2200 fpm, I am loosing 1650 feet per turn. It won't take more than 2 turns or so to get me out of the clouds, so there is not a lot of opportunity for the maneuver to decay into something nasty if indeed it is possible.

Matt