Performance Observation

[Bob Axsom]

I made an interesting observation due to the ALTRAK altitude hold system in the airplane during a quick trip to New Orleans last week. First of all the system performance with my Pictorial Pilot and Altrak autopilot system for IFR flight is nothing short of fantastic! Ammendments to clearance, changing charts as part of taking care of business is no longer a problem. That is everything I had hoped for. Now for the interesting observation in cruise at altitude (8,000 ft. in this case) with the Altrak holding altitude when I open the vent in the cockpit the angle of attack increases and the TAS decreases 3-5 kts. It is dramatic and consistent without altitude hold I would never have noticed this. Note that the NACA inlets are always open this change is strictly due to opening a single eyeball vent (the big plastic ones) in the cockpit. It makes perfect sense of course but I never expected to see such a difference nor did I expect it to be so dramatically observable.

Bob Axsom

[Bob Martin]

Thanks Bob!
Thats all I need to edge past the long-ez we fly with....wink!
What a wonderful and easy tip!!!
Bob Martin, RV-6

[mark manda]

can you tape off the NACA ducts and make a run?

[N916K]

Don't worry Mark, the speed comes back when the Altrak returns to what it thinks is the correct altitude. It's just flying a constant pressure altitude, the vents change the pressure inside the plane so the Altrak thinks it's too low. Bob probably did what most others do, not hook the Altrak up to the static ports and just use the pressure inside the plane. I use that trick to demo pressure altitude to my fellow engineers. Sometimes engineers just have to be beat over the head with something to believe it. can't say I'm not guilty myself.

[Bob Axsom]

WOW - you are probably right. I did not connect the input of the controller to the external static source. I could put another "T" in the plumbing back there and correct that and recheck it to get the real contributing but I hate taking that bulkhead back out. We are going to St. Louis Saturday I will check it out over time but my intuition says you are correct. [I just ordered the parts to connect the controller static port to the aircraft static system - can't have Altrak chasing an unstable source all over the sky.]

EE*

Bob Axsom

* Embarassed but Enlightened (again)

[N916K]

Bob, why change it?? It's only a small altitude change if any. Most of the time the altitude goes back to the original altitude after the pressure inside the plane stabilizes. It's just that an in rush of air that causes a small pressure wave in the plane.

[Bob Axsom]

We are flying in the cross country air race from Dayton to Oshkosh and as much as I would like to tape the inlets and keep everything closed, I know that's not going to happen. I want the Pictorial Pilot and Altrak to fly a perfectly straight line. I just can't stand the porposing as my wife's vent goes in and out to retain conscousness - she's a trooper but I can only ask so much. I also ordered the $145 apiece aluminum vent controls to replace the original plastic ones - mine is OK but hers just flops wherever the pressure takes it. In normal circumstances I would do exactly as you suggest because the oscillation is kind of trivial. I know it is not going to make much difference in the race either but if I could just beat one in "360 RV Class" it would make my day. Jeanine says if we finish last again she will never fly in another race - I really love the competion and at 68 it is hard to play football.

Bob Axsom

[N916K]

Oh, now I see Bob. The Van's cheap parts causing pitch oscillations problem. I've seen this before. I had that really nice JC Whitney hose pop off the NACA duct on one of my first flights. Outside temps where around 20F at the time. I love it that Vans leaves the JC Whitney label on the box.

As far as going faster in the race. That's easy. You need to advance your throttle all the way to 11. (bonus points if you get that one)

[Fallguy]

Do I get points?

[1911pilot]

As far as going faster in the race. That's easy. You need to advance your throttle all the way to 11. (bonus points if you get that one)

Thats the special Conversion that also effects the radio music level too right?