new performance data, for those that care....

I had a chance last Sunday to get out through the Oregon weather and do a short three way speed run.

Engine 21.4 map, 2400 rpm. ALT 8750 oat 26 F

153 kts @ 119
172 kts @ 37
149 @ 252

The TASCALC.XLS spread sheet gives the following answer:

Wind Speed 16 @ 194
TAS 158 kts

I was burning 8 gph.

I am happy, except that the pitot must have a leak because the EFIS was showing 149 kts.

Kent

kentb said:

I am happy, except that the pitot must have a leak because the EFIS was showing 149 kts.

Click to expand...

A pitot leak check won't take long to do, so that is any easy thing to rule out.

It could also be caused by a static source leak, but that is unlikely as they usually lead to a too high IAS, and yours is too low. I'm assuming the 149 kt you mentioned is a TAS reading on your EFIS, and not an IAS reading.

Another possibility is static source position error. Fitting flush static ports rather than Van's pop rivet ports have caused IAS to read about 10 kt too low on some other RVs.

Kevin Horton said:

Fitting flush static ports rather than Van's pop rivet ports have caused IAS to read about 10 kt too low on some other RVs.

Click to expand...

So you're saying that the Vans pop rivet static ports appear to be more accurate (on average) than the flush static ports, is that correct?

I had that exact problem on my 9A using the old Cleaveland flush static ports. My EFIS and steam gauge airspeed was almost exactly 10 MPH slower than the 3-way speed run indicated. I replaced the flush mounted ports with Cleavelands newer domed ports and then the EFIS and steam gauge speeds were right where they should be.

airguy said:

So you're saying that the Vans pop rivet static ports appear to be more accurate (on average) than the flush static ports, is that correct?

Click to expand...

I've seen several reports from RV flyers who had RVs with flush static ports who found the ASI read about 10 kt too low. One RV-4 guy first noted the problem as an altimeter that read about 100 ft below field elevation whenever he made a high speed pass down the runway - this was just another symptom of the same problem - a static port that sensed a pressure that was higher than the ambient pressure.

It seems that the pressure on the side of the aft fuselage is a bit higher than free stream ambient pressure. The airflow has to accelerate over the head of the pop rivet static port, and the pressure decreases as the velocity increases, bringing the pressure at the static port close to the free stream pressure - Bernoulli's Law at work.

Some manufacturers of the truly flush static ports eventually realized that they didn't provide an accurate static source on RVs. They came up with updated designs that had some sort of bump, to attempt to get the same reduction in sensed static pressure that is achieved by Van's recommended domed pop rivet. I haven't seen any data that tells me how well these other static ports work, but I haven't seen any complaints, so I suspect they work reasonably well.

As a point of reference, on my RV-8, with the pop rivet static port, the static system position error contributes less than two kt to the airspeed error in most of the flight envelope. ASI instrument error contributes about another two kt error.

I have a every expensive high tech static port.

It came from Vans and I installed it with a rivet puller.

I think the problem is due to removal of one of my BMA EFIS units that I sold. I just cut the tubes off and doubled them over and then safety wired them in a crimped position.

I am hoping that the other EFIS sales soon and then I will redo the plumbing.

Kent

I can vouch for a static leak causing high A/S indication. A couple of weeks ago I took off in my 8A and at cruise power I was indicating 180 kts. For a quick moment I'm thinking "this is great" but something has to be wrong. At stall speeds the indication was right on. The hose to my encoder had split and the static system was basically vented to the cabin.

Paul Danclovic
Jamestown NC
RV-8A N181SB

Here is more data.

I have redone my pitot/static system. I switched over to using the sweet connectors and red piping that you can get from Stein. Ground testing is less then 100 ft in one minute with alt at 2000 ft. The airspeed dropped only 2 kts in one minute at 160 kts.

Now for the flight data ( hope the formating work in the post)add periods in the hope that the columns would work)

Presure Alt...MP.... RPM....OAT gph peek bearing GS ASI
8030...........22.....2690....67 10.7 100 141........169 144
8030...........22.....2690....67 10.7 100 44.........166 145
8030...........22.....2690....67 10.7 100 283........163 144
8030..........22.2....2590....67 10.5 100 282........161 142
8030..........22.2....2500....67 10.2 100 281........160 141
8030...........22.1...2500....67 7.6 -13 281........155 136
8030...........22.1...2500....67 7.5 -26 172........158 136
8030...........22.1...2500....67 7.5 -26 51..........156 137
8030...........22.2...2400....67 7.3 -34 251........154 134
8030...........22.3...2300....67 7.3 -44 191........156 134
8030........... 22.....2210....67 7.5 .... 190........156 132
8030...........21......2090....67 7.7 .... 161........146 127
8030...........20......2000....67 6.8 .... 161........139 120
8030...........18.9...1900....67 6.2 .... 161........134 114
8030...........17.9...1790....67 5.6 .... 161.........127 108

This worked out to 166 kts at 2700 rpm and 10.7 gph.
Or
156 kts at 2500 rpm at 7.5 gph.

Kent