Need advice for porpoising

[PhantomPholly]

I could probably experiment with this in the air over a period of time and finally get it right, but am hoping you can save me a lot of trial-and-error.

Out flying my Lancair 320 small tail short nose, elevator servo connected to the inner hole (less movement per adjustment of the servo) on a nearly calm day, cruising around 190mph IAS at 7,500'. Initial settings:

Vrt Activity 3
Vrt Torque 12
Min Airspeed 80
Max Airspeed 300
Static Lag 0
Micro Activity 0
Half Step? No

Flying straight and level this resulted in mild porpoising, not unexpected as I experienced the same in my previous Lancair with TT autopilot.

Changed Static Lag to 1 - better.
Changed Static Lag to 2 - smooth.

Dial in a 180 degree course change - roll in smooth, then - HOLD ON!!!

:eek:

In turn I tried:
Reduce Static Lag to 1
Change Micro Activity to 1
Increase activity to 5
Change half-step to Yes

Repeat the exercise after each change to test for improvement. No dice, no single change resulted in any noticeable improvement.

So, before I try every permutation searching for a good solution, does anyone have a recommendation of settings to try?

[TrutrakTech]

With increasing the static lag progressively smoothing it out, it appears you have a static lag problem. Change static lag back to 2, turn the half-step back off. The other two are probably OK.

Vent the static port on the AP to the cabin to eliminate any source of static lag, I bet you'll find it performs better. Let me know how it does. Thanks!

[PhantomPholly]

With increasing the static lag progressively smoothing it out, it appears you have a static lag problem. Change static lag back to 2, turn the half-step back off. The other two are probably OK.

Vent the static port on the AP to the cabin to eliminate any source of static lag, I bet you'll find it performs better. Let me know how it does. Thanks!

Hehe - I've gone down that road before. Opening a vent inflight in that circumstance causes a really wild ride! Yes, I can do this for a trial (I have an internal selector) but would really prefer not to (it would also affect my altimeter unless I re-plumb the system).

My static system is a two-port setup using standard ports from Aircraft Spruce and "hard" tubing. It was just tested and there are no leaks. Why would it have significant static lag?

[TrutrakTech]

Hehe - I've gone down that road before. Opening a vent inflight in that circumstance causes a really wild ride! Yes, I can do this for a trial (I have an internal selector) but would really prefer not to (it would also affect my altimeter unless I re-plumb the system).

My static system is a two-port setup using standard ports from Aircraft Spruce and "hard" tubing. It was just tested and there are no leaks. Why would it have significant static lag?

It is most certainly a test. I don't recommend using the AP in this manner other than to eliminate lag in the static system as a testing procedure.

Any instrument connected to the static system becomes a reservoir. They cause a lag in the system. Altimeters don't care, VSI's don't care. But the AP DOES care. It is incredibly sensitive to static lag because of what it uses the static pressure for. Ideally, the AP should be on a dedicated static system. Lag in the system is completely unrelated to leaks. While leaks can cause all kinds of undesired results, having lag does not mean you have a leak.

[PhantomPholly]

It is most certainly a test. I don't recommend using the AP in this manor other than to eliminate lag in the static system as a testing procedure.

Any instrument connected to the static system becomes a reservoir. They cause a lag in the system. Altimeters don't care, VSI's don't care. But the AP DOES care. It is incredibly sensitive to static lag because of what it uses the static pressure for. Ideally, the AP should be on a dedicated static system. Lag in the system is completely unrelated to leaks. While leaks can cause all kinds of undesired results, having lag does not mean you have a leak.

Ah, then if I used smaller diameter tubing for my static system the lag would be reduced, because the tubing itself is a resevoir. Wish I'd known that before I instlled it all, I'd have used a microtube!

Ok, so the ideal installation is a tiny tube to a dedicated port, and it doesn't have to be accurate in the same way the altimeter port is.

Sounds like a tiny tube run straight to the engine compartment would work good?

[TrutrakTech]

Ah, then if I used smaller diameter tubing for my static system the lag would be reduced, because the tubing itself is a resevoir. Wish I'd known that before I instlled it all, I'd have used a microtube!

Ok, so the ideal installation is a tiny tube to a dedicated port, and it doesn't have to be accurate in the same way the altimeter port is.

Sounds like a tiny tube run straight to the engine compartment would work good?

Correct. Large tubing is a reservoir in and of itself. Length of tube can also affect lag. Something else to keep in mind.

Probably not a good idea to put one in the engine compartment as the positive pressure of the cooling air will probably throw off the reading.

[PhantomPholly]

My other thought would be the gear well, but that might have similar effects when raising/lowering the gear as a cockpit solution when opening/closing a vent.

Refresh my memory - isn't the only critical factor that the pressure reading be consistent relative to the altitude? Although air in the engine compartment has a high flow rate, I would think pressure would be relatively stable and correlated to altitude.

I really don't want to drill another hole in my nice fiberglass plane...

[glider]

You need a location where the pressure ONLY varies with altitude. The pressure in the engine compartment will vary with airspeed and possibly angle of attack. A port inside the engine compartment will act more like a pitot tube than a static port, and in fact the pressure plenum has been successfully used as an alternate pitot source.

Pat

[PhantomPholly]

Thanks. I suppose as a stop-gap I can put it in the wheel well and just be prepared for pitch change when the gear is activated. Grrr, don't want to have to install yet another static port...

[PhantomPholly]

Found a combo which is at least acceptable so thought I'd post it. I have NOT yet created an alternate static source with a smaller diameter tube anywhere.

Vrt Activity 6 (increased by 3)
Vrt Torque 12
Min Airspeed 80
Max Airspeed 300
Static Lag 2 (Increased to Max)
Micro Activity 0
Half Step? No

In light chop it flew straight and level acceptably. Upon commanding a 90 degree heading change, it was initially smooth, imperfect but tolerable in the turn, and did a couple of light porpoises on rollout.

Still not perfect, will eventually try another static source. May end up using cabin air...