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Bob Axsom

Well Known Member
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
 
More Speed for the RV's....

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
 
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. :D can't say I'm not guilty myself.
 
Duh!

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)
 
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No need to change anything, Bob

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.
 
Well this is why

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
 
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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. :rolleyes:

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)
 
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)
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Thats the special Conversion that also effects the radio music level too right? ;)
 
On another performance note

Speaking of performance. I just got back from a trip with my buddy in a Seminole. ooh neat a twin :cool: ... It seems that the only thing the 2nd o-360 does is carry the extra weight from its fuel and itself. 140knots... I can't wait to finish my 9a and do that for way less than 20+ gallons per hour!

On the other hand I probably wouldn't fly across lake MI like we did today in a single.

Jeff
9emp in the mail
 
1911pilot said:
On the other hand I probably wouldn't fly across lake MI like we did today in a single.

Jeff
9emp in the mail
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Yes you will, because you built the plane and will trust it. I flew to a waterski tournament a week or so ago in Milwaukee from Detroit. Flew over the widest part of Lake Michigan at an angle. I know my plane and I know what I'll do if I have a problem. You'll be the same way. Plus it's only a few minutes of flying that you can't glide to land. I plan 4 Lake Michigan crossings during Oshkosh this year.


Fallguy, yes you get points for that, talk to Van about cashing them in.
 
Yes you will, because you built the plane and will trust it.
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Good point. I'll sure trust it a lot more than that poor beat down seminole. after I landed up at MNM I realized that I could see through a hole in the floor. The dust tape holding the door and ceiling molding on was a nice touch too.
 
St. Louis Check Results

I tookoff into a 700 ft overcast returning from St. Louis today and it was IFR all the way back but I did have a chance to check the earlier observation. I guess "Cam's" observation and my earlier interpretation have some validity. When I open the vent the TAS drops off 3-5 kts. There appears to be some settling to the low end of the difference but the margine did not disapear. Anyway, the "clear" aluminum vents ($290) arrived while I was in St. Louis and they are NICE. Will get then in well before Oshkosh.

Bob Axsom
 
Static Port on Altrak Connected to Static System

I completed the plumbing to the static port on the Altrak controller last night so I will be able to check the "cockpit vent open, nose up, slow down" characteristic in altitude hold mode. That probably will not happen until the flight to Dayton on Saturday because there is still so much to get done before then. The approach I took to making the change was to cut the 1/4" static tube a few inches forward of the "T" between the 1/8" tubes from the two static ports and insert another "T" in that line. I installed a 1/8" tube from that "T" down to the Altrak controller's static port. I used a MS21919DG to keep the 1/8" tube from flopping around because I already had a anchornut clip there from a previous ELT coax cable routing. I don't remember the size of the clamp but it is snug without restricting the tube size due to squeezing. I used silicon adhesive to secure the now longer (because of the "T" insert) 1/4" tube to the side skin a few inches from the "T" in its routing forward to the instrument panel.

Bob Axsom
 
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Test Today

I took the plane up to 3500 ft and ran at 2450 RPM and as many inches as I could get (something over 25) with plane trimmed for level, Altrak and pictorial pilot on. Every time I opened the new aluminum vent (man do they look good!) the speed went down 3-5 kts. When I closed the vent the speed came back. Did not thry both vents. Tested approximately 5 times in each of two oposing directions. Very consistent.

Bob Axsom
 
I tried this on a flight this morning. Only going about 130 knots at the time. I got a small altitude change 20-300 feet but couldn't get more than a knot speed change. The speed always came back after a few seconds and the plane leveled off. Interesting....... Wonder whats going on here???

Do you have EI and is it inside the plane?

Cam
 
130 Knots ?

130 knots is like a slow walk in the park - I don't think you will find much sensitivity to drag down at that level and 3-5 kts is a greater percentage change at that level than at 170 kts which is the speed I most usually fly at. I was doing 180+ knots on the GPS one direction and 160 in the other with the vent closed. When I opened the vent there was a slower bleed off of speed than the previous condition but the magnitude was the same 175-177 and 155-157 respectively. When I closed the vent the speed gradually climbed back up the the original level. I remaind in each condition far longer than necessary to achieve a steady state condition and of course constant GS. I didn't bother looking at the TAS. I wanted a same source digital readout for the two vent conditions with every thing constant in a series of repeated tests. Since I have hooked up the Altrak to the aircrafts static system I do not notice the aircraft pitchup trying to correct for the perceived altitude change from the Altrak setting. After more than $300 of upgrades I am convinced that the test method was adequate and valid for the specific effect being evaluated and the difference is real.

Bob Axsom
 
I believe you Bob, I'm just surprised that the airvents will change the speed that much. I can only do about 164 knots in my little engined RV but I'll give it a shot the next time up. I generally don't fly that fast anymore. I like the fuel savings af 7.5 gallons and hour.

Cam
 
Thanks Cam

Thanks for your critical review Cam it caused me to do work and make changes I should have but was too cheap and lazy to do. Keep up the good work.

Bob Axsom
 
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